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Study of membrane fouling in a membrane enhanced biological phosphorus removal process Geng, Zuohong 2006

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STUDY OF MEMBRANE FOULING IN A MEMBRANE ENHANCED BIOLOGICAL PHOSPHORUS REMOVAL PROCESS by ZUOHONG GENG A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA November 2006 ©Zuohong Geng, 2006 Abstract Membrane fouling is an inherent problem that has been associated with membrane processes since the emergence of membrane technologies. In the present research, membrane fouling in a membrane enhanced biological phosphorus removal (MEBPR) process was, for the first time, investigated systematically with respect to membrane fouling mechanisms, roles of sludge constituents in membrane filtration, characteristics of membrane foulants, and the relation of sludge properties to fouling. It was revealed that membrane fouling in the pilot scale M E B P R process was hydraulically irreversible and was mainly due to the adsorption of dissolved organic matter in activated sludge mixed liquor, particularly extracellular polymeric substances (EPS) such as carbohydrates and humic or humic-like substances. Large sludge floes were found to likely exert dual effects on membrane filtration. At low flux, they tended to induce hydraulic resistance via sludge deposition. At high flux, large sludge floes seemed to be able to mitigate fouling by sterically hindering the transport of soluble and colloidal sludge components to membrane surfaces. Unlike in other wastewater treatment membrane bioreactors, biofouling in the form of microbial growth was not significant in the studied M E B P R process due to the vigorous aeration and frequent backflushing applied. Inorganic fouling (i.e. precipitation of struvite) was also not observed. The accumulation of foulants (i.e. carbohydrates and humic or humic-like substances) on membrane surfaces resulted in an increase of surface hydrophobicity, which in turn might have accelerated the fouling process. The physical and biochemical properties of sludge, including floe size distribution, zeta potential, relative hydrophobicity, bound and unbound (soluble) EPS content, were also examined in the present research. Compared to a conventional sludge, the higher content of soluble EPS in activated sludge mixed liquor and the smaller floe size distribution were very likely responsible for the higher fouling propensity of the M E B P R sludge. Thus, the content of soluble EPS in mixed liquor was suggested to be a key property to evaluate the fouling potential of activated sludge mixed liquor. Contrary to the literature, the content of EPS bound in the sludge matrix was found not to be a direct influencing factor to membrane fouling. i i Table of Contents Abstract ii Table of Contents iii List of Tables .. ix List of Figures xi List of Symbols xv Acknowledgements xviii Dedication xix Chapter 1. Introduction 1 Research Background 1 Introduction to Membrane Coupled Activated Sludge Processes 3 Conventional Activated Sludge Processes 3 Basics of Membrane Filtration 4 Membrane Coupled Activated Sludge Processes 5 Literature Review on Membrane Fouling 9 Mechanisms of Membrane Fouling 9 Factors Influencing Membrane Fouling 13 Physical and Empirical Fouling Models 23 Fouling Prevention and Control Strategies 26 Research Scope and Objectives 28 References 30 Chapter 2. Investigation of Mechanisms of Membrane Fouling in a Membrane Enhanced Biological Phosphorus Removal Process 35 Introduction 35 Methods 36 The U B C Wastewater Treatment Pilot Plant 36 The Membrane Filtration Module 39 On-line and Off-line Filtration Tests 43 SMP Effect on Sludge Filterability 45 iii Results and Discussion ; 46 Filtration Performance of the Pilot Scale Membrane Module in the M E B P R Process 46 Membrane Fouling Behavior in the Off-line Filtration Tests 48 Effect of Soluble Microbial Products on Membrane Fouling 52 A Mathematical Model for the Long-term Fouling in the M E B P R process 53 Conclusions 58 References 59 Chapter 3. Roles of Various Sludge Constituents in Membrane Filtration of Activated Sludge Mixed Liquor 61 Introduction 61 Methods 62 Sludge Sampling and Fractionation 62 Bench Scale Filtration Tests 64 Analysis of the Soluble Fraction of Sludge 66 Results and Discussion 66 Contribution of Sludge Constituents to Membrane Fouling at a Low Permeate Flux 66 Behavior of Large Sludge Floes in Membrane Filtration at High Permeate Fluxes 72 Steric Hindrance Effect of Large Sludge Floes 75 Characteristics of the Soluble Fraction of Sludge 77 Conclusions 81 References 81 Chapter 4. Characterization of Fouled Membranes from a Membrane Enhanced Biological Phosphorus Removal System 83 Introduction 83 Methods 85 The M E B P R Process 85 iv Membrane Operation 86 Membrane Sampling 86 Analytical Methods 87 Results and Discussion 90 Surface Morphology of Fouled Membrane Fibres 90 Composition of Membrane Foulants 93 M A L D I - M S Analysis 95 Effect of Fouling on Membrane Hydrophobicity 97 Conclusions 98 References 99 Chapter 5. A Comparative Study of Fouling-related Properties of Sludge from a Membrane Enhanced Biological Phosphorus Removal Process and a Conventional Enhanced Biological Phosphorus Removal Process 102 Introduction 102 Methods 104 The Wastewater Treatment Pilot Plant 104 Assessment of the Fouling Propensity of Activated Sludge Mixed Liquor 105 Measurement of Sludge Floe Size and Surface Properties 106 EPS Extraction and Quantification 107 UV-visible Absorbance of Bound and Soluble EPS and Foulant Extract 109 Results and Discussion 110 Fouling Propensity of Activated Sludge Mixed Liquor 110 Floe Size and Surface Properties of Sludge 112 Bound and Soluble EPS in Activated Sludge Mixed Liquor 113 UV-visible Absorbance of Bound and Soluble EPS in Comparison to Foulant Extract 117 Conclusion.... 118 References 119 v Chapter 6. Research Overview 122 Some Points about the Research Discrepancies 122 Bound vs. Unbound (Soluble) EPS and EPS Components 122 Roles o fMLSS 123 An Extended Discussion about the Effects of the Sizes of Membrane Pores and Sludge Constituents on Membrane Fouling 124 Strategies for Prevention and Control of Membrane Fouling 127 Application of Appropriate Aeration Intensity 128 Optimization of the Biological Process Design 128 Addition of Porous Materials 128 Evolution of Membrane Materials 129 References 130 Chapter 7. Conclusions, Engineering Significance and Future Work 133 Conclusions 133 Engineering Significance of the Research 136 Future Work 137 Effect of SRT on Membrane Fouling 137 Removal of Foam Layer in the Anoxic Zone 138 Effect of Aerobic vs. Anaerobic Conditions on Membrane Fouling ... 138 Optimization of Aeration Intensity 139 References 139 Appendices 141 Appendix 1. The U B C Wastewater Treatment Pilot Plant 142 Appendix 2. Clean and Fouled Membrane Modules at the U B C Pilot Plant 143 Appendix 3. The On-line and Off-line Filtration Apparatus 144 Appendix 4. Determination of I C 5 0 Value of CuS0 4 -5H 2 0 With Respect to the M E B P R Sludge 145 Appendix 5. Filtration Performance of the M E B P R Membrane Module During the Entire Experimental Period at the U B C Pilot Plant 146 Appendix 6. Data Summary for Fouled Membranes Filtering Clean Water in Comparison With Virgin Membranes 149 VI Appendix 7. Data Summary of Development and Breakdown of the Transmembrane Pressure of the Off-line Membrane Loops Filtering the Aerobic M E B P R Sludge 151 Appendix 8. Data Summary of Sludge Filtration Tests With/Without C u 2 + Inhibition.. 152 Appendix 9. Data Summary for Modeling the Long-term Fouling in the M E B P R . Process 153 Appendix 10. Sample Preparation for GPC Analysis 155 Appendix 11. Data Summary of the Filtration Tests With the Four Sludge Fractions ... 156 Appendix 12. Data Summary of the Dissolved TOC Level in the Mixed Liquor Collected from Both the M E B P R and CEBPR Processes at the U B C Pilot Plant 159 Appendix 13. Data Summary of the Fine Particle Size Distributions of the Soluble Fractions of Both the M E B P R and CEBPR Sludge 160 Appendix 14. Additional S E M Images of Virgin and Fouled Membranes Sampled from the UBC Pilot Plant 162 Appendix 15. Additional X-ray Microanalysis Profiles of Fouled Membranes Sampled from the U B C Pilot Plant 164 Appendix 16. Additional Examples of the M A L D I - M S Profiles of the Foulants Extracted from the M E B P R Membrane Fibers 165 Appendix 17. Calculation of the Extraction Efficiency of Organic Nitrogen from Fouled Membrane Fibers 167 Appendix 18. Measurement of Contact Angles of Virgin and Fouled Membrane Fibers 168 Appendix 19. Flux-step Filtration Tests of the M E B P R Sludge and the CEBPR Sludge Collected in the Second Experimental Run 169 Appendix 20. S E M Images of the Aerobic M E B P R Sludge and the Aerobic CEBPR Sludge .-, 170 Appendix 21. Examples of Sludge Floe Size Measurement 171 Appendix 22. Data Summary of Sludge Floe Size Measurement 172 Appendix 23. Examples of Sludge Zeta Potential Measurement 173 Appendix 24. Data Summary of Sludge Zeta Potential Measurement 174 vii Appendix 25. Data Summary of Sludge Relative Hydrophobicity Measurement 175 Appendix 26. The Initial Measurement of Various EPS Components Bound in Activated Sludge Floes 176 Appendix 27. Data Summary of Measurement of EPS Components Bound in Activated Sludge Floes 178 Appendix 28. Data Summary of Measurement of Soluble EPS in Activated Sludge Mixed Liquor 179 viii List of Tables Table 2.1 Influent characteristics and overall treatment performance of the M E B P R process and the CEBPR process 38 Table 2.2 Specifications and operating limits of the major membrane module used in the MEBPR process;..:........ ... .43 Table 2.3 Model parameters for the two experimental runs................. 54 Table 3.1 Characteristics of the four fractions of both the MEBPR and CEBPR sludge 64 Table 3.2 Dissolved TOC retained in the M E B P R system 78 Table 3.3 Molecular weight distribution of the soluble fractions of sludge and permeate 79 Table 4.1 Characteristics of the influent to the MEBPR system 85 Table 4.2 Elemental composition of virgin and completely fouled membrane skins .. ..93 Table 4.3 Chemical analysis of foulants extracted from completely fouled membranes 94 Table 4.4 Comparison of contact angles between virgin and completely fouled membranes 97 Table 5.1 Operating regime and characteristics of the U B C pilot plant 105 Table 5.2 Run time of the MEBPR membrane module in different experimental runs 110 Table 5.3 Ratios of protein to carbohydrate and protein to total EPS in bound and soluble EPS of the two types of sludge 116 Table 6.1 Selected results from previous studies on membrane fouling in water and wastewater treatment applications 125 Table 7.1 Run time of the major membrane module in different experimental runs ... 137 Table A4.1 Respiration rate of the M E B P R sludge at different concentrations of CuS0 4 -5H 2 0 145 Table A5.1 Filtration performance of the M E B P R membrane module during the experimental period 146 Table A6.1 Filtration test of clean water using fouled membranes 149 ix Table A6.2 Filtration test of clean water using virgin membranes 150 Table A7.1 Development and breakdown of the transmembrane pressure of the off-line membrane loops filtering the aerobic membrane sludge 151 Table A8.1 Sludge filtration tests with/without C u 2 + inhibition 152 Table A9.1 Raw data for modeling the long-term fouling in the MEBPR process 153 Table A 11.1 Filtration tests with the four fractions of both the aerobic MEBPR sludge and the aerobic CEBPR sludge at the flux of 23 L/m 2.h 156 Table A l 1.2 Filtration tests with the four fractions of the aerobic MEBPR sludge at the increased fluxes of 33 L/m 2.h and 68 L/m 2.h 157 Table A l 1.3 Relative fouling rates of the four fractions of both the aerobic MEBPR sludge and the aerobic CEBPR sludge at the flux of 23 L/m 2.h 158 Table A l 1.4 Relative fouling rates of the four fractions of the MEBPR sludge at the increased fluxes of 33 L/m 2.h and 68 L/m 2 .h 158 Table A12.1 Dissolved TOC level in the mixed liquor collected from both the MEBPR and CEBPR processes 159 Table A 17.1 Extraction efficiency of organic nitrogen from fouled membrane fibres 167 Table A22.1 Sludge floe size measurement 172 Table A24.1 Sludge zeta potential measurement ..174 Table A25.1 Sludge relative hydrophobicity measurement 175 Table A27.1 Measurement of carbohydrates and proteins bound in activated sludge floes 178 Table A27.2 Measurement of humic substances and total EPS bound in activated sludge floes 178 Table A28.1 Measurement of soluble carbohydrates and proteins in activated sludge mixed liquor 179 Table A28.2 Measurement of soluble humic substances and total EPS in activated sludge mixed liquor 179 x List of Figures Figure 1.1 Conventional activated sludge process 3 Figure 1.2 Schematic of M C A S P with (a) external and (b) internal membrane filtration 6 Figure 1.3 Schematic representation of the maximum performance of an M C A S P compared to a conventional ASP with enlargements for advanced wastewater treatment 7 Figure 1.4 The UCT-type biological phosphorus removal process 9 Figure 1.5 Schematic illustration of major events in membrane biofouling process 11 Figure 1.6 Membrane fouling in relation to various influencing factors 14 Figure 1.7 Theoretical flux evolution as a function of the trans-membrane pressure .... 24 Figure 2.1 Schematic of the U B C Wastewater Treatment Pilot Plant comprising the MEBPR process and the CEBPR process 37 Figure 2.2 Daily performance of the M E B P R and CEBPR processes at the U B C Wastewater Treatment Pilot Plan 42 Figure 2.3 An illustration of the on-line membrane filtration apparatus 44 Figure 2.4 Experimental setup of bench-scale off-line filtration tests 45 Figure 2.5 The TMP accumulation process during a filtration run of the pilot scale membrane module in the MEBPR process at the U B C pilot plant 47 Figure 2.6 The TMP profiles of the major membrane module at the U B C pilot plant during the entire experimental period 48 Figure 2.7 The TMP profiles of fouled membranes filtering clean water at a flux of 33 L/m 2-h in comparison with virgin membranes 49 Figure 2.8 Development and breakdown of the TMP of the off-line membrane loops filtering the aerobic M E B P R mixed liquor of 5.7 g M L S S / L at a constant flux of 33 L/m 2-h 50 Figure 2.9 Filtration performance of the MEBPR sludge inhibited by C u 2 + in comparison to that of the control sludge at a volumetric airflow rate of 60 m 3/m 3.h , 52 x i Figure 2.10 Model development for the long-term irreversible fouling in the pilot scale MEBPR process during the two experimental runs 55 Figure 3.1 Scheme for fractionation of activated sludge mixed liquor 63 Figure 3.2 Transmembrane pressure during the filtration of the four fractions of (a) an aerobic M E B P R sludge and (b) an aerobic CEBPR sludge at a flux of 23 L/m 2.h 67 Figure 3.3 Hydraulic resistance due to fouling in filtration of the four fractions of (a) the aerobic MEBPR sludge and (b) the aerobic CEBPR sludge at a flux of 23 L/m 2-h 68 Figure 3.4 The relative fouling rates of the individual sludge fractions to the whole sludge fouling at a flux of 23 L/m 2.h 69 Figure 3.5 Relative contribution of the different constituents of (a) the aerobic MEBPR sludge and (b) the aerobic CEBPR sludge to the short-term membrane fouling at a flux of 23 L/m 2-h 70 Figure 3.6 Comparison of the absolute fouling rates of the different M E B P R sludge constituents to those of the CEBPR sludge constituents at a flux of 23 L / m 2 h 70 Figure 3.7 The fouling rates of the individual M E B P R sludge fractions relative to the whole sludge fouling at the increased fluxes of (a) 33 L/m~.h and (b) 68 L/m 2.h 73 Figure 3.8 Fouling rates of the four fractions of the aerobic M E B P R sludge at different operating fluxes 74 Figure 3.9 Postulated mechanism for the steric hindrance effect of large sludge floes on membrane filtration 75 Figure 3.10 TOC contents of the influent, CEBPR effluent, M E B P R permeate and the soluble fractions of the sludge collected from the three compartments of both the M E B P R process and the CEBPR process in May 2004 77 Figure 3.11 Molecular weight distribution of the THF-dissolved substances in the MEBPR permeate and the soluble fractions of the sludge collected from the aerobic zones of the M E B P R and CEBPR processes 79 xii Figure 3.12 Fine particle size distributions of the soluble fractions of sludge collected from the aerobic zones of the M E B P R process and the CEBPR process .... 80 Figure 4.1 The pilot scale membrane enhanced biological phosphorus removal (MEBPR) system 85 Figure 4.2 Overall scheme for characterization of fouled membranes and foulant extract : 87 Figure 4.3 S E M images of virgin membrane surfaces (a-b) and completely fouled membrane surfaces (c-h) at different magnifications 91 Figure 4.4 S E M images of (a) clean and (b) fouled inner fabric support of membrane fibre 92 Figure 4.5 X-ray microanalyses of (a) the foulant layer on membrane skin and (b) the crystal-like particles found on the fouled inner fabric support 92 Figure 4.6 The M A L D I mass spectra of the foulants extracted from the inner support of completely fouled membrane fibres that were sampled on April 27, 2004 96 Figure 4.7 The trans-membrane pressure profile of the M E B P R process for the entire filtration run from March 7 to April 27, 2004 98 Figure 5.1 Flux-step filtration tests of (a) the MEBPR sludge and (b) the CEBPR sludge collected in Run I I l l Figure 5.2 Variation of (a) sludge floe size distribution; (b) zeta potential; (c) relative hydrophobicity during the two experimental runs 112 Figure 5.3 Content of EPS bound in activated sludge floes during Run I and Run II... 114 Figure 5.4 Content of soluble EPS in activated sludge mixed liquor during Run I and RunH 115 Figure 5.5 UV-visible absorbance of bound and soluble EPS in comparison to that of membrane foulant extract 118 Figure 6.1 A generic relationship between the pore size of a membrane filter and the nature of fouling in membrane coupled activated sludge processes 126 Figure 7.1 The modified MEBPR process with the membrane module submerged in the anoxic zone 138 Figure A4.1 Inhibition of Cu 2 + on the microbial activity of the M E B P R sludge 145 x i n Figure A13.1 Number-based fine particle size distribution of the soluble fractions of both the MEBPR and CEBPR sludge 160 Figure A13.2 Volume-based fine particle size distribution of the soluble fractions of both the MEBPR and CEBPR sludge 161 Figure A 14.1 Additional S E M images of virgin membranes at the magnifications of 100 K (a) and 25 K (b and c) 162 Figure A14.2 Additional S E M images of the fouled membranes at the magnifications of 5 K (a), 9 K (b), 10 K (c and d), 20 K (e) and 25 K (f, g and h) 163 Figure A15.1 X-ray microanalysis of the foulant layer on the surface of fouled membrane fibers 164 Figure A15.2 X-ray microanalysis of the crystal-like particles on the surface of the inner fabric support of fouled membrane fibers 164 Figure A 16.1 M A L D I mass spectra of the foulants extracted from the outer skin of completely fouled M E B P R membrane fibers sampled on April 27, 2004 (DHB as the matrix) 165 Figure A 16.2 M A L D I mass spectra of the foulants extracted from the inner support of completely fouled M E B P R membrane fibers sampled on April 27, 2004 (DHB as the matrix) 166 Figure A19.1 Flux-step filtration tests of (a) the M E B P R sludge and (b) the CEBPR sludge collected in Run II 169 Figure A20.1 S E M images of the aerobic M E B P R sludge collected in Run I at the magnifications of 6 K (a) and 1 K (b) 170 Figure A20.2 S E M images of the aerobic CEBPR sludge collected in Run I at the magnifications of 5 K (a) and 1 K (b) 170 Figure A21.1 Measurement of the floe size distribution of (a) the aerobic MEBPR sludge and (b) the aerobic CEBPR sludge collected on July 30, 2003 171 Figure A23.1 Zeta potential measurement of (a) the aerobic M E B P R sludge and (b) the aerobic CEBPR sludge 173 Figure A26.1 Initial measurement of EPS components bound in activated sludge floes 177 xiv List of Symbols 0 a function of the mass transfer properties of a membrane system [i permeate viscosity AP pressure drop across the membrane At or t filtration time A V permeate volume A surface area ASP activated sludge process B A P biomass-associated products BNR biological nutrient removal BSA bovin serum albumin C concentration of organic substances in the liquid phase of mixed liquor CEBPR conventional enhanced biological phosphorus removal CER cation exchange resin COD chemical oxygen demand C O M conventional optical microscopy DDW distilled deionized water D N A deoxyribonucleic acid DO dissolved oxygen DOC dissolved organic carbon D O M dissolved organic matter E D T A ethylenediaminetetraacetic acid E D X energy dispersive x-ray microanalysis EPS extracellular polymeric substances F / M food to microorganism ratio GPC gel permeation chromatography HRT hydraulic retention time IC50 50% inhibitory concentration J flux kL organic loading-based irreversible fouling coefficient xv kt time-based irreversible fouling coefficient kv volume-based irreversible fouling coefficient L cumulative organic loading on the membrane surfaces M A L D I - M S matrix assisted laser desorption ionization - mass spectrometry M B R membrane bioreactor M C A S P membrane coupled activated sludge process MEBPR membrane enhanced biological phosphorus removal M F microfiltration MLSS mixed liquor suspended solids M W molecular weight M W C O molecular weight cut off NF nanofiltration NH4 -N ammonia nitrogen NO3 -N nitrate nitrogen N O M natural organic matter P hydraulic permeability PAC powdered activated carbon PO4-P phosphate phosphorus P-total total phosphorus PVDF polyvinylidene fluoride Q flow rate R a hydraulic resistance due to adsorptive fouling R a p hydraulic resistance due to adsorption and deep pore clogging R c hydraulic resistance due to cake formation Rf hydraulic resistance due to fouling R g hydraulic resistance due to gel layer formation R m hydraulic resistance due to membrane itself R n hydraulic resistance due to irreversible fouling RO reverse osmosis R p hydraulic resistance due to pore blocking R s hydraulic resistance due to filtration of activated sludge xvi R t orR total hydraulic resistance SBR sequencing batch reactor S E M scanning electronic microscopy SMP soluble microbial products SRT sludge retention time THF tetrahydrofuran T K N total Kjeldahl nitrogen TMP transmembrane pressure TOC total organic carbon TSS total suspended solids U A P utilization-associated products U B C The University of British Columbia UCT The University of Cape Town UF ultrafiltration V F A volatile fatty acids VSS volatile suspended solids xvii Acknowledgements First of all, I would like to thank my supervisor Dr. Eric R. Hall for providing the opportunity for me to do this research. Through his supervision and guidance, I learned how to work as a decent scholar. I also want to thank my supervisory committee members Dr. Donald S. Mavinic, Dr. Pierre R. Berube, and Dr. James Piret for their valuable advice on my research work. In addition, I'm very grateful to Susan Harper and Paula Parkinson for their great assistance to my experimental work. Special thanks go to Dr. Noel A . Buskard (Haematologist) and Dr. James V. Dunne (Rheumatologist). Without their kind care and outstanding medical skills, it would not be possible for me to recover quickly from a serious health condition and finally complete this research. The financial support to the MEBPR project from the Natural Sciences and Engineering Research Council of Canada (NSERC), and the technical and financial aid from Zenon Environmental Inc., Stantec Consulting, and Dayton and Knight Ltd., are also acknowledged. xviii To my parents & my husband xix Chapter 1. Introduction Research Background Membrane coupled activated sludge processes (MCASP), or membrane bioreactors (MBR), constitute a class of innovative wastewater treatment technologies that have gained growing attention in recent decades. These processes are actually modified types of conventional activated sludge processes (ASP), in which solids-liquid separation is completed by means of membrane filtration, and the secondary settling tank is thus eliminated. These new activated sludge processes combine biological waste degradation with membrane separation, leading to a number of advantages such as superior effluent quality, a small space requirement, lower sludge production, and the decoupling of hydraulic retention time (HRT) and sludge retention time (SRT) (Giinder, 2001; van der Roest et al, 2001; Ramphao et al, 2005). As effluent discharge standards become more and more stringent and land use is becoming an increasing concern in many cities, membrane coupled activated sludge processes have been considered as plausible alternatives to conventional wastewater treatment processes that are particularly suited for expansion and upgrading of existing activated sludge systems (Ahn et al, 1999). Therefore, the widespread application of M C A S P in both industrial and municipal wastewater treatment is a predictable trend in the near future (de Korte et al, 2001). Like other membrane technologies, however, membrane coupled activated sludge processes are constrained by the tendency of membranes to foul (Gander et al, 2000; Judd, 2005). Fouling is a general term that refers to an increase of membrane hydraulic resistance. Membrane fouling seems to be an inherent problem with membrane processes, which not only affects the long term operational stability, but also leads to significant operational costs due to increased membrane replacement frequency and added energy consumption. In this regard, considerable research and engineering effort has been devoted to understanding the mechanisms of membrane fouling and to working out fouling prevention and control strategies. At present, most existing knowledge of membrane fouling is based on the research and practices in the water treatment area. Due to the relative scarcity of membranes in wastewater treatment applications at the present time and the very complicated nature of membrane fouling, knowledge of fouling 1 mechanisms, characteristics, and influencing factors in membrane coupled activated sludge processes is extremely limited. Therefore, extensive research work on membrane fouling in wastewater treatment is definitely necessary to achieve long-term stable membrane filtration and to benefit the increasing application of M C A S P in the future. A few years ago, preliminary research was conducted at The University of British Columbia (UBC) Wastewater Treatment Pilot Plant to assess the feasibility of an improved membrane-coupled activated sludge process for removal of phosphorus (F. Koch, Department of Civi l Engineering, UBC, Vancouver, Canada, personal communications). The studied process integrated membrane filtration technology with a conventional enhanced biological phosphorus removal process (CEBPR) and was thus . named as a membrane enhanced biological phosphorus removal (MEBPR) process. The" experimental data on the performance of the M E B P R process indicated that membrane filtration in place of secondary clarification enabled the process to operate at substantially higher organic loadings and lower hydraulic retention times than a conventional process, without compromising the phosphorus removal efficiency. Although the results were quite encouraging, it was still a pioneering and very preliminary research work, and many aspects of this new MEBPR process including the interactions between different reaction zones, microbial evolution and adaptation, membrane fouling mechanisms and control strategies, effects of operational parameters on the treatment performance, design criteria and process control, etc., were basically unknown or uncertain. Therefore, further research work was required in order to gain an in-depth understanding of the M E B P R process and finally to optimize the process for future application. In view of such needs, an integrated research project was proposed, which involved investigation of the M E B P R process in comparison to a conventional process, process modelling, and the study of membrane fouling in the M E B P R process. The present membrane fouling research is therefore a sub-project of the above integrated project. It was intended to make use of the existing experimental facilities at the UBC Wastewater Treatment Pilot Plant to explore the nature of membrane fouling in the MEBPR process, to examine the effects of plant design and operating conditions on sludge fouling behaviour, and to develop solutions to the fouling problem. It was expected that upon the completion of this project, a good understanding of membrane fouling in the 2 MEBPR process would be established and the corresponding fouling prevention and control strategies would be formulated, which would in turn provide a valuable basis for the overall optimization of the process and contribute to the long-term stable operation of the MEBPR system. Introduction to Membrane Coupled Activated Sludge Processes Conventional Activated Sludge Processes The activated sludge process (ASP) is the most commonly used secondary wastewater treatment process. It utilizes a so-called "activated sludge", a suspension containing active biomass, to oxidize organic contaminants to CO2, H 2 0 , N H 4 and some intermediate products, and at the same time, to grow new cell biomass. Conventional ASP is an aerobic treatment and air is provided by diffused or mechanical aeration to meet the oxygen demand of microbial growth and metabolism. Treated wastewater is separated from the biomass through gravitational settling. Figure 1.1 is a graphical presentation of the conventional ASP and its major components are described as follows (Britton, 1994). Sludge digester Figure 1.1. Conventional activated sludge process sludge Primary clarifier is mainly employed for removal of settleable and floating materials in the raw wastewater. It usually provides a quiescent environment that allows solids to settle to the bottom of the tank. The liquid portion of the flow then goes to the aeration tank, and the solids containing portion goes to sludge digesters. Aeration tank is also called activated sludge tank, where aerobic oxidation of organic matter is carried out. Primary effluent is introduced and mixed with returned 3 activated sludge to form the mixed liquor, which contains 1500-2500 mg/L of suspended solids (or activated sludge). The recycling of a large portion of the biomass makes the sludge retention time (SRT) much greater than the hydraulic retention time (HRT) of the wastewater. This helps maintain a significant amount of microorganisms that effectively oxidize organic compounds in a relatively short time. The wastewater detention time in the aeration tank usually varies between 4 hours and 8 hours. Secondary/final clarifier, also called sedimentation tank, is used for separation of treated wastewater from the biomass by means of sedimentation of activated sludge floes produced during the oxidation phase in the aeration tank. A portion of the settled sludge in the clarifier is recycled back to the aeration tank and the remainder is wasted to maintain a proper concentration of biomass in the mixed liquor. Basics of Membrane Filtration Membrane filtration is primarily a physical process for the separation of solids/solutes from liquids. In pressure-driven membrane processes, liquids and all the species that are smaller than the membrane pores are transported across the membrane, while others that have larger particle sizes are unable to permeate and thus are retained by the membrane. In this regard, membrane pore size is a very important characteristic of a porous membrane. In the order of descending pore size (or molecular weight cut-off), membranes are generally classified as microfiltration (MF) membranes, ultrafiltration (UF) membranes, nanofiltration (NF) membranes, and reverse osmosis (RO) membranes (Giinder, 2001). In pressure-driven filtration, the most important concern is the permeate production capacity of the membranes. Therefore, the specific permeate volumetric flow, also called permeate flux (J), is the most straightforward measure for evaluating the hydraulic performance of a membrane system. Under steady state conditions, permeate flux can be calculated from permeate volumetric flow (Q) divided by the membrane surface area (A), or the permeate volume (AV) relating to both the membrane surface area (A) and the filtration period (At) (Equation 1.1) (Giinder, 2001). In the case of non-steady state conditions, only an average flux can be specified. J = Q / A = A V / ( A At) (1.1) 4 It is usually assumed that permeate flux follows Darcy's law, that is, flux is directly proportional to the pressure drop across the membrane (AP) or the transmembrane pressure (TMP), reciprocally proportional to the absolute viscosity (\x) of the liquid to be filtered and the hydraulic resistance of the filtration system (Wiesner and Aptel, 1996). When filtering clean water across a virgin membrane, the permeate flux (J) can be expressed as J = A P / 0 i R m ) (1.2) Where, R m is the hydraulic resistance of the clean membrane with a dimension of reciprocal length. In addition, hydraulic permeability (P) is also employed to characterize the filtration ability of membranes. It is determined by the relationship between permeate flux and trans-membrane pressure, as described in the following equation (Grinder, 2001). P = J / A P (1.3) By integrating Equation 1.2 with Equation 1.3, a second expression for hydraulic permeability is obtained. P = l / ( t t R m ) (1.4) Membrane Coupled Activated Sludge Processes The combination of an activated sludge tank and a membrane filtration unit for the separation of activated sludge from treated wastewater is defined as a membrane coupled activated sludge process (MCASP) (Grinder, 2001), or a membrane bioreactor (MBR) process (Manem and Sanderson, 1996). When used for description of activated sludge systems, the terms " M C A S P " and " M B R " are actually synonymous. Membrane filtration has also been applied in drinking water production and in the biochemical engineering industry (Denis and Boyaval, 1991; Urbain et al, 1996). It should be pointed out that the M C A S P or M B R process is definitely different from the wastewater reclamation processes, in which membrane filtration is installed downstream of a biological treatment system as a refining stage or a tertiary treatment step. The major alteration from conventional ASP to M C A S P is the replacement of the secondary clarifier with a membrane filtration unit. Therefore, separation of activated sludge from treated wastewater in an M C A S P is no longer dependent on sludge 5 settleability. Instead, it largely relies on the pore size or the molecular weight cut-off of the filtration membranes used. In practice, both M F membranes and UF membranes are utilized for wastewater / activated sludge separation. Since the maximum pore size of M F membranes typically used in M C A S P is 0.4 |im (Giinder, 2001), the effluent or permeate is free of suspended solids and bacteria, and has a very high quality for wastewater reuse (van Dijk and Roncken, 1997). According to the location of membrane filtration unit, M C A S P is usually classified in one of two categories: M C A S P with external membrane filtration (or recirculated MBR) and M C A S P with internal membrane filtration (or submerged MBR) (Manem and Sanderson, 1996; Giinder, 2001), as shown in Figure 1.2. The latter has gained more popularity due to its lower energy consumption (Gander et al, 2000). To avoid fast clogging of the membranes, crossflow filtration is the only suitable filtration procedure for activated sludge processes (Giinder, 2001). Activated External sludge tank membrane filtration Air T Concentrate Excess (returned sludge) sludge Activated Internal, submerged sludge tank membrane filtration Influent ZZ1 Air Permeate Excess sludge (a) (b) Figure 1.2. Schematic of M C A S P with (a) external and (b) internal membrane filtration Considering the exceptional performance of M C A S P in comparison to conventional ASP, the development of M C A S P represents a revolutionary shift in the field of wastewater treatment (Giinder, 2001), as elucidated in Figure 1.3. The shaded portions represent the enlargements and reconstructions, which are necessary in order to improve the effluent quality to meet stringent requirements. These enlargements and reconstructions can be replaced with the installation of membrane filtration units in the 6 activated sludge tank, so that the conventional ASP is upgraded to an M C A S P that can produce identical or even better effluent quality than the enlarged and reconstructed ASP. In brief, M C A S P has the following characteristics when compared to a conventional activated sludge process. 1) Long sludge retention time (SRT) and high concentration of mixed liquor suspended solids (MLSS). The typical SRT ranges from 10 to 60 days and MLSS falls in the range of 10-30 g/L. Up to 35 g/L biomass concentration has also been shown to be feasible (Manem and Sanderson, 1996; van Dijk and Roncken, 1997; Defrance et al, 2000). 2) Low food/microorganisms (F7M) ratio and reduced sludge production. Due to the extremely high MLSS and long SRT, an M B R is somewhat like an aerobic sludge digester, where microorganisms are in an endogenous growth phase with very low growth rates. Zero sludge production has even been reported (van Dijk and Roncken, 1997; Gander et al, 2000). Influent • M C A S P Permeate Influent Activated Sludge tank Enlarge-ment Final clarifier Sand Dis-filtration infection Effluent • Figure 1.3. Schematic representation of the maximum performance of an M C A S P compared to a conventional ASP with enlargements for advanced wastewater treatment 7 3) Superior treatment performance. Typical COD (chemical oxygen demand), BOD (biochemical oxygen demand), and SS (suspended solids) removal efficiencies are often greater than 95%, 98% and 99%, respectively (Manem and Sanderson, 1996). Nitrification efficiency may be 100%. 4) High volumetric loading due to enormously increased specific substrate removal rates. A treatment capacity of up to 5.7 kg COD/m3-day has been demonstrated in an aerobic M B R system treating dairy effluent (Bouhabila et al, 2001). 5) Total separation of SRT and HRT and better control of biological activity (Manem and Sanderson, 1996). 6) Reduced concern about floe settleability and sludge bulking problems (Ramphao et al, 2005). 7) Good flexibility in handling fluctuations in both influent flow and organic loading (Manem and Sanderson, 1996). 8) High energy consumption due to the requirement of mixed liquor recirculation in a recirculated M B R or crossflow aeration in a submerged M B R (van Dijk and Roncken, 1997). In addition to the conventional activated sludge process, membrane filtration has also been coupled to a sequencing batch reactor (SBR) (Ng et al, 2000), a biological nutrient removal (BNR) system (Lesjean et al, 2002), and an anaerobic digestion system (Choo et al, 2000) for different wastewater treatment purposes. The previously mentioned membrane enhanced biological phosphorus removal (MEBPR) process, which is installed at the U B C pilot plant and was investigated in the present study, is one of the first attempts worldwide to integrate membrane technology with the UCT-type biological phosphorus removal process in order to achieve a superior quality effluent. The typical UCT process, illustrated in Figure 1.4, contains three bioreactors in series that are operated under anaerobic, anoxic and aerobic conditions, respectively (Britton, 1994). The principle of phosphorus removal is based on the fact that exposing activated sludge mixed liquor to an anaerobic/aerobic sequence allows the selection of a large population of phosphorus-removing microorganisms. In the anaerobic reactor, these microorganisms 8 release phosphorus to produce energy for uptake of fermentation products. While in the aerobic reactor, they oxidize stored fermentation products and accumulate high levels of intracellular phosphate. The incorporation of the anoxic reactor into the process enables the simultaneous removal of nitrogen. Through coupling with membrane technology, this type of process has been demonstrated to reduce the phosphorus level of an effluent to less than 0.2 mg P t o t _ / L (Lawrence et al, 2001). Mixed liquor recycle Influence Anaerobic Anoxic Aerobic Sludge recycle Clarifier' Effluent Figure 1.4. The UCT-type biological phosphorus removal process Literature Review on Membrane Fouling Mechanisms of Membrane Fouling Membrane fouling is a phenomenon that is expressed as decline of membrane performance such as an increase in trans-membrane pressure (TMP) or a decrease in permeate flux due to the accumulation of substances within membrane pores and/or on the membrane surface (Hong et al, 2002). Substances that cause membrane fouling are collectively referred to as foulants. Fouling is a time-controlled process that may be both reversible and irreversible (Wiesner and Aptel, 1996). For reversible fouling, foulants are weakly associated with the membrane substratum and thus can be physically removed without much effort. Irreversible fouling represents stronger interactions between foulants and membrane materials (i.e., chemical bonds), by which foulants remain on the membrane surface or within the membrane structure even after intensive chemical cleaning. 9 The mechanisms that lead to this time-controlled process are of very different natures. Six fouling mechanisms have been described in M B R applications: scaling, biofouling, organic adsorption, pore blocking, cake formation, and feed debris accumulation (Van Bentem et al, 2001). Scaling is usually caused by mineral precipitation or deposition. Biofouling is defined as biofilm formation that is specifically related to microbial cells, aggregates, and their bio-products, and that results in an unacceptable degree of system performance loss. Organic adsorption is the association of macromolecules with the membrane substratum through van der Waals forces, hydrogen bonds, electrostatic interactions, or strong chemical bonds, and can be classified as either physical adsorption or chemical adsorption. Pore blocking means the clogging of membrane pores by fine colloids that are of similar size to the membrane pore size. Cake formation refers to the development of a layer of solutes on the membrane surface due to the concentration gradient. Fouling by feed debris accumulation, e.g. hair, paper, plastic, grease balls, etc., is a macro-phenomenon and can be prevented or alleviated by influent pre-treatment. Normally, several or all of the above mechanisms take part in the membrane fouling process simultaneously, with one or two dominating. Due to the coincidence of these mechanisms, it is often difficult to attribute performance decline exclusively to one mechanism or the other. In municipal wastewater reclamation applications, however, there is a clear dominance of biofouling in membrane deterioration due to the high concentrations of microbial organisms in activated sludge. Wastewater microbes, e.g. bacteria, fungi, and microalgae, may be passively transported to the membrane surface by diffusion (i.e., Brownian motion), gravitational settling, or bulk fluid convection. Motile bacteria may also actively seek surfaces via chemotactic processes (Ridgway and Flemming, 1996). It's known that bacteria have evolved elaborate adhesion mechanisms, which include stereo-specific or non-specific bonding interactions between the synthetic polymeric membrane substratum and the adhesive structures of microbes, e.g. flagella, fimbria, etc. Once attached to membrane material, a cell will grow and multiply using nutrients transported from the bulk fluid and will thus eventually form a biofilm that covers the membrane surface (Figure 1.5) (Ridgway and Flemming, 1996), resulting in an increase of hydraulic 10 resistance and the deterioration of membrane filtration. In this sense, biofouling can be seen as a "biofilm reactor in the wrong place" (Flemming et al, 1997). Cell #1 Cell #2 Primary adhesion Surface EPS change Secondary adhesion • EPS. Figure 1.5. Schematic illustration of major events in membrane biofouling process Bacterial bonding to a membrane is often mediated and strengthened by extracellular polymeric substances (EPS), which are biosynthesized, secreted, and released by microbial cells due to cell metabolism and auto-lysis (Brown and Lester, 1980; Ridgway and Flemming, 1996). These biopolymers typically consist of polysaccharides, proteins (including enzymes), DNA, lipids, and uronic acids (Spaeth and Wuertz, 2000). It has also been reported that EPS associated with wastewater activated sludge matrices may also contain humic substances which are adsorbed from the wastewater medium (Urbain et al, 1993; Jahn and Nielsen, 1995). Most of the EPS components are either tightly bound to cells as a capsule or loosely associated with cells as a slime. Only a small portion of EPS drifts in the bulk fluid as soluble EPS. Since extracellular polymeric substances are adhesive and viscous, the cells enveloped by them are more tightly attached to the membrane surface through the intimate interactions (i.e., adhesion, adsorption) between bound EPS and membrane surface material, and as a result, the biofilm integrity is reinforced remarkably. Furthermore, EPS enhances the survival and robustness of the biofim microorganisms by absorbing and storing nutrients in the hydrated gel-like matrix for microbial growth and metabolism. Bound EPS also serves as a chemically reactive 11 diffusional transport barrier retarding convective flow and slowing the penetration of anti-microbial agents into the biofilm (Ridgway and Flemming, 1996). Thus, in a certain sense, EPS forms a protective buffer zone that cushions microorganisms against adverse environmental changes such as desiccation, pH value extremes, salt exposure, biocides and hydraulic shears (Spaeth and Wuertz, 2000). High contents of bound EPS have been measured in M B R processes and these were believed to be responsible for both reversible and irreversible biofouling (Nagaoka et al, 1996; Kraemer, 2002; Ng and Hermanowicz, 2005). It has been known that EPS could build up on membrane surfaces and within pore structures through physical and chemical adsorption, leading to smaller filtration areas and greater hydraulic resistance and finally a decrease in membrane permeability (Liao et al, 2004). On the other hand, it was conceived that the water and solute permeation properties of the membrane were altered due to the chemical bonding of EPS (Ridgway and Flemming, 1996). Nagaoka and co-workers (1996) elucidated the influences of EPS on the performance of a submerged membrane separation activated sludge process. It was indicated that EPS accumulated both in the aeration tanks and on the membrane surfaces, causing an increase of viscosity of the mixed liquor and an increase in the filtration resistance. Chang and Lee (1998) confirmed that the predominant flux limitation component within activated sludge is EPS. The higher the content of EPS bound in the activated sludge floes, the greater was the membrane fouling observed. Therefore, they suggested that EPS content be used as an index for assessing membrane fouling tendency in membrane coupled activated sludge systems. Some researchers have studied the biofouling phenomenon from a different perspective: they evaluated the contributions to membrane fouling of different fractions of activated sludge mixed liquor in either recirculated or submerged M B R processes (Wisniewski and Grasmick, 1998; Tardieu et al, 1999; Bouhabila et al, 2001). Under their respective experimental conditions, a similar conclusion was drawn: fouling was mainly caused by the soluble and colloidal fraction of the biological suspension. As the soluble fraction was composed essentially of bacterial residual compounds released by the cells, these findings seemed to support the results of other research work concerning the significant role of EPS in membrane biofouling. 12 Liao et al. (2004) reviewed the biofouling issue in membrane separation bioreactors and indicated that biofouling normally takes place in three patterns: adsorption of EPS to the membrane surface; pore clogging by cells; and sludge cake formation arising from adhesion and/or deposition of individual cells and their aggregates. The three mechanisms are likely to occur simultaneously, and the relative importance of each mechanism is quite case-dependent. Irrespective of the prevalence of biofouling in aerobic M C A S P , inorganic precipitates were revealed to be the most significant foulants in membrane coupled anaerobic bioreactors (Choo and Lee, 1996; Choo et al, 2000). Struvite (MgNH3P04.6H 20), rather than anaerobic microbial floes, was found to have severe fouling effects on both ceramic and polymeric membranes filtering alcohol distillery wastewater. In this regard, the mechanisms and behaviours of membrane fouling in combined anaerobic/aerobic membrane systems, like membrane coupled BNR processes or the MEBPR process in the present research project, are very hard to predict before a complete investigation is carried out. Factors Influencing Membrane Fouling Fouling occurs only when the filtration membrane and the liquid to be filtered interact with each other under favourable hydrodynamic conditions. In view of this prerequisite, the nature and extent of membrane fouling in membrane coupled activated sludge processes are proposed to be determined by the following three aspects (Van Bentem etal, 2001; Liao et al., 2004). • Physico-chemical nature of the membrane • Physico-chemical and biological nature of the mixed liquor • System design, operating conditions and hydrodynamics Each of these aspects involves a number of factors influencing the membrane fouling, which are illustrated in Figure 1.6. The most important factors are discussed below. 13 System design SRT, HRT, volumetric ratio of reaction zones, re-circulation rate, etc. Influent Composition and characteristics of wastewater Biological system Activated sludge properties, e.g., hydrophobicity, surface charge, EPS contents, etc. Operating conditions Organic loading rate, DO, pH, etc. Filtration system Flux, TMP, aeration intensity / re-circulation velocity, backwashing frequency, etc. Membrane Membrane materials, pore structure, surface roughness, module configuration, etc. Figure 1.6. Membrane fouling in relation to various influencing factors Physico-chemical Nature of the Membrane Based on the pore size or molecular weight cut-off, filtration membranes can be classified as micro-, ultra-, nanofiltration membranes and reverse osmosis membranes, as described in the previous section. They can also be categorized as organic (e.g. polyamide, polysulfone, polypropylene, polyethylene, polytetrafluorethylene, and polyvinyl), inorganic (e.g. ceramic, metallic, porous glass), and modified natural products (e.g. cellulose acetate and cellulose nitrate) according to the nature of the membrane material, or symmetric, asymmetric and composite membranes according to the membrane structure (Giinder, 2001). The types of filtration units available include hollow fiber, tubular, flat sheet, etc. For M C A S P applications, hollow fiber M F and UF membranes made of organic polymers are becoming more and more popular, considering the unique characteristics of activated sludge mixed liquor and the relatively low energy consumption of the system. Hong et al. (2002) studied the permeate flux decline with time during the membrane filtration of activated sludge with four different UF and M F membranes. The experimental results showed that the rate of flux decline increased with increasing membrane pore size. The faster decline of permeate flux observed with M F membranes was attributed to enhanced foulant transfer to the membrane surface at high initial operating flux. Thus, the M F membranes experienced more severe fouling due to pore clogging than the UF membranes at the initial stage of M B R operation. On the other hand, it was suggested that to limit fouling due to pore blocking, the pore size distribution of the membrane should have as little overlap as possible with the size distribution of the particles to be filtered. Highly porous membranes with evenly distributed pores enhance filtration efficiency (Manem and Sanderson, 1996). Two membrane surface properties are most important in membrane fouling studies: membrane hydrophobicity and membrane surface charge (Cho and Lee, 1997). As the surfaces of bacteria in activated sludge floes contain hydrophobic molecules such as lipids and thus exhibit hydrophobic areas, some bacteria have a natural tendency to avoid hydrated environments and adhere preferentially to the hydrophobic substratum (Urbain et al, 1993; Chang and Lee, 1998). As a result, hydrophobic membranes usually show a greater fouling tendency than hydrophilic membranes due to the hydrophobic interactions between the surfaces of the membrane and activated sludge microorganisms (Chang et al., 15 1999; Choi et al, 2002). On the other hand, surface charge also plays an important role in biofilm development. It has been demonstrated that most bacteria found in activated sludge are negatively charged with a zeta potential ranging from -10 mV to -30 mV (Kraemer, 2002). So, in general, negatively and neutrally charged membranes are not attractive electro-statically to the bacteria in activated sludge floes. Conversely, positively charged membranes encourage microbial attachment and thus have greater fouling potential. Numerous studies have revealed that in addition to hydrophobicity and surface charge, many other properties of membrane material, i.e. surface roughness and structure, charge distribution, polymer homogeneity and density, surface free energy, surface tension, membrane porosity, etc., all influence bacterial affinity and thus have significant impacts on membrane biofouling (Ridgway and Flemming, 1996; Choo and Lee, 2000; Kraemer, 2002). The relative impact of these properties is hard to determine, and is strongly dependent on the environmental conditions. Finally, the chemical nature of the membrane material should not be neglected. If the membrane material is chemically incompatible with the mixed liquor or favours certain intimate interactions between the membrane and the solutes or bioparticles, the membrane will absolutely deteriorate and fouling will occur. Physico-chemical and Biological Nature of the Mixed Liquor Activated sludge mixed liquor is composed of two parts: wastewater influent coming from the outside of the treatment system and activated sludge floes retained within the system. Therefore, the physico-chemical and biological properties of the mixed liquor are governed by both the nature of the wastewater influent and the characteristics of the activated sludge floes. The fouling effects of the influent are generally attributed to such properties as wastewater composition, types and characteristics of contaminants, organic loading, temperature, pH, etc. An increase in wastewater temperature not only reduces the viscosity of the mixed liquor, but also improves the backtransport of solutes and biosolids from the membrane surface to the bulk fluid by increasing the diffusion ^coefficient and thus is conducive to the alleviation of membrane fouling (Chiemchaisri and Yamamoto, 1994). 16 When the influent contains non-negligible amounts of inorganic salts, pH adjustment is usually necessary to maintain a proper acidic/basic environment to prevent fouling induced by mineral precipitation. Some types of industrial wastewaters are characterized by high concentrations of organic macromolecules, which may cause membrane fouling through organic adsorption and thus need particular attention. Although the effect of influent organic loading on membrane fouling has received limited attention, it has been shown that soluble chemical oxygen demand (COD) of the influent was responsible for an increase of membrane hydraulic resistance (Sato and Ishii, 1991). For activated sludge floes, surface characteristics such as hydrophobicity and surface charge have been reported to affect the fouling process through hydrophobic and electrostatic interactions between the surfaces of biosolids and membranes (Chang and Lee, 1998; Kraemer, 2002). In addition, other physico-chemical and biological properties of activated sludge, i.e. sludge morphology or floe structure, floe size distribution, content and composition of EPS in the sludge matrix, etc., also play significant roles in membrane fouling in activated sludge MBRs. Chang et al (1999) studied the effect of different floe structure (normal, pinpoint, and bulking activated sludge) on membrane fouling behaviour and found that the fouling tendency was in the order of bulking sludge > pinpoint sludge > normal sludge. The experimental results of Wisniewski and Grasmick (1998) illustrated that a higher hydraulic resistance was, to some extent, associated with the smaller floe size distribution of activated sludge, induced by the intensive recirculation in a recirculated MBR. To date, the great importance of the bound EPS content'of activated sludge to membrane biofouling has been well recognized and documented, as stated previously. On the other hand, it is worth pointing out that the liquid phase of activated sludge mixed liquor has received particular attention in the most recent studies due to the significant effect of soluble microbial products (SMP) or EPS in the sludge water phase on membrane filtration performance (Evenblij and Graaf, 2004; Judd, 2005; Lesjean et al, 2005). Although various physico-chemical properties of activated sludge have been investigated separately with regard to their effects on sludge fouling potential, these properties reflect the nature of activated sludge from different perspectives and they are essentially interrelated and integrated as a whole when sludge floes interact with the membrane surface. The relative importance of the sludge properties depends on a number 17 of factors and is hard to assess conclusively due to the different environmental conditions encountered. So far, the correlations between these physico-chemical properties have been little studied. Ridgway and Flemming (1996) investigated the microbial affinity for membranes and found that the physico-chemical properties of biomass were more or less related to the physiological state of cells. The attachment of starved cells to membrane surfaces was less than that of unstarved bacteria, suggesting that the physiological status of bacteria could influence their adhesion. On the other hand, EPS was observed to be synthesized intensively at the endogenous phase of biomass growth and flux reduction occurred more severely in the endogenous phase than in the log growth phase (Chang and Lee, 1998). The variation in growth phase could also explain the difference between the relative hydrophobicity of foaming and non-foaming activated sludge. A study on the influence of growth rates of an E. coli suspension on the performance of a ceramic membrane showed that an increase in the growth rate was associated with a higher membrane fouling rate and a more hydrophobic microorganism surface (Manem and Sanderson, 1996). Since the metabolic state and activity of bacteria determine the nature and extent of biofilm development and the effects of biofouling on membrane performance (Ridgway and Flemming, 1996), examination of the physiology of activated sludge may provide an useful tool for a biofouling study. Finally, viscosity is another physico-chemical property of the mixed liquor that is considered when dealing with membrane filtration (Itonaga et al, 2004). Although viscosity does not directly influence the membrane fouling, it does influence the filtration performance through an inversely proportional relationship with permeate flux (Equation 1.2). In addition, suspension viscosity has considerable impact on the head loss of a recirculated M B R especially at higher recirculation velocities (Manem and Sanderson, 1996). The characteristics of activated sludge, the concentration of mixed liquor suspended solids (MLSS), and the temperature are known to contribute to the rheological behaviour of mixed liquor and affect the suspension viscosity (Gunder, 2001). An exponential relationship was suggested between suspended solids concentration and viscosity of mixed liquor for aerobic sludge (Manem and Sanderson, 1996). Unfortunately, for a non-Newtonian liquid such as activated sludge mixed liquor, 18 viscosity is not a constant factor and cannot be used for a complete characterization of the liquid flow properties (Giinder, 2001). System Design and Operation and Hydrodynamics Design and operation of an M C A S P system involve both the biological treatment system and the membrane filtration system. However, they are not two separate steps. Since the filtration membrane used in M C A S P is not a simple replacement of the secondary clarifier, it should be considered together with the activated sludge system as a single unit operation which has to be considered as a whole (Lawrence et al., 2001). Any design and operation parameters that affect the biological process will undoubtedly have some effects on the membrane filtration performance. Therefore, not only the filtration hydrodynamics such as permeate flux, trans-membrane pressure (TMP), recirculation velocity and/or aeration intensity have important influences on membrane fouling, but also the parameters for design and operation of the biological system, e.g., sludge retention time (SRT), hydraulic retention time (HRT) or organic loading rate, F / M ratio, nutrient conditions, etc., play significant roles in the membrane performance. The difference is that the former exerts direct shear stress on the foulant layer, while the latter impose their influences indirectly by altering or modifying the biological process and consequently changing the activated sludge properties. The following is a short review on these influencing factors. Permeate flux and trans-membrane pressure (TMP) are the two most important operating parameters of a membrane filtration process. To control and minimize fouling, the filtration system should be run within a proper operation window, and both flux and TMP cannot be set too high. Field and co-workers (1995) first introduced the concept of critical flux for microfiltration fouling. Based on the experimental results, they hypothesized that on start-up of membrane filtration, there exists a flux below which a constant flux can be achieved at a constant TMP; above which a constant flux has to be maintained by gradually increasing TMP from an appropriate starting value to cope with the developed fouling. This flux is called critical flux. The value of critical flux depends on the hydrodynamics (e.g., cross-flow velocity) and probably other variables. This critical flux hypothesis was strongly substantiated by the results of Defrance and Jaffrin 19 (1999) and Tardieu et al. (1998). The former investigated the reversibility of membrane fouling by activated sludge in an M B R process, and found that when the permeate flux was set below the critical flux, the TMP remained stable and fouling was reversible. On the contrary, when the critical flux was exceeded, the TMP increased and did not stabilize. The fouling was found to be partly irreversible when the flux was lowered again. It was agreed that selecting an appropriate permeate flux was of crucial importance to obtain the best compromise between flux and TMP and to reduce the degree of fouling as well. Recirculation velocity / crossflow aeration intensity are two measures that are used for recirculated M B R and submerged M B R , respectively, to ensure sufficient turbulence around membrane filtration units in order to prevent adhesion and deposition of bioparticles onto the membrane surface. In a submerged M B R process, as expected, the rate of permeate flux decline was reduced with an increase of aeration intensity. It was reported that by increasing the air flow rate from 1.2 to 3.6 m3/m2-hr., it was possible to decrease the total hydraulic resistance and thus increase the permeate flux by a ratio of 3 (Bouhabila et al, 2001). Like flux, there also existed a critical aeration rate above which flux enhancement no longer occurred (Hong et al, 2002). This might be due to the increased fluid resistance to uplifting air bubbles and the existence of irreversible fouling on the membrane surface or within the pore structures. As crossflow aeration could also result in an over-supply of dissolved oxygen that could impede the denitrification process in a BNR process, a suitable aeration intensity should be determined to meet the requirements for both fouling control and nitrogen removal. On the other hand, a recirculated M B R operated under laminar conditions with a recirculation velocity of 0.5 m/s was reported to experience a severe fouling due to deposition of biological floes. Under completely turbulent conditions with a recirculation velocity of 4 m/s, however, no sludge deposit was observed and the total membrane hydraulic resistance was 100-1000 times smaller than that at lower velocity (Tardieu et al, 1998). Obviously, for both types of M B R processes, adequate crossflow can remarkably reduce the reversible fouling and improve the membrane productivity. Concentration of mixed liquor suspended solids (MLSS) is an important process parameter for operation of activated sludge system. Generally, it was believed that the higher the MLSS concentration, the more severe the membrane fouling, depending on the 20 nature of the biological process (e.g., aerobic and anaerobic). A study on the effect of activated sludge properties on water flux in a membrane separation system revealed an almost linear correlation between the filtration resistance and the concentration of MLSS (R - (MLSS) 0 9 2 6 ) in the range of 20,000-29,000 mg MLSS IL (Sato and Ishii, 1991). However, other investigators reported that fouling was independent of MLSS concentration in relatively dilute mixed liquor until a critical value was reached, beyond which a sharp decline in permeate flux occurred. Yamamoto et al. (1989) found that the critical MLSS concentration was about 30,000-40,000 mg/L, and it varied with the operating conditions. The experimental results of Hong et al. (2002) indicated that MLSS concentrations exhibited very little influence on permeate flux for the range of 3600-8400 mg/L. It appeared that MLSS concentration was not always an important factor and it was probably responsible for the hydraulic resistance increase only in its upper range, say, greater than 30,000 mg/L. Sludge retention time (SRT) has long been a major concern with regard to its influences on both the biological conversion process and the membrane filtration process. The most common discrepancy in the literature regarding membrane fouling in M B R systems is the effect of SRT on filtration performance. Chang and Lee (1998) monitored membrane fouling at SRTs of 3, 8, and 33 days, respectively, and measured the greatest EPS content of activated sludge and hence the greatest filtration resistance at an SRT of 3 days and the least at an SRT of 33 days. Accordingly, it was concluded that the fouling propensity of activated sludge mixed liquor increased as SRT decreased. Similar results were reported by Fan et al. (2000), who ran their M B R at SRTs of 5, 10, and 20 days, respectively, and found that the membrane filtration run time, or the interval between two chemical cleanings, was a function of SRT. A longer cleaning interval (or a lower membrane fouling rate) corresponded to a longer SRT. Conversely, van Houten et al. (2001) discovered that at a long SRT, microorganisms in activated sludge tended to disintegrate and excreted soluble microbial products, leading to high sludge viscosity and high filtration resistance. Considering the diverse research results of the effect of MLSS concentration on membrane fouling and the close relationship of SRT to MLSS concentration (i.e., prolonging SRT correspondingly increases MLSS concentration and decreases F / M ratio, provided that the influent characteristics are unchanged), it can be 21 assumed that a certain value may exist for SRT, below which the fouling potential of activated sludge is reduced with the lengthening of SRT, and above which fouling becomes more severe as SRT increases. Nevertheless, the different experimental results reported by different research groups strongly revealed the complexity of membrane fouling in the M C A S P processes. Organic loading rate is determined by influent organic concentration and hydraulic retention time. It was reported that organic loading rate affected membrane fouling through its influence on the production of EPS (Nagaoka et al, 2000). A study on the effect of organic loading rate on membrane fouling in a submerged M B R process indicated that a reactor with a higher loading rate showed a sudden increase in TMP, while a reactor with a lower loading rate demonstrated delayed increase in TMP (Nagaoka et al., 2000). Moreover, organic loading rate significantly influenced the membrane fouling process only when flux was low (< 4.2 L/m2-hr.) and organic loading rate was low (< 2 g TOC /L-day). Otherwise, it was not very influential and could be increased without damaging the reactor performance. Nutrient condition is a factor that has profound impact on the physiological status of microorganisms and hence the physico-chemical properties of activated sludge. In an adhesion assay, a lower adhesion tendency of starving cells to membrane surface was observed, compared to "fat" cells of the same strains. Also, starving cells adhered normally in an island structure with free membrane areas in between, and non-starving cells were capable of covering the surface completely and did not show the island structure (Flemming and Schaule, 1988). In addition, cells that were cultivated under carbon-limited conditions had a strong tendency to excrete extracellular polymers (van Houten et al, 2001). Conversely, the activated sludge acclimated to nitrogen-deficient substrates was observed to produce less EPS than normal activated sludge due to the incomplete metabolism (Chang and Lee, 1998). Moreover, the COD:N:P ratio of influent wastewater was found to affect the hydrophobicity, surface charge and the EPS composition of activated sludge floes in a membrane coupled sequential batch reactor (SBR) process (Bura et al, 1998). By altering the properties of activated sludge, unusual nutrient conditions play an important part in membrane biofouling. 22 Aerobic/anoxic/anaerobic conditions undoubtedly affect the microbial composition of activated sludge and the biological activity and specificity of the system, which in turn determine the nature and extent of membrane fouling. Anaerobic sludge was reported to produce 10-20 mg EPS /g SS compared to 70-90 mg EPS /g SS for aerobic sludge. Also, anaerobic sludge polymers differed from aerobic sludge polymers with protein being the most dominant fraction in the former compared to carbohydrate in the latter (Morgan et al, 1990). In addition, it was observed that degradation of EPS was accelerated by intermittent aeration, suggesting that introduction of anoxic/anaerobic environment to the membrane separation process is possibly advantageous in prevention of membrane fouling in the BOD-removal-type membrane process (Nagaoka, 1999). From the above literature review, two general points are apparent. First, the influencing factors are not totally independent of each other. Instead, they are interrelated and contribute collectively to the fouling of a membrane. Therefore, the nature and extent of fouling is determined not simply by a few individual factors, but by the combined effects of all the physico-chemical and biological properties of the system as well as the process design and operating conditions. Second, contradictory experimental results have been frequently reported in the literature. This, from another standpoint, reflects the complexity of membrane fouling in membrane coupled activated sludge processes, and suggests that fouling in M C A S P is quite case-specific. Physical and Empirical Fouling Models Based on the fundamental membrane filtration theory and the understanding of membrane fouling mechanisms and influencing factors, a number of physical and empirical fouling models have been developed. The most frequently used model is the resistance-in-series model, in which individual fouling mechanisms were considered to have additive effects and thus the total hydraulic resistance (R t) can be straightforwardly broken down into two, three or more components in accordance with the knowledge of fouling mechanisms and the different research needs. In the study of Cheryan (1986), it was believed that the total fouling effect in an M B R process is the sum of three filtration resistance components: resistance caused by membrane itself (Rm), resistance caused by 23 irreversible fouling (Rn), and resistance due to formation of a sludge cake layer (Rc). Thus, the permeate flux can be expressed by the following equation. R t = R m + R n + R c (1.5) J = A P / ( u ^ t ) = A P / [ | i ( R m + R n + Rc)] (1.6) van Houten et al. (2001) suggested that the resistance due to filtration of activated sludge (Rs) could be divided into cake/gel layer resistance (Rg), pore blocking resistance (Rp) and adsorptive fouling resistance (Ra). Thus, the total hydraulic resistance can be calculated as follows. R t = R m + Rs = Rm + Rg + Rp + R a (1-7) Other investigators simply regarded the total membrane filtration resistance as the sum of the initial resistance caused by a clean membrane (Rm) and the resistance due to fouling (Rf) (Bouhabila et al, 2001), as shown in Equation 1.8. R t = Rm + Rf (1-8) According to Equation 1.2 (page 5), permeate flux and trans-membrane pressure (TMP) seem to exhibit a linear relationship: flux increases with an increase of TMP. However, this is true only when trans-membrane pressure is lower than a certain value (Manem and Sanderson, 1996). Once this value is exceeded, permeate flux will remain constant no matter how large a TMP is imposed (Figure 1.7). This phenomenon was Figure 1.7. Theoretical flux evolution as a function of the trans-membrane pressure 24 explained by the change of hydraulic resistance R c due to cake layer formation, which is a function of the concentration and composition of suspended matter as well as the applied hydraulic conditions. Usually, this relationship can be described as R c = 0 AP, where 0 is a function of the system's mass transfer properties. Therefore, Equation 1.6 can be rewritten as J = AP / (u, R0= AP / [u. (R m + R n + 0 AP)] (1.9) In this case, the whole filtration process can be divided into two regions: a low-pressure region where the hydraulic resistance caused by the membrane itself and the irreversible fouling dominates the process (R m + R n » © AP) and the flux is directly proportional to trans-membrane pressure, and a high-pressure region where the cake layer resistance is the governing factor (R m + R n « 0 AP) and the flux is constant at the limiting flux. The trans-membrane pressure under which the cake layer resistance (0 AP) is equal to the membrane hydraulic resistance (R m + R n ) is called the critical or optimal trans-membrane pressure (TMPC). Beyond this value, increasing TMP only exacerbates membrane fouling without any significant increase in the membrane performance. The gel layer model provides a similar explanation to this limiting flux phenomenon (Cheryan, 1986). Research efforts have also been made to establish the relationships between membrane flux (J) and biomass concentration (MLSS), and between filtration resistance (R) and organic loading (COD), biomass concentration (MLSS), trans-membrane pressure (AP), and viscosity of an activated sludge suspension (p:). Empirical expressions for such relationships are illustrated in Equation 1.10 and Equation 1.11 (Magara and Itoh, 1991; Sato and Ishii, 1991), respectively. J = 1.571 log(MLSS) + 7.84 (1.10) R = 842.7 AP ( M L S S ) 0 9 2 6 ( C O D ) 1 3 6 8 (u ) 0 3 2 6 (1.11) Where, J was expressed in m/d, MLSS in mg/L, R in 1/m, AP in Pa, COD in mg/L, and | i in centipoises or cP. Nagaoka and co-workers (1998; 2000) developed a mathematical model which consisted of a series of kinetic and hydrodynamic equations, to simulate the temporal 25 changes of suction pressure, permeate flux and filtration resistance in a submerged M B R process. They used the classic activated sludge growth theory and made the assumption that accumulation, detachment and consolidation of EPS on the membrane surface and in the mixed liquor followed first order kinetics. The experimental results were predicted well by the developed model. Fouling Prevention and Control Strategies From the preceding sections, it can be seen that almost every wastewater treatment step has an effect on membrane fouling. Therefore, fouling prevention and control strategies could be carried out through the entire M B R process. Influent pre-treatment is the first step to prevent potential foulants from entering the filtration system. It involves most of the common preliminary and primary treatment procedures such as bar screening, a grit chamber, sedimentation with/without coagulation, etc., and removes or reduces the feed debris and colloidal particles that are detrimental to a filtration membrane. In addition, pH adjustment is especially important for mitigation of membrane fouling due to mineral precipitation. Selection of membrane and module configuration is another strategic measure, because bacteria have different affinities for different membranes, or in other words, membrane characteristics have strong influence on fouling propensity of the membranes. Normally, the membranes used for filtering biological suspensions should.be preferably hydrophilic and be either negatively charged or neutral in order to limit biomass adsorption (Manem and Sanderson, 1996). To reduce fouling potential of filtration membranes, photochemical modification of a membrane surface has been employed by means of U V photolysis and graft polymerization of hydrophilic monomers onto the membrane surface to create more hydrophilic and more fouling-resistant membrane surfaces (Pieracci et al, 1999). In addition, module configuration is also known to have an influence on membrane fouling. Loose fiber configuration has been found to be able to promote lateral sway in a multi-fiber module for dual-phase cross-flow, which could increase the physical contact between the membrane fibers and enhance the permeate flux by mechanically eroding the foulant layer (Berube and Lei, 2006). The greatest permeate 26 flux is observed in the module configuration that maximizes the impact of shear force so as to minimize the sludge accumulation On the membrane surface (Hong et al, 2002). Hydrodvnamic control of filtration system for fouling alleviation includes sub-critical flux operation, non-continuous filtration operation and turbulent aeration / recirculation. An operating tactic of a low starting TMP and thus a low starting flux (below the critical flux) is widely used to reduce the fouling extent (Field et al, 1995). Also, the constant-flux operation mode was believed to be superior to the constant-TMP mode. Further, non-continuous filtration, i.e., intermittent suction, was discovered to significantly improve membrane productivity (Hong et al, 2002). This could be explained by the enhanced backtransport of foulants under pressure relaxation. During relaxation periods, the foulants not irreversibly attached to the membrane surface are thought to diffuse away from the membrane surface because of the concentration gradient. Moreover, the effectiveness of air scouring is greatly enhanced in the absence of TMP, resulting in higher removal of foulants accumulated on the membrane surface. Finally, maintaining turbulent conditions around a membrane surface through either recirculation of activated sludge at a high velocity or cross-flow aeration at a high aeration rate is the most straightforward way to prevent foulant adhesion and deposition (Gander et al, 2000). Addition of adsorbents or coagulants has been proven to be effective in alleviating membrane fouling. Powdered activated carbon (PAC) is a common adsorbent that could remove most of hydrophobic natural organic matter in raw surface water and thus greatly mitigate the fouling caused by this fraction of organics (Seo et al, 2005). With respect to wastewater treatment using M B R , coagulants such as alum and zeolite were found to be able to enhance membrane filtration performance by reducing the unsettleable organic substances in activated sludge mixed liquor and improving floe structure and strength (Lee et al, 2001; Holbrook et al, 2004). However, pre-coagulation with polyaluminium chloride could not reduce the degree of irreversible fouling during ultrafiltration of a surface water (Kimura et al., 2005). Proper design and operation of biological system to produce activated sludge mixed liquor with high filterability is a very promising way to alleviate membrane fouling. It has been demonstrated that through optimization of the biological system in an M B R process, EPS could be minimized or produced in a form that did not worsen the 27 dewaterability or filterability of the sludge. The latter resulted in an activated sludge mixed liquor with similar characteristics to those of conventional secondary sludge (Lawrence et al, 2001). However, the present knowledge of the effect of biological process design and operation on sludge properties and membrane fouling is rather sparse, thus a vast amount of research work is required to fill this gap. Membrane cleaning is the most straightforward resolution for fouling control, which is usually classified as maintenance cleaning and recovery cleaning. The former mainly refers to the clean-up of membrane using mechanical methods such as permeate/air backpulsing and backflushing. The latter is more like an end-of-pipe fouling control method that usually involves a cleaning step using chemical oxidants like NaOCl, NaOH, H2O2, CI2, etc. An alternative to intensive membrane cleaning is the chemical modification of EPS molecular structure using ethylenediaminetetraacetic acid (EDTA), a divalent-cation chelating agent, or pronase, a proteolytic enzyme, to increase the water permeability of EPS matrix (Ridgway and Flemming, 1996). These treatments did not change the packing density, compressibility, or 3-D arrangement of cells in the biofilm, but only the porosity and diffusional properties of the EPS matrix in which the cells were embedded. To date, many cleaning methods have been developed to cope with various fouling problems. As a general rule, the type of fouling determines the type of cleaning required. Thus, the effectiveness of a particular cleaning procedure directly relies on the fouling mechanisms occurring in the membrane process. Research Scope and Objectives As stated earlier, membrane coupled activated sludge processes or MBRs have been demonstrated to be promising technologies for wastewater treatment with a number of advantages over the conventional activated sludge processes. MBRs are gaining more and more popularity in both industrial and domestic applications, and are even considered as the "third generation treatment plant" (de Korte et al, 2001). However, membrane fouling, the so-called "the Achilles heel of membrane technologies", largely limits the further development of these processes (Flemming et al, 1997). Solutions to this challenging problem are urgently needed. In particular, since the membrane enhanced 28 biological phosphorus removal (MEBPR) process is a rather new type of M B R , studies on membrane fouling in this emerging process have not yet been reported. The present study was initiated in order to fill this gap, complement the existing knowledge of membrane fouling, and make an important contribution to better control and alleviation of fouling problems. The scope of this research was defined to focus on the correlation of the activated sludge process to the membrane fouling in an M E B P R system. The overall objective was, through the biological process optimization, to reduce membrane fouling and achieve long-term stable membrane filtration without compromising the biological treatment performance. The specific research objectives are listed as follows. • Identifying the prevailing fouling mechanisms (biofouling, inorganic precipitation, organic adsorption or others) under the M E B P R design and operating conditions • Revealing the nature of fouling in the M E B P R system by characterizing fouled membranes and analyzing the composition of membrane foulants • Clarifying the roles of various activated sludge components in the membrane filtration process and comparing their relative importance to membrane fouling • Exploring the correlation between the physico-chemical and biological properties of activated sludge and membrane fouling and determining the key sludge properties that significantly influence the fouling propensity of mixed liquor • Recommending fouling prevention and control strategies for the MEBPR process with respect to the system design and operation It is interesting to note that aerobic MBRs normally display much higher specific substrate removal rates and much lower sludge production than conventional aerobic activated sludge processes. On the contrary, anaerobic MBRs do not seem to favour high specific degradation rates with respect to carbonaceous substrate and they demonstrate sludge production equivalent to conventional processes (Manem and Sanderson, 1996). These results suggest that use.of membrane bioreactors modifies the aerobic ecosystem considerably but may have less effect on anaerobic or anoxic biomass. As the MEBPR process consists of three reaction zones (aerobic, anoxic and anaerobic), the influence of 29 membrane installation on this 3-in-l process is hard to infer. Based on the literature review and the current understanding of the biological phosphorus removal process, however, it is hypothesized that in the MEBPR process, (1) biofouling is the major fouling mechanism because of the elevated biomass concentration in the system; (2) EPS plays an important role in M E B P R membrane fouling; (3) mineral deposition and/or inorganic precipitation are not negligible due to the existence of the anaerobic and anoxic environment; (4) design and operation of the biological phosphorus removal process greatly influence the physico-chemical and biological properties of activated sludge and consequently have significant impact on membrane filtration performance. In addition, fouling in the M E B P R process may be not so severe as in a conventional M B R with an aerobic activated sludge tank, because it has been reported that degradation of EPS is accelerated when anoxic/anaerobic conditions are introduced (Nagaoka, 1999). 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McGraw-Hill Companies, Inc. Wisniewski, C. and Grasmick, A. (1998). Floe size distribution in a membrane bioreactor and consequences for membrane fouling. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 138(2-3), 403-411. Yamamoto, K., Hiasa, M . , Mahmood, T. and Matsuo, T. (1989). Direct solid-liquid separation using hollow fiber membrane in an activated sludge aeration tank. Water Science and Technology, 21(4-5 pt 1), 43-54. 34 Chapter 2. Investigation of mechanisms of membrane fouling in a membrane enhanced biological phosphorus removal process Introduction Membrane fouling is known as an inherent problem that has been accompanying membrane processes since the emergence of membrane technologies. In recent decades, membrane coupled activated sludge processes, or membrane bioreactors (MBRs), have shown great potential to revolutionize conventional wastewater treatment because of their exceptional competency in achieving water sustainability (van der Roest et al, 2002). However, membrane fouling is still a particular concern that considerably restricts the widespread use of MBRs in the wastewater industry. There are a number of mechanisms that have been reported to cause membrane fouling in MBRs used for wastewater treatment. Deposition of sludge on membrane surfaces, cake or gel-layer formation and microbial growth were reported to be common fouling phenomena in submerged MBRs (Liu et al, 2000; Gui et al, 2003). It was also demonstrated that membrane fouling was mainly attributable to initial pore blocking by colloidal particles, followed by cake formation (Lim and Bai, 2003). In some cases, macromolecule adsorption was found to induce irreversible fouling and thus played an important role in membrane performance decline (Ognier et al, 2002). For certain anaerobic MBRs, significant inorganic precipitation was observed either inside the membrane pores or on the membrane surfaces (Choo et al, 2000; Kang et al, 2002). It was pointed out in a recent literature review that because of the extreme complexity of membrane fouling processes, many research findings, like those mentioned above, are contradictory and no single fouling model has yet been accepted (Laine et al, 2003; Le-Clech et al, 2006). This may imply that there is actually no single mechanism that is universally applicable. Fouling processes and the relative importance of each mechanism are case specific and could differ substantially in different membrane applications. In this regard, membrane fouling studies are necessary for any new membrane process in order to A version of this chapter will be submitted for publication. Geng, Z., Hall, E.R., and Berube, P.R. Investigation of mechanisms of membrane fouling in a membrane enhanced biological phosphorus removal process 35 take appropriate measures to prevent and control fouling, though extensive research on membrane fouling has been done in the past decades. Installing membrane filtration in biological nutrient removal processes is a latest effort to integrate membrane technologies with activated sludge processes (Lesjean et al, 2002; Adam et al, 2003; Ramphao et al, 2005). Although a study on membrane fouling behaviour in this type of M B R was performed recently (Rosenberger et al., 2006), an in-depth and systematic research on membrane fouling mechanisms in such membrane applications has not yet been attempted. In particular, the specific causes and patterns of membrane fouling formation are totally unknown, which makes the corresponding fouling control ineffective. To fill these important knowledge gaps, a pilot scale membrane enhanced biological phosphorus removal (MEBPR) process, one of the above types of MBRs, was investigated in the present study with a focus on membrane fouling mechanisms. As the MEBPR process consists of three compartments, namely anaerobic, anoxic, and aerobic reaction zones in series, it is more complex than common MBRs that comprise only one compartment (either aerobic or anaerobic). Therefore, the membrane fouling behaviour may be quite different from usual, which could be any modification and/or combination of such commonly reported mechanisms as sludge deposition or cake formation, pore clogging, organic adsorption, and inorganic precipitation, due to the combined effect of the three reaction zones. The objective of this study was to determine the prevailing mechanisms that govern the membrane fouling in the newly developed M E B P R process. Both long-term pilot scale membrane filtration experiments and short-term bench scale membrane filtration tests were carried out in the present study. A simple mathematical model was also developed to further elucidate the fouling process. Methods The UBC Wastewater Treatment Pilot Plant The studied MEBPR process was operated in the University of British Columbia (UBC) Wastewater Treatment Pilot Plant (see Appendix 1 for photos). For comparison purposes, a conventional enhanced biological phosphorus removal (CEBPR) process was run in parallel to the MEBPR process, as illustrated in Figure 2.1. The two processes 36 shared the same raw sewage influent from the U B C campus and both operated under comparable process design and operating conditions, except that the former used submerged membranes to separate sludge floes from treated water and the latter used secondary clarification for sludge separation. The liquid volumes of the anaerobic (A), anoxic (B), and aerobic (C) compartments of the two processes were 0.23, 0.59, and 1.31 m , respectively. Sludge was recycled at the ratio of 1:1. Air was supplied constantly in the aerobic zone of the CEBPR process at the rate of 3-5 m3/m3-h. In the counterpart MEBPR process, aeration was supplied in a cyclic mode of 10 second ON and 10 second OFF, with an airflow rate of 15-20 m3/m3-h in order to alleviate fouling and maintain a similar dissolved oxygen (DO) level to that of the CEBPR process (2-4 mg/L). For more details of the design and operation of the U B C Wastewater Treatment Pilot Plant, please refer to Monti (2006). Underflow ^ Primary clarifier \ Air R 1 X Permeate • On-line filtration apparatus ' Major membrane module Waste sludge Membrane Bioreactor Side A: the M E B P R process | Air 1 1 — 1 A i l R l l C ° o > ° O i o o ° « o i n—f~ 1 Effluent :=:=::! ^ Waste sludge Bioreactor Secondary clarifier Side B: the CEBPR process Figure 2.1. Schematic of the U B C Wastewater Treatment Pilot Plant comprising the MEBPR process and the CEBPR process. A : anaerobic reaction zone; B: anoxic reaction zone; C: aerobic reaction zone 37 An intensive sampling program was designed for use at the UBC pilot plant during the experimental period. In addition to the daily operational measurements like pH, temperature and dissolved oxygen (DO) in all reaction zones, influent, effluent and mixed liquor samples were collected every day from the two treatment processes, except for weekends and statutory holidays. Parameters such as total and filtered chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammonia nitrogen (NH 4-N), nitrate nitrogen (NO3 -N), total phosphorus (P-total) and phosphate phosphorus (PO4-P) were analyzed for both the influent and the two effluents. Total suspended solids (TSS) and volatile fatty acids (VFA) of the influent were also measured. At the same time, grab samples of mixed liquor from the three compartments of each process were analyzed for total and filtered COD, T K N , N H 4 - N , N O 3 - N , P-total, P0 4 -P, V F A and mixed liquor suspended solids (MLSS). For detailed sampling protocols, please refer to Monti (2006). Both the M E B P R and CEBPR processes underwent two consecutive experimental runs during the period between March 2003 and June 2004, after nearly four months of start-up operation. The sludge retention time (SRT) was set at 12 days for both runs and only the hydraulic retention time (HRT) was changed from 10 hours in Run I (March 2003 - December 2003) to 7 hours in Run II (December 2003 - June 2004). As a result, the MLSS concentration of the aerobic MEBPR sludge was 3.3 g/L in Run I and 4.1 g/L in Run n. For the aerobic CEBPR sludge, the MLSS was 2.7 g/L and 3.3 g/L in the two runs, Table 2.1. Influent characteristics and overall treatment performance of the MEBPR process and the CEBPR process (all values in mg/L unless specified) Run I Run II Parameter Influent SRT=12 days HRT=10 hours SRT=12 days HRT=7 hours MEBPR effluent CEBPR effluent MEBPR effluent CEBPR effluent TSS 110±50 0 31 ±38 0 18 ± 9 COD (total) 330± 100 25 + 21 74 ±95 36 ±26 93 ±62 COD (sol.) 100 ±90 25 ±21 34 ±26 36 ±26 42 ±37 P-total 4.3 ± 1.1 0.97 ± 0.68 1.4 ±0.7 0.81 +0.99 1.7 ± 1.1 P O 4 - P 2.7±0.9 0.49 ± 0.61 0.39 ± 0.45 0.54 ± 1.25 0.57 ± 0.64 TKN 34 ± 8 0.81 ±0.57 2.3+1.2 1.15 ±0.86 2.7 ±3.4 N H 4 - N 26 + 5 0.065 ± 0.079 0.20 ± 0.44 0.14 ±0.88 0.16±0.15 NO3-N 0.057 ± 0.087 11 ± 3 8.6 ±3.4 12±2 9.5 ± 1.6 PH 7.3 ±0.3 - — Temperature 21 ± 2 (°C) - -Note: (a) The influent characteristics were the mean values over the period February 2003 - June 2004. The number of samples was between 120 and 320. (b) The treatment performance was evaluated based on 140-200 effluent samples collected in Run I and 30-120 effluent samples collected in Run II. (c) + followed by standard deviations, (d) - not analyzed. 38 respectively. The influent characteristics and the overall treatment performance of the two pilot scale processes are summarized in Table 2.1. The day-to-day variation of the U B C pilot plant performance is illustrated in Figure 2.2(a-i). It can be seen from Figure 2.2(a-i) that four major process failures occurred during the whole experimental period due to either extreme weather conditions (i.e. sudden drop of temperature in January 2004) or carbon limitation in the influent (i.e. insufficient volatile fatty acids in the influent during September-October 2003 and April 2004) or other unexplained factors (i.e. May-June 2003). In addition, operational accidents and equipment failures took place from time to time. A l l these led to occasional upsets of the two biological processes and, in particular, imposed significant effect on the phosphorus removal (Figure 2.2(f)). Despite these perturbations, the M E B P R process was generally superior to the CEBPR process in terms of the mean effluent quality (Table 2.1). Even with the occasional process upsets, the average contents of total COD, total phosphorus, and total organic nitrogen (TKN) in the M E B P R effluent were less than 40, 1.0 and 1.2 mg/L, respectively, in both experimental runs. In contrast, the CEBPR process discharged effluent with concentrations of total COD, total phosphorus and total organic nitrogen (TKN) greater than 74, 1.4 and 2.3 mg/L, respectively (Table 2.1). Ammonia nitrogen was effectively removed in both processes. It should be noted that the CEBPR effluent contained less nitrate nitrogen than the M E B P R effluent probably because of the potential denitrification occurring in the CEBPR secondary clarifier (Figure 2.2 (i)). The Membrane Filtration Module The major membrane unit used in the MEBPR process was a custom-built ZeeWeed module (Zenon Environmental Inc., Oakville, Ontario, Canada) that comprised bundles of hollow membrane fibres. The module specifications and operating limits are listed in Table 2.2. When in service, the module was submerged in the aerobic compartment and was operated under vacuum for 9 minutes and 30 seconds and then was backflushed with the permeate for 30 seconds in a cyclic form. No relaxation was applied and thus each filtration cycle lasted 10 minutes. As mentioned previously, aeration was supplied at a volumetric rate of 15-20 m3/m3-h in a mode of 10 second ON and 10 second 39 200 150 100 50 Run I • 4 * • • • • • • • 4» • • / i 4 Run II • • 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-04 Time • Influent m CEBPR effluent A MEBPR effluent 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-i Time • Influent m CEBPR effluent A MEBPR effluent 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-04 Time • Influent • CEBPR effluent A MEBPR effluent 40 o i_ CD 05 CD CO CO CD c o N 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-04 Time HCEBPR A MEBPR 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-04 Time • Influent • CEBPR effluent A MEBPR effluent 41 c o •1 =d •s E o 60 40 20 0 Run I Run II 7 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-04 Time • Influent m CEBPR effluent A MEBPR effluent 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-04 Time • Influent n CEBPR effluent A MEBPR effluent 10-Dec-02 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 23-Apr-04 01-Aug-04 Time • Influent m CEBPR effluent A MEBPR effluent Figure 2.2. Daily performance of the MEBPR and CEBPR processes at the U B C wastewater treatment pilot plant: (a) SS; (b) total COD; (c) filtered COD (0.1 \im); (d) MLSS; (e) total phosphorus (P-total); (f) phosphate phosphorus (P0 4-P); (g) total Kjeldahl nitrogen (TKN); (h) ammonia nitrogen (NH 4-N); and (i) nitrate nitrogen (NO3-N) 42 Table 2.2. Specifications and operating limits of the major membrane module used in the MEBPR process Model Custom-built ZeeWeed, submersible Configuration Outside/In hollow fiber Membrane material Polyvinylidene fluoride (PVDF) Nominal Membrane Surface Area 11.9 m 2 Nominal Membrane Pore Size 0.04 urn Membrane Surface Chemistry Neutral and hydrophilic Dimension (approximately) 100 cm x 100 cm x 15 cm Maximum Trans-membrane Pressure 69 kPa(10ps i ) Maximum Operating Temperature 40°C Operating pH range 5 - 9 OFF. A constant-flux mode was adopted in the M E B P R membrane operation and the flux was maintained at 23 L/m 2-h in Run I and 33 L/m 2-h in Run II. To track the filtration performance and decide whether a chemical cleaning was required, the trans-membrane pressure (TMP) of the MEBPR membrane module was closely monitored during the whole experimental period. Once the TMP reached the maximum operating limit (69 kPa), the module was removed from the aerobic compartment and subjected to a recovery cleaning, which encompassed an overnight soak in a diluted bleach solution (200 mg OCF /L) followed by an additional soak in citric acid solution (2000 mg/L) for 24^-8 hours. Over 95% of membrane permeability was recovered after each chemical cleaning. For a qualitative comparison of clean and fouled membrane modules at the U B C pilot plant, see the photo in Appendix 2. On-line and Off-line Filtration Tests To investigate the membrane fouling mechanisms in the M E B P R process, a series of membrane filtration tests, either long-term on-line or short-term off-line, was designed in the present study to filter activated sludge, membrane permeate, or clean water. First of all, an on-line filtration apparatus, which accommodated twenty-four 0.5 m-long membrane loops, as illustrated in Figure 2.3, was placed beside (-10 cm away from) the pilot scale membrane module in the MEBPR process (Figure 2.1). Thus, it was assumed 43 that the on-line system was subjected to similar hydrodynamic conditions as for the membrane module in the pilot scale MEBPR system. Moreover, it was operated in the same way as the pilot module in order to provide fouled membrane fibers for off-line clean water filtration tests and accordingly to examine the reversibility of fouling developed under the M E B P R conditions. To make this on-line filtration apparatus, the two ends of membrane fibers were first immobilized in a small piece of plastic pipe with epoxy and then connected to the manifold via Swageloc stainless steel fittings. See Appendix 3 for a photo of the system. Figure 2.3. An illustration of the on-line membrane filtration apparatus Figure 2.4 shows the experimental setup of the short-term bench scale off-line membrane filtration tests (for details, please see Appendix 3). Except for its smaller size, the off-line filtration apparatus had exactly the same configuration as the on-line apparatus and could be readily connected via Swageloc fittings to four membrane loops, either 0.1 m-long virgin membrane loops for short-term sludge filtration tests or 0.5 m-long fouled membrane loops removed from the on-line system for fouling reversibility tests. The bioreactor that housed the off-line filtration apparatus had a liquid volume of 3 L . Aeration was supplied in the bioreactor according to the needs of different types of filtration tests. Two Masterflex peristaltic pumps were employed for influent feeding and suction/backflushing purposes. The TMP was measured using a pressure transducer installed on the permeate line. The experimental setup illustrated in Figure 2.4 was used for several types of bench scale filtration tests. To facilitate the understanding of the 44 experimental results, the procedure for each type of filtration tests is described in detail under Results and Discussion. Pressure transducer Feed pump Permeate m Membrane loops Air K Membrane bioreactor Sewer Figure 2.4. Experimental setup of bench scale off-line filtration tests It should be stressed that the short-term off-line filtration tests were designed with different filtration conditions from those used in the long-term M E B P R membrane filtration, such as continuous suction without backflushing or with only limited aeration, in an attempt to simplify the fouling processes and to reveal the various fouling mechanisms that might be involved. In addition, the hydrodynamic conditions for the on-line and off-line systems were different, though effort was made to simulate the on-line hydrodynamic conditions in the off-line system by using similar volumetric airflow rates. It is also necessary to add that, depending on the needs of the filtration tests, two sets of membrane loops, of 0.5 m and 0.1 m length each, were prepared in the present research. Also, both the on-line and off-line membrane loops were made of the same fibres as the pilot scale membrane module. Thus, all the membranes used in this study had identical chemical and physical properties. SMP Effect on Sludge Filterability The effect of soluble microbial products (SMP) on membrane fouling in the M E B P R process was examined in the present study. As microorganisms tend to produce SMP via cell lysis in a toxic environment (Barker and Stuckey, 1999), microbial inhibition 45 of activated sludge by addition of a C u 2 + solution was implemented in order to compare the filterability of sludge with and without C u 2 + inhibition and accordingly test the SMP effect on membrane fouling. A total of 3.0 L of activated sludge was sampled from the aerobic zone of the M E B P R process. Half of the sludge was used as a control and was centrifuged at 2,000 x g for 12 minutes. The supernatant was decanted and the solids were re-suspended in 1.5 L distilled water. The suspension was then aerated for 24 hours before a filtration test. The other half of the sludge was microbiologically inhibited through centrifuging the sludge at 2,000 x g for 12 minutes, re-suspending the solids in 1.5 L CuS04-5H 20 solution, and aerating the mixture for 24 hours before filtration test. The 50% inhibitory concentration (IC50) of CuS04-5HiO to the M E B P R sludge sample was used to make up the C u 2 + solution and was determined to be 1000 mg/L in this study, according to the method of Wong et al. (1997) (see Appendix 4 for details). The bench scale filtration tests with the control sludge and the inhibited sludge were performed in a continuous-suction and constant-flux mode, using the apparatus depicted in Figure 2.4. Aeration was supplied during the filtration tests at a volumetric flow rate of 60 m 3 air / m 3 liquid • h. Results and Discussion Filtration Performance of the Pilot Scale Membrane Module in the MEBPR Process It is widely recognized that hydraulic resistance, which is expressed as the TMP of a filtration apparatus in a constant-flux mode, increases as fouling proceeds. However, the filtration performance, including the pattern of the TMP increase and the duration of the filtration run, are case specific and characteristic of the M B R system used. To investigate the mechanisms of membrane fouling in the M E B P R process, the filtration performance of the pilot scale membrane module at the U B C pilot plant was first examined. As described under Methods, the pilot scale membrane module of the MEBPR process was operated in a cyclic mode and each cycle included 9 minutes and 30 seconds of suction followed by 30 seconds of backflushing. Therefore, an entire filtration run, which was defined as the membrane operation between two sequential recovery cleanings, actually embraced thousands of 10-minute filtration cycles that could continue for months 46 before a chemical cleaning was required. Figure 2.5 delineates the TMP accumulation process in a filtration run and highlights the variation of TMP within the small filtration cycles on certain days of the run as well as the overall TMP trend during the entire run. After each backflushing, the net TMP increase between the two neighbouring 10-minute cycles was so negligible that it was even hard to perceive. However, it was in this manner that foulants accumulated day by day, finally forming the long-term fouling in the M E B P R process. In this regard, Figure 2.5 is a zoom-in picture of the long-term fouling at the UBC pilot plant. 100 50 CL a." 0 -50 H -100 , 10 min., 10 min. , 10 min., , 10 min., (10 t h day) (59 , n day) (76 , n day) D) C 'sz to __ J2 o CO 00 o o CO Run time, days Figure 2.5. The TMP accumulation process during a filtration run of the pilot scale membrane module in the M E B P R process at the U B C pilot plant (Dotted lines indicate the TMP trend over the filtration run) The overall filtration performance of the pilot scale membrane module at the UBC pilot plant during the entire experimental period (March 2003 - June 2004) is illustrated in Figure 2.6 (see Appendix 5 for details). The data in Figure 2.6 were collected once a day and represented the maximum TMP values during suction and backflushing of a filtration cycle of the day. After start-up, a total of seven filtration runs was carried out in this study. The first, second, fifth, and sixth filtration runs were completed under the pseudo-steady state conditions and were thus considered to be representative of the membrane performance in the two experimental runs. The third filtration run spanned the transition period from Run I to Run II. The fourth filtration run encompassed a period of extreme 47 weather conditions, and the seventh run included an operational accident, thus the TMP profiles of these three were not regarded as typical. 80 40 CO CL CL I --40 -80 Runl J * J ? * J / / 1 1 © \ ©' 1 1 c x: _g **— o CO CO c o o 00 20-Mar-03 28-Jun-03 06-Oct-03 14-Jan-04 Time 23-Apr-04 01-Aug-04 Figure 2.6. The TMP profiles of the major membrane module at the UBC pilot plant during the entire experimental period (the arrows indicate recovery cleanings) It can be observed from Figure 2.6 that the TMP, or the hydraulic resistance, increased slowly at the beginning of each filtration run. After a few weeks of sludge filtration, however, the TMP increased at a much faster pace than before and soon after reached the maximum operating limit. Due to the decreased HRT applied in Run U, the fouling process was accelerated, resulting in shorter filtration run times (46-51 days) than in Run I (84-85 days). Membrane Fouling Behaviour in the Off-line Filtration Tests As illustrated in Figures 2.5 and 2.6, the membrane fouling in the M E B P R pilot plant was a long-term accumulation process. Then two questions arose: what mechanisms led to this accumulated fouling, and, was this fouling reversible? To address these research issues, two types of bench scale off-line filtration tests were designed. In the first test, fouled membrane loops (in a length of 0.5 m) were removed from the on-line filtration apparatus after 50 days of sludge filtration. Since the same membrane fibres were used under the same operating and hydrodynamic conditions, these membrane loops were assumed to have similar fouling characteristics to those of the pilot scale membrane module. Once removed from the on-line apparatus, they were immediately reconnected to 48 the off-line filtration apparatus depicted in Figure 2.4. The apparatus was filled with clean water and the fouled membrane loops were submerged in the clean water and operated in a repeated suction-relaxation-backflushing-relaxation mode with a constant-flux setting. Virgin membrane loops, which had been washed with a diluted bleach solution (200 mg OCTAL), were used as controls, and these were operated in the same way as the fouled membrane loops. The filtration results are shown in Figure 2.7. -100 J 1 0 20 40 60 80 Time elapsed, min. Figure 2.7. The TMP profiles of fouled membranes filtering clean water at a flux of 33 L/m 2-h in comparison with virgin membranes (Rlx.: relaxation; BF: backflushing) It is evident that the membranes previously fouled in the pilot scale M E B P R process exhibited a TMP profile that was similar to that of the virgin membranes, that is, after an initial quick pressure development, the TMP (especially when in a suction mode) always returned to a constant value regardless of the number of clean water filtration cycles applied (Figure 2.7). This suggested that the fouling developed in the MEBPR process was mostly hydraulically irreversible, at least under the hydraulic conditions of the off-line tests. The foulants were so intimately associated with the membranes that they acted as a whole, just like the virgin membranes. Otherwise, it would be anticipated that a more loosely attached foulant layer would be sloughed off due to the repeated backflushing and relaxation applied, leading to a decrease in the absolute TMP values. Obviously, this irreversible fouling was the result of long-term foulant accumulation on the membranes in the manner illustrated in Figure 2.5. 49 The second type of off-line filtration test was then designed to identify the major mechanisms that resulted in the above irreversible fouling. Virgin membrane loops were used in these tests. The apparatus depicted in Figure 2.4 was first filled with the aerobic M E B P R mixed liquor and the system was operated under vacuum in a constant-flux mode. Slight aeration, rather than the vigorous aeration applied in the pilot scale M E B P R process, was supplied in order to prevent the sludge from settling. As shown in Figure 2.8, the TMP increased gradually during the first 150 minutes (Phase I). At this point, the activated sludge mixed liquor was replaced with distilled deionized water (DDW) and the membrane filtration was continued. A stable but significantly lower TMP was then observed (Phase II). After a quick backflushing, the TMP decreased again but settled out at a higher value than that of the virgin membranes filtering clean water (Phase III). The total hydraulic resistance (R) encountered during the filtration with sludge was apparently the sum of the resistance caused by the membrane itself (Rm) and the resistance caused by activated sludge mixed liquor (Rs). Time elapsed, min. -8.5 Figure 2.8. Development and breakdown of the TMP of the off-line membrane loops filtering the aerobic MEBPR mixed liquor of 5.7 g M L S S / L at a constant flux of 33 L/nT-h (Phase I: suction in sludge; Phase II: suction in DDW; Phase UL suction in DDW after backflushing. The dashed line represents the final steady TMP of the virgin membranes filtering DDW) According to the results presented in Figure 2.8, Rs could be further broken down to three components. The first component is the resistance caused by sludge deposition or sludge cake layer formation due to concentration polarization (Rc), which is assumed to 50 be eliminated either by the removal of bulk sludge, as in the above tests, or by applying shear stress on the membrane surfaces via tangential flow, as in the pilot scale MEBPR process, to eliminate the cake layer formation. The second component is the resistance due to superficial pore blocking and is assumed to be reduced through backflushing (Rp). The third component is the resistance induced mainly by adsorption of organic substances and also possibly by pore clogging when macromolecules or fine colloids accumulate deep in the membrane structure (Rap). The absorption resistance is assumed to be hydraulically irreversible and can only be removed through chemical cleaning. It is noted in Figure 2.8 that Rap accounted for a very small portion of the total Rs in these short-term tests. It was shown previously that the long-term membrane fouling in the pilot scale MEBPR process was hydraulically irreversible (Figure 2.7). Based on the above analysis, it was thus asserted that organic adsorption and deep pore clogging (Rap) were the major mechanisms that caused the irreversible fouling in the MEBPR system. As stated earlier, the long-term sludge filtration in the pilot scale M E B P R process comprised thousands of 10-minute small filtration cycles. Although the hydrodynamic conditions at the pilot plant were not exactly the same as those in the off-line bench scale filtration system, each 10-minute filtration cycle in the pilot scale M E B P R process was somewhat like the off-line short-term filtration tests described in Figure 2.8, where sludge deposition (Rc) and superficial pore blocking (Rp) were diminished to almost nil through the strong shear stress induced by aeration and backflushing. After each small filtration cycle, only the fouling due to organic adsorption and deep pore clogging (Rap) was retained on the membranes. This hydraulically irreversible fouling accumulated little by little as illustrated in Figures 2.5 and 2.6 and eventually brought the system to the maximum T M P operating limit. It should be noted that, according to other studies (Nagaoka et al, 1998; Hong et al, 2002; Berube et al, 2007), some reversible fouling, such as Rc and Rp, could become irreversible (Rap) through the consolidation of accumulated foulants on the membrane surface. In these studies, the extent of consolidation, or the magnitude of the resulted Rap, appeared to be a function of applied TMP and/or the duration of the applied TMP. The longer the duration, the larger the magnitude of Rap. This was probably why the irreversible fouling (Rap) was noticeable in the 180-minute bench scale filtration test 51 (Figure 2.8), but was insignificant in any 10-minute filtration cycle of the M E B P R system (Figure 2.5). In this sense, shorter filtration cycles or more frequent maintenance cleaning are conducive to reducing not only reversible fouling but also irreversible fouling. Effect of Soluble Microbial Products on Membrane Fouling The above experimental results suggested that the membrane fouling in the pilot scale M E B P R process was mainly due to irreversible organic adsorption and probably deep pore clogging as well (Rap). Since soluble microbial products (SMP), also called soluble extracellular polymeric substances in the literature (Laspidou and Rittmann, 2002), which consist of utilization-associated products (UAP) and biomass-associated products (BAP), have been found to play an important role in membrane fouling in submerged MBRs (Cho et al, 2003; Shin and Kang, 2003), examination of the effect of SMP on the irreversible fouling in the MEBPR process was thus necessary. SMP production is affected by many factors including microbial growth, substrate metabolism, nutrient availability, and environmental stress. SMP are especially excreted in response to toxic substances (Barker and Stuckey, 1999). As described under Methods, a bench scale filtration test was designed to compare the filtration performance of a control M E B P R sludge and a M E B P R sludge that was inhibited by a high concentration of Cu solution and which consequently produced SMP under the stress. The experimental results are illustrated in Figure 2.9. As expected, the sludge that was inhibited by C u 2 + and D. 10 20 30 Time elapsed, min. o Control sludge (46 l_/mzh.) • Control sludge (68 L/m2h.) A Sludge inhibited (46 L/m2h.) • Sludge inhibited (68 L/m2-h.) 40 Figure 2.9. Filtration performance of the M E B P R sludge inhibited by Cu in comparison to that of the control sludge at a volumetric airflow rate of 60 m3/m3-h 52 hence excreted SMP showed a higher fouling tendency, as indicated by its higher TMP, than the control sludge, especially at a high flux. The subsequent measurement of the TOC content of the soluble fraction of sludge further indicated that the dissolved organic carbon of the sludge containing C u 2 + was 39.4 mg/L, 10 times higher than that of the control sludge (3.9 mg/L). Therefore, the addition of C u 2 + ions not only inhibited the respiration of sludge microorganisms, but also likely stimulated the excretion of SMP, resulting in an enormous release of intracellular substances into the mixed liquor. As such SMP were mainly cellular macromolecules or B A P (Jarusutthirak and Amy, 2006), they could either readily adsorb onto the membrane surface or clog the membrane pores, leading to a sharp increase in hydraulic resistance and accordingly an exacerbation of membrane fouling. It should be noted that, as intensive aeration was applied in the above filtration tests, the fouling developed was very likely due to adsorption and pore blocking (Rap and Rp) that was not completely irreversible. Nonetheless, the results in Figure 2.9 confirmed the important role of SMP in membrane fouling by showing that the sludge with more SMP had greater tendency to foul the membranes than the sludge with less SMP. A Mathematical Model for the Long-term Fouling in the MEBPR Process It could have been noticed from Figure 2.6 that the long-term membrane fouling in the pilot scale MEBPR process proceeded in a similar manner in each filtration run during either Run I or Run II: a slow TMP buildup in the first 1-2 months followed by a sharp increase in TMP near the end of the filtration run. Since the long-term fouling in the MEBPR process has been characterized as irreversible fouling caused mainly by organic adsorption and possibly by deep pore clogging as well (Rap), the TMP profiles shown in Figure 2.6 can be viewed as foulant adsorption curves that are unlimited in adsorption capacity. A non-linear regression analysis of the data in Figure 2.6 further revealed that the TMP developed in the pilot scale MEBPR process exhibited a simple exponential relationship with the filtration time, as described in the following mathematical expression. TMP = T M P 0 e k , t (2.1) 53 Where, TMPo is the initial TMP at filtration time t = 0 day when no fouling occurs (kPa). It is equal to the steady state TMP when filtering clean water. The exponent kt is the time-based fouling coefficient that is determined by the characteristics of the M E B P R system (1/d). Since the long-term fouling at the UBC pilot plant was primarily irreversible, Jct here actually represents the time-based irreversible fouling coefficient. Once k, is known, the filtration run time, i.e. the time when the maximum allowable TMP (69 kPa in this study) is reached and a recovery cleaning is required, can be predicted according to the Equation 2.1. Four typical filtration runs, i.e., the first, second, fifth and sixth filtration runs as illustrated in Figure 2.6, were modeled in the present study. The model parameters were obtained by fitting Equation 2.1 to the experimental data collected in these filtration runs and the results are given in Figure 2.10 (a). It is evident that the exponential model satisfactorily fits each set of data, although there is some divergence for the last few data points in Run I. Due to the change of the operating conditions in the second experimental run, the model parameters, in particular the time-based irreversible fouling coefficients, kt, for Run II were quite different from those for Run I (Table 2.3), resulting in the two distinctive groups of mathematical models present in Figure 2.10 (a). Thus, the time-based fouling model (Equation 2.1) appeared to be considerably affected by the system operating conditions, especially the operating flux (or HRT). Increasing the permeate flux from 23 L/m 2-h in Run I to 33 L/m 2-h in Run U (or, decreasing the HRT from 10 hours in Run I to 7 hours in Run II) greatly enhanced the mass flux to the membrane surfaces, causing a quicker membrane fouling and a greater time-based fouling coefficient (0.0249 - 0.0271 1/d for Run II compared to 0.0121 - 0.0178 1/d for Run I). Table 2.3. Model parameters for the two experimental runs Experimental Flux (J) Soluble TMPo Time-based Volume-based Organic loading C O D a fouling coeff. fouling coeff. -based fouling run L/m2-h mg/L kPa {kt), 1/d (Jfcv). 1/m 3 coeff. (ki), 1/kg Run 1 23 25.8 b 15.28-20.58 0.0121-0.0178 0.0018-0.0027 0.0714-0.1048 Run II 33 3 1 . 4 c 18.22-19.27 0.0249-0.0271 0.0026-0.0029 0.0841-0.0916 a. COD in the soluble fraction of the aerobic MEBPR activated sludge mixed liquor b. The average soluble COD in Run I. The standard error was 1.9 mg/L, and the number of samples was 65. c. The average soluble COD in Run II. The standard error was 6.1 mg/L, and the number of samples was 20. 54 100 80 CO CL 60 CL 40 20 0 y = 1 9 . 2 7 2 e 0 0 2 7 1 x r , ? y = 1 5 . 2 8 3 e0 0 1 7 8 x R = 0.972 y j R 2 = 0.932 r . y = 18.218e 0 0249x^r> R 2 = 0 . 9 7 4 2 j f e ^ y = 20.581 e 0 0 1 2 1 x R 2 = 0.9804 a 0 20 40 60 80 Filtration time, days 100 80 60 co CL * 40 CL ? 20 0 A y= 19.272e 0 0 0 2 9 x R 2 = 0.972 j/j? a y= 18.218e 0 0 0 2 6 x ^C^j R 2 = 0 . 9 7 4 2 j a ^ > £ ^ ^ ^ = 20.581 e 0 0 0 1 8 x R 2 = 0.9804 . y = 15.283e 0 0 0 2 7 x R 2 = 0.932 b 200 400 Cumulative volume filtered, m 3 600 80 60 40 20 0 y= 15.283e' R 2 = 0.932 0.1048X A ^ y= 19.272e o.cei6x o y= 20.581 e u a y=18.218e U U M R 2 = 0.9742 0.9804 0 5 10 15 Cumulative organic loading, kg soluble C O D • The 1st filtration run (Run I) o The 2nd filtration run (Run I) A The 5th filtration run (Run II) • The 6th filtration run (Run II) • The 1st filtration run (Run I) o The 2nd filtration run (Run I) • The 5th filtration run (Run II) • The 6th filtration run (Run II) • The 1st filtration run (Run I) o The 2nd filtration run (Run I) A The 5th filtration run (Run II) • The 6th filtration run (Run II) Figure 2.10. Model development for the long-term irreversible fouling in the pilot scale M E B P R process during the two experimental runs, (a) time-based model; (b) cumulative volume-based model; (c) cumulative organic loading-based model 55 For the M E B P R system that was operated in a constant-flux mode, the volume (V) of permeate filtered is the product of the permeate flux (J), the membrane surface area (A) and the filtration time (t) (Equation 2.2). Therefore, Equation 2.1 can be readily converted into a function of the cumulative volume of permeate filtered instead of the filtration time, as shown in Equation 2.3. V = J A t (2.2) TMP = T M P 0 e k , t = T M P 0 e [ ^ / ( J A ) ] V = T M P 0 e k v V (2.3) kv = k,/(]-A) (2.4) Where, kv is defined as the volume-based irreversible fouling coefficient, 1/m3. Accordingly, the evolution of TMP in the pilot scale M E B P R system can also be expressed as a simple exponential relationship with the volume of the permeate filtered. Figure 2.10 (b) presents the volume-based irreversible fouling models obtained by fitting Equation 2.3 to the experimental data of the four filtration runs. It is clear that the two groups of mathematical models, which described the fouling process in Run I and Run II, respectively, now get close to each other in Figure 2.10 (b) and are not as distinctively different as shown in Figure 2.10 (a). This was also reflected in the corresponding volume-based fouling coefficients: the range of kv for Run I (0.0018 - 0.0027 1/m3) had a slight overlap with that of Run U (0.0026 - 0.0029 1/m3) (Table 2.3). As the operating fluxes in the two experimental runs were quite different, the modeling results in Figure 2.10 (b) suggested that the volume-based fouling model (Equation 2.3) or the volume-based irreversible fouling coefficient kv was not very much affected by the operating flux. Moreover, the extent of the fouling in the pilot scale M E B P R system was directly associated with the cumulative volume of the permeate that had been filtered through the membranes. Berube and Watai (2005) also reported that when filtering a raw drinking water source, the increase in TMP could be modeled using an exponential model that was a function of the volume of permeate filtered. A recent study on a MEBPR-type system by Rosenberger et al. (2006) indicated that the non-settable fraction of sludge, i.e. soluble polysaccharides, proteins and organic colloids, had significant impact on the membrane fouling process. The preceding experimental results also suggested that soluble microbial products, measured as dissolved TOC, played an important role in the membrane fouling during filtration of the aerobic 56 MEBPR sludge. Therefore, the concentration of soluble organic material in activated sludge mixed liquor could be an important index in determining the irreversible fouling coefficient. A positive relationship was assumed between the volume-based irreversible fouling coefficient kv and the concentration of soluble substances (C) in activated sludge mixed liquor (e.g. kv oc C or kv = k^C), since the volume of permeate filtered and the content of soluble organic matter in mixed liquor together established the total amount of soluble foulants that were brought into contact with the membranes in a defined period of time. If this is true, Equation 2.3 could be rewritten as follows. TMP = TMPo e k v ' V = TMP 0 e k l ' C ' V = TMP 0 G k l ' L (2.5) L = C V (2.6) Where, L is the cumulative organic loading on the membrane surfaces, expressed as kg soluble COD. kL is defined as the organic loading-based irreversible fouling coefficient, 1/kg. C is the concentration of organic substances in the soluble fraction of mixed liquor and is expressed in mg soluble COD/L in this study. To verify the above assumption, the cumulative organic loading on the membrane surfaces was calculated by multiplying the cumulative volume of the permeate with the average COD in the soluble fraction of the aerobic MEBPR mixed liquor (Table 2.3). Then, the cumulative organic loading was plotted against the TMP and the exponential model (Equation 2.5) was applied to fit the data. The results are shown in Figure 2.10 (c). It is clear that the organic loading-based model depicted the fouling process very well, proving that the previous assumption (kv oc C) was tenable. Most interestingly, the resulting mathematical models for Run I almost "merged" with those for Run II. There is visually no distinction between the two groups of models. The corresponding fouling coefficients ki for Run II varied between 0.0841 and 0.0916 1/kg, which fall into the range of the fouling coefficient for Run I (0.0714 - 0.1048 1/kg) (Table 2.3). From a practical standpoint, the organic loading-based fouling coefficients were basically identical for the two experimental runs, if the process fluctuations and/or variations were considered. The modeling results in Figure 2.10 (c) suggested that regardless of the operating conditions, the long-term irreversible fouling in the pilot scale MEBPR process was primarily determined by the cumulative organic loading carried by the soluble fraction of activated 57 sludge mixed liquor to the membrane surfaces. Obviously, this was in an agreement with the preceding findings. Compared to the time-based fouling model (Equation 2.1) and the volume-based fouling model (Equation 2.3), the organic loading-based model (Equation 2.5) appeared to be able to use a single kL value to reasonably elaborate the TMP evolution in the two experimental runs. In this regard, Equation 2.5 could be useful as a generic model to describe the irreversible fouling in the M E B P R process. To date, different mathematical models have been developed to simulate the wastewater treatment performance and the membrane fouling process in submerged MBRs (Nagaoka et al, 1998; Cho et al, 2003). However, these models were all based on short-term bench scale experiments and involved dozens of differential equations and multiple model parameters. In comparison to the existing fouling models, the exponential model proposed in the present study was developed on the basis of the long-term pilot scale M E B P R process. It generalized the effect of various influencing factors on membrane fouling and thus was generic, practical and simple, with only two parameters involved. In addition to its prediction of the TMP evolution, the exponential model is advantageous in providing an improved understanding of the long-term irreversible fouling in the studied M E B P R process, because the combined effect of the system design and operating conditions on membrane fouling is captured in a single parameter, the irreversible fouling coefficient. Conclusions The long-term membrane fouling developed in the pilot scale MEBPR process was hydraulically irreversible and was likely caused by organic adsorption and deep pore clogging. Due to the vigorous aeration and frequent backflushing applied in the process, reversible fouling was reduced to a minimum. Soluble microbial products in the activated sludge mixed liquor, measured as dissolved TOC in this study, were likely responsible for the irreversible fouling. The TMP evolution in the pilot scale M E B P R process followed a simple exponential relationship with the filtration time, the cumulative volume of permeate filtered, or the cumulative soluble organic loading on the membranes. In nature, the degree of fouling mainly depended on the amount of soluble substances that had been carried 58 onto the membranes by the soluble fraction of activated sludge mixed liquor. Operating flux and/or hydraulic retention time had little impact on the organic loading-based irreversible fouling coefficient. Although the anaerobic-anoxic-aerobic configuration of the M E B P R system was quite different from the configuration of common MBRs that comprise single reaction zone, organic adsorption and pore clogging by soluble and colloidal substances, which were found to govern the membrane fouling in the pilot scale M E B P R process, have also been reported as the fouling mechanisms for certain MBRs other than MEBPR-type systems. References Adam, C , Kraume, M . , Gnirss, R. and Lesjean, B. (2003). Membrane bioreactor configurations for enhanced biological phosphorus removal. Water Science and Technology: Water Supply, 3(5-6), 237-244. Barker, D. J. and Stuckey, D. C. (1999). A review of soluble microbial products (SMP) in wastewater treatment systems. Water Research, 33(14), 3063-3082. Berube, P. R. and Watai, Y . (2005). Impact of operating flux, hydrodynamic conditions and system configuration on the operating pressure in a submerged hollow fiber membrane system. AWWA Membrane Technology Conference, Phoenix, Arizona, USA. Berube, P. R., Wong, J. and Lin, H. (2007). Evolution of the trans-membrane pressure during successive filtration cycles in a submerged hollow fiber membrane system. AWWA Membrane Technology Conference, Tampa, Florida, USA (in submission). Cho, J., Ahn, K.-PL, Seo, Y . and Lee, Y . (2003). Modification of A S M No . l for a submerged membrane bioreactor system: Including the effects of soluble microbial products on membrane fouling. Water Science and Technology, 47(12), 177-181. Choo, K.-PL, Kang, I.-J., Yoon, S.-PL, Park, PL, Kim, J.-H., Adiya, S. and Lee, C.-H. (2000). Approaches to membrane fouling control in anaerobic membrane bioreactors. Water Science and Technology, 41(10), 363-371. Gui, P., Huang, X . , Chen, Y . and Qian, Y . (2003). Effect of operational parameters on sludge accumulation on membrane surfaces in a submerged membrane bioreactor. Desalination, 151(2), 185-194. Hong, S. P., Bae, T. H. , Tak, T. M . , Hong, S. and Randall, A . (2002). Fouling control in activated sludge submerged hollow fiber membrane bioreactors. Desalination, 143(3), 219-228. Jarusutthirak, C. and Amy, G. (2006). Role of soluble microbial products (SMP) in membrane fouling and flux decline. 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Lim, A. L. and Bai, R. (2003). Membrane fouling and cleaning in microfiltration of activated sludge wastewater. Journal of Membrane Science, 216(1-2), 279-290. Liu, R., Huang, X . , Chen, L. , Wang, C. and Qian, Y . (2000). Pilot study on a submerged membrane bioreactor for domestic wastewater treatment. Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 35(10), 1761-1772. Monti, A. (2006). A comparative study of biological nutrient removal processes with gravity and membrane solids-liquid separation, PhD thesis, The University of British Columbia, Vancouver, Canada. Nagaoka, H. , Yamanishi, S. and Miya, A . (1998). Modeling of biofouling by extracellular polymers in a membrane separation activated sludge system. Water Science and Technology, 38(4-5), 497-504. Ognier, S., Wisniewski, C. and Grasmick, A. (2002). Influence of macromolecule adsorption during filtration of a membrane bioreactor mixed liquor suspension. Journal of Membrane Science, 209(1), 27-31. Ramphao, M . , Wentzel, M . C , Ekama, G. A. , Merritt, R., Young, T. and Buckley, C. A. (2005). Impact of membrane solid-liquid separation on design of biological nutrient removal activated sludge systems. Biotechnology and Bioengineering, 89(6), 630-646. Rosenberger, S., Laabs, C , Lesjean, B. , Gnirss, R., Amy, G., Jekel, M . and Schrotter, J.-C. (2006). Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment. Water Research, 40(4), 710-720. Shin, H. and Kang, S. (2003). Performance and membrane fouling in a pilot scale SBR process coupled with membrane. Water Science and Technology, 47(1), 139-144. van der Roest, H. , van Bentem, A. and Lawrence, D. (2002). MBR-technology in municipal wastewater treatment: Challenging the traditional treatment technologies. Water Science and Technology, 46(4-5), 273-280. Wong, K . -Y . , Zhang, M.-Q., L i , X . - M . and Lo, W. (1997). A luminescence-based scanning respirometer for heavy metal toxicity monitoring. Biosensors and Bioelectronics, 12(2), 125-133. 60 Chapter 3. Roles of various sludge constituents in membrane filtration of activated sludge mixed liquor* Introduction Due to the rapid development of membrane technology and the formulation of increasingly stringent environmental regulations, coupling membrane filtration with activated sludge processes, that is the so called membrane bioreactor (MBR), has recently become one of the major choices in the wastewater treatment industry (Stephenson et al, 2000). Despite the many superior features of MBRs, membrane fouling caused by accumulation of sludge constituents on the membrane surface and/or within the membrane pore structure is of particular concern, since it limits the total treatment performance if the fouling problem is not well addressed (Judd, 2005). Research work has been carried out on the roles of different sludge fractions in membrane fouling. Wisniewski and Grasmick (1998) quantified the contributions of three main families of particles (settleable, supracolloidal-colloidal and soluble) to the membrane fouling and found that half of the total resistance was due to the soluble compounds and the setteable sludge floes exerted only a small influence on membrane performance. Subsequent studies further revealed that soluble and colloidal substances in activated sludge mixed liquor were responsible for the majority of membrane filtration deterioration (Shin and Kang, 2003a; Itonaga et al, 2004; Rosenberger et al, 2006). Previous experimental results from the present study suggested that soluble microbial products (SMP), measured as dissolved total organic carbon (TOC), played an important role in the membrane fouling in a membrane enhanced biological phosphorus removal system, since a sludge with a higher SMP concentration exhibited a greater fouling propensity than a sludge with less SMP (Chapter 2). In view of all these research works, it seems that the soluble fraction of sludge is the major contributor to membrane fouling and the particulate fraction of sludge, on the other hand, can be neglected due to its insignificant impact on membrane filtration. If this is true, then the membrane filtration of activated sludge would be, to some extent, analogous to a filtration of the liquid fraction * A version of this chapter will be submitted for publication. Geng, Z., Hall, E.R., and Berube, P.R. Roles of various sludge constituents in membrane filtration of activated sludge mixed liquor 61 of mixed liquor. In contrast, Lee et al. (2001) compared the filtration performance of attached and suspended growth microorganisms in two submerged membrane bioreactors and the results were not in agreement with the above deduction. They found that despite the similar characteristics of the soluble fraction in the two reactors, the rate of membrane fouling of the attached growth system was about 7 times higher than that of the suspended growth system. This implied that the presence of sludge floes somehow mitigated the membrane fouling in the suspended growth system and thus the influence of the particulate fraction of sludge could not be neglected. Obviously, the research findings on the roles of various sludge fractions and/or components in membrane fouling are not consistent with one another. To address this controversy and unify the understanding, a thorough and systematic investigation of the behaviour of different sludge constituents in membrane fouling process was required. The present study was thus initiated to meet the above research needs. The objectives were to clarify the roles of various sludge constituents in membrane filtration of sludge and to gain insight into the interactions among the sludge constituents and their effect on membrane fouling. Size exclusion-based sludge fractionation techniques were used to separate sludge fractions. Bench scale filtration tests were then applied to the resultant sludge fractions at different fluxes to study the fouling behaviour of sludge constituents under different filtration conditions. An examination of the physical and biochemical characteristics of the soluble fraction of sludge was also performed in order to further elucidate the irreversible fouling induced by this sludge fraction. Methods Sludge Sampling and Fractionation Two types of activated sludge mixed liquor, i.e. an M B R sludge and a conventional sludge, were used in the present study. They were collected, respectively, from the aerobic zones of a membrane enhanced biological phosphorus removal (MEBPR) process and a conventional enhanced biological phosphorus removal (CEBPR) process at the University of British Columbia (UBC) Wastewater Treatment Pilot Plant. Both processes utilized three compartments, i.e. anaerobic, anoxic, and aerobic reaction 62 zones in series. A custom-built ZeeWeed module (Zenon Environmental Inc., Oakville, Ontario, Canada), which had a nominal membrane pore size of 0.04 fim, was installed in the aerobic zone of the pilot scale M E B P R process for separation of sludge from treated wastewater. In contrast, a secondary clarifier was used in the pilot scale C E B P R process for sludge separation. For the purpose of comparison, the two treatment processes shared the same sewage influent and the same design and operating parameters (except for the aeration intensity). Two experimental runs were carried out at the U B C pilot plant during the period of this study: March 2003 - December 2003 for Run I and December 2003 -June 2004 for Run II. A description of the two processes and their treatment performances in the two experimental runs were presented in detail under Methods of Chapter 2. After collection in Run I, activated sludge grab samples (about 4 L of sludge each) were immediately transported to an environmental lab for sludge fractionation. Figure 3.1 illustrates the steps and means for fractionation of activated sludge mixed liquor. First, approximately 1 L of a sludge sample was retained as the original whole sludge, and the remaining 3 L was centrifuged at 2000 x g for 5 minutes and the supernatant was collected. Second, 2 L of the supernatant was then filtered through 8.0 | im membrane filters and the filtrate was collected and designated as Filtrate I. Third, half of this filtrate Filtration (0.04 |i) Sludge fractions Sludge constituents ; Permeate i Filtration (0.45 u) .V. Soluble substances Filtration (8 u) Filtrate II Colloids Centrifugation (2000 x g) Filtrate I Unsettleable microflocs Large sludge floes j Original whole sludge Figure 3.1. Scheme for fractionation of activated sludge mixed liquor 63 was immediately subjected to a second filtration using 0.45 u.m filter papers and the resulting filtrate was designated as Filtrate II, or the soluble fraction of the sludge. In the present study, the term "soluble" was defined in accordance with the traditional definition, which referred to the substances that were able to pass through 0.45 urn filter paper. It was evident that the sludge fractionation here was mainly based on the principle of size exclusion. To facilitate the analysis of the experimental results, the characteristics of the resultant sludge fractions are presented in Table 3.1, of which total suspended solids (TSS) and total organic carbon (TOC) were measured according to Standard Methods (APHA etal, 1995). In addition to the steps for sludge fractionation, Figure 3.1 also demonstrates the relationship of the sludge fractions to sludge constituents. Clearly, individual sludge constituents were the difference between any two neighboring sludge fractions. By comparing the filtration performance of any two sludge fractions (i.e. supernatant and Filtrate I), the effect of particular sludge constituents (i.e. unsettleable microflocs) on membrane filtration could be roughly assessed. An exception was that membrane permeate could be regarded as either a sludge fraction or a collection of sludge constituents whose sizes were smaller than 0.04 u\m. Table 3.1. Characteristics of the four fractions of both the M E B P R and CEBPR sludge Sludge Fraction M E B P R C E B P R T S S T O C T S S T O C Whole sludge (large floes + microflocs + colloids + solutes) 2330 - 1230 -Supernatant (microflocs + colloids + solutes) 88 12.5* 37 6.5* Filtrate 1 (colloids + solutes) - 9.0 - 4.5 Filtrate II (solutes) - 7.5 - 3.0 Note: (1) - not applicable / analyzed; (2) * floating materials removed. Bench Scale Filtration Tests The resultant sludge fractions, namely the original whole sludge, supernatant, Filtrate I, and Filtrate II or the soluble fraction of sludge, were filtered separately at constant permeate fluxes using the bench scale filtration apparatus depicted in Figure 2.4 of Chapter 2. The membrane module used in this work comprised of four 0.1-m membrane hollow fiber loops, which had a polyvinylidene fluoride (PVDF) skin with a nominal membrane pore size of 0.04 um. In the filtration tests, the membrane loops were 64 submerged in a sludge fraction and the permeate was collected at room temperature (20 -22 °C) via a Masterflex peristaltic pump. No backflushing or relaxation was applied and the system was operated with continuous suction in a constant-flux mode for 30 minutes. Aeration at a volumetric flow rate of 15 m 3 air / m 3 liquid • h was provided around the membrane loops during the suction and the change of transmembrane pressure (TMP) with time was closely monitored. As stated earlier under Introduction, previous literature reports about the effect of various sludge fractions on membrane fouling are very contradictory. Considering that different M B R systems with different hydrodynamics were used in those studies, it was anticipated that the differences in the system design and especially in the membrane operating conditions might have led to the apparent discrepancies. Permeate flux is the most influential operating parameter with respect to membrane fouling (Nagaoka et al., 1998). In the present study, therefore, three constant fluxes (23 L/nr-h, 33 L/nr-h and 68 L/nr-h) were applied in the bench scale filtration tests with the sludge fractions, in an attempt to examine whether the sludge constituents exhibited different fouling behavior under different filtration conditions. Brand new membrane loops were used in each filtration test. Before filtering a sludge fraction, a clean water filtration test was performed to estimate the hydraulic resistance caused by membrane itself, according to Equation 3.1. Afterwards, the sludge fraction was filtered using the same set of membrane loops and the total hydraulic resistance was calculated in the same way as with clean water. Then, the hydraulic resistance due to fouling was derived by subtracting the resistance induced by membranes from the total hydraulic resistance, as indicated in Equation 3.2. Darcy'slaw: J = AP/ (uR t ) (3.1) Resistance-in-series: R, = R m + Rf (3.2) Where, J - membrane flux, m3/m2-s; AP - transmembrane pressure (TMP), Pa; ]1 -permeate viscosity, Pas; R t - total hydraulic resistance, 1/m; R m - hydraulic resistance due to membrane itself, 1/m; R f - hydraulic resistance due to fouling, 1/m. 65 Analysis of the Soluble Fraction of Sludge To further elucidate the role of soluble organic substances (<0.45 u,m) in the irreversible membrane fouling, the chemical and physical characteristics of the soluble fraction of sludge were analyzed in terms of total organic carbon (TOC), molecular weight distribution and fine particle size distribution. A Phoenix 8000 UV-Persulfate TOC analyzer (Dohrmann) was used to measure the content of TOC in accordance with Standard Method 5310C (APHA et al., 1995). The number average and weight average molecular weight (Mn and Mw, respectively) were estimated by gel permeation chromatography (GPC) using a Agilent 1100 HPLC system equipped with an autosampler, an isocratic pump, a thermostatted column compartment, a multiple wavelength detector (MWD), a refractive index detector (RID), and two Waters Styragel columns (HR5E and HR1) in tandem. Tetrahydrofuran (THF) was used as the eluent and was supplied at 1 mL/min. Column temperature was 50 °C and the columns were calibrated with polystyrene standards. Sample preparation for the GPC analysis is described in detail in Appendix 10. Both samples and polystyrene standards were detected by M W D at 280 nm and 254 nm, respectively. For the measurement of the fine particle size distribution, a Malvern Hydro 2000S was applied in the present study. Since the particles contained in the soluble fraction of sludge (Filtrate II) were smaller than 0.45 p:m in size and close to the lower limit of the instrument (0.020-1000 p:m), the measurement was challenging and very susceptible to noise and errors. For this reason, the measurement cell and all the parts that might have contacted the sample were thoroughly cleaned. Distilled deionized water was used for background measurement. In addition, by using the Degas function of the instrument, a degassing operation was performed prior to the measurement to eliminate the interference of fine air bubbles. Results and Discussion Contribution of Sludge Constituents to Membrane Fouling at a Low Permeate Flux The four fractions of both the M E B P R sludge and the CEBPR sludge, i.e. the original whole sludge, the supernatant after centrifugation, the Filtrate I after filtration of the supernatant with 8.0 urn membrane filters, and the Filtrate n, or the soluble fraction of 66 sludge that could pass through 0.45 \im membrane filters, were first subjected to a series of short-term bench scale filtration tests at a constant low flux of 23 L/m 2-h, the operating flux in the MEBPR process at the U B C pilot plant. Figure 3.2 (a) illustrates the TMP over time during the filtration of the M E B P R sludge fractions. It is clear that after the initial lag period required for the pump to establish a stable vacuum throughout the system, the TMP increased linearly with time. Similar results were also obtained for the CEBPR sludge fractions as shown in Figure 3.2 (b). CO CL 0 a Time elapsed, min. o Whole sludge • Supernatant A Filtrate I o Filtration II Time elapsed, min. CO CL o Whole sludge • Supernatant A Filtrate I o Filtrate II Figure 3.2. Transmembrane pressure during the filtration of the four fractions of (a) an aerobic M E B P R sludge and (b) an aerobic CEBPR sludge at a flux of 23 L/m 2-h From the above measured TMP values, the hydraulic resistances due to fouling after the initial lag period were then calculated in accordance with the resistance-in-series model and Darcy's law (Equations 3.1 and 3.2), and the results are shown in Figure 3.3. It can be seen that for both the M E B P R sludge and the CEBPR sludge, the resistances due to 67 C 3 o LU O 2 E CD ^ "O 0 O c ro "So 'co CD C 3 o CD =3 T3 0 O C ro GO 'co CD rr LU o 0 . 0 7 8 X - 0.326 R 2 = 0.990 o Whole sludge • Supernatant A Filtrate I o Filtrate II 40 Elapsed time, min. 2.0 1.5 1.0 0.5 0.0 -0.5 y = 0.059x - 0.253 R 2 = 0.996 y = 0.057x - 0.208 R 2 = 0.988 ^ y = 0.056x - 0.302 R 2 = 0.998 y = 0.038x-0.109 R2 = 0.967 10 20 30 o Whole sludge • Supernatant A Filtrate I o Filtrate II 40 Elapsed time, min. Figure 3.3. Hydraulic resistance due to fouling in the filtration of the four fractions of (a) the aerobic M E B P R sludge and (b) the aerobic CEBPR sludge at a flux of 23 L/m 2-h fouling increased linearly with time at different rates for the different sludge fractions. The rate of increase in hydraulic resistance due to fouling, that is the slope of each line in Figure 3.3, was defined as the fouling rate. From Figure 3.3(a), for example, the fouling rate of the supernatant of the M E B P R sludge was 0.213x10" 17m •min. Then, the rate of fouling induced by the original whole sludge was taken as a baseline, the individual fouling rates measured for each fraction were compared to this baseline to estimate the percentages of the whole sludge fouling caused by the individual sludge fractions. The results are illustrated in Figure 3.4. It is evident that the supernatant, Filtrate I and Filtrate II of the M E B P R sludge accounted, respectively, for about 93%, 66%, and 34% of the whole sludge fouling under the experimental conditions used. In other words, the relative 68 150 s D) C 100 = 50 o 100 93 66 34 Whole sludge Supernatant Filtrate I Filtrate II S ludge fraction CD 03 I— CD c o Whole sludge Supernatant Filtrate I Filtrate II Sludge fraction Figure 3.4. The relative fouling rates of the individual sludge fractions to the whole sludge fouling at a flux of 23 L/m 2-h: (a) the aerobic MEBPR sludge; (b) the aerobic CEBPR sludge fouling rates of these fractions to the whole sludge were 93%, 66%, and 34%, respectively. For the CEBPR sludge, the relative fouling rates as percentages of the whole sludge fouling were 96%, 95%, and 65%, respectively. It is not difficult to realize that the differences between the percentages in Figure 3.4 represented the contribution of some particular sludge constituents to membrane fouling. For example, the difference of 7% between the M E B P R whole sludge (100%) and its supernatant (93%) indicated the contribution of the large MEBPR sludge floes to the total fouling. Similarly, the differences in the percentages between the supernatant and Filtrate I and between the Filtrate I and Filtrate II represented the contribution of 69 unsettleable rnicroflocs and colloids to the whole sludge fouling, respectively. The relative contribution of the different sludge constituents to the whole sludge fouling was thus calculated and the results are demonstrated in Figure 3.5. Moreover, to compare the direct inputs of these sludge constituents to membrane fouling, the absolute values of the fouling rates of these constituents were derived from Figure 3.3 and are illustrated in Figure 3.6. A l l the above calculations were based on the assumption that the fouling caused by the different sludge fractions/constituents is simply additive. Figure 3.5. Relative contribution of the different constituents of (a) the aerobic MEBPR sludge and (b) the aerobic CEBPR sludge to the short-term membrane fouling at a flux of 23 L/m 2-h C E B P R sludge M E B P R sludge f j <0.45 nm soluble l ~ l between 0.45-8.0 |im H >8.0 urn unsettleable E*3 large sludge floes 0.00 0.05 0.10 0.15 0.20 0.25 Fouling rate, 1.0E-11 1/m-min. Figure 3.6. Comparison of the absolute fouling rates of the different MEBPR sludge constituents to those of the CEBPR sludge constituents at a flux of 23 L/m 2-h 70 It is readily noticed in Figure 3.5 that for both types of sludge, large sludge floes accounted only for a very small portion of the fouling (4-7%). Fine particles and colloids between 0.45 and 8.0 |Ltm were responsible for about one third of the total short-term fouling (30-32%). A large part of the membrane fouling (33-65%) was attributable to the soluble fraction of the mixed liquor. Obviously, these results were congruent with the aforementioned research work (Wisniewski and Grasmick, 1998; Shin and Kang, 2003a; Itonaga et ah, 2004; Rosenberger et ah, 2006), indicating that under the experimental conditions applied, the soluble and colloidal substances in the activated sludge mixed liquor were the major sludge constituents causing the membrane fouling in the M E B P R process and, by contrast, large sludge floes appeared to play a small role in the deterioration of membrane performance. It is also noted in Figure 3.5 that, due to the membrane retaining, there was a greater proportion of unsettleable microflocs or supra-colloids (> 8.0 um) in the M E B P R mixed liquor than in the counterpart CEBPR mixed liquor, resulting in a larger contribution of these sludge constituents to the total fouling than that of the CEBPR sludge (28% vs. 1%). This was also the reason for the poorer settleability of the M E B P R sludge relative to the CEBPR sludge. In addition, because the concentrations of the various sludge constituents (especially the soluble and colloidal substances expressed as TOC) in the MEBPR sludge fractions were as much as 2 times higher than those in the CEBPR sludge fractions (Table 3.1), greater absolute fouling rates were measured for the former (Figure 3.6), indicating that not only the type, but also the amount of sludge constituents had significant influence on the fouling process. It was indicated in Chapter 2 that the long-term membrane fouling in the pilot scale MEBPR process was hydraulically irreversible and was likely caused by organic adsorption and deep pore clogging (Rap). The reversible fouling induced by sludge deposition (Rc) and superficial pore blocking (Rp) was reduced to a minimum due to the vigorous aeration and frequent backflushing applied in the system. If looking further into the fouling process, certain links could be established between the particular sludge constituents and the fouling mechanisms. It was not hard to understand that the soluble and fine colloidal substances in activated sludge mixed liquor tended to adsorb onto the membrane surfaces and were thus most likely associated with the irreversible adsorption 71 and deep pore clogging (Rap). Colloids and unsettleable microflocs mainly contributed to pore blocking (Rp), and large sludge floes were apparently most responsible for sludge deposition and cake formation (Rc). In the submerged membrane systems such as the above bench scale filtration system and the M E B P R process, in which strong shear stress was applied on the membrane surfaces and the irreversible fouling (Rap) was dominant, the soluble and fine colloidal sludge constituents likely played a key role in the membrane performance decline. Just as the dominance of Rap, Rp and Rc was affected by the hydrodynamic conditions and the mass transfer processes, the relative importance of the various sludge constituents to membrane fouling also very much depended on the system hydrodynamics. Behavior of Large Sludge Floes in Membrane Filtration at High Permeate Fluxes After the filtration tests at the flux of 23 L/m 2-h were completed, the permeate flux was increased to 33 L/m 2-h and 68 L/m 2-h successively. To cut down the experimental work, this time only the M E B P R sludge fractions were subjected to the high-flux membrane filtration. The TMP was monitored and the fouling rate of each sludge fraction in the high-flux filtration tests was derived in the same manner as for the low-flux tests. Figure 3.7 shows the relative fouling rates of the four MEBPR sludge fractions at the fluxes of 33 L/m 2-h and 68 L/m 2-h. Recall that at the low flux of 23 L/m 2-h, the whole sludge had the highest relative fouling rate (100%), and the supernatant, Filtrate I and Filtrate II exhibited fouling rates in a descending order (Figure 3.4). When the permeate flux was increased, however, things seemed not to work in the same "normal" logic. At the flux of 33 L/m -h, the fouling rate of the supernatant unexpectedly exceeded that of the whole sludge (Figure 3.7 (a)). This phenomenon became more apparent when the flux was further increased to 68 L/m 2-h, since not only the supernatant but also Filtrate I demonstrated higher fouling propensity than the original whole sludge: the relative fouling rate of the supernatant was 166% and Filtrate I 121% of that of the whole sludge (Figure 3.7 (b)). This suggests that when suspended solids were removed from the mixed liquor, the fouling process was accelerated. In reverse, it could be inferred that if sludge floes were added back to the membrane feed, the filtration resistance might be mitigated to 72 some extent. Therefore, the presence of large sludge floes appeared to be beneficial to the alleviation of membrane fouling. D) o 150 100 Whole sludge Supernatant Filtrate I Filtrate II Sludge fraction 250 £ 00 D) C "5 o Whole sludge Supernatant Filtrate I Sludge fraction Filtrate II Figure 3.7. The fouling rates of the individual M E B P R sludge fractions relative to the whole sludge fouling at the increased fluxes of (a) 33 L/m 2-h and (b) 68 L/m 2-h Analysis of the fouling rates of the four M E B P R sludge fractions at the different membrane fluxes provided further insight into the above phenomenon. Figure 3.8 (a) shows the absolute fouling rates of the four sludge fractions at fluxes of 23, 33 and 68 L/m 2-h, respectively. It was evident that for any sludge fraction, the absolute fouling rate increased with an increase of the permeate flux. However, when the fouling rates were divided by the actual flux (23, 33 or 68 L/m2-h), the resultant fouling rates at the unit flux of 1 L/m -h, or the normalized fouling rates, did not change with an increase in the permeate flux for the three sludge fractions of supernatant, Filtrate I and Filtration II 73 (Figure 3.8 (b)). This was consistent with the model developed in the previous study (Chapter 2), which indicated that the degree of irreversible fouling was a function of cumulative soluble organic loading on the membranes and was not directly related to operating flux. On the other hand, the normalized fouling rates for the large sludge floes exhibited a declining trend with increasing flux, as demonstrated in Figure 3.8 (b). This implied that the role of large sludge floes in the membrane filtration of sludge was considerably affected by the membrane operating conditions. As the flux increased, the relative contribution of large sludge floes to fouling decreased, or, the impact that large solids had on minimizing fouling increased. 0 20 40 60 80 100 0 20 40 60 80 100 Flux, L /m 2 .h. Flux, L /m 2 .h. —c— Whole sludge —o— Supernatant —a— Filtrate I —o— Filtrate II Figure 3.8. Fouling rates of the four fractions of the aerobic MEBPR sludge at different operating fluxes: (a) absolute fouling rates; (b) fouling rates normalized to 1 L/m 2.h. These experimental results support the research work of Lee et al. (2001), in which better filtration performance was found in a suspended growth M B R rather than in an attached growth M B R . Similar findings were also reported from a different standpoint by Defrance et al. (2000), who investigated the additivity of filtration resistance induced by individual sludge components and found that the sum of resistance due to each constituent at the same respective concentration as in the sludge was 50% higher than the measured total resistance. Therefore, the fouling caused by individual sludge constituents was antagonistic, rather than additive. 74 Steric Hindrance Effect of Large Sludge Floes The results presented in Figures 3.7 and 3.8 are understandable if the large sludge floes are considered as "moving barriers" that intercept fine particles and dissolved matter before they reach the membranes. A postulation about this steric hindrance effect of sludge floes on membrane filtration is proposed in Figure 3.9. In a quiescent environment, sludge floes tend to attach to or deposit on the membrane surface to form the cake layer. While in a flowing setting, large sludge floes are believed to be able to sterically hinder the movement of other sludge constituents when they move around and thus aggregate or adsorb fine particles and soluble organics onto their porous structures. This process is to some extent like a sweep flocculation. A number of mechanisms can cause particle aggregation, including Brownian motion, velocity gradients and turbulent diffusion. As the flocculation rate constant is proportional to velocity gradient and the second or third power of particle size (Letterman et al, 1999), the aggregation process is obviously favored when the velocity gradient becomes greater at an increased flux and large particles are present. This is probably why the steric hindrance effect of large sludge floes became more prominent at higher fluxes. Figure 3.9. Postulated mechanism for the steric hindrance effect of large sludge floes on membrane filtration Activated sludge Permeate Membrane 75 It could be imagined that at an increased flux, more particles and dissolved substances have chances to collide with large sludge floes and are subsequently entrapped within the floe structure. As a result, fewer particles and soluble organics are transported to the vicinity of the membranes to cause fouling. When filtering sludge supernatant, great numbers of fine particles and organic substances are readily transported to the membrane surfaces without any obstruction due to the absence of large sludge floes. Many of these small particles become adsorbed onto the membranes, causing a severe deterioration of membrane performance. Therefore, not all the sludge constituents result in membrane fouling at all times: large sludge floes may play a positive role in filtration of activated sludge especially at high flux, due to their steric hindrance effect. The experimental results obtained at both low and high fluxes suggested that sludge floes very likely have dual effects on membrane filtration. They could either act as membrane foulants causing membrane fouling via sludge deposition and cake formation, or serve as "moving barriers" entrapping soluble and colloidal substances and thus mitigating the fouling. The two attributes may coexist and change with the system design and hydrodynamic conditions. In some cases, one effect might outweigh the other. For example, in the suspended growth membrane system as reported by Lee et al. (2001) and in the present high-flux filtration tests as well, the positive effect of intercepting fine foulants surpassed the negative effect of sludge deposition. In other cases, such as the previous low-flux filtration tests, the two effects offset each other and most of time the positive effect was masked. As a result, the outstanding negative fouling effect was observed, as reported in many previous research papers (Wisniewski and Grasmick, 1998; Defiance et al., 2000). In any case, however, the contribution of the soluble fraction was significant particularly to the irreversible fouling. Considering the possible dual effects of large sludge floes on membrane filtration, the research findings mentioned earlier, which seemed to be inconsistent with one another in regard to the roles of various sludge fractions and/or components in membrane fouling, may not conflict in nature and could be unified under the steric hindrance model. 76 Characteristics of the Soluble Fraction of Sludge Although the roles of various sludge constituents in membrane filtration could change with the system design and operating conditions, Filtrate II or the soluble fraction that contained the soluble and fine colloidal substances was still the most important sludge fraction with respect to the irreversible membrane fouling. To further elucidate the mechanisms involved, activated sludge grab samples were collected from the three compartments of both the MEBPR and CEBPR processes during the pseudo-steady state operation in Run II, and the soluble fractions of these sludge samples were analyzed, in a comparative approach, in terms of the fouling-related physical and chemical properties such as total organic carbon (TOC), molecular weight (MW) distribution, and fine particle size distribution. Figure 3.10 shows the TOC levels in the soluble fractions of sludge as well as in the influent, CEBPR effluent and MEBPR permeate. As in most activated sludge systems, the soluble TOC content in the CEBPR process exhibited a consistent decline from the influent, through the anaerobic, anoxic, and aerobic zones sequentially, to the final effluent. In contrast, there was a measurable increase in the TOC content of the soluble fraction of sludge in the aerobic zone of the M E B P R process. The dissolved TOC level in • MEBPR sludge • CEBPR sludge Influent Anaerobic Anoxic Aerobic Perm./Eff. Source of soluble fraction Figure 3.10. TOC contents of the influent, CEBPR effluent, M E B P R permeate and the soluble fractions of the sludge collected from the three compartments of both the MEBPR process and the CEBPR process in May 2004 77 the aerobic zone was significantly higher than that of the permeate and of the preceding anoxic zone as well. This was likely because of the continuing accumulation of soluble microbial products and macromolecules within the aerobic zone due to the membrane barriers (Huang et al, 2000; Lu et al, 2002). Table 3.2 shows the membrane retaining efficiencies of the dissolved TOC in the M E B P R process. About 42-76 % of the soluble organic carbon compounds was retained in the system by the membranes. These retained substances likely aggravated fouling and affected the filtration process. It has been reported that permeate flux was inversely proportional to the log values of the differential TOC between membrane feed and permeate (Ishiguro et al, 1994). Therefore, the bigger the difference in the TOC levels between the two sides of membranes (i.e. the mixed liquor to be filtered and the permeate), the larger the filtration resistance. The enrichment of dissolved organic carbon in the aerobic zone of the MEBPR process may have been an important factor that led to the higher fouling propensity of the MEBPR sludge than that of the CEBPR sludge, as shown in Figure 3.6. Table 3.2. Dissolved TOC retained in the M E B P R system Experimental run Sampling date T O C in the soluble fraction of the aerobic M E B P R sludge, mg/L T O C in the permeate, mg/L T O C retained, % 18/08/2003 19.0 11.0 42 Run 1 12/11/2003 12.5 4.0 68 Run II 29/03/2004 31/05/2004 18.3 12.0 4.4 4.6 76 62 The molecular weight (MW) distributions of the soluble fraction of sludge and the permeate were measured using gel permeation chromatography (GPC) and the results are presented in Figure 3.11 and Table 3.3. Only the substances that could be dissolved in the eluent tetrahydrofuran (THF) were mapped in this GPC analysis. As large molecules were eluted first, followed by small molecules, it can be concluded from Figure 3.11 that both the aerobic M E B P R mixed liquor and the corresponding permeate contained a large quantity of macromolecules (-40,000 Da) as well as a lot of small molecules (-300 Da). Thus, the widths of their M W distributions were very broad, as indicated by their high polydispersity (Table 3.3). In contrast, the soluble fraction of the aerobic CEBPR mixed 78 liquor mainly consisted of small molecules (-300 Da), and few large molecules were measured, leading to a monodispersed M W distribution (Table 3.3). Since large molecules contributed more TOC than small molecules, these results were basically consistent with the preceding TOC analysis, which showed a higher TOC content in the soluble fraction of the aerobic MEBPR mixed liquor than in the counterpart CEBPR mixed liquor as a result of the accumulation of soluble microbial products and macromolecules due to the membrane barriers. The research of Shin and Kang (2003b) indicated that the compounds accumulated in an M B R comprised large, aromatic, hydrophobic and double-bond-rich organics that originated from decayed biomass. Retention time, min Figure 3.11. Molecular weight distribution of the THF-dissolved substances in the MEBPR permeate and the soluble fractions of the sludge collected from the aerobic zones of the M E B P R and CEBPR processes Table 3.3. Molecular weight distribution of the soluble fractions of sludge and permeate Molecular weight distribution MEBPR sludge MEBPR Permeate CEBPR sludge Number average molecular weight (Mn), g/mol 3,739 8,918 326 Weight average molecular weight (Mw), g/mol 47,411 46,264 327 Polydispersity (D = Mw / Mn) 12.68 5.19 1.00 Figure 3.12 illustrates the fine particle size distributions of the soluble fractions of the activated sludge mixed liquor in the aerobic zones of the two processes. It was not 79 surprising that, due to the vigorous aeration normally employed in an immersed M B R , the MEBPR mixed liquor contained much more small supra-dissolved particles (0.1-0.45 |i,m), as defined in Poele et al. (2004), than did the CEBPR mixed liquor. Since the closer the diameters of the particles to the membrane pore size, the greater the effect of these particles on the membrane filtration process (Choo and Lee, 1996), the distributions shown in Figure 3.12 explain the lower filterability of the M E B P R mixed liquor than the CEBPR mixed liquor in terms of the fouling mechanism of pore clogging. Particle Size (um) Figure 3.12. Fine particle size distributions of the soluble fractions of sludge collected from the aerobic zones of the MEBPR process and the CEBPR process, (a) number based particle size distribution, (b) volume based particle size distribution. These results indicate that the soluble fraction of the aerobic M E B P R sludge was mainly characterized by high levels of soluble organic carbon compounds that were rich in both macromolecules and fine dissolved particles, which tended to cause irreversible fouling likely via organic adsorption and deep pore clogging. 80 Conclusions Due to its high content of organic carbon compounds, macromolecules and small supra-dissolved particles that were most likely to adsorb onto and/or clog in the membrane structure, the soluble fraction (<0.45 Lim) of activated sludge mixed liquor ranked first in terms of its contribution to the total sludge fouling, followed by the colloidal fraction (0.45-8.0 Lim) and the unsettleable microfloc fraction (>8.0 Lim). Large sludge floes might exert dual effects on membrane filtration. At low permeate flux, they tended to induce hydraulic resistance probably via sludge deposition or cake layer formation, though this effect was relatively small and even negligible under strong aeration conditions. At high flux, however, large sludge floes seemed to be able to mitigate fouling, because the membranes were fouled more severely when filtered with the sludge supernatant than the whole sludge. A "steric hindrance effect" mechanism was postulated, which explained that large sludge floes could intercept soluble and colloidal substances in their porous structures before they reached the membrane surfaces. This positive effect of large sludge floes on membrane fouling outweighed the negative effect of sludge deposition under high permeate flux and strong aeration conditions. The finding of fouling alleviation by large sludge floes further revealed the complexity of membrane fouling processes in activated sludge membrane bioreactors. This undoubtedly helps to improve the understanding of membrane fouling and the roles of various sludge constituents in membrane filtration of sludge. References APHA, A W W A and WEF. (1995). Standard Methods for the Examination of Water and Wastewater. (19th) American Public Health Association (APHA), American Water Works Association (AWWA), Water Environment Federation (WEF), Washington, D.C. Choo, K . -H . and Lee, C.-H. (1996). Effect of anaerobic digestion broth composition on . membrane permeability. Water Science and Technology, 34(9), 173-179. Defrance, L. , Jaffrin, M . Y. , Gupta, J3., Paullier, P. and Geaugey, V . (2000). Contribution of various constituents of activated sludge to membrane bioreactor fouling. Bioresource Technology, 73(2), 105-112. Huang, X . , Liu, R. and Qian, Y . (2000). Behaviour of soluble microbial products in a membrane bioreactor. Process Biochemistry, 36(5), 401-406. 81 Ishiguro, K., Imai, K. and Sawada, S. (1994). Effects of biological treatment conditions on permeate flux of UF membrane in a membrane/activated-sludge wastewater treatment system. Desalination, 98(1-3), 119-126. Itonaga, T., Kimura, K. and Watanabe, Y . (2004). Influence of suspension viscosity and colloidal particles on permeability of membrane used in membrane bioreactor (MBR). Water Science and Technology, 50(12), 301-309. Judd, S. (2005). Fouling control in submerged membrane bioreactors. Water Science and Technology, 51(6-7), 27-34. Lee, J., Ann, W.-Y. and Lee, C.-H. (2001). Comparison of the filtration characteristics between attached and suspended growth microorganisms in submerged membrane bioreactor. Water Research, 35(10), 2435-2445. Letterman, R. D., Amirtharajah, A. and O'Melia, C. R. (1999). Coagulation and flocculation. Water Quality and Treatment, Letterman, R. D. (ed). McGraw-Hill, Inc., 6.44 - 6.48. Lu, S. G., Imai, T., Ukita, M . , Sekine, M . and Higuchi, T. (2002). Modeling prediction of membrane bioreactor process with the concept of soluble microbial product. Water Science and Technology, 46(11-12), 63-69. Nagaoka, H. , Yamanishi, S. and Miya, A . (1998). Modeling of biofouling by extracellular polymers in a membrane separation activated sludge system. Water Science and Technology, 38(4-5), 497-504. Poele, S. T., Roorda, J. H. and van der Graaf, J. (2004). Influence of the size of membrane foulants on the filterability of WWTP-effluent. Water Science and Technology, 50(12), 111-118. Rosenberger, S., Laabs, C , Lesjean, B. , Gnirss, R., Amy, G., Jekel, M . and Schrotter, J.-C. (2006). Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment. Water Research, 40(4), 710-720. Shin, H. and Kang, S. (2003a). Performance and membrane fouling in a pilot scale SBR process coupled with membrane. Water Science and Technology, 47(1), 139-144. Shin, H.-S. and Kang, S.-T. (2003b). Characteristics and fates of soluble microbial products in ceramic membrane bioreactor at various sludge retention times. Water Research, 37(1), 121-127. Stephenson, T., Judd, S., Jefferson, B. and Brindle, K. (2000). Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London. Wisniewski, C. and Grasmick, A. (1998). Floe size distribution in a membrane bioreactor and consequences for membrane fouling. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 138(2-3), 403-411. 82 Chapter 4. Characterization of fouled membranes from a membrane enhanced biological phosphorus removal system Introduction As environmental standards become more and more stringent and the cost for membrane manufacture is progressively decreased, application of membrane technologies in water and wastewater treatment is gaining more and more popularity (Mallevialle et al, 1996; Stephenson et al, 2000). As with other membrane applications, however, membrane fouling has been an inherent problem that has restricted the environmental application of membrane technologies. In general, fouling is a phenomenon that is expressed as a decline of membrane performance such as an increase in trans-membrane pressure (TMP) or a decrease in permeate flux due to accumulation of substances on the membrane surface and/or within the membrane pores (Hong et al, 2002). Undoubtedly, understanding the characteristics of foulants and fouled membranes is crucial for exploring the nature of membrane fouling and formulating effective strategies for fouling prevention and control. Previous investigations of foulants and fouled membranes have focused mainly on those used for drinking water treatment purposes. A number of studies revealed that natural organic matter (NOM) in raw water was a major component of membrane foulants in drinking water treatment processes (Mallevialle et al, 1989; Laine et al, 2003). In addition, biofouling played an important role in deterioration of membrane performance, in which microorganisms tended to adhere to membrane surfaces, and then to biosynthesize extracellular polymeric substances (EPS) to form a gel layer (biofilm) that eventually led to an increase in filtration resistance (Ridgway and Flemming, 1996; Flemming et al, 1997). In membrane-coupled activated sludge wastewater treatment processes, biofouling in the form of microbial growth is believed to be the dominant mechanism of membrane fouling, because both the abundant nutrients in wastewater and the large number of microorganisms in sludge suspensions particularly favour biofouling. Different microbial community structures of membrane biofilms were actually found in * A version of this chapter has been presented at 2006 IWA World Water Congress and will be published in Water Science and Technology. Geng, Z. and Hall, E.R. Characterization of fouled membranes from a membrane enhanced biological phosphorus removal (MEBPR) system. 83 intermittently and continuously aerated membrane bioreactors (Lim et al, 2004). However, in anaerobic membrane bioreactors treating magnesium-containing wastewater, inorganic precipitates (i.e. struvite) were reported to be one of the most significant foulants (Choo et al, 2000). It is thus known from the above literature that the nature of membrane fouling largely depends on the water treated and the membrane process used. For the same source of wastewater, different membrane treatment processes may result in totally different types of fouling. As a new application of membrane technology in wastewater treatment, the membrane enhanced biological phosphorus removal (MEBPR) process, which encompasses an anaerobic zone, an anoxic zone and an aerobic zone in series, is receiving increasing attention. However, the studies that have been initiated on the MEBPR-type processes have focused mostly on the evaluation of the treatment performance and the effect of membrane operation on biological phosphorus (Bio-P) removal (Adam et al, 2003; Lesjean et al, 2003). Only few studies have been reported on the specific features of fouling in this type of membrane application (Rosenberger et al, 2006). To date, it is not known whether microbial growth is significant on the membrane surfaces in the MEBPR process, just as in many other wastewater treatment membrane bioreactors. It is also unclear what material tends to accumulate on the membrane surfaces. Most importantly, since an anaerobic environment tends to preserve ammonia and phosphate, it is anticipated that inorganic fouling such as the formation of struvite (MgNH 4P04-6H 20) might coexist with biofouling due to the possible accumulative effect of the anaerobic conditions in the MEBPR process. To address these research issues, membrane fouling in a pilot scale M E B P R system was investigated in this study with respect to the characteristics of foulants and fouled membranes. The objective of the study was to characterize fouled membranes, determine the extent of inorganic fouling, identify and quantify the major fouling-causing substances, and gain insight into the nature of membrane fouling in the MEBPR-type processes. 84 Methods The MEBPR Process Figure 4.1 is a schematic of the pilot scale membrane enhanced biological phosphorus removal process utilized in the present study. The influent of the system was from a municipal wastewater source and its characteristics are described in Table 4.1. The Permeate Underflow •+ Primary clarifier Waste sludge Membrane Bioreactor Figure 4.1. The pilot scale membrane enhanced biological phosphorus removal (MEBPR) system. A - anaerobic compartment; B - anoxic compartment; C - aerobic compartment Table 4.1. Characteristics of the influent to the M E B P R system (all values in mg/L except where indicated) Parameter Average Std. Dev. Parameter Average Std. Dev. T S S 110 50 Boron (B) 0.16 0.04 C O D (total) 330 100 Calcium (Ca) 4.8 0.3 C O D (0.1 urn filtered) 100 90 Copper (Cu) 0.014 0.005 VFA (total) 20 9 Iron (Fe) 0.24 0.07 P-total 4.3 1.1 Magnesium (Mg) 1.3 0.2 P 0 4 - P 2.7 0.9 Manganese (Mn) 0.017 0.002 TKN 34 8 Potassium (K) 8.2 0.7 NH4-N 26 5 Silicon (Si) 2.5 0.1 N O 3 -N 0.057 0.087 Sodium (Na) 79 42 Hardness (total C a C 0 3 ) 17 2 Strontium (Sr) 0.018 0.002 Temperature (°C) 21 2 Sulphur (S) 20 3 PH 7.3 0.3 Zinc (Zn) 0.012 0.003 Note: a). The parameters listed in the left column (except for hardness) were the averages over the period February 2003 - June 2004. The number of samples (n) was between 120 and 320. b). The concentrations of the metals listed in the right column (including hardness) were based on the results of an ICP scan of the influent performed during May 2-9, 2003. The number of samples (n) was 7. Those measurements that were below their method detection limits were not presented in the table. 85 three compartments of the membrane bioreactor, i.e. the anaerobic (A), anoxic (B), and aerobic (C) compartments, operated with liquid volumes of 0.23, 0.59, and 1.31 m 3 , respectively. The membrane module was submerged in the aerobic compartment and the permeate was collected via a vacuum pump. Air was supplied in this compartment in a cyclic mode of 10 seconds ON and 10 seconds OFF with an aeration rate of 20-26 m /h. Sludge was recycled between the neighbouring compartments at the ratio of 1:1. The MEBPR experimental program was designed with two consecutive runs during the period December 2002 - June 2004. In Run I (December 2002-December 2003), the sludge retention time (SRT) was controlled at 12 days and the hydraulic retention time (HRT) was 10 hours. In Run II (December 2003- June 2004), the SRT was unchanged but the HRT was decreased to 7 hours. Overall, the M E B P R process worked well in both experimental runs. The average concentration of total COD in the effluent was below 30 mg/L, total phosphorus below 1.0 mg/L, and ammonia nitrogen less than 0.2 mg/L. Details about the operation and treatment performance of the M E B P R process are presented in Chapter 2. Membrane Operation A custom-built ZeeWeed membrane filtration module (Zenon Environmental Inc., Oakville, Ontario, Canada) with a surface area of 11.9 m 2 was used in the M E B P R system (see Appendix 2 for the photo of the module). The module consisted of hollow fibres with a nominal membrane pore size of 0.04 |im. The hollow fibres encompassed two parts: an outer membrane skin and an inner fabric support. The former was made of polyvinylidene fluoride (PVDF) and its surface was neutral and hydrophilic. The membrane module was operated in a cyclic mode of 9 minutes and 30 seconds of suction followed by a 30 second backflushing with permeate. The flux was constant at each experimental run: 23 L/m h in Run I and 33 L/m 2-h in Run II. Membrane Sampling A l l fouled membrane fibres analyzed in the present study were sampled from the membrane module of the M E B P R process at the ends of filtration runs at which times the biological process was operating under pseudo-steady state conditions and the maximum 86 allowable transmembrane pressure (10 psi. or 69 kPa) had been reached. Such collected membranes were thus defined as "completely fouled membranes". They were cut off with scissors at random sites in the fouled membrane module right after the module was removed from the bioreactor. The original fibres were resealed with Epoxy glue and left for at least one hour to let the glue harden before the module was chemically cleaned. The collected membrane fibres were immediately transported to an environmental engineering lab for further study, including the microscopic examination of membrane fibres, conventional chemical analysis and advanced instrumental analysis of the foulant extract. The overall analytical scheme is depicted in Figure 4.2. Foulant layer Membrane skin. Inner support Conventional Chemical analysis M A L D I - MS C O M S E M E D X Figure 4.2. Overall scheme for characterization of fouled membranes and foulant extract Analytical Methods The completely fouled membrane fibre samples were first rinsed with deionised distilled water (DDW) to remove scum. Then the outer membrane skin was peeled off from the inner fabric support, and both the skin and the support were cut into small pieces (~5 mm) with a sterile scalpel blade for the following analyses. Virgin membranes were used as a control and were first soaked in a 200 mg/L chlorine (OCT) solution overnight and then filtered with DDW to remove preservatives prior to analysis. As the fouled membranes had experienced frequent backflushing and high shear stress due to the strong aeration applied, the fouling studied here was considered to be hydraulically irreversible (see Chapter 2). 87 Microscopic study The samples for scanning electronic microscopy (SEM) and energy dispersive X -ray (EDX) microanalysis were prepared according to a microwave S E M processing protocol. (1) The sliced outer skins and inner support of both virgin and completely fouled membrane fibres were fixed in 2,5% glutaraldehyde solution at 22 °C in a Pelco laboratory microwave in the order of 1 minute OFF, 1 minute 40 second ON at power 2 (-200 W) and 3 minute OFF for vacuum hold. (2) The fixed samples were rinsed with 0.1 M sodium cacodylate buffer at 22 °C in the order of 40 second ON at power 1 (-100 W), changing buffer outside the microwave and 40 second ON at power 1 again. (3) The buffer-rinsed samples were post-fixed in 1% osmium tetroxide at 22 °C in the order of 1 minute OFF, 1 minute 40 second ON at power 1, and 3 minute OFF for vacuum hold. (4) The post-fixed samples were rinsed with distilled water at room temperature. (5) The rinsed samples were dehydrated sequentially in 50%, 70%, 85%, 95%, and 100% ethanol solutions at 22 °C and each for 40 seconds at power 1. (6) The dehydrated samples were then dried with hexamethyldisilazane (HMDS) and coated with gold (Nanotech sputter coater) for final treatment. After fixation, dehydration, drying and gold coating, the slices of outer membrane skin and inner fabric support were placed under a Hitachi S4700 S E M for high resolution imaging. Step (3) was omitted when x-ray microanalysis was performed. Contact angles of fouled and virgin membranes were measured using the modified sessile drop method (Cho et al., 1998) to study the effect of fouling on the hydrophobicity of membrane surface. The membrane fibre samples were stored in a sealed Ziploc® freezer bag to avoid drying out. When analyzed, a 1.0 j iL aliquot of DDW was dropped onto the membrane fibre surface using a 10-piL glass syringe and the angle between the water droplet and the membrane surface was immediately imaged under a conventional optical microscope (COM) (Motic B3 Professional Series) and measured through a computer that was connected to the C O M . Extraction of membrane foulants Basic extraction: samples of sliced membrane skin and inner support, of a total length of 1.8-2.0 m, were soaked in 9.0 mL of 0.1 M NaOH solution. After sonication 88 (Aquasonic Model 550HT, VWR) at 40 °C for 1 hour, the solution was neutralized with 3.0 mL of 0.15 M H2SO4. The resultant liquid was then collected as the membrane foulant extract. Acidic extraction: the extraction procedures were the same as above, except that 9.0 mL of 0.5 M H 2 S 0 4 and 3.0 mL of 3.0 M NaOH were used as the acidic extraction solution and the neutralizer, respectively. Conventional chemical analysis The extracted membrane foulants were analyzed in terms of such gross parameters as total Kjeldahl nitrogen (TKN), total phosphorus (TP), and total organic carbon (TOC) as described in Standard Methods (APHA et al, 1995). Extracellular polymeric substances (EPS) were also measured on the foulant extract, of which carbohydrates were determined according to Fr0lund et al. (1996), proteins and humic or humic-like substances were quantified using the modified Lowry method (Jahn and Nielsen, 1995). MALDI-MS analysis To gain more information about the membrane foulants, the extracts from both the outer membrane skin and the inner fabric support were further subjected to matrix assisted laser desorption/ionization - mass spectrometry (MALDI-MS). As the name implies, this is basically an MS technique with the assistance of the innovative separation technique of M A L D I . Selection of M A L D I matrix is critical, which must be able to embed and isolate analyte, soluble in solvents compatible with analyte, vacuum stable, absorb laser wavelength, cause co-desorption of the analyte upon laser irradiation and promote analyte ionization. By using a suitable matrix, M A L D I can uniformly disperse analyte molecules, enhance energy transfer from laser pulse to analyte, and thus facilitate the analyte desorption and ionization processes. As a result, M A L D I makes the MS analysis of macromolecules and polymers easier and more sensitive. In the present study, the foulant extract samples (-0.5 mL each) were submitted to the U B C chemistry lab for M A L D I - M S analysis. Bruker Biflex IV, a time-of-flight mass spectrometer equipped with a M A L D I ion source, was used to analyze the samples, and 2,5-dihydroxybenzoic acid (DHB) and a-cyano-4-hydroxycinnamic acid (CHCA) were chosen as the matrices, respectively. 89 Results and Discussion Surface Morphology of Fouled Membrane Fibres Direct exarnination of fouled membranes using a microscopic technique is usually the most straightforward method for exploring the nature of fouling. Figures 4.3 (a)-(h) are S E M images of the outer polymeric skin of virgin and completely fouled membrane surfaces at different magnifications. It is evident from Figures 4.3 (a) and (b) that virgin membranes had a relatively homogeneous, cross-linked and porous structure. In contrast, completely fouled membrane surfaces were quite heterogeneous. As expected, microorganisms were observed on the fouled membranes (Figures 4.3 (c)-(e)). However, unlike some literature reports (Ridgway and Flemming, 1996; Liao et al, 2004), the microbial colonies were scattered sparsely and were not dominant over the fouled membrane skins. Among the many microscopic observations made in the present study, only a very few indicated microbial growth. The majority of the membrane area was covered with a layer of material (Figure 4.3 (f)-(h)), which had a porous structure that was quite similar in appearance to that of the virgin membrane surface (Figures 4.3 (a)-(b)). This porous foulant layer could also be seen surrounding microbial colonies as illustrated in Figures 4.3 (d) and (e). The observed void space of this foulant layer seemed to be slightly less than that of the membrane substratum. This was probably because, as filtration proceeded, some of pores were either clogged with fine colloids or blocked with adsorbed foulants, resulting in a decrease in the total filtration area. The adjacent foulant thus accumulated, expanded and eventually covered the membrane surface, forming a porous layer of foulants under the repeated suction and backflushing. On the other hand, the vigorous aeration in the aerobic zone of the M E B P R process and the strong hydrodynamic shear conditions within the membrane module apparently made microbial attachment so difficult that only a few microorganisms could adhere to the membrane surfaces. Once they survived, they proliferated and developed into microbial colonies under the shelter of the foulant layer. Figure 4.4 (a)-(b) are the S E M images of the permeate-facing side of the inner fabric support of new and fouled membrane fibres, respectively. Like the outer membrane skin, the inner fabric support was also fouled after a period of filtration, as a result of 90 Figure 4.3. S E M images of virgin membrane surfaces (a-b) and completely fouled membrane surfaces (c-h) at different magnifications: c. 10 K; a, d, e, and g: 25 K; b, f, and h: 100 K 91 frequent backflushing with permeate that contained soluble organic substances. X-ray microanalyses by S E M - E D X showed that the foulants in the membrane structure were mainly organic carbon compounds (Figure 4.5 (a)), except that the crystal-like particles arrowed in Figure 4.4 (b) also included inorganic components such as the oxides of aluminium and manganese (Figure 4.5 (b)), indicating a possible contamination of the permeate with dust due to its exposure to the open air. Additionally, more quantitative information about the elemental composition of fouled and virgin membranes is given in Table 4.2. Compared to the virgin PVDF Figure 4.4. S E M images of (a) clean and (b) fouled inner fabric support of membrane fibre S keV (a) (b) Figure 4.5. X-ray microanalyses of (a) the foulant layer on membrane skin and (b) the crystal-like particles found on the fouled inner fabric support 92 Table 4.2. Elemental composition of virgin and completely fouled membrane skins Concentration, wt% Completely fouled membrane skin A (EDX analyzed) Completely fouled membrane skin B (EDX analyzed) Virgin membrane skin (Calculated) Carbon 63.89 67.30 38.71 Oxygen 6.38 4.84 -Fluorine 29.73 27.86 58.06 Hydrogen - - 3.23 Note: a). The completely fouled membrane skins A and B were collected on July 8, 2003 and April 27, 2004, respectively. B). The elemental composition of virgin membrane (PVDF) was calculated according to its molecular formula [ - C H 2 C F 2 - ] „. c). - Not detected or not contained. membrane, the surfaces of the two completely fouled membrane skins, which were sampled at different times, were found to contain more carbon and oxygen and less fluorine (Table 4.2). Since the fluorine detected by E D X very likely originated from the membrane material itself (PVDF), due to the penetration of x-rays through the foulant layer to the membrane substratum, it was deduced that carbon was the backbone element of the membrane foulants. On the other hand, inorganic deposits and precipitates such as silicates and struvite (MgNFUPO^FEO), which have been reported as important foulants in some membrane processes (Speth et al, 1998; Choo et al, 2000; Luo and Wang, 2001), were not detected on the fouled membrane surfaces in the present M E B P R study. Although the influent characteristics (Table 4.1) and the presence of the anaerobic environment in the MEBPR process suggested the possibility of struvite formation, the strong oxidation conditions and the Bio-P uptake mechanism in the M E B P R aerobic compartment made the PO4-P and NH4 -N concentrations as low as about 0.54 mg/L and 0.47 mg/L, respectively, in the membrane filtration zone. These concentrations were far below the solubility product of struvite and thus did not favour its precipitation (Doyle and Parsons, 2002). Composition of Membrane Foulants The study of the composition of membrane foulants was furthered by conducting conventional chemical analysis on the foulant extract, and the results are presented in Table 4.3. It is evident that more foulants were extracted using the basic extraction method than with the acidic extraction method. For both the alkaline and acidic extractions, the amount of foulants extracted from the completely fouled membranes of Run II (March 7 -93 Table 4.3. Chemical analysis of foulants extracted from completely fouled membranes Extraction Runtime Type of sample Arrount of extracted foulants, uc/m method T K N T P T O C Carbohydrates Proteins Hunic-l ike substances T O C est. 85days outerskin 28(47-17) - 570(47-10) 1160(-t/-20) - 410(47-10) 670 (Jul. 9 - O c t . 2,2003) ' Run I : wholefibre 60(47-9) 7(47-4) 670(47-10) 1680(47-40) - 560(47-40) 950 Bas ic (outer skin + imer support) extraction 52 days outerskin 78 (47- 3) 2 3 (47- 8) 950(47-20) 1600 (-t/-130) - 760(47-50) 1020 (Mar. 7 - Apr. 27,2004) Runl imer support 31(47-2) 18 (-+7- 2) 720(47-80) 480(-t7-50) - 460 (+7-40) 420 85days outerskin 34(47-5) - 150(47-17) 860(47-160) - 340(47-10) 510 ( J J . 9 - O c t . 2,2003) Run l wholefibre 60(47-10) - 160(47-10) 1040(47-90) - 700(47-120) 760 Ac id ic (outer ski n 4- i m e r support) extraction 5 2 d a y s outerskin 22(47-1) 4(47-3) 470(47-10) 1180(47-140) - 420(47-50) 680 (Mar. 7 - Apr. 27,2004) Run II ~ ' imer support 29(47-1) 3(47-1) 250(47-10) 270(47-150) - 230(47-10) 220 a. "Whole fibre" means that the outer skin of membrane fibre was not peeled off and the fibre (both the outer skin and the inner support) was processed as a whole. b. " - " means that the measurement result was below the method detection limit (MDL). The MDL of total P and protein was 3 and 90 ug/m, respectively. c. The values in parentheses indicate standard deviation. d. 'TOC est." is the TOC estimated from the measured carbohydrates and humic-like substances using the formula of TOC est. = amount of carbohydrates x 40% + amount of humic-like substances x 50%. Glucose is used as the standard for carbohydrates, of which 40% is elemental carbon. Humic acids and fulvic acids are used to represent the humic-like substances, which contain 52-62 % and 44-49 % of organic carbon, respectively. An estimate of 50% is selected as the content of carbon in humic-like substances. April 27, 2004) was slightly more than the amount of foulants extracted from the completely fouled membranes of Run I (July 9 - October 2, 2003). Considering the possible variation associated with membrane sampling, storage, foulant extraction and the laboratory analysis errors, the amounts of foulants extracted from the completely fouled membranes of the two experimental runs were concluded to be approximately the same. This implies that the process design and operating conditions imposed here did not significantly affect the total amount of foulants that caused the TMP to reach the operating limit, or, which led to a complete fouling. Obviously, this was consistent with the finding of the earlier study on fouling mechanisms that the degree of fouling mainly depended on the cumulative organic loading on the membrane surface (Chapter 2). One meter of completely fouled membrane fibre skin carried about 1100-1800 pig of carbohydrates (or 2 2 220-360 mg/m ) and 400-800 pig of humic or humic-like substances (or 80-160 mg/m ) that were extractable in a basic solution. A quick estimation of TOC on the basis of measured carbohydrates and humic-like substances revealed that for both pilot plant runs, carbohydrates and humic or humic-like substances, which are the primary components of EPS, were the major extractable membrane foulants, since the estimated TOC was fairly comparable to the measured TOC (Table 4.3). On the other hand, nitrogen-containing compounds and phosphorus-containing compounds were present in minor amounts, and in particular, no measurable amount of protein was detected in the foulant extract. Evidently, these results are very consistent with the previous S E M examination and x-ray microanalysis. With regard to the carbohydrates, Kraemer (2002) visualized some specific carbohydrates like glucose, mannose, N-acetylglucosamine, and galactose on fouled membrane surfaces using confocal laser scanning microscopy. MALDI-MS Analysis The characterization of membrane foulants was supplemented by matrix assisted laser desorption/ionization - mass spectrometry (MALDI-MS), which is an effective tool for mass analysis of peptides and proteins, carbohydrates, oligonucleotides, and polymers (Fenselau, 1997). An example of the M A L D I mass spectra obtained in the present study is shown in Figure 4.6, in which the abundance of analyte ions (a.i.) is plotted against mass-95 to-charge ratio (m/z). The constant mass intervals between the neighbouring major peaks clearly indicated a polymeric substance in the foulant extract. Since most of the peaks appeared in the low m/z range (500-2000) and no peaks with m/z exceeding 3000 were detected, the foulants assessed by M A L D I - M S were primarily polymers with small molecular weights (MW). These might be the oligosaccharides or low M W humic or humic-like substances as measured in the conventional chemical analysis (Table 4.3). Given that the membranes used in this study had a nominal molecular weight cut off (MWCO) of about 100,000 daltons, these small M W foulants were retained on the membranes not because of their molecular sizes, but as a result of other fouling mechanisms, such as organic adsorption, as described in Chapter 2. a.i 3 0 0 0 m / z Figure 4.6. The M A L D I mass spectra of the foulants extracted from the inner support of completely fouled membrane fibres that were sampled on April 27, 2004 (DHB as the matrix) In addition, the M A L D I mass spectral data of the foulants extracted from the outer skins of completely fouled membrane fibres in the M E B P R process were basically identical to those for the foulants extracted from the inner support as illustrated in Figure 4.6, suggesting that the substances leading to the fouling of both the inside and outside of 96 membrane fibres were the same or similar (Appendix 16). Since the foulants retained within the inner support could only come from permeate, this implies that the soluble or filterable substances were able to cause fouling, probably via organic adsorption after passage through the membranes. It should be pointed out that no protein was detected in the MALDI-MS analysis, which was in agreement with the chemical analysis results presented in Table 4.3. However, the extraction efficiency for organic nitrogen was only about 5-10% in this study, which was estimated by comparing the total Kjeldahl nitrogen (TKN) of the foulant extract with that of the whole fouled membrane fibre (Appendix 17). It must be assumed that the extraction efficiencies for other foulants may also have been modest. In this regard, the M A L D I - M S technique may have been restricted by the low concentration of the foulant extract, and hence it may have been unable to reveal other important information on the membrane foulants. Effect of Fouling on Membrane Hydrophobicity The contact angle between a membrane surface and a water droplet on the surface is indicative of the hydrophobicity of the membrane surface material. The larger the angle, the more hydrophobic the surface is assumed to be (Cho et al, 1998). In the present study, measurements of contact angle were performed on both virgin membranes and the completely fouled membranes sampled at the end of the filtration run from March 7 to April 27, 2004, and the results are summarized in Table 4.4 (see Appendix 18 for images). The statistical analysis of these results indicated that the fouled membranes were less hydrophilic than the virgin membrane fibre at a significance level of a=0.05, as shown by Table 4.4. Comparison of contact angles between virgin and completely fouled membrane Type of membrane Number of measurements Average contact angle Standard deviation Virgin membrane 8 51.5 5.6 Completely fouled membrane (1) 8 61.1 4.2 Completely fouled membrane (2) 8 59.8 4.9 Note: The two completely fouled membrane fibres (1) and (2) were sampled from different sites of the membrane module in the MEBPR process on April 27, 2004. 97 their increased contact angles. From the previous foulant analysis results (Table 4.3), it is known that the extractable membrane foulants consisted primarily of carbohydrates and humic or humic- like substances. Since the former tends to be hydrophilic and the latter is essentially hydrophobic, the hydrophobicity of fouled membranes is thus determined by the combined effect of the two major constituents. As a result, the surface of the fouled membrane fibres in the MEBPR process appeared to be less hydrophilic than that of the virgin fibres after over one month of sludge filtration. This probably in turn accelerated the fouling process by facilitating the hydrophobic interactions between the fouled membrane surfaces and hydrophobic substances in the M E B P R mixed liquor and caused an exponential rise in TMP as filtration runs progressed, as demonstrated in Figure 4.7. CO CL • Suction • Backflush Run time, days Figure 4.7. The trans-membrane pressure profile of the M E B P R process for the entire filtration run from March 7 to April 27, 2004 Conclusions Different from the observations reported for many other wastewater treatment membrane bioreactors, microbial growth on membrane surfaces was minimal in the M E B P R process studied here, and was most likely limited by the vigorous aeration applied around the membranes. Instead, the fouling in the M E B P R system was predominantly expressed as accumulation of a porous layer of substances on the membrane surfaces and in the membrane pore structures. In addition, a microscopic 98 examination and chemical analysis of fouled membranes further revealed the following features of the fouling. (1) Membrane fouling in the M E B P R process was mainly of an organic nature. Precipitation of struvite was not found in the fouled membrane structure, probably because of the strong oxidation conditions and the Bio-P uptake mechanism in the M E B P R aerobic zone. Inorganic deposition on the inner fabric support was observed occasionally due to the possible contamination of permeate with dust. (2) Carbohydrates and humic or humic-like substances, the key EPS components, were the major extractable foulants. A change in the operating conditions, such as a reduction in HRT, did not have significant effect on the amount of foulants that led to the maximum trans-membrane pressure of the membrane module. One square meter of completely fouled membrane skin carried about 220-360 mg of carbohydrates and 80-160 mg of humic or humic-like substances that could be extracted in a basic solution. (3) Filtration of activated sludge increased the hydrophobicity of membrane surfaces. This may have facilitated the hydrophobic interactions between the foulant layer and mixed liquor constituents that may then have accelerated the fouling process, resulting in an exponential fouling curve. (4) Filterable polymers with small M W likely caused membrane fouling via organic adsorption. References Adam, C , Kraume, M . , Gnirss, R. and Lesjean, B. (2003). Membrane bioreactor configurations for enhanced biological phosphorus removal. Water Science and Technology: Water Supply, 3(5-6), 237-244. A P H A , A W W A and WEF. (1995). Standard Methods for the Examination of Water and Wastewater. (19th) American Public Health Association (APHA), American Water Works Association (AWWA), Water Environment Federation (WEF), Washington, D.C. Cho, J., Amy, G., Pellegrino, J. and Yoon, Y . (1998). Characterization of clean and natural organic matter (NOM) fouled NF and UF membranes, and foulants characterization. Desalination, 118(1-3), 101-108. 99 Choo, K . -H . , Kang, I.-J., Yoon, S.-H., Park, H., Kim, J.-H., Adiya, S. and Lee, C.-H. (2000). Approaches to membrane fouling control in anaerobic membrane bioreactors. Water Science and Technology, 41(10), 363-371. Doyle, J. 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Characterization of microbial aggregates in relation to membrane biofouling in submerged membrane bioreactors, MASc thesis, Ryerson University, Toronto, Canada. Laine, J.-M., Campos, C , Baudin, I. and Janex, M.-L . (2003). Understanding membrane fouling: A review of over a decade of research. Water Science and Technology: Water Supply, 3(5-6), 155-164. Lesjean, B., Luck, F., Gnirss, R., Adam, C. and Kraume, M . (2003). Enhanced biological phosphorus removal process implemented in membrane bioreactors to improve phosphorous recovery and recycling. Water Science and Technology, 48(1), 87-94. Liao, B. Q., Bagley, D. M . , Kraemer, H . E., Leppard, G. G. and Liss, S. N . (2004). A review of biofouling and its control in membrane separation bioreactors. Water Environment Research, 76(5), 425-436. Lim, B.-R., Ahn, K . -H . , Songprasert, P., Cho, J. W. and Lee, S. H. (2004). Microbial community structure of membrane fouling film in an intermittently and continuously aerated submerged membrane bioreactor treating domestic wastewater. Water Science and Technology, 49(2), 255-261. Luo, M . and Wang, Z. (2001). Complex fouling and cleaning-in-place of a reverse osmosis desalination system. Desalination, 141(1), 15-22. Mallevialle, J., Anselme, C. and Marsigny, O. (1989). Effects of Humic Substances on Membrane Processes. Acs Symposium Series, 219, 749-767. Mallevialle, J., Odendaal, P. E. and Wiesner, M . R. (1996). Chapter 1. The emergence of membranes in water and wastewater treatment. Water Treatment Membrane Processes, Mallevialle, J., Odendaal, P. E. and Wiesner, M . R. (eds). McGraw-Hi l l , New York. Ridgway, H. F. and Flemming, H.-C. (1996). Chapter 6. Membrane Biofouling. Water Treatment Membrane Processes, Mallevialle, J., Odendaal, P. E. and Wiesner, M . R. (eds). McGraw-Hill, New York. 100 Rosenberger, S., Laabs, C , Lesjean, B., Gnirss, R., Amy, G., Jekel, M . and Schrotter, J.-C. (2006). Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment. Water Research, 40(4), 710-720. Speth, T. F., Summers, R. S. and Gusses, A. M . (1998). Nanofiltration foulants from a treated surface water. Environmental Science and Technology, 32(22), 3612-3617. Stephenson, T., Judd, S., Jefferson, B. and Brindle, K. (2000). Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London. 101 Chapter 5. A comparative study of fouling-related properties of sludge from a membrane enhanced biological phosphorus removal process and a conventional enhanced biological phosphorus removal process* Introduction Membrane fouling in activated sludge membrane bioreactors (MBRs) is referred to as the accumulation of sludge foulants on the membrane structure due to the interactions between the membrane and the sludge to be filtered (Liao et al, 2004). In this regard, the physical, chemical and biological properties of activated sludge mixed liquor, which have significant impact on the membrane-foulant interactions, are major factors that determine the sludge fouling propensity and accordingly influence the membrane filtration performance (Chang et al, 2002; Judd, 2004). Among many sludge properties, the surface characteristics of activated sludge floes, such as zeta potential and hydrophobicity, are assumed to play an important part in membrane fouling, because it is believed that the attachment of foulants to a membrane is somewhat based on electrostatic and/or hydrophobic interactions between the two (Kraemer, 2002; Liao et al, 2004). In addition, sludge floe size distribution also affects membrane permeability. Smaller floe sizes have been found to be associated with greater hydraulic resistance and poorer filtration behavior (Wisniewski and Grasmick, 1998). Other sludge properties, including the concentration of mixed liquor suspended solids (MLSS) and sludge viscosity, which were previously considered to influence membrane filtration (Sato and Ishii, 1991; Lee et al, 2001), were later found to have no impact on the filterability of sludge (Rosenberger and Kraume, 2003; Fan et al, 2006). In recent years, the content of extracellular polymeric substances (EPS) in activated sludge, which includes polysaccharides, proteins, humic substances, uronic acids, and nucleic acid substances, grew to be of the greatest research interest with respect to membrane fouling. EPS in activated sludge have two different origins: (1) from metabolism or lysis of microorganisms (e.g. proteins, polysaccharides, D N A and lipids) and (2) from the wastewater itself (e.g. humic acids) (Urbain et al, 1993; Jahn and * A version of this chapter will be submitted for publication. Geng, Z. and Hall, E.R. A comparative study of fouling-related properties of sludge from conventional and membrane enhanced biological phosphorus removal processes 102 Nielsen, 1995). Considerable effect of EPS on membrane performance has been widely reported (Nagaoka et al, 1996; Nagaoka et al, 1998; Cho and Fane, 2003; Ng and Hermanowicz, 2005). It has been found that the higher the content of EPS bound in the activated sludge floes, the more rapidly membrane fouling proceeded. Therefore, the bound EPS content of activated sludge was suggested as an probable index for membrane fouling in membrane coupled activated sludge systems (Chang and Lee, 1998). However, with improved understanding of the fouling mechanisms, soluble microbial products including the soluble EPS released from sludge floes to the liquid phase, which were ignored in the previous studies, have received more attention because of their important role in sludge filterability and irreversible fouling (Rosenberger and Kraume, 2003; Evenblij and Graaf, 2004; Holbrook et al, 2004). The latest studies have revealed that critical flux was almost solely related to the colloidal total organic carbon (0.04 - 1.5 Lim) that was attributed to soluble EPS (Fan et al, 2006). In particular, the concentration of polysaccharides present in the sludge water phase exhibited a linear and positive relationship with the rate of membrane fouling (Lesjean et al, 2005; Rosenberger et al, 2006). As reported in Chapter 4 on characterization of membrane foulants, EPS components, such as carbohydrates and humic or humic-like substances, were found to accumulate on membrane structures. Moreover, the study of sludge constituents, which is presented in Chapter 3, has revealed that the soluble fraction of sludge accounted for a major part of the total sludge fouling. Based on the literature and our previous findings, a research hypothesis was proposed: it is actually soluble EPS rather than bound EPS that plays a critical role in membrane fouling in activated sludge MBRs. It was further hypothesized that the bound EPS of sludge influences membrane filtration indirectly through the release of EPS components into the liquid phase of activated sludge mixed liquor. Although there have been a few studies that focused on the properties of M B R sludge and their comparison with those of conventional sludge, little is known about the properties of sludge from the membrane enhanced biological phosphorus removal (MEBPR) process, a new type of M B R , and their relationship to membrane fouling. To fill this knowledge gap and test the above hypothesis, the present study was initiated and 103 such sludge properties as floe size distribution, zeta potential, relative hydrophobicity, bound and unbound (soluble) EPS contents, which were believed to be closely related to membrane fouling, were investigated. A comparative approach was adopted, which involved a pilot scale M E B P R process and a pilot scale conventional enhanced biological phosphorus removal (CEBPR) process operated in parallel throughout two experimental runs. The selected sludge properties were measured regularly on both the M E B P R sludge and the CEBPR sludge during the entire experimental period. The specific objectives of this study were to evaluate the effect of M E B P R operation, in comparison to the CEBPR operation, on the fouling-related sludge properties, and to clarify the relative importance of these properties (especially soluble EPS and bound EPS) to the fouling propensity of sludge. Methods The Wastewater Treatment Pilot Plant The two test processes, i.e. the MEBPR process and the CEBPR process, were run at the University of British Columbia (UBC) Wastewater Treatment Pilot Plant. Both shared the same primary clarifier effluent from a municipal sewage source and employed a typical UCT (The University of Cape Town) treatment scheme with a sequence of an 3 3 3 anaerobic zone (0.2 m J), an anoxic zone (0.6 m ), and an aerobic zone (1.3 m J). A l l the system design and operating parameters were identical for the two processes except for the means of sludge separation and the aeration intensity. A secondary clarifier was used in the CEBPR system for separating treated wastewater from activated sludge. In the M E B P R system, the secondary clarifier was replaced with a custom-built ZeeWeed membrane module (Zenon Environmental Inc., Oakville, Ontario, Canada) submerged in the aerobic zone. The module comprised thousands of polyvinylidene fluoride (PVDF) hollow fibres with a total membrane surface area of 11.9 m 2 and a nominal pore size of 0.04 |im. For the CEBPR process, air was supplied continuously at a rate of 4-6 m3/h in the aerobic zone. In the M E B P R process, a cyclic aeration mode of 10 second ON and 10 second OFF was applied at an airflow rate of 20-26 m /h, in order to maintain a similar dissolved oxygen level of 2^4 mg/L to that of the CEBPR process. 104 The two parallel treatment processes at the U B C pilot plant went through two consecutive experimental runs during the period December 2002 - June 2004. In Run I, the sludge retention time (SRT) was set at 12 days and the hydraulic retention time (HRT) was 10 hours for both processes. In Run II, the HRT was reduced to 7 hours and the SRT was unchanged. The operating regime and some characteristics of the pilot plant are summarized in Table 5.1. Details about the process design and operation and treatment performance of the two processes are presented in Chapter 2. Table 5.1. Operating regime and characteristics of the U B C pilot plant Run Duration Process S R T (d) HRT (hr) M L S S (g/L) F/M (1/d) Organic loading (kgCOD/m 3-d) M E B P R 12 10 3.3 0.32 0.88 1 12 /2002-12/2003 C E B P R 12 10 2.7 0.39 0.88 II 12 /2003-06/2004 M E B P R C E B P R 12 12 7 7 4.1 3.3 0.31 0.40 1.03 1.03 Note: M L S S - mixed liquor suspended solids; F/M - food to microorganism ratio Assessment of the Fouling Propensity of Activated Sludge Mixed Liquor The flux-step method was used to determine the apparent critical flux and accordingly to compare the fouling propensity of activated sludge mixed liquor (Le Clech et al, 2003). The greater the critical flux, the lower the potential of sludge to foul membranes. A l l the sludge tested in this study was collected as grab samples from the aerobic zones of the M E B P R process and the CEBPR process. To assess the fouling propensity of sludge, freshly collected sludge was subjected to off-line membrane filtration tests using the bench scale filtration apparatus depicted in Figure 2.4 of Chapter 2. The system was operated at fluxes that were increased every 30 minutes with a step increment of 9 L/m -hr. That is, the initial flux was kept constant for 30 minutes, then the flux was increased by 9 L/irr-hr., after which the system was operated in a constant flux mode again, and so on. Transmembrane pressure (TMP) was closely monitored during the entire test period. The apparent critical flux was defined as the maximum flux, below which stable TMP was achieved and above which an increase in TMP was observed. 105 Measurement of Sludge Floe Size and Surface Properties Floe size distribution and zeta potential Floe size distribution and zeta potential were analyzed using a Malvern MasterSizer 2000 and a Malvern ZetaSizer 2000, respectively. Fresh sludge samples were collected from the aerobic zones of the two pilot scale processes and these were then diluted 100-200 times with MEBPR permeate prior to analysis. For floe size measurement, a Masterflex peristaltic pump was used to transfer the diluted sludge to the measurement cell at a low flow rate of 70 mL/min. in order to minimize the disturbance of the sludge floes. For zeta potential measurement, the diluted sludge was pre-filtered with 8.0 urn filter paper to remove settleable particles, which might interfere with the measurement, and the filtrate was then subjected to the zeta potential measurement. The samples were all analyzed in triplicate. Relative hydrophobicity Relative hydrophobicity of activated sludge was estimated according to Kraemer (2002). Sludge floes were first washed with distilled water twice by centrifuging the mixture of sludge floes and distilled water at 2000 x g for 5 minutes and discarding the supernatant. Then, based on the measurement of mixed liquor suspended solids (MLSS), a calculated amount of sludge suspension was transferred to a 10-mL test tube to make a total volume of 6.0 mL with an MLSS of 500 mg/L. This sludge suspension was immediately homogenized for 30 seconds using a vortex mixer and the absorbance at 400 nm (Io) was measured at room temperature on a Hach 2000 spectrophotometer. Then, 1.0 mL of hexadecane was added to the test tube and the sludge-hexadecane mixture was thoroughly agitated for 2 minutes using the vortex mixer. The mixture was allowed to settle down for 10 min. and the absorbance of the aqueous phase at 400 nm (I) was measured. The relative hydrophobicity (Hr) of sludge was estimated as Hr (%) = [(I0 - I) / I0] x 100. 106 EPS Extraction and Quantification EPS extraction Extraction of EPS was carried out using the cation exchange resin method (Fr0lund et al, 1996). In this method, it is assumed that EPS is mainly bound to cell aggregates through the bridging of divalent cations such as calcium and magnesium. By removing these metal ions using cation exchange resin (CER), the EPS matrix is weakened and thus EPS is more easily released from sludge floes into the liquid phase (Fr0lund et al, 1996; Flemming et al, 2000). The procedure of EPS extraction was as follows. (1) Exactly 50.0 mL of activated sludge mixed liquor was centrifuged at 2600 x g for 20 minutes at room temperature. The supernatant was collected for unbound or soluble EPS measurement. (2) The sludge pellets were washed with about 50 mL of EPS extraction buffer (2 m M Na 3 P0 4 , 4 mM NaPLPCu, 9 mM NaCl and 1 mM KC1 at pH 7) by re-suspending the pellets in the buffer, centrifuging the suspension at 2600 xg for 20 minutes and decanting the supernatant. (3) The washed sludge pellets were transferred to a 50-mL Erlenmeyer conical glass flask after mixing with 10.0-20.0 mL of EPS extraction buffer. Additional 5.0 mL of the buffer was used to rinse the centrifuge tube and this was also transferred to the flask. (4) A measured amount of cation exchange resin (DOWEX 50X 8, 20-50 mesh, Fluka 44445) was added to the flask in line with the rule of 60 g CER/g SS. (5) The flask was placed on a refrigerated incubator shaker (New Brunswick Scientific, Edison, NJ, USA) and agitated at 400 rpm at 4 °C for 2 hours. (6) After agitation, the sludge/CER mixture was immediately centrifuged at 12,000 x g for 20 minutes at room temperature. The supernatant was collected as the crude EPS extract for the subsequent analysis of bound EPS. A l l the sludge samples were processed in triplicate. The bound EPS extract, as well as the soluble EPS samples, were stored at -20°C if the EPS quantification was not conducted immediately. When analyzed, the frozen samples were thawed at 4 °C and centrifuged at 12,000 x g for 5 minutes to remove any remaining floe components. 107 EPS quantification Quantification of EPS was accomplished using colorimetric methods. The concentration of carbohydrates in EPS was determined according to the anthrone method, in which glucose was used as the standard (Fr0lund et al., 1996). Anthrone reagent, prepared prior to the analysis, contained 0.125% (w/v) anthrone in 94.5% (v/v) H2SO4. First, 0.40 mL of EPS extract was mixed with 0.80 mL of the anthrone reagent and the mixture was immediately placed in an oven set at 100 °C for 15 minutes and cooled in a water bath at 4 °C for 5 minutes. Absorbance of the mixture at 625 nm was then measured and corrected subsequently by subtracting the blind value due to the non-anthrone specific colour development, which was estimated in the same way as above except that the anthrone reagent was omitted. Proteins and humic or humic-like compounds were quantified using the corrected Lowry method, in which bovine serum albumin (BSA) and humic acids were used as the standards, respectively (Fr0lund et al., 1995; Jahn and Nielsen, 1995). The Lowry assay for total proteins was based on the use of the Folin phenol reagent (Lowry et al., 1951). A wide range of organic and inorganic substances, including humic acids, fulvic acids, some aromatic compounds, certain reactive amino acids, Fe 2 + , M n 2 + , etc., could reduce the Folin phenol reagent (phosphomolybdic and phosphotungstic acids) to heteropoly blue in alkaline solution (Box, 1983). The majority of amino acids do not reduce the Folin reagent. Only such reactive amino acids as tyrosine, tryptophan, and cysteine react with the Folin reagent to produce heteropoly blue. This colour development can be enhanced by treatment with alkaline C u S 0 4 and the enhancement with alkaline copper is specific to proteins but not to humic substances. In the corrected Lowry method, therefore, the colour development was measured with and without Q1SO4 addition. For a mixture of B S A and humic acids, the colour development in the presence of Q1SO4 is the sum of the colour induced by humic compounds and the colour induced by total proteins. When C U S O 4 is omitted, the colour development is due to humic acids and certain reactive amino acids in B S A and the latter decreased to 20% of the value obtained in the presence of C U S O 4 (Fr0lund et al, 1995). The mutual interference of proteins and humic compounds is described in the following expressions. 108 (w/o CuS0 4 ) (w/ CuSQ 4) (ii) (i) A, proteins - 1.25 (Atotal - Ablind) (iii) Ahumic — AbHnd — 0.2Ai 'proteins (iv) Where A t o t a i is the total absorbance of the mixture with addition of CuS0 4 , AbHnd is the total absorbance of the mixture without addition of CuS0 4 , A h u m i C is the absorbance due to humic compounds, and Aproteins is the absorbance due to total proteins. The concentrations of proteins and humic substances in the EPS extract could be calculated by fitting the values of Aproteins and A n u m j c into the standard curves of B S A and humic acids, respectively. Five reagents were prepared prior to the analysis of proteins and humic or humic-like substances (Fr0lund et al, 1996). Reagent 1 contained 143 m M NaOH and 270 mM N a 2 C 0 3 . Reagent 2 contained 57 m M CuS0 4 . Reagent 3 was a 124 mM Na-tartrate solution. Reagent 4 was a mixture of Reagents 1 to 3 in the proportion 100:1:1. Reagent 5 was the Folin reagent diluted with distilled water by 2. When analyzed, 0.50 mL of EPS extract was first mixed using a vortex mixer with 0.70 mL Reagent 4 and 0.1 mL Reagent 5 at room temperature for 45 minutes. Then, the absorbance of the mixture at 750 nm was measured and recorded as A t 0t ai- Abimd was estimated in the same way as above except that Reagent 2 (CuS0 4 solution) was replaced with distilled deionized water. UV-visible Absorbance of Bound and Soluble EPS and Foulant Extract To compare the similarity and difference among bound EPS, soluble EPS and membrane foulant extract, which originated from the same source of activated sludge, and to facilitate the study of the relationship of bound and soluble EPS to membrane fouling, the UV/visible absorbance of the previously prepared soluble EPS samples, bound EPS samples and extract of membrane foulants was measured on a U V 300 UV-Visible Spectrometer (Spectronic Unicam). The foulant extract was obtained by soaking sliced outer membrane skin and inner fabric support of fouled membrane fibres in a basic solution (0.1 M NaOH), applying sonication to the mixture for 1 hour, neutralizing the solution, and collecting the resultant liquid, as described in Chapter 4. 109 Results and Discussion Fouling Propensity of Activated Sludge Mixed Liquor The U B C pilot plant was operated in the two consecutive experimental runs from December 2002 to June 2004. The overall membrane performance in the pilot scale MEBPR process is shown in Figure 2.6 of Chapter 2, and the filtration run time (the interval between two chemical cleanings) of the typical filtration runs that were operated at pseudo-steady state are summarized in Table 5.2. It was evident that the MEBPR mixed liquor fouled the membranes more quickly in Run II than it did in Run I, as indicated by the shorter filtration run times in Run U (46-51 days) compared to those in Run I (84-85 days). This suggested that the fouling propensity of the M E B P R mixed liquor was increased in Run II due to the reduced HRT and thus the increased organic loading rate. Table 5.2. Run time of the M E B P R membrane module in different experimental runs Experimental run 1 (SRT=12 days HRT=10 hours) II (SRT=12 days HRT=7 hours) Filtration run time, days 8 4 - 8 5 46 - 51 Number of filtration runs 2 2 The assessment of the fouling propensity of activated sludge mixed liquor in the two parallel processes (MEBPR vs. CEBPR) was accomplished through the comparison of apparent critical flux using the flux-step method. Sludge samples were collected in both experimental runs and typical results of the flux-step filtration tests are shown in Figure 5.1. It is not hard to estimate that the apparent critical flux of the M E B P R mixed liquor was around 27 L/m2-hr, while the critical flux of the CEBPR mixed liquor was close to 45 L/m2-hr., which was much higher than that of the M E B P R sludge. Since the lower the critical flux, the greater the fouling tendency of the sludge, the M E B P R mixed liquor was more inclined to foul membranes than the CEBPR mixed liquor. The greater fouling potential of M B R sludge was also demonstrated in the research of Cicek (1999), who found that the specific membrane resistance of M B R sludge was three orders of magnitude higher than that of conventional activated sludge. 110 0 30 60 90 120 150 Time elapsed, min. 0 30 60 90 120 150 Time elapsed, min. Figure 5.1. Flux-step filtration tests of (a) the M E B P R sludge and (b) the CEBPR sludge collected in Run I Similar experimental results to those shown in Figure 5.1 were also obtained for the two types of sludge collected in Run II (Appendix 19). It can be concluded that the MEBPR mixed liquor consistently exhibited higher fouling propensity than the CEBPR mixed liquor during both Run I and Run II. Moreover, the fouling tendency of sludge was greater in Run II due to the increased organic loading rate. A preliminary examination of the two types of sludge using scanning electron microscopy (SEM) showed that there was little difference in the morphology of the M E B P R sludge and the CEBPR sludge (Appendix 20). Therefore, further analysis of the physical and biochemical properties of the two types of sludge was performed in order to find out the key factors that caused the high fouling propensity of the sludge. Floe Size and Surface Properties of Sludge The sludge floe size distribution and the surface properties such as zeta potential and relative hydrophobicity were measured on randomly selected samples during the pseudo-steady state periods of the two experimental runs, and the results are illustrated in Figure 5.2. The most prominent point that can be drawn from the graph is that at either a E n CO 21 Q CD N CO o o CO o CL B "CD N i f o la o .c Q_ O i _ T3 -C CD > J O CD rr —•— MEBPR sludge —O— -CEBPR sludge a — E Run 1 Run II 250 100 50 0 28-Jun-03 11 -Sep-03 25-Nov-03 08 -Feb -04 23-Apr-04 Date of samp l ing Date of samp l ing 28-Jun-03 11 -Sep-03 25-Nov-03 08 -Feb -04 23-Apr-04 0 -10 -20 -30 -40 100 75 50 25 0 •MEBPR sludge -CEBPR sludge Run I Run II —•— MEBPR sludge —o— - CEBPR sludge c Run I Run II 20-Mar-03 28-Jun-03 06-Oct -03 14-Jan-04 23-Apr-04 01-Aug-04 Date of samp l ing Figure 5.2. Variation of (a) sludge floe size distrubtion; (b) zeta potential; (c) relative hydrophobicity during the two experimental runs 112 10-hour HRT (Run I) or a 7-hour HRT (Run II), the M E B P R sludge floes were always smaller in size than the CEBPR sludge floes as indicated by the volume weighted mean diameter D[4,3], though the former increased in size when the organic loading rate was enhanced in Run H This is in agreement with the findings of Cicek et al. (1999) and Merlo et al. (2003), but is contrary to the findings of Sperandio et al. (2005), who reported a larger floe size (240 |im) of a M B R sludge compared to that of a conventional sludge (160 urn) under similar operating conditions (SRT = 9.5 days). In the present study, the intense aeration applied in the aerobic zone of the M E B P R process in order to reduce sludge deposition on the membrane surface was thought to be among the major reasons for the shift towards the smaller floe sizes. On the other hand, small CEBPR sludge floes were not easy to settle out and mostly exited the clarifier of the CEBPR system, resulting in a distribution of bigger sludge floes. Overall, the greater quantity of small floes in the MEBPR sludge very likely contributed to the more severe membrane fouling via pore blocking, if compared to the CEBPR sludge. As for the surface properties, none of the zeta potential and the relative hydrophobicity values of sludge appeared to be related to either the types of sludge or the operating conditions, such as HRT (Figure 5.2 (b)-(c)). The zeta potentials of the two types of sludge were comparable, in the range of -16 ~ -26 mV. The relative hydrophobicity seemed to be a highly variable sludge property, which varied from 15% to 55% during the two experimental runs. Bound and Soluble EPS in Activated Sludge Mixed Liquor The initial measurement of EPS components, including carbohydrates, proteins, humic substances, uronic acids, and DNA, indicated that the latter two accounted for no more than 5% of the total measurable EPS (Appendix 26). Moreover, they were not detected in the extract of membrane foulants in the previous study (Chapter 4). Therefore, only carbohydrates, proteins and humic or humic-like substances were subsequently measured throughout the study, and the results are summarized in Figure 5.3 and Figure 5.4. It seemed that the two types of sludge contained similar amounts of EPS bound in sludge floes, particularly in Run II. The CEBPR sludge did exhibit a slightly higher level of bound proteins and a slightly higher level of total EPS in Run I (Figure 5.3). As the 113 co 3 T3 >* o JD i CO o T3 C 3 O CO CO CO > E CO 1 % 2 > § E o CO CO CO > E CO CD o c 05 oo . Q co o 'E T3 C o CO CO £] co "D C o "CO o CO > D) Q) 40 30 20 H 10 0 a T —•—MEBPR sludge —o— CEBPR sludge Run I y / Run II 29-Jan-03 28-Jun-03 25-Nov-03 23-Apr-04 20-Sep-04 Date of sampling 120 90 60 30 0 29-Jan-03 b J —•—MEBPR sludge A , —o— CEBPR sludge Run I Run II 28-Jun-03 25-Nov-03 23-Apr-04 20-Sep-04 Date of sampling 60 40 20 H 0 — • — M E B P R sludge C - o — C E B P R sludge ii Run I Run II 1 29-Jan-03 28-Jun-03 25-Nov-03 23-Apr-04 20-Sep-04 Date of samping 150 100 50 A 0 —•—MEBPR sludge A. —o— CEBPR sludge Run I Run II 29-Jan-03 28-Jun-03 25-Nov-03 23-Apr-04 Date of sampling 20-Sep-04 Figure 5.3. Content of EPS carbohydrates; (b) proteins; bound in activated sludge floes during Run I and Run JJ: (a) (c) humic substances; (d) total EPS. 114 co £ i _ T3 SZ o JQ v. 03 O O JZ\ J3 O CO 03 c 'o o i Q_ J!> _Q _ZS O CO 30 20 10 0 —•— MEBPR mixed liquor I —o— CEBPR mixed liquor ; a i R u n l | 7^ Run II axn> 29-Jan-03 28-Jun-03 25-Nov-03 23-Apr-04 20-Sep-04 Date of sampl ing 12 E 4 H 0 b !—•—MEBPR mixed liquor A J i - o—CEBPR mixed liquor Run I o £ — ; a j ^ ^ Run II 29-Jan-03 28-Jun-03 25-Nov-03 23-Apr-04 20-Sep-04 Date of sampl ing co OJ o c ro "to . Q 13 CO o 'E sz £ _ 3 o CO CO Q. LU a> JD o to "TO o 40 30 20 10 0 29-Jan-03 28-Jun-03 25-Nov-03 23-Apr-04 20 -Sep-04 Date of samp l ing 60 —•—MEBPR mixed liquor C —o— CEBPR mixed liquor R u n l fir t r 4 X 0 Run II D) E 40 20 m MEBPR mixed liquor d - o — C E B P R mixed liquor R u n , > l Run II I- 29-Jan-03 28-Jun-03 25-Nov-03 23-Apr-04 20 -Sep-04 Date of samp l ing Figure 5.4. Content of soluble EPS in activated sludge mixed liquor during Run I and Run IF (a) carbohydrates; (b) proteins; (c) humic substances; (d) total EPS. 115 M E B P R sludge exhibited significantly greater fouling propensity than the CEBPR sludge (Figure 5.1), the content of bound EPS was obviously not indicative of the fouling tendencies of the test sludges. On the other hand, a clear contrast between the soluble EPS content in the M E B P R sludge and in the CEBPR sludge is demonstrated in Figure 5.4: The concentrations of soluble carbohydrates, proteins and humic substances were all consistently higher in the M E B P R mixed liquor than in the counterpart CEBPR mixed liquor, and this phenomenon became more pronounced in Run II after the organic loading rate had been increased. More importantly, this difference in soluble EPS content coincided well with the difference in sludge fouling behaviour. Along with the notable escalation in the concentration of soluble EPS (especially carbohydrates and humic substances) of the M E B P R sludge in Run II, the fouling propensity of the sludge increased remarkably and the membrane filtration run time decreased from 84-85 days in Run I to 46-51 days in Run II (Table 5.2), as discussed earlier. This suggests that the content of soluble EPS in the mixed liquor is strongly associated with the membrane performance and that it might be a key property to predict the fouling potential of activated sludge mixed liquor. From Figures 5.3 and 5.4, it was also noted that both the M E B P R and the CEBPR mixed liquor contained large amounts of bound proteins but very small amounts of soluble proteins. The relative contents of proteins in bound and soluble EPS are detailed in Table 5.3. For both types of sludge, proteins constituted 47-72% of the total bound EPS. In contrast, carbohydrates and humic substances dominated in the soluble EPS, and proteins accounted for only 12-28% of the total soluble EPS. It was very likely that under the experimental conditions used, the proteins bound in sludge floes were not as readily Table 5.3. Ratios of protein to carbohydrate and protein to total EPS in bound and soluble EPS of the two types of sludge Run I Run II Ratio Type of E P S -M E B P R C E B P R M E B P R C E B P R Bound E P S 3.3 4.3 4.9 6.6 Protein/carbohydrate Soluble E P S 0.90 0.76 0.35 0.69 Protein/Total E P S Bound E P S Soluble E P S 0.58 0.28 0.72 0.20 0.47 0.12 0.49 0.13 116 released and/or dissolved in the liquid phase as carbohydrates and humic substances. The very low content of soluble proteins corresponded well to the previous finding, reported in Chapter 4, that no measurable amount of protein was detected in the extract of membrane foulants. This good correspondence strongly supported a cause-effect relationship between soluble EPS and membrane foulants, suggesting that the soluble EPS in the mixed liquor, particularly the soluble carbohydrates and humic substances, were the major substances that were accumulated on the membranes and consequently caused membrane fouling. Soluble proteins had little influence on fouling due to their very low concentrations. Based on these analyses, a model describing the relationship among bound EPS, soluble EPS and membrane fouling is postulated in the present study. In this model, the EPS bound in sludge floes does not directly relate to fouling, except when the EPS components are released from the sludge matrix to the aqueous phase. In this sense, the soluble EPS acts as an important medium between the bound EPS and membrane foulants, as illustrated by the following expression. Bound EPS * » Soluble EPS « ' Membrane Foulants UV-visible Absorbance of Bound and Soluble EPS in Comparison to Foulant Extract The relative importance of bound and soluble EPS in membrane fouling was more straightforwardly reflected through the comparison of the UV-visible absorbance of bound and soluble EPS to the absorbance of membrane foulant extract (Figure 5.5). It was clear that the bound EPS exhibited two distinct absorbance peaks in the range of 200-210 nm and 250-260 nm, respectively. In contrast, the soluble EPS of both types of sludge exhibited absorbance only in the range of 200-210 nm. No significant absorbance was observed around 250-260 nm. More importantly, the foulant extract displayed a similar single-peak absorbance profile to the soluble EPS. Since these samples were derived from the same source of activated sludge mixed liquor, the UV-visible absorbance spectra revealed that the composition of the foulant extract was similar in nature to that of the soluble EPS, but also that it was different from the composition of the bound EPS. In other words, this further indicated that the membrane foulants originated from the soluble EPS, rather than the bound EPS. 117 E o <D O c cc J D i o co .o < CD O c CO J D o CO < 4.0 3.0 H 2.0 1.0 0.0 4.0 -j E 3.0 -o 1 — CD" 2.0 -o CO J D 1.0 -O 00 J D < 0.0 -2.0 1.5 1.0 0.5 0.0 Bound EPS(MEBPR) Bound EPS(CEBPR) 150 200 250 300 Wavelength, nm 350 400 b Soluble EPS (MEBPR) A — S o l u b l e EPS (CEBPR) \ 150 200 250 300 350 400 Wavelength, nm • Foulants on outer skin •Foulants on inner support 150 200 250 300 350 400 Wavelength, nm Figure 5.5. UV-visible absorbance of bound and soluble EPS in comparison to that of membrane foulant extract: (a) bound EPS; (b) soluble EPS; and (c) foulant extract Conclusion Despite parallel operation, the physical differences between the M E B P R and the CEBPR processes resulted in a number of different physical and biochemical properties of mixed liquor and consequently different fouling behaviors of the two types of sludge. The 118 MEBPR sludge exhibited consistently higher fouling propensity than the CEBPR sludge, which was somewhat associated with the smaller M E B P R sludge floes, if compared to the CEBPR sludge floes. In the present study, sludge surface properties such as zeta potential and relative hydrophobicity did not appear to be linked to the difference in the fouling behaviors of the two types of sludge. As hypothesized, the sludge property that had the most profound effect on the fouling propensity of sludge was the content of soluble EPS in the liquid phase of the mixed liquor. The higher the soluble EPS content, the greater potential the sludge had to foul the membranes. On the other hand, the content of EPS bound in the sludge floes did not show a direct association with sludge fouling tendency. It was further confirmed that it was the soluble EPS rather than the bound EPS that was most directly related to membrane fouling. Bound EPS may influence the sludge filterability through the release of EPS from sludge floes to the bulk liquor. Contrary to earlier literature reports, the content of soluble EPS (especially carbohydrates and humic substances) in the water phase of activated sludge mixed liquor was concluded to be a key index to assess the fouling propensity of sludge, if adsorptive fouling is dominant. 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Desalination, 120(3), 221-233. 119 Cho, B. D. and Fane, A. G. (2003). Fouling phenomena in a MBR: Transmembrane pressure transients and the role of EPS (extracellular polymeric substances). Water Science and Technology: Water Supply, 3(5-6), 261-266. Cicek, N . , Franco, J. P., Suidan, M . T., Urbain, V . and Manem, J. (1999). Characterization and comparison of a membrane bioreactor and a conventional activated-sludge system in the treatment of wastewater containing high-molecular-weight compounds. Water Environment Research, 71(1), 64-70. Evenblij, H . and Graaf, J. H. J. M . v. d. (2004). Occurrence of EPS in activated sludge from a membrane bioreactor treating municipal wastewater. Water Science and Technology, 50(12), 293-300. Fan, F., Zhou, H . and Husain, H. (2006). Identification of wastewater sludge characteristics to predict critical flux for membrane bioreactor processes. Water Research, 40(2), 205-212. Flemming, H . - C , Szewzyk, U . and Griebe, T. (2000). 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A review of fouling of membrane bioreactors in sewage treatment. Water Science and Technology, 49(2), 229-235. Kraemer, H. E. (2002). Characterization of microbial aggregates in relation to membrane biofouling in submerged membrane bioreactors, MASc thesis, Ryerson University, Toronto, Canada. Le Clech, P., Jefferson, B., Judd, S. J. and Chang, I. S. (2003). Critical flux determination by the flux-step method in a submerged membrane bioreactor. Journal of Membrane Science, 227(1-2), 81-93. Lee, J., Ahn, W.-Y. and Lee, C.-H. (2001). Comparison of the filtration characteristics between attached and suspended growth microorganisms in submerged membrane bioreactor. Water Research, 35(10), 2435-2445. Lesjean, B. , Rosenberger, S., Laabs, C , Jekel, M . , Gnirss, R. and Amy, G. (2005). Correlation between membrane fouling and soluble/colloidal organic substances in membrane bioreactors for municipal wastewater treatment. Water Science and Technology, 51(6-7), 1-8. Liao, B. Q., Bagley, D. M . , Kraemer, H. E., Leppard, G. G. and Liss, S. N . (2004). A review of biofouling and its control in membrane separation bioreactors. Water Environment Research, 76(5), 425-436. 120 Lowry, O. H., Rosebrough, N . J., A .L . , F. and Randall, R. J. (1951). Protein measurement with the Folin phenol reagent. Journal of Biological Chemistry, 193, 265-275. Merlo, R. P., Trussell, R. S., Ng, PL Y. , Hermanowicz, S. W. and Jenkins, D. (2003). Biomass Characteristics in Membrane Bioreactors. WEFTEC 2003 Workshop #110 Membrane Technologies: Application in Wastewater Treatment, Los Angeles, USA, 11-15. Nagaoka, PL, Ueda, S. and Miya, A . (1996). Influence of bacterial extracellular polymers on the membrane separation activated sludge process. Water Science and Technology, 34(9), 165-172. Nagaoka, PL, Yamanishi, S. and Miya, A . (1998). Modeling of biofouling by extracellular polymers in a membrane separation activated sludge system. Water Science and Technology, 38(4-5), 497-504. Ng, H. Y . and Hermanowicz, S. W. (2005). Specific resistance to filtration of biomass from membrane bioreactor and activated sludge: Effects of exocellular polymeric substance and dispersed microorganisms. Water Environment Research, 77(2), 187-192. Rosenberger, S. and Kraume, M . (2003). Filterability of activated sludge in membrane bioreactors. Desalination, 151(2), 195-200. Rosenberger, S., Laabs, C , Lesjean, B., Gnirss, R., Amy, G., Jekel, M . and Schrotter, J.-C. (2006). Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment. Water Research, 40(4), 710-720. Sato, T. and Ishii, Y . (1991). Effects of activated sludge properties on water flux of ultrafiltration membrane used for human excrement treatment. Water Science and Technology, 23(7-9), 1601-1608. Sperandio, M . , Masse, A. , Espinosa-Bouchot, M . C. and Cabassud, C. (2005). Characterization of sludge structure and activity in submerged membrane bioreactor. Water Science and Technology, 52(10-11), 401-408. Urbain, V. , Block, J. C. and Manem, J. (1993). Bioflocculation in activated sludge: An analytic approach. Water Research, 27(5), 829-838. Wisniewski, C. and Grasmick, A. (1998). Floe size distribution in a membrane bioreactor and consequences for membrane fouling. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 138(2-3), 403-411. 121 Chapter 6. Research overview Some Points about the Research Discrepancies Research on membrane fouling has been done for several decades since the emergence of membrane technology. Due to the complexity of fouling processes and the diversity of membrane systems used, inconsistent or even completely contrary results often have been reported in the literature. In the present research, it has been noted that some findings are not in congruity with the results of other studies. Due to their significance to this research and to the field of activated sludge membrane filtration, it is worthwhile to highlight these findings in the following context for the purpose of special attention. Bound vs. Unbound (Soluble) EPS and EPS Components As described in Chapter 5, it has long been thought that the amount of EPS bound in sludge floes was a decisive factor in membrane fouling in activated sludge membrane bioreactors (Nagaoka et al, 1996; Chang and Lee, 1998; Cho et al, 2005; Ng and Hermanowicz, 2005). However, the results of the present research demonstrated that membrane fouling was affected most by the soluble EPS in the water phase of sludge rather than the EPS bound in the activated sludge matrix. The latter may influence the fouling process through release of EPS to the bulk liquor. In addition, protein and carbohydrates (or polysaccharides) were previously reported to have more profound impact on sludge properties and membrane filtration performance than the total EPS and other EPS components, with protein being more significant than carbohydrates (Hoa et al, 2003; Evenblij and Graaf, 2004). For membrane filtration of surface water, on the other hand, fouling by humic substances was a major factor limiting water purification processes (Mallevialle et al, 1989; Yuan et al, 2002). By contrast, the current study revealed that in the M E B P R process, carbohydrates and humic substances played a leading role in membrane fouling, and protein showed little influence. This was probably due to the low concentration of protein (1.9-3.6 mg/L) in the water phase of the sludge throughout this research, in contrast to the average protein concentration of 23 mg/L in the M B R used by Evenblij and Graaf (2004). The work of 122 Rosenberger (2006) was somewhat in.agreement to the findings of the present research, by demonstrating that high polysaccharide concentrations in the supernatant corresponded to high fouling rates while low concentrations corresponded to low fouling rates. In addition, it should be pointed out that soluble EPS is actually soluble microbial products (SMP). Although the two terms have been used separately for a long time, it was proven that based on the analysis of their origins and chemical compositions, SMP and soluble EPS are analogous in nature (Laspidou and Rittmann, 2002). In this research, both terms were used according to the needs of the context. Role of MLSS Similarly, the previous understanding of the significance of sludge concentration to membrane fouling is now challenged by the findings of the present research. Unlike reports in the literature that the higher the concentration of MLSS, the more severe the fouling (Sato and Ishii, 1991; Nagaoka et al, 1996), sludge concentration in the range of 0.8^1.2 g/L in the current research exhibited little adverse impact on the membrane filtration performance, whereas the soluble fraction of sludge accounted for most of the fouling. Furthermore, large sludge floes were found to alleviate fouling probably by sterically "filtering out" potential foulants before they arrived at the membrane surfaces. Therefore, it is deduced that simply increasing the solids concentration would help mitigate membrane fouling. This was also observed in the study of Lee et al. (2001a). The above discrepancies on the relative importance of bound and soluble EPS and the role of MLSS could be explained from two perspectives. First, the soluble fraction or the water phase of activated sludge mixed liquor was often ignored in many earlier studies. Since a high MLSS concentration or a high content of bound EPS is usually accompanied by a high level of soluble EPS in the water phase, the nature of fouling by dissolved organic matter is very easily confounded by the apparent factors such as MLSS concentration and/or the amount of bound EPS in sludge floes. Second, as the degree of reversible fouling was related to the concentration of MLSS and the irreversible fouling was largely caused by organic adsorption, the fouling mechanisms in MBRs determine the relative importance of bound and soluble EPS and the role of MLSS. In this regard, some 123 of the findings of the present research may only apply to the studied M E B P R process, in which adsorptive fouling was the prevailing fouling mechanism. It was reported that there might exist an upper limit of sludge concentration, below which the concentration of MLSS exhibits very little influence on permeate flux (Hong et al, 2002; Itonaga et al, 2004). Therefore, it could be inferred that, once sludge concentration exceeds the critical value, irreversible fouling due to organic adsorption no longer dominates the fouling process. Instead, the impact of sludge concentration on fouling outweighs that of the soluble fraction of sludge and thus may dominate in membrane fouling via sludge deposition and cake formation. An Extended Discussion about the Effects of the Sizes of Membrane Pores and Sludge Constituents on Membrane Fouling It is known from the findings of this research that different sizes of sludge constituents have different effects on membrane filtration performance. Through a detailed review of literature on membrane fouling in both water and wastewater treatment applications (Table 6.1), it was found that this understanding can be further extended to most MBRs, which utilize membranes with different pore sizes and activated sludge mixed liquor containing ions, small molecules, macromolecules, fine colloids, supra-dissolved particles, and large sludge floes. The research summary in Table 6.1 clearly indicates that for microfiltration (MF) and ultrafiltration (UF) of wastewater using membrane pore sizes of 0.04-0.2 Lim, the influential foulants were the colloidal and soluble fractions of activated sludge mixed liquor, which fouled membranes via adsorption and pore clogging. On the other hand, for nanofiltration (NF) and reverse osmosis (RO) systems used for water treatment and wastewater reclamation, which had molecular weight cutoffs (MWCO) of <200-80,000 Da, small dissolved organic matter was the major contributor to membrane fouling by means of adsorption. Hong et al. (2002) studied the filtration performance of four different UF and M F membranes and found that the M F membranes experienced more severe fouling due to pore clogging than the UF membranes. Choo and Lee (1996) investigated the effect of anaerobic digestion broth composition on the permeability of M F and UF membranes. 124 Table 6.1. Selected results from previous studies on membrane fouling in water and wastewater treatment applications Purpose of treatment Type of membrane feed Filtration apparatus M W C O / Pore size, Da / urn Influential foulants Key fouling mechanism Reference Water Pre-treated surface water Cross-flow element < 200 Biofilm Biofouling (Speth etal., 1998) supply Surface water Cross-flow unit 200 - 10,000 Hydrophilic and hydrophobic natural organic mater (NOM) Adsorption (Cho etal., 1998) Anaerobic A S Stirred cell 0.1; 20,000 Fine colloids Adhesion and pore blocking (Choo and Lee, 1996) Conventional A S Cross-flow filtration unit 0.2 Settleable (>100 urn) (24%) Supracolloidal-colloidal (0.05-100 urn) (24%) Interaction between (Wisniewski and soluble products and Grasmick, 1998) membrane material Soluble (< 0.05 urn) (52%) A S from a S B R Stirred cell 0.1 Soluble microbial products (SMP) Cake formation (Shin and Kang, process 2003) Wastewater treatment Anaerobic A S Cross-flow filtration unit Extracellular polymeric substances (EPS) E P S deposition (Cho and Fane, 2003) Granular / bulking A S Hollow fibre module 0.1 Small particles Pore blocking and cake formation (Lim and Bai, 2003) Conventional/ pre-coagulated A S Flat plate membranes, dead-end test 0.1 S S (12-18%) Colloidal (18-63%) Soluble (25-64%) Irreversible fouling (Itonaga et al., 2004) A S from B P R processes Hollow fibre membrane loops 0.04 S S (4-7%) Unsettleable (>8.0 urn) (1-28%) Colloidal (0.45-8.0 urn) (30-32%) Soluble (<0.45 um) (33-65%) Irreversible fouling via adsorption and pore clogging The present research WWTP-effluent Stirred cell 200; 8,000; Effluent organic matter (e.g. humic fractions and Adsorption via electrostatic or (Jarusutthirak and Amy, 2001) Wastewater reclamation 10,000; 20,000 polysaccharides) hydrophobic interactions WWTP-effluent Capillary UF in 5 0 , 0 0 0 -dead-end 80,000 mode S S (0.45-450 um) (12%) Supra-dissolved (0.1-0.45 um) (60%) Dissolved (<0.1 um) (28%) Pore blocking & adsorption (Poele etal., 2004) Note: AS - activated sludge; BPR - biological phosphorus removal; SBR - sequencing batch reactor; SS - suspended solids in activated sludge mixed liquor; WWTP - wastewater treatment plant. They found that an M F membrane with a pore size of 0.1 urn exhibited minimal fouling potential and thus suggested that an optimal pore size existed due to the correlation between the sizes of membrane pores and broth constituents. Another study conducted by Jarusutthirak and Amy (2001) confirmed that M W C O of membranes was one of the significant factors influencing flux decline, organic matter rejection and fouling mechanisms. Based on the existing literature and the findings of the present research, a generic relationship between membrane pore size and the nature of membrane fouling in membrane coupled activated sludge processes is proposed and depicted in Figure 6.1. Irreversible fouling Fouling Mechanisms Reversible fouling Adsorption Pore blocking Sludge deposition Microbial growth Pore size, plm-(Log scale) 0.001 0.01 0.1 1.0 10 100 1000 Separation Processes NF M F Fine screen RO UF Fine particle filtration Sludge constituents Small M W Macro M W Colloids Soluble fraction Supra-colloids Sludge floes Insoluble fraction Figure 6.1. A generic relationship between the pore size of a membrane filter and the nature of fouling in membrane coupled activated sludge processes Among the potential fouling mechanisms in MBRs, adsorptive fouling dominates in NF and UF applications (Figure 6.1), probably because of the indispensable hydrophobic and electrostatic interactions between membranes and the constituents of the 126 membrane feed (Bellona et al, 2004). As the pore size increases (i.e. MF), pore blocking due to colloidal entrapment becomes important. When the membrane pore size is large enough, the role of fouling caused by adsorption decreases and pore blocking and sludge deposition become the prevailing fouling mechanisms successively. It should be noted that first, biofouling is possible in all the membrane filtration applications as long as a favorable environment exists for microbial growth. Second, sludge deposition can be considerably reduced or even eliminated under strong hydrodynamic conditions (i.e. vigorous aeration). As illustrated in Figure 6.1, the pore sizes or M W C O of membranes somewhat determine the influential foulants and prevailing fouling mechanisms. For nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) membranes, substances at the micron and sub-micron levels, i.e. natural organic matter (NOM) in surface water, dissolved organic matter (DOM) in wastewater, or soluble microbial products (SMP) or EPS in activated sludge mixed liquor, are the major substances that cause fouling via organic adsorption and/or pore blocking. Generally, the soluble fraction of activated sludge mixed liquor (<0.45 (im) is responsible for the irreversible fouling, while the insoluble fraction accounts for the reversible fouling. Since fouling is a very complex phenomenon, Figure 6.1 is obviously not a quantitative analysis but a qualitative description of the correspondence among the membrane pore size, fouling mechanisms, and sludge constituents at different sizes. Strategies for Prevention and Control of Membrane Fouling Prevention and control of membrane fouling is the ultimate goal of any fouling study. The engineering strategies for alleviation of membrane fouling in MBRs are mostly described in Chapter 1, which include influent pre-treatment, low-flux operation, periodic air or permeate backflushing, intermittent suction operation, high-shear slug flow aeration, addition of coagulants (i.e. alum) or absorbents (i.e. powered activated carbon), and improvement of module configuration (Chang et al, 2002; Fane et al, 2002; Holbrook et al, 2004; Berube and Lei, 2006). In addition to these common practices, the following measures are particularly recommended according to the results of the present research, in 127 order to improve the process design and operation and effectively address the membrane fouling issue. Application of Appropriate Aeration Intensity In addition to supplying oxygen for microbial metabolism, aeration was also used to minimize membrane fouling due to sludge deposition in submerged MBRs through an air scouring effect (Sofia et al, 2004). However, it was found in the present research work that the content of dissolved organic matter, such as soluble EPS, was the governing factor in membrane fouling. As the shear stress induced by aeration can break sludge floes and liberate colloidal particles and soluble organics from the floe matrix to the bulk liquor, aeration intensity should be limited to a level at which the sludge cake layer (reversible fouling) is effectively removed, but above which severe floe breakage (accordingly irreversible fouling) would occur and membrane performance would deteriorate sharply. From both the viewpoint of economics and the perspective of fouling control, an excess supply of air is not beneficial at all. Therefore, an appropriate or optimal aeration intensity should be sought during the operation of submerged MBRs. Optimization of the Biological Process Design As revealed in this research work, the physical and biochemical properties of activated sludge mixed liquor such as the content of soluble EPS and floe size distribution are decisive factors that govern the fouling propensity of sludge. Therefore, any engineering design approach that could either reduce the production of sludge EPS or improve the floe structure and rigidity should enhance fouling control in MBRs. According to the experience in operating the M E B P R process, it seemed that long HRT or low organic loading rate helped maintain long-term membrane filtration. Considering the effluent quality requirement, optimization of the process design and operation is definitely desired in order to minimize membrane fouling and, at the same time, not to compensate the biological treatment performance. Addition of Porous Materials According to the finding that large sludge floes were probably able to entrap and 128 absorb fine particles and soluble organic matter and thus facilitate the alleviation of membrane fouling, the addition to MBRs of material, which have the same porous structure as sludge floes and have stable physical and chemical properties, may be a wise option for fouling control. Lee and the co-workers (2001b) investigated the potential and limitation of addition of zeolite powder (>45 (im) to improve the performance of a submerged M B R . The results demonstrated that the previous loose structure of sludge became rigid and stable due to the attached microbial growth and the concentration of soluble organic substances in the mixed liquor was decreased to one third of the original, resulting in a remarkable improvement of membrane permeability. Evolution of Membrane Materials By nature, membrane fouling is determined by the interactions between the membrane and the liquid to be filtered. In activated sludge M B R systems, engineering options to mitigate fouling are usually very limited in effect, since they can only partially reduce the amount of sludge foulants or passively remove the sludge foulant layer. On the other hand, foulants are usually determined by the corresponding specific membrane material especially in the case of adsorptive fouling. If the membrane materials are completely altered to other types, the original foulants like soluble EPS may not foul the membrane any more. Therefore, membrane material is a root reason for fouling and development of new fouling-resistant membrane materials is probably a revolutionary measure for prevention of fouling. It has been reported that ceramic membranes can maintain high fluxes with almost no fouling (Judd, 2005). Research is being done on the modification of polymeric membrane surface by U V irradiation or Ti02 deposition/entrapment (Kim et al, 2002; Bae and Tak, 2005). It seemed that these modifications were simple and powerful for reduction of adsorptive fouling in activated sludge membrane filtration. 129 References Bae, T.-H. and Tak, T . -M. (2005). Effect of TiOa nanoparticles on fouling mitigation of ultrafiltration membranes for activated sludge filtration. Journal of Membrane Science, 249(1-2), 1-8. Bellona, C , Drewes, J. E. J. E., Xu , P. and Amy, G. (2004). Factors affecting the rejection of organic solutes during NF/RO treatment—a literature review. Water Research, 38(12), 2795-2809. Berube, P. R. and Lei, E. (2006). The effect of hydrodynamic conditions and system configurations on the permeate flux in a submerged hollow fiber membrane system. Journal of Membrane Science, 271(1-2), 29-31. Chang, I.-S., Le-Clech, P., Jefferson, B. and Judd, S. (2002). Membrane fouling in membrane bioreactors for wastewater treatment. Journal of Environmental Engineering, 128(11), 1018-1029. Chang, I.-S. and Lee, C.-H. (1998). Membrane filtration characteristics in membrane-coupled activated sludge system - the effect of physiological states of activated sludge on membrane fouling. Desalination, 120(3), 221-233. Cho, B.-D. and Fane, A. G. (2003). Fouling phenomena in a MBR: Transmembrane pressure transients and the role of EPS (extracellular polymeric substances). Water Science and Technology: Water Supply, 3(5-6), 261-266. Cho, J., Amy, G., Pellegrino, J. and Yoon, Y . (1998). Characterization of clean and natural organic matter (NOM) fouled NF and UF membranes, and foulants characterization. Desalination, 118(1-3), 101-108. Cho, J., Song, K. G., Yun, PL, Ahn, K. PL, Kim, J. Y . and Chung, T. H. (2005). Quantitative analysis of biological effect on membrane fouling in submerged membrane bioreactor. Water Science and Technology, 51(6-7), 9-18. Choo, K . -H . and Lee, C.-H. (1996). Effect of anaerobic digestion broth composition on membrane permeability. Water Science and Technology, 34(9), 173-179. Evenblij, H . and Graaf, J. H . J. M . v. d. (2004). Occurrence of EPS in activated sludge from a membrane bioreactor treating municipal wastewater. Water Science and Technology, 50(12), 293-300. Fane, A. G., Chang, S. and Chardon, E. (2002). Submerged hollow fibre membrane module - Design options and operational considerations. Desalination, 146(1-3), 231-236. Hoa, P. T., Nair, L . and Visvanathan, C. (2003). The effect of nutrients on extracellular polymeric substance production and its influence on sludge properties. Water SA, 29(4), 437-442. Holbrook, R. D., Higgins, M . J., Murthy, S. N . , Fonseca, A . D., Fleischer, E. J., Daigger, G. T., Grizzard, T. J., Love, N . G. and Novak, J. T. (2004). Effect of alum addition on the performance of submerged membranes for wastewater treatment. Water Environment Research, 76(7), 2699-2702. Hong, S. P., Bae, T. H. , Tak, T. M . , Hong, S. and Randall, A . (2002). Fouling control in activated sludge submerged hollow fiber membrane bioreactors. Desalination, 143(3), 219-228. 130 Itonaga, T., Kimura, K. and Watanabe, Y . (2004). Influence of suspension viscosity and colloidal particles on permeability of membrane used in membrane bioreactor (MBR). Water Science and Technology, 50(12), 301-309. Jarusutthirak, C. and Amy, G. (2001). Membrane filtration of wastewater effluents for reuse: Effluent organic matter rejection and fouling. 225-232. Judd, S. (2005). Fouling control in submerged membrane bioreactors. Water Science and Technology, 51(6-7), 27-34. Kim, D. S., Kang, J. S., Kim, K. Y . and Lee, Y . M . (2002). Surface modification of a p o l y v i n y l chloride) membrane by U V irradiation for reduction in sludge adsorption. Desalination, 146(1-3), 301-305. Laspidou, C. S. and Rittmann, B. E. (2002). 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Influence of bacterial extracellular polymers on the membrane separation activated sludge process. Water Science and Technology, 34(9), 165-172. Ng, H. Y . and Hermanowicz, S. W. (2005). Specific resistance to filtration of biomass from membrane bioreactor and activated sludge: Effects of exocellular polymeric substance and dispersed microorganisms. Water Environment Research, 77(2), 187-192. Poele, S. T., Roorda, J. H. and van der Graaf, J. (2004). Influence of the size of membrane foulants on the filterability of WWTP-effluent. Water Science and Technology, 50(12), 111-118. Rosenberger, S., Laabs, C , Lesjean, B., Gnirss, R., Amy, G., Jekel, M . and Schrotter, J.-C. (2006). Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment. Water Research, 40(4), 710-720. Sato, T. and Ishii, Y . (1991). Effects of activated sludge properties on water flux of ultrafiltration membrane used for human excrement treatment. Water Science and Technology, 23(7-9), 1601-1608. Shin, H.-S. and Kang, S.-T. (2003). Characteristics and fates of soluble microbial-products in ceramic membrane bioreactor at various sludge retention times. Water Research, 37(1), 121-127. Sofia, A. , Ng, W. J. and Ong, S. L . (2004). Engineering design approaches for minimum fouling in submerged M B R . Desalination, 160(1), 67-74. 131 Speth, T. F., Summers, R. S. and Gusses, A. M . (1998). Nanofiltration foulants from a treated surface water. Environmental Science and Technology, 32(22), 3612-3617. Wisniewski, C. and Grasmick, A. (1998). Floe size distribution in a membrane bioreactor and consequences for membrane fouling. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 138(2-3), 403-411. Yuan, W., Kocic, A . and Zydney, A. L. (2002). Analysis of humic acid fouling during microfiltration using a pore blockage-eake filtration model. Journal of Membrane Science, 198(1), 51-62. 132 Chapter 7. Conclusions, engineering significance and future work Conclusions Being an extremely complex phenomenon, membrane fouling is influenced by a multitude of factors and thus displays different faces in various circumstances. Despite these difficulties, effort has been made in this research to cover the key aspects of fouling in a membrane enhanced biological phosphorus removal (MEBPR) process, in order to systematically investigate the membrane fouling and to obtain a complete picture of the issue. The features of membrane fouling studied in the present research include membrane foulants or substances that cause fouling, membrane fouling mechanisms, roles of sludge constituents in membrane filtration, and sludge properties that are related to fouling. The latter three were examined through parallel experiments in comparison to a conventional enhanced biological phosphorus removal (CEBPR) process. The major conclusions drawn from each part of study are summarized as follows. (1) Under the experimental conditions examined, the M E B P R system appeared to undergo an irreversible fouling via organic adsorption and deep pore clogging. Due to the vigorous aeration and frequent backflushing applied, the reversible fouling caused by sludge deposition and superficial pore blocking was reduced to a minimum (Chapter 2). (2) The long-term irreversible fouling developed in the pilot scale MEBPR process followed a simple exponential relationship with the filtration time, the volume of permeate filtered, or the cumulative soluble organic loading on the membranes. In particular, the organic loading-based irreversible fouling coefficient was not affected by operating flux. The extent of fouling largely depended on the amount of soluble organic substances that had been carried onto the membranes by the liquid phase of activated sludge mixed liquor (Chapter 2). (3) The high content of dissolved organic carbon and the large amount of small supra-dissolved particles (0.1-0.45 Lim) in the soluble fraction of activated sludge mixed liquor made this fraction the primary contributor to the irreversible fouling in the 133 MEBPR system. Comparatively, the fouling induced by the particulate fraction (suspended solids) was insignificant (Chapter 3). (4) Large sludge floes could play a positive role in reducing membrane fouling when irreversible fouling is dominant, probably by sterically hindering the transport of other sludge constituents to membrane surfaces and entrapping or adsorbing colloidal particles and soluble organics onto their porous structures. This steric hindrance effect seemed to become more prominent at higher flux (Chapter 3). (5) Fouled membrane fibers in the M E B P R system were coated with a thin and uneven layer of organic foulants. Microbial attachment and growth were sparse on the membrane surfaces (Chapter 4). (6) Low molecular weight (MW) organic material such as some carbohydrates and humic or humic-like substances constituted the majority of the membrane foulants that could be extracted in a basic solution. High M W compounds like proteins were minor constituents in the foulant extract (Chapter 4). (7) Gradual accumulation of foulants on membrane surfaces increased the hydrophobicity of fouled membranes and consequently accelerated the fouling process (Chapter 4). (8) As for the sludge properties, neither the content of extracellular polymeric substances (EPS) bound in sludge floes nor the sludge surface characteristics, such as zeta potential and relative hydrophobicity, were related to the high fouling propensity of the M E B P R sludge. Only the content of soluble EPS (e.g. carbohydrates and humic or humic-like substances) in the water phase of sludge and the floe size distribution corresponded well to the fouling behavior of the sludge. Due to its profound influence on irreversible membrane fouling, the soluble EPS content, in particular, was suggested to be an important index for assessment of the fouling tendency of activated sludge mixed liquor (Chapter 5). It is evident that, although the above conclusions are drawn from the different parts of this research, they are very supportive and complementary to each other. For instance, the study of sludge properties (Chapter 5) revealed the essential role of soluble EPS in membrane fouling and is accordingly in good agreement with the study of fouling 134 mechanisms and the study of sludge constituents (Chapters 2 and 3), in which the soluble fraction of sludge, or, the soluble substances in mixed liquor, were found to contribute most to the irreversible membrane fouling. Moreover, the characterization of fouled membranes (Chapter 4) further indicated that carbohydrates and humic or humic-like substances were the two major EPS components that tended to accumulate on the membrane surfaces and caused the deterioration of filtration performance. This was explained indirectly in the study of sludge properties (Chapter 5), where the concentrations of soluble carbohydrates and humic or humic-like substances in the mixed liquor were found to be closely associated with the fouling propensity of sludge. Overall, the present research on membrane fouling in the MEBPR process was very consistent. Compared to most previous studies, in which fouling issues were investigated separately on various M B R systems and thus it was hard to generate a definite and/or complete picture of membrane fouling at one time due to the extreme complexity of fouling, this research focused on the specific MEBPR system and explored the nature of membrane fouling from four different perspectives at the same time, and thus a whole picture of the fouling phenomenon in the M E B P R process was obtained. By integrating the results of these four aspects of work, it can be concluded that the membranes in the MEBPR-type systems mainly suffer irreversible adsorptive fouling, which is induced by the soluble fraction of activated sludge mixed liquor, or more accurately, the soluble EPS components such as carbohydrates and humic or humic-like substances dissolved in the water phase of sludge. Lowering the content of soluble EPS in the mixed liquor is therefore a key step to control membrane fouling. It should be pointed out that some of the research hypotheses proposed at the beginning of the research were found to be incorrect. It has been known that, (1) although EPS played an important role in membrane fouling, biofouling in the form of microbial attachment and growth was not significant in the studied M E B P R process, due to frequent backflushing and intensive aeration applied; (2) inorganic precipitation did not occur due to the bio-P removal mechanism and the strong oxidation conditions in the aerobic zone of the M E B P R process. On the other hand, it was proven that design and operation of the biological process, i.e. decrease of hydraulic retention time (HRT), affected the sludge 135 properties and thus influenced the membrane fouling process. So, it is feasible to improve the membrane performance through optimization of the biological treatment process. Engineering Significance of the Research As stated in the introductory chapter, the integration of membrane filtration with biological phosphorus removal processes, i.e. the present MEBPR process, is a new research initiative in the field of M B R application. To date, only two other groups have conducted research on the MEBPR-like processes (Adam et al, 2003; Lesjean et al, 2003; Ramphao et al, 2005), and very limited information has been published on the membrane fouling of this type of M B R (Lesjean et al, 2005; Rosenberger et al, 2006). Thus, the present research work took the lead in investigating the membrane fouling in the MEBPR process. It was for the first time that the fouling mechanisms in the MEBPR-type processes were systematically studied, the fouled membranes and foulants were characterized, and the effect of sludge properties on the membrane filtration performance was examined. Undoubtedly, the results of this research provide valuable insight into the nature of the membrane fouling phenomenon and lay a solid foundation for further exploration of the MEBPR process in order to achieve a better combination of the biological phosphorus removal and membrane filtration of sludge. Based on the research findings of the present work, a number of strategies for prevention and control of membrane fouling are proposed in the preceding chapter. Although the feasibility of these strategies needs to be studied, they are undoubtedly helpful in exploring practical measures for fouling control in the MEBPR-like processes. As the expense incurred due to fouling accounts for a fairly large portion of the total cost of MBRs (Gander et al, 2000; Yoon et al, 2004), the results of this fouling research will definitely contribute to the effort of minimizing M B R cost and promote the widespread applications of membrane technologies in the wastewater industry. In this research work, a number of new techniques were developed for membrane fouling study, including the use of the M A L D I - M S technique in foulant characterization, measurement of fine particle size distribution of the soluble fraction of sludge using Malvern MasterSizer 2000, modified EPS extraction procedure, quantitative analysis of membrane foulants, application of the concept of fouling rate, etc. 136 Future Work Although the present research addressed four different aspects of membrane fouling, it is the first attempt to systematically investigate the fouling issue in the M E B P R process and thus there is still research that needs to be carried on in the future. In particular, the following work is suggested in order to fill research gaps and gain more in-depth understanding of membrane fouling in the MEBPR-like processes. Effect of SRT on Membrane Fouling As stated earlier, two experimental runs were carried out in this research with a constant solids retention time (SRT) and varied hydraulic retention time (HRT). In addition to these two runs, a third experimental run was also carried out at the U B C Wastewater Treatment Pilot Plant with a longer SRT (20 days). The membrane filtration performance during this run is compared to the performance in the first two runs and the result is summarized in Table 7.1. The effect of SRT on membrane fouling can thus be roughly observed from the apparent membrane performance. It is evident that extending the SRT from 12 days to 20 days alleviated membrane fouling and resulted in an increase in membrane run time from 46-51 days to 68-73 days. Based on the results of this research, it is assumed that this improvement is very likely because of the decreased concentration of soluble EPS in the water phase of sludge and/or the enhanced steric hindrance effect of sludge floes, since the concentration of MLSS increases with SRT. Although a similar effect of SRT on membrane performance was reported for other types of MBRs (Chang and Lee, 1998; Fan et al, 2000; Cho et al, 2005), no EPS data were collected during the third experimental run and the above postulation is short of direct data Table 7.1. Run time of the major membrane module in different experimental runs I II III Experimental run SRT=12 days HRT=10 hours SRT=12 days HRT=7 hours SRT=20 days HRT=7 hours Filtration run time, days 8 4 - 8 5 46 - 51 68 - 73 Number of filtration runs 2 2 2 137 support. Therefore, a study of the effect of SRT on membrane fouling is quite necessary in order to optimize the design and operation of the MEBPR process. Removal of Foam Layer in the Anoxic Zone It was noticed that in the anoxic zone of the M E B P R process, residual air bubbles from the sludge recycle line were gradually lifted up and accumulated on the top of the mixed liquor, forming a thick layer of foam. The current practice was to disperse this foam layer back into the bulk liquor by pressurized water. As the air bubbles often carry fine particles and soluble organics when they rise, removal of this foam layer will probably reduce the contents of colloidal and soluble matters in the aerobic zone, where the membrane module is installed, and thus benefit the fouling alleviation. In this regard, it may be worthwhile to conduct a comparative study on membrane fouling before and after the removal of the foam layer. Effect of Aerobic vs. Anaerobic Conditions on Membrane Fouling It has been reported that the EPS production of sludge was lower in anaerobic than in aerobic processes (Morgan et al, 1990; Hoa et al, 2003). Since EPS, especially the soluble EPS, is the most important membrane foulant in the MEBPR process, it is speculated that displacement of the major membrane module in the anoxic zone may be able to improve the control of fouling (Figure 7.1). This measure can also be combined influent Anaerobic :ii Aerobic Anoxic Permeate • Sludge Figure 7.1. The modified MEBPR process with the membrane module submerged in the anoxic zone (with reference to Lesjean et al. (2005)) 138 with the removal of the foam layer in the anoxic zone so as to work in synergy. As for the treatment performance, Lesjean et al. (2003) investigated two types of MBRs similar to the above process and found that efficient Bio-P removal could be achieved in both of the reactors. To test the above speculation, a study should be carried out in the future to compare the treatment performance and the fouling control efficiency between the current MEBPR process with the membrane, module installed in the aerobic zone and the modified M E B P R process with the membrane module installed in the anoxic zone. Optimization of Aeration Intensity As discussed earlier, aeration in MBRs could neither be too strong nor too weak in order to maintain microbial growth, scour off sludge deposition, and minimize membrane adsorptive fouling. However, a detailed investigation of the relationship among these three aspects is absent in the publication. It is thus necessary to initiate such a study to see if an optimal aeration intensity exists and to find out more favourable hydrodynamic conditions for operation of activated sludge MBRs. References Adam, C , Kraume, M . , Gnirss, R. and Lesjean, B. (2003). Membrane bioreactor configurations for enhanced biological phosphorus removal. Water Science and Technology: Water Supply, 3(5-6), 237-244. Chang, I.-S. and Lee, C.-H. (1998). Membrane filtration characteristics in membrane-coupled activated sludge system - the effect of physiological states of activated sludge on membrane fouling. Desalination, 120(3), 221-233. Cho, J., Song, K. G., Yun, H. , Ahn, K. H. , Kim, J. Y . and Chung, T. H. (2005). Quantitative analysis of biological effect on membrane fouling in submerged membrane bioreactor. Water Science and Technology, 51(6-7), 9-18. Fan, X. - j . , Urbain, V. , Qian, Y . and Manem, J. (2000). Ultrafiltration of activated sludge with ceramic membranes in a cross-flow membrane bioreactor process. Water Science and Technology, 41(10), 243-250. Gander, M . , Jefferson, B. and Judd, S. (2000). Aerobic MBRs for domestic wastewater treatment: a review with cost considerations. Separation and Purification Technology, 18(2), 119-130. Hoa, P. T., Nair, L . and Visvanathan, C. (2003). The effect of nutrients on extracellular polymeric substance production and its influence on sludge properties. Water SA, 29(4), 437-442. 139 Lesjean, B., Luck, F., Gnirss, R., Adam, C. and Kraume, M . (2003). Enhanced biological phosphorus removal process implemented in membrane bioreactors to improve phosphorous recovery and recycling. Water Science and Technology, 48(1), 87-94. Lesjean, B., Rosenberger, S., Laabs, C , Jekel, M . , Gnirss, R. and Amy, G. (2005). Correlation between membrane fouling and soluble/colloidal organic substances in membrane bioreactors for municipal wastewater treatment. Water Science and Technology, 51(6-7), 1-8. Morgan, J. W., Forster, C. F. and Evison, L . (1990). Comparative study of the nature of biopolymers extracted from anaerobic and activated sludges. Water Research, 24(6), 743-750. Ramphao, M . , Wentzel, M . C , Ekama, G. A. , Merritt, R., Young, T. and Buckley, C A . (2005). Impact of membrane solid-liquid separation on design of biological nutrient removal activated sludge systems. Biotechnology and Bioengineering, 89(6), 630-646. Rosenberger, S., Laabs, C , Lesjean, B. , Gnirss, R., Amy, G., Jekel, M . and Schrotter, J.-C. (2006). Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment. Water Research, 40(4), 710-720. Yoon, S.-PL, Kim, H.-S. and Yeom, I.-T. (2004). The optimum operational condition of membrane bioreactor (MBR): Cost estimation of aeration and sludge treatment. Water Research, 38(1), 37-46. 140 Appendices 141 Appendix 1. The UBC Wastewater Treatment Pilot Plant Photo a: the trailer in which the M E B P R and CEBPR systems are accommodated in parallel Photo b: the inside of the trailer 142 Appendix 2. Clean and Fouled Membrane Modules at the UBC Pilot Plant 143 Appendix 3. The On-line and Off-line Filtration Apparatus Photo a: The on-line membrane filtration apparatus (not including controls) Photo b: The off-line membrane filtration apparatus J Appendix 4. Determination of the IC 5 0 Value of CuS04-5H20 With Respect to the MEBPR Sludge bo o • 0 mg/L CuS04.5H20 • 50 mg/L CuS04.5H20 A 1000 mg/L CuS04.5H20 x 2000 mg/L CuS04.5H20 Figure A4.1 Inhibition of Cu 2 + on the microbial activity of the M E B P R sludge (6.9 g/L MLSS) Table A4.1 Respiration rate of the M E B P R sludge at different concentrations of CuSQ 4 -5H 2 0 Concentration of CuSCV5H 2 0, mg/L 0 50 1000 2000 Respiration rate, mg 0 2/L-min. 0.39 0.40 0.20 0.22 Therefore, the 50% inhibitory concentration (IC5o) of CuS0 4 -5H 2 0 with respect to the M E B P R sludge at a MLSS of 6.9 g/L is about 1000 mg/L. 145 Appendix 5. Filtration Performance of the MEBPR Membrane Module During the Entire Experimental Period at the UBC Pilot Plant Table A5.1 Filtration performance of the MEBPR membrane module during the experimental period Date TMP, K P a Date TMP, K P a Suction Backflush Suction Backflush 17-Apr-03 -20.7 10.3 09-Jul-03 -21.0 7.6 18-Apr-03 -19.7 11.0 11-Jul-03 -22.8 6.2 21-Apr-03 -20.7 12.0 14-Jul-03 -22.4 7.6 27-Apr-03 -21.0 13.8 17-Jul-03 -22.4 7.6 05-May-03 -20.7 15.2 21-Jul-03 -25.5 4.8 08-May-03 -21.4 15.2 23-Jul-03 -24.5 4.1 13-May-03 -21.7 15.2 25-Jul-03 -24.5 4.8 14-May-03 -23.1 14.5 28-Jul-03 -25.5 5.5 15-May-03 -22.8 15.9 29-Jul-03 -26.9 4.1 16-May-03 -22.4 17.3 31-Jul-03 -27.9 4.1 20-May-03 -23.8 17.3 01-Aug-03 -27.6 4.8 21-May-03 -24.8 17.3 04-Aug-03 -26.9 4.1 22-May-03 -25.5 16.6 05-Aug-03 -27.6 6.2 23-May-03 -25.9 16.6 06-Aug-03 -27.9 6.9 25-May-03 -27.3 17.9 07-Aug-03 -27.6 8.3 28-May-03 -27.3 20.0 12-Aug-03 -29.3 8.3 29-May-03 -28.6 19.3 13-Aug-03 -30.7 9.0 30-May-03 -29.3 20.0 17-Aug-03 -32.1 8.3 31-May-03 -30.7 21.4 18-Aug-03 -33.1 8.3 02-Jun-03 -31.7 24.2 20-Aug-03 -33.1 10.4 04-Jun-03 -33.5 24.8 22-Aug-03 -34.8 10.4 05-Jun-03 -35.5 23.5 25-Aug-03 -36.2 11.7 11-Jun-03 -40.4 29.7 28-Aug-03 -38.3 12.4 12-Jun-03 -41.4 29.7 04-Sep-03 -41.1 13.8 13-Jun-03 -42.4 30.4 05-Sep-03 -42.4 15.2 18-Jun-03 -47.3 32.4 08-Sep-03 -43.8 20.7 19-Jun-03 -48.3 33.8 09-Sep-03 -45.2 20.7 23-Jun-03 -51.4 37.3 10-Sep-03 -44.5 22.8 24-Jun-03 -53.5 39.3 11-Sep-03 -44.5 24.2 25-Jun-03 -54.5 39.3 15-Sep-03 -45.2 26.9 26-Jun-03 -55.5 38.6 16-Sep-03 -45.9 27.6 27-Jun-03 -56.9 37.3 17-Sep-03 -45.5 30.4 30-Jun-03 -60.4 40.0 18-Sep-03 -46.2 29.7 02-Jul-03 -61.8 43.5 19-Sep-03 -47.6 29.7 03-Jul-03 -62.4 44.2 22-Sep-03 -50.0 33.1 04-Jul-03 -65.6 47.6 23-Sep-03 -50.7 33.1 07-Jul-03 -74.2 49.7 24-Sep-03 -50.0 33.1 08-Jul-03 -75.9 52.4 26-Sep-03 -54.9 40.0 End of the 1st filtration run 30-Sep-03 -63.5 44.9 146 (Conf) Date T M P , K P a Date T M P , KPa Suction Backflush Suction Backflush 01-Oct-03 -64.5 44.9 02-Dec-03 -27.6 24.8 End of the 2nd filtration run 03-Dec-03 -26.6 27.6 02-Oct-03 -18.6 4.5 05-Dec-03 -30.0 26.2 03-Oct-03 -18.3 4.6 08-Dec-03 -30.0 29.0 06-Oct-03 -19.3 4.8 End of Run I and Start of Run II I 07-Oct-03 -19.3 5.0 09-Dec-03 -45.5 41.4 08-Oct-03 -19.3 5.0 10-Dec-03 -47.3 42.1 09-Oct-03 -17.9 5.5 11-Dec-03 -48.6 42.1 10-Oct-03 -17.3 6.2 12-Dec-03 -50.0 43.5 14-Oct-03 -17.3 7.6 14-Dec-03 -56.9 47.6 15-Oct-03 -17.6 8.3 15-Dec-03 -56.9 50.4 16-Oct-03 -18.3 8.3 16-Dec-03 -57.6 51.1 17-Oct-03 -20.0 8.3 17-Dec-03 -57.6 54.5 20-Oct-03 -20.7 8.3 18-Dec-03 -62.1 55.9 21-Oct-03 -21.0 8.3 End of the 3rd filtration run 22-Oct-03 -21.7 8.3 18-Dec-03 -20.7 24.2 23-Oct-03 -18.6 11.7 19-Dec-03 -31.7 25.5 24-Oct-03 -17.9 13.1 27-Dec-03 -41.4 28.3 27-Oct-03 -20.4 12.4 02-Jan-04 -48.3 40.2 28-Oct-03 -22.4 10.4 05-Jan-04 -55.2 41.4 29-Oct-03 -20.7 13.1 06-Jan-04 -61.1 38.0 30-Oct-03 -18.6 15.9 12-Jan-04 -57.6 48.8 31-Oct-03 -17.9 18.6 13-Jan-04 -58.0 54.5 03-Nov-03 -21.0 15.2 15-Jan-04 -64.2 55.2 04-Nov-03 -20.4 16.6 16-Jan-04 -66.2 56.6 05-Nov-03 -20.0 17.3 19-Jan-04 -66.2 59.3 06-Nov-03 -20.7 17.3 End of the 4th filtration run 07-Nov-03 -21.4 17.9 21-Jan-04 -20.0 20.0 08-Nov-03 -22.4 15.2 22-Jan-04 -22.4 20.0 10-Nov-03 -23.5 15.2 23-Jan-04 -23.1 19.3 12-Nov-03 -22.1 16.6 26-Jan-04 -23.5 20.7 13-Nov-03 -23.1 16.6 27-Jan-04 -23.5 20.7 17-Nbv-03 -24.5 16.6 28-Jan-04 -24.2 20.7 18-Nov-03 -23.5 17.9 29-Jan-04 -25.2 19.3 19-Nov-03 -22.1 20.7 30-Jan-04 -25.2 21.4 20-Nov-03 -21.7 21.4 03-Feb-04 -26.2 24.2 * 21-Nov-03 -21.7 22.1 04-Feb-04 -25.9 25.5 24-Nov-03 -24.2 21.4 05-Feb-04 -25.9 26.9 25-Nov-03 -25.5 20.7 06-Feb-04 -27.9 27.6 26-Nov-03 -24.2 22.8 09-Feb-04 -29.7 30.4 27-Nov-03 -24.5 21.4 10-Feb-04 -31.1 31.1 28-Nov-03 -25.9 22.1 11-Feb-04 -32.1 30.4 01-Dec-03 -26.2 25.5 12-Feb-04 -33.5 30.4 147 (Conf) Date TMP, KPa Date T M P , KPa Suction Backflush Suction Backflush 13-Feb-04 -34.5 31.1 22-Mar-04 -26.6 22.1 16-Feb-04 -37.3 33.1 24-Mar-04 -27.3 24.2 17-Feb-04 -37.6 34.5 25-Mar-04 -28.3 24.2 18-Feb-04 -38.0 35.2 26-Mar-04 -28.3 25.5 19-Feb-04 -40.0 38.0 29-Mar-04 -29.7 25.5 20-Feb-04 -41.7 38.0 30-Mar-04 -31.4 26.2 23-Feb-04 -45.9 40.7 31-Mar-04 -37.3 26.9 24-Feb-04 -48.3 42.1 06-Apr-04 -40.4 33.1 25-Feb-04 -49.7 44.2 . 09-Apr-04 -42.4 35.9 26-Feb-04 -50.7 46.2 15-Apr-04 -47.6 41.4 28-Feb-04 -56.0 54.0 19-Apr-04 -51.4 45.5 01-Mar-04 -61.1 58.7 21-Apr-04 -53.5 51.1 02-Mar-04 -62.4 62.8 23-Apr-04 -55.2 52.4 03-Mar-04 -63.8 65.6 26-Apr-04 -66.9 69.0 04-Mar-04 -65.2 68.3 End of the 6th filtration run 05-Mar-04 -68.0 73.1 27-Apr-04 -20.7 22.1 06-Mar-04 -70.7 74.5 28-Apr-04 -21.7 23.5 End of the 5th filtration run 03-May-04 -23.0 23.0 07-Mar-04 -15.9 20.0 08-May-04 -26.0 24.5 09-Mar-04 -18.6 20.0 12-May-04 -32.0 28.0 10-Mar-04 -18.3 21.4 18-May-04 -36.3 34.2 11 -Mar-04 -23.1 22.1 21-May-04 -38.0 36.5 12-Mar-04 -21.7 21.4 23-May-04 -40.0 38.0 15-Mar-04 -21.7 22.1 26-May-04 -44.9 40.7 16-Mar-04 -22.8 22.1 27-May-04 -46.2 44.9 17-Mar-04 -24.2 22.8 02-Jun-04 -48.3 48.3 18-Mar-04 -26.2 22.1 07-Jun-04 -59.0 59.0 19-Mar-04 -24.2 24.2 End of the 7th filtration run 148 Appendix 6. Data Summary for Fouled Membranes Filtering Clean Water in Comparison with Virgin Membranes Table A6 .1 Filtration test of clean water using fouled membranes Time, min. T M P , K P a Note Time, min. T M P , K P a Note 0.0 -1.4 34.0 87.4 0.2 -11.2 34.5 87.5 Backflush 0.3 -20.9 35.0 87.8 0.5 -26.8 37.0 -0.3 0.7 -28.6 38.0 -0.6 Relaxation 0.8 -33.5 40.0 -1.0 1.0 -35.7 40.3 -35.3 1.3 -38.9 40.5 -40.4 1.5 -40.7 41.0 -47.7 1.8 -42.6 41.5 -51.3 2.0 -44.4 42.0 -54.9 2.5 -47.5 42.5 -56.0 3.0 -49.4 43.0 -57.5 3.5 -51.0 Suction 43.5 -57.9 Suction 4.0 -52.6 44.0 -58.5 4.5 -53.5 44.5 -58.6 5.0 -54.4 45.0 -58.9 5.5 -55.1 45.5 -58.9 6.0 -55.3 46.0 -59.0 6.5 -55.7 47.0 -59.1 7.0 -56.0 50.0 -58.9 7.5 -56.3 51.0 -6.9 8.0 -56.6 55.0 -1.4 Relaxation 8.5 -56.7 55.3 41.2 9.0 -56.7 55.5 89.1 9.5 -56.8 57.0 64.8 Backflush 10.0 -56.9 59.0 64.4 12.0 -57.1 60.0 65.6 15.0 -56.9 61.0 24.2 20.0 -57.1 65.0 15.4 Relaxation 22.0 -8.3 70.0 11.9 27.0 -1.7 Relaxation 70.5 -2.3 30.0 -1.7 71.0 -12.1 30.2 23.0 71.5 -21.5 30.5 74.1 72.0 -27.7 31.0 91.2 72.5 -33.2 Suction 31.5 87.6 Backflush 73.0 -38.2 32.0 86.3 73.5 -42.6 32.5 86.7 74.0 -46.1 33.0 86.3 74.5 -49.1 33.5 86.8 75.0 -52.1 149 (Conf) Time, min. TMP, K P a Note Time, min. T M P , K P a Note 75.5 -54.9 77.5 -60.1 76.0 -56.8 Suction 78.0 -60.9 Suction 76.5 -58.0 79.0 -61.3 77.0 -59.0 80.0 -61.9 Table A6.2 Filtration test of clean water using virgin membranes Time, min. TMP, KPa Note Time, min. T M P , KPa Note 0.0 -1.2 40.5 -7.2 0.2 -6.6 41.0 -6.9 0.3 -7.0 41.5 -6.7 0.5 -6.4 42.0 -6.5 0.7 -6.7 42.5 -6.6 0.8 -6.1 43.0 -6.6 1.0 -6.3 43.5 -6.7 1.3 -6.3 44.0 -6.8 1.5 -6.2 Suction 45.0 -6.6 1.8 -6.3 46.0 -6.8 2.0 -6.1 50.0 -6.6 2.5 -6.1 51.0 -1.6 3.0 -6.1 55.0 -1.2 Relaxation 5.0 -6.3 55.3 4.8 10.0 -6.2 55.5 34.1 15.0 -6.4 57.0 33.5 Backflush 20.0 -6.3 59.0 33.3 22.0 -2.8 60.0 33.1 27.0 -1.7 Relaxation 61.0 10.4 30.0 -1.3 65.0 6.2 Relaxation 30.2 6.6 70.0 0.6 30.5 34.1 70.5 -7.6 31.0 33.1 71.0 -6.8 31.5 32.8 71.5 -6.9 32.0 32.4 72.0 -6.8 32.5 32.6 73.0 -6.7 Suction 33.0 32.6 74.0 -6.6 34.0 33.0 75.0 -6.8 35.0 33.1 76.0 -6.6 37.0 3.2 Relaxation 78.0 -6.7 40.0 2.3 80.0 -6.6 40.3 -7.7 Note: a. The above filtration tests were carried out on April 26, 2004. b. Permeate flux : 33 L/m .h Aeration rate: 15 m /m .h 150 Appendix 7. Data Summary of Development and Breakdown of the Transmembrane Pressure of the Off-line Membrane Loops Filtering the Aerobic MEBPR Sludge Table A7.1 Development and breakdown of the transmembrane pressure of the off-line membrane loops filtering the aerobic membrane sludge Time, min. T M P , KPa Note 0 -1.24 0.5 -6.00 1 -6.14 2 -6.21 3 -6.28 5 -6.21 7 -6.42 10 -6.42 Suction in sludge 15 -6.56 mixed liquor 20 -6.83 25 -6.83 30 -6.97 35 -6.83 40 -6.97 70 -7.38 100 -7.73 130 -7.87 150 -8.00 151 -6.69 152 -6.69 155 -6.76 Suction in DDW 157 -6.62 160 -6.76 165 -6.62 168 -6.00 170 -6.21 Suction in DDW 180 -6.14 after backflushing 185 -6.14 Note: a. The above filtration test was performed on June 16, 2004. b. Concentration of the aerobic M E B P R mixed liquor: 5.7 g / L (MLSS) 2 3 3 c. Permeate flux: 33 L/m -h Slight aeration: ~3 m /m .h 151 Appendix 8. Data Summary of Sludge Filtration Tests With /Without Cu 2 + Inhibition Table A8.1 Sludge filtration tests with/without Cu inhibition Trans-membrane pressure (TMP), KPa w/o C u S 0 4 whole sludge w/ C u S 0 4 whole sludge J=46 L/m 2.h J=68 L/m 2.h J=46 L/m 2.h J=68 L/m 2.h 0 2.62 2.76 2.76 2.69 0.17 5.52 7.25 5.24 8.21 0.33 6.14 8.35 6.14 10.07 0.5 6.35 8.69 6.35 10.56 1 6.62 8.97 6.62 11.45 1.5 6.69 9.11 6.83 12.14 2 6.76 9.18 7.04 12.77 5 6.90 9.38 8.00 16.22 10 7.04 9.87 10.97 24.08 15 7.18 10.28 12.83 37.67 20 7.31 10.70 15.46 50.30 25 7.45 11.11 18.15 65.41 30 7.59 11.52 21.39 -Fouling rate, KPa/min. 0.07 0.14 0.52 2.18 Note: a. The above filtration tests were carried out during July 21-23, 2004. b. Aeration rate: 60 m 3/m 3.h 152 Appendix 9. Data Summary for Modeling the Long-term Fouling in the MEBPR Process Table A9.1 Raw data for modeling the long-term fouling in the M E B P R process Time Accumulative Accumulative T M P Time Accumulative Accumulative T M P volume organic loading volume organic loading day m 3 kg soluble C O D K P a day m 3 kg soluble C O D KPa The 1st filtration run (April 17 - July 8, 2003) The 2nd filtration run (July 9 - October 1, 2003) 0 0 0.0 20.7 0 0 0.0 21.0 1 7 0.2 19.7 2 13 0.3 22.8 4 26 0.7 20.7 5 33 0.8 22.4 10 66 1.7 21.0 8 53 1.4 22.4 18 118 3.1 20.7 12 79 2.0 25.5 21 138 3.6 21.4 14 92 2.4 24.5 26 171 4.4 21.7 16 105 2.7 24.5 27 177 4.6 23.1 19 125 3.2 25.5 28 184 4.7 22.8 20 131 3.4 26.9 29 190 4.9 22.4 22 145 3.7 27.9 33 217 5.6 23.8 23 151 3.9 27.6 34 223 5.8 24.8 26 171 4.4 26.9 35 230 5.9 25.5 27 177 4.6 27.6 36 236 6.1 25.9 28 184 4.7 27.9 38 250 6.4 27.3 29 190 4.9 27.6 41 269 6.9 27.3 34 223 5.8 29.3 42 276 7.1 28.6 35 230 5.9 30.7 43 282 7.3 29.3 39 256 6.6 32.1 44 289 7.5 30.7 40 263 6.8 33.1 46 302 7.8 31.7 42 276 7.1 33.1 48 315 8.1 33.5 44 289 7.5 34.8 49 322 8.3 35.5 47 309 8.0 36.2 55 361 9.3 40.4 50 328 8.5 38.3 56 368 9.5 41.4 57 374 9.7 41.1 57 374 9.7 42.4 58 381 9.8 42.4 62 407 10.5 47.3 61 401 10.3 43.8 63 414 10.7 48.3 62 407 10.5 45.2 67 440 11.4 51.4 63 414 10.7 44.5 68 447 11.5 53.5 64 420 10.8 44.5 69 453 11.7 54.5 68 447 11.5 45.2 70 460 11.9 55.5 69 453 11.7 45.9 71 466 12.0 56.9 70 460 11.9 45.5 74 486 12.5 60.4 71 466 12.0 46.2 76 499 12.9 61.8 72 473 12.2 47.6 77 506 13.0 62.4 75 493 12.7 50.0 78 512 13.2 65.6 76 499 12.9 50.7 81 532 13.7 74.2 77 506 13.0 50.0 82 539 13.9 75.9 79 519 13.4 54.9 83 545 14.1 63.5 84 552 14.2 64.5 153 (Conf) Time Accumulative Accumulative T M P Time Accumulative Accumulative T M P volume organic loading volume organic loading day m 3 kg soluble C O D KPa day m 3 kg soluble C O D K P a The 5th filtration run (January 21 - March 6, 2004) The 6th filtration run (March 7 - April 26, 2004) 0 0 0.0 20.0 0 0 0.0 15.9 1 9 0.3 22.4 2 19 0.6 18.6 2 19 0.6 23.1 3 28 0.9 18.3 5 47 1.5 23.5 4 38 1.2 23.1 6 57 1.8 23.5 5 47 1.5 21.7 7 66 2.1 24.2 8 75 2.4 21.7 8 75 2.4 25.2 9 85 2.7 22.8 9 85 2.7 25.2 10 94 3.0 24.2 13 123 3.8 26.2 11 104 3.3 26.2 14 132 4.1 25.9 12 113 3.6 24.2 15 141 4.4 25.9 15 141 4.4 26.6 16 151 4.7 27.9 17 160 5.0 27.3 19 179 5.6 29.7 18 170 5.3 28.3 20 188 5.9 31.1 19 179 5.6 28.3 21 198 6.2 32.1 22 207 6.5 29.7 22 207 6.5 33.5 23 217 6.8 31.4 23 217 6.8 34.5 24 226 7.1 37.3 26 245 7.7 37.3 30 283 8.9 40.4 27 254 8.0 37.6 33 311 9.8 42.4 28 264 8.3 38.0 39 368 11.5 47.6 29 273 8.6 40.0 43 405 12.7 51.4 30 283 8.9 41.7 45 424 13.3 53.5 33 311 9.8 45.9 47 443 13.9 55.2 34 320 10.1 48.3 50 471 14.8 66.9 35 330 10.4 49.7 36 339 10.7 50.7 38 358 11.2 56.0 40 377 ,11.8 61.1 41 386 12.1 62.4 42 396 12.4 63.8 43 405 12.7 65.2 44 415 13.0 68.0 45 424 13.3 70.7 154 Appendix 10. Sample Preparation for GPC Analysis Procedure: (1) Exactly 50.0 mL of the soluble fraction of aerobic MEBPR or CEBPR sludge was collected, by filtering fresh activated sludge using 0.45 um membrane filter. (2) At the same time, exactly 50.0 mL of membrane permeate was collected from the U B C pilot plant. (3) The two types of soluble fractions of sludge and the membrane permeate were poured into three 1000 mL beakers, respectively, and freeze-dried for 2-3 days. (4) About 1.0-1.5 mL of Tetrahydroftiran (THF) was added to each beaker containing dried solids and shaken for 5 minutes. As THF was very volatile, a portion of THF was lost and the resulted THF solutions were collected in three 1.5 mL glass vials. (5) The three vials were then submitted to a lab of Department of Forestry, UBC, for GPC analysis. 155 Appendix 11. Data Summary of the Filtration Tests With Four Sludge Fractions Table A 11.1 Filtration tests with the four fractions of both the aerobic MEBPR sludge and the aerobic CEBPR sludge at the flux of 23 L/m 2.h Type Elapsed Whole sludge Supernatant Filtrate I Filtrate II of time TMPcw TMP Rm Rf TMPcw TMP Rm Rf TMPcw TMP Rm Rf TMPcw TMP Rm Rf sludge min. psi psi -1 m m psi psi m"1 m 1 psi psi m"1 -1 m psi psi m~1 m"' 0 0 0 0 0 0 0 0 0 0.25 -0.55 -7.53E+11 -0.61 -2.61 E+11 -0.42 -2.93E+11 -0.49 -4.89E+11 0.5 -0.82 -4.59E+11 -0.79 -6.52E+10 -0.56 -1.41E+11 -0.68 -2.82E+11 1 -1.07 -1.88E+11 -0.85 0.00E+00 -0.61 -8.69E+10 -0.77 -1.85E+11 MEBPR 1.5 -1.15 -1.01E+11 -0.90 5.43E+10 -0.62 -7.60E+10 -0.84 -1.09E+11 sludge 2 -1.24 -1.23 1.35E+12 -1.42E+10 -0.85 -0.90 9.23E+11 5.43E+10 -0.69 -0.63 7.49E+11 -6.52E+10 -0.94 -0.90 1.02E+12 -4.34E+10 5 -1.22 -2.51 E+10 -1.00 1.63E+11 -0.69 0.00E+00 -0.94 0.00E+00 10 -1.34 1.05E+11 -1.15 3.26E+11 -0.76 7.60E+10 -0.98 4.34E+10 15 -1.47 2.46E+11 -1.25 4.34E+11 -0.83 1.52E+11 -1.03 9.77E+10 20 -1.57 3.55E+11 -1.34 5.32E+11 -0.89 2.17E+11 -1.05 1.19E+11 25 -1.63 4.20E+11 -1.43 6.30E+11 -0.96 2.93E+11 -1.09 1.63E+11 30 -1.66 4.53E+11 -1.50 7.06E+11 -1.04 3.80E+11 -1.12 1.95E+11 0 0 0 0 0 0 0 0 0 0.25 -1.64 -2.72E+11 -1.11 -1.30E+11 -1.16 -2.75E+11 -0.70 -7.60E+10 0.5 -2.13 -9.41E+10 -1.33 -5.07E+10 -1.61 -1.12E+11 -0.90 -3.62E+09 1 -2.31 -2.90E+10 -1.37 -3.62E+10 -1.80 -4.34E+10 -0.90 -3.62E+09 1.5 -2.32 -2.53E+10 -1.40 -2.53E+10 -1.82 -3.62E+10 -0.92 3.62E+09 CEBPR 2 -2.39 -2.35 8.65E+11 -1.45E+10 -1.47 -1.42 5.32E+11 -1.81E+10 -1.92 -1.85 6.95E+11 -2.53E+10 -0.91 -0.89 3.29E+11 -7.24E+09 sludge 5 -2.41 7.24E+09 -1.47 0.00E+00 -1.92 0.00E+00 -0.91 0.00E+00 10 -2.47 2.90E+10 -1.58 3.98E+10 -1.98 2.17E+10 -0.99 2.90E+10 15 -2.56 6.15E+10 -1.67 7.24E+10 -2.07 5.43E+10 -1.06 5.43E+10 20 -2.66 9.77E+10 -1.73 9.41 E+10 -2.15 8.33E+10 -1.11 7.24E+10 25 -2.72 1.19E+11 -1.79 1.16E+11 -2.22 1.09E+11 -1.14 8.33E+10 30 -2.81 1.52E+11 -1.88 1.48E+11 -2.30 1.38E+11 -1.18 9.77E+10 Table A l 1.2 Filtration tests with the four fractions of the aerobic MEBPR sludge at the increased fluxes of 33 L/m 2.h and 68 L/m 2.h Flux Elapsed Whole sludge Supernatant Filtrate I Filtrate II L/m2.h. time TMPcw TMP Rm Rf TMPcw TMP Rm Rf TMPcw TMP Rm Rf TMPcw TMP Rm Rf min. psi psi m-1 m psi psi m"1 m psi psi m"1 m psi psi m"1 m"1 0 0 0 0 0 0.25 -0.91 -6.53E+11 -0.88 -7.26E+11 -0.83 -7.34E+11 -0.51 -5.51 E+11 0.5 -1.15 -4.72E+11 -1.13 -5.37E+11 -1.05 -5.68E+11 -0.68 -4.23E+11 1 -1.56 -1.63E+11 -1.51 -2.50E+11 -1.33 -3.56E+11 -0.91 -2.49E+11 1.5 -1.77 -4.18E+09 -1.77 -5.42E+10 -1.54 -1.98E+11 -1.05 -1.44E+11 32.8 2 -1.78 -1.83 1.34E+12 4.11E+10 -1.84 -1.84 1.39E+12 -1.35E+09 -1.80 -1.71 1.36E+12 -6.95E+10 -1.24 -1.11 9.36E+11 -9.83E+10 5 -1.95 1.32E+11 -1.98 1.04E+11 -1.87 5.13E+10 -1.29 3.76E+10 10 -2.09 2.37E+11 -2.17 2.48E+11 -2.03 1.72E+11 -1.38 1.05E+11 15 -2.25 3.58E+11 -2.46 4.67E+11 -2.17 2.78E+11 -1.43 1.43E+11 20 -2.49 5.39E+11 -2.58 5.57E+11 -2.35 4.14E+11 -1.53 2.19E+11 25 -2.73 7.20E+11 -2.85 7.61 E+11 -2.45 4.89E+11 -1.60 2.72E+11 30 -2.87 8.26E+11 -3.02 8.89E+11 -2.61 6.10E+11 -1.67 3.24E+11 0 0 0 0 0 0.25 -1.85 -4.53E+11 -1.60 -2.86E+11 -1.35 -2.35E+11 -1.49 -3.73E+11 0.5 -2.69 -1.48E+11 -2.44 1.81E+10 -1.90 -3.62E+10 -2.20 -1.16E+11 1 -3.36 9.41E+10 -3.12 2.64E+11 -2.29 1.05E+11 -2.61 3.26E+10 1.5 -3.64 1.95E+11 -3.38 3.58E+11 -2.41 1.48E+11 -2.79 9.77E+10 68.4 2 -3.10 -3.88 1.12E+12 2.82E+11 -2.39 -3.41 8.65E+11 3.69E+11 -2.00 -2.49 7.24E+11 1.77E+11 -2.52 -2.85 9.12E+11 1.19E+11 5 -4.45 4.89E+11 -4.12 6.26E+11 -2.80 2.90E+11 -3.15 2.28E+11 10 -4.95 6.70E+11 -5.21 1.02E+12 -3.57 5.68E+11 -3.56 3.76E+11 15 -5.38 8.25E+11 -5.85 1.25E+12 -3.99 7.20E+11 -3.98 5.29E+11 20 -5.85 9.96E+11 -6.39 1.45E+12 -4.64 9.56E+11 -4.26 6.30E+11 25 -6.34 1.17E+12 -6.56 1.51E+12 -5.25 1.18E+12 -4.58 7.46E+11 30 -6.80 1.34E+12 -6.71 1.56E+12 -5.88 1.40E+12 -4.94 8.76E+11 Note: a. TMP: transmembrane pressure when filtering sludge fractions; TMPcw: stable TMP of virgin membranes filtering clean water Rm: hydraulic resistance caused by membrane itself; Rf: hydraulic resistance due to fouling b. u = 1.002*10(-3)Pa.s; 1 psi = 6.89*103 N/m2 = 6.89*103 Pa or 1 bar = 10 5 N/m2 = 14.5 psi J=22.8: Rm = TMP/(uJ) = TMPcw * 6.89*103 Pa / [(1.002*10 3 Pa.s) * (22.8*10"3 m3/(m2.60*60 s))] = TMPcw * 1.09*1012 m"1 J=32.8: Rm = TMP/(uJ) = TMPcw * 6.89*103 Pa / [(1.002*10 3 Pa.s) * (32.8*10"3 m3/(m2.60*60 s))] = TMPcw * 7.55*1011 m"1 J=68.4: Rm = TMP/(uJ) = TMPcw * 6.89*103 Pa / [(1.002*10 3 Pa.s) * (68.4*10"3 m3/(m2.60*60 s))] = TMPcw * 3.62*1011 m"1 c. At J=22.8 L/m2.hr., Rf = 1.09*1012 TMP - Rm; at J=32.8 L/m2.h, Rf = 7.55*101' TMP - Rm; at J=68.4 L7m2.hr., Rf = 3.62*1011 TMP - Rm Table A l 1.3 Relative fouling rates of the four fractions of both the aerobic MEBPR sludge and the aerobic CEBPR sludge at the flux of 23 L/m 2.h M E B P R sludge C E B P R sludge Sludge fraction Fouling rate Relative fouling rate Standard error Fouling rate Relative fouling rate Standard error 1.0E-11, m'1min."1 % % 1.0E-11, m"1min."1 % % Whole sludge 0.228 100 26 0.059 100 9 Supernatant 0.213 93 16 0.057 96 15 Filtrate 1 0.150 66 3 0.056 95 6 Filtrate II 0.078 34 5 0.038 65 17 Table A l 1.4 Relative fouling rates of the four fractions of the MEBPR sludge at the increased fluxes of 33 L/m 2.h and 68 L/m 2 .h 32.8 L/m 2.h 68.4 L/m2.h Sludge fraction Fouling rate Relative fouling rate Standard error Fouling rate Relative fouling rate Standard error 1.0E-11, m"1min."1 % % 1.0E-11, m"1min."1 % % Whole sludge 0.292 100 13 0.361 100 10 Supernatant 0.317 109 13 0.599 166 51 Filtrate 1 0.222 76 6 0.436 121 7 Filtrate II 0.115 39 4 0.265 73 7 Appendix 12. Data Summary of the Dissolved TOC level in the Mixed Liquor Collected from Both the MEBPR and CEBPR Processes at the UBC Pilot Plant Table A 12.1 Dissolved TOC level in the mixed liquor collected from both the MEBPR and CEBPR processes (unit: mg TOC/L) Sampling Influent M E B P R Sludge C E B P R Sludge Date Anaerobic Anoxic Aerobic Permeate Anaerobic Anoxic Aerobic 18/08/2003 - 43.0 (±1.5) 13.0 (±1.0) 19.0 (±1.0) 11.0 (±0.4) 61.0 22.0 (±1.0) (±0.3) 11.5 (±0.5) 12/11/2003 - - - 12.5 (±0.5) 4.0 (±0.2) - 6.5 (±0.7) 29/03/2004 19.0 (±1.0) 12.3 (±0.4) 14.4 (±0.6) 18.3 (±0.5) 4.4 (±0.2) -5.8 (±0.4) 31/05/2004 28.3 (±1.2) 16.4 (±0.2) 10.0 (±0.0) 12.0 (±0.4) 4.6 (±0.1) 18.2 8.4 (±0.4) (±0.1) 6.5 (±0.2) Note: The data enclosed in the parentheses indicate standard deviation. 159 Appendix 13. Data Summary of the Fine Particle Size Distributions of the Soluble Fractions of Both the MEBPR and CEBPR Sludge M A S T E R S I Z E R Result Analysis Report Sample Name: AER-B soluble fraction SOP Name: Measured: February 21, 2005 5:C 6:52 AM Sample Source & type: Measured by: UBC EnviroGroup Analysed: February 21, 2005 5:3 2:09 AM Sample bulk lot ref: Result Source: Edited Particle Name: Water droplets Accessory Name: Hydro 2O0OS (A) Analysis model: General purpose Sensitivity: Enhanced Particle Rl: 1.330 Absorption: 0.01 Size range: 0.020 to 1000 000 um Obscuration: 0.17 % Dispersant Name: Water Dispersant Rl: 1.330 Weighted Residual: 17.G&1 % Result Emulation: Off Concentration: 0.0004 WAD Span : 0 a.;.n Uniformity: 0.245 Result units: Number Specific Surface Area: 19.9 nV-g Surface Weighted Mean D[3.2]: 0.301 um Vol. Weighted Mean D[4.3]: 0.323 um d(0.1): 0.169 um d(O.S): 0.244 um d(0.9i: 0.364 um Rmcte Uze [isti ilxtion 10 Particle Size (pm) 1000 -AER-Asduble fraction. February 21, 20054:23:01 AM -AER-B soluble fraction. February 21. 2005 5:06:52 AM 0010 0011 0013 0015 mm OCBO 0.023 0026 OfflO 0 0 * O040 o c « oos OO90 0039 0079 0091 010 0 0 3 0 0 3 0)10 0 0 3 0 0 3 0 0 3 an 0 0 3 aco oco 0 0 3 0 0 3 0 0 3 0 0 3 0 0 3 OCO 0 0 3 0109 0123 01351 0 1 * | 0182 O Z B I 0 2 C ' 02n ayts 0 3 8 3 0417 a » | asc 0-53I 0724 am a** m GOO 000 434 1233, 148»l 1644 era 1441 1111 ear 104 017 000 000 GOO 000 GOO I OB 1.239 1445 MS) 1.935 2 « 8 2312 2854 3111 3802 4353 3.012 sral Cw* 7.586 «710 IjCCO 114S2 0 0 3 oct> oco 0 0 3 OCO aco 0 0 3 0 0 3 oco aco aco aco aco aco aco •3)33 aco 11482 13183 15135 U 3 » 1 9 * 8 229C& 28305 3020:' 34674 39811 4*70? 32481 802*. 89183 794331 91201 1347131 120225 00C' ooo OOC' 000 Ml ooo 000 ooo ooo '300 000 000 .3001 ooo OCO ooo ooo d=i-:jji> Nu-rtiHn% 120226 T35G33 1314* 181.970 201930 226853 271423 315228 351078 411859 47*rfS3 549341 62095-724436 •tail 9349» 1095478 123S923 aco •00 aco OCO aco aco ' 133 M i 0 0 3 OCO Ml aco oco aco 0 0 3 oco aco •.k»i)jrn 1258923 MVU4'.' 1559387 1935461 2187.762 2511685 2884932 3311311 3631.694 4263135| 3011372 5J5439&I 9506934 T333.7H5 870963: lotoooco 000 OOC' 000 OOC' ooo 000 000 000 000 000 OOC' 0 * OOC' OOC' OOC' Figure A 13.1 Number-based fine particle size distribution of the soluble fractions of both the MEBPR and CEBPR sludge 160 MASTERSIZER Result A n a l y s i s Report Sample Name: AER-B soluble fraction Sample Source 4 type: Sample bulk lot tef: SOP Name: Measured by: UBC EnviroGroup Result Source: Edited Measured: February 21. 2005 5:05:47 AM Analysed: February 21. 2005 5:47:07 AM Particle- Name: Water droplets Particle Rl: 1.33C-Dispersant Name: Water Accessory Name: Hydro2000S (AS Absorption: 0.01 Dispersant Rl: 1.330 Analysis model: Gen-Mai purpose Size range: 0.020 to 1000.000 um Weighted Residual: 16.845 % Sensitivity: Enhanced Obscuration: 0.17 % Result Emulation: Off Concentration: 0 0CO4 %Vol Specific Surface Area: 20 rrrVg Span : 0.875 Surface Weighted Mean 0(3.2]: 0.300 um Uniformity: 0.209 Vol. Weighted Mean D[4.3J: 0.323 um Result units: Volume d(0.1j: 0.214 mm d(0.5): 0.322 um d(0.9): 0.431 um Panda azo UstnbUion 20 15 10 5 01 1 10 Particle Size •; pm) 100 1iX0 AER-A soluble fraction. February 21. 20054:23:01 A M -AER-B s u b t r a c t i o n . February 21. 2005 5:05:47 A M 0010 am a n 0013 0017 oceo 00Z3 0038 OC80 0030 00* ON) O0S2 0090 0099 0QJ9 OCPI o n '.•du*n&ln%| Ml MB am aco am am aco OOD am am Ml am am em Mi am Mi MB 01* 0136 0106 0182 02* 0210 027? 0516 Q3SS 0417 047* 0300 0631 0724 0«3E osos UM aco aCO 072 283 529 8J39 1280 17.CT 19.73 i&eo 1272 120 0)32 aco aco aco aco »tjjfn vmi 12» 1.90»| 21f»| 2312 2864 1311 JSCC 435S -.012 3.754 ear 7386 6710 MOB 11482 aco occ ooo ooo 0* 0*' 000 0* 000 Mi 000 0* 0* 000 0* Q00 0* 11.482 11183 15.136 17.370 t i953 2293* 2i3u3 M M 34674 36811 45-n» 32481 a:i2B «!183 TS433 P1.2CI 104713 120226 CMC oco oco MB oco 01225 138.036 1334* BUM 2:aat> 23M8J 273.423 316226 363.076 41a!* 478.-530 349M1 7244K 811.784 9548® 1068476 OCO oco aco aco aco aoo OCO oco oco oco oco oco oco oco oco oco oco •jizs. ipni t238S25 14*3440 1339387 1M3461 2187.62 2311.89!. 2384032 3111311 3S01J8WI 436313?| 3W1.872 37343S&I 8709635 10C0O.OO: aoo Ml 000 000 000 Mi 000 0* 000 aoo ooo Mi a * 000 0* Figure A13.2 Volume-based fine particle size distribution of the soluble fractions of both the MEBPR and CEBPR sludge 161 Appendix 14. Additional SEM Images of Virgin and Fouled Membranes Sampled from the UBC Pilot Plant Figure A14.1 Additional S E M images of virgin membranes at the magnifications of 100 K (a) and 25 K (b and c) 162 Figure A 14.2 Additional S E M images of the fouled membranes at the magnifications of 5 K (a), 9 K (b), 10 K (c and d), 20 K (e) and 25 K (f, g and h) 163 Appendix 15. Additional X-ray Microanalysis Profiles of Fouled Membranes Sampled from the UBC Pilot Plant Counts 2000 Concentration (Carbon 65.68 wt% (Oxygen 6.16 wt% Fluorine 28.17 wt% Figure A 15.1 X-ray microanalysis of the foulant layer on the surface of fouled membrane fibers 3000 H 2000 Concentration jCarbon 36.21 wt% |Nitrogen 8.80 wt% (Oxygen 27.57 wt% (Fluorine 15.79 wt% [Sodium 0.70 wt% , |Aluminum 1.31 wt% Silicon 0.18 wt% Phosphoru s 0.29 wt% Sulfur 0.65 wt% Manganese^ 6.57 wt% Iron i 0.45 wt% Arsenic 0.41 wt% Palladium 0.29 wt% Osmium 0.41 wt% Gold 0.37 wt% Figure A15.2 X-ray microanalysis of the crystal-like particles on the surface of the inner fabric support of fouled membrane fibers 164 Appendix 16. Additional Examples of the MALDI-MS Profiles of the Foulants Extracted from the MEBPR Membrane Fibres 250 150 160O Figure A16.1 M A L D I mass spectra of the foulants extracted from the outer skin of completely fouled M E B P R membrane fibers sampled on April 27, 2004 (DHB as the matrix) 165 900 19 8 I 21OO Figure A16.2 M A L D I mass spectra of the foulants extracted from the inner support of completely fouled M E B P R membrane fibers sampled on April 27, 2004 (DHB as the matrix) 166 Appendix 17. Calculation of the Extraction Efficiency of Organic Nitrogen from Fouled Membrane Fibres Table A17.1 Extraction efficiency of organic nitrogen from fouled membrane fibres Membrane sample Description Average T K N , (ag/m Extraction efficiency Basic Extraction (a) Acid Digestion (b) E = a*100/b, % Fouled membrane (Apr.17-Jul.7, 2003) Skin only Whole fibre 30.8 74.8 328 959 9.4 7.8 Fouled membrane (Jul.8-Oct.2, 2003) Skin only Whole fibre 28.0 30.0 466 528 6.0 5.7 Note: (1) The T K N contents of both virgin membranes and the extract from the virgin membranes were regarded as negligible; (2) The content of total organic nitrogen in membrane foulants was determined after the basic extraction; (3) The content of total organic nitrogen retained on fouled membranes was estimated using acid digestion method. 167 Appendix 18. Measurement of Contact Angles of Virgin and Fouled Membrane Fibers Photo a: virgin membrane fiber Photo b: fouled membrane fiber sampled from the UBC pilot plant on July 8, 2003 Photo b: fouled membrane fiber sampled from the UBC pilot plant on October 2, 2003 168 Appendix 19. Flux-step Filtration Tests of the MEBPR Sludge and the CEBPR Sludge Collected in the Second Experimental Run CO D. CO Q _ r-16 12 15.5 L/rrf.h 30 60 90 Time elapsed, min. 120 40 30 20 1(H 42 Um .h 10L/rrV\h f 21 L/m z.h f* » • • • - • 32 L/rri .h 30 60 90 Time elapsed, min. 120 Figure A 19.1 Flux-step filtration tests of (a) the M E B P R sludge and (b) the CEBPR sludge collected in Run II 169 Appendix 20. SEM Images of the Aerobic MEBPR Sludge and the Aerobic CEBPR Sludge Figure A20.1 S E M images of the aerobic M E B P R sludge collected in Run I at the magnifications of 6 K (a) and 1 K (b) Figure A20.2 S E M images of the aerobic C E B P R sludge collected in Run I at the magnifications of 5 K (a) and 1 K (b) 170 Appendix 2 1 . Examples of Sludge Floe Size Measurement Concentration: Span : Result units: d(0.1): • 0772 2.368 Volume 24.311 %Vol um Vol. Weighted Mean D[ 4,3]: 97 699 um Uniformity: 0.799 d(0.S): 73.ES9 um Specific Surface Area: 0.12B m'tg Surface Weighted Mean D[3,2]: 46 as I um dC0.9): 198.714 um 8 g 6 « 5 1 4 > 3 2 1 %. Parti d ir* Distribution r / \ / \ / \ 31 0.1 1 10 Particle Size (um) 100 1000 3000 A E R - A . 07/30/03 15:54:01 (a) the aerobic MEBPR sludge Concentration: 0.0S1S %Vol Span: 2.076 Result units: Volume d(0.1): 39.793 um Vol. Weighted Mean 0(4,3]: Uniformity: 139.029 0.643 d(0.S): 113.473 um Specific Surface Area: 0.0838 m-.'g Surface Weighted Mean D[3,2]: 71.622 um df0.9): 275.367 um 9 8 7 6 5 4 3 2 1 %.01 Part ic le Size D is t r ibu t ion 0.1 1 10 Particle Size Qjm) 100 1000 3000 - A E R - B , 07/30M3 16:02:31 (b) the aerobic CEBPR sludge Figure A21.1 Measurement of the floe size distribution of (a) the aerobic MEBPR sludge and (b) the aerobic CEBPR sludge collected on July 30, 2003. 171 Appendix 22. Data Summary of Sludge Floe Size Measurement Table A22.1 Sludge floe size measurement Date of Type of i d(0.5), urn Ave. Stdev I C(4,3], um Ave. Stdev sampling sludge j 1 2 3 d(0.5) 1 2 3 CT4,3] AER-A | 74 71 j 69 71 3 97 91 89 92 4 ANO-A j 77 70 69 72 4 115 92 91 99 14 30-JUJ-03 ANA-A | 67 66 I "63 | 65 2 92 87 I 77 85 8 AER-B | 113 114 113 113 1 139 145 141 142 3 ANO-B ) 92 94 97 94 3 120 124 134 126 7 " 7 J N ^ B ~ | 87 86 83 I 85 2 114 115 107 I 112 4 AER-A j 65 65 66 66 0 82 83 83 83 1 ANO-A j 63 62 62 62 0 78 79 79 79 l _ 0 ANA-A i 58 58 61 59 2 77 78 87 81 5 I AER-B I 104 97 95 98 5 150 134 135 140 9 ANO-B | 93 92 ~ 1 79 88 8 146 116 109 124 19 ANA-B 94 100 99 98 3 127 147 134 136 10 AER-A 74 75 74 74 1 96 100 95 97 3 ANO-A 69 j 67 68 1 89 90 85 88 3 10-Oct-03 ANA-A 70 70 70 70 0 95 96 93 95 2 AER-B 146 142 145 "139 " 131 141 9 172 155 167 10 21 ANO-B 130 137 6 195 ^ 165 155 172 ANA-B | 134 130 123 129 5 168 156 145 156 12 AER-A | 107 97 93 99 7 146 136 123 135 12 ANO-A | 84 79 80 79 3 124 108 119 113 8 15-Jar>04 ANA-A i 100 79 j 79 79 1 160 110 112 111 1 AER-B | 102 106 j 103 104 2 138 141 137 139 2 ANOB~| 7 9 ~ 78 | 78 78 1 122 105 102 110 11 ANA-B | 75 75 I 74 75 1 121 106 103 105 2 AER-A j 96 99 j 97 97 2 133 135 135 135 2 ANO-A J 76 76 73 75 2 109 116 101 109 7 04-Fer>04 ANA-A j 85 92 | 94 90 5 136 152 150 145 9 AER-B | 110 115 j 118 114 4 140 163 160 154 13 ANO-B J 80 91 j 87 86 6 106 143 126 125 19 A N ^ B | 68 65 | 64 66 2 93 86 80 86 7 AER-A j 101 99 | 94 98 3 140 145 133 139 7 ANO-A 96 93 | 92 95 2 140 136 131 136 5 17-Mar-04 ANA-A 94 98 | 97 96 2 149 150 140 146 5 | AER-B I 111 123 | 135 123 12 155 162 170 162 8 ANO-B ! 75 85 | 92 84 8 107 119 135 121 14 ANA-B j 89 128 I 130 116 23 124 166 165 152 24 Note: (1) d(0.5): particle size at 50% volume; (2) D[4,3]: volume based mean (= E d 4 / 2d 3); (3) Stdev: standard deviation; (4) A E R - A : the sludge collected from the aerobic zone of the M E B P R process; ANO-A: the sludge collected from the anoxic zone of the M E B P R process; ANA-A: the sludge collected from the anaerobic zone of the M E B P R process; A E R - B : the sludge collected from the aerobic zone of the C E B P R process; ANO-B: the sludge collected from the anoxic zone of the C E B P R process; ANA-B: the sludge collected from the anaerobic zone of the C E B P R process. 172 Appendix 23. Examples of Sludge Zeta Potential Measurement -100 0 100 Zeta Potential (mV) (a) Aerobic M E B P R sludge 20 (b) Aerobic CEBPR sludge | 10 i PS 1 1 A -100 0 100 Zeta Potential (mV) Figure A23.1 Zeta potential measurement of (a) the aerobic M E B P R sludge and (b) the aerobic CEBPR sludge 173 Appendix 24. Data Summary of Sludge Zeta Potential Measurement Table A24.1 Sludge zeta potential measurement Date of Type of Zeta potential, mv Ave. St dev. Date of Type of Zeta potential, mv Ave. Stdev. sampling sludge 1 2 3 sampling sludge r i ~ 3 AER-A -15.3 -17.4 -15.3 -16.0 1.2 AER-A -17.5 -19.3 -19.9 -18.9 1.2 ANO-A -15.6 -19.3 -19.4 -18.1 2.2 ANO-A -18.1 -18.9 -20.7 -19.2 1.3 29Jul-03 ANA-A -20.0 -20.6 -19.9 -20.2 0.4 15>^r>04 ANA-A -18.8 ™ -21.1 -19.2 -19.7 AER-B -20.3 -18.1 I -22.1 -202 2.0 AER-B -16.3 -17.8 -17.0 -17.0 0.8 ANO-B -16.6 -17.4 -17.8 -17.3 0.6 ANO-B -17.2 -17.3 -15.6 -16.7 1.0 ANA-B -17.6 -19.3 -17.2 -18.0 1.1 j ANA-B -19.3 -19.1 -15.8 -18.1 2.0 AER-A -17.0 - -17.3 -17.2 0.2 . _ _ AER-A -18.7 -20.2 -17.9 -18.9 1.2 ANO-A -19.0 -17.8 z§i2 r -18.6 ANO-A -20.3 -21.6 r -20.7 0.8 08-Aug-03 ANA-A -25.0 -23.5 -25.7 •^47 1.1 04-Feb-04 ANA-A -21.2 -18.5 -20.1 -19.9 1.4 " AER-B -20.7 -15.5 -19.9 [___ 2.8 AER-B -18.6 -19.1 -20.4 r -19.4 0.9 ANO-B -22.0 -17.3 -19.2 -19.5 2.4 ANO-B -20.9 -20.1 -18.1 -19.7 1.4 ANA-B -19.1 -20.4 -22.4 -20.6 1.7 ANA-B -19.4 -16.3 -20.6 -18.8 2.2 AER-A -16.3 -16.4 - -16.4 0.1 AER-A -25.1 -19.5 -23.9 -22.8! | I 2.9 ANO-A -15.3 -18.7 - -17.0 2.4 ANO-A -25.2 -24.1 -24.4 -24.6 0.6 29-Aug-03 ANA-A -19 8 -20.8 - -20.3 0.7 19-Fet>04 ANA-A -223 -22.5 -25.0 -23.3 1.5 AER-B -15.1 -22.0 - -18.6 4 9 _ .... AER-B -16.4 -18.6 -18.2 -17.7 " 1.2 ANO-B ~ -19.7 -19.1 ANO-B -20.2 -16 5 -18.3 -18.3 ANA-B -19.7 -19.8 I -19.8 0.1 ANA-B -20.2 -21.3 -23.7 -21.7 1.8 AER-A -18.2 - -18.2 0.9 AER-A -28.9 -32.2 -24.6 -28.6 3.8 10-OctO3 " ANO-A -20.7 - - -20.7 2.1 A t O A -22.2 -32.7 -32.6 -29.2 6.0 ANA-A -19.3 - - -19.3 0.5 17-Mar-04 ANA-A -30.1 ' -31.9 -27.5 -29 8 AER-B -15.8 - - -15.8 ~ 0 . 1 AER-B -24.6 -27.5 -18.4 -23.5 4.6 ANO-B -18.3 - - - -18.3 2.0 ANO-B -17.5 -16.8 -20.3 -18.2 1.9 ANA-B -21.5 - - -21.5 1.5 ANA-B -15.9 -19.5 -20.3 -18.6 2.3 Note: (1) Stdev: standard deviation (2) AER-A : the sludge collected from the aerobic zone of the M E B P R process; ANO-A: the sludge collected from the anoxic zone of the M E B P R process; ANA-A: the sludge collected from the anaerobic zone of the M E B P R process; A E R - B : the sludge collected from the aerobic zone of the C E B P R process; ANO-B: the sludge collected from the anoxic zone of the C E B P R process; ANA-B: the sludge collected from the anaerobic zone of the C E B P R process. Appendix 25. Data Summary of Sludge Relative Hydrophobicity Measurement Table A25.1 Sludge relative hydrophobicity measurement Date of Type of Relative hydrophobicity, % Ave. Stdev. Date of Type of Relative ryclnophobicity, % Ave. Stdev. sampling sludge 1 2 sampling sludge 1 2 \ AER-A 20.1 L 26.5 23.3 4.5 AER-A 14.3 14.5 14.4 0.1 ANO-A 24.8 21.2 . ^ . . . . . . . . . . . ANO-A 10.5 9.5 10.0 0.8 04-Jul-O3 ANA-A 16.2 18.9 17.6 1.9 11-Feb-04 ANA-A 18.8 17.9 18.3 0.6 AER-B 10.9 15.7 13.3 3.4 AER-B 40.0 48.5 44.2 6.0 ANO-B 18.81 14.5 16.7 3.0 ANO-B 43.8 37.0 40.4 4.8 1 ANA-B 22.8 27.1 25.0 3.0 ANA-B 38.1 33.3 35.7 3.4 AER-A 20.6 24.6 22.6 2.8 i AER-A 15.4 16.0 15.7 0.4 ANO-A 23.8 19.6 21.7 3.0 | ANO-A 15.6 12.8 14.2 2.0 07-Jul-03 ANA-A 19.6 26.3 23.0 4.7 20-Feb-04 ANA-A 17.1 21 1 19.1 2.8 AER-B 33.8 30.6 32.2 2.3 AER-B 1 47.4 45.3 46.3 1.5 ANO-B 35.7 33.8 34.8 1.3 ANO-B 42.6 34.4 38.5 5.7 ANA-B 32.1 39.1 35.6 4.9 ANA-B 33.3 38.2 35.8 3.4 AER-A 60.4 49.2 54.8 7.9 AER-A 30.0 26.0 28.0 2.8 ANO-A" 47.8 50.9 49.4 ANO-A 21.1 22.4 21.7 0.9 04-Aug-03 ANA-A 41.2 49.1 45.2 5.6 23-Mar-04 ANA-A 32.5 2A 28.6 & 6 AER-B 58.1 49.3 53.7 6.2 AER-B — — 5 4 _ 2 49.4 51.8 3.4 A N O - B j 41.1 43.9 42.5 2.0 ANO-B 506 45.7 48.2 3.5 ANA-B 51.3 58.9 55.1 5.4™ ANA-B 56.6 - 56.6 2.6 AER-A 30.1 28.8 29.5 0.9 AER-A 40.2 45.1 42.7 3.5 ANO-A 31.7 166 24.2 10.7 ANO-A 44.6 51.1 47 9 4.6 02-Dec-03 ANA-A 24.7 22.0 21-Apr-04 ANA-A 48.2 43.2 3.6 AER-B 51.7 44.8 48.3 4.9 AER-B 51.9 49.4 50.6 1.8 ANO-B 33.4 35.8 34.6 1.7 ANO-B 49.4 48.7 49.0 0.5 ANA-B 34.0 31.4 ' 32.7 1.8 j ANA-B 57.5 48.8 53.2 6.1 Note: (1) Stdev: standard deviation (2) AER-A : the sludge collected from the aerobic zone of the M E B P R process; ANO-A: the sludge collected from the anoxic zone of the M E B P R process; ANA-A: the sludge collected from the anaerobic zone of the M E B P R process; A E R - B : the sludge collected from the aerobic zone of the C E B P R process; ANO-B: the sludge collected from the anoxic zone of the C E B P R process; ANA-B : the sludge collected from the anaerobic zone of the C E B P R process. Appendix 26. The Initial Measurement of Various EPS Components Bound in Activated Sludge Floes 15.0 - p > 12.0 -9.0 -6.0 -1 •b 3.0 -0.0 -Aerobic Anoxic Anaerobic Aerobic Anoxc Anaerobic Source of sludge 40.0 30.0 --20.0 --10.0 --0.0 M E B P R C E B P R Aerobic Anoxc Anaerobic Aerobic Anoxc Anaerobic Source of sludge M E B P R C E B P R (Q 60.0 f . 30.0 4 -E 15.0 J3 D 0.0 w O Aerobic Anoxc Anaerobic Aerobic Anoxc Anaerobic § Source of sludge 176 •2? "8 o 1 D 5.0 4.0 3.0 2.0 1.0 0.0 M E B P R C E B P R t \ t i -Aerobic Anoxic Anaerobic Aerobic Sou rce of s ludge Anoxc Anaerobic Aerobic Anoxic Anaerobic Aerobic Anoxc Anaerobic Source of sludge Figure A26.1 Initial measurement of EPS components bound in activated sludge floes Note: (1) The content of bound EPS is expressed in mean value ± standard deviation. (2) The bound EPS was extracted in accordance with the following procedure. a. A measured amount of cation exchange resin (CER) was added to sludge suspension in line with the rule of 60 g CER/g SS, and was then mixed by magnetic stirring at room temperature for 1 hour. b. The mixture of sludge and CER was centrifuged at 2600 xg for 20 minutes. The supernatant was collected as bound EPS extract and was stored at -20°C for further analysis. 177 Appendix 27. Data Summary of Measurement of EPS Components Bound in Activated Sludge Floes Table A27.1 Measurement of carbohydrates and proteins bound in activated sludge floes Date of Bound carbohydrates, mg/g VSS Bound protein, mg/g VSS sampling MEBPR sludge! stdev CEBPR sludge! stdev MEBPR sludge stdev CEBPR sludge stdev Run 1 08-May-O3 13-May-03 8.6 1.3 10.7 1.8 34.2 3.3 41.4 4.5 11.8 1.9 9.7 1.2 5.8 53.6 3.2 65.0 4.1 25-Jun-03 16.3 3.0 26.1 41.0 6.8 92.5 11.8 02-JUI-03 19.5 1.4 14.6 1.3 49.4 8.1 78.0 12.8 05-Aug-03 3.7 0.2 ! 107 j _ _ 25.7 3.9 46.5 3.2 31-Oct-03 15.4 1.0 15.1 45.3 15.0 47.6 3.2 Run II 26-Feb-04 9.4 | 0.3 6.3 0.5 24.8 0.6 31.5 3.2 27-Fer>04 4.1 | 0.3 3.5 0.5 39.6 3.6 31.3 8.8 03-Mar-04 3.1 I 0.4 2.5 0.8 22.6 5.4 29.9 4 0 16-Mar-04 3.5 0.7 2.5 0.6 33.5 6.8 33.3 5.5 22-Mar-04 6.4 1.3 3.5 0.8 34.4 3.3 25.6 7.2 29-Mar-04 9.4 0.6 8.4 0.7 31.1 9.6 32.7 5.9 23-Apr-04 8.0 0.4 7.7 0.2 30.2 12.4 42.2 5.9 Table A27.2 Measurement of humic substances and total EPS bound in activated sludge floes Date of Bound hurric substances, mg/g VSS Total bound EPS, mg/g VSS sampling MEBPR sludge stdev CEBPR sludge! stdev MEBPR sludge stdev CEBPR sludge, stdev Run I 08-May-03 11.2 3.8 9.6 3.0 54.0 8.4 61.8 9.3 13-May-03 10.3 i 5.1 12.8 6.7 75.6 10.2 87.5 12.0 25-Jun-03 29.6 3.4 8.5 3.1 86.9 13.2 127.1 20.7 02-Jul-03 32.1 11.0 100.9 13.9 103.6 20.1 05-Aug-03 7.9 ^ 1.9 4.7 2.0 37.3 61.9 5.5 31-Cct-03 61.7 15.9 62.6 3.7 Run II I 26-Feb-04 26.5 | 0.4 37.4 2.8 60.7 1.3 75.2 6.5 27-Fet>04 20.6 | 4.6 27.2 1.5 64.3 8.5 62.0 | 10.8 03-Mar-O4 23.2 3.9 30.7 7.4 48.9 9.7 63.1 12.2 16-Mar-04 20.5 4.0 27.4 5.0 57.5 11.5 63.2 11.1 22-Mar-04 41.8 7.6 29.4 2.4 82.6 12.2 58.5 10.4 29-Mar-04 49.5 0.3 39.8 0.5 90.0 10.5 80.9 7.1 23-Apr-04 15.4 4.0 14.4 0.7 53.6 16.8 64.3 6.8 178 Appendix 28. Data Summary of Measurement of Soluble EPS in Activated Sludge Mixed Liquor Table A28.1 Measurement of soluble carbohydrates and proteins in activated sludge mixed liquor | Soluble carbohydrates, rrg/L Soluble protein, mg/L Date of MEBPR | j CEBPR MEBPR CEBPR sampling rrixed liquor) stdev | mixed liquor stdev nixed liquor stdev rrixed liquor stdev Run 1 i I 13-May-03 4.8 | 1.2 j 2.3 0.6 4.2 1.1 1.8 0.0 02Jul-03 3.0 i 1.3 i 2.5 0.4 7.5 0.7 4.6 0.7 05-AugO3 4.0 | 1.2 | 2.7 1.4 3.3 0.4 2.3 1.0 31-Oct-03 8.0 ! 2.3 5.7 1.0 4.8 1.5 1.2 0.6 Run II 26-Feb-04 17.0 0.3 0.9 0 1.8 0 1.8 0 03-Mar-04 7.5 1.8 0.9 0 1.8 _ _ | 1.8 0 16-Mar-04 9.9 2.4 0.9 0 3.5 1.3 1.8 0 22-Mar-04 6.9 2.0 0.9 0 4.1 1.6 1.8 0 29-Mar-04 19.2 0.8 | 0.9 0 1.8 0 1.8 0 23-Apr-04 20.8 3.1 | 11.0 1.8 6.8 1.0 1.8 0 Table A28.2 Measurement of soluble humic substances and total EPS in activated sludge mixed liquor | Soluble hurric substances, mg/L JTotal soluble EPS, mg/L Date of MEBPR I | CEBPR MEBPR CEBPR sampling rrixed liquor stdev rrixed liquor stdev rrixed liquor stdev rrixed liquor stdev Run I [ 13-May-03 7.8 j 1.0 11.9 1.6 16.8 3.3 16.0 2.2 02-Jul-03 8.0 I 0.8 3.5 0 18.4 2.8 10.6 1.1 05-Aug-03 4.5 j 3.6 3.5 0 11.7 5.1 8.5 2.3 31-Oct-03 10.9 | 2.1 6.9 1.0 23.7 13.8 2.6 Run II 26-Feb-04 24.2 0.0 j 6.0 0.6 43.0 0.3 8.7 0.6 03-Mar-04 24.0 j 1.9 | 11.0 4.2 • 33.3 5.4 13.7 4.2 16-Mar-04 23.9 | 1.2 12.3 0.2 37.3 4.9 15.0 0.2 22-Mar-04 29.8 1.6 ^~ 13.0 1.1 40.8 5.2 15.7 1.1 29-Mar-04 0.3 46.1 2.9 10.6 0.3 23-Apr-04 12.7 | 0.1 ! 6.5 0 40.3 4.2 19.3 1.8 179 

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