Applied Science, Faculty of
Civil Engineering, Department of
DSpace
UBCV
Caufield, Christopher
Master of Applied Science - MASc
University of British Columbia
Sewage sludge is a by-product of wastewater treatment. It is considered a waste product and is generally disposed of in landfills; yet sludge is rich in nutrients like nitrogen and phosphorus â essential for plant life and valuable in agricultural fertilizers. However, getting these nutrients out of sludge is not always easy. The microwave-enhanced advanced oxidation process (MW-AOP) is a treatment method effective at releasing these nutrients from sewage sludge. The MW-AOP incorporates microwave irradiation with hydrogen peroxide, a combination that attacks the organic fraction of the sewage sludge and releases nutrients, proteins, and amino acids. MW-AOP-treated sludge also shows promise for anaerobic digestion, a process where organic matter is converted to methane. Previous studies in this research group used pilot-scale MW-AOP systems to treat sludge and other organic slurries. This current study focused on two aspects of MW-AOP: installing and operating a larger 25 kW MW-AOP system to treat municipal wastewater sludge; and studying the effects of MW-AOP-treated sludge used as a feed substrate for anaerobic digestion. The 25 kW MW-AOP system was consistent with expected results: volatile fatty acids (VFA) and solubilized chemical oxygen demand (COD) increased; total suspended solids (TSS) decreased; nitrogen and phosphorus were solubilized, as were calcium, magnesium, and potassium. The system was easily able to treat 100 litres of sludge, as opposed to 20 litres in past experiments using a 5 kW MW-AOP system. Based on the results of this study, the 25 kW MW-AOP system would be capable of treating larger quantities of sludge to desired standards. This study also showed that MW-AOP-treated sludge could be a good candidate for high-rate anaerobic digestion, especially in terms of sludge reduction. Using the MW-AOP-treated sludge as a feed substrate for anaerobic digestion showed that volatile solids (VS) and COD were greatly reduced when compared with raw sludge as a feed substrate, especially at very low hydraulic retention times (HRT). Additionally, using MW-AOP-treated sludge resulted in higher levels of alkalinity being produced, implying a strong buffer capacity able to withstand higher loading rates. This would make MW-AOP a suitable pre-treatment where maximizing sludge reduction is a priority.
A MICROWAVE-ENHANCED ADVANCED OXIDATION PROCESS TO TREAT MUNICIPAL WASTEWATER SLUDGE AND AS A PRETREATMENT FOR ANAEROBIC DIGESTION by Christopher Caufield B.A.Sc, The University of British Columbia, 2017 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in The Faculty of Graduate and Postdoctoral Studies (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2019 © Christopher Caufield, 2019 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis/dissertation entitled: A Microwave-Enhanced Advanced Oxidation Process to Treat Municipal Wastewater Sludge and as a Pretreatment for Anaerobic Digestion submitted by Christopher Caufield in partial fulfilment of the requirements for the degree of Master of Applied Science in Civil Engineering Examining Committee: Victor Lo, Civil Engineering Supervisor Don Mavinic, Civil Engineering Supervisory Committee Member Supervisory Committee Member Additional Examiner Additional Supervisory Committee Members: Supervisory Committee Member Supervisory Committee Member iii Abstract Sewage sludge is a by-product of wastewater treatment. It is considered a waste product and is generally disposed of in landfills; yet sludge is rich in nutrients like nitrogen and phosphorus â essential for plant life and valuable in agricultural fertilizers. However, getting these nutrients out of sludge is not always easy. The microwave-enhanced advanced oxidation process (MW-AOP) is a treatment method effective at releasing these nutrients from sewage sludge. The MW-AOP incorporates microwave irradiation with hydrogen peroxide, a combination that attacks the organic fraction of the sewage sludge and releases nutrients, proteins, and amino acids. MW-AOP-treated sludge also shows promise for anaerobic digestion, a process where organic matter is converted to methane. Previous studies in this research group used pilot-scale MW-AOP systems to treat sludge and other organic slurries. This current study focused on two aspects of MW-AOP: installing and operating a larger 25 kW MW-AOP system to treat municipal wastewater sludge; and studying the effects of MW-AOP-treated sludge used as a feed substrate for anaerobic digestion. The 25 kW MW-AOP system was consistent with expected results: volatile fatty acids (VFA) and solubilized chemical oxygen demand (COD) increased; total suspended solids (TSS) decreased; nitrogen and phosphorus were solubilized, as were calcium, magnesium, and potassium. The system was easily able to treat 100 litres of sludge, as opposed to 20 litres in past experiments using a 5 kW MW-AOP system. Based on the results of this study, the 25 kW MW-AOP system would be capable of treating larger quantities of sludge to desired standards. iv This study also showed that MW-AOP-treated sludge could be a good candidate for high-rate anaerobic digestion, especially in terms of sludge reduction. Using the MW-AOP-treated sludge as a feed substrate for anaerobic digestion showed that volatile solids (VS) and COD were greatly reduced when compared with raw sludge as a feed substrate, especially at very low hydraulic retention times (HRT). Additionally, using MW-AOP-treated sludge resulted in higher levels of alkalinity being produced, implying a strong buffer capacity able to withstand higher loading rates. This would make MW-AOP a suitable pre-treatment where maximizing sludge reduction is a priority. v Lay Summary Sewage sludge is a by-product of wastewater treatment. It is considered a waste product and is generally disposed of in landfills. However, sludge is rich in nutrients like nitrogen and phosphorus â essential for plant life and valuable in agricultural fertilizers. However, getting these nutrients out of sludge is not always easy. The microwave-enhanced advanced oxidation process (MW-AOP) is a treatment method effective at releasing these nutrients from sewage sludge. The MW-AOP incorporates microwave irradiation with hydrogen peroxide, a combination that attacks the organic fraction of the sewage sludge and releases nutrients, proteins, and amino acids. MW-AOP-treated sludge also shows promise for anaerobic digestion, a process where organic matter is converted to methane â the largest constituent of natural gas. This study looked at using a 25 kW MW-AOP system to treat sludge for nutrient release, as well as to study the effects of MW-AOP-treated sludge on methane production in anaerobic digestion. vi Preface The experiments and research conducted in this thesis were initiated and supervised by Professor Emeritus Dr. Victor Lo, Department of Civil Engineering, The University of British Columbia. The experiments were designed in collaboration with Dr. Asha Srinivasan and Dr. Ping Liao. They provided their guidance throughout the research. A version of the chapter Anaerobic Digestion Using MW-AOP-Treated Sludge is under preparation for submitting to publish. The paper is being written by Christopher Caufield, Dr. Asha Srinivasan, and Dr. Ping Liao, and revised by Dr. Victor Lo. The experiments were designed in collaboration with Dr. Asha Srinivasan and Dr. Ping Liao. I was responsible for conducting the experiments, chemical analyses, interpretation of the results, and writing. Sarah Ries, Luke Burrows, Patrick Zhang, Connor Beardmore, and Eric Doroshuk assisted with the chemical analyses. The work in other chapters of this thesis is unpublished work designed in collaboration with Dr. Asha Srinivasan and Dr. Ping Liao, and supervised by Dr. Victor Lo. I was responsible for conducting the experiments, chemical analysis, interpretation of the results, and writing. Indre Tunile, Sarah Ries, Luke Burrows, Patrick Zhang, Connor Beardmore, and Eric Doroshuk assisted with the chemical analyses. vii Table of Contents Abstract .............................................................................................................................................................. iii Lay Summary ....................................................................................................................................................... v Preface ............................................................................................................................................................... vi Table of Contents ............................................................................................................................................... vii List of Tables ........................................................................................................................................................ x List of Figures ..................................................................................................................................................... xii List of Abbreviations .......................................................................................................................................... xiii Acknowledgements ............................................................................................................................................ xv 1 Introduction ................................................................................................................................................ 1 1.1 Background and Present State of the Problem ......................................................................................... 1 1.2 Municipal Sewage Sludge .......................................................................................................................... 2 1.3 Overview of MW-AOP Treatment ............................................................................................................. 2 1.4 Overview of Anaerobic Digestion .............................................................................................................. 3 1.5 Research Gap ............................................................................................................................................. 4 1.6 Research objectives ................................................................................................................................... 5 2 Literature Review ....................................................................................................................................... 6 2.1 Microwave-Enhanced Advanced Oxidation Process (MW-AOP) ............................................................... 6 2.1.1 Solids Destruction ................................................................................................................................. 9 2.1.2 Nutrient and Metal Release ................................................................................................................ 11 2.2 Anaerobic Digestion ................................................................................................................................ 14 2.2.1 MW-AOP as Pre-treatment for AD ...................................................................................................... 16 3 A 25 kW MW-AOP Pilot-Scale System to Treat Sludge ............................................................................... 18 viii 3.1 Introduction ............................................................................................................................................. 18 3.2 Materials and Methods ........................................................................................................................... 19 3.2.1 Microwave Unit .................................................................................................................................. 19 3.2.2 Substrate ............................................................................................................................................. 20 3.2.3 System setup ....................................................................................................................................... 21 3.2.4 Experimental Design ........................................................................................................................... 23 3.2.5 Experiment Run Operation ................................................................................................................. 23 3.2.6 Sample and Data analysis ................................................................................................................... 24 3.3 Experimental Results ............................................................................................................................... 25 3.3.1 Solids Destruction ............................................................................................................................... 25 3.3.2 VFA production and composition ........................................................................................................ 27 3.3.3 Nutrient and Metal Release ................................................................................................................ 28 3.4 Discussion ................................................................................................................................................ 30 3.5 Conclusions .............................................................................................................................................. 35 4 Anaerobic Digestion Using MW-AOP-Treated Sludge ................................................................................ 37 4.1 Introduction ............................................................................................................................................. 37 4.2 Materials and Methods ........................................................................................................................... 39 4.2.1 Substrate and Inoculum ...................................................................................................................... 39 4.2.2 System Setup ....................................................................................................................................... 41 4.2.3 Experimental Design ........................................................................................................................... 42 4.2.4 Sample and Data Analysis .................................................................................................................. 45 4.3 Experimental Results ............................................................................................................................... 46 4.3.1 Solids reduction ................................................................................................................................... 46 4.3.2 VFA, Alkalinity and pH ........................................................................................................................ 49 4.3.3 Biogas ................................................................................................................................................. 54 4.3.4 Methane ............................................................................................................................................. 56 ix 4.4 Discussion ................................................................................................................................................ 58 4.5 Conclusions .............................................................................................................................................. 61 5 Summary and Conclusions ........................................................................................................................ 64 5.1 Using a 25 kW MW-AOP Pilot-Scale System to Treat Sludge .................................................................. 64 5.2 Anaerobic Digestion Using MW-AOP-treated Sludge .............................................................................. 65 5.3 Recommendations for Future Research .................................................................................................. 66 5.3.1 Using a 25 kW MW-AOP Pilot-Scale System to Treat Sludge .............................................................. 66 5.3.2 Anaerobic Digestion Using MW-AOP-treated Sludge ......................................................................... 67 References ......................................................................................................................................................... 69 Appendices ........................................................................................................................................................ 78 Appendix A: Change in Solids for Anaerobic Digesters .......................................................................................... 78 Appendix B: Gas flow charts .................................................................................................................................. 80 Appendix C: Compiled Data Tables ........................................................................................................................ 83 Appendix D: VFA to Alkalinity Ratios ..................................................................................................................... 86 Appendix E: VFA composition ................................................................................................................................ 87 Appendix F: Methane ............................................................................................................................................ 89 Appendix G: Theoretical Methane Yield ................................................................................................................ 92 x List of Tables Table 1 â Averaged characteristics of MUN WWTP thickened secondary sludge.. .................................................... 21 Table 2 â Experimental conditions for MW-AOP treatment of MUN TWSS ............................................................... 23 Table 3 - Solids disintegration of MW-AOP-treated sludge ........................................................................................ 26 Table 4 - Solids disintegration values normalized by gram of TS ................................................................................ 26 Table 5 - Nutrient release of MW-AOP-treated sludge .............................................................................................. 29 Table 6 - Nutrient release values ................................................................................................................................ 29 Table 7 - Metals release of MW-AOP-treated sludge ................................................................................................. 29 Table 8 - Metals release values normalized by gram of TS ......................................................................................... 30 Table 9 - Description of substrates fed to anaerobic digesters .................................................................................. 40 Table 10 â Characteristics of inoculum, raw substrate, and MW-AOP-treated substrate ........................................ 40 Table 11 - Hydraulic retention times and operating modes used in experiment. ...................................................... 43 Table 12- VFA destruction in control and microwave reactors .................................................................................. 52 Table 13 - VFA, alkalinity, and pH for MUN WWTP substrate .................................................................................... 52 Table 14 - Daily average biogas production ............................................................................................................... 54 Table 15 - Biogas production per gram VS destroyed and per gram VS added .......................................................... 55 Table 16 - Biogas produced per VFA destroyed .......................................................................................................... 59 Table 17 - Biogas production in terms of VFA and VS fed .......................................................................................... 59 Table 18 - Compiled data for 16-day expanded bed reactor ...................................................................................... 83 Table 19 - Compiled data for 12-day expanded bed reactor ...................................................................................... 83 Table 20 - Compiled data for 12-day CSTR reactor ..................................................................................................... 83 Table 21 - Compiled data for 8-day CSTR reactor ....................................................................................................... 84 Table 22 - Compiled data for 5-day CSTR reactor ....................................................................................................... 84 Table 23 - Compiled data for 5-day MUN CSTR reactor ............................................................................................. 84 Table 24 - Compiled data for 3.6-day MUN CSTR reactor .......................................................................................... 85 Table 25 - VFA to alkalinity ratio ................................................................................................................................. 86 xi Table 26 - Methane percentage of biogas produced ................................................................................................. 90 Table 27 - Comparison of methane content before and after feeding ...................................................................... 90 Table 28 - Assumed methane values compared with theoretical methane values .................................................... 91 xii List of Figures Figure 1 - Plan view of microwave trailer and system layout ..................................................................................... 22 Figure 2 - Change in VFA composition after MW-AOP treatment ............................................................................. 27 Figure 3 - Diagram of feed tank used in MW-AOP system ......................................................................................... 31 Figure 4 - Diagram of anaerobic digester used in experiments .................................................................................. 42 Figure 5 - a. Volatile solids reduction b. Total solids reduction c. SCOD d. TCOD ...................................................... 48 Figure 6 - SCOD removal in control and microwave reactors ..................................................................................... 49 Figure 8 - Change in VFA for control reactor; Change in VFA for microwave reactor; Change in alkalinity for control reactor; Change in alkalinity for microwave reactor .................................................................................................. 53 Figure 9 - Biogas per day VS added in the control reactor; Biogas per day VS added in the MW reactor; Cumulative biogas produced per day in MW reactor; Cumulative biogas produced per day in the control reactor .................... 57 Figure 10 - Control reactor change in total solids; MW reactor change in total solids; MW reactor change in volatile solids; control reactor change in volatile solids ......................................................................................................... 79 Figure 11 - Biogas production for 16-day expanded bed, 12-day expanded bed, 12-day CSTR, and 8-day CSTR operation .................................................................................................................................................................... 81 Figure 12 Biogas production for 5-day CSTR, 5-day MUN CSTR, and 3.6-day MUN CSTR .......................................... 82 Figure 13 - VFA composition of the substrate collected at MUN ............................................................................... 87 Figure 14 - VFA composition of effluent from anaerobic digesters fed with MUN substrate .................................... 87 Figure 15 â Change in VFA concentration for anaerobic digesters using AGR sludge ................................................ 88 xiii List of Abbreviations ANOVA: Analysis of Variance ATH: Advanced thermal hydrolysis BOD: Biochemical oxygen demand COD: Chemical oxygen demand EB: Expanded bed EPS: Extracellular polymeric substances FOG: Fats, oils, and grease H2O2: Hydrogen peroxide HRT: Hydraulic retention time MW: Microwave MW-AOP: Microwave-enhanced advanced oxidation process Ortho-P: Orthophosphate SCOD: Soluble chemical oxygen demand SRT: Solids retention time TCOD: Total chemical oxygen demand TKN: Total Kjeldahl nitrogen TH: Thermal hydrolysis TP: Total phosphorus TS: Total solids TSS: Total suspended solids TWSS: Thickened waste secondary sludge xiv UBC: University of British Columbia VFA: Volatile fatty acids VS: Volatile solids WAS: Waste activated sludge WWTP: Wastewater treatment plant xv Acknowledgements I would like to sincerely thank Professor Emeritus Dr. Victor Lo for giving me the opportunity to work with his research group on these projects, and for his funding, guidance and supervision. I would like to thank Dr. Ping Liao for his guidance and questioning, and always making sure I understood what we were doing and why. I would especially like to thank Dr. Asha Srinivasan for her daily guidance, support, and advice. She and I spent many days standing in the cold trying to troubleshoot the microwave system. I would also like to thank Professor Emeritus Dr. Don Mavinic for being my second reader, and for always bringing great stories, inspiration, and motivation towards wastewater treatment. I would like to thank Dr. Otman Abida for his help with the instrumental analysis; and Doug Hudniuk for helping to build and repair so many things for the projects. I would also like to thank Indre Tunile, Sarah Ries, Luke Burrows, Patrick Zhang, Connor Beardmore, and Eric Doroshuk for helping with the chemical analyses, assisting with the microwave setup, and helping to feed the anaerobic digesters. 1 1 Introduction 1.1 Background and Present State of the Problem As cities and populations grow, so does the water demand and the volume of wastewater produced. Wastewater comes from many sources that may be municipal, agricultural, or industrial in origin. Municipal wastewater is treated at local wastewater treatment plants (WWTPs), and an unavoidable by-product of wastewater treatment is sludge. Sludge is an ever-growing waste product that poses health risks to humans and the environment if treated or disposed of improperly. However, sludge is also rich in nutrients and potential energy. As such, this increase in sludge production provides an opportunity for researchers and engineers to capitalize upon. Nutrients can be recovered from sludge and turned into fertilizer; and methane â the main constituent of natural gas â can be captured and used for energy. Solutions such as these are viable answers to the increasing pressure to develop sustainable waste reduction and treatment strategies. The microwave-enhanced advanced oxidation process (MW-AOP) is a thermal-chemical treatment used to treat wastewater sludge (and other organic slurries) with the goal of making the sludge more suitable for nutrient and energy recovery. The MW-AOP has been researched for the last fifteen years and has been used in different scale studies with substrates that include municipal wastewater, dairy cow manure, source separated organics, and fats, oils, and grease (FOG) (Kenge et al., 2009; Lo et al., 2015, 2016, 2018a; Nkansah-Boadu et al., 2015; Remya and Lin, 2011; Serpone et al., 2010; Srinivasan et al., 2018; Tyagi and Lo, 2013; Yu et al., 2010). 2 1.2 Municipal Sewage Sludge Sludge is a collective term used to describe the different constituents removed during wastewater treatment. These constituents may include grit and scum from preaeration, primary sludge from sedimentation, and secondary sludge from biological treatment. Sludge is treated in order to stabilize it, which prevents its natural fermentation. From there the sludge may be used as a biosolid â depending on which treatment it has undergone â or may simply be disposed of. Sludge disposal often simply takes the form of being trucked to the local landfill. Overall, sludge treatment and disposal is an expensive operation and may require up to half of a WWTPs annual operating budget (Ãegaard, 2004). Metcalf and Eddy have summed up sludge treatment, reuse, and disposal as âperhaps the most complex problem in the field of wastewater treatmentâ (Tchobanoglous et al., 2014). As such, much research has gone into sludge minimization technologies. 1.3 Overview of MW-AOP Treatment Microwaves constitute a wide range of wavelength frequencies from 300 MHz to 300 GHz. Household microwave ovens generate microwaves at 2450 MHz, whereas industrial applications â including the MW-AOP â generally generate microwaves at 915 MHz. This lower frequency results in a longer wavelength and allows microwave irradiation to penetrate deeper within a given substrate. The microwave-enhanced advanced oxidation process uses microwave irradiation in combination with hydrogen peroxide to treat wastewater sludge or other organic slurries. Hydrogen peroxide (H2O2) is an oxidizing agent; however, when used in combination 3 with microwave irradiation, the hydrogen peroxide is broken into two hydroxyl radicals (OH-). Hydroxyl radicals are highly reactive and are much stronger oxidants than hydrogen peroxide. Thus, the MW-AOP uses the synergy between microwave irradiation and hydroxyl radicals to degrade organics in wastewater sludge at a faster rate than other oxidation methods (Remya and Lin, 2011). These degraded organics are further oxidized to volatile fatty acids, nutrients such as nitrogen and phosphorus are solubilized, and the overall sludge mass and volume are reduced (Liao et al., 2007; Lo et al., 2017; Wong et al., 2006). 1.4 Overview of Anaerobic Digestion Anaerobic digestion is a common treatment option in which sewage sludge â or other substrate â is stabilized by bacteria in an environment devoid of oxygen. The main benefits of anaerobic digestion are its ability to reduce the volume of sludge requiring ultimate disposal, while producing biogas rich with methane as a by-product. However, a drawback of anaerobic digestion is the long hydraulic retention time (HRT) necessary to ensure adequate stabilization of sludge. Anaerobic digestion can be simplified into three main steps: hydrolysis, fermentation, and methanogenesis. First, hydrolysis is where cells and extracellular polymeric substances (EPS) are broken down into particles small enough to be transported across bacterial cell membranes. In the fermentation step, the products from hydrolysis are converted by bacteria â acidogens and acetogens â into organic acids, and eventually into volatile fatty acids (VFAs). Finally, a unique group of Archaea called methanogens consume the VFAs and produce methane, carbon dioxide, and water as final products. 4 Hydrolysis is generally considered to be the rate limiting step in anaerobic digestion, and thus is directly related to the HRT (Abelleira et al., 2012; Appels et al., 2011; Rittmann and McCarty, 2012; Tchobanoglous et al., 2014). As such, much research has gone into pre-treatment methods for improving the digestion efficiency in anaerobic digestion (Ariunbaatar et al., 2014; Carlsson et al., 2012; Zhang et al., 2014). Thermal-chemical processes such as the MW-AOP may be good candidates for pre-treatment for anaerobic digestion as they result in both thermal hydrolysis and the creation of VFAs (Liao et al., 2007). 1.5 Research Gap The present study was a continuation of the MW-AOP research being done at the University of British Columbia (UBC). In the first part of this study, the MW-AOP was used to treat sewage sludge from a local WWTP. This study used a larger microwave system than had previously been used; thus, it was unknown whether similar optimal operating conditions remain constant as the size of MW-AOP systems increases. By increasing the size of pilot-scale studies, a final goal of implementing real-time sludge treatment in a WWTP is being pursued. The second part of this study looked at the differences between MW-AOP-treated and untreated sewage sludge when used as feed substrates for anaerobic digestion. The MW-AOP could be useful in improving the efficiency of anaerobic digestion by increasing the biodegradability of the sewage sludge: solubilizing much of the nutrients and COD, breaking down the floc structure of the sludge, and producing a solution rich in VFAs. 5 1.6 Research objectives The objectives of this study involved the use of the MW-AOP to fill in the research gaps listed previously. The specific objectives were as follows: â¢ Confirm that the previously determined optimal MW-AOP operating conditions hold at this larger 25 kW pilot-scale system. This involved, but was not limited to, comparing the results from specific hydrogen peroxide dosages and the temperatures at which the hydrogen peroxide is introduced. Key parameters studied included COD solubilization, VFA creation, nutrient solubilization, and TSS reduction. â¢ Evaluate the effects of MW-AOP-treated sludge in anaerobic digestion. Key parameters studied included the effects on solids destruction, biogas production, methane content, and hydraulic retention times. 6 2 Literature Review 2.1 Microwave-Enhanced Advanced Oxidation Process (MW-AOP) The microwave-enhanced advanced oxidation process (MW-AOP) is a thermal-chemical hydrolysis process which uses microwave irradiation in combination with hydrogen peroxide to generate hydroxyl radicals. The technology has been shown to be effective in the treatment of wastewater sludge (Remya and Lin, 2011; Serpone et al., 2010; Tyagi and Lo, 2013). In particular, MW-AOP has been shown to be an effective pre-treatment for nutrient recovery and anaerobic digestion, as well as reducing sludge volume (Chan et al., 2010; Liao et al., 2007; Lo et al., 2018b, 2017, 2016, 2015; Wong et al., 2006; Yi et al., 2014). Thermal hydrolysis itself is an effective pre-treatment for sewage sludge, and can use conventional or dielectric heating (Eskicioglu et al., 2006, 2007c). Both Cambi (Cambi, Norway) and Bio Thelys (Veolia Water Technologies, France) are industrial thermal hydrolysis processes currently commercialized and used in WWTPs, and see sludge conventionally heated to 150-180ËC for 30-60 minutes (Carrère et al., 2010). MW-AOP, however, is a dielectric heating process using microwaves. As opposed to conventional heating, dielectric heating brings the advantage of alleviating temperature gradients between the surface and centre of the substrate, which can arise as substrates thickness increases (Laughton and Warne, 2003). North America generally uses two different microwave frequencies: 915 MHz and 2450 MHz. Household microwave ovens typically use 2450 MHz, whereas industrial processes, including MW-AOP, more commonly use 915 MHz. This lower frequency of 915 MHz allows for greater penetration of the microwaves into a given substrate. Additionally, 915 MHz systems are 7 associated with higher overall system efficiency and lower operating costs (Laughton and Warne, 2003). Hydrogen peroxide (H2O2) is an oxidizing agent; however, when used in combination with microwave irradiation, the hydrogen peroxide is broken into two hydroxyl radicals (OH-). Hydroxyl radicals are highly reactive and are much stronger oxidants than hydrogen peroxide. Thus, the MW-AOP uses the synergy between microwave irradiation and hydroxyl radicals to degrade organics in wastewater sludge at a faster rate than other oxidation methods (Remya and Lin, 2011). These degraded organics are further oxidized to volatile fatty acids, nutrients such as nitrogen and phosphorus are solubilized (Liao et al., 2007; Lo et al., 2017; Wong et al., 2006). The efficacy of the MW-AOP has been shown to be dependent on a combination of four variables: hydrogen peroxide dosage, temperature, heating time, and microwave intensity. Previous studies have shown that the amount of H2O2 added during the MW-AOP has a significant effect of the efficacy of the treatment, notably the solubilization of COD, reduction of TSS, and increase of VFA (Liao et al., 2007; Lo et al., 2017, 2016, 2014; Wang et al., 2009; Wong et al., 2006). Kenge et al. (2009) found that for best solids destruction using municipal sewage sludge with a TS percentage between 1.2 â 2.8%, an ideal dose of hydrogen peroxide is approximately 0.3 %H2O2/%TS at 120ËC. However, at treatment temperatures of around 90ËC, less variance occurred between doses of 0.4-1.0 %H2O2/%TS (Lo et al., 2017). Temperature of the substrate during treatment has two important functions in the MW-AOP treatment of aerobic municipal wastewater sludge. The first is the deactivation of catalase, an enzyme present in all aerobic cells. Catalase is a terminal respiratory enzyme which breaks 8 down hydrogen peroxide into water and molecular oxygen, protecting cells from reactive oxygen species (Wang et al., 2009). Thus, when catalase is present the MW-AOP requires a higher dose of H2O2 to achieve the same results as when no catalase is present. Catalase is temperature sensitive, and gradually becomes inactive at temperatures above 40ËC. It has been found that by first heating the MW-AOP substrate to at least 60ËC before hydrogen peroxide is dosed, better results were achieved and a significant increase in treatment efficiency was seen (Lo et al., 2016, 2015, 2014; Nkansah-Boadu et al., 2015; Wang et al., 2009). The second function of temperature is its effect on hydrogen peroxide. At temperatures above 80ËC there is a possible increase in the formation of hydroxyl radicals due the disintegration of hydrogen peroxide (Eskicioglu et al., 2008). Using the MW-AOP to treat secondary aerobic sludge in a batch mode, Wong et al. (2006) found that at a treatment temperature of 80ËC with a hydrogen peroxide dose of 1.5% (i.e. an H2O2 weight equal to 1.5% of the weight of the 30 mL sludge sample), 65% of COD was solubilized; and at 100ËC, 77% of COD was solubilized. An increase in hydrogen peroxide dosage with high temperature (90-110ËC), and thus an increase in hydroxyl radicals, has been shown to bring other effective results, notably an increase in solubilized total phosphate (TP), TKN, SCOD, and VFA, as well as reduced total suspended solids and improved sludge dewaterability (Lo et al., 2017). Heating time is also significant in solubilizing COD, although not as significant as temperature and hydrogen peroxide dosage (Lo et al., 2014; Wong et al., 2006; Yu et al., 2010). Heating time is differentiated by the mode of operation in which the MW-AOP is used. In batch mode the heating time is the time needed to elevate the substrate temperature to the desired temperature. This is controlled by the âramp rateâ (ËC/min). It has been shown that for a batch 9 mode system the most effective treatment occurs with the highest ramp rate and a holding time of ten minutes (Chan et al., 2010). The ten minute hold time, however, was dependent on a specific substrate and may change accordingly with different substrates. In continuous-flow mode the heating rate is âcontrolled by its flow rate through the microwave applicatorâ (Lo et al., 2017). This flow through the applicator controls the hydraulic retention time (HRT), which could be considered as an operational parameter of continuous-flow MW-AOP systems. It was shown that a longer applicator HRT was beneficial to the efficacy of the MW-AOP in terms of organic matter solubilization, and a shorter HRT was beneficial in terms of orthophosphate release (Yu et al., 2010). 2.1.1 Solids Destruction 2.1.1.1 Chemical Oxygen Demand (COD) An important result of the MW-AOP is its solubilization of chemical oxygen demand (COD). Two distinct processes are hypothesized to be involved in this increase in soluble chemical oxygen demand (SCOD). The first is thermal decomposition, which involves the breakdown of extracellular polymeric substances (EPS), as well as cell lysis (Appels et al., 2010). This causes the âbreakdown of large particulate organic matter into smaller and more soluble organic componentsâ (Lo et al., 2016). The second process involves the oxidation of the decomposed organic products (Shanableh and Shimizu, 2000a). As such, an increase in hydrogen peroxide used in the process should give rise to an increase in SCOD (Lo et al., 2016). Too much hydrogen peroxide, however, could result in extra H2O2 remaining in the effluent, and could give erroneous results in the lab determination of COD (Lo et al., 2016). 10 The timing of H2O2 dosing is critical in the MW-AOP due to catalase. Catalase is a âterminal respiratory enzyme, [and] is present in all aerobic cellsâ (Lo et al., 2014). If H2O2 is added while active catalase is present the catalase will consume a portion of the H2O2, requiring higher dosing amounts. Catalase gradually becomes inactive at temperatures above 40ËC, and is essentially fully deactivated at temperatures of around 60ËC (Lo et al., 2014); hence, it has been shown that SCOD values are much higher when H2O2 is added once the substrate has already exceeded 60ËC (Lo et al., 2016, 2014). The final process temperature also has an effect on SCOD. Systems open to atmospheric pressure are unable to exceed 100ËC. In these cases treatment temperatures in a continuous-flow system of 90ËC showed a COD solubilization of 52% with an H2O2 dose of 0.5% H2O2/%TS (Lo et al., 2016). For pressurized continuous-flow systems temperatures of 110ËC showed a COD solubilization of 81% with an H2O2 dose of 0.4% H2O2/%TS (Lo et al., 2017). Flow rate has also been shown to have an effect on COD solubilization: higher flow rates give rise to higher SCOD values (Lo et al., 2016). This is likely related to the fact that mixing also improves COD solubilization during the MW-AOP treatment (Kenge et al., 2008). However, higher flow rates also lead to a lower substrate HRT in the microwave applicator, resulting in lower amounts microwave radiation reaching the substrate (Yu et al., 2010). 2.1.1.2 Suspended Solids The MW-AOP has been shown to be effective in reducing total suspended solids (TSS) (Lo et al., 2017, 2016). TSS reduction is controlled by H2O2 dosage and system flow rate: a higher H2O2 dosage resulted in greater TSS reduction, whereas a lower flow rate also resulted in 11 a greater TSS reduction (Lo et al., 2016). Although thermal treatment does damage the sludge membrane, it does not affect the floc structure. With the addition of H2O2 the TSS reduction was significantly greater due to the thermal-chemical treatment process also reacting with and degrading âextracellular and intracellular materials to produce soluble organic materialsâ (Lo et al., 2016). As such, the profile for particle size distribution is altered from its initial state. 2.1.1.3 Volatile Fatty Acids Volatile fatty acids (VFA) are produced during the MW-AOP, and follow the same trend as SCOD: higher H2O2 dosages favour VFA production, as do higher flow rates (Kenge et al., 2009; Liao et al., 2007; Lo et al., 2016). A proposed model for this is that âintermediate oxidation products and thermal decomposition products are formed first in the oxidation processesâ, and are then âfurther oxidized to form final products, such as VFA and CO2â (Liao et al., 2007; Shanableh and Shimizu, 2000b). The increase in VFA and other intermediate oxidation products also causes a drop in the pH of the treated solution (Lo et al., 2014). 2.1.2 Nutrient and Metal Release The MW-AOP solubilizes nutrients and metals, five of which have been regularly studied in UBCâs MW-AOP research: nitrogen, phosphorus, calcium, magnesium, potassium. These five nutrients are of interest as MW-AOP-treated substrates have proven to be good candidates for nutrient recovery in the form of struvite, a slow-release fertilizer (Liao et al., 2005a, 2005b; Wei et al., 2003). 12 2.1.2.1 Nitrogen The MW-AOP breaks down EPS and causes cell lysis, resulting in SCOD and soluble TKN concentrations increasing in the treated solution. Compared with thermal treatment, MW-AOP treatment produces much higher concentrations of soluble TKN (Lo et al., 2016). It has been shown that the MW-AOP liberates protein from particulate into soluble form, and that nearly half of those proteins are then converted into amino acids (Yi et al., 2014). Ammonia is also released during the MW-AOP treatment, and microwave temperature and H2O2 dosage have been shown to be the most important factors affecting soluble TKN and ammonia release (Lo et al., 2016; Wong et al., 2006). 2.1.2.2 Phosphorus Phosphorus release has been shown to be due mostly to treatment temperature (Kuroda et al., 2002), and is not significantly affected by H2O2 (Lo et al., 2016). A proposed mechanism for this release is âthe cell membrane of sludge microorganisms was partially disrupted by heatingâ, allowing polyphosphates to âdiffuse out of the cytoplasmâ (Danesh et al., 2008; Kuroda et al., 2002). It was also concluded that âthe release of phosphorus was dependent on its existent phosphorus forms in the sludge and EPSâ (Lo et al., 2016). These polyphosphates may then be broken down into orthophosphate (Chan et al., 2010). However, it has also been seen that above 60ËC orthophosphate concentrations decrease. It was proposed that due to the heating, âhigh energy phosphate-to-phosphate bonds may be formed to yield phosphate anhydride (pyrophosphate), resulting in a decrease in orthophosphate in the treated solutionâ (Lo et al., 2016). 13 2.1.2.3 Metals Metal release occurs during MW-AOP treatment and is important particularly for struvite recovery. Magnesium is one of the three constituents of struvite, along with ammonia and phosphate (Le Corre et al., 2009). Additionally, potassium may be used in lieu of magnesium in K-struvite (potassium ammonium phosphate). Calcium is detrimental to the formation of struvite, but may be precipitated out of solution using oxalic acid (Lo et al., 2017; Zhang et al., 2015). According to Wong et al. (2006) âmetal concentrations of the sewage sludge were found to be independent of microwave temperature and hydrogen peroxide concentrationâ. Furthermore, âit appears that microwave heating alone is equally as effective as the combination of microwave heating and hydrogen peroxideâ (Wong et al., 2006). In the previous experiments done at UBC, soluble metal concentrations increased after MW-AOP treatment (Liao et al., 2005a; Lo et al., 2016; Wong et al., 2006). Accordingly, âthese results most likely indicate that the metals in the sewage sludge are completely released into solution with bacterial cell lysis under microwave treatmentâ (Wong et al., 2006). 14 2.2 Anaerobic Digestion Anaerobic treatment is a widely used process for stabilizing municipal wastewater sludges in an environment devoid of oxygen. The main benefits of anaerobic digestion are its ability to reduce the mass and volume of sludge while producing biogas rich with methane as a by-product. However, drawbacks of anaerobic digestion are the long hydraulic retention time (HRT) necessary to ensure adequate reduction of sludge, as well as elevated temperatures for successful operation. Anaerobic digestion produces far less biological solids than aerobic systems, reducing costly requirements for solid waste disposal; only 5 â 15% of biochemical oxygen demand (BOD) is converted into biological solids, whereas aerobic treatment can see up to half of BOD converted into biological solids (Rittmann and McCarty, 2012). The synthesis of biomass requires essential nutrients such as nitrogen and phosphorus, but as anaerobic treatment produces less biomass, its nutrient requirements are also lower. Finally, anaerobic treatment can withstand a higher organic loading rate (OLR) compared with aerobic treatment.; therefore, anaerobic treatment may be suitable for more concentrated wastewaters (Rittmann and McCarty, 2012). Anaerobic digestion can be simplified into three main steps: hydrolysis, fermentation, and methanogenesis. First, hydrolysis is where cells and extracellular polymeric substances (EPS) are broken down into particles small enough to be transported across bacterial cell membranes. In the fermentation step, the products from hydrolysis are converted by bacteria â acidogens and acetogens â into organic acids, and eventually into volatile fatty acids (VFAs). Finally, bacteria 15 called methanogens consume the VFAs and produce methane, carbon dioxide, and water as final products (Tchobanoglous et al., 2014). Due to the slow nature of anaerobic digestion, a long hydraulic retention time and/or solids retention time is necessary. A typical continuously-stirred tank reactor (CSTR) anaerobic digester may have an HRT of 15 â 25 days (Rittmann and McCarty, 2012). Hydrolysis, fermentation, and methanogenesis are all related to the HRT (or SRT). As such, changing either of these steps could affect the HRT and, conversely, changing the HRT (or SRT) could affect any of these three reactions (Tchobanoglous et al., 2014). It is desirable to have a lower HRT as this brings benefits of either allowing more volume to be treated in existing tanks, or else allowing for the reduction of tank volumes in a WWTP. Hydrolysis is generally considered to be the rate limiting step in anaerobic digestion, and thus is directly related to the HRT (Abelleira et al., 2012; Appels et al., 2011; Rittmann and McCarty, 2012; Tchobanoglous et al., 2014). As such, much research has gone into pre-treatment methods for improving the digestion efficiency in anaerobic digestion including thermal, mechanical, ultrasonic, and chemical processes (Ariunbaatar et al., 2014; Carlsson et al., 2012; Zhang et al., 2014). Thermal-chemical processes such as the MW-AOP may be good candidates for pre-treatment for anaerobic digestion as they result in both thermal hydrolysis and the creation of VFAs (Liao et al., 2007). However, few anaerobic digestion studies have actually been performed using MW-AOP-treated substrate. 16 2.2.1 MW-AOP as Pre-treatment for AD Conventional thermal pre-treatment (thermal hydrolysis) was shown to produce upwards of twenty times the amount of total methane, and it was found that a high temperature and long holding time (90ËC at 30 and 60 minutes) had the most beneficial effects. The temperature and holding time greatly increased the amount of VFA and slightly increased the amount of SCOD in the substrate (Appels et al., 2010). Athermal pre-treatment using microwaves has also been studied, and has shown increased biogas production as well as better biodegradability (Eskicioglu et al., 2007c, 2007b, 2007a, 2006). Abelleira-Pereira et al. (2012, 2015) compared the effects of using thermal hydrolysis (TH) through steam injection versus advanced thermal hydrolysis (ATH) on methane production in anaerobic digestion. ATH is thermal hydrolysis with H2O2 added as an oxidant to increase solubilization. They performed their experiments at three different temperatures and three different H2O2 doses. Their results showed that all pre-treated samples produced more total methane than the control. Additionally, it was found that generally TH performed better than AHT (i.e. H2O2 had an inhibitory effect when compared to HT without H2O2) except at the temperature of 115ËC (Abelleira-Pereira et al., 2015). Liu et al. (2017) used the MW-AOP at 100ËC to study the effects of H2O2 on anaerobic digestion. They found that low H2O2 dosages (0.2 %H2O2/%TS) had positive effects on methane production, whereas higher doses (0.6 and 1.0 %H2O2/%TS) had negative effects on methane production. Their conclusion was that any unreacted excess H2O2 remaining in the pre-treated sample had inhibitory effects in all three anaerobic digestion stages (Liu et al., 2017). Following this, Liu et al. (2018) found that mixing pre-treated and untreated substrate in a 1:1 ratio 17 negated the negative effects of excess H2O2, possibly through the re-introduction of live microorganisms, most of which would be destroyed during the MW-AOP treatment. These microorganisms include catalase which breaks down H2O2 into water and oxygen (Liu et al., 2018). Finally, Eskicioglu et al. (2008) performed anaerobic digestion experiments using MW-AOP-treated waste activated sludge (WAS) in the temperature range of 60-120ËC. In all temperatures tested the MW-AOP-treated samples showed less biodegradability and lower ultimate methane yields compared to untreated samples and samples only treated with microwave irradiation. 18 3 A 25 kW MW-AOP Pilot-Scale System to Treat Sludge 3.1 Introduction Sludge is an unavoidable by-product of wastewater treatment and is âby far the largest in volume, and its processing, reuse, and disposition present perhaps the most complex problem in the field of wastewater treatmentâ (Tchobanoglous et al., 2014). General sludge handling involves stabilization, dewatering, and disposal. Municipal WWTP sludge is nutrient-rich and is becoming more frequently viewed as a potential resource: nitrogen and phosphorus can be recovered and used as fertilizer; and methane is a by-product of anaerobic treatment processes. Pre-treatment processes are often employed in order to maximize the potential for resource recovery from wastewater sludge (Ariunbaatar et al., 2014; Carlsson et al., 2012). Thermal hydrolysis is a popular pre-treatment currently commercialized and used in WWTPs (Tchobanoglous et al., 2014). The microwave-enhanced advanced oxidation process (MW-AOP) is a thermal-chemical hydrolysis process that uses microwave irradiation in combination with hydrogen peroxide to generate hydroxyl radicals. The MW-AOP has been shown to be an effective pre-treatment process for WWTP sludge. Notably, the MW-AOP is able to solubilize a large fraction of COD, and results in a nutrient-rich solution ideal for nutrient recovery and/or anaerobic digestion. Additionally, the process results in a high overall reduction in solids and brings the added benefit of enhanced settling and improved dewaterability (Liu et al., 2018; Lo et al., 2016; Wang et al., 2009; Wong et al., 2006). Pilot-scale studies using the MW-AOP have been previously undertaken at the University of British Columbia (UBC) using wastewater sludge from the UBC Pilot Plant and local WWTPs 19 and farms. The studies initially used a benchtop 1 kW microwave generator to treat small samples of sludge. Eventually the studies were upgraded to a pilot-scale 5 kW microwave generator to treat modest volumes of sludge (20 L per run), and producing a set of optimal operating conditions. (Lo et al., 2018b, 2017, 2016; Srinivasan et al., 2018; Tan, 2017; Wong et al., 2006). With the goal of eventually achieving full-scale operation in a WWTP, the next step was a larger pilot-scale experiment using a 25 kW microwave generator and aiming to treat much larger volumes per batch (100-300 L per run) with more rapid heating times. The 25 kW microwave system used in this study was set up and installed near a local WWTP. The objective of this study was to use the 25 kW system to treat thickened waste secondary sludge (TWSS) and confirm that results were consistent with previous studies of low temperature (90ËC), semi-batch experiments, as well as to make any recommendations necessary to improve future set-ups. Furthermore, based on the results of this study, future studies would see the current 25 kW microwave system upgraded to a high temperature, high pressure system. This will allow treatment of sludge of 110-120ËC which has been shown to be an optimal temperature range for MW-AOP treatment (Kenge et al., 2009). 3.2 Materials and Methods 3.2.1 Microwave Unit The 25 kW 915 MHz continuous-flow microwave system consisted of a GMG-925 microwave generator (China) connected to an isolator, an automated impedance tuner, and a custom-designed applicator cavity. The system was housed in a 20-foot steel shipping 20 container. The generator used a 915 MHz magnetron and integrated isolator with a water-cooled load. An automated three-stub impedance tuner helped match the varying load impedance to the generator, ensuring efficient power transfer to the load. The automated tuner was integrated to WR975 flanges and wave-guide. The applicator was constructed of welded aluminum panels 24 inches (60 cm) tall by 34 inches (86 cm) long. A two-inch (5 cm) inner diameter silicone applicator tube with stainless steel end fittings was installed inside the aluminum cavity. The applicator was a four-pass design consisting of four silicone pipes with tri-clamp fittings attached. The four sections were joined together in series by stainless steel U sections and elbows. The total exposure length in the applicator was 192 inches (4.88 m) and a total volume of 9.6 L. The experiments were run at approximately 22 - 25 L/min, giving a residence time of about 20 - 25 seconds per pass. The microwave unit was operated between 10 â 22 kW, depending on the desired temperature rise per pass. 3.2.2 Substrate Sludge used in the experiments was collected from a WWTP located in British Columbia (referred to in this study as MUN). MUN WWTP provides secondary treatment using trickling filters, and has a large catchment area which receives a variety of influents. MUN influent can be generalized as 50% residential, 20% commercial and institutional, 5% industrial, and 25% inflow and infiltration. Its thickened waste secondary sludge (TWSS) characteristics are summarized in Table 1. 21 Table 1 â Averaged characteristics of MUN WWTP thickened secondary sludge. All values normalized per gram of TS. Note: TS% was between 3.6 â 5.1% Characteristic Value Characteristic Value TSS (g/L) 7.9 Total TP (mg/L) 13.5 TCOD (mg/L) 1540.0 Soluble TP (mg/L) 4.2 SCOD (mg/L) 249.0 Ortho-P (mg/L) 3.4 VFA (mg/L) 125.0 Potassium (mg/L) 4.6 TKN (mg/L) 61.0 Magnesium (mg/L) 1.5 Soluble TKN (mg/L) 19.5 Calcium (mg/L) 1.0 Ammonia (mg/L) 9.7 3.2.3 System setup The microwave unit and necessary equipment was kept in a 20-foot steel shipping container (MW trailer) (Figure 1). Sludge delivered from MUN was kept in a 6000 L container inside a storage building near the MW trailer. Since only a fraction of the sludge was needed at any time, a 350 L conical feed tank was installed beside the MW trailer. A submersible pump was lowered into the 6000 L holding container and sludge could be pumped to the feed tank as needed. A feed pump circulated sludge from the feed tank through the applicator and back into the feed tank, and was operated at 22 L/min. Hydrogen peroxide was injected at a port located just before the sludge entered the applicator, and was controlled by a separate peristaltic dosing pump. The sludge feed lines also included a heat exchanger, and this was used to capture heat from the microwaved sludge exiting the applicator and transfer it to the incoming sludge about to enter the applicator. Generally, the heat exchanger resulted in a temperature rise of between 5-10ËC. 22 The main pump hose connected near the bottom of the feed tank; due to this, the sludge in the very bottom of the tank experienced limited mixing. To mitigate this another small pump was used to circulate the sludge from the very bottom of the tank to the top, ensuring adequate mixing throughout the tank. Figure 1 - Plan view of microwave trailer and system layout. Not to scale Temperature probes were installed at four different locations in the system: inside the feed tank; right before the sludge entered the heat exchanger; right after the sludge exited the applicator; and the hose outlet returning to the feed tank. These probes were used to help adjust the power settings of the microwave, ensuring the applicator temperature did not exceed 100ËC; and to determine the end of the experiment, i.e. when the feed tank temperature reached 90ËC. 23 3.2.4 Experimental Design Four different experimental runs were conducted, and are presented in Table 2. All experiments were performed using the provided MUN TWSS. The runs were considered complete when the bulk temperature of the feed tank reached 90ËC. The temperature was not held for any amount of time as was done in past experiments (Lo et al., 2014). Even though previous MW-AOP research has shown the best solids disintegration and nutrient release occurred at 120ËC and 0.80 g H2O2/g dry sludge (approximately 0.3 %H2O2/%TS) (Kenge et al., 2009), the current 25 kW system was not pressurized, thus the target temperature was only 90ËC. Additionally, in previous research done at UBC, microwave runs reaching 90ËC found that the solubilization differences that occur between H2O2 doses of 0.4 â 1.0% did not vary greatly (Lo et al., 2017). To remain conservative, the experiments at the MUN were dosed on the lower end: the target temperature was 90ËC, and the H2O2 varied between 0.1 â 0.3 %H2O2/%TS. Table 2 â Experimental conditions for MW-AOP treatment of MUN TWSS Set Number Run Date H2O2 Dosage (% H2O2/%TS) Temperature of H2O2 Injection (ËC) 1 Nov. 30, 2018 0.1 60 2 Dec. 11, 2018 0.2 60 3 Dec. 1, 2018 0.2 70 4 Dec. 12, 2018 0.3 70 3.2.5 Experiment Run Operation Sludge was moved from the 6000 L indoor holding tank into the feed tank outside the MW trailer. The microwave generator was allowed to warm up, and the feed and mixing pumps 24 were turned on. The sludge was heated by the microwaves until the bulk temperature in the feed tank reached either 60ËC or 70ËC, depending on the experimental plan. At this point the MW systemâs magnetron was turned off and the desired amount of H2O2 was dosed through a peristaltic pump. The magnetron was turned off to ensure the temperature of the substrate in the applicator did not exceed 100ËC: due to unlevelled lines in the system (see Discussion), the reaction between hydrogen peroxide and substrate sometimes caused gas buildup in the lines and lead to unsteady flow and temperature spikes. Once the H2O2 was fully dosed the magnetron was again turned on and the substrate was irradiated until the bulk temperature of the feed tank reached 90ËC. At this point the experimental run was considered complete. The treated substrate was allowed to cool, drained from the feed tank, and returned to UBC for storage and analysis. 3.2.6 Sample and Data analysis Both the raw and treated samples were centrifuged at 15000 rpm for ten minutes and vacuum-filtered using 0.45-μm fiberglass filter paper. The soluble portions were analyzed for soluble chemical oxygen demand (SCOD), ammonia, volatile fatty acids (VFA), orthophosphate (ortho-P), soluble total phosphate (soluble TP), soluble total Kjeldahl nitrogen (STKN), soluble calcium (Ca), soluble potassium (K), and soluble magnesium (Mg). In addition, the total portions of the samples were analyzed for total solids (TS), total suspended solids (TSS), volatile solids (VS), volatile suspended solids (VSS), total phosphate (TP), total Kjeldahl nitrogen (TKN), total chemical oxygen demand (TCOD), Ca, K, and Mg. All chemical analyses were conducted in accordance with APHA standard methods (Rice and American Public Health Association, 2012). 25 Ammonia, ortho-P, TP, and TKN analyses were done using an automatic ion analyzer. Ca, K, and Mg were determined using inductively coupled plasma (ICP) optical emission spectroscopy (Optima 7,300 DV, PerkinElmer, Massachusetts); COD was analyzed using a Hach DR2008 Spectrophotometer. For VFA, a Hewlett Packard 6890 Series II gas chromatograph equipped with a flame ionization detector was used. Total VFA was calculated in terms of acetic acid equivalent. Statistical analyses were carried out using Microsoft Excel. Regression and one-way Analysis of Variance (ANOVA) were performed to compare the effect of multiple levels of temperature and dosage on nutrient release and solids solubilization. 3.3 Experimental Results 3.3.1 Solids Destruction The ratio of SCOD to TCOD increased in all four of the experimental sets compared with the initial substrates, with the highest degree of solubilization occurring in set 4 (Table 3 and Table 4). (Note that Table 4 â as with other tables in this thesis â has been normalized by grams of TS. As the TS varies between each experimental run (see Discussion), normalizing by this value allows an easier comparison between results). The TCOD values remained relatively constant throughout the experiments, signifying that hydrogen peroxide was not overdosed and was not remaining in excess. It was expected that as dosing temperature and H2O2 doses increased so would the amount of COD solubilized. Apart from set 1 â which is explained in the Discussion section of this chapter â the data seem to follow the predicted trend; regression and ANOVA support this relationship between H2O2 dose, dosing temperature, and solubilized COD 26 found in this study. Previous research had also shown the strong relationships between these parameters (Chan et al., 2010; Wong et al., 2006). TSS levels did not change in set 1, and is likely due to the low dose of H2O2 applied, as well as incomplete mixing and treatment (see Discussion). SCOD in set 1 did increase, however, and this may have been attributed more to thermal decomposition. Thermal decomposition will lead to some solubilization of COD, but does not greatly affect TSS levels (Lo et al., 2016). Sets 2 and 4 saw the highest TSS reductions of approximately 40%, whereas set 3, had lower TSS reduction of only 21%. Possible reasons for the differences seen between actual results and expected results are examined in the Discussion section of this chapter. Table 3 - Solids disintegration of MW-AOP-treated sludge Set pH TS (%) TSS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Initial 5.69 5.09 46.9±3.4 13927±432 82963±4672 5914±195 1 5.44 4.1 37.4±0.8 18616±1032 63890±1906 4981±323 2 4.59 2.29 13.1±6.4 24825±1225 40538±1540 5032±125 3 4.55 5.55 40.4±2.8 32228±1728 80542±1526 12668±1204 4 4.53 3.98 21.6±0.2 32456±566 60869±1217 7467±291 Table 4 - Solids disintegration values normalized by gram of TS Set TSS (g/L) TSS Reduction (%) SCOD (mg/L) TCOD (mg/L) SCOD/TCOD VFA (mg/L) VFA/SCOD Initial 9.16±3.3 - 272±8 1622±133 0.17 115±4 0.43 1 9.12±0.2 0 454±25 1558±46 0.29 121±8 0.27 2 5.72±2.8 38 1084±54 1770±67 0.61 219±5 0.20 3 7.28±0.5 21 580±31 1451±27 0.40 228±22 0.39 4 5.43±0.5 41 815±14 1529±31 0.53 187±7 0.23 27 3.3.2 VFA production and composition VFA increased in sets 2-4 but remained the same in set 1 (Table 4 and Figure 2); the drop seen in pH for the treated samples is generally correlated with this increase in VFA. However, the increase was not as significant as in previous research which saw VFA levels increase by as much as 20 times for similar H2O2 dosing and temperatures (Lo et al., 2017, 2016); however, it is also likely that the VFA production in the MW-AOP is dependent on the initial characteristics of the raw substrate, such as the composition of the organic matter. Acetic acid was found to increase the most in sets 2 and 3. The MW-AOP is considered a potentially viable pre-treatment method for anaerobic digestion due to its ability to solubilize organics and produce VFA. Furthermore, methanogens in anaerobic digestion convert acetic acid very efficiently to methane gas; hence, it would be desirable to produce MW-AOP-treated sludge with the highest levels of acetic acid. Figure 2 - Change in VFA composition after MW-AOP treatment. VFA values were normalized by gram of TS in the samples in order to give a clearer comparison 050100150200250Initial Set 1 Set 2 Set 3 Set 4VFA (mg/L as acetic acid)VFA Composition Normalized by Grams of TSAcetic Propionic Iso-Butyric Butyric Iso-Valeric Valeric 28 3.3.3 Nutrient and Metal Release For each of the treated sets an increase was seen in the ratio of soluble TKN to total TKN (Table 5Table 6). Thermal-chemical treatments such as MW-AOP are thought to decompose extracellular polymeric substances (EPS) and cause cell lysis â and thus the release of amino acids and proteins â leading to increases in soluble TKN in the treated solution (Lo et al., 2016). Sets 2-4 saw their STKN:TKN ratios approximately double, whereas set 1, with its low H2O2 dose, saw only a 30% increase. It is likely that set 1 did not receive full treatment as detailed in the Discussion section of this chapter. Ammonia did not increase as a fraction of TKN and decreased as a fraction of STKN, implying that the TKN solubilized under MW-AOP was not converted to ammonia. Neither thermal nor thermal-chemical processes are very effective in releasing ammonia from sludge (Liao et al., 2007; Lo et al., 2017, 2016). Phosphorus was effectively solubilized during the MW-AOP treatment (Table 6). However, previous research has shown that phosphorus release is determined mostly by treatment temperature rather than by H2O2 dose (Danesh et al., 2008; Lo et al., 2017, 2016). Except for set 1, orthophosphate increased relative to total phosphorus. Phosphorus release is likely related to the makeup of each individual substrate, and it was concluded by Lo et al. (2016) that âthe release of phosphorus was dependent on its existent phosphorus forms in the sludge and EPSâ. Except for set 1, metals were all effectively released during MW-AOP treatment (Table 8). Soluble calcium increased from 30% of total calcium to between 45-80% in sets 2-4; magnesium increased from 50% to 60-90% in sets 2-4; and potassium was nearly completely solubilized in sets 2 and 3. 29 Table 5 - Nutrient release of MW-AOP-treated sludge Set Ortho-P (mg/L) Sol TP (mg/L) Total TP (mg/L) Ammonia (mg/L) Sol TKN (mg/L) Total TKN (mg/L) Initial 212±23 268±18 623±33 459±5 1055±33 3041±427 1 150±0.5 248±1 530±28 409±2 1140±127 2653±96 2 157±4 259±26 382±8 393±7 1530±26 2034±146 3 188±49 364±6 414±15 405±59 1520±216 2357±150 4 191±1 361±2 609±1 473±5 1750±90 2657±360 Table 6 - Nutrient release values; normalized by gram of TS of each sample for clearer comparison Set Ortho-P (mg/L) Sol TP (mg/L) Total TP (mg/L) Sol TP/ Tot TP Ortho-P/TP Ammonia (mg/L) Sol TKN (mg/L) Total TKN (mg/L) STKN/TKN Initial 4.2±0.5 5.3±0.4 12.2±0.6 0.43 0.35 9.0±0.1 20.6±0.6 59±8 0.35 1 3.7±0.0 6.1±0.0 12.9±0.7 0.47 0.28 10.0±0.0 27.8±3.1 65±2 0.43 2 6.9±0.2 11.3±1.1 16.7±0.3 0.68 0.41 17.2±0.3 66.9±1.1 89±6 0.75 3 3.4±0.9 6.6±0.1 7.5±0.3 0.88 0.45 7.3±1.1 27.4±1.1 43±1 0.64 4 4.8±0.0 9.1±0.1 15.3±0.0 0.59 0.31 11.9±0.1 44.0±2.3 67±9 0.66 Table 7 - Metals release of MW-AOP-treated sludge Set Sol K (mg/L) Total K (mg/L) Sol Mg (mg/L) Total Mg (mg/L) Sol Ca (mg/L) Total Ca (mg/L) Initial 211±7 251±56 86±3 179±10 14±1 48±3 1 150±3 170±7 82±0.8 165±7.8 12±0.1 41±2 2 164.98±4.11 160.78±8.33 84±7.6 110±3.2 15±1.4 24±1 3 189±5 195±5 110±1.9 121±4.3 19±0.2 24±0.7 4 244.06±42.8 292.6±13.38 109±1.4 180±12 20±0.2 44±2.9 30 Table 8 - Metals release values normalized by gram of TS; normalized by gram of TS of each sample for clearer comparison Set Sol K (mg/L) Total K (mg/L) Sol K/ Tot K Sol Mg (mg/L) Total Mg (mg/L) Sol Mg/ Tot Mg Sol Ca (mg/L) Total Ca (mg/L) Sol Ca/ Tot Ca Initial 4.13±0.14 4.92±1.10 0.85 1.68±0.07 3.50±0.19 0.48 0.27±0.01 0.94±0.06 0.29 1 3.66±0.07 4.15±0.17 0.88 2.00±0.02 4.02±0.19 0.5 0.29±0.00 1.00±0.05 0.29 2 7.2±0.18 7.02±0.36 1.03 3.67±0.33 4.80±0.14 0.76 0.66±0.06 1.05±0.04 0.63 3 3.41±0.09 3.51±0.09 0.97 1.98±0.03 2.18±0.08 0.91 0.34±0.00 0.43±0.01 0.79 4 6.13±1.08 7.35±0.34 0.83 2.74±0.04 4.52±0.30 0.61 0.50±0.01 1.11±0.07 0.45 3.4 Discussion There are several reasons why characteristics like SCOD, TSS, and VFA did not always follow the predicted trend, especially in set 1. The first is likely due to poor mixing, leading to incomplete treatment in set 1. The feed tank had a conical shape with two ports. One near the bottom for sampling, and a second port about one third of the way up (Figure 3). This second upper port was the feed port into the microwave system, and initially the volume of substrate lying below this port was not being mixed and was excluded from the continuous feed loop. To mitigate this problem a second pump was used to pull substrate out of the bottom sampling port and feed it back into the top of the tank. This modification occurred after set 1 had already been run; hence, this lack of mixing likely resulted in incomplete treatment of the bulk sludge, and was responsible for the results seen in set 1. Furthermore, to ensure complete mixing was taking place the feed tank was frequently manually stirred by hand, and a separate, independent temperature reading was taken from the bottom of the tank to ensure that the bulk temperature was uniform. 31 Figure 3 - Diagram of feed tank used in MW-AOP system. Not to scale Sampling practices may also have led to differences seen in the lab-determined values of TS%. When the desired experimental temperature of 90ËC had been reached and the experiment was deemed complete, the treated substrate was circulated in the system and allowed to cool to around 60ËC before it was collected. This generally took between 15-30 minutes depending on the ambient outdoor temperature. During that time it is likely that the substrate settled to some extent in the feed tank. MW-AOP-treated sludge generally shows very good settling characteristics (Lo et al., 2017); although it wasnât quantified in this study, the samples visually demonstrated good settling in the lab. The treated substrate was drained from the bottom sampling port into several 20-litre containers. Due to the probable settling that occurred it is likely that the total solids content of the treated effluent in each 20-litre 32 container did vary a little. For example, the first container filled may have contained a higher solids content than the final container filled; however, the containers werenât labeled in a way that indicated in which order they were filled. In the lab, a one-litre sample was taken for each experimental set from one of the 20-litre containers for analysis, but again it was unknown which container was sampled from for each set. Although this kept the sampling random, some discrepancies may have arisen from this sampling technique. For example, the sample tested for set 2 may have been the last container filled (with the lowest solids content), whereas the sample tested for set 3 may have come from the first container filled that day (with the highest solids content). Because of this, the data in the Experimental Results section of this chapter were also presented in a normalized form. This was done by dividing each value by the grams of TS in each respective sample, which allows for clearer comparison. Finally, some H2O2 doses were inaccurate because of settling occurring in the 6000 L holding tank. Initially, MUN TWSS was analyzed in the UBC lab before experimental runs began, and an average TS% of 3.6% was determined. Hydrogen peroxide doses in these experiments were calculated using %H2O2/%TS, and were thus based on this initial TS% value of 3.6%. During two of the four experimental runs an additional sample of the initial substrate was taken from the 6000L holding tank and later analyzed at UBC. The TS% determined afterwards was found to have an average of 5%. This would suggest that the H2O2 doses administered during the experiments were approximately 22% lower than planned. As shown in Table 2 the hydrogen peroxide doses for the experiments were from 0.1 â 0.3 %H2O2/%TS. In fact, the original experimental plan had called for doses in the range of 0.2 â 0.4 %H2O2/%TS. 33 As stated above, this difference in the TS% of the initial substrate was most likely caused by settling occurring in the 6000 L holding tank in which the MUN TWSS was stored. The TWSS remained in that tank over the course of nearly three weeks. At the time of the experiments any qualitative differences in the sludge solids between sets were not explicitly visible, especially as only 100 L of TWSS was withdrawn from the tank per day. However, at the end of the experiments the remaining sludge was released into waste drains. During this draining period that the sludge first exiting the tank was visibly thicker than the sludge coming out near the end of the draining period. This implied that some settling did indeed occur over its storage period. To mitigate this potential problem in future work it is recommended to either ensure the holding tank is completely mixed, or else transfer the sludge to the feed tank a day ahead of time and allowing for TS% to be measured overnight. Aside from mixing problems, flow problems also arose. The MW-AOP system setup used in this study did not have levelled lines, i.e. the sludge hoses had many changes in elevation. Additionally, there were several areas of expansions and contractions, bends, as well as other losses associated with things like the heat exchanger. As such, operating the feed pump at flow rates less than approximately 20 L/min resulted in unsteady flow. This became more apparent at high temperatures with hydrogen peroxide added as unsteady flow would result in gasification in the applicator and sudden temperature spikes. It was found that running the feed pump between 22-25 L/min produced relatively steady flow throughout all stages of the experiments. However, it is generally desirable to run the MW-AOP system at a low flow rate. This allows the sludge a longer residence time in the applicator and increases its exposure to 34 microwave irradiation. The upshot of this is that the sludge will see a greater change in temperature per pass, similar to the ramp rate used in batch mode treatments. Studies show that a higher ramp rate gives better results in terms of COD solubilization and VFA production (Chan et al., 2010; Lo et al., 2010). Furthermore, most previous studies by the UBC research group used a flow rate much lower than 20 L/min (Lo et al., 2016; Yu et al., 2010). To improve this system in future studies all lines should be levelled, losses minimized, and the flow rate kept as low as possible. Finally, it was felt that bad weather and exposure may have had negative impacts on the overall experiments. The four experiments were run from November 30 to December 12, 2018. During the runs the weather was cold, rainy, and occasionally snowy. The feed tank was located outside and adjacent to the microwave trailer, and was exposed to the elements. A lid on the tank prevented rain and snow from entering, but the tank itself was constructed of polyethylene and released much heat to the ambient environment. This likely reduced the overall efficiency of the system as more power was needed to attain the required bulk temperature of 90ËC. Insulation of the feed tank should be considered for any future MW-AOP system where holding tanks are located in exposed areas. 35 3.5 Conclusions The 25 kW MW-AOP system was a large power jump relative to the pilot-scale systems previously used by the research group at UBC. Overall, the experiments using this new system were considered a success. The results generally followed expected trends: VFA and solubilized COD increased; TSS decreased; nutrients such as nitrogen and phosphorus were solubilized; and calcium, magnesium, and potassium were all greatly solubilized. As such, the resulting MW-AOP-treated sludge could be a good candidate for energy recovery such as feed for anaerobic digestion, or for nutrient recovery such as struvite. Set 1 â at 0.1% %H2O2/%TS â seemed to be the outlier, and did not show the expected results of MW-AOP treatment. This is likely due to poor mixing and low hydrogen peroxide dosing â as related in the previous Discussion section â as well as possible negative effects from inclement weather. Based on this study of the initial trials of the 25 kW MW-AOP system, recommendations can be made to improve the system for future experiments: â¢ Transfer sludge to feed tank 24 hours before experimental run: Settling of the sludge seemed to occur in the 6000 L holding tank. As sludge for the feed tank was withdrawn from the bottom of the holding tank, a higher TS% was found when compared with the initial TS% measurements taken at the beginning of the experiments. Hydrogen peroxide dosage is based on TS%, therefore the hydrogen peroxide doses in this experiment were lower than anticipated. By transferring the sludge to the feed tank at least one day before the experimental run, sufficient time will be available to perform a TS% analysis to ensure proper H2O2 dosing volumes. 36 â¢ Install a mixer in the feed tank: Incomplete mixing can lead to unequal treatment of the sludge, difficulties ascertaining bulk temperature in the feed tank, as well as inconsistent solids content when sampling from the feed tank. By installing a mixer the tank can be considered as a CSTR and would help mitigate these problems. â¢ Level lines and reduce bends, expansions and other losses: Unlevel lines and other sources of losses caused uneven flow during low pumping rates. This required the feed pump to be run at a higher flow rate than desired, reducing the HRT in the applicator and the exposure to microwave irradiation per pass. Levelling lines and reducing other losses as much as possible will allow for lower pump flow rates, and will likely improve the overall efficacy of the MW-AOP. â¢ Insulate feed tank: As the size of the MW-AOP pilot-scale systems increase, so does the amount of sludge that can be treated. As such, the feed tank used in this study was large enough to require outdoor placement. During the experiments the outside weather was cold (~5ËC) and much heat was lost from the feed tank. Since the end point for the experiment was a bulk temperature of 90ËC, more energy from the microwave unit was used than necessary due to this lost heat. Insulating the feed tank would ensure greater system efficiency and would make the MW-AOP more versatile in inclement weather conditions. 37 4 Anaerobic Digestion Using MW-AOP-Treated Sludge 4.1 Introduction As cities and populations grow, so does the volume of wastewater produced on a daily basis. An unavoidable by-product of wastewater treatment is sludge, and the treatment and disposal of this sludge is a complex problems with many approaches. In many cities treated sludge from WWTPs is trucked to landfills or other land-based disposal sites (e.g. rock quarries, mine tailing sites, etc.) â an expensive undertaking. Hence, one of the important goals in sludge treatment is to reduce the final volume or mass of sludge as much as possible. Anaerobic digestion is a popular treatment option for sludge, and is a widely used process in WWTPs. In the absence of oxygen anaerobic treatment effectively stabilizes sludge and produces methane as a by-product (Rittmann and McCarty, 2012; Tchobanoglous et al., 2014). The predominant bacteria in anaerobic treatment are slow-growing microorganisms, and much of the substrate consumed by these bacteria is used for energy rather than growth. As such, anaerobic treatment produces far less biological solids than aerobic treatment: 5 â 15% of BOD is converted into biological solids, whereas aerobic treatment may have as much as 50% of BOD converted into biological solids (Tchobanoglous et al., 2014). However, anaerobic treatment is a slow process generally requiring a long sludge retention time; from 10 â 30 days in order âto allow significant destruction of volatile suspended solids (VSS) to occurâ (Tchobanoglous et al., 2014). This results in the requirement of either large tank volumes and elevated temperatures, or low flow rates and additional heat being supplied. Anaerobic treatment is often simplified to three key steps: hydrolysis, fermentation, and methanogenesis. Hydrolysis sees particulate matter converted into soluble compounds by 38 extracellular enzymes, which are then able to be transported through the cell membranes of bacteria. Fermentation includes acidogenesis and acetogenesis, and sees the soluble compounds converted into organic acids and hydrogen by the bacteria, and ultimately converted into acetate (Rittmann and McCarty, 2012). Finally, methanogens convert the acetate and some carbon dioxide into methane. Hydrolysis is generally considered to be the rate-limiting step in anaerobic treatment for complex organic substrates; methanogenesis is considered rate-limiting when the substrates are easily biodegradable (Abelleira-Pereira et al., 2015; Appels et al., 2010; Liu et al., 2017; Rittmann and McCarty, 2012; Tchobanoglous et al., 2014). To enhance the rate-limiting step of hydrolysis many pre-treatment methods have been explored, including chemical, physical, electrical, and thermal treatments (Ariunbaatar et al., 2014; Carlsson et al., 2012; Rice and American Public Health Association, 2012). The microwave-enhanced advanced oxidation process (MW-AOP) is a suitable candidate for effective pre-treatment before anaerobic treatment. As described in the previous chapter, the MW-AOP solubilizes a large fraction of COD and nutrients, and results in an increase of VFA, particularly acetic acid â the desired electron donor for methanogens. The objective of this study was to operate two anaerobic digesters in a lab-scale experiment. One digester was fed raw, untreated thickened waste secondary sludge (TWSS). The second digester was fed TWSS that had been treated using the MW-AOP. Key parameters monitored included changes in TS, VS, TCOD, SCOD, VFA, and alkalinity. As well, biogas was collected and analyzed for methane content. It was hypothesized that an anaerobic digester fed with MW-AOP-treated sludge would more efficiently treat the sludge, i.e. an equivalent amount of volatile solids destruction and methane production would occur at a lower HRT when 39 compared to the control reactor. This stems from the MW-AOP-treated sludge having a greater degree of solubilized COD, lower viscosity, and a higher VFA content. 4.2 Materials and Methods 4.2.1 Substrate and Inoculum Secondary sludge was collected from two WWTPs in British Columbia. One WWTP was located in a more rural, agricultural area, and the second was located in a more metropolitan area. The two WWTPs will be referred to in this study as AGR (for agricultural) and MUN (for municipal) respectively. Wastewater treated at AGR was comprised of roughly 50% municipal wastewater and 50% agricultural wastewater. The predominant constituent of the agricultural portion was wastewater created by a milk processing industry. Wastewater treated at MUN came from a large catchment area with a variety of influents. MUN influent can be generalized as 50% residential, 20% commercial and institutional, 5% industrial, and 25% inflow and infiltration. AGR sludge underwent MW-AOP using a pilot-scale setup run by UBC. The system utilized a slightly pressurized 5 kW MW system capable of treating substrate to temperatures above 100ËC. The MUN sludge underwent MW-AOP using a 25 kW MW system (see previous chapter for full details of treatment). Table 9 details the different MW-AOP conditions applied to the substrate used in this experiment. Table 10 shows the main characteristics of each of the substrate, and the âMW Treatmentâ column can be read as in the following example for MW-1: 0.5% H2O2 per TS%, added at 80ËC and subsequently heated to 110ËC. 40 Table 9 - Description of substrates fed to anaerobic digesters. âMW Treatmentâ column to be read (using MW-1 as an example): 0.5% H2O2 per TS%, added at 80ËC and subsequently heated to 110ËC Set Number Substrate MW Treatment AGR AGR TWSS Initial none MW-1 AGR TWSS 0.5% @ 80ËC - 110ËC MW-2 AGR TWSS 0.5% @ 90ËC - 110ËC MW-3 AGR TWSS 1.0% @ 75ËC - 110ËC MW-4 AGR TWSS 0.75% @ 70ËC - 110ËC MW-5 AGR TWSS 0.7% @ 70ËC - 110ËC MW-6 AGR TWSS 0.5% @ 60ËC - 110ËC MUN MUN TWSS Initial none MW-9 MUN TWSS 0.3% @ 70ËC - 90ËC Table 10 â Characteristics of inoculum, raw substrate, and MW-AOP-treated substrate used in anaerobic digestion Set Number pH TS (%) VS (g/L) VS/TS SCOD (mg/L) TCOD (mg/L) VFA (mg/L) INOCULUM 7.36 0.69±0.01 2±1 0.29 628±44 9157±271 10±4 AGR 6.17 3.39±0.63 26±5 0.77 1876±799 46586±6287 1331±624 MW-1 3.32 3.30±0.03 28±1 0.85 21668±1302 35524±2402 728±21 MW-2 3.38 3.90±0.10 32±1 0.82 26465±372 37593±3701 800±10 MW-3 3.71 2.20±0.10 18±1 0.82 16299±156 25244±5450 1377±41 MW-4 3.34 2.02±0.08 16±2 0.79 13755±561 25622±2114 1521±183 MW-5 3.21 2.50±0.01 21±2 0.84 14211±741 32185±2967 2046±262 MW-6 3.43 4.17±0.01 36±2 0.86 20713±605 41306±1059 2213±94 MUN 5.67 5.14±0.03 39±2 0.76 12867±204 81102±1980 5854±193 MW-9 4.55 5.55±0.05 43±2 0.77 32228±1728 80542±1526 4185±1418 41 The MW-AOP-treated substrate had a much lower pH due to the MW-AOP treatment. This is attributed to âthe formation of VFA and other forms of intermediate oxidation productsâ (Lo et al., 2014). It was desired for all substrates to have a similar pH; thus, the MW substrate was pH corrected to approximately 6.20. Initially the MW substrate pH was adjusted using 0.43 M sodium bicarbonate solution (Na2HCO3). Due to reactor problems (see next section) the solution was changed to a 50:50 wt/wt mixture of 1.0 M sodium hydroxide (NaOH) and 1.0 M potassium hydroxide (KOH). Approximately 75 g of the NaOH:KOH solution was required for every 1000 g of MW substrate. Inoculum used to start the reactors was collected as effluent from the anaerobic digesters at the AGR WWTP. The inoculum was allowed to settle overnight and then decanted such that only the settled solids remained. These solids were then used start up the reactors. The characteristics of the unsettled inoculum were determined in previous experiments and the basic characteristics are presented in Table 10 (Lo et al., 2018a). 4.2.2 System Setup Two reactors were used in the experiments: a control reactor and an experimental reactor (known as âcontrol reactorâ and âMW reactorâ) as shown in Figure 4. The reactors were constructed of acrylic cylindrical containers and each had a total volume of six litres. Each reactor was equipped with a two-paddle mixer. Two ports were located on each reactor: one for feeding and one for wasting. A port on top was fitted with polyethylene hose and connected to a programmable mass flow meter and totalizer (model: FMA-4304-ST; Omega Engineering), which in turn was connected to a laptop computer for data logging. Two peristaltic pumps were 42 connected to each reactor, one for feeding and the other for wasting. The reactors were situated in a water bath maintained at 35ËC. Figure 4 - Diagram of anaerobic digester used in experiments. Reactor volume equal to six litres. Not to scale 4.2.3 Experimental Design The anaerobic digesters were run at a number of different HRTs, starting at 16 days and ending at 3.6 days. Once each reactor reached steady-state at a given HRT, that HRT was then run for 2-3 cycles. The total sludge volume in the reactors was maintained between four and five litres. Two different waste sludges were used (see Substrate section above): AGR sludge was used from 16-day HRT to 5-day HRT; MUN sludge was used only for 5- and 3.6-day HRT. Initially the reactors were operated as expanded bed (EB) reactors: after feeding they were mixed for 10-30 minutes, but were then left undisturbed. During the 12-day HRT cycle the mode of operation was changed to a continuous stirred-tank reactor (CSTR) to compare with 43 the EB data. This mode of operation was used for the remainder of the experiment, and the two-paddle mixers were continuously run at 10 rpm. Throughout the remainder of this thesis the graphs and tables will display the HRT and operating mode. For example, 16 EB refers to a 16-day HRT run as an expanded bed; 8 CSTR refers to an 8-day HRT run as a CSTR; 5 MUN refers to a 5-day HRT using MUN sludge and run as a CSTR. Table 11 - Hydraulic retention times and operating modes used in experiment HRT (days and operation mode) Control Substrate Fed Microwave Substrate Fed Control Reactor OLR (kg VS/m3*day) Microwave Reactor OLR (kg VS/m3*day) 16 EB AGR MW-1 2 1.4 12 EB AGR MW-1 & 2 2.7 2.2 12 CSTR AGR MW-3 2.7 1.6 8 CSTR AGR MW-3,4, & 5 4.6 1.9 5 CSTR AGR MW-6 6.2 6.1 5 MUN MUN MW-9 5.2 6.8 3.6 MUN MUN MW-9 6.2 8.5 The organic loading rate (OLR) in the control reactor increased as the HRT decreased, as would be expected. However, there was a wide variance in the VS content of the MW-AOP-treated substrate: between 14 â 43 g/L. Therefore, as seen in Table 11, OLR does not always follow a linear trajectory as would be expected if influent VS was constant. 4.2.3.1 First Attempt at Reactor Start-up The reactors each had one litre of inoculum and one litre of feed substrate (raw or treated) added to them. 250 mL of substrate was added every day until the sludge volume of each reactor reached five litres. 44 The MW reactor seemed to fail after about two weeks: no biogas was being produced. Sodium bicarbonate had been used to adjust the pH of the MW substrate and it was felt that sodium toxicity may have been inhibiting the reactor. Sodium at levels between 3500-5500 mg/L are moderately inhibitory, and levels above 8000 mg/L are strongly inhibitory to anaerobic digesters (Tchobanoglous et al., 2014). Additionally, both reactor vessels were found to have minor air leaks. These leaks were fixed by replacing the gaskets and sealing any potential leak areas with silicone. 4.2.3.2 Second Attempt at Reactor Start-up Since the control reactor was operating well from the first start-up attempt, it now became the new inoculum for re-starting the MW reactor. The control sludge reactorâs contents were completely removed from the reactor and one litre of the sludge (now the inoculum) was added to each reactor. The reactors were daily fed 400 mL of substrate while 200 mL of sludge was wasted, giving a daily increase in working sludge volume of 200 mL. The MW substrate was this time pH-corrected with a mixed solution of sodium hydroxide and potassium hydroxide (see Substrate and Inoculum). The feeding/wasting continued until each reactorâs sludge volume reached four litres. Once at four litres the reactors had 250 mL fed and wasted each day, giving a 16-day HRT. During this start-up period the both reactors were producing biogas and were felt to be healthy. 45 4.2.4 Sample and Data Analysis Samples of feed substrate and wasted effluent were collected from each reactor daily, a total of four samples per day. Samples were centrifuged at 15000 rpm for 10 minutes and vacuum-filtered using 0.45-μm fiberglass filter paper. The soluble portions were analyzed for soluble chemical oxygen demand (SCOD), ammonia, volatile fatty acids (VFA), orthophosphate (ortho-P), soluble total phosphate (soluble TP), soluble total Kjeldahl nitrogen (soluble TKN). In addition, the total portions of the samples were analyzed for total solids (TS), total suspended solids (TSS), volatile solids (VS), volatile suspended solids (VSS), total phosphate (TP), total Kjeldahl nitrogen (TKN), and total chemical oxygen demand (TCOD) and alkalinity. All chemical analyses were conducted in accordance with APHA standard methods (Rice and American Public Health Association, 2012). Ammonia, ortho-P, TP, and TKN analyses were done using an automatic ion analyzer. COD was analyzed using a Hach DR2008 Spectrophotometer. Biogas production was measured using a programmable mass flow meter and totalizer (model: FMA-4304-ST; Omega Engineering). For VFA and methane, a Hewlett Packard 6890 Series II gas chromatograph, equipped with a flame ionization detector, was used. Total VFA was calculated in terms of acetic acid equivalent. Biogas was collected using one-litre Tedlar sample bags with a polypropylene septum fitting. 46 4.3 Experimental Results 4.3.1 Solids reduction The performance of the microwave reactor exceeded the control reactor in terms of total solids destruction and volatile solids destruction ( Figure 5). In the microwave reactor, total solids reduction ranged from 18-53% and volatile solids reduction ranged from 30-58% depending on the HRT; whereas the control reactor showed 11-26% and 10-28% respectively. According to Metcalf & Eddy a 30-day HRT in a mesophilic CSTR can bring a VS reduction of 50-65%, and a 20-day HRT can bring VS reduction of 50-60% (Tchobanoglous et al., 2014). Furthermore, Nges and Liu showed that a thermophilic CSTR â effective at HRTs of 15 days or more â drops below 40% VS reduction somewhere between a twelve- and nine-day HRT (Nges and Liu, 2010). The microwave reactor in this experiment showed a consistent VS reduction between 40-50% all the way down to a five-day HRT; only the 3.6-day HRT dropped below 40%. Total and volatile solids removal in terms of g/L can be found in Appendix A, and complete data tables for each HRT and operating mode can be found in Appendix C. VS reduction was calculated using the Van Kleeck equation, a simplification of the mass-balance equation. The Van Kleeck equation assumes no change in fixed solids during digestion, which can sometimes lead to an underestimation of the volatile solids reduced (Switzenbaum et al., 2003; Tchobanoglous et al., 2014). !"## = %"# '( )**+ â%"# '( +'-*./*+ .01+-*%"# '( )**+ â (%"# '( +'-*./*+ .01+-*)(%"# '( )**+) Ã 100 where RVSS = volatile solids destruction, % 47 WVS in feed = Weight fraction of digested sludge volatile content per total dry solids WVS in digested sludge = Weight fraction of volatiles out of digester per total dry solids The microwave reactor also showed consistently greater TCOD reduction than the control reactor. On the other hand, the control reactor generally showed higher SCOD reduction than the microwave reactor. However, this is misleading as the magnitude of the SCOD concentration in the MW-AOP-treated substrate was significantly higher than the untreated control substrate (Figure 6) â one of the effects of the MW-AOP treatment is that COD is solubilized during the process. In this respect the magnitude of the SCOD removed in the microwave reactor also exceeded that of the control reactor. 48 Figure 5 (clockwise starting at top-left) - a. Volatile solids reduction b. Total solids reduction c. SCOD d. TCOD 010203040506016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNVolatile Solids Reduction (%)Volatile Solids ReductionControl MW010203040506016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNTotal Solids Reduction (%)Total Solids ReductionControl MW0102030405060708016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNReduction (%)TCOD ReductionControl MW0102030405060708016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNReduction (%)SCOD ReductionControl MW 49 Figure 6 - SCOD removal in control and microwave reactors. Note that EB refers to expanded bed, CSTR refers to continuous stirred-tank reactor, and MUN refers to sludge from MUN used in CSTR mode 4.3.2 VFA, Alkalinity and pH VFA comparisons in this study were based on VFA concentrations in both the influent and effluent of the anaerobic digesters. However, it must be clearly noted that this is not the full picture: in a proper mass balance VFA are also produced by bacteria in the anaerobic digester, and are consumed and converted to biogas by bacteria and archaea. In this thesis, VFA production and the associated kinetics were not studied; therefore the comparisons made in the following section are simplifications. Based on comparisons of VFA concentrations in both the influent and effluent of the anaerobic digesters, the control reactor showed better VFA destruction both in terms of 05000100001500020000250003000016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNSCOD (mg/L)SCOD RemovalControl Feed Control Effluent MW Feed MW Effluent 50 percentage of VFA destroyed as well as the magnitude of VFA destroyed (Table 12). The exception to this was when the feed was changed to MUN substrate. At this point the microwave reactor began to outperform the control reactor, especially at the 3.6-day HRT where the MW reactor destroyed 68% of VFA compared to the control reactorâs 51% destruction. The control reactorâs VFA destruction remained relatively consistent, even with variations in the loading rate of influent VFA, destroying roughly 90% of total VFA. The MW reactor showed much less consistency, and varied between 11-91% destruction. Figure 7 shows the changes in VFA concentrations in both reactors. The VFA concentrations in MUN sludge was significantly higher than AGR; therefore, the data are also presented in Table 13 to maintain clarity of the graphs. It is unclear if the relatively lower VFA destruction seen in the MW reactor is due to differences in the makeup of the microbial communities, or perhaps something inhibitory was present in the MW-AOP-treated substrate. However, this only seemed to be the case for AGR sludge; VFA destruction of MW-AOP-treated MUN sludge was similar to that of the raw MUN sludge. Metals such as sodium and potassium are inhibitory to anaerobic microbial communities at high concentrations (Tchobanoglous et al., 2014); however, measured values remained well within allowable concentrations. Excess hydrogen peroxide is also inhibitory (Liu et al., 2017); but consistent TCOD values signified that no excess hydrogen peroxide was present in the MW-AOP-treated substrate. The VFA measured in both the influent and effluent of the MW reactor was constituted mostly of acetic acid, possibly implying that there were not enough methanogens present for complete consumption. A table of the VFA composition of the 51 influent and effluent samples can be found in Appendix E, as well as the VFA to alkalinity ratios in Appendix D. Figure 7 also shows the change in alkalinity in the two reactors. As before, alkalinity using the MUN sludge is found in Table 13. Alkalinity measured in mg/L of calcium carbonate (CaCO3) increased for all HRTs in both reactors and is likely due to VFA destruction and pH changes caused by the microbial communities. However, the MW reactorâs increase in alkalinity was generally double the increase seen in the control reactor. As well, its average pH was 7.52 compared to the control reactorâs average pH of 7.15. Alkalinity acts as a pH buffer and is a key parameter in anaerobic digesters. If VFA are being produced faster than they are consumed the alkalinity will drop accordingly in response. The microwave reactor showed decreasing alkalinity and pH at the 5-day HRT, which is in accordance with the increase in the effluent VFA that was seen. 52 Table 12- VFA destruction in control and microwave reactors HRT and Operation Control Reactor VFA Destroyed MW Reactor VFA Destroyed % mg/L % mg/L 16 EB 97 1259 56 515 12 EB 78 1474 11 64 12 CSTR 91 1941 87 1503 8 CSTR 96 1460 72 980 5 CSTR 86 1812 51 936 5 MUN 94 17459 91 8817 3.6 MUN 51 6789 68 8115 Table 13 - VFA, alkalinity, and pH for MUN WWTP substrate VFA (mg/L) Alkalinity (mg/L) pH Influent Effluent Destruction (%) Influent Effluent Influent Effluent Control 5 MUN 18538 1079 94 4320 7872 5.99 7.29 MW 5 MUN 9689 284 91 3848 11380 6.25 7.61 Control 3.6 MUN 13356 6567 51 2685 5995 5.66 7.23 MW 3.6 MUN 11939 3824 68 3610 12170 6.05 7.65 53 Figure 7 (listed clockwise beginning at top-left) - Change in VFA for control reactor; Change in VFA for microwave reactor; Change in alkalinity for control reactor; Change in alkalinity for microwave reactor 5.05.56.06.57.07.58.00500100015002000250016 EB 12 EB 12 CSTR 8 CSTR 5 CSTRpHVFA (mg/L)Control Reactor Change in VFAInf Eff pH5.05.56.06.57.07.58.00500100015002000250016 EB 12 EB 12 CSTR 8 CSTR 5 CSTRpHVFA (mg/L)MW Reactor Change in VFAInf Eff pH5.05.56.06.57.07.58.0020004000600080001000016 EB 12 EB 12 CSTR 8 CSTR 5 CSTRpHAlkalinity (mg/L as CaCO3)Control Reactor Change in AlkalinityInf Eff pH5.05.56.06.57.07.58.0020004000600080001000016 EB 12 EB 12 CSTR 8 CSTR 5 CSTRpHAlkalinity (mg/L as CaCO3)MW Reactor Change in AlkalinityInf Eff pH 54 4.3.3 Biogas Biogas was continuously recorded and logged during the experiments (Table 14). The cumulative biogas produced in a day was often used as a rough indicator of the reactorsâ health. Changing to a new HRT required an adjustment period for the reactors. Therefore, the biogas production was monitored closely and once the reactors were felt to have reached a steady state, biogas data points before this point were discarded. Biogas was also periodically collected and sampled for methane content, and is described more thoroughly in the next section. During the 8-day HRT period a gas hose connected to the gas flow meter of the microwave reactor was found to be loose, likely jostled during methane collection. Tightening the hose clamp produced a significant jump in gas flow for the microwave reactor. To account for this any data felt to have been collected during the loose hose time interval was removed from the datasets. Table 14 - Daily average biogas production HRT Control Reactor Biogas (mL/day) MW Reactor Biogas (mL/day) 16 EB 489±109 380±90 12 EB 736±128 1026±181 12 CSTR 2033±309 650±144 8 CSTR 2886±545 2627±526 5 CSTR 3776±203 3729±104 5 MUN 8011±365 10380±1057 3.6 MUN 8421±310 14845±1437 Table 15 compares the biogas data in two different ways. The first is millilitres of biogas produced per day per gram of VS destroyed; and the second is millilitres of biogas produced per 55 day per gram of VS added. All values have been normalized by the working volume of sludge in their respective reactors. The VS destroyed data has a large amount of variance attached to them. This arises from day-to-day variance in the VS content of the effluent, i.e. VSinfluent minus VSeffluent varied from day to day; whereas VSinfluent remained relatively constant. Both reactors produced similar amounts of biogas per gram of VS added. Paired t-tests at a 95% confidence level for each HRT showed that only the two 12-day HRTs (both expanded bed and CSTR) showed statistical differences between means. Additionally, both reactors performed much better when fed MUN substrate, reinforcing the fact that the results of anaerobic digestion are highly dependent on the substrate (Mottet et al., 2010). Table 15 - Biogas production per gram VS destroyed and per gram VS added mL biogas/g VS destroyed/L/day mL biogas/g VS added/L/day HRT Control MW Control MW 16 EB 55±17 32±15 15±5 16±7 12 EB 58±27 49±1 18±4 31±2 12 CSTR 113±76 44±21 29±4 24±5 8 CSTR 114±75 58±40 23±12 25±15 5 CSTR 270±86 65±9 25±1 24±1 5 MUN 496±131 212±32 81±9 72±20 3.6 MUN 270±72 175±23 54±4 56±7 When compared with the organic loading rate (OLR) the biogas produced per VS added also did not vary greatly (Figure 8). Only when the feed was changed to MUN substrate did significant changes occur. The total biogas produced per day increased as the OLR increased, even at the lowest HRT of 3.6 days. Gas production over a 24-hour period, as recorded by the 56 mass flow meters, is included in Appendix B. From the gas production in these graphs it appears that both reactors responded well to the increase in OLR, even at the lowest HRT. 4.3.4 Methane Methane sampling in this experiment did not produce reliable results. This is likely due to biogas samples being collected right after feeding the reactors, as opposed to before. Due to this, samples appeared to have their methane contents diluted. A full write-up on the problems encountered, as well as theoretical methane yields based on biogas production, can be found in Appendices F and G. 57 Figure 8 (clockwise from top-left) â Biogas per day VS added in the control reactor; Biogas per day VS added in the MW reactor; Cumulative biogas produced per day in MW reactor; Cumulative biogas produced per day in the control reactor0102030405060708090012345678916 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNmL biogas per gram VS addedOLR (kg VS/m^3/day)Control Biogas vs OLROLR Biogas0102030405060708090012345678916 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNmL biogas per gram VS addedOLR (kg VS/m^3/day)MW Biogas vs OLROLR Biogas02000400060008000100001200014000012345678916 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6MUNBiogas (mL/day)OLR (kg VS/m^3/day)Daily Control Biogas Produced versus OLROLR Biogas02000400060008000100001200014000012345678916 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6MUNBiogas (mL/day)OLR (kg VS/m^3/day)Daily MW Biogas Produced versus OLROLR Biogas 58 4.4 Discussion From the results presented in this chapter it is clear that differences in the initial substrates had significant effects on the performance of the anaerobic digesters. This may have been in terms of its biodegradability, as well as its suitability for anaerobic digestion. Every substrate is different, and thus a lab-scale or pilot-scale analysis becomes invaluable. However, the makeup of the microbial communities in the reactors may have also contributed significantly to the differences seen between the reactorsâ performance. Likely, differences in both the substrate, as well as the microbial communities, were impactful, and were causes of some of the unexplained results. For example, both reactors produced similar amounts of biogas based on OLR; however, the MW reactor showed much higher values for VS and COD destruction. Based on the destruction occurring, it generally would have been expected to see higher biogas production. On the other hand, VFA destruction was much higher in the control reactor, both magnitude- and percentage-wise; although, as explained in the VFA, Alkalinity, and pH section of this chapter, comparisons of VFA used an incomplete mass balance, and thus were greatly simplified. When comparing these apparent inconsistencies, a relationship between VFA destroyed and biogas produced was found. As seen in Table 16, the biogas produced per VFA destroyed remained consistent between both reactors at similar HRTs. The reactors also showed similar biogas production, both in terms of VFA concentration in the influent, as well as per gram of VS added (Table 17). The exception to this is the production of biogas per VFA fed using MUN sludge. In this case the MW reactor produced 2.3 times the amount of the biogas as the control reactor. 59 Table 16 - Biogas produced per VFA destroyed HRT and Operation Biogas Produced per VFA destroyed (mL/mg) Control Reactor MW Reactor 16 EB 4.00 4.00 12 EB 2.94 2.94 12 CSTR 2.38 2.94 8 CSTR 1.60 1.60 5 CSTR 1.00 1.00 5 MUN 1.00 1.11 3.6 MUN 0.80 0.80 Table 17 - Biogas production in terms of VFA and VS fed; normalized by reactor volume Control Reactor Microwave Reactor AGR MUN AGR MUN Biogas per VS fed (mL/g/L) 23±5 62±8 25±4 69±3 Biogas per VFA fed (mL/g/L) 415±58 94±6 358±80 220±32 These differences in performance could stem from the influents that constitute each wastewater. AGR sees a high amount of agricultural wastewater, especially from the dairy milk industry; whereas MUN sees a wider variety of influents from industries producing high-BOD wastes such as poultry processing and beer and wine brewery waste. Although the AGR substrate underwent MW-AOP treatment at 110ËC â which has been shown to increase VFA and SCOD much more than 90ËC treatment the MUN sludge received â the MUN sludge still showed itself to be far more suitable for high-rate anaerobic digestion. 60 Since both raw and MW-AOP-treated substrates seem to have different advantages and disadvantages (regardless of which WWTP they originated at), it is worth considering mixing the raw and treated substrates together. Liu et al. (2017) got the best results in their anaerobic digestion studies when mixing MW-AOP-treated substrate with untreated substrate in a 1:1 ratio, as an example of this approach. As for methane production, it is clear that sampling times were poorly chosen for this experiment. The comparisons in feeding time and methane content detailed in the previous Methane section and Appendix F shows that a large difference likely remains between what was measured and what the actual values were. Although there seemed to be a build-up of VFA in the anaerobic digesters during the 3.6-day HRT cycle, the reactors themselves still hadnât failed. In the MW reactor 30% of volatile solids were still being destroyed and biogas production was relatively high. It would have been informative to continue the experiments, lowering the HRT until the reactors completely failed. However, because of the setup of the reactors, namely the location of the feeding and wasting ports, it was impossible to get a lower HRT than 3.6 days. Finally, as MUN sludge was only used for 5- and 3.6-day HRT trials, not much can be said about this substrateâs suitability at higher HRTs. However, based on the results from low HRTs, the MUN sludge looks to have very good potential in this respect. Furthermore, MW-AOP treatment of MUN sludge at 110ËC could make it even more suitable for anaerobic digestion through greater release of SCOD and an increase in VFA (Chan et al., 2010; Kenge et al., 2008; Lo et al., 2017; Wong et al., 2006). 61 4.5 Conclusions This study showed that MW-AOP-treated sludge could be a good candidate for high-rate anaerobic digestion. The MW-AOP effectively solubilizes COD and nutrients, and makes the substrate more biodegradable. As such, by using the MW-AOP-treated sludge as feed substrate, the VS and COD destruction were increased in the MW anaerobic reactor compared to the control reactor. Additionally, the microwave reactor showed high levels of alkalinity, implying a strong buffer capacity able to withstand higher loading rates. This would make MW-AOP a suitable pre-treatment where maximizing sludge reduction at higher loading rates is a priority. Both the MW and control reactors performed similarly in terms of biogas produced per gram of VS added, as well as per gram of VFA added. However, even though the MW reactor was better at destroying VS, the control reactor consistently destroyed a higher percentage of VFA. This observation may just be related to the raw substrate. As discussed in the VFA, Alkalinity, and pH section, when AGR sludge was replaced with MUN sludge, the microwave reactor showed better VFA destruction than the control reactor, especially at low HRTs. Based on the results of this study some recommendations can be made for further anaerobic studies involving the MW-AOP: â¢ Identify predominant microbial communities in the reactors: It is clear that results were different based on whether the feed substrate had undergone MW-AOP treatment or not. However, it was not clear what role the microbial community had in that difference, nor how the makeup of the microbial community was affected by the MW-AOP-treated sludge. This is important to understand and is research worthwhile pursuing. It may provide further insights how to 62 best optimize an anaerobic digester using MW-AOP-treated sludge in terms of VS and VFA reduction, and/or biogas and methane production. â¢ Study effects of mixtures of raw and treated sludge: A previous study showed that mixing raw and MW-AOP-treated sludge together in a 1:1 ratio resulted in better methane production and more stable start-up than either substrate alone, particularly if any residual H2O2 was present; however, it must be noted that the setup in that experiment differed in some aspects, nor did our experiments show signs of excess H2O2 (Liu et al., 2018). Nonetheless, this is worthwhile researching further since results in the present study show the microwave reactor having the benefits of greater VS and COD destruction, whereas the control reactor showed greater VFA destruction. It would be an interesting study to try and combine these strengths. â¢ Study results of MUN sludge at different HRTs and treatment temperatures: As was seen in this study, the MUN sludge performed better at 5- and 3.6-day HRTs than its AGR counterpart. It would thus be beneficial to research the results of using MUN sludge at longer HRTs as some improvements in VFA or VS destruction may be immediately apparent. Furthermore, it has been shown that using the MW-AOP at 110ËC to treat sludge produces more VFA and solubilizes more COD and nutrients than treatment at 90ËC. MUN sludge should undergo this 110ËC MW-AOP treatment and be used for further anaerobic digestion studies. â¢ Further methane sampling: This study did not achieve conclusive methane production results. As discussed in the Methane section, sampling was done right after feeding and was later shown to be 63 highly inaccurate. As methane production is generally one of the two main goals of anaerobic digestion, the results of this study were not satisfactory nor conclusive. It is recommended that sampling be done at different points of the day, but especially before feeding. This will provide more accurate results and will mitigate the problems stemming from collection right after feeding. 64 5 Summary and Conclusions This study had two parts to it. The first was setting up and running a 25 kW MW-AOP system and using it to treat municipal TWSS. Different treatment parameters were tested and compared with previous studies of MW-AOP systems. The second part used MW-AOP-treated sludge â including some that had been treated by the new 25 kW system â as feed substrate for anaerobic digesters to study the results of using this treated substrate. Findings from this study would be useful in furthering the pilot-scale MW-AOP systems towards real-time treatment in WWTPs; and assessing the viability of using MW-AOP-treated substrate in sludge reduction treatments such as anaerobic digestion. A summary of the research conducted in this study will be presented in this chapter. 5.1 Using a 25 kW MW-AOP Pilot-Scale System to Treat Sludge The experiments run using the 25 kW MW-AOP system were generally considered a success, and the results followed expected trends demonstrated in previous smaller scale research: VFA and solubilized COD increased; TSS decreased; nutrients such as nitrogen and phosphorus were solubilized; and calcium, magnesium, and potassium were all greatly solubilized. Furthermore, experiments were run by treating 100 litres of sludge, as opposed to 20 litres in past experiments using a 5 kW pilot-scale MW-AOP system. Based on the results of these experiments, the 25 kW MW-AOP system would be capable of treating larger quantities of sludge to desired standards. However, some problems were attributed to the set-up and operation of the system, and some modifications to the system set-up could be performed to 65 improve overall efficiency and help attain desired results. The following Recommendations chapter summarizes these modifications. 5.2 Anaerobic Digestion Using MW-AOP-treated Sludge This study showed that MW-AOP-treated sludge could be a good candidate for high-rate anaerobic digestion, especially in terms of sludge reduction. It was also seen that the anaerobic digestion results were affected by the feed substrate; in this case, two different source sludges. With both sludge types, using the MW-AOP-treated sludge as a feed substrate showed that the VS and COD were greatly reduced compared to the control reactor, especially at very low HRTs â the MW reactor still showed almost 40% VS reduction at a 5-day HRT. Additionally, the MW reactor showed high levels of alkalinity, implying a strong buffer capacity able to withstand higher loading rates. This would make MW-AOP a suitable pre-treatment where maximizing sludge reduction at higher loading rates is a priority. Both reactors produced similar amounts of biogas per gram of VS added, and biogas production seemed to be much more strongly affected by the initial feed substrateâs quality and makeup, rather than whether it had undergone MW-AOP treatment or not. Using MW-AOP-treated sludge as a feed substrate for anaerobic digestion has a lot of potential, and further research will help identify the best ways to utilize the benefits this treated sludge brings. Recommendations for furthering this research are summarized in the next chapter. 66 5.3 Recommendations for Future Research Based on this study some recommendations can be made. The recommendations are summarized here, but are discussed more fully in their respective chapters. The recommendations for the 25 kW MW-AOP are more related to the set-up and operation of the system, whereas the recommendations for the anaerobic digestion are more focused on potentially beneficial paths to further the research on this topic. 5.3.1 Using a 25 kW MW-AOP Pilot-Scale System to Treat Sludge â¢ Transfer sludge to feed tank 24 hours before experimental run: Due to settling of sludge during the three-week experimental window, higher values of TS% were recorded when compared to initial measurements. Hydrogen peroxide dosage is based off of TS%, therefore the hydrogen peroxide doses in this experiment were lower than planned. By transferring the sludge to the feed tank at least a day before the experimental run, sufficient time will be available to perform a TS% analysis and ensure proper H2O2 dosing volumes. â¢ Install a mixer in the feed tank: Incomplete mixing may have been responsible for unequal treatment of the sludge, and gave rise to difficulties ascertaining bulk temperature in the feed tank, as well as inconsistent solids content when sampling from the feed tank. By installing a mixer the tank can be considered as a CSTR and would help mitigate these problems. â¢ Level lines and reduce bends, expansions and other losses: 67 Unlevel lines and other sources of friction losses caused uneven flow during low pumping rates. This required the feed pump to be run at a higher flow rate, reducing both the HRT in the applicator and exposure to microwave irradiation per pass. Ramp rate has been shown to be an important parameter in MW-AOP treatment. Levelling lines and reducing other losses as much as possible will allow for lower pump flow rates, and will likely improve the overall efficacy of the MW-AOP. â¢ Insulate feed tank: During the experiments the outside weather was cold and much heat was lost from the feed tank. As the end point for the experiment was a bulk temperature of 90ËC, more energy from the microwave unit was used than necessary due to this lost heat. Insulating the feed tank would ensure greater system efficiency and would make the MW-AOP more versatile in inclement weather conditions. 5.3.2 Anaerobic Digestion Using MW-AOP-treated Sludge â¢ Identify predominant microbial communities in the reactors: Results from the two reactors were different based on whether the feed substrate had undergone MW-AOP treatment or not. However, it was not clear what role the microbial community had in that difference, nor how the makeup of the microbial community was affected by the MW-AOP-treated sludge. It is recommended to undertake a study of the microbial makeup in the reactors which may provide further insights how to best optimize an aerobic digester using MW-AOP-treated sludge in terms of VS reduction and/or biogas and methane production. 68 â¢ Study effects of mixtures of raw and treated sludge: A previous study showed that mixing raw and MW-AOP-treated sludge together in a 1:1 ratio resulted in better methane production and more stable start-up than either substrate alone. The results in the present study show the microwave reactor having the benefits of greater VS and COD destruction, whereas the control reactor showed greater VFA destruction. It would be beneficial if these strengths could be combined through substrate mixing. â¢ Study results of MUN sludge at higher and lower HRTs, and treatment temperatures: MUN sludge performed better at 5- and 3.6-day HRTs than its AGR counterpart. However, MUN was not studied at higher HRTs. 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Water Research 37, 4453â4467. https://doi.org/10.1016/S0043-1354(03)00441-X Wong, W.T., Chan, W.I., Liao, P.H., Lo, K.V., 2006. A Hydrogen Peroxide/ Microwave Advanced Oxidation Process for Sewage Sludge Treatment. Journal of Environmental Science and Health, Part A 41, 2623â2633. https://doi.org/10.1080/10934520600928086 Yi, W.G., Lo, K.V., Mavinic, D.S., 2014. Effects of microwave, ultrasonic and enzymatic treatment on chemical and physical properties of waste-activated sludge. Journal of Environmental Science and Health, Part A 49, 203â209. https://doi.org/10.1080/10934529.2013.838880 Yu, Y., Chan, W. i., Lo, I. w., Liao, P. h., Lo, K. v., 2010. Sewage sludge treatment by a continuous microwave enhanced advanced oxidation process. Canadian Journal of Civil Engineering 37, 796â804. https://doi.org/10.1139/L10-044 Zhang, C., Su, H., Baeyens, J., Tan, T., 2014. Reviewing the anaerobic digestion of food waste for biogas production. Renewable and Sustainable Energy Reviews 38, 383â392. https://doi.org/10.1016/j.rser.2014.05.038 77 Zhang, H., Lo, V.K., Thompson, J.R., Koch, F.A., Liao, P.H., Lobanov, S., Mavinic, D.S., Atwater, J.W., 2015. Recovery of phosphorus from dairy manure: a pilot-scale study. Environmental Technology 36, 1398â1404. https://doi.org/10.1080/09593330.2014.991354 78 Appendices Appendix A: Change in Solids for Anaerobic Digesters The following graphs display the change in total solids and volatile solids for both the control and microwave reactors used in the anaerobic digestion studies. Figure 8 - TCOD removal in control and microwave reactors 0100002000030000400005000060000700008000016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNCOD (mg/L)TCOD RemovalControl Feed Control Effluent MW Feed MW Effluent 79 Figure 9 (clockwise from top-left) - Control reactor change in total solids; MW reactor change in total solids; MW reactor change in volatile solids; control reactor change in volatile solids0102030405016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNTotal Solids (g/L)Control Reactor Change in Total SolidsInfluent Effluent0102030405016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNTotal Solids (g/L)MW Reactor Change in Total SolidsInfluent Effluent051015202530354016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNVolatile Solids (g/L)Control Reactor Change in Volatile SolidsInfluent Effluent051015202530354016 EB 12 EB 12 CSTR 8 CSTR 5 CSTR 5 MUN 3.6 MUNVolatile Solids (g/L)MW Reactor Change in Volatile SolidsInfluent Effluent 80 Appendix B: Gas flow charts The following graphs show the daily biogas production from both of the reactors under different HRTs and operating conditions. The days depicted were under steady-state conditions, but were selected randomly. 81 Figure 10 - Biogas production for 16-day expanded bed, 12-day expanded bed, 12-day CSTR, and 8-day CSTR operation 05101520253035400 6 12 18 24Biogas Production (mL/hour)Elapsed Time (hours)16-day Expanded Bed Biogas ProductionControl Microwave010203040506070800 6 12 18 24Biogas Production (mL/hour)Elapsed Time (hours)12-day Expanded Bed Biogas ProductionControl Microwave01020304050600 6 12 18 24Biogas Production (mL/hour)Elapsed Time (hours)12-day CSTR Biogas ProductionControl Microwave01020304050600 6 12 18 24Biogas Production (mL/hour)Elapsed Time (hours)8-day CSTR Biogas ProductionControl Microwave 82 Figure 11 Biogas production for 5-day CSTR, 5-day MUN CSTR, and 3.6-day MUN CSTR operation0501001502002500 6 12 18 24Biogas Production (mL/hour)Elapsed Time (hours)5-day CSTR Biogas ProductionControl Microwave01002003004005006000 6 12 18 24Biogas Production (mL/hour)Elapsed Time (hours)5-day MUN CSTR Biogas ProductionControl Microwave01002003004005006007000 6 12 18 24Biogas Production (mL/hour)Elapsed Time (hours)3.6-day MUN CSTR Biogas ProductionControl Microwave 83 Appendix C: Compiled Data Tables Table 18 - Compiled data for 16-day expanded bed reactor 16-day Expanded Bed pH TS (g/L) VS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Alkalinity (mg/L) Control Feed 6.04 36.5±0.3 32.1±0.4 2150±399 54230±5641 1293±110 1420±20 Control Effluent 7.25 30.8±1.3 25.8±1.3 523±35 41152±7871 35±5 4308±249 MW Feed 6.9 32.3±2.9 23.1±2.1 18660±2823 41241±4322 924±54 2565±405 MW Effluent 7.69 16.7±0.7 9.4±0.3 6932±325 11670±177 409±70 9392±155 Table 19 - Compiled data for 12-day expanded bed reactor 12-day Expanded Bed pH TS (g/L) VS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Alkalinity (mg/L) Control Feed 6.08 36.3±0.2 31.4±0.3 1565±361 28889±3267 1884±95 1545±15 Control Effluent 7.07 26.7±1.1 22.5±0.9 855±1590 17609±3111 410±36 3619±288 MW Feed 6.9 34.6±2.9 25.4±2.6 12300±2561 32936±3995 574±53 1772±364 MW Effluent 7.47 16.3±2.1 8.6±1.5 9233±2311 20588±3190 510±102 8298±326 Table 20 - Compiled data for 12-day CSTR reactor 12-day CSTR pH TS (g/L) VS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Alkalinity (mg/L) Control Feed 5.94 36.8±1.2 31.6±0.6 2840±506 54505±7170 2135±202 1490±262 Control Effluent 7.25 31.2±13.9 26.2±11.5 1195±293 40994±0 194±35 4320±387 MW Feed 6.64 25.6±2.9 18.4±1.9 14027±1421 31224±7242 1730±145 2202±103 MW Effluent 7.69 16.1±22.4 9.4±0.8 5877±1497 13714±937 227±54 9565±109 84 Table 21 - Compiled data for 8-day CSTR reactor 8-day CSTR pH TS (g/L) VS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Alkalinity (mg/L) Control Feed 6.32 36.7±0.4 31.8±0.2 4360±130 56420±2796 1521±183 2044±37 Control Effluent 7.1 31.1±1 26.1±0.5 987±216 46357±7242 61±12 3750±150 MW Feed 5.93 22.5±0.2 15.5±0.2 16190±690 42129±6800 1363±450 773±60 MW Effluent 7.49 15.2±3.4 8.7±2.1 6490±84 20771±2274 383±53 9033±193 Table 22 - Compiled data for 5-day CSTR reactor 5-day CSTR pH TS (g/L) VS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Alkalinity (mg/L) Control Feed 6.46 36.6±0.3 31.4±0.4 3920±210 63942±6824 2101±122 1936±473 Control Effluent 6.95 33.3±0.1 28.1±0 825±125 57092±4385 289±89 3607±7 MW Feed 6.18 39.1±0.2 30.6±0.2 21690±290 48473±248 1167±85 1460±200 MW Effluent 7.07 32.2±0.6 22±0.5 16333±137 41149±2381 4411±843 6874±47 Table 23 - Compiled data for 5-day MUN CSTR reactor 5-day MUN CSTR pH TS (g/L) VS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Alkalinity (mg/L) Control Feed 5.99 29.9±0 23.4±0 14550±977 53128±0 18538±552 4320±424 Control Effluent 7.29 23.2±1 17.2±0.8 6065±786 37280±1988 1079±412 7872±167 MW Feed 6.25 44.1±5.9 34.2±4.3 27385±637 61515±661 9689±704 3848±322 MW Effluent 7.61 25.4±1.3 17.5±0.9 10358±245 30088±4305 248±18 11380±434 85 Table 24 - Compiled data for 3.6-day MUN CSTR reactor 3.6-day MUN CSTR pH TS (g/L) VS (g/L) SCOD (mg/L) TCOD (mg/L) VFA (mg/L) Alkalinity (mg/L) Control Feed 5.66 31.2±0 24.9±0 13082±0 59525±0 13356±336 2685±403 Control Effluent 7.23 25.8±2 19.6±1.6 6591±336 40431±367 909±137 5995±166 MW Feed 6.05 44±11 34±8.2 27400±433 70961±0 11939±1179 3610±312 MW Effluent 7.65 29.7±1.1 20.7±0.9 13734±258 38733±98 3997±428 12170±380 86 Appendix D: VFA to Alkalinity Ratios Table 25 - VFA to alkalinity ratio. A properly functioning anaerobic digestor should fall between 0.05 and 0.25 VFA to Alkalinity Ratio HRT and Operation Control Reactor MW Reactor 16 EB 0.01 0.04 12 EB 0.11 0.06 12 CSTR 0.04 0.02 8 CSTR 0.02 0.04 5 CSTR 0.08 0.13 5 MUN 0.14 0.02 3.6 MUN 1.10 0.31 87 Appendix E: VFA composition Figure 12 - VFA composition of the substrate collected at MUN and fed to anaerobic digesters Figure 13 - VFA composition of effluent from anaerobic digesters fed with MUN substrate 05000100001500020000Control 5 ARC MW 5 ARC Control 3.6 ARC MW 3.6 ARCVFAs (mg/L as acetic acid)MUN Influent VFAsAcetic Propionic Iso-Butyric Butyric Iso-Valeric Valeric01000200030004000500060007000Control 5 ARC MW 5 ARC Control 3.6 ARC MW 3.6 ARCVFAs (mg/L as acetic acid)MUN Effluent VFAsAcetic Propionic Iso-Butyric Butyric Iso-Valeric Valeric 88 Figure 14 â Change in VFA concentration and composition for anaerobic digesters using AGR sludge0.00500.001000.001500.002000.002500.0016 EB12 EB12 CSTR8 CSTR5 CSTRVFAs (mg/L as acetic acid)Control Feed VFAsAcetic Propionic Iso-Butyric Butyric Iso-Valeric Valeric0.00500.001000.001500.002000.002500.0016 EB12 EB12 CSTR8 CSTR5 CSTRVFAs (mg/L as acetic acid)Control Effluent VFAsAcetic Propionic Iso-Butyric Butyric Iso-Valeric Valeric0.00500.001000.001500.002000.002500.0016 EB12 EB12 CSTR8 CSTR5 CSTRVFAs (mg/L as acetic acid)Microwave Feed VFAsAcetic Propionic Iso-Butyric Butyric Iso-Valeric Valeric0.00500.001000.001500.002000.002500.0016 EB12 EB12 CSTR8 CSTR5 CSTRVFAs (mg/L as acetic acid)Microwave Effluent VFAsAcetic Propionic Iso-Butyric Butyric Iso-Valeric Valeric 89 Appendix F: Methane Biogas was generally collected a few times during each HRT and tested for methane. Tedlar sampling bags were used, and were connected to the gas outlet hoses exiting the anaerobic reactors. Based on the samples analyzed, the average methane content values are presented below (Table 26). The expected methane content in anaerobic digestion is roughly 60% (Tchobanoglous et al., 2014), and even at low HRTs average methane content generally remains around 60% (Nges and Liu, 2010). As can be seen, the methane contents determined in these experiments are considerably lower than the expected results. These differences in methane contents are likely due to a combination of air getting into the reactors during feeding, as well as the time at which the methane samples were collected. Substrate was fed to the reactors using a peristaltic pumps. Though pumping air into the reactors was actively avoided, it is possible that the some air remained in the hose connected to the pumps. Additionally, the raw control feed may have contained air. The feed was stored at 4ËC, but was left on the bench top overnight to bring it to room temperature before feeding. During this warming up period the sludge volume would often increase by up by 100 mL, implying air and gasses trapped in the sludge. It is likely that some air trapped in the sludge entered the reactors. 90 Table 26 - Methane percentage of biogas produced HRT and Operation Control Methane (%) MW Methane (%) 16 EB 62 63 12 EB 49 13 12 CSTR 51 no data 8 CSTR 53 53 5 CSTR 35 51 5 MUN 34 42 3.6 MUN 34 45 The collection time of methane was also a problem. Sample collection bags were connected to the gas-release hoses coming from the headspace of the reactors. Due to scheduling, these collection bags were connected shortly after feeding the reactors. Therefore, if outside air and gasses were entering the reactors during feeding, it is very probable that the gas samples collected shortly after feeding were diluted with air and not indicative of the true methane content. This hypothesis was tested near the end of the experiments. Gas samples were collected from the reactors shortly before and after feeding. The results are presented below in Table 27. Table 27 - Comparison of methane content before and after feeding Methane Content Before Feeding (%) Methane Content After Feeding (%) Control Reactor 61 70 37 Microwave Reactor 59 64 53 46 As can be seen, the methane content values before feeding are much closer to expected values (~60% methane). Thus it seems the above hypothesis is plausible. Unfortunately these 91 before-and-after data points were collected near the end of the experiments, and well after the substrate had been changed from AGR substrate to MUN substrate. As such, it can only be speculated that methane content remained around 60% during the entirety of the experiments. Using that assumption, Table 28 compares the methane assumed to have been produced with the theoretical methane yield. The theoretical methane yield was determined assuming the methane content remained at 60-70%, and that 50-80% of TCOD was biodegradable. The values were calculated based on Metcalf & Eddy (Tchobanoglous et al., 2014), and the formulae used can be found in the next section. Table 28 - Assumed methane values compared with theoretical methane values. Note: assumed methane values were calculated assuming a methane content of 60-70% in the measured biogas; theoretical methane range values were calculated assuming 50-80% of TCOD was biodegradable HRT and Operation Assumed CH4 Range in Control Reactor (mL/day) Theoretical CH4 Range for Control (mL/day) Assumed CH4 Range in MW Reactor (mL/day) Theoretical CH4 Range for MW (mL/day) 16 EB 293 - 342 648 - 1001 228 - 266 1438 - 2264 12 EB 442 - 515 763 - 1171 616 - 718 824 - 1282 12 CSTR 1220 - 1423 1125 - 1733 390 - 455 1150 - 1818 8 CSTR 1732 - 2020 2165 - 3345 1576 - 1839 2575 - 4062 5 CSTR 2266 - 2643 1347 - 2116 2237 - 2610 1376 - 2223 5 MUN 4807 - 5608 3029 - 4809 6228 - 7266 5404 - 8584 3.6 MUN 5053 - 5895 4716 - 7233 8907 - 10392 7767 - 12208 92 Appendix G: Theoretical Methane Yield Theoretical methane yield was calculated by using the following equations found in Metcalf and Eddy (Tchobanoglous et al., 2014). !"#$ = (0.40)[(,- â ,)(/) 0 12310435 â 1.4278] where !"#$ = volume of methane produced at 35ËC and 1 atm, m3/d 0.40 = theoretical conversion factor for the amount of methane produced, m3, from the conversion of 1 kg of bCOD at 0ËC Q = flowrate, m3/d S0 = bCOD in influent, g/m3 S = bCOD in effluent, g/m3 Px = net mass of cell tissue produced per day, kg/d 78 = :/(,- â ,) 0 123104351 + <(,=>) where Y = yield coefficient, g VSS/g bCOD b = endogenous coefficient, d-1 SRT = solids retention time, d Based off of typical results listed in Metcalf and Eddy, 0.04 g VSS/g bCOD was used as the yield coefficient, Y; and 0.02 d-1 was used for the endogenous coefficient, b. Because bCOD was not explicitly measured for any of the anaerobic samples a range of values was simply 93 calculated. The range took the TCOD values of the influent and effluent, and assumed that between 50 â 80% of TCOD would be bCOD.
Thesis/Dissertation
eng
Civil Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
A microwave-enhanced advanced oxidation process to treat municipal wastewater sludge and as a pretreatment for anaerobic digestion
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