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Studies of the control and operation of the aerobic digestion process applied to waste activated sludges… Koers, D. Antonie 1979

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STUDIES OF THE CONTROL AND OPERATION OF THE AEROBIC DIGESTION PROCESS APPLIED TO WASTE ACTIVATED SLUDGES AT LOW TEMPERATURES by D. Antonie Koers Diploma I r . , Delft University of Technology, 1967 A DISSERTATION SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of C i v i l Engineering) We accept this dissertation as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1979 (c) Dirk Antonie Koers, 1979 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l . f u l f i lment o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag r ee tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f C i v i l Engineering The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date September 25, 1979 [j ABSTRACT Continuous flow, d a i l y f i l l and draw, and batch aeration digesters were studied on a laboratory scale, to develop low temperature characteristics and design c r i t e r i a for aerobic digestion of waste activated sludge. These results were compared against f u l l - s c a l e data from three indepen-dent sources. Raw sludge used i n these studies was obtained from a municipal high rate activated sludge plant. The digestion systems were operated at l i q u i d temperatures of 20, 10, and 5°C, and at s i x different sludge ages. Measurement of a l l parameters studied for the continuous feed systems were made under steady-state conditions. Parameters studied were divided into three main groups, namely: (1) Parameters related to aerobic digestion k i n e t i c s , such as solids destruction and oxygen uptake rate; (2) Parameters related to aerobic digestion sludge cha r a c t e r i s t i c s , such as biochemical oxygen demand, viable bacteria, organic carbon, nitrogen forms, and odour values; (3) Parameters r e l a t i n g to supernatant quality, such as dissolved s o l i d s , organic carbon, biochemical oxygen demand, chemical oxygen demand, nitrogen forms, and pH. The results show that the effect of low temperature on aerobic digestion performance was pronounced. The combined effect of sludge age and tem-perature was shown to be an important design parameter. Kinetic reaction i i rates and temperature s e n s i t i v i t y c o e f f i c i e n t s were calculated for the various conditions studied on the basis of v o l a t i l e suspended s o l i d s . I t was shown that reaction rates for batch digestion and continuous feed digestion systems were not interchangeable. This i s s i g n i f i c a n t , as most p i l o t plant and bench-scale studies on aerobic digestion are being conducted using batch digestion, the results of which are then being used for design of continuous feed digesters. I t appears that, as temperature decreases, the da i l y f i l l and draw method of digester operation resulted i n s i g n i f i c a n t l y higher reaction rates than the continuous feed method of digester operation. At higher temperatures, the two were about equal. Oxygen uptake rate was not considered a r e l i a b l e indicator of digested sludge s t a b i l i t y ; instead, mixed liquor BOD,- was introduced as a possible means of determining such s t a b i l i t y . N i t r i f i c a t i o n and d e n i t r i f i c a t i o n i s shown to be appreciable during aerobic digestion i n a l l systems and at a l l temperatures studied. Although somewhat tentative, the results show important n i t r i f i c a t i o n and d e n i t r i f i c a t i o n trends at temperature and pH levels w e l l below optimum values for these processes. The difference between batch and continuous feed digestion i s shown i n various ways, but probably most c l e a r l y through monitoring supernatant characteristics. The continuous feed systems show increased s o l u b i l i z a t i o n i i i of organics with increased sludge age, whereas the batch systems do not show any s o l u b i l i z a t i o n of organics. Reasonable correlation i s demonstrated between the laboratory and f u l l -scale aerobic digestion data. i v TABLE OF CONTENTS ABSTRACT LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS CHAPTER I - INTRODUCTION II - LITERATURE REVIEW A - Kinetics of Aerobic Digestion B - Effect of Temperature on Aerobic Digestion C - Previous Research i n Aerobic Digestion D - Need for Further Research I I I -IV -MATERIALS AND METHODS General Procedures A n a l y t i c a l Procedures RESULTS AND DISCUSSION A - Aerobic Digestion Kinetics 1. Solids Destruction 2. Oxygen Uptake Rate B - Digested Sludge Characteristics 1. Biochemical Oxygen Demand 2. Chemical Oxygen Demand and Total Organic Carbon 3. Nitrogen Mineralization, N i t r i f i c a t i o n , and D e n i t r i f i c a t i o n 4. Viable Bacteria i n Digested Sludge 5. V o l a t i l e Fraction of Digested Sludge C - Supernatant Characteristics D - Evaluation of Full-Scale Data V - CONCLUSIONS A - Aerobic Digestion Kinetics B - Digested Sludge Characteristics C - Supernatant Characteristics D - Suggestions for Further Research REFERENCES PREVIOUS PUBLICATIONS ON THIS RESEARCH APPENDICES APPENDIX A APPENDIX B VITA AUCTORIS Explanation of Calculations Regarding Solids Mass Balance Application of Research Data - Design Example v LIST OF TABLES TABLE PAGE 1. Steady-State Suspended Solids Mass Balance 94 2. Steady-State Suspended Solids Mass Balance 95 3. Steady-State Suspended Solids Mass Balance 96 4. Steady-State Suspended Solids Mass Balance 97 5. Steady-State Suspended Solids Mass Balance 98 6. Steady-State Suspended Solids Mass Balance 99 7. Steady-State Suspended Solids Mass Balance 100 8. Steady-State Suspended Solids Mass Balance 101 9. Steady-State Suspended Solids Mass Balance 102 10. Steady-State Suspended Solids Mass Balance 103 11. Steady-State Suspended Solids Mass Balance 104 12. Steady-State Suspended Solids Mass Balance 105 13. Steady-State Suspended Solids Mass Balance 106 14. Steady-State Suspended Solids Mass Balance 107 15. Steady-State Suspended Solids Mass Balance 108 16. Steady-State Suspended Solids Mass Balance 109 17. Steady-State Suspended Solids Mass Balance 110 18. Steady-State Suspended Solids Mass Balance 111 19. Batch Aeration Suspended Solids Mass Balance 112 20. Batch Aeration Suspended Solids Mass Balance 113 21. Batch Aeration Suspended Solids Mass Balance 114 22. V o l a t i l e Suspended Solids Reduction at Steady-State 115 23. Oxygen Uptake Rate at Steady-State 160 24. Sludge and Supernatant Data for 20°C Systems 182 25. Sludge and Supernatant Data for 10°C Systems 184 26. Sludge and Supernatant Data for 5°C Systems 186 27. Summary of Sludge and Supernatant Data at Steady-State 188 28. B0D5 of Digested Sludge at Steady-State 189 29. Digested Sludge Odour Value Test Results 196 30. Nitrogen Data for 20°C Systems 201 31. Nitrogen Data for 10°C Systems 202 32. Nitrogen Data for 5°C Systems 204 33. Summary of Nitrogen Data and steady-State 206 34. Viable Bacteria Counts for 20°C Systems 222 35. Viable Bacteria Counts for 10°C Systems 223 36. Viable Bacteria Counts for 5°C Systems 224 37. Summary of Viable Bacteria Counts at Steady-State 225 38. Summary of Steady-State Supernatant Data at 20°C 233 39. Summary of Steady-State Supernatant Data at 10°C 234 40. Summary of Steady-State Supernatant Data at 5°C 235 41. Summary of Smithers STP Digester Operating Data (1975-76) (From Reference 128) 243 42. Summary of Denver STP Digester Operating Data (1972-73) (From Reference 38) 244 43. I n i t i a l Endogenous Decay Rate Constants for Waste Activated Sludge, Daily Feed/Manual Decant Mode (VSS Basis) 251 44. Comparison of Streeter-Phelps Temperature S e n s i t i v i t y Coefficient, 9, for Endogenous Decay Rate (VSS Basis) 251 v i LIST OF FIGURES FIGURE PAGE 1. Aerobically Digested Sludge (Jaworski) 17 2. Relationship between V o l a t i l e Solids Reduction and Sludge Age (Norman) 20 3. Aerobic Digester Oxygen U t i l i z a t i o n Rates (Ahlberg) 40 4. Schematic of Laboratory Digester 69 5. Schematic of Laboratory Digester 71 6. Photograph of Experimental I n s t a l l a t i o n Continuous 74 Feed Digesters 7. Photograph of Experimental I n s t a l l a t i o n Semi-Continuous Feed Digesters 75 8. Solids Mass Balance i n Continuous Digesters 79 9. Photograph of Instrument Arrangement Oxygen Uptake 83 Rate Monitoring 10. Effect of Sludge Age and Temperature on VSS Reduction 116 (Continuous Feed Units) 11. Effect of Sludge Age and Temperature on VSS Reduction 117 (Batch Aeration Units) 12. Combined Effect of Sludge Age and Temperature on VSS Reduction 120 13. Effect of Digestion Time on VSS Reduction Batch Aeration @ 20°C 122 14. Effect of Digestion Time on VSS Reduction Batch Aeration @ 10°C 123 15. Effect of Digestion Time on VSS Reduction Batch Aeration @ 5°C 124 16. Determination of Endogenous Decay Rates - VSS Basis. Continuous Feed Systems @ 20°C 126 17. Determination of Endogenous Decay Rates - VSS Basis. Continuous Feed Systems @ 10°C 127 18. Determination of Endogenous Decay Rates - VSS Basis. Continuous Feed System @ 5°C 128 19. Determination of Endogenous Decay Rates - VSS Basis. Batch Aeration @ 20°C 130 20. Determination of Endogenous Decay Rates - VSS Basis. Batch Aeration @ 10°C 131 21. Determination of Endogenous Decay Rates - VSS Basis. Batch Aeration @ 5°C 132 22. Temperature Coefficients for I n i t i a l Endogenous Decay Rates. Continuous Feed Systems. VSS Basis 133 23. Temperature Coefficients for I n i t i a l Endogenous Decay 134 Rates. Batch Aeration Systems. VSS Basis 24. Effect of Temperature on I n i t i a l and Secondary 136 Endogenous Decay Rates. VSS Basis 25. Digester pH and Oxygen Uptake Rate 138 26. Digester pH and Oxygen Uptake Rate 139 v i i LIST OF FIGURES (Continued) FIGURE PAGE 27. Digester pH and Oxygen Uptake Rate 140 28. Digester pH and Oxygen Uptake Rate 141 29. Digester pH and Oxygen Uptake Rate 142 30. Digester pH and Oxygen Uptake Rate 143 31. Digester pH and Oxygen Uptake Rate 144 32. Digester pH and Oxygen Uptake Rate 145 33. Digester pH and Oxygen Uptake Rate 146 34. Digester pH and Oxygen Uptake Rate 147 35. Digester pH and Oxygen Uptake Rate 148 36. Digester pH and Oxygen Uptake Rate 149 37. Digester pH and Oxygen Uptake Rate 150 38. Digester pH and Oxygen Uptake Rate 151 39. Digester pH and Oxygen Uptake Rate 152 40. Digester pH and Oxygen Uptake Rate 153 41. Digester pH and Oxygen Uptake Rate 154 42. Digester pH and Oxygen Uptake Rate 155 43. Digester pH and Oxygen Uptake Rate 156 44. Digester pH and Oxygen Uptake Rate 157 45. Digester pH and Oxygen Uptake Rate 158 46. Effect of Sludge Age on Oxygen Uptake Rate @ 20°C. Continuous Feed Systems 161 47. Effect of Sludge Age on Oxygen Uptake Rate @ 10°C. Continuous Feed Systems 162 48. Effect of Sludge Age on Oxygen Uptake Rate @ 5°C. Continuous Feed Systems 163 49. Effect of Sludge Age on Oxygen Uptake Rate @ 20°C. Batch Aeration 165 50. Effect of Sludge Age on Oxygen Uptake Rate @ 10°C. Batch Aeration 166 51. Effect of Sludge Age on Oxygen Uptake Rate @ 5°C. Batch Aeration 167 52. Effect of Temperature on Oxygen Uptake Rate. Continuous Feed Systems 168 53. Effect of Temperature on Oxygen Uptake Rate. Batch Aeration Systems 169 54. Determination of Temperature S e n s i t i v i t y Coefficients for Oxygen Uptake Rate. Continuous Feed Systems 170 55. Determination of Temperature S e n s i t i v i t y Coefficients for Oxygen Uptake Rate. Batch Aeration Systems 171 56. Variation i n Temperature Response with Respect to Oxygen Uptake Rate. Continuous Feed Systems 173 57. Variation i n Temperature Response with Respect to Oxygen Uptake Rate. Batch Aeration Systems 174 58. Effect of Sludge Age on Oxygen Uptake Rate @ 20°C 176 59. Effect of Sludge Age on Oxygen Uptake Rate @ 10°C 177 v i i i LIST OF FIGURES (Continued) FIGURE PAGE 60. Effect of Sludge Age on Oxygen Uptake Rate @ 5°C 178 61. Determination of Temperature S e n s i t i v i t y Coefficients for Oxygen Uptake Rate. A l l Systems Combined 179 62. Variation i n Temperature Response with Respect to Oxygen Uptake Rate. A l l Systems Combined 180 63. Effect of Sludge Age on B0D5 @ 20°C 190 64. Effect of Sludge Age on B0D5 @ 10°C 191 65. Effect of Sludge Age on BOD5 @ 5°C 192 66. Effect of Temperature on BOD5 Reduction i n VSS Fraction. Continuous Feed and Batch Aeration Systems 194 67. Relationship between Digested Sludge Odour Value and Digested Sludge BOD5, after Storage @ 20°C 197 68. Nitrogen Balance during Aerobic Digestion 208 69. Histogram of Nitrogen Forms Expressed i n mgN/gVSS Destroyed 210 70. Nitrogen Balance @ 20°C 211 71. Nitrogen Balance @ 10°C 212 72. Nitrogen Balance @ 5°C 213 73. Nitrate Nitrogen as a Function of Digester Sludge Age @ 20°C 214 74. Nitrate Nitrogen as a Function of Digester Sludge Age @ 10°C 216 75. Nitrate Nitrogen as a Function of Digester Sludge Age @ 5°C 218 76. Viable Bacteria Counts, 20°C Systems 227 77. Viable Bacteria Counts, 10°C Systems 228 78. Viable Bacteria Counts, 5°C Systems 229 79. Digester pH as a Function of Sludge Age 236 80. Digester Supernatant Characteristics @ 20°C 237 81. Digester Supernatant Characteristics @ 10°C 238 82. Digester Supernatant Characteristics @ 5°C 239 83. Combined Effect of Sludge Age and Temperature on VSS Reduction. Correlation of Full-Scale Data with Laboratory Data 245 84. Smithers STP, Determination of Endogenous Decay Rates - VSS Basis 247 85. Denver STP - Determination of Endogenous Decay Rates - VSS Basis 248 86. Effect of Temperature on I n i t i a l and Secondary Endogenous Decay Rates. VSS Basis. Correlation of Full-Scale Data with Laboratory Data 249 87. Comparison of Full-Scale Oxygen Uptake Rate Data with Laboratory Data 252 ix ACKNOWLEDGEMENTS The author i s indebted to many individuals who assisted and supported him throughout twelve, sometimes tr y i n g , years at two u n i v e r s i t i e s , which eventually culminated i n the completion of this d i s s e r t a t i o n . No one, however, deserves greater gratitude than my wife, Marjorie, and children, Naomi and Nadine, whose understanding, perseverance, and s p i r i t u a l support made the completion of my studies possible. With fondness, I also thank my mother and father, whose f a i t h and f i n a n c i a l s a c r i f i c e allowed me to f i r s t attend university. I am grateful for the technical assistance and moral encouragement received from Dr. P. H. Jones of the University of Toronto, who started me on the right track, and guided me through some periods of personal indecision and d i f f i c u l t i e s . The author would l i k e to thank Dr. A. H. Benedict and Dr. W. K. Oldham, who were instrumental i n his decision to recommence his studies at th i s university. The successful completion of the second stage of his doctoral studies, however, would not have been possible without the f i n a n c i a l and moral support of the Directors of Associated Engineering Services Ltd., Consulting Engineers; i n p a r t i c u l a r Messrs. J. R. O'Brien and J. D. Tudor, who have been most encouraging. x I also wish to express my sincere gratitude to Dr. D. S. Mavinic, who inherited me as his student during the l a s t leg of these studies; his help and patience are much appreciated. Fond thanks are also due to Mrs. E. McDonald, head laboratory technician, and her sta f f for thei r valuable technical assistance during the research portion of my work. Special personal gratitude i s due to my colleague, Mervin J. Stewart, Ph.D., for his advice during the course of my studies, and his assistance during the preparation of th i s dissertation. I would also l i k e to specially thank the sta f f of the Mamquam Sewage Treatment Plant i n Squamish, B. C, and the Sewage Treatment Plant i n Smithers, B. C, for their assistance i n providing the raw material for my research work, and their assistance during the f i e l d evaluation portion of my research, respectively. Lastly, the author wishes to thank the Central Mortgage & Housing Corpo-ration and the National Research Council for their f i n a n c i a l assistance i n support of t h i s work. x i 1 I. INTRODUCTION Sludge treatment and disposal i s perhaps the most controversial aspect of sewage treatment today. Statements l i k e : "The enigma of sewage treatment i s the disposal of ever-accumulating sludge," "The problem of sludge disposal i s as great or greater than that of purifying the sewage," and "Disposal of sludge solids presents an increasingly d i f f i -c ult problem to communities due to high land values and increasing labour costs," or statements to that effect (2, 23), have appeared i n numerous a r t i c l e s discussing contemporary sewage treatment practices. As most forms of sewage p u r i f i c a t i o n merely provide means of converting impurities i n raw sewage to s o l i d materials that can be physically separated from the treated effluent and collected as sludge, treatment and disposal of th i s sludge has to be a necessary and in t e g r a l part of sewage treatment system design. Although the theory of sludge accumulation i n activated sludge treatment systems has been well established during the past decades (138, 155), i t i s to this day not unusual to read claims that certain activated sludge variants or systems produce so l i t t l e sludge that, f i g u r a t i v e l y speaking, i t can be removed by the bucke t - f u l l . This attitude towards sludge production i s most prevalent when i t comes to the small "package type" activated sludge systems, designed to handle sewage flows from communities of 10,000 persons or less. 2 Most modern p o l l u t i o n control authorities require no less than secondary treatment of domestic sewage for most diposal conditions. Thus, the number of activated sludge plants, necessarily operated with intentional removal of excess sludge to maintain required treatment e f f i c i e n c i e s , are increasing at a rapid irate. Existing plants, designed under less stringent regulations, i n many cases require upgrading of their sludge treatment and disposal f a c i l i t i e s to meet new regulations. Aerobic digestion i s only one method of sludge treatment prior to u l t i -mate disposal. I t results i n a reduction of t o t a l suspended solids and p a r t i a l dewatering of the sludge. The product of aerobic digestion i s a s t a b i l i z e d sludge that can be stored or further dewatered, more or less nuisance free, i n the open a i r prior to f i n a l disposal. For reasons of economics, the means of sludge s t a b i l i z a t i o n i n small activated sludge plants (less than about 1 mgd) i s generally r e s t r i c t e d to aerobic digestion, with other available methods, such as anaerobic digestion and incineration being too costly from both a c a p i t a l and operating cost point of view. I t has been shown that aerobic digestion can be economically competitive with anaerobic digestion for plants up to a size of 8 US mgd (30,300 m /day) (126). However, for larger plants, i t may be more advantageous to consider separate methods of sludge s t a b i l i z a t i o n for the primary and secondary sludges, aerobic digestion being more suitable to handle waste activated sludge separately (30, 38). 3 Because of the foregoing, and the fact that activated sludge treatment w i l l be the most common type of secondary treatment for domestic sewage for some time to come, aerobic digestion f a c i l i t i e s can be expected to increase around the world; this i s p a r t i c u l a r l y so as land disposal of aerobically digested sludge, for use as a s o i l conditioner on a g r i c u l t u r a l land, gains more acceptance (41, 121). I t i s a we l l known fact that microbial a c t i v i t y decreases with decreasing temperatures. Aerobic digestion, being a purely b i o l o g i c a l process, i s no exception. However, under conditions of endogenous respiration, the main driving force of aerobic digestion, microorganisms are already at a minimum l e v e l of a c t i v i t y . The author f e l t that, under such conditions, i t would not be unreasonable to expect a different pattern of temperature dependency for aerobic digestion systems, than for b i o l o g i c a l systems operating at higher levels of metabolism. As far as i s known, no work has previously been published on the behaviour of the continuous-flow aerobic digestion process at temperatures below 15°C, under controlled conditions. Aerobic digestion, however, i s almost universally applied i n small activated sludge sewage treatment plants, even i n northern and northern-interior locations. The author f e l t , therefore, that a study of the type presented here would f i l l a d e f i n i t e need for designers and operators of aerobic digestion f a c i l i t i e s , as well as researchers. I I . LITERATURE REVIEW Kinetics of Aerobic Digestion For microorganisms to perform t h e i r l i f e function i n organic waste treatment, they must oxidize organic matter either aerobically or anaerobically i n order to obtain energy and organic compounds necessary to the synthesis of new c e l l material. The aerobic b i o l o g i c a l process of organic waste s t a b i l i z a t i o n can be i l l u s t r a t e d by the following basic equation: Organic Matter + 0 2 + NH^ •* Sludge Cells + CT>2 + H20 (1) C e l l material can be represented by the empirical chemical formula C^ -H^ NO,,, which has been suggested by Hoover, et a l (61), to be representative of the r a t i o of the primary elemental constituents of activated sludge. I t was found to be representative of the s t a t i s t i c a l average composition of the complex organic compounds constituting c e l l material. Accordingly, the stoichiometric relationship of Equation 1 may be stated as follows, for a range of organic materials found i n domestic wastewater (158): 2(C xH y0 z) + 2(x + y/ 4 - z/ 2 - 5)0 2 + 2NH3  2C 5H ?N0 2 + 2(x - 5)C0 2 + (y - 4)H 20 (2) When there i s an unlimited food supply, the organisms are i n the log-growth phase, and growth i s limited only by the a b i l i t y of the 5 organisms to reproduce; the a c t i v i t y of the organisms per unit mass i s maximum. As oxidation of organic materials proceeds, the food supply w i l l become l i m i t i n g , a declining growth phase i s reached, and the organism a c t i v i t y i s something less than maximum. The l a t t e r i s usually the case i n conventional domestic waste treatment systems. In situations where the amount of organic substrate i s l i m i t i n g , production of c e l l material i s accompanied by auto-oxidation of c e l l material (137). C e l l material i t s e l f , therefore, then becomes an additional source of energy and metabolites for mainte-nance and synthesis of c e l l materials. Thus, when the organic substrate i s unable to supply s u f f i c i e n t materials for energy and synthesis, and the rate of c e l l destruction exceeds the rate of c e l l growth, the microorganisms obtain the i r energy and c e l l building blocks from auto-oxidation of c e l l proto-plasm, a process often termed "endogenous respiration". The l a t t e r process i s the p r i n c i p l e of aerobic digestion and can be i l l u s t r a t e d by the basic equation: Sludge Cell s + 0 *»» Non-biodegradable C e l l Material The stoichiometry of the above reaction, when applied to the biodegradable portion of the sludge c e l l s only, i s as follows (61): 3 ....(3) C 5H ?N0 2 + 50 2 6 As aerobic digestion proceeds and as temperatures permit, more oxygen i s needed by n i t r i f y i n g bacteria to convert the ammonia released to n i t r i t e s and n i t r a t e s , for which the following stoichio-metry has been proposed (114): C 5H 7N0 2 + 70 2 •» 5C0 2 + 3H20 + H + + NC>3~ (5) In any b i o l o g i c a l system, net protoplasm accumulation can be expressed i n terms of increase through synthesis, and decrease through endogenous respiration: Sludge accumulation 4^  = k 4^  ~~ k,M ....(6) dt a dt d where: M = quantity of active mass i n system (VSS), S = quantity of substrate removed from system (BOD), k^ = fr a c t i o n of substrate removed, which i s synthesized into new active mass, and k^ = f r a c t i o n of active mass i n the system, which i s destroyed per day by endogenous respiration. In aerobic digestion of waste activated sludge, i t can be assumed that the dissolved organic matter in" the medium surrounding the organisms has been removed, and that stored and adsorbed food materials have been f u l l y metabolized. Equation 6 then becomes: — = -kjM or, upon integration: dt d M -k,t M - k J t t d t d — - e , or log — - _ > ( 7 ) o o The l a t t e r equation represents a f i r s t order reaction, where k^ i s the rate constant of auto-oxidation. 7 Since M never becomes zero, but tends to a non-biodegradable residue, M^ (135), Equation 7 i s commonly refined to: (M - M ) lc^t log (M — M/) = " 2^03 • • " ( 8 ) o 1 where: k^ i s the fr a c t i o n of biodegradable c e l l mass, which i s destroyed per day by endogenous respiration. Equation 7 i s v a l i d only for a batch fed system or a continuous feed, plug flow reactor (16). Most systems, used i n practice, however, are of the continuous feed, completely mixed type. When using that type of system, a mass balance around the system leads to the following expression (16): M 1 + k.t 1 o d When the system i s of the flow-through type, without r e c i r c u l a t i o n , t' i s equal to the hydraulic retention time; however, when the system employs recycling and thickening of the solids under aeration, t' has to be taken as the average residence time of the sludge p a r t i c l e s i n the system, better known as "sludge age". Sludge age i s correctly defined as follows: Mass of solids under aeration Sludge Age (S.A.) = Mass of digested solids wasted per day Using the reaction Equations 4 and 5, the theoretical oxygen requirements for complete oxidation of c e l l material can be calcu-lated. The c e l l material, having the empirical chemical formula C^H^NC^, has a pseudo-molecular weight of 113. The theoretical amount of oxygen required when ammonia i s the f i n a l form of nitrogen, thus, would be 1.42 lb G^/lb c e l l oxidized. When n i t r i f i c a t i o n i s complete, i n other words, a l l ammonia has been converted to n i t r a t e , 1.98 lb would be required to completely oxidize 1 lb of biode-gradable c e l l material and convert a l l ammonia to n i t r a t e . B a r r i t (14), showed that n i t r i f i c a t i o n may be inhibited by accumu-lated CO^ and/or NH^+, or by i n s u f f i c i e n t aeration. He recognized that continued aeration of organic solids drove the pH down below 5.5, which caused n i t r i f i c a t i o n to cease. He proposed a complex c y c l i c reaction to be the cause of a continued drop i n pH, without producing additional n i t r a t e s : 3HN02 %• (H + + N03~) + 2N0 + H20 2N0 + H20 + l / 2 0 2 2HN02 Accordingly, i t can be expected that, i f the pH of digested sludge f a l l s below 5.5, the n i t r i f i c a t i o n process w i l l be incomplete, and that the amount of oxygen required for aerobic digestion would be a value i n between 1.42 and 1.98 lb 0 2/lb c e l l mass oxidized. Effect of Temperature on Aerobic Digestion Kinetics The rates of biochemical reactions are increased by an increase i n temperature and vice versa. Since b i o l o g i c a l waste treatment processes, including aerobic digestion systems, are not normally 9 heated, these systems are exposed to f l u c t u a t i o n s i n ambient temperatures determined by the c l i m a t i c conditions. This f a c t , then, exerts an influence on the b i o l o g i c a l process e f f i c i e n c i e s involved. Temperature dependence can be described mathematically only between c e r t a i n l i m i t s , because i t i s a complex and poorly understood phenomenon. A symbol often used as an index for temperature dependence i s Q-^g' a factor i n d i c a t i n g how many times the reaction rate w i l l increase i f the temperature i s raised by 1 0°C. Jones ( 6 9 ) , showed that the c l a s s i c a l " r u l e of thumb", which states that the growth rate of most organisms i n most conditions doubles for every 1 0°C r i s e i n temperature, could not be substan-t i a t e d f o r a v a r i e t y of organisms he studied. He suggested that the growth rate - temperature r e l a t i o n s h i p may be substrate dependent, and could be described by a two-stage l i n e a r expression. Although Benedek, et a l . ( 1 6 ) , reported a Q^Q of approximately 2 . 0 for t h e i r experiments on batch-fed aerobic digestion systems, i n the temperature range between 0°C and 2 5°C, they concluded i n a previous paper ( 1 5 ) , that the temperature dependence of continuously fed systems i s usually less than would be expected from non-steady state measurements. Possible reasons for t h i s d i f f e r e n c e brought forward were Eckenfelder's theory of p a r t i a l anaerobiosis within the f l o e at higher temperatures, p a r t i a l l y o f f s e t t i n g increased reaction rates, plus their own observation that c o l l o i d removal was only s l i g h t l y temperature dependent. Thus, a system containing a large proportion of c o l l o i d a l organic material, as compared to dissolved organic material, may exhibit a larger degree of thermal independence than a system containing largely dissolved organic matter. The same authors (15), concluded that endogenous metabolism follows the Arrhenius law (see Equation 10), as opposed to the metabolism of dissolved substrate, which does not. The authors contributed the l a t t e r phenomenon to the Crozier theory, which proposes that substrate metabolism, because of i t s complexity, cannot be described with one simple Arrhenius equation. At different temperatures, substrate metabolism would have different "master reactions", characterized by different activation energies. I t was proposed by the authors (15), that, since endogenous respiration readily followed the Arrhenius law, this type of metabolism had only one * "master reaction" i n the 0° - 25°C temperature range. In the authors' view (15), endogenous metabolism would have.to be very • stable, because i t would be more important, from the point of view of survival and resistance to adverse environmental conditions, than substrate metabolism. The Arrhenius law can be stated as follows: ^ - - - ^ • • - . d o ) RT where: H = the heat of reaction, or activation heat ( c a l . ) , k = the equilibrium constant of the reaction, i . e . the r a t i o of rate constants of the reactions, R = the universal gas constant (1.986 cal/°K), and T = temperature i n °K. Upon integration, Equation 10 yields the following expression: log k = Log A - 23Q3R ' T .... (11) where: k^ or k^ may be substituted for k, and A i s a constant. When p l o t t i n g log k d or log k^ versus temperature, a straight l i n e results i f the Arrhenius law i s followed. The Streeter-Phelps empirical formula i s used most frequently to describe the temperature dependency of a biochemical reaction rate constant (18): kd2 ( T 2 - T 1 ) = 9 ....(12) k d l where: k,n and k,„ are the rate constants corresponding to temperatures d l d/ T^ and T^ respectively. Eckenfelder et al.(45), concluded that the temperature c o e f f i c i e n t , 9, was greatly dependent upon the process c h a r a c t e r i s t i c s , i . e . dispersed growth vs. flocculated growth. The range of 9 for activated sludge, between the temperature l i m i t s of 4° and 45°C, was found to 12 be 1.000 - 1.041. 9 would also depend on the degree of mixing, as th i s would affect the floe size and rate of oxygenation of the inner portions of the f l o e . 9 would further depend on the type of organism prevailing i n the system. Hence, a system operating i n the mesophilic range would experience a retardation of b a c t e r i a l a c t i v i t y as the temperature approaches freezing; however, r e l a t i v e l y high reaction rates may s t i l l exist for psychrophilic organisms at low temperatures (58). Values for 9, determined for different processes and under widely fluctuating conditions, have been published frequently. For the most part, such determinations showed very c o n f l i c t i n g results (127). This would enforce Eckenfelder's conclusion that 9 i s greatly dependent upon the process c h a r a c t e r i s t i c s , and would support Benedict's thesis (17) that the response of the temperature coeffic i e n t to substrate concentration i s highly variable, and must be assessed i n terms of the basic parameters c o n t r o l l i n g the oxidation reactions. C. Previous Research i n Aerobic Digestion The f i r s t reports on aerobic digestion originated over 45 years ago when Rudolfs and Heukelekian (119), i n 1932, and Heukelekian (59), i n 1933, reported that aerobic s t a b i l i z a t i o n of raw primary sludge took place at a greater rate, i n terms of v o l a t i l e s o l i d s , f a t , t o t a l nitrogen, and BOD reduction, than anaerobic s t a b i l i z a t i o n . These studies were conducted in batch units over a 92 day period, presumably at room temperature. It was found that proper seeding of the aerobic process reduced the time of decomposition. Aerobi-cally decomposed sludge yielded further decomposition, when subjected to anaerobic conditions, as did anaerobically decomposed sludge, when subjected to aerobic conditions. At approximately the same time that the above studies were conducted, the activated sludge process was gaining in popularity in North America. However, i t was not un t i l the early 1950's that serious research into the aerobic digestion process of primary and waste activated sludges commenced. In 1950, Coackley (36, 37), studied aerobic digestion of sludge previously subjected to anaerobic digestion. This research was primarily concerned with dewatering characteristics of the digested sludge; however, reduction in solids and organic nitrogen were also observed. Digestion at 18°C showed l i t t l e reduction of volatile solids after 48 days, with and without inoculation of aerobic organisms. At 37°C, volatile solids reductions of 64 per-cent were obtained in the inoculated series and of 43 percent in the non-inoculated series, after 47 days of aeration. A decrease of organic nitrogen was noted in the aerobic series (inoculated and non-inoculated) which did not occur in the anaerobic series. The aerobic series produced a stable sludge after 47 days, showing no signs of decomposition when l e f t unaerated. Anaerobic digestion could not be i n i t i a t e d i n this series, even after inoculation with anaerobic organisms. In 1952, Hoover et al.(61), studied the endogenous respiration rate of sludge grown on skim milk at room temperature. Based on this and previous studies by the same authors, they concluded that endogenous respiration proceeded at about one tenth the rate of oxidation of milk sol i d s . The stoichiometry of the endogenous respiration reaction was presented. Hood and Spoohr (60), doing work i n 1956 on activated sludge systems at Ridgewood, New Jersey, studied the correlation of ecological c l a s s i f i c a t i o n s with oxidation-reduction potential. They showed that a well-environed aerobic digester i s i n the "oligosaprobic" state, characterized by reducing numbers of bacteria, high mineralization, low organic nitrogen, and a high steady positive oxidation-reduction potential of 700 to 800 mV, with a f l a t slope with respect to time. Eckenfelder (44), i n 1956, studied aerobic digestion using waste activated sludge from a conventional activated sludge treatment plant. F o u r - l i t r e digesters were aerated for a period of 7 days at a constant temperature of 25°C. At the end of the 7-day period of aeration, his data showed a reduction i n COD of 48.5 percent, a reduction i n t o t a l suspended solids of 38.2 percent, and a decrease i n the v o l a t i l e f r a c t i o n from 76.5 to 63.5 percent of t o t a l sus-pended so l i d s . I n i t i a l t o t a l suspended solids were 4,380 mg/1. The soluble nitrogen increased from 46 ppm to 94 ppm i n the 7 days of aeration. He also reported auto-oxidation rates of about 10 percent per day. In 1958, Moriarty (92), compared aerobic and anaerobic digestion of raw primary sludge at a temperature of 97°F, an average loading rate of 0.042 lb TVS/day/cu f t (0.673 kg TVS/day/m3), and an average hydraulic detention time of 35 days. Digester feeding was on a da i l y f i l l and draw basis. Based on CO^ production, he found aerobic digestion approximately 300 percent more complete; however, he did not account for CH^ liberated i n anaerobic digestion. Total carbon removal measurements indicated removals of 0.255 g C/g TVS added during aerobic digestion versus 0.302 g C/g TVS added during anaerobic digestion, thus suggesting anaerobic digestion to be more complete than aerobic digestion under the part i c u l a r operating conditions. Kountz and Forney (76), used dry skim milk as a source of organic matter i n their work. The results of their study on energy balances, i n a t o t a l oxidation activated sludge unit, were reported i n 1959 and showed that t o t a l endogenous oxidation of b i o l o g i c a l sludge i s not possible, as a residual material accumulated at a rate of 20 to 25 percent of the weight of new activated sludge produced, or 0.6 percent of the t o t a l weight of activated sludge i n the system. 16 Akers (5), i n 1959, studied aerobic digestion of mixed-liquor soli d s . He found an average t o t a l v o l a t i l e solids reduction of 20 percent at 23°C i n 8 days, and a corresponding v o l a t i l e suspended solids reduction of 30 percent. Auto-oxidation rates were found to be higher at lesser sludge age. Murphy (94), i n 1959, studied the effects of aeration on the f i l t e r a b i l i t y and s e t t l e a b i l i t y of sewage sludges. This was the f i r s t of a series of excellent studies done at the University of Wisconsin under Dr. G. A. Rohlich. Sludges were obtained from the Nine Springs Plant at Madison which treats, by conventional activated sludge, a mixture of one-third pre-treated meat packing waste and two-thirds domestic sewage. Murphy used primary and waste activated sludge, mixed at a r a t i o of 1:1 by volume. Most of his work was done at 15°C, and he concluded that the reduction of v o l a t i l e solids at 15°C, with digestion times up to 6 days, was not appreciable, and that vigorous aeration for short periods reduced the f i l t e r -a b i l i t y and s e t t l e a b i l i t y of the sludge. Jaworsky (66, 67, 68), i n 1960, continued the program of aerobic digestion research at the University of Wisconsin, using sludge from the same source. He used mixtures of primary and waste activated sludge of 1.75:1 (dry solids basis). Digester feed was maintained at 3.2 percent t o t a l solids (70-80% v o l a t i l e ) and the digester volume at 4 L. Feeding of the digester was semi-continuous on a d a i l y fill-and-draw, flow-through basis, the size of the da i l y withdrawal and loading rate being controlled by the volumetric detention time. Air supply was kept constant at 0.20 cfm (0.0056 3 m /min) per digester. The variables studied were as follows: temperature: 15° - 35°C detention time: 5 - 6 0 days (volumetric) solids loading: 0.290 - 0.024 lb v o l a t i l e solids/day/cu f t (4.640 - 0.384 kg v o l a t i l e solids/day/m 3) The study pointed to the following conclusions: 1. Reduction of v o l a t i l e solids was a function of detention time. However, beyond detention times of 15 days, only small increases i n reduction were obtained (see Figure 1). 2. In general, greater reductions i n v o l a t i l e solids were obtained at high temperatures of digestion (see Figure 1). 3. In general, higher v o l a t i l e solids removal was obtained at lower loading rates (see Figure 1). 4. Set t l i n g characteristics of sludges digested for 30 days or less were generally poorer than those of undigested sludges. 5. Sludges digested over 5 days showed satisfactory d r a i n a b i l i t y . 6. Supernatant liquors showed low BOD values when compared to anaerobic digester liquors. 7. The sludges dried with no objectionable odour. D E T E N T I O N T I M E - Days Figure 1. Aerobically Digested Sludge (Jaworski) 8. Values of pH increased for detention times up to about 10 days to a maximum of approximately 8.0 and then decreased gradually to values near 5.0. However, no adverse effects were reported at these low pH values. This lowering of the pH at prolonged aeration was thought to be due to increased n i t r a t e ion concentration and corresponding loss of system buffering capacity from decreasing a l k a l i n i t y (Ludzack). 9. Nitrate concentrations increased gradually with longer detention times. Nitrate values i n excess of 900 mg/1 were measured. Norman (79, 97), i n 1961, reported on further studies i n aerobic digestion at the University of Wisconsin. These studies were also done under controlled laboratory conditions, and the same equipment was used as i n the previous study. Mainly, only waste activated sludge was used. At constant temperatures (20°C), the three major variables studied were detention time, unit solids loading, and a i r rate. Again, as i n a l l of the other studies, the main c r i t e r i o n for digestion e f f i c i e n c y was v o l a t i l e solids reduction. I t was found that a pH as low as 5 (this was found to occur at detention times greater than 30 days by other workers also) did not s i g n i f i -cantly affect the digestion e f f i c i e n c y , i n terms of v o l a t i l e solids reduction. When pH was controlled by use of NaOH to pH 7, a reduction i n v o l a t i l e solids removal occurred, possibly due to sodium ion t o x i c i t y . An i n i t i a l series of tests was done on a 1.75:1.00 mixture of primary and waste activated sludge at varying detention times (10 and 15 days) and temperatures (15° and 20°C). The conclusions were as follows: 1. The higher temperature resulted i n highly s i g n i f i c a n t increases i n v o l a t i l e solids reduction. 2. An increased detention time resulted i n a s i g n i f i c a n t increase i n v o l a t i l e solids reduction. 3. The effects of temperature and detention time were independent of each other. The remaining series of tests employed waste activated sludge only. A i r rate studies indicated that increased a i r rates from 0.20 to 0.35 cfm (0.0056 to 0.0098 m3/min) did not produce a s i g n i f i c a n t improvement i n v o l a t i l e solids reduction. I t was found that at a constant loading of 0.1125 lb v o l a t i l e solids/day/cu .ft 3 (1.8000 kg VS/day/m ), v o l a t i l e solids removal increased s i g n i f i -cantly with increasing detention time. I t was also shown that v o l a t i l e solids reductions between 15 and 30 days detention time increased much more at higher v o l a t i l e solids loadings (0.1125 lb/day/cu-ft) than i t did at a low loading of 0.0417 lb/day/cu.ft 3 (0.6672 kg/day/m ), probably indicating that, at the lower loading, digestion was more complete i n 15 days. Two-stage digestion studies showed that with 12 days detention i n the primary, and 15 days i n the seconday digester, and with 36.6 and 21.9 percent v o l a t i l e solids reduction respectively, the ov e r a l l reduction of 50.4 percent through both units was not as great as the 53.1 percent reduction obtained i n a 30-day single-stage unit, at i d e n t i c a l loading. Probably the most s i g n i f i c a n t finding of this research project was that sludge age i s the cont r o l l i n g parameter i n terms of digester e f f i c i e n c y . Since detention time and solids loading were shown to be dependent variables, attempts were made to determine some other parameter to correlate with v o l a t i l e solids removal. By use of three-parameter p l o t t i n g , the controlling parameter was found to be sludge age, which i s d i r e c t l y related to food-to-organism r a t i o . A plot of v o l a t i l e solids reduction vs. sludge age indicated a s i g n i f i c a n t degree of correlation (coefficient of correlation = 0.78), and seemed to j u s t i f y the use of sludge age as the major c r i t e r i o n , r e l a t i v e to the ef f i c i e n c y of v o l a t i l e solids reduction i n aerobic digestion. (Sludge age was defined as the r a t i o of the weight of v o l a t i l e solids i n the digester to the weight of v o l a t i l e solids removed d a i l y ) . The relationship obtained by Norman (97) i s i l l u s t r a t e d i n Figure 2: io. [ I ! ! [ i l l ! l i [ i i [ 2 3 K 5 t> 7 8 9 15 20 30 *0 50 60 Sludge Age (Cays) Figure 2. Relationship between V o l a t i l e Solids Reduction and Sludge Age (Norman) Further conclusions from this study were the following: 1. For any given degree of v o l a t i l e solids reduction, the sludge age observed i s expected to vary with the composition of the particular sludge and the frequency of feeding employed. I t appears that frequent automatic feeding, with automatic decant, would result i n higher v o l a t i l e s o l i d s reductions than i n manual, feed-starve cycles for loading the digester; i n the l a t t e r case, the number of viable organisms at the end of the starve cycle would be too small to effect proper auto-digestion of new sludge. 2. For accumulative feeding, seed sludge which has not undergone extensive periods of auto-digestion i s expected to produce more e f f i c i e n t v o l a t i l e solids reduction. 3. Sludges digested for periods greater than 5 days showed satisfactory d r a i n a b i l i t y characteristics. 4. Supernatant liquors from aerobic digestion showed r e l a t i v e l y low BOD. Woodley (162), i n 1961, conducted studies i n aerobic digestion of raw primary sludge at mesophilic (35°C) and thermophilic (52°C) temperatures. Single units were operated at each temperature, on a d a i l y fill-and-draw basis at loading rates of 0.031, 0.062, 0.094 and 0.125 lb TVS/day/cu. f t (0.496, 0.992, 1.504, and 2.000 kg TVS/day/m ). Each:.loading:.rate was maintained for 45 days. He found satisfactory solids destructions at both temperatures for a l l loading rates. In general, however, higher v o l a t i l e solids reductions were achieved i n the mesophilic process at the loading rates investigated. The s e t t l e a b i l i t y of mesophilic sludge was found to be much superior than that of thermophilic sludge. The pH i n both systems was about equal and always basic. Thermophilic digestion appeared to convert or destroy more nitrogenous material, as indicated by the greater release of ammonia from these units and the lower organic nitrogen content of the thermophilic sludges. In 1961, Barnhart (11), reported on aerobic digestion studies at Manhattan College applied to several i n d u s t r i a l and domestic sludges. He described the auto-roxidation reaction as a retardant one; however, for purposes of his particular studies, he showed that, i f the v o l a t i l e non-oxidizable portion of the sludge was subtracted from the t o t a l v o l a t i l e s o l i d s , the oxidation rate of the oxidizable portion followed f i r s t order k i n e t i c s . He concluded that the solids reductions obtained with a l l sludges under aerobic digestion were comparable to anaerobic digestion. Temperatures below 20°C were s i g n i f i c a n t l y retardant to aerobic digestion. The rate of oxidation for the various sludges varied widely but 15 days detention time was s u f f i c i e n t for acceptable sludge digestion. Oxygen u t i l i z a t i o n measurements showed that, i n most cases, a i r supplied for mixing was governing. S o l i d - l i q u i d separation after digestion did not appear to be a problem with any of the sludges. Reyes and Kruse (112), reported i n 1962 that aerobic digestion of night s o i l , as measured by CO^ production and v o l a t i l e solids reduction (40 - 50 percent at 45°C), may be accomplished i n 20 days. As compared to common practice of heated anaerobic night s o i l digestion i n the Orient, aerobic digestion provided more rapid s t a b i l i z a t i o n , smaller unit volumes were required, and produced a drainable product free from nuisance odours. A single run at 8°C showed a decrease i n v o l a t i l e solids reduction, which was more pronounced as the i n i t i a l solids concentration increased. Carpenter and Blosser (34) investigated aerobic digestion of waste activated papermill sludges (boardmill and deinking). The following conclusions were drawn from their reported results i n 1962: 1. L i t t l e v o l a t i l e solids reduction occurred after 27 days digestion. 2. Nitrogen and phosphorus supplements resulted i n small increases i n v o l a t i l e solids reduction. 3. The rate of solids decomposition was doubled as the temperature increased from 20° to 30°C. 4. The system oxygen requirements per 1000 ppm v o l a t i l e solids decreased from 9 - 1 2 ppm per hour before digestion to 4 - 7 ppm per hour and 2.5-6 ppm per hour after one and two days digestion respectively. 5. Sludge aerated for long periods floated unless degasified by a s l i g h t vacuum. 6. Aerobically digested sludge did not dewater as wel l as raw sludge. This might have been due to mechanical breakdown of the sludge floe during prolonged agitation. In 1963, Dreier (152), presented an excellent review of the state of the art of aerobic digestion. In addition, he reported on f i e l d data from several i n s t a l l a t i o n s i n the U. S. No data on sp e c i f i c plant operating parameters, l i k e sludge age or treatment cha r a c t e r i s t i c s , were provided. Reported digested sludge character-i s t i c s indicated stable sludges from a l l types of plants, i.e. waste activated sludge only, primary and waste activated sludge, and primary and t r i c k l i n g f i l t e r sludge. No odours were noticeable at any of the plants investigated. I t was observed that, at a l l the plants investigated, the a i r requirements for aerobic digestion of sludges did not exceed 15 - 20 cfm per 1000 cu f t (1.5 - 2.0 3 3 m /min per 1000 m ) of digester capacity. Burton (30, 31), studied aerobic digestion of primary sludge at 35°C. Radioactive phosphorus tracers were used to measure hydraulic detention times. The laboratory systems were fed on a once d a i l y , flow-through basis. Organic loadings employed were 0.10 and 0.14 3 lb VS/day/cu f t (1.60 and 2.24 kg VS/day/m ). Measured detention times were approximately half of the theoretical detention time, indicating a s i g n i f i c a n t degree of s h o r t - c i r c u i t i n g . The results indicated that a greater v o l a t i l e solids reduction took place at the higher loading than at the lower loading. Oxidation of organic and ammonia nitrogen i n the supernatant to nitrates was 98 percent complete at both rates. In a report prepared for the City of Regina, Associated Engineering Services Ltd. (8), conducted batch studies on aerobic digestion of waste activated sludge at approximately 1°C and 20°C l i q u i d temperature. V o l a t i l e suspended solids reductions were reported as 28 and 50 percent, respectively, after 24 days aeration. When in t e r v a l feeding, without thickening of the digester s o l i d s , was employed, the average rate of v o l a t i l e suspended solids reduction, at an ambient temperature of about 0°C, was 0.013 lb VSS/day/cu f t 3 (0.208 kg VSS/day/m ) l i q u i d digester capacity, which was equal to 3.5 percent per day, based on the weight of VSS contained i n the digester. In 1964, Hostetler (63), compared aerobic and anaerobic digestion of primary sludges at 35°C. He investigated the effect of three different loading rates, namely, 0.14, 0.17 and 0.20 lb VS/day/cu f t 3 (2.24, 2.72 and 3.20 kg VS/day/m ) and a hydraulic detention time of 15 days. He concluded that only a limited amount of organic solids were degraded under aerobic conditions, with the v o l a t i l e solids reduction increasing more than two-fold as the loading rate was increased from 0.14 to 0.20 lb VS/day/cu f t (2.24 to 3.20 kg 3 VS/day/m ). I t was further determined that the d r a i n a b i l i t y of anaerobically digested sludges was superior to that of aerobically digested sludge. No offensive odors were detected upon drying of either of the digested sludges. 26 Irgens and Halvorson (64), showed i n 1964 that aerobic digestion of primary sludge ties up a s i g n i f i c a n t portion of the available nitrogen and phosphorus i n the microbial sludge, leaving very l i t t l e i n solution. Aerobic digestion of the sludges was performed at 23° and 30°C i n 20 1 reactors, fed on a once da i l y basis after solids were concentrated to about 3 percent t o t a l solids concentration. Once the desired t o t a l solids concentration was established, feeding of the digesters was done on a flow-through basis at detention times of 7, 10, 13, 20 and 40 days. BOD and COD of the supernatant was p r a c t i c a l l y constant for a l l detention times investigated. Less than one percent of the t o t a l phosphorus was contained i n the supernatant, the phosphorus content of the raw sludge being the same as that of the digested sludge. The reduction i n t o t a l nitrogen was about 200 mg/1 for each detention time, and was attributed to d e n i t r i f i c a t i o n . Depending on the rate of aeration, nitrates would be produced upon oxidation of ammonia at higher rates, whereas at lower rates of aeration, d e n i t r i f i c a t i o n , occurring i n the anaerobic inner portion of the microbial f l o e s , would reduce n i t r i t e s and nitrates to free nitrogen, as soon as these compounds were formed. The authors also showed that, i n order to obtain good f l o c c u l a t i o n with aerobic digestion of primary sludge, an inoculum of waste activated sludge was necessary. Design considerations of aerobic digestion f a c i l i t i e s were d i s -cussed i n a review paper by Loehr. (83), i n 1965. He observed that the most economical sludge disposal system i n a conventional activated sludge plant could be one where the primary and secondary sludge i s handled separately. This would mean anaerobic digestion of primary sludge and aerobic digestion of waste activated sludge. He reported that when aerobically digesting a mixture of primary and waste activated sludge, the oxygen supply would have to be increased almost s i x times over the amount required when digesting waste activated sludge alone, and would therefore be uneconomical. Viraraghaven (150), investigated the f e a s i b i l i t y of aerobic diges-t i o n of primary sludge under the climatic conditions of Madras, India i n 1965. At an average temperature of 31°C, detention times of 5, 10, 15 and 20 days were studied i n batch aeration (no feed) systems. Close to 40 percent reduction of t o t a l v o l a t i l e solids was obtained after 15 days with l i t t l e further increase up to 20 days. Supernatant BOD^  doubled after 5 days of aeration, then dropped to i t s o r i g i n a l value after 15 days. N i t r i f i c a t i o n did take place; however, not nearly as much as was reported by others. A possible reason offered for this was the low rate of airflow employed i n this study. Bruemmer (27), i n 1966, presented the f i r s t report on the use of pure oxygen i n sludge s t a b i l i z a t i o n . The aerobic digestion experi-ments were conducted using primary sludge of domestic o r i g i n at 30°C. Both batch aeration and fill-and-draw reactors were used. Hydraulic detention times of 5, 7 and 10 days were studied. Each run consisted of a duplicate set-up, using pure oxygen i n one and 28 air in the other. At a total solids concentration of approximately 70,000 mg/1, i t was found that oxygen transport rates from normal air sparging were inadequate to meet the oxygen demand of the sludge under digestion, particularly when feed sludge was added in a once-daily cycle. When the feed cycle was shortened to even out the total feed addition, the oxygenation requirement diminished; however, i t was shown that oxygen was s t i l l many times more efficient than air per volume of gas sparged. A further advantage of the use of pure oxygen and i t s associated smaller gasflow rate, was the possible elimination of problems due to disintegration of the sludge floe by high sparging rates. Digestion efficiency was determined mainly by measurement of f i l t r a t e BOD^  and COD, and i t was stated that primary sludge would be suitably stabilized in about four days at 30°C, based on a levelling out of f i l t r a t e BOD,. and COD at about four to five days. Reynolds (113), in 1967, aerobically digested waste activated sludges from the Austin, Texas biosorption plant, treating a mixture of domestic and slaughterhouse wastes. Rate of decay constants varied from 0.53 to 0.85 day \ based on removal of biodegradable volatile solids and were found to be constant for each of the four test runs. The degree of stabilization achieved wasL,95 percent, in times-varying between 3.5 and 5.7 days. He reported that after digestion, the sludge had a dark brown colour and did not have an offensive odour. Temperatures and sludge age prior to digestion were not reported. He reported on further similar studies i n 1973 (114). As i n the previous studies, he used waste activated sludge i n batch reactors. In addition, a number of f i e l d studies were conducted and design examples given. The main variable studied was i n i t i a l solids concentration, which varied from 8,400 to 22,700 mg/1. Based on these studies, he recommended the following design c r i t e r i a : (a) hydraulic detention time =10.5 days 3 (b) capacity = 1.73 cu f t / c a p i t a (0.048 m /capita) (c) organic loading = 0.046 lb TVS/day/cu f t (0.737 kg TVS/day/m3) Digestion temperatures were not mentioned. In 1968, Burd (29), presented a trea t i s e on aerobic digestion as part of an ov e r a l l sludge disposal study. Other than reviewing the l i t e r a t u r e , he summarized the advantages and disadvantages of aerobic digestion as follows: Advantages: 1. A humus-like, b i o l o g i c a l l y stable end product i s produced. 2. The stable end product has no odours; therefore, simple land disposal, such as i n lagoons, i s feasible. 3. When compared with anaerobic digestion and other schemes, c a p i t a l costs for an aerobic digestion system are low. 4. Aerobically digested sludge usually has good dewatering characteristics. When applied to sand drying beds, i t drains well and redries quickly i f rained upon. 5. V o l a t i l e solids reduction equal to anaerobic digestion i s possible with aerobic systems. 3 0 6 . Supernatant liquors from aerobic digestion have a lower BOD than those from anaerobic digestion. This advantage i s important because the efficiency of many treatment plants i s reduced as a result of recycling high BOD supernatant liquors. 7 . There are fewer operational problems with aerobic digestion than with the more complex anaerobic form because the system i s more stable. As a r e s u l t , less s k i l l e d labour can be used to operate the f a c i l i t y . 8 . In comparison with anaerobic digestion, more of the sludge's basic f e r t i l i z e r values are recovered. Disadvantages: 1 . Aerobic digestion requires high power input for aeration pur-poses. This w i l l be s i g n i f i c a n t when compared to the power requirements for anaerobic digestion especially at the larger plants. 2 . Some sludges do not thicken or dewater easily after aerobic digestion. 3 . There i s no production of a useful by-product, such as CH^ gas. 4 . Aerobic digestion results i n variable solids reduction at varying temperatures. Saunders ( 1 2 0 ) , i n 1 9 6 8 , authored the f i r s t of a series of laboratory studies on aerobic digestion of waste activated sludge, conducted at the V i r g i n i a Polytechnic I n s t i t u t e between 1 9 6 8 and 1 9 7 4 , under Dr. C. W. Randall. The i n i t i a l study was conducted at 20°C, and employed 6 l i t e r batch digesters at three different i n i t i a l t o t a l suspended solids concentrations of 10,980, 15,390 and 26,015 mg/1. The activated sludge used i n the study was obtained from a f u l l -scale sewage treatment plant. The digesters were aerated for a period of 30 days. Although the lower i n i t i a l concentration showed a lower t o t a l and v o l a t i l e suspended solids reduction than the other two digesters, i t could not be concluded with certainty that the i n i t i a l solids concentration had an effect on solids reduction. Apparently, the former digester experienced an upset during the 30 day period, immediately after which the rate of solids reduction was decidedly lower, possibly due to temporary anaerobiosis of the digester contents. Throughout the study, reductions of t o t a l and v o l a t i l e suspended solids were approxi-mately equal, indicating reduction of fixed suspended solids to the same extent as v o l a t i l e suspended s o l i d s . Total and v o l a t i l e suspended solids reductions reached 70 percent after 30 days for the two systems not interrupted by the upset. The r a t i o of c e l l u l a r carbohydrates to t o t a l suspended solids remained constant, whereas the r a t i o of c e l l u l a r protein to t o t a l suspended solids increased with increasing time of digestion. Sludge s e t t l e a b i l i t y and d r a i n a b i l i t y were poor i n i t i a l l y , and did not improve with further digestion. Further studies were recommended to determine the factors c r i t i c a l to sludge d r a i n a b i l i t y . In 1968, Randall and Koch (75, 106), conducted p i l o t plant studies on the dewatering characteristics of aerobically digested sludge. Sludges from various plants, having a variety of problems with respect to sludge dewaterability, were compared on model sand drying beds. The main object of the study was to find a parameter which could be correlated with dewaterability. Although not perfect the parameters best suiting this requirement were oxygen uptake rate ( a c t i v i t y of the sludge) and the amount of hygroscopic material present i n the sludge. S e t t l i n g characteristics showed absolutely no correlation with dewaterability of the sludges. The studies showed that drainage and drying properties of the sludges could be vastly improved by further aerobic digestion. The authors further observed that aerobic digestion of waste activated sludge did not result i n a change i n the percent v o l a t i l e suspended s o l i d s ; i n other words, no build-up of fixed solids occurred. Digester supernatants contained large concentrations of inorganic nutrients and minerals, but very l i t t l e BOD. Drastic drops i n pH were observed under progressive digestion. Randall et al.(105), i n 1969, reported on the b i o l o g i c a l and chemical changes i n activated sludge under aerobic digestion. Both p i l o t scale and laboratory scale digesters were used. The average daily temperature for the p i l o t runs was 85°F (29°C), and the laboratory runs were conducted at a constant temperature of 70°F (21°C). A l l digesters were run on a batch basis with detention times ranging from 14 to 30 days. I t was concluded that the solids reduction continued to be si g n i f i c a n t after 15 days of aerobic digestion. pH was observed to drop d r a s t i c a l l y i n most cases, a value of 5.6 being the l i m i t below which i n h i b i t i o n and 33 destruction of activated sludge organisms occurred. Solids reduction s t i l l took place, although at a limited rate, below a pH of 5.0. The carbohydrate f r a c t i o n of sludge solids was found to remain r e l a t i v e l y constant during digestion, but the protein f r a c t i o n increased steadily with increasing detention time. In a discussion by H e i e r l i (57), of Vrijburg's paper (151), i n 1969, i t was evident that promising results with aerobic digestion of waste activated sludge were being obtained, i n both p i l o t and f u l l - s c a l e i n s t a l l a t i o n s , i n the Netherlands and Switzerland. In these countries, aerobic digestion f a c i l i t i e s are being planned for communities as large as 100,000 people. Although past practice showed a preference for sludge drying beds, i t was pointed out that for larger i n s t a l l a t i o n s , mechanical dewatering and thermal drying would be required. Also i n 1969, Laubenberger and Hartmann (78), reported on the physical structure of activated sludge i n aerobic s t a b i l i z a t i o n . They assumed floe formation and disruption to be a purely physical process. An activated sludge f l o e , consisting of a mixed population of b a c t e r i a l colonies, would not be stable, but could be disrupted into i n d i v i d u a l p a r t i c l e s by changing the shearing forces i n the mixed liquor. I t was stated that the physical structure of activa-ted sludge floes can be analyzed and described to some extent by measuring the p a r t i c l e size and the Thomas <=K -value, where <o< i s the r a t i o of volume of water i n the floe to volume of sludge solids i n the fl o e . Both factors would undergo changes during aerobic digestion, p a r a l l e l l e d by a reduction of organic matter within the floes. The larger the p a r t i c l e sizes within the f l o e , the lower the water content was inside the f l o e , and the greater the effect of aerobic digestion would be i n reducing the water content and increasing the density of the fl o e . The authors found that, although continued aerobic digestion produced increasingly denser floes, the difference i n density, with respect to inorganic chemical fl o e s , would always be a factor two or greater. Turpin (144), studied c e l l u l a r parameters of aerobically digested waste activated sludge i n r e l a t i o n to sludge d r a i n a b i l i t y . This research was conducted i n 1969 on three different sludges. One sludge originated from an activated sludge plant treating domestic sewage, another was the same sludge, but grown on a synthetic substrate, to change the properties of the sludge and produce a high carbohydrate content. The th i r d sludge was obtained from a highly loaded activated sludge plant, treating an equal mixture of domestic and i n d u s t r i a l wastewater. Digestion was performed at 20°C i n three batch reactors, each aerated for 30 days. C e l l u l a r parameters studied were dehydrogenase enzyme a c t i v i t y , c e l l u l a r carbohydrate content, and c e l l u l a r protein content. There was no apparent relationship between dehydrogenase a c t i v i t y and sludge d r a i n a b i l i t y . The c e l l u l a r protein content increased as aerobic digestion progressed, whereas there was no predictable pattern as far as carbohydrate content was concerned. Aerobic digestion took place over a pH range between 5.0 to 9.0, and solids destruction was s t i l l s i g n i f i c a n t at 30 days. Continued digestion improved sludge d r a i n a b i l i t y . In 1970, Moore (91) studied the effects of pH on aerobic digestion of waste activated sludge. Two runs were conducted, the f i r s t consisting of 3 digesters operating as batch reactors, where two of the digesters were controlled to a pH of 5.0 and 7.0, and the t h i r d digester was allowed to establish i t s own pH l e v e l . The second run consisted of 3 digesters also, but more extreme pH levels were employed: 3.5 and 9.0, with the t h i r d digester uncontrolled. A l l experiments were conducted at 20°C. Aeration i n a l l units took place for 23 days. The study results indicated that aerobic digestion e f f i c i e n c y , as measured by solids reduction, was r e l a t i v e l y insensitive to changes i n pH over the range studied. Solids reduction, however, was always greater i n pH controlled units. Except at pH 9.0, a l l digested sludges were well s t a b i l i z e d after 23 days. Sludge digested at pH 9.0 became ea s i l y anaerobic upon storage. The low pH sludges showed considerably improved fl o c c u l a t i o n c h a r a c t e r i s t i c s , corresponding to a large predator population. This, i n turn, s i g n i f i c a n t l y enhanced the s e t t l e a b i l i t y and d r a i n a b i l i t y of the low pH sludges. R i t t e r (116), i n 1970, observed the aerobic digestion process at three contact s t a b i l i z a t i o n plants i n eastern Pennsylvania. Besides digesting waste activated sludge, one of the plants also 36 digested primary sludge aerobically. A l l digesters were operated according to the fill-and-draw p r i n c i p l e with the a i r shut off while decanting took place. The author pointed out that, as the a i r was shut o f f , the hydraulic back pressure would force the sludge into the diffusers, and frequent clogging occurred as soon as the a i r was turned on; this dried the deposited sludge. The use of sock-type diffusers minimized clogging at a l l three plants. At any of the plants, the a i r requirements did not exceed 20 cfm 3 3 per 1000 cu f t (2.0 m /min per 1000 m ) of digester capacity, which was required to keep the solids i n suspension. No wasting of sludge took place within 5 days from the time that digested sludge would be discharged from the digester. The use of froth sprays was not recommended for digesters, as this would increase the required frequency of decanting. The study showed that a l l plants produced a stable sludge. I t was therefore suggested that the then current design c r i t e r i a of 3.0 to 5.0 cu.ft/cap. (0.084 to 0.140 m /cap) were adequate. Anaerobic digestion was shown to accomplish greater v o l a t i l e solids reduction than aerobic digestion. Orthophosphate tests at one of the plants showed that manual decanting resulted i n increased effluent phosphate concentrations, explained by the release of adsorbed phosphates from sludge c e l l s upon stopping aeration. I t was suggested that i n order to reduce phosphate concentrations i n the supernatant, automatic decanting should be practiced. Power costs i n the plants studied amounted to $2.18/yr/lb BOD ($4.80/yr/kg BOD) received, including digester aeration. Construction costs were less than 37 for anaerobic digesters. Cold temperature operation was not mentioned, nor were the sludge characteristics prior to digestion. The Ontario Ministry of the Environment (formerly Ontario Water Resources Commission [OWRC]) (3, 4), undertook an extensive study i n 1970 of aerobic digesters i n operation i n Ontario during the preceding f i v e years. The intent of the study was to evaluate the v a l i d i t y of design parameters developed as a result of laboratory scale studies reported previously, as well as to evaluate the performance of the aerobic digestion process at seven sewage treatment plants i n the Province of Ontario. The areas i n v e s t i -gated i n the study were b i o l o g i c a l oxygen requirements, method of operation, type of treatment prior to digestion, supernatant quality, p r a c t i c a l operating range for suspended solids concentration, digested sludge c h a r a c t e r i s t i c s , disposal methods following diges-tio n , v o l a t i l e solids destruction, seasonal operating problems, and operational problems related to ether soluble materials, and metals. I t was determined that a i r flow rates of 20 cfm/1000 cu f t (2.0 m /min 3 per 1000 m ) were inadequate for both mixing and oxygen requirements i n some i n s t a l l a t i o n s . Factors affecting a i r supply were not as much the b i o l o g i c a l oxygen requirements but tank configuration and method of digester operation, i.e. more a i r was required to resus-pend digester solids after a i r had been shut off for sludge wasting and decanting. This problem was.especially prominent when t o t a l solids concentrations exceeded 3 percent. The conditions at the 38 plants studied indicated that a i r rates of 50 cfm/1000 cu f t 3 3 (5.0 m /min per 1000 m ) were required for proper operation at the solids levels and sludge ages encountered. The digesters investigated varied i n t o t a l solids concentration from less than 1 to almost 6 percent, with most of the plants at greater than 2 percent. D i f f i c u l t i e s with sludge s e t t l i n g and supernatant removal were encountered when the solids l e v e l exceeded 3 percent. I t was therefore suggested that at such high concentrations, two-stage digestion should be provided. The f i r s t stage would maintain a low solids concentration (1.5 to 2.5 percent t o t a l s o l i d s ) , to permit frequent decanting and also to minimize oxygen requirements where maximum s p e c i f i c oxygen uptake rates existed. The second stage would have greater than 3 percent s o l i d s , to permit concentration of the sludge where lower s p e c i f i c oxygen uptake rates prevailed and where infrequent decanting would be required. S e t t l e a b i l i t y of aerobically digested sludge was found to be a function of solids concentration, dissolved oxygen concentration, and type of sludge fed to the digester. Excessively long detention time tended to deteriorate s e t t l i n g e f f i c i e n c y and so did low (less than 1 ppm) dissolved oxygen levels. V o l a t i l e solids reductions ranged from 24 to 50 percent i n two-stage digesters, and from 10 to 25 percent i n f i r s t stage or single stage units. I t was pointed out that v o l a t i l e s o l i d s 39 reduction alone would not indicate the performance of the digester, as the i n i t i a l sludge age of the sludge wasted to the digester has a direct bearing on t h i s . Deterioration of supernatant quality was observed at plants having single-stage digesters and at plants where the solids l e v e l approached 6 percent. The strength of supernatant i n terms of soluble BOD, however, was always less than the strength of normal domestic sewage, and, with the low super-natant volumes, did not affect the treatment processes. Solids recycled to the treatment process from aerobic digester liquors were i n a highly oxidized state (as compared to solids i n anaerobic digester liquors that are i n a reduced state) and would therefore not have a pronounced effect on the treatment process. Specific oxygen uptake rates ranged from 0.5 to 6.3 mg O^ /g VSS/hr i n single stage digesters. The range for second stage digesters was 0.5 to 2.4 mg 0 2/g VSS/hr. However, most of the data for single stage units were between 2 and 4 mg 02/g VSS/hr. The uptake rates depend on the i n i t i a l sludge age and i t was found that digesters used with conventional activated sludge plants had s p e c i f i c uptake rates that are approximately 50 percent greater than digesters used with contact s t a b i l i z a t i o n plants. Uptake rates were also found to be extremely temperature dependent. Digested sludges with s p e c i f i c uptake rates between 0.5 and 1 mg O^ /g VSS/hr were well s t a b i l i z e d ( i . e . did not decompose anaerobically when l e f t unaerated). This did not hold true at temperatures below 5°C; however, i t was argued that ultimate sludge disposal would probably be impossible at such temperatures. The data presented i n Figure 3 indicate that t o t a l sludge s t a b i l i -zation, as measured by s p e c i f i c oxygen uptake rate, did not take place u n t i l the t o t a l sludge age was about 180 days. Total sludge age was defined as the sludge age of the digesting sludge plus the sludge age prior to digestion. A E R O B I C DIGESTER O X Y G E N U T I L I Z A T I O N S A T E S 20 40 60 80 100 120 HO 160 160 200 220 240 T O T A L S L U D G E A G E ( D a y . ) Figure 3. Aerobic Digester Oxygen U t i l i z a t i o n Rates (Ahlberg) I t was f e l t that the temperature effect was greatest between 0° and 5°C. Temperature extremes could be avoided by physical design such as decreasing hydraulic retention time, u t i l i z a t i o n of heat sources and heat sinks i n terms of a i r supply and sewage flow through the treatment plant, and the construction characteristics of the digester. Smith (127), i n 1970, was the f i r s t to direct his research e n t i r e l y to the low temperature aspects of aerobic digestion of waste activated sludge. He developed the sludge to be digested i n a f i l l and draw type reactor using a synthetic substrate. The sludges had a 10 day sludge age and were developed at the same temperature as they would be subjected to during aerobic digestion. Digestion took place i n batch reactors at 5° and 10°C. Digested sludge was analysed regularly for COD, t o t a l and suspended s o l i d s , v o l a t i l e suspended s o l i d s , and pH. I n i t i a l oxygen uptake rates of the two sludges were 5.5 and 6.5 mg 0 2/g VSS/hr at 5° and 10°C respectively. V o l a t i l e suspended solids reductions reported were 11, 13, 19, and 29 percent at 5°C and detention periods of 10, 20, 30, and 50 days, respectively, and 14, 25, 34, and 51 percent at 10°C and the same detention periods. Tebbutt (142), started his studies i n aerobic digestion i n 1970, and reported on continuing studies on that subject i n 1971 (124). Digestion was done by batch procedure at 15° and 20°C on a p i l o t scale and laboratory scale, respectively. Activated sludge was used i n a l l tests. I n i t i a l solids concentrations for a l l digestion runs i n the f i r s t study were much lower than i n any of the previous investigations. Tebbutt concluded that aerobic digestion of activated sludge mixed-liquor brought about s i g n i f i c a n t v o l a t i l e suspended solids reductions: 45 percent after 8 days, and 55 percent after 16 days. Rate of aeration did not have a s i g n i f i c a n t effect on v o l a t i l e suspended solids reduction. Digested sludge had good dewatering cha r a c t e r i s t i c s , and there was l i t t l e or no increase i n organic content of the supernatant. /He suggested that more concentrated return sludges would not s t a b i l i z e as well at normal aeration rates; however, this conclusion could not be substantiated from the results published i n this paper, as a l l digester runs used approximately the same (low) i n i t i a l solids concentration of about 1200 mg/1 t o t a l suspended so l i d s . In a second research program (143), Tebbutt used higher i n i t i a l s o lids concentrations. These studies confirmed his e a r l i e r con-clusion that the eff i c i e n c y of aerobic digestion tended to be reduced as the i n i t i a l solids content of the sludge was increased. Cook et al.(40), reported i n 1971 on their p i l o t plant results from aerobic digestion of primary sludge. Their studies were done at 25°C using an intermittent feeding system. Digested sludge and supernatant were monitored for COD, ammonia and n i t r a t e nitrogen,, t o t a l phosphorus, and t o t a l and v o l a t i l e s o l i d s . Considering a l l these parameters, i t was concluded that the 4-day detention time out-performed the other detention times (2, 8, and 12 days). Aerobic digestion f a c i l i t i e s i n operation at M i l l e r s v i l l e , Pa. i n 3 a 1.0 mgd (3785 m /day) c o n t a c t - s t a b i l i z a t i o n , activated sludge plant were b u i l t u t i l i z i n g an old Imhoff tank. Levis et al.(81), reported very satisfactory operation. The digested sludge produced was suitable for direct l i q u i d hauling, as well as for dewatering on sludge drying beds. 43 The Oklahoma State University (100), i n 1971, i n a project sponsored by the Environmental Protection Agency, conducted a study of aerobic digestion of primary, mixed primary and secondary, and secondary sludge at 25°C. Detention times between 2 and 30 days i n semicontinuous feed, flow-through reactors were investigated on a p i l o t scale. Digested sludges were dried on p i l o t sand beds. The results were i n general agreement with previous studies, except that i n this study, f i l t r a t e COD of aerobically digested sludge was greater than f i l t r a t e COD of anaerobically digested sludge. The report also stated that continuous feeding of aerobic digesters would produce approximately a f i v e percent increase i n the reductions of the various parameters studied. Roy F. Weston, Inc. (159), i n a design manual for upgrading of existing treatment plants prepared for the Environmental Protection Agency i n 1971, described aerobic digestion as an effec t i v e means for upgrading existing overloaded anaerobic digestion f a c i l i t i e s . Existing design parameters were l i s t e d and discussed, and several case studies were presented. One of the more si g n i f i c a n t observations was the fact that normal effluent orthophosphate concentrations were exceeded by as much as 200 percent during decanting of f i l l -and-draw type digesters. The explanation offered was that during aeration, phosphate i s adsorbed to the sludge c e l l s , whereas when aeration i s terminated, the phosphate would be released by the sludge c e l l s and leave the digester with the supernatant. I t was suggested that, i f low effluent phosphate concentrations were 44 required, then automatic decanting, without stopping aeration, should be used. Parker et al.(101), reported i n 1971 on the effects of commonly used sludge handling procedures on the subsequent f i l t e r a b i l i t y of waste activated sludge. They used laboratory scale batch digesters at 19°C. I n i t i a l suspended solids concentration of the sludges was between 1.2 and 2.0 percent. The results showed that the way sludge i s handled prior to dewatering can have s i g n i f i c a n t effects on the dewaterability of the sludge. Dewaterability can be adversely affected by anaerobic storage, excessive mixing, chlorination, and rapid changes i n temperatures; whereas, i t can be improved or restored by further aeration. For optimum dewaterability, i t was found that the sludge organisms must be maintained aerobically i n the endogenous respiration phase and be active and intact when subjected to dewatering procedures. In 1972, B o k i l (24, 25, 26), studied the effect of blending on aerobic digestion of waste activated sludge. These experiments were conducted i n batch digesters at room temperature, while blending of the sludge was performed either as a batch procedure or continuously. Both the auto-oxidation rate and s e t t l e a b i l i t y of the digester sludges improved with blending. The increase i n auto-oxidation rate was found to be a linear function of the energy input index. Washing the sludge i n i t i a l l y with 0.05 M phosphate buffer resulted i n a f a i r l y constant pH during the digestion process; whereas, unwashed sludges showed a drop i n pH to between 5.0 and 6.0 as digestion progressed. When accompanied by blending, washing with phosphate buffer further increased the auto-oxidation rate. Under optimum conditions of blending, the blended sludge showed a 67 percent higher auto-oxidation rate than unblended sludge. Cameron (33), i n 1972, developed operating c r i t e r i a for the separate aerobic digestion of waste activated sludge at the 6 mgd F l i n t River Water P o l l u t i o n Control Plant i n Atlanta. He found that the low pH i n the digester, caused by the destruction of natural a l k a l i n i t y during n i t r i f i c a t i o n , could be controlled by increasing the rate of wasting to the digester at decreased solids concentra-tions, thus providing additional a l k a l i n i t y to the digester. I t was suggested that continuous gravity thickening of the aerobic digester contents would greatly simplify digester operation, as opposed to thickening and decanting while shutting off the a i r ; this was especially so when wanting to waste d i l u t e sludge i n order to control pH. A gravity thickener, with i t s prevailing anaerobic conditions, would also provide continuous d e n i t r i f i c a t i o n of the digester contents. Although the s e t t l i n g and dewatering characteristics of digested waste activated sludge were better than that of raw waste activated sludge, i t was not as good as that of primary sludge. Properly digested sludge contained approxi-mately 66 percent v o l a t i l e s and could be gravity thickened to 2 to 3 percent s o l i d s , when a slow s t i r r i n g mechanism was employed i n the thickener. Rivera-Cordero (117), completed i n 1972 an extensive research project i n the effects of aerobic digestion on sludge f i l t e r a b i l i t y and dewaterability. Digestion was carried out i n batch digesters at a constant temperature of 20°C. The raw sludge was obtained from three different activated sludge plants. The change i n f i l t r a t i o n characteristics of sludge, brought about by aerobic digestion, was found to be simultaneous to changes i n the s p e c i f i c resistance and compressibility factor of the sludge. Specific resistance reached a minimum value after one to f i v e days of aeration, and increased thereafter to values w e l l over that of the o r i g i n a l sludge. The s p e c i f i c resistance of activated sludges, and thus the f i l t e r a b i l i t y , was i n d i r e c t l y related to the median sludge p a r t i c l e size. I t was shown that the factors most affecting p a r t i c l e size were pH and the change i n exocellular polymer concen-tr a t i o n . Chlorination of digested sludges reduced the median p a r t i c l e s i z e , p a r t i c u l a r l y when the sludge had been aerated for a long time. Stein et al.(134), i n 1972, compared aerobic digestion of waste activated sludge using pure oxygen and a i r for oxygenation. Sludges were obtained from an activated sludge brewery waste treatment system. Ten laboratory scale digestion units were employed; half of those were aerated with a i r , the other half with pure oxygen. Eight units were fed continuously on a flow-through basis at detention times of 3, 5, 10 and 15 days; i n addition, two units were operated as a batch system over an 18-day period. A l l digestion took place at a temperature of 30°C. The authors concluded that, for both the batch and continuous feed systems, there seemed to be very l i t t l e advantage i n the use of pure oxygen as compared with a i r i n aerobic digestion. In 1973, Andrews and Kambhu (7), reviewed the theory of thermophilic aerobic digestion of organic s o l i d wastes and presented a simulated model of thermophilic aerobic digestion of organic s o l i d s . The in-depth review of the stoichiometry, thermochemistry, and k i n e t i c s of thermophilic aerobic digestion i s also applicable to a large extent to lower temperature situations. An e a r l i e r report on the same studies appeared i n 1969 (70). Under the terms of the Canada-Ontario Agreement on Great Lakes Water Quality i n 1973, Ganczarczyk and Hamoda (52) studied the effects of aluminum and f e r r i c s a l t s on the aerobic digestion process. Eighteen l i t r e batch systems and semi-continuous, once da i l y fed systems, at an average loading of 0.06 lb VS/day/cu f t 3 (0.96 kg VS/day/m ), were investigated at varying chemical dosages. Digestion temperature was 20°C for a l l experiments. The results showed that the performance of aerobic digestion of alum and f e r r i c sludges was not s i g n i f i c a n t l y different from that of the conventional activated sludge control. I t was shown that sludge dewatering characteristics deteriorated with prolonged aeration periods. Although batch operation resulted i n higher v o l a t i l e solids destruction, semi-continuous operation achieved better dewaterability and supernatant quality. Cohen and Puntenney (38), i n 1973, reported on the results of a 13-month f u l l - s c a l e aerobic digestion study at the Metropolitan Denver Sewage Treatment Plant. The federally funded project resulted from e a r l i e r promising small-scale results (82), and premature overloading of the existing solids handling f a c i l i t i e s at the plant ( f l o t a t i o n , incineration, l a n d f i l l ) . Other than the e a r l i e r OWRC f i e l d evaluation project (3, 4), th i s i s the only f u l l - s c a l e evaluation known of aerobic digestion syterns. The Denver plant has a capacity of 100 US mgd (378,500 3 m /day), and aerobic digestion was incorporated by u t i l i z i n g one-3 t h i r d of the existing 24 MG (91,000 m ) aeration tank capacity for aerobic digestion of a l l waste activated sludge. The study program was intended to provide complete information on the aerobic digestion process and included a large number of v a r i -ables, such as, time of s t a b i l i z a t i o n , v o l a t i l e solids loading, a i r supply, dissolved oxygen concentration, oxygen uptake rates, sludge s e t t l e a b i l i t y , supernatant quality, odour l e v e l s , sludge dewaterability, and physical, chemical and microbiological characteristics. The digestion study included a wide range of solids loadings and temperatures. 49 Temperatures during the course of the study i n the digester l i q u i d did not drop below 15.8°C on a monthly average basis or below 12°C on a d a i l y basis. V o l a t i l e solids destruction during the 13 month period ranged from 11.2 percent i n January to 47.2 percent i n October. Attempts by the authors to relate that performance to one single variable, such as hydraulic residence time, s o l i d s residence time, or temperature, were unsuccessful. The authors concluded that the most s i g n i f i c a n t correlation was observed between performance measured as percent v o l a t i l e solids destruction and sludge retention time multiplied by temperature (SRT x T), with sludge retention time between the l i m i t s of 0 - 12 days, and temperature between the l i m i t s of 12° - 22°C. The authors also conclude that sludge prehistory (sludge age p r i o r to digestion) has l i t t l e effect on digestion e f f i c i e n c y , a claim which i s substantiated by comparing the September and January performance when pre-digestion sludge ages were approximately equal, but VSS destruction markedly lower i n January. However, when comparing the May and November records, one notices that the pre-digestion sludge ages are quite d i f f e r e n t , as are the sludge retention times during digestion, but the t o t a l sludge age i n the system (pre-digestion + digestion) and temperature are approxi-mately equal i n these two months, and so i s the VSS destruction. I t seems, therefore, that the author's conclusion might not be correct, and that the t o t a l sludge age rather than pre-digestion sludge age or digestion sludge age alone governs digestion e f f i -ciency at any temperature. 50 The authors found si x major taxonomic groups of invertebrate organisms i n aerobic digester sludges, namely: f l a g e l l a t e s , motile c i l i a t e s , stalked c i l i a t e s , r o t i f e r s , amoeba, and nematodes. It was observed that, when the digester ecosystem underwent stress ( i . e . low temperatures or rapid change i n loading rate), the ecological d i v e r s i t y declined. A high correlation was found between the r o t i f e r f r a c t i o n of the invertebrate biomass and performance. No s i g n i f i c a n t correlation was found between the ATP contents of aerobically and anaerobically digested sludges, although, i n the one example ci t e d , the ATP/VSS r a t i o of aerobic digester sludge was almost 50 percent higher than that of anaerobic digester sludge. The non-bacterial ATP source i n aerobic digester sludge was a t t r i -buted to the invertebrate protozoa and metozoa. I t was recognized that r o t i f e r s play a key role i n the aerobic s t a b i l i z a t i o n process; however, the mechanism by which r o t i f e r metabolism influences performance was not established. Liquid-solids separation was measured as sludge volume index and as zone s e t t l i n g v e l o c i t y , but a poor correlation between the two was indicated. The average sludge s e t t l i n g rate over the year was less than 2 f t / h r (608 mm/hr). The practice of shutting off the a i r to provide s u f f i c i e n t supernatant for manual decanting was not favoured, as depletion of oxygen with accompanying decline i n oxidation-reduction potential (ORP) i n less than two hours caused odours, d e n i t r i f i c a t i o n , and fl o a t i n g sludge. I t was suggested that separate c l a r i f i e r capacity be i n s t a l l e d for continuous decanting, i f concentration of digested sludge i s desired. The desired overflow rate was suggested at less than 300 US gpd/sq f t (12.2 3 2 m /day/m ); however, no mention was made of allowable c l a r i f i e r solids loading. N i t r i f i c a t i o n was observed to decline as temperatures dropped below 22°C. No correlation was found between n i t r i f i c a t i o n and dissolved oxygen concentration; however, the correlation between n i t r i f i c a t i o n and SRT x temperature was s i g n i f i c a n t . No correlation was found between performance and phosphorus removal. Oxygen uptake rates averaged 7.1 mg O^/gVSS/hr throughout the year and correlated closely with temperature. The range of oxygen used per unit of VSS destroyed was 1.2 - 3.7 mg/g/hr, with an average value at 2.2. The most e f f i c i e n t oxygen u t i l i z a t i o n occurred at a loading of 0.08 lb VSS/day/cu f t (1.28 kg VSS/day/m3). At loadings greater than 0.15, active mass was washed out before a high degree of s o l u b i l i z a t i o n could occur. At loadings less than 0.05, the amount of readily biodegradable VSS was marginal per unit oxygen supplied, leading to apparent oxygen wastage. Better dewaterability was indicated for aerobically digested sludge as compared to anaerobically digested sludge, when measured on a volumetric basis. When corrected for differences i n solids concentrations, however, aerobically digested sludge drained 2.5 times slower than anaerobically digested sludge. On the same solids weighted basis, undigested sludge drained 20 percent faster than aerobically digested sludge. Odour panel tests indicated that, i f nuisance odours were to be avoided, the aerobically^ digested v o l a t i l e f r a c t i o n had to be reduced to about 60 percent. This had been impossible under the Denver study conditions with aerobic digestion alone. I t was shown that anaerobic digestion subsequent to aerobic digestion could reduce the v o l a t i l e f r a c t i o n to 60 percent i n less than 3 weeks, but the same degree of s t a b i l i z a t i o n could be achieved by pure oxygen/ozonation i n 2-3 hours. The mechanism of th i s treatment was not described, and i t was stated that further studies i n th i s regard would be conducted. In 1974, Cohen (39), reported on continuing studies i n aerobic digestion at Denver. The main objective of these studies was to investigate the f e a s i b i l i t y of pure oxygen aerobic digestion by means of p i l o t scale digesters. The pure oxygen digesters were set up on a batch and continuous feed basis. The batch studies showed an endogenous decay rate of 0.27 day \ A li n e a r r e l a t i o n -ship was found between oxygen uptake rate and biodegradable v o l a t i l e suspended solids above 2000 mg/1, according to the equation: Oxygen uptake rate (mg/l/hr) = 0,0127 VSS (biodegradable) (mg/1) +39.7 The only reference to digester temperature was made i n the discus-sion of the continuous feed system, where i t was stated that at 53 the highest loading rate investigated of 0.433 lb VSS/day/cu f t 3 (6.928 kg VSS/day/m ), the temperature i n the digester was 28.6°C, whereas, the average ambient temperature was 8.4°C. At the lower loading rate of 0.083 lb VSS/day/cu f t (1.328 kg VSS/day/m3), there was no such temperature d i f f e r e n t i a l . The increase i n loading rate from the low value to the high value resulted i n only a nominal decrease i n v o l a t i l e suspended solids reduction (47.1% versus 38.8%). The results further indicated that, with the type of rotary diffuser used for pure oxygen digestion, a high degree of mass reduction could be achieved at economic oxygen u t i l i z a t i o n levels i n thickened waste activated sludge (5% TSS), without screening problems. When comparing diffused a i r and oxygen aerobic digestion performance, i t was shown that the ra t i o of v o l a t i l e to t o t a l suspended solids was consistently lower for pure oxygen digested sludge, as compared to a i r digested sludge. The flow-through oxygen systems showed very l i t t l e n i t r i f i c a t i o n occurring, as compared to the oxygen batch systems and a i r systems. V o l a t i l e s o l i d s reductions were higher i n a i r systems below a loading rate of 0.08 lb VSS/day/cu f t 3 3 (1.28 kg VSS/day/m ). Above 0.14 lb VSS/day/cu f t (2.24 kg VSS/day/m ), the performance of the a i r system declined rapidly, whereas the oxygen system continued to perform we l l up to loading rates as 3 high as 0.60 lb VSS/day/cu f t (9.60 kg VSS/day/m ). Randall et al.(107), i n 1973, reported on the changes that occur i n waste activated sludge, when i t i s processed by common handling procedures, and the effect of these changes on subsequent dewatering of the sludge. In order to ide n t i f y the conditions that make activated sludge more d i f f i c u l t to dewater and to formulate pro-cedures that would optimize dewatering, the following handling procedures were studied extensively: aerobic digestion, anaerobic storage, chlorination, polymer conditioning. Among the operational parameters studied were solids concentration, temperature, mixing, dissolved oxygen concentration, aeration rate, and digester flow conditions. Methods used for measuring changes i n dewaterability were vacuum f i l t e r a b i l i t y , s p e c i f i c resistance, and compressibility. Except for the temperature studies, a l l work was conducted at 20°C. The temperature studies were conducted at 20°, 30°, and 35°C, and one set of tests at 6°C. Other than a comparison test of batch digestion versus continuous digestion, a l l digesters i n the study were operated as batch units. The major conclusions reached i n these studies were as follows: 1. The various handling procedures (conditioning or otherwise) applied to activated sludges could have a s i g n i f i c a n t effect on subsequent dewaterability. 2. Anaerobic conditions were detrimental to the f i l t r a t i o n characteristics of activated sludge. 3. Aerobic digestion had a considerable effect on the f i l t r a t i o n characteristics of waste activated sludge, as i t produced changes i n both s p e c i f i c resistance and compressibility. 4. Increased temperature during aerobic digestion increased f i l t e r a b i l i t y ; however, this effect could be decreased by increased detention time. 5. Continuously fed digested sludge dewatered more readily than batch digested sludge, but the difference was small. In general, for best dewatering, activated sludge had to be held i n a viable, aerobic condition during the handling procedures imposed. Procedures, which would destroy the b i o l o g i c a l c e l l s and break up the f l o e , apparently, resulted i n poorer f i l t e r a b i l i t y . The dewaterability of activated sludge was most c l e a r l y reflected by the median p a r t i c l e size of the floe and the amount of organic matter i n the supernatant. Measurements of these parameters could be used to control sludge handling processes. In 1974, Notebaert et al.(99), concluded that for aerobically digested sludge to be properly s t a b i l i z e d , the oxygen uptake rate should be less than 100 mg O^ /g organic matter/day and the ether soluble fat content less than 40 mg/g organic matter. Of the two parameters, oxygen uptake rate was the more r e l i a b l e . Sludges from oxidation ditches and activated sludge plants were used i n these experiments. Digestion was performed i n batch units at a temperature of 20°C. With respect to n i t r i f i c a t i o n , the authors noted that, after reaching a high n i t r a t e concentration of 200 mg/1 as N, the n i t r i f i c a t i o n process slowed down while the ammonia and n i t r i t e concentrations would quickly increase. This was thought to be due to the low pH and the high n i t r a t e concentration. Upon anaerobic storage, d e n i t r i f i c a t i o n took place. Nitrates and n i t r i t e s disappeared and the ammonia concentration quickly rose to 100 - 200 mg/1 as N. Randall et al.(109), reported on aerobic digestion of t r i c k l i n g f i l t e r humus i n 1974. The experiments were conducted i n batch units of 20°C. I t was shown that t r i c k l i n g f i l t e r sludge goes through an active stage of metabolism, prior to entering the endogenous respiration stage. This i s i n contrast to activated sludge, which normally enters the endogenous stage immediately. I t was also pointed out that, although the rate constant 1c of Stein et al.(135), (which described the rate of biodegradable solids destruction per unit time) adequately predicts the detention time required to achieve a certain degree of solids destruction, the rate constant k, (which expresses the rate of t o t a l v o l a t i l e d suspended solids destruction per unit time) would describe the aerobic digestion process more adequately. Fixed suspended solids were destroyed to about the same extent as v o l a t i l e suspended solid s . I t was therefore suggested that t o t a l suspended solids should be used i n k i n e t i c analyses. Overall t o t a l suspended solids destruction, during aerobic digestion of t r i c k l i n g f i l t e r sludge, was found to be similar to that of waste activated sludge, although supernatant quality for the former was s i g n i f i c a n t l y worse. 57 The effects of temperature on aerobic digestion and subsequent dewatering of activated sludge were investigated by Randall et a l . (108), i n 1974 also. Temperatures of 5°, 10°, 20°, 30°, 35°, and 45°C were used i n batch-type experiments, with digestion times of up to 15 days. Although the systems below 20°C conformed more or less to established reaction k i n e t i c s , the systems above 20°C showed a decrease i n reaction rate at around 30°C and a subsequent peak i n reaction rate around 35° to 40°C at detention times greater than 5 days. The studies c l e a r l y established that 45°C was not a desirable temperature for aerobic digestion, and that there was l i t t l e difference i n aerobic digestion performance i n the tempera-ture range between 20° and 35°C. Bishop and LePage (21), conducted low temperature studies on aerobic digestion of primary sludge i n 1974. Temperatures studied were 5, 10, 15, 20, 25, and 30°C using d a i l y fill-and-draw type aerobic 3 digesters, loaded at 0.013 lb B0D5/day/cu f t (0.208 kg B0D5/day/m ). The study concluded that greater reductions i n BOD, COD, t o t a l s o l i d s , and t o t a l v o l a t i l e solids occurred at higher temperatures. I t was also concluded that digestion periods longer than 15 days produced l i t t l e improvement i n the degree of s t a b i l i z a t i o n , except at 5°C, and that the effect of temperature on the degree of s t a b i -l i z a t i o n was more s i g n i f i c a n t at shorter detention times. In this respect, i t should be noted that only one sludge loading rate was investigated, i.e. only a single sludge age, and that comments r e l a t i v e to detention time relate to examination of the systems during the f i r s t 15 days of operation, before equilibrium was reached. Further conclusions of the study were that there was no apparent correlation between temperature and sludge s e t t l e a b i l i t y , but that the s e t t l e a b i l i t y of digested sludge was always less than that of the feed sludge. F i l t e r a b i l i t y of digested sludge generally improved as digestion temperatures increased; however, i t dropped off considerably beyond 15 days digestion at the higher temperatures. F i l t e r a b i l i t y of digested sludges was not as good as that of raw sludge. Aerobically digested sludge never had a disagreeable odour, either during aeration or f i l t r a t i o n . Eikum et al.(46), i n 1975, investigated the phosphorus release during storage of aerobically digested mixed primary-chemical sludge. Sludges were taken from daily-fed, flow-through laboratory digesters, operated at temperatures of 7°, 12°, 18°, and 25°C. Storage of the digested sludges took place at 10°C. The authors concluded that t o t a l phosphorus does not change during aerobic sludge s t a b i l i z a t i o n ; however, i n the s o l i d phase, the phosphorus content increases during digestion for both primary and mixed primary-chemical sludges. During storage, phosphorus i s always released from the s o l i d phase to the l i q u i d phase i n primary sludges; however, this was not the case for mixed primary-chemical sludges. The ultimate orthophosphate release, per gram of v o l a t i l e suspended solids and per viable c e l l , decreased with increasing detention time i n the aerobic digester. Hamoda and Ganczarczyk (56) , reported i n 1975 on continuing aerobic digestion studies on mixed primary-chemical sludges. This l a t e s t study reports on the effects of varying lime dosages on the e f f i -ciency of the aerobic digestion process. At a temperature of 20°C, lime dosages between 140 and 650 mg/1 were investigated on a batch and d a i l y fill-and-draw basis, with loading rates of 0.063, 0.044 and 0.030 lb TVS/day/cu f t (1.008, 0.704 and 0.480 kg TVS/day/m3 Aerobic digestion of lime-primary sludge proved to be feasible, but the k i n e t i c s of the digestion process were adversely affected by high dosages of lime. Lime additions had the effect of increasing the buffering capacity of the digested sludge. The pH never dropped below a value of 8.5. I t was further determined that the VSS reduction rate was inversely proportional to the t o t a l a l k a l i n i t y . Oxygen uptake rates did not seem to be affected by the presence of lime. Aerobically digested lime-primary sludges had good s e t t l e -a b i l i t y and f i l t e r a b i l i t y . The major difference between semi-continuous and batch operation was that the former mode of operation resulted i n higher oxygen uptake rates, better dewaterability, and higher supernatant quality. Also i n 1975, Matsch and Drnevich (88), reported on autothermal aerobic digestion studies done by the Union Carbide Corporation. Large volumes of gas required for a i r oxygenation of aerobic digester contents were i d e n t i f i e d as a major source of heat loss, and i t was shown that by using pure oxygen, this source of heat loss could be largely eliminated. By using other heat conservation 60 measures, such as covered tankage (also inherent i n a pure oxygen process), underground i n s t a l l a t i o n , and high influent v o l a t i l e solids concentrations (greater than 3.5 percent), i t was shown that the aerobic digestion process of waste activated sludge and mixed primary-waste activated sludge was autothermal, independent of influent sludge temperature and ambient temperatures. The process was self-regulating, with respect to temperature, as above 60°C, the digestion rate dropped off automatically. Digester temperature was always between 45° and 60°C. Beside higher digestion e f f i c i e n c i e s during thermophilic digestion, other advantages of higher tempera-tures were lower oxygen requirements because of i n h i b i t i o n of n i t r i f i c a t i o n , and the digested sludge being free of pathogens because of the pasteurization effect of the higher temperatures. Process evaluation tests were conducted on a p i l o t and f u l l - s c a l e basis, using single stage and two-stage digestion f a c i l i t i e s . Randall et al.(110), i n 1975, studied aerobic digestion of activated sludge at elevated temperatures (20°C - 45°C). Batch operation was" compared against d a i l y f i l l and draw operation on a laboratory scale. I t was concluded that temperatures above 30°C inhibited n i t r i f i c a t i o n , as shown by a lesser drop i n pH during aerobic digestion. Higher temperatures also resulted i n decreased oxygen uptake rates and increased s o l u b i l i z a t i o n of sludge s o l i d s , without metabolism of the re s u l t i n g soluble organics during the early stages of digestion. Solids reduction at higher temperatures could not be predicted by the Arrhenius equation. There was a substantial difference i n e f f i c i e n c y between the two modes of operation studied, with the batch process being 60 percent more e f f i c i e n t than the fill-and-draw process, at equivalent detention times. In 1976, Ganczarczyk et al.(53), published the results of a study on the effects of solids concentration and digester operation on the aerobic digestion process. The work was conducted at 20°C, using primary and waste activated sludge, and compared batch and semi-continuous operation at solids concentrations ranging between 10,000 and 60,000 mg/1. Although the aerobic digestion e f f i c i e n c y was found to be s l i g h t l y lower at higher suspended solids concen-trations than at lower suspended solids concentrations, i t was concluded that the treatment of a sludge with a higher solids concentration was an effective means of u t i l i z i n g digester volume. It was also found that the amount of v o l a t i l e suspended solids removed per unit digester volume was comparable for waste activated sludge and primary sludge. Batch digestion appeared to be more effective than the semi-continuous feeding process, although i t was pointed out that the k i n e t i c behaviour of the two processes is. different. N i t r i f i c a t i o n was s i g n i f i c a n t l y reduced at increased solids concentrations. Benefield and Randall (19) i n 1978, discussed a new approach to a design model of aerobic digestion. They point out that, i n addition to the destruction of v o l a t i l e suspended s o l i d s , fixed suspended 62 solids are destroyed during aerobic digestion as w e l l . They a t t r i -buted t h i s to the l y s i s of b a c t e r i a l c e l l s , releasing inorganic as well as organic compounds into solution. Expressions were developed for aerobic digestion of waste activated sludge only and for a mixture of primary and waste activated sludge. These expressions described the relationship between the physiological state of the sludge to be digested, the rate of endogenous respiration and the solids retention time i n the digester. I t i s further suggested that, for the waste activated sludge model and a constant solids loading, the solids retention time must be increased as the active f r a c t i o n of the biomass i n the feed sludge decreases, i n order to achieve a similar solids reduction; however, i t was pointed out that this may result i n unreasonably high solids retention time requirements i n the case of a feed sludge produced from an activated sludge plant, operated at high sludge age (such as extended aeration). Also i n 1978, Bishop and Farmer (22) presented the results of a study of the fate of nutrients during aerobic digestion. They found that s o l i d phase organic nitrogen i s mineralized to form ammonium during aerobic digestion, which i s then readily oxidized to n i t r a t e s . N i t r i f i c a t i o n results i n consumption of a l k a l i n i t y and a drop i n pH, p a r t i c u l a r l y when the feed sludge i s low i n a l k a l i n i t y . N i t r i f i c a t i o n i n the pH range from 5.0-5.5 was sub-s t a n t i a l . B i o l o g i c a l d e n i t r i f i c a t i o n was found to occur at dissolved oxygen levels less than 1 mg/1. The percentage of organic nitrogen mineralized was roughly equal to the percentage of v o l a t i l e suspended solids reduction. The s o l i d phase nitrogen content of aerobically digested sludge was found to be 6-7 percent of the t o t a l suspended soli d s . Phosphorus was released i n approximately a stoichiometric r a t i o to the v o l a t i l e solids destroyed, producing a digested sludge with a phosphorus content of about 2 percent of the t o t a l suspended so l i d s . I t was pointed out that the potential exists i n aerobic digesters for the supernatant to contain more than 50 percent of the nitrogen and phosphorus i n the digester, when digesting waste activated sludge only. Need for Further Research The foregoing was an attempt to summarize approximately 25 years of research into aerobic digestion. Although a considerable amount of good research has been conducted i n aerobic digestion, a number of areas s t i l l require further knowledge. During the early part of the period covered by the foregoing review, the majority of investigations concentrated on aerobic digestion of primary sludge. I t seems to have been shown rather c l e a r l y that aerobic digestion i s not i d e a l l y suited to the treat-ment of primary sludge, mainly because of i t s non-biological nature. The main concensus seems to be that primary sludge i s best digested by anaerobic means, or treated d i f f e r e n t l y altogether More recently, because of the knowledge gained with primary sludge digestion, and the upsurge of small activated sludge treatment systems without primary sedimentation, most research e f f o r t s were related to digestion of waste activated sludge, which, because of i t s aerobic, b i o l o g i c a l nature, lends i t s e l f i d e a l l y to aerobic digestion. A large number of the experiments using waste activated sludge were concerned with the factors affecting sludge dewaterability after aerobic digestion, since the predominant means of ultimate disposal of digested sludge was, at the time, into l a n d f i l l s after drying on sand beds. With the increasing interest i n land disposal of digested sludge, which could use the sludge i n the l i q u i d form, sludge dewaterability may become a less c r i t i c a l factor i n the future. A number of other investigations seem to have been directed towards establishing the optimum conditions to achieve maximum aerobic digestion e f f i c i e n c y , rather than determining the process charac-t e r i s t i c s and requirements under normal operating conditions. Furthermore, the number of studies into the low temperature aspects of aerobic digestion have been very limited. Because of the almost universal application of aerobic digestion systems, especially i n small activated sludge plants without primary s e t t l i n g , i n low temperature regions, there i s a great need f research data on low-temperature aerobic digestion of waste activated sludge. \ 66 I I I . MATERIALS AND METHODS The rationale governing the design of the experiments and a n a l y t i c a l methodology, was to investigate the performance characteristics of the aerobic digestion process under conditions encountered i n actual practice. An a l y t i c a l techniques used were d i r e c t l y applicable to the realm of the ty p i c a l sewage treatment plant laboratory, yet s u f f i c i e n t l y accurate and s p e c i f i c for the scope of this study. The primary objective of the investigations was to provide comparative data between the systems studied under the controlled conditions imposed on them. I t was f e l t that such a comparative analysis could be used and applied to any system, independent of the scale factor. Since the scale factor was the only aspect of th i s investigation not allowing extrapolation of the absolute study results to f u l l - s c a l e situations, a limited f i e l d evaluation of aerobic digesters was conducted to provide a basis for comparison. General Procedures The waste activated sludge used i n th i s study was obtained from the Mamquam Sewage Treatment Plant at Squamish, B. C., approximately 40 miles north of Vancouver, B. C. This source of sludge was selected after an extensive period of evaluating the conditions at a l l activated sludge plants within a radius of 60 miles (100 km) around the City of Vancouver. 67 At the time of this study, the City of Vancouver i t s e l f did not have any activated sludge sewage treatment f a c i l i t i e s , and only small plants 3 of less than 1 migd (4540 m /day) capacity were available within reasonable distance, for use as a source of activated sludge. The Mamquam Sewage Treatment Plant treats domestic sewage using the high rate activated sludge process, without primary sedimentation. The plant i s of the Chicago Pump "Modulaire" design, and has a capacity of 3 0.5 US mgd (1900 m /day). The aeration tank loading, at capacity, i s 125 lb BOD5/day/1000 cu f t (2000 g B0D5/day/m3), and the average sludge age maintained i n the plant during the length of this study was 5.0 days. Waste activated sludge i s digested aerobically, after which i t i s stored i n a sludge storage basin of limited capacity, or allowed to leave the plant with the treated effluent. The l a t t e r occurred regularly (due to the limited disposal f a c i l i t i e s ) throughout the study period. Waste activated sludge used i n the experiments was collected from the plant's sludge return l i n e on a twice weekly basis. During the 50-mile car t r i p between the sewage treatment plant and the laboratory at the University of B r i t i s h Columbia campus, the freshly collected sludge was allowed to s e t t l e i n two 5-gallon (20-litre) carboys. Within two hours after c o l l e c t i o n , supernatant was decanted off the carboy contents i n the laboratory; the remaining thickened sludge (average t o t a l suspended solids concentration about 10,000 mg/1) was strained twice through a 1/8-inch x 1/4-inch (3 mm x 6 mm) screen i n order to remove hairs, seeds and other large solids that might interfere with the feed and recycle pumps used i n the experimental apparatus. The strained sludge was then transferred to a 5-gallon (20-litre) capacity holding tank, which was aerated and mechanically mixed, and kept at a temperature of 5°C or less at a l l times. The sludge stored i n this fashion was used to feed a l l experimental digesters. After each 3 or 4 day period, the remaining sludge i n the holding tank was discarded and replaced by a batch of freshly collected feed sludge. The feed sludge was analysed d a i l y for TSS and VSS content, pH, and dissolved oxygen content. The digesters used i n these experiments were of three different types: (a) Continuous Feed, Continuous Decant Digesters A schematic drawing of this type of digester i s shown i n Figure 4. Each digester consisted of an aeration chamber of approximately 5 l i t r e l i q u i d volume, followed by a c l a r i f i e r of approximately 1 l i t r e l i q u i d capacity. Raw sludge was pumped into the aeration compartments by means of Masterflex p e r i s t a l t i c pumps. The pumps were controlled by timers and time-delay relays to deliver short, high v e l o c i t y bursts of sludge at approximately 30-minute intervals (in order to prevent deposition of solids i n the feed l i n e s ) . The aeration chambers were aerated continuously with compressed a i r , using porous ceramic di f f u s e r s , and were kept i n a completely mixed condition by a mechanical s t i r r i n g device. The mixed liquor i n each unit was displaced by gravity into the c l a r i f i e r RAW R C^r^ SLUDGE ^ CONTINUOUS FEED / AUTOMATIC DECANT LABORATORY DIGESTER Figure 4 - Schematic of Laboratory Digester 70 section, where the sludge was thickened and then returned to the aeration compartment by another Masterflex p e r i s t a l t i c pump. The overflow, displaced by gravity from the c l a r i f i e r , consisted of supernatant and digested sludge, and was collected from each unit and measured for volume on a daily basis. Digested sludge, instead of being withdrawn separately, was allowed to leave the systems with the supernatant overflow, thus causing the solids i n the digestion systems to bui l d up to maximum concentrations; t h i s resulted i n maximum sludge ages i n the systems. The sludge age, which established i t s e l f automatically at steady-state, was therefore dependent only on the hydraulic feed rate, the s e t t l i n g characteristics of the sludge, and the hydraulic properties of the c l a r i f i e r . (b) Once Daily Feed, Once Daily Decant Digesters (Semi-Continuous Digesters) A schematic drawing of this type of digester i s shown i n Figure 5. Each digester consisted of an aeration chamber of 2.0 l i t r e l i q u i d volume. The digesting sludge was aerated with compressed a i r , using porous ceramic diffusers. Diffused aeration was s u f f i c i e n t to maintain the solids i n suspension throughout the reactors. Once d a i l y , the a i r to the digesters was shut off for a period of 2 to 3 hours to allow the sludge to s e t t l e . A quantity of supernatant, determined by the digester loading rate, was then drawn o f f , using the overflow RAW SLUDGE SUPERNATANT AND AIR DAILY FEED / MANUAL DECANT LABORATORY DIGESTER Figure 5 Schematic of Laboratory Digester device shown i n Figure 5, and a manually controlled pump. An equal quantity of fresh activated sludge was added to the digesters, after which aeration was resumed. As i n the previously described continuous digesters, digested sludge was not withdrawn separately, but was allowed to leave the systems with the supernatant overflow; this caused the solids i n the digesters to build up to maximum concentrations, res u l t i n g i n maximum sludge ages i n the systems. (c) Batch Digester A 2.0 l i t r e volume of fresh activated sludge was aerated continuously at each temperature, without adding any further feed sludge (Figure 5, again, shows t y p i c a l apparatus). The digesters were set up inside a Bel-Par Industries walk-in incubator, i n which the temperature could be controlled to within 0.5°C. Three experimental runs were conducted at l i q u i d temperatures of 20°C, 10°C, and 5°C. At each temperature, three digester solids loading rates were i n v e s t i -gated, namely 0.021, 0.042, and 0.084 lb VSS/day/cu f t (0.336, 0.672, 3 and 1.344 kg VSS/day/m ); this corresponded to digester size c r i t e r i a 3 of 6, 3, and 1.5 cu f t / c a p i t a (0.168, 0.084, and 0.042 m /cap) respec-t i v e l y , when assuming a t y p i c a l excess solids production i n an activated sludge plant of 0.17 lb TSS/day/capita (0.077 kg TSS/day/capita), with a 73 v o l a t i l e content of 75 percent. Feed sludge concentrations and feed rates were such as to remain close to the above loading rates; however, once steady-state conditions were established and the c o l l e c t i o n of data begun, the feed rates were kept constant, and the feed sludge solids concentration was allowed to vary within reasonable l i m i t s (+15 percent). This resulted i n s l i g h t l y varying solids loadings, but the individual loading rates of the various reactors were always i n constant proportion to each other. At each temperature and solids loading rate, the three previously described modes of digester operation were studied. Therefore, for each of the three temperatures, three of the continuous digesters (operating at the three loading rates mentioned e a r l i e r ) , three of the semi-continuous digesters (operating at the same three loading rates), and one batch digestion unit were operated simultaneously. Photographs of the t y p i c a l experimental arrangement are shown i n Figures 6 and 7. The three loading rates used would normally result i n three di f f e r e n t sludge ages. As i t turned out, the maximum solids concentrations obtained i n the semi-continuous units was lower than i n the continuous units, resulting i n b a s i c a l l y s i x different steady-state sludge ages per run. At the start of each run, the digesters were f i l l e d with fresh activated sludge, then operated i n the manners outlined above, u n t i l steady-state conditions developed. Steady-state conditions were assumed to be 74 Figure 7 Photograph of Experimental I n s t a l l a t i o n Semi-Continuous Feed Digesters reached when the suspended s o l i d s concentration i n the reactors attained a maximum value and remained r e l a t i v e l y constant. This took an average of 4 weeks for each of the temperature runs. A f t e r reaching steady-state conditions, the systems were operated an average of 8 weeks, to allow for the c o l l e c t i o n of data at steady-state. Evaporation losses i n the semi-continuous and i n the batch digesters were compensated for by adding tap water on a d a i l y basis. In the continuous digesters, evaporation losses were made up from the d a i l y feed volume. This resulted i n smaller e f f l u e n t volumes and therefore, d a i l y feed volumes for the l a t t e r units needed to be measured and checked r e g u l a r l y at the i n f l u e n t l i n e . Sludge s o l i d s "build-up", on the reactor walls, was removed and returned to the reactors twice d a i l y . For the continuous feed reactors, t h i s was done by scraping; for the semi-continuous feed and batch reactors, s o l i d s "build-up" was removed by increasing the a i r supply for a short period, as the increased a g i t a t i o n of the l i q u i d resuspended the s o l i d s attached to the reactor walls. Heavy foaming occurred i n the continuous and semi-continuous feed u n i t s throughout the 20°C run at steady-state. The use of a s i l i c o n e spray anti-foaming agent prevented loss of s o l i d s from the semi-continuous reactors; however, t h i s was not possible for the continuous reactors. Foam, accumulating i n these reactors, was removed manually as required and c o l l e c t e d separately. At the time a s o l i d s mass balance was per-formed, s o l i d s removed as foam was weighed for each reactor, and added 77 to the effluent solids wasted from each system. This explains why, during the worst foaming periods, the effluent solids concentration shown i n the mass balance summary was sometimes higher than the digester solids concentration. The a i r supply to each unit was not care f u l l y controlled; however, i t was s u f f i c i e n t at a l l times to maintain a dissolved oxygen concentra-t i o n i n excess of 2.0 mg/1 i n each reactor. A n a l y t i c a l Procedures 1. Suspended Solids Determination: Because of the extremely large number of samples to be analyzed at any one time, and the generally extremely slow f i l t r a t i o n rates of the sludges, a method other than the Gooch Crucible Method (132), was developed to determine suspended solids (TSS) concentrations. A suspended solids determination, therefore, consisted of introducing duplicate 25 ml sludge samples i n 50 ml centrifuge tubes, which were spun for 30 minutes at 2000 rpm i n an International Equipment Co. centrifuge, Model CS. The supernatants from the tubes were discarded and the solids were washed into separate 100 ml evaporation dishes, using d i s t i l l e d water. The dishes were then placed over-night i n a Fisher S c i e n t i f i c Isotemp forced draft drying oven at a temperature of 105°C. Total suspended solids of the sample were determined gravimetrically as described i n Standard Methods (132) for t o t a l solids analysis using a Sartorius Type 2442 Balance. 78 The sample value was taken as the average of the two duplicate samples. V o l a t i l e suspended solids (VSS) were determined by i g n i t i n g the dried residue at 550°C for 1 hour i n a Hotpack muffle furnace, i n accordance with Standard Methods (132). The above method of suspended solids determination was calibrated several times against the standard Gooch Crucible Method (132) and was found to be i n good agreement with the l a t t e r , and highly reproducible (within +5%). 2. Total Solids Determination: Total s o l i d s (TS) and t o t a l v o l a t i l e solids (TVS) were determined using the Evaporation Dish technique as described i n Standard Methods (132), using duplicate 25 ml samples. Total dissolved solids (TDS) and v o l a t i l e dissolved solids (VDS) were determined by calculating the difference between the values obtained under 1. and 2. above. 3. Solids Mass Balance: V o l a t i l e suspended solids reductions were determined by performing a complete and continuous solids mass balance across a l l systems during steady-state operation. A solids mass balance was completed, on the average, once every 9 days. A schematic mass balance diagram i s shown i n Figure 8. Cp = Solids Concentration i n Feed Sludge Vp = Daily Volume of Feed Sludge A C D X V d = Increase i n Digester Solids (+) increase (-) decrease C^ , = Solids Concentration i n Effluent V = Total Volume of Effluent Cg = Solids Concentration i n Sample V = Volume of Sample A CD X VD C E X V E * ( C s * V . SOLIDS DESTROYED = - £ ( 0 ^ ) - (C^xV^) - -£(CQxVQ) - (AC nxV n) E E S S; D D % SOLIDS DESTROYED = SOLIDS DESTROYED x 1 0 0 % Figure 8 Solids Mass Balance i n Continuous Digesters Solids i n the influent stream were determined d a i l y from the central feed sludge container by measuring the v o l a t i l e suspended solids and the d a i l y volume of feed sludge introduced into the reactors (Z(C xV ) i n Figure 8). Solids i n the effluent stream were determined by accumulating composite samples of the indi v i d u a l reactor effluents, over the mass balance period. The composite samples were made up from samples taken from the d a i l y volumes of effluent, i n exact propor-tion to the d a i l y volume discharged. The composites were thoroughly mixed at the end of the mass balance period. Solids discharged over the balance period were determined by using the solids con-centration i n the composite samples and the t o t a l volumes of effluent discharged during the mass balance period, for each reactor (CxV i n Figure 8). t E The increase or decrease i n solids concentration i n the reactors was determined by measuring the solids concentration at the beginning and the end of each balance period (AC^xV^ i n Figure 8). Just prior to sampling from the reactors, each unit was prepared such that a l l solids were evenly distributed throughout. This was done by bringing any solids "build-up" on the reactor walls back into the l i q u i d , and by s t i r r i n g up any pockets of deposited materials. In the case of the continuous feed digesters, i t was necessary to empty the c l a r i f i e r sections by pumping into the aeration compartments; this ensured a homogeneous mixture of the t o t a l digester content. \ An allowance was made i n the solids balance for solids l o s t i n the samples withdrawn during the balance period (E(C xV ) i n Figure 8). Special care was taken at a l l times during steady-state operation to prevent or minimize the loss of solids i n the aerosol from the reactors. A i r flow rates were kept to a minimum, made possible by providing mixing for the reactors using mechanical s t i r r e r s . The freeboard on the continuous feed reactors was large, and any solids accumulating on the walls were scraped back into the mixed liqu o r , once d a i l y . The configuration of the semi-continuous feed reactors was such that only a small opening was l e f t at the top, thus preventing a s i g n i f i c a n t loss of solids i n the aerosol. There was no s i g n i f i c a n t build-up of solids on the white enamel incubator walls or any other surfaces i n the v i c i n i t y of the reactors. The t o t a l quantity of solids destroyed and the percentage of solids destroyed during the balance period were calculated as shown i n Figure 8. Note that i n the case of the batch digesters, E(C_xV_) and (C„xV_) are both zero, r r ii t S e t t l e a b i l i t y : At high suspended solids concentrations (greater than 10,000 mg/1), as were obtained at steady-state i n these experiments, the sludge-volume index (SVI), determined i n accordance with Standard  Methods (132), becomes meaningless, as 30-minute s e t t l e a b i l i t y 82 cannot be measured properly. An indication of sludge s e t t l e a b i l i t y i n each reactor could be obtained by observing the maximum suspended solids concentration obtained at steady-state under similar conditions of decanting. 5. pH.: pH was measured using a Fisher S c i e n t i f i c Accumet Model 210 pH meter. Prior to each set of pH measurements, the instrument was calibrated using a standard buffer solution. 6. Dissolved Oxygen: Individual dissolved oxygen concentration measurements were made using a Yellow Springs Instruments (YSI) Model 51 oxygen meter and probe. The instrument was calibrated frequently against samples analyzed using the Winkler Azide Modification Method as described i n Standard Methods (132). 7. Oxygen Uptake Rate: Oxygen uptake rate measurements were made using a YSI Model 53 oxygen monitor with dual probe and s e l f - s t i r r i n g water bath assembly. A photograph of the instrument arrangement i s shown i n Figure 9. A Haake Model FJ constant temperature bath and pump assembly was used to regulate the temperature of the sample chambers i n the s e l f - s t i r r i n g water bath assembly. The Haake water bath contains a heater and a cooling c o i l , through which refrigerated water Figure 9 - Photograph of Instrument Arrangement Oxygen Uptake Rate Monitoring 84 was circulated to achieve 20°C and 10°C temperatures i n the s e l f -s t i r r i n g water bath. Water was pumped d i r e c t l y from a 3°C r e f r i -gerated water bath to the cooling c o i l of the s e l f - s t i r r i n g water bath to achieve a 5°C temperature i n the oxygen electrode sample chambers. A Moseley Autograf Model 680, continuous strip-chart recorder was used to record the oxygen monitor output. The recorder speed range was from 1/16-inch per minute to 8 inches per minute (1.5 mm/min to 200 mm/min). The oxygen monitor readout i s i n percent saturation, and could be calibrated to 100 percent saturation at temperatures greater than about 15°C. At temperatures below 15°C, the maximum meter setting became less than 100 percent. Therefore, at lower temperatures, the following procedure was adopted: The meter was calibrated to a value less than 100 percent, say 70 percent using a water sample saturated with a i r at the temperature under investigation. The oxygen uptake rate of a sludge sample at that temperature was then calculated propor-t i o n a l l y as follows: 0^ uptake rate (mg/l/hr) = % decrease recorded P.O. @ saturation (mg/1) meter setting @ saturation time lapse (hr) For each measurement, 5 ml sludge samples were introduced into the probe chambers, with a i r being added to the sample i n i t i a l l y , when required. 85 8. Biochemical Oxygen Demand: Biochemical Oxygen Demand (BOD^) was determined i n accordance with Standard Methods (132). The d i l u t i o n water was not seeded. No pretreatment of the samples was required. Sludge samples were blended for 2 minutes i n a Waring Blendor, i n order to homogenize the samples. Depending on the suspended solids concentration of the sludge sample, the blended sludge was diluted 100 to 1 or 10 to 1 prior to d i s t r i b u t i o n to the incubation bottles. Two duplicate dilutions, were incubated for each sludge sample. Supernatant samples, analyzed to determine soluble BOD, were obtained by centrifugation of a sample of sludge withdrawn from the reactors. The centrate obtained was then f i l t e r e d through No. 4 Whatman f i l t e r paper, res u l t i n g i n a clear l i q u i d . Two duplicate d i l u t i o n s were incubated for each supernatant sample. Dissolved oxygen measurements prior to and after incubation were made using a YSI BOD bottle dissolved oxygen probe and Model 54 oxygen meter; t h i s was calibrated for each run using the Winkler Azide Modification Method (132). 9. Chemical Oxygen Demand: Chemical Oxygen Demand (COD) was determined i n accordance with Standard Methods (132). Sludge samples were blended for 2 minutes i n a Waring Blendor i n order to homogenize the samples. The blended sludge was diluted 100 to 1 or 50 to 1, depending on the suspended solids concentration of the sludge. Supernatant samples, analyzed to determine soluble COD, were obtained as for the BOD supernatant samples. Total Organic Carbon: Total Organic Carbon (TOC) was determined using a Beckman Model 915 Total Organic Carbon Analyzer. Sludge samples were homogenized i n a 7 ml Tenbroeck Tissue Grinder, then diluted 100 to 1 or 50 to 1, depending on the suspended solids concentration i n the sludge samples. Supernatant samples, analyzed to determine soluble TOC, were obtained as for the BOD supernatant samples. Prior to analysis i n the Beckman Analyzer, one drop of concentrated HCl was added to each of the samples to drive off any CO^ - The instrument was calibrated at the start of each set of analyses, using a 100 mg/1 organic carbon standard. Instrument readings were then converted to equivalent ppm CO2, using a c a l i b r a t i o n curve supplied with the instrument. 87 11. Total Kjeldahl Nitrogen: Total Kjeldahl Nitrogen was determined using the digestion, d i s t i l -l a t i o n , and t i t r a t i o n method as described i n Standard Methods (132) . For the sludge determinations, 5 or 10 ml l i q u i d samples were analyzed; however, for the supernatant determinations, after f i l t r a t i o n , 100 ml samples were used. 12. Ammonia Nitrogen: Ammonia Nitrogen was determined for supernatants only, primarily using an Orion Model 95-10 ammonia electrode and Fisher S c i e n t i f i c Accumet 320 Model potentiometer, used on an expanded scale. The instrument was calibrated for each run, using ammonia nitrogen standards; these were prepared fresh each time, using d i l u t i o n s of Orion Standard ammonium chloride solution. 13. Organic Nitrogen: Organic Nitrogen was determined by subtracting the ammonia nitrogen values from the t o t a l Kjeldahl nitrogen values. 14. Nitrate Nitrogen: Nitrate Nitrogen was determined using the u l t r a v i o l e t absorption technique described i n Standard Methods (132). The instrument used was a Spectronic 600 Bausch & Lomb Spectrophotometer. Standards were prepared using s i l v e r n i t r a t e and a c a l i b r a t i o n curve made up for absorbance @ 220 my. Nitrate nitrogen was determined only for supernatants, obtained after f i r s t suitably d i l u t i n g a sludge sample, then f i l t r a t i o n through No. 5 Whatman paper. Viable Bacteria Count: Viable bacteria counts were made using the "spread plate" method, as described i n Manual of Microbiological Methods (130). The semi-solids culture medium used to provide the growth surface was composed as follows: 2.0 g/1 tryptone 1.0 g/1 glucose 1.0 g/1 yeast extract 9.0 g/1 agar tap water S e r i a l d i l u t i o n s were made i n s t e r i l e saline solutions. The sludge samples were prepared by homogenization i n a 7 ml Tenbroeck Tissue Grinder, and thorough mixing on a Vortex mixer, prior to innoculation into the d i l u t i o n tubes. Plates were incubated i n duplicate at the temperature under i n v e s t i -gation and also at 20°C. 89 Counting of the colonies was done on a Quebec Colony Counter, using a hand t a l l y . 16. Microscopic Examination: Microscopic examinations of the sludge micro-organisms were made using a Nikon Model L-Ke phase-contract microscope with a Microflex Model EFM photo-micrograph attachment. 17. Odour Panel Test: Odour tests were conducted on stored sludges using a f i v e member odour panel. Each panel member was given a choice of f i v e ratings, ranging from 0 (no detectable odour) to 5 (extremely objectionable odour). For each test, the arithmetic average of the ratings reported was used to determine the odour value of the sample. 18. General Sampling Procedures: As was pointed out previously, solids l o s t from the systems by sampling were a l l accounted for i n the mass balance calculations. In general, the samples were kept as small as possible. Also, wherever possible, samples which were not affected by the test (such as pH or sludge centrifuged to obtain supernatant), were returned immediately to the respective reactors. IV. RESULTS AND DISCUSSION The main objective of this research was to create a better understanding of the effect of low l i q u i d temperatures on the characteristics of the aerobic digestion process applied to a t y p i c a l , domestic, waste activated sludge. In addition, two methods of continuous feeding of aerobic digesters were investigated, namely continuous feed/automatic decant, and dai l y feed/manual decant (the l a t t e r method i s also referred to as semi-continuous feed or intermittent feed), as well as the batch aeration method of aerobic digestion. Process characterization consisted of the analysis and study of four main groups of parameters: a.. Parameters r e l a t i n g to process k i n e t i c s , such as solids destruction and oxygen uptake rate. b_. Parameters r e l a t i n g to digested sludge c h a r a c t e r i s t i c s , such as v o l a t i l e content, viable bacteria, biochemical oxygen demand, chemical oxygen demand, organic carbon content, nitrogen content, and odour values. c_. Parameters r e l a t i n g to supernatant quality, such as dissolved s o l i d s , organic carbon, biochemical oxygen demand, chemical oxygen demand, various forms of nitrogen, and pH. <1. F i e l d evaluation of process k i n e t i c s parameters, such as solids destruction and oxygen uptake rate. The majority of experiments were done on a laboratory scale, where conditions could be best controlled. A limited program of f u l l - s c a l e f i e l d evaluations was conducted i n an attempt to bridge the gap between laboratory and f u l l - s c a l e r e s ults. Throughout the analysis of data, i t has been assumed that sludge age i n continuous and semi-continuous digestion was comparable to elapsed time i n batch digestion units. In the author's view, t h i s was j u s t i f i e d , subject to the following assumptions: (1) Pre-digestion sludge age of the waste activated sludge used i n these experiments was constant. (2) Complete mixing was achieved i n a l l reactors, resulting i n a normal d i s t r i b u t i o n of p a r t i c l e s having a residence time ranging from zero to the time elapsed since start-up of the reactor. (3) The solids residence time, or sludge age, calculated i n accordance with the formula presented l a t e r i n this chapter, represented the average length of time that the p a r t i c l e s i n the continuous and semi-continuous reactors were under digestion. 92 (4) Any sample taken from the completely mixed continuous and semi-continuous units manifested i t s e l f as a homogeneous mixture of p a r t i c l e s , having a sludge age equal to the calculated average sludge age. Digester sludge age (SA) i s defined as follows: s ^ _ Weight of VSS i n digester  Weight of VSS wasted from digester per day Sludge age was calculated i n accordance with this d e f i n i t i o n for a l l continuous and semi-continuous digesters throughout t h i s study. When sludge age was used i n conjunction with batch digestion units, i t was taken as the time elapsed since start-up of these units. Although t o t a l suspended solids and v o l a t i l e suspended solids were both measured throughout this research, only v o l a t i l e suspended solids data have been u t i l i z e d to characterize the aerobic digestion process. Except for certain r e l a t i v e l y complex biochemical indicators, v o l a t i l e suspended solids are believed to most closely approximate the active portion of waste activated sludge, and are therefore believed to consti-tute a more universally usable parameter, i n r e l a t i o n to aerobic digestion k i n e t i c s , than t o t a l suspended soli d s . In this respect i t should be remembered that one of the self-imposed conditions of t h i s research was to use a n a l y t i c a l techniques that would be applicable to the realm of a normal sewage treatment plant laboratory. This eliminated the a p p l i c a b i l i t y of such biochemical parameters as ATP, DNA, or Dehydrogenase enzyme a c t i v i t y . Unless stated otherwise, a l l data points representing steady-state conditions, and which involved suspended solids measurements ( i . e . VSS reduction, sludge age), were determined by performing a continuous solids mass balance across the systems, and represent average values over a period of approximately 8 weeks of steady-state operation. A. AEROBIC DIGESTION KINETICS 1. Solids Destruction The operating conditions of the digestion systems studied have been described previously i n Chapter I I I . Summaries of the steady-state mass balance data for the continuous and semi-continuous feed systems are contained i n Tables 1 to 18. Solids destruction data for the batch aeration systems are contained i n Tables 19 to 21. Loading rates, percent v o l a t i l e , suspended solids reduction, and sludge age for the continuous and semi-continuous feed systems have been calculated i n accordance with the example calculations shown i n Appendix A. A summary of steady-state, v o l a t i l e suspended solids reductions for the various systems i s contained i n Table 22. Figures 10 and 11 show the VSS reductions for the various systems and temperatures studied, as a function of sludge age. Table 1. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.02 lb VSS/day/cu f t (0.32 kg VSS/day/m3) Temperature: 20°C Digester Volume: 6.4 l i t r e s Days @ Steady State Feed Sludge Effluent Sludge Digester Sludge Volume (ml/day) TSS (mg/1) VSS Volume (ml/day) TSS (mg/1) VSS TSS (mg/1) VSS mg/1 % of TSS mg/1 % of TSS mg/1 % of TSS 0 13,420 10,170 75.8 267.0 9,695 7,795 80.4 130.6 2,620 1,940 74.0 8 14,430 11,125 77.1 267.0 9,470 7,633 80.6 149.4 1,070 890 83.2 16 15,295 12,120 79.2 267.0 8,957 7,255 81.0 156.4 8,615 6,635 77.0 23 15,290 11,955 78.2 267.0 9,965 7,872 79.0 154.0 17,295 13,490 78.0 28 14,620 11,010 75.3 267.0 8,871 7,106 80.1 153.2 16,080 11,900 74.0 45 11,390 8,590 75.4 267.0 7,596 6,001 79.0 151.9 6,855 5,210 76.0 53 11,415 8,540 74.8 Table 2. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.04 lb VSS/day/cu f t (0.64 kg VSS/day/m3) Temperature: 20°C Digester Volume: 6.2 l i t r e s Days @ Steady State Feed Sludge Effluent Sludge Digester Sludge Volume (ml/day) TSS (mg/1) VSS Volume (ml/day) TSS (mg/1) VSS TSS (mg/1) VSS mg/1 % of TSS mg/1 % of TSS mg/1 % of TSS 0 12,870 9,850 76.5 505.0 9,695 7,795 80.4 378.1 5,585 4,230 75.7 8 13,360 10,405 77.9 12 13,175 10,200 77.4 505.0 8,570 6,856 80.0 352.5 4,795 3,790 79.0 16 13,665 10,765 78.8 505.0 8,957 7,255 81.0 365.7 7,940 6,190 77.9 23 13,665 10,800 79.0 505.0 9,965 7,872 79.0 386.0 7,905 6,250 79.1 28 13,590 10,865 79.9 505.0 8,871 7,106 80.1 367.0 6,720 5,310 79.0 31 14,715 11,030 75.0 505.0 8,871 7,106 80.1 376.4 5,800 4,455 76.8 45 15,230 11,890 78.1 505.0 7,596 6,001 79.0 342.5 10,870 8,370 77.0 53 13,555 10,320 76.1 Table 3. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.08 lb VSS/day/cu f t (1.28 kg VSS/day/m3) Temperature: 20°C Digester Volume: 6.4 l i t r e s Days @ Steady State Feed Sludge Effluent Sludge Digester Sludge Volume (ml/day) TSS (mg/D VSS Volume (ml/day) TSS (mg/1) VSS TSS (mg/1) VSS mg/1 % of TSS mg/1 % of TSS mg/1 % of TSS 0 12,835 10,090 78.6 16 10,390 8,050 77.5 1018.0 8,989 7,254 80.7 881.4 7,930 6,105 77.0 23 10,545 8,100 76.8 1018.0 9,965 7,872 79.0 864.0 7,290 5,685 78.0 28 11,425 8,955 78.4 1018.0 8,871 7,106 80.1 903.5 7,955 6,125 76.9 45 10,255 7,735 75.4 1018.0 7,596 6,001 79.0 803.1 6,910 5,115 74.0 53 11,775 8,830 75.0 Table 4. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.02 lb VSS/day/cu f t (0.32 kg VSS/day/m3) Temperature: 10°C Digester Volume: 6.4 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 9,030 6,795 75.2 10 272.0 9,503 7,194 75.7 211.5 470 355 75.5 10,580 8,000 75.6 18 272.0 9,589 7,115 74.2 207.5 555 380 68.5 12,705 9,210 72.5 25 272.0 9,466 7,014 74.1 208.6 440 370 84.1 14,610 10,670 73.0 38 272.0 9,616 7,385 76.8 197.7 385 315 81.8 16,775 12,475 74.4 48 272.0 10,762 8,082 75.1 233.5 6,790 4,760 70.1 19,195 13,210 68.8 58 272.0 ' 12,278 9,049 73.7 255.0 8,490 5,935 69.9 19,840 13,820 69.7 64 272.0 10,847 8,233 75.9 170.8 4,000 2,810 70.3 20,740 14,210 68.5 VO Table 5. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.04 lb VSS/day/cu f t (0.64 kg VSS/day/m3) Temperature: 10°C Digester Volume: 6.2 l i t r e s Days @ Steady State Feed Sludge Effluent Sludge Digester Sludge Volume (ml/day) TSS (mg/1) VSS Volume (ml/day) TSS (mg/1) VSS TSS (mg/1) VSS mg/1 % of TSS mg/1 % of TSS mg/1 % of TSS 0 12,100 9,295 76.8 509.0 9,503 7,194 75.7 460.0 4,350 3,200 73.6 10 14,100 10,550 74.9 509.0 9,589 7,115 74.2 474.4 5,260 3,685 70.1 18 15,220 11,015 72.4 509.0 9,466 7,014 74.1 466.4 5,175 3,840 74.2 25 16,645 12,390 74.4 509.0 9,616 7,385 76.8 459.2 5,865 4,310 73.5 38 17,365 13,220 76.1 509.0 . 10,762 8,082 75.1 489.0 8,200 5,935 . 72.4 48 20,220 14,495 71.7 509.0 12,278 9,049 73.7 498.5 9,755 7,065 72.4 58 20,300 14,665 72.2 509.0 10,847 8,233 75.9 425.8 6,950 4,970 71.5 64 20,810 15,015 72.2 OO Table 6. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.08 lb VSS/day/cu f t (1.28 kg VSS/day/nP) Temperature: 10°C Digester Volume: 6.4 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 11,850 9,210 77.7 10 1011.0 9,503 7,194 75.7 943.5 5,990 4,545 75.9 13,875 10,615 76.5 18 1011.0 9,589 7,115 74.2 953.1 6,390 4,605 72.1 15,330 11,255 73.4 25 1011.0 9,466 7,014 74.1 917.1 8,105 6,000 74.0 15,780 11,690 74.1 38 1011.0 9,616 7,385 76.8 928.5 7,900 5,910 74.8 15,820 12,000 75.9 48 1011.0 10,762 8,082 75.1 965.0 9,000 6,450 71.7 18,720 13,570 72.5 58 1011.0 12,278 9,049 73.7 1003.5 9,540 7,010 73.5 19,425 14,100 72.6 64 1011.0 10,847 8,233 75.9 905.0 9,795 7,055 72.0 19,490 14,235 73.0 Table 7. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.02 lb VSS/day/cu f t (0.32 kg VSS/day/m3) Temperature: 5°C Digester Volume: 6.4 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 18,490 13,960 75.5 9 254.0 9,400 7,313 77.8 247.2 5,530 4,620 77.0 19,040 14,545 76.4 18 306.0 8,931 7,127 79.8 290.6 5,655 4,320 76.4 19,120 14,815 77.5 29 306.0 9,495 7,596 80.0 272.3 7,795 6,070 77.9 21,140 15,420 72.9 41 306.0 9,171 7,089 77.3 242.1 9,560 7,100 74.3 19,850 14,740 74.3 47 306.0 10,649 8,051 75.6 260.0 11,050 8,175 74.0 19,790 14,475 73.1 53 306.0 10,469 8,051 76.9 254.2 10,040 7,355 73.3 20,010 14,490 72.4 o o Table 8. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.04 lb VSS/day/cu f t (0.64 kg VSS/day/m3) Temperature: 5°C Digester Volume: 6.2 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 18,465 13,965 75.6 9 503.3 9,400 7,313 77.8 486.7 10,375 7,940 76.5 17,630 13,535 76.8 18 589.0 8,931 7,127 79.8 561.7 7,580 5,865 77.4 17,060 13,250 77.7 29 591.0 9,495 7,596 80.0 545.9 8,950 7,040 78.7 18,720 13,765 73.5 41 591.0 9,171 7,089 77.3 567.5 10,075 7,530 74-7 17,360 13,400 77.2 47 591.0 10,649 8,051 75.6 541.7 10,035 7,620 75.9 17,730 13,275 74.9 53 591.0 10,469 8,051 76.9 537.5 9,905 7,385 74.6 16,985 12,770 75.2 r—1 O Table 9. Steady-State Suspended Solids Mass Balance System: Continuous Feed/Automatic Decant Approximate Loading Rate: 0.08 lb VSS/day/cu f t (1.28 kg VSS/day/m3) Temperature: 5°C Digester Volume: 6.4 l i t r e s Days @ Steady State Feed Sludge Effluent Sludge Digester Sludge Volume (ml/day) TSS (mg/1) VSS Volume (ml/day) TSS (mg/1) VSS TSS (mg/1) VSS mg/1 % of TSS mg/1 % of TSS mg/1 % of TSS 0 14,940 11,180 74.8 1006.2 9,400 7,313 77.8 968.3 9,620 7,305 75.9 9 13,805 10,580 76.6 1239.6 8,931 7,127 79.8 1200.6 8,110 6,260 77.2 18 13,985 10,890 77.9 1256.2 9,495 7,596 80.0 1243.6 8,970 7,130 79.5 29 14,220 11,130 78.3 1266.0 9,171 7,089 77.3 1261.7 9,930 7,615 76.7 41 15,865 12,010 75.7 1266.0 10,649 8,051 75.6 1241.7 9,565 7,250 75.8 47 15,785 12,095 76.6 1266.0 10,469 8,051 76.9 1225.8 9,580 7,230 75.5 53 15,730 12,000 76.3 I—1 O ro Table 10. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.02 lb VSS/day/cu f t (0.32 kg VSS/day/m3) Temperature: 20°C Digester Volume: 2.0 l i t r e s Days @ Steady State Feed Sludge Effluent Sludge Digester Sludge Volume (ml/day) TSS (mg/1) VSS Volume (ml/day) TSS (mg/1) VSS TSS (mg/1) VSS mg/1 % of TSS mg/1 % of TSS mg/1 % of TSS 0 9,390 6,800 72.2 88 9,799 7,908 80.7 88 6,785 5,120 75.5 11 9,480 7,045 74.3 88 9,570 7,694 80.4 88 6,120 4,550 74.3 19 9,350 6,895 73.7 88 9,551 7,673 80.6 88 4,340 3,305 76.2 27 9,840 7,300 74.2 88 9,927 8,041 81.0 88 5,490 4,130 75.2 34 10,220 7,710 75.4 88 9,999 7,899 79.0 88 6,110 4,670 76.4 39 10,070 7,615 75.6 88 8,909 7,136 80.1 88 5,965 4,595 77.0 56 9,305 6,960 74.8 88 7,653 6,046 79.0 88 5,360 4,075 76.0 64 9,115 6,850 75.2 o u> Table 11. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.04 lb VSS/day/cu f t (0.64 kg VSS/day/m3) Temperature: 20°C Digester Volume: 2.0 l i t r e s Days @ Steady State Feed Sludge Effluent Sludge Digester Sludge Volume (ml/day) TSS (mg/1) VSS Volume (ml/day) TSS (mg/1) VSS TSS (mg/1) VSS mg/1 % of TSS mg/1 % of TSS mg/1 % of TSS 0 10,345 7,580 73.3 176 9,799 7,908 80.7 176 6,780 5,140 75.8 11 10,745 8,135 75.7 176 9,570 7,694 80.4 176 8,060 6,070 75.3 19. 9,915 7,500 75.6 176 9,551 7,673 80.6 176 4,670 3,645 78.1 27 11,060 8,420 76.1 176 9,927 8,041 81.0 176 6,660 5,090 76.4 34 11,435 8,760 76.6 176 9,999 7,899 79.0 176 7,050 5,480 77.7 39 11,325 8,785 77.6 176 8,909 7,136 80.1 176 6,460 4,995 77.3 56 10,790 8,300 76.9 176 7,653 6,046 79.0 176 6,055 4,655 76.9 64 9,770 7,445 76.2 o 4>-Table 12. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.08 lb VSS/day/cu f t (1.28 kg VSS/day/m3) Temperature: 20°C Digester Volume: 2.0 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 12,035 9,025 75.0 11 352 9,799 7,908 80.7 352 8,300 6,370 76.7 10,920 8,485 77.7 19 352 9,570 7,694 80.4 352 8,345 6,405 76.8 9,480 7,260 76.6 27 352 9,551 7,673 80.6 352 6,075 4,780 78.7 10,475 8,410 80.3 34 352 9,927 8,041 81.0 352 8,145 6,300 77.3 9,965 7,775 78.0 39 352 9,999 7,899 79.0 352 8,470 6,655 78.6 9,340 7,340 78.6 56 352 8,909 7,136 80.1 352 6,825 5,330 78.1 10,180 7,880 77.4 64 352 7,653 6,046 79.0 352 6,460 4,980 77.1 9,345 7,170 76.7 o Steady-State Suspended Solids Mass Balance Approximate Loading Rate: 0.02 lb VSS/day/cu f t (0.32 kg VSS/day/m3). Digester Volume: 2.0 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 8,025 5,960 74.3 10 88 9,811 7,427 75.7 88 2,665 2,040 76.5 10,650 7,910 74.3 18 88 9,720 7,212 74.2 88 5,595 4,060 72.6 11,955 8,630 72.2 25 88 9,649 7,150 74.1 88 6,780 5,060 74.6 11,460 8,300 72.4 38 88 9,732 7,474 76.8 88 5,820 4,340 74.6 11,750 8,555 72.8 48 88 11,087 8,326 75.1 88 8,970 6,520 72.7 11,790 8,505 72.1 58 88 12,069 8,895 73.7 88 8,500 6,225 73.2 11,930 8,555 71.7 64 88 11,195 8,497 75.9 88 3,330 2,340 70.3 10,504 7,255 69.7 o Table 13. System: Daily Feed/Manual Decant Temperature: 10°C Table 14. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.04 lb VSS/day/cu f t (0.64 kg VSS/day/nr3) Temperature: 10°C Digester Volume: 2.0 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 10,330 7,850 76.0 10 176 9,811 7,427 75.7 176 7,445 5,685 76.4 10,770 8,050 74.7 18 176 9,720 7,212 74.2 176 4,380 3,165 72.3 13,125 9,545 72.7 25 176 9,649 7,150 74.1 176 7,990 5,970 74.7 12,735 9,335 73.3 38 176 9,732 7,474 76.8 176 8,255 6,175 74.8 11,555 8,490 73.5 48 176 11,087 8,326 75.1 176 6,535 4,670 71.5 14,780 10,555 71.4 58 176 12,069 8,895 73.7 176 9,210 6,765 73.5 15,790 11,210 71.0 64 176 11,195 8,497 75.9 176 7,950 5,660 71.3 13,245 9,380 70.8 o Table 15. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.08 lb VSS/day/cu f t (1.28 kg VSS/day/m3) Temperature: 10°C Digester Volume: 2.0 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 10,355 8,030 77.6 10 352 9,811 7,427 75.7 352 8,595 6,545 76.2 11,705 8,760 74.8 18 352 9,720 7,212 74.2 352 7,580 5,565 73.4 12,950 9,470 73.1 25 352 9,640 7,150 74.1 352 9,600 7,165 74.6 12,110 8,940 73.8 38 352 9,732 7,474 76.8 352 8,005 5,965 74.5 12,950 9,655 74.6 48 352 11,087 8,326 75.1 352 8,310 6,015 72.4 15,580 11,420 73.3 58 352 12,069 8,895 73.7 352 9,320 6,915 74.2 17,210 12,575 73.1 64 352 11,195 8,497 75.9 352 10,710 7,740 72.3 15,590 11,275 72.3 o oo Table 16. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.02 lb VSS/day/cu f t (0.32 kg VSS/day/m3) Temperature: 5°C Digester Volume: 2.0 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 14,370 10,455 72.8 9 88 9,509 7,398 77.8 88 8,195 6,085 74.3 14,060 10,230 72.8 18 96 9,025 7,202 79.8 96 8,855 6,555 74.0 13,440 9,910 73.7 29 100 9,659 7,727 80.0 100 9,210 7,040 76.4 12,810 9,620 75.1 41 100 11,153 8,621 77.3 100 9,410 7,210 76.6 13,135 9,670 73.6 47 100 10,833 8,189 75.6 100 7,525 5,680 75.5 12,970 9,595 74.0 53 100 10,689 8,220 76.9 100 9,500 7,090 74.6 12,780 9,390 73.5 o Table 17. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.04 lb VSS/day/cu f t (0.64 kg VSS/day/m3) Temperature: 5°C Digester Volume: 2.0 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 14,265 10,420 73.1 9 176 9,509 7,398 77.8 176 10,250 7,600 74.2 12,390 9,230 74.5 18 192 9,025 7,202 79.8 192 8,270 6,205 75.0 12,385 9,400 75.9 29 200 9,659 7,727 80.0 200 9,245 7,210 78.0 11,730 9,215 78.6 41 200 11,153 8,621 77.3 200 9,555 7,395 77.4 12,460 9,430 75.7 47 200 10,833 8,189 75.6 200 9,615 7,335 76.3 11,935 .9,025 75.6 53 200 10,689 8,220 76.9 200 9,005 6,815 75.7 12,690 9,560 75.3 Table 18. Steady-State Suspended Solids Mass Balance System: Daily Feed/Manual Decant Approximate Loading Rate: 0.08 lb VSS/day/cu f t (1.28 kg VSS/day/nP) Temperature: 5°C Digester Volume: 2.0 l i t r e s Days Feed Sludge Effluent Sludge Digester Sludge @ Steady State Volume TSS VSS Volume TSS VSS TSS VSS (ml/day) (mg/1) mg/1 % of TSS (ml/day) (mg/1) mg/1 % of TSS (mg/1) mg/1 % of TSS 0 17,010 12,645 74.3 .9 352 9,509 7,398 77.8 352 9,760 7,400 75.8 15,460 11,690 75.6 18 384 9,025 7,202 79.8 384 8,150 6,300 77.3 15,545 12,090 77.8 29 400 9,659 7,727 80.0 400 9,260 7,375 79.6 14,230 11,250 79.1 41 400 11,153 8,621 77.3 400 9,725 7,665 78.8 15,965 12,185 76.3 47 400 10,833 8,189 75.6 400 10,115 7,720 76.3 15,260 11,420 74.8 53 400 10,689 8,220 76.9 400 6,695 5,105 76.3 17,760 13,465 75.8 Table 19. Batch Aeration Suspended Solids Mass Balance Temperature: 20°C Digester Volume: 2.0 l i t r e s Day Digester Sludge VSS Destroyed TSS (mg/1) VSS *Net VSS (mg/1) mg/1 % of TSS mg/1 % 0 4,340 3,600 83.0 3,600 0 0 12 3,000 2,140 71.3 2,140 1,460 40.6 18 2,760 2,000 72.5 1,909 1,691 47.0 29 2,350 1,675 71.3 1,636 1,964 54.6 37 2,130 1,490 70.0 1,500 2,100 58.3 48 2,300 1,675 72.8 1,727 1,873 52.0 56 2,315 1,620 70.0 1,705 1,895 52.6 64 2,280 1,640 71.9 1,750 1,850 51.4 71 2,060 1,415 68.7 1,568 2,032 56.4 76 1,900 1,375 72.4 1,614 1,986 55.2 93 1,630 1,135 69.6 1,386 2,214 61.5 101 1,585 1,070 67.5 1,341 2.259 62.7 *VSS concentration adjusted for samples withdrawn, which were replaced with tap water. 113 Table 20. Batch Aeration Suspended Solids Mass Balance Temperature: 10°C Digester Volume: 2.0 l i t r e s Day Digester Sludge VSS Destroyed TSS (mg/1) VSS *Net VSS (mg/1) mg/1 % of TSS mg/1 % 0 4,740 3,655 77.1 3,655 0 0 11 4,280 3,065 71.6 3,065 590 16.1 22 3,735 2,800 75.0 2,863 792 21.7 32 3,455 2,480 71.8 2,636 1,019 27.9 40 3,320 2,335 70.3 2,568 1,087 29.7 47 3,150 2,305 73.2 2,614 1,041 28.5 55 2,900 2,070 71.4 2,455 1,200 32.8 60 2,650 2,050 77.4 2,500 1,155 31.6 65 2,580 1,805 70.0 2,318 1,337 36.6 70 2,240 1,525 68.1 2,091 1,564 42.8 76 2,265 1,600 70.6 2,205 1,450 39.7 80 2,140 1,580 73.8 2,250 1,405 38.4 86 1,875 1,435 76.5 2,136 1,519 41.6 *VSS concentration adjusted for samples withdrawn, which were replaced with tap water. 2000 i . e . , Net VSS = VSS x ( 2 0 0 Q - m l s a m p l e > Table 21. Batch Aeration Suspended Solids Mass Balance Temperature: 5°C Digester Volume: 2.0 l i t r e s Day Digester Sludge VSS Destroyed TSS (mg/1) VSS *Net VSS (mg/1) mg/1 % of TSS mg/1 % 0 11,145 8,265 74.2 8,265 0 0 20 9,410 6,835 72.6 6,835 1,430 17.3 31 8,520 6,200 72.8 6,522 1,743 21.1 40 8,170 5,890 72.1 6,045 2,220 26.9 49 7,640 5,430 71.1 5,750 2,515 30.4 60 7,170 5,255 73.3 5,727 2,538 30.7 70 6,665 4,755 71.3 5,364 2,901 35.1 72 6,520 4,750 72.9 5,432 2,833 34.3 76 6,670 4,680 70.2 5,568 2,697 32.6 78 6,395 4,485 70.1 5,432 2,833 34.3 80 6,385 4,455 69.8 5,545 2,720 32.9 84 6,190 4,280 69.1 5,400 2,865 34.7 *VSS concentration adjusted for samples withdrawn, which were replaced with tap water. Table 22. V o l a t i l e Suspended Solids Reduction at Steady-State Mode of Operation 20°C 10°C 5°C S.A. (days) % VSS Reduction S.A. (days) % VSS Reduction S.A. (days) % VSS Reduction Continuous 69.2 49.81 53.8 37.25 53.5 20.58 Feed Automatic Decant 30.9 10.1 39.10 26.00 26.7 12.4 26.02 20.51 21.2 8.3 9.92 3.96 Daily 32.6 38.59 32.4 33.64 29.9 20.89 Feed Manual 17.7 31.80 16.7 24.42 13.2 11.07 Decant 7.8 24.51 7.9 13.05 8.5 8.59 60 i — i — i — i — r i—i \ i o M H U o w co CO > 50 40 30 20 10 O Continuous Feed / Automatic Decant O Daily Feed / Manual Decant I I I 10 20 50 DIGESTER SLUDGE AGE (DAYS) 100 Figure 10 Effect of Sludge Age and Temperature on VSS Reduction (Continuous Feed Units) 2 5 10 20 50 100 DIGESTER SLUDGE AGE (DAYS) Figure 11 - Effect of Sludge Age and Temperature on VSS Reduction (Batch Aeration Units) As would be expected, VSS reduction increased with increasing temperature and increasing sludge age. In the continuous and semi-continuous feed systems (Fig. 10), there was no apparent difference between the two modes of operation at 20°C and 10°C; however, at 5°C the continuous feed system showed a d e f i n i t e lesser degree of VSS reduction than the semi-continuous feed system at a l l sludge ages. This i s contrary to e a r l i e r reports (100), although con-ducted at higher temperatures, which suggested that uninterrupted feeding would improve the degree of solids reduction. The results obtained i n the batch reactors are shown i n Figure 11. For comparison, the continuous feed system curves of Figure 10 are superimposed on this figure as broken l i n e s . The f i r s t four data points of the 20°C batch system are not considered representative, as during the f i r s t 40 days of that run excessive build-up of solids occurred on the reactor walls. These solids were d i f f i c u l t to remove. After a different procedure was adopted for resuspending these solids on a daily basis, further build-up was avoided. VSS reductions i n the 20°C batch reactor were approximately 15 percent higher than those for the continuous feed systems, whereas the 10°C VSS reductions were about'15 percent lower than the corres-ponding continuous feed ones. The VSS reductions i n the batch digester at 5°C were very close to those of the 5°C semi-continuous feed system. Although there was a detectable difference i n the response to operating temperature for the various digestion systems studied, there did not appear to be a predictable pattern between the systems with respect to solids destruction. I t i s evident, however, that the response of the batch systems to temperature was quite different than that of the continuous feed systems. I t was apparent that temperature and sludge age had a similar effect on the VSS reduction obtained i n a given system. Figures 10 and 11 show that a similar VSS reduction may be obtained at a high temperature and low sludge age and at a low temperature and high sludge age. A combination of those two parameters, i n a common function with VSS reduction, should therefore enable an assessment of the overall process l i m i t a t i o n s of aerobic digestion, with respect to VSS reduction. For this purpose, a plot was prepared of percent VSS reduction versus the product of temperature and sludge age, as shown i n Figure 12. Although there i s no s c i e n t i f i c basis to such a plo t , i t did provide a means of three-parameter p l o t t i n g , which, i n th i s case, resulted i n extremely good correlation. The curve appears to reduce to two straight l i n e s , with a deflection occurring between the values 150 and 250 on the abscissa. This would thus indicate a s i g n i f i c a n t l y slower increase i n VSS reduction with increased temperature or sludge age above a product value of between 150 and 250, for the temperature and sludge age range studied. TEMPERATURE (°C) X SLUDGE AGE (DAYS) Figure 12 - Combined Effect of Sludge Age and Temperature on VSS Reduction 121 The data points for the continuous feed/automatic decant system show a slower i n i t i a l increase i n VSS reduction; however, the point of deflection i n the two curves does not vary much from that for the other systems studied. The significance of this emperical relationship i s rather important,as the deflection point indicates a d e f i n i t e optimum combination of system sludge age and operating temperature, beyond which i t would become disproportionally more d i f f i c u l t to achieve higher levels of VSS reduction. Aerobic digestion k i n e t i c s have often been expressed i n terms of degradable VSS (DVSS). The reason for this was that t o t a l VSS reduction does not follow f i r s t order reaction k i n e t i c s ; instead i t has been observed to follow a pattern of decreasing rate of endogenous decay with time, as the easier to metabolize components of the sludge disappeared f i r s t . By eliminating the non-degradable portion of the VSS from the rate equation, i t was claimed, a constant rate of endogenous decay would res u l t . The non-degradable VSS would t y p i c a l l y be determined by aerating a volume of sludge u n t i l the VSS remaining levels o f f , as shown i n Figure 13 for the 20°C batch aeration system. I t can be seen, however, from Figures 14 and 15 (which show the percent VSS remaining for the 10°C and 5°C batch aeration systems respectively) that the non-degradable VSS remaining was much higher at the lower temperatures. This points out a very serious shortcoming i n the usefulness of DVSS as a universal parameter describing endogenous decay k i n e t i c s . Not only i s the magnitude of the degradable VSS dependent on the 122 l 1 1 1 1 i 1 1 r Figure 13 Effect of Digestion Time on VSS Reduction Batch Aeration @ 20°C Figure 14 Effect of Digestion Time on VSS Reduction Batch Aeration @ 10°C Figure 15 - Effect of Digestion Time on VSS Reduction Batch Aeration @ 5°C temperature at which i t was determined, but i t would also appear to be dependent on the type of sludge and type of digester operation, and, i n fact, i t would appear to be a function of the endogenous decay rate i t s e l f . Such results therefore, would only apply to the particular s i t u a t i o n studied, and could not be used beyond that s p e c i f i c application. I t i s for these reasons that this author has not used this p a r t i c u l a r parameter and therefore, unless otherwise noted, VSS implies t o t a l v o l a t i l e suspended s o l i d s . The rate equations for endogenous decay, i n terms of VSS removal, have been described i n Chapter I I for batch digestion, and for continuous digestion with digested sludge recycle. The rate equation for continuous feed with sludge recycle i s as follows: M /M = , , } r or M /M = 1 + k J . t ' t o 1 + k j . t ' o t d d When pl o t t i n g M /M versus sludge age ( t ' ) , a straight l i n e results with slope k,, i f this equation i s v a l i d . Figures 16 to 18 show d these plots for the three temperatures studied. The rate equation for batch digestion i s as follows: -k .t M /M = e t o 2.1 2.0 1.9 1.8 1.7 1.6 S° 1.5 1.4 1.3 1.2 1.1 1.0 1 1 i I 1 1 I" -Continuous Systems M /M = 1 + k,.t' ^ o t d 20°C - kJ = 0.0099 -- -- ° Continuous Feed / Automatic Decant _ - ^ Daily Feed/ Manual Decant -J k d = 0.0388 (day" -i • i i i i i — 0 10 20 30 40 50 60 SLUDGE AGE (DAYS) 70 Figure 16 - Determination of Endogenous Decay Rates - VSS Basis Continuous Feed Systems @ 20°C t—• ON ^ 1 i i i r Figure 17 - Determination of Endogenous Decay Rates - VSS Basis Continuous Feed Systems @ 10°C r-N i 1 1 1 r Continuous Systems M /M = 1 + k,.t' J o t d 0 10 20 30 40 50 60 70 SLUDGE AGE (DAYS) Figure 18 - Determination of Endogenous Decay Rates - VSS Basis Continuous Feed Systems @ 5°C NJ oo When plo t t i n g In M versus time of digestion ( t ) , a straight l i n e , with slope k^, would result i f this equation i s v a l i d . Figures 19 to 21 show these plots for the three temperatures studied. As already discussed before, a decreasing reaction rate (k^) with time would be expected when using t o t a l VSS as the process parameter It appears, however, that t h i s decreasing rate can be adequately represented by a pair of straight l i n e s of best f i t through the data points. This would indicate a rapid i n i t i a l rate of decay followed by a much slower, secondary rate of decay. As was already shown i n Figure 12, the change between i n i t i a l and secondary rate of decay occurred at that sludge age or digestion time, where the product of temperature and sludge age was between 150 and 250, for the batch systems as well as the continuous feed systems. Note that only single rate values were found for the 5°C continuous feed systems, as the maximum sludge age i n these systems barely exceeded 50 days. The i n i t i a l decay rates have been plotted against temperature for the continuous and semi-continuous feed systems i n Figure 22 and for the batch aeration systems i n Figure 23. The decay rates were calculated graphically from the l i n e s of best f i t i n the preceding graphs. From the semi-logarithmic plots of decay rate versus temperature, the temperature s e n s i t i v i t y c o e f f i c i e n t s , 9, can be calculated, i n accordance with the Streeter-Phelps formula given e a r l i e r : (T--T ) k d 2 / k d l = 9 Figure 19 - Determination of Endogenous Decay Rates Batch Aeration @ 20°C - VSS Basis Figure 20 - Determination of Endogenous Decay Rates - VSS Ba Batch Aeration @ 10°C Figure 21 - Determination of Endogenous Decay Rates - VSS Basis Batch Aeration @ 5°C 133 = 1.156 TEMPERATURE (°C) Figure 22 - Temperature Coefficients for I n i t i a l Endogenous Decay Rates. Continuous Feed Systems. VSS Basis 134 Figure 23 - Temperature Coefficients for I n i t i a l Endogenous Decay Rates. Batch Aeration Systems. VSS Basis 135 For discussion purposes, the results from Figures 22 and 23, together with the secondary decay rates, have been plotted i n a common graph i n Figure 24. It i s apparent that the i n i t i a l decay rates were at least a factor two, although more generally about a factor four, greater than the corresponding secondary rates. The secondary k-values would there-fore be of comparatively minor importance. On the basis of i n i t i a l decay rates, there was probably an i n s i g n i -ficant difference between the batch and continuous feed systems at 20°C. At 10°C, the batch decay rate was substantially lower than that of both continuous feed systems. At 5°C, there was a s i g n i f i -cantly different behaviour between the continuous and semi-continuous feed systems, with the l a t t e r exhibiting almost twice the decay rate than the former. The batch decay rate at 5°C was approximately half-way between those of the two continuous feed systems. These results indicate that at very low temperatures, i t would be advantageous to operate on a daily fill-andrdraw basis, rather than feeding the digester i n many small increments. A reason for this may be that by adding small quantities of fresh sludge at a time, a lesser proportion of the feed sludge may adapt to the unfavourable digester environment, whereas, when larger quantities of feed sludge are added at once, the degree of adaptation could be greater, because of greater t o t a l numbers of organisms. 136 0.10 0.05 0.04 0.03 0.02 % 0.01 Q 0.005 0.004 0.003 0.002 0.001 O • Continuous Feed / Automatic Decant Daily Feed / Manual Decant Batch Aeration I n i t i a l Endogenous Decay Rates Secondary Endogenous Decay Rates \ \ \ Values for Q ^ Q = 2 J_ 20 15 TEMPERATURE (°C) 10 Figure 24 - Effect of Temperature on I n i t i a l and Secondary Endogenous Decay Rates. VSS Basis I t i s apparent that the k-values of batch and continuous feed systems are not interchangeable, which presents a further caution against the use of batch data i n predicting the behaviour of continuous feed digesters. The latter, finding i s supported by Ganczarczyk et a l . (53), i n work done at 20°C, but i s contrary to results presented by Adams et a l . ( 1 ) , on work done at an unspecified temperature. Both groups based their results on degradable VSS removal. In the batch system, there appeared to be l i t t l e difference i n temperature s e n s i t i v i t y between the 20°C - 10°C and the 10°C - 5°C temperature range. On the other hand, there was a great increase i n temperature s e n s i t i v i t y of the continuous feed systems i n the lower temperature range. On an ove r a l l basis, the temperature s e n s i t i v i t y of the aerobic digestion systems studied appeared to be greater than that corresponding to a value of Q = 2 (9 = 1.072), reported by others to be representative of aerobic digestion (15). Oxygen Uptake Rate Specific oxygen uptake rate data, measured for a l l systems as a function of time elapsed since start-up of the units, are plotted for a l l systems i n Figures 25 to 45 inclusive. In the same figures, the progression of digester pH with digestion time i s plotted. Continuous Feed Automatic Decant 20°C 0.02 lbVSS/day/cu f t (0.32 kgVSS/day/m ) foaming (1.66) _L 10 20 30 40 50 60 70 80 90 TIME ELAPSED SINCE START-UP (DAYS) 100 Figure 25 - Digester pH and Oxygen Uptake Rate rt (TO C H 0 2 UPTAKE RATE (mg/gVSS/hr) Ul ON 1~ Ni ON (TO tu CO rt (D i-i TJ a P L O (TO ro 0 c rt po ro CO K M rt f CO rt n M CO H H C CO Ni o Co o •p-o O ON o o 00 o o o o /-N o NJ > o o • O c o o o rt ON J> O O rt 0 H-r-1 Co 0 cr rt C (TO < H- O < CO n C to CO cn CO \ a \ Cu ro rt CL fa o ro CO ^ Co ro V) \ CL ^ O rt 0 c CO ^ r+i rt cn JL ON »-J oo 6CT 0 I I I I I I I L I I l _ 0 10 20 30 40 50 60 70 80 90 100 TIME ELAPSED SINCE START-UP (DAYS) Figure 27 - Digester pH and Oxygen Uptake Rate o 0 UPTAKE RATE (mg/gVSS/hr) O M K> U> - P ^ L n O N »J 4^ Ln O 00 Figure 29 - Digester pH and Oxygen Uptake Rate 0 2 UPTAKE RATE (mg/gVSS/hr) r—1 N3 UJ -P> U l ON P -OQ C H f D U> O H-00 f D CO r t f D i - i T3 PCI 0> a-o OP ro (=! r t OJ ! ^ f D 0) 1—' o N3 O o o Un O O M S O r—1 • o 0) 0) • O o 3 H-ho 00 n c r-> 00 r—1 al ?r cr < a f D < C O f D C O C O o CL. C O CL. an CL. P r t o e UJ v ' ft ON o o 00 o o o o 00 Hd en T 1 1 1 i r i i i i i i i i_ i i _ 0 10 20 30 40 50 60 70 80 90 100 TIME ELAPSED SINCE START-UP (DAYS) Figure 31 - Digester pH and Oxygen Uptake Rate t-1 . c CO CO > 60 e w H H -T" 1 10 20 Continuous Feed Automatic Decant 10°C 0.02 lbVSS/day/cu f t CO.32 kgVSS/day/m3) 30 40 (0.93) 50 60 70 80 90 TIME ELAPSED SINCE START-UP (DAYS) 8 H 7 6 5 4 100 Figure 32 - Digester pH and Oxygen Uptake Rate Figure 33 - Digester pH and Oxygen Uptake Rate u , c CO CO > 6 0 6 0 s H 3 H P-i £0 Continuous Feed Automatic Decant 10°C 0.08 lbVSS/day/cu f t (1.28 kgVSS/day/m ) 4i 1.80) J_ _L 10 20 30 40 50 60 70 80 90 TIME ELAPSED SINCE START-UP (DAYS) 100 Figure 34 - Digester pH and Oxygen Uptake Rate l 1 1 1 1 1 I nr I i i i i i i l I I I 0 10 20 30 40 50 60 70 80 90 100 TIME ELAPSED SINCE START-UP (DAYS) Figure 35 - Digester pH and Oxygen Uptake Rate 6n T 1 i 1 1 1 i i r I i i i i i i i i I 1 0 10 20 30 40 50 60 70 80 90 100 TIME ELAPSED SINCE START-UP (DAYS) Figure 37 - Digester pH and Oxygen Uptake Rate Figure 38 - Digester pH and Oxygen Uptake Rate rt H-09 C H tt u> 0 2 UPTAKE RATE (mg/gVSS/hr) NJ On "T" o 00 ro rt ro H Tt a Co 0 Cu o 00 ro a X) rt fu ?r ro rt Co S w w rt rt C O w 2 w CO H > ES i c n rt a CO NJ o o J> o O CTN O ^1 O 00 o VO O 00 00 ?—* o On > o o • o C o • o n rt 0 OJ NJ O rt NJ 3 H-h-1 0 iV cr rt C 00 < H- o < CO O c CO CO CO CO o Cu Cu ro rt CO o ro CO <<! Co ro 0 Cu O rt c OJ V ' r-h o o 7 4 Continuous Feed Automatic Decant 5°C 0.04 lbVSS/day/cu f t (0.64 kgVSS/day/m ) (1.06) 10 20 30 40 50 60 70 80 90 TIME ELAPSED SINCE START-UP (DAYS) 100 Figure 40 - Digester pH and Oxygen Uptake Rate Ln 0 £ UPTAKE RATE (mg/gVSS/hr) 00 0 H ro a OQ ro CO r t fD H a OJ 3 Cu O X OQ fD 0 Xi r t P i ?r ro & 0> M CO w o CO H > 3 CO I—1 o o O0 o 4>-o Ul o o o 00 o VC o .-—s o > o r-1 • o 0 o • o o rt 3 S3 00 O rt CO s H-h-1 OJ 0 IV cr rt C OQ <! H- O <3 CO O 0 CO CO CO CO —. t) '—^  a. fD >*i Cu n fD OJ OJ fD o nt Cu 0 "— ft o t-1 o o J L ON u CO CO > 6 0 6 0 B H H 7 r-3 2 10 20 Daily Feed Manual Decant 5°C 0.02 lbVSS/day/cu f t (0.32 kgVSS/day/m ) (1-20) JL 30 40 50 60 70 80 90 TIME ELAPSED SINCE START-UP (DAYS) - 8 - 7 6 - 5 - 4 100 Figure 42 - Digester pH and Oxygen Uptake Rate T 1 1 1 1 1 1 r Daily Feed Manual Feed 5°C 0.04 lbVSS/day/cu f t (0.64 kgVSS/day/m ) (1-62) _L 10 20 30 40 50 60 70 80 90 TIME ELAPSED SINCE START-UP (DAYS) 100 Figure 43 - Digester pH and Oxygen Uptake Rate ON l 1 1 r 1 r 0 10 20 30 40 50 60 70 80 90 100 TIME ELAPSED SINCE START-UP (DAYS) Figure 45 - Digester pH and Oxygen Uptake Rate Problems encountered i n the f i r s t continuous feed run (Figures 25, 26, and 27 @ 20°C), such as foaming and s p l i t pump tubing, are indicated i n the plots. In most cases, i t i s apparent that steady-state conditions were re-established after the problem was corrected. A l l plots c l e a r l y show the establishment of steady-state conditions beyond 40-50 days after start-up of the digesters, resulting i n a l e v e l l i n g off at a minimum pH and a minimum oxygen uptake rate. Steady-state s p e c i f i c oxygen uptake rate values were calculated as the arithmetic average of the individual oxygen uptake rate values measured beyond a digestion time of 50 days. These values are shown i n brackets on each of the curves. Figures 30 and 37 show an i n i t i a l sharp drop i n s p e c i f i c oxygen uptake rate, after which a higher steady-state value i s established. In both cases this happened i n the highest loaded intermittent feed units (20°C and 10°C). No explanation can be offered, however, for th i s behaviour. The steady-state s p e c i f i c oxygen uptake rate values obtained i n Figures 25-45 are summarized i n Table 23 and are plotted against sludge age, for the three temperatures studied i n Figures 46, 47, and 48. At any temperature, the s p e c i f i c oxygen uptake rate decreased asymptotically as the sludge age increased. There appeared to be l i t t l e , i f any, difference between the two modes of continuous feed at any of the temperatures studied. Table"23. Oxygen Uptake Rate at Steady-State Mode of Operation 20°C 10°C 5°C S...A. (days) Oo Uptake mg/gvSS/hr S.A. (days) O2 Uptake mg/gVSS/hr S.A. (days) Oo Uptake mg/gVSS/hr Continuous Feed Automatic Decant 69.2 30.9 10.1 1.66 1.52 4.00 53.8 26.7 12.4 0.93 1.25 1.80 53.5 21.2 8.3 0.88 1.06 1.40 Daily Feed Manual Decant 32.6 17.7 7.8 1.98 3.29 4.80 32.4 16.7 7.9 0.96 1.70 2.65 29.9 13.2 8.5 1.20 1.62 1.32 r—1 ON O Figure 46 - Effect of Sludge Age on Oxygen Uptake Rate @ 20°C Continuous Feed Systems. Figure 47 - Effect of Sludge Age on Oxygen Uptake Rate @ 10°C Continuous Feed Systems. Figure 48 - Effect of Sludge Age on Oxygen Uptake Rate @ 5°C Continuous Feed Systems. ON 164 The batch aeration s p e c i f i c oxygen uptake rate values were replotted from Figures 31, 38, and 45, and are shown i n Figures 49, 50, and 51. For comparison, the curves for the continuous feed systems are plotted as dotted lines i n the same figures. The f i n a l s p e c i f i c oxygen uptake rate values for the batch aeration systems were s i g n i -f i c a n t l y lower than those for the continuous feed systems at a l l temperatures. I t i s interesting to note that, at the lower tempera-tures, the rate of decrease of oxygen uptake rate values was con-siderably slower for the batch aeration systems, as compared to that for the continuous feed systems. Figures 52 and 53 show composites of the indi v i d u a l oxygen uptake rate curves for the continuous feed and batch aeration systems respectively, transposed from Figures 46 to 51 inclusive. The effect of temperature on s p e c i f i c oxygen uptake rate was pronounced for both systems. Curves plotted for selected sludge ages i l l u s t r a t e the combined effect of decreasing temperatures and increasing sludge age on the lowering of the s p e c i f i c oxygen uptake rate. Figures 54 and 55 show plots of the ratios of oxygen uptake rates at the lower temperatures, r e l a t i v e to that at 20°C, for the conti-nuous feed and batch aeration systems, respectively. This pl o t , on a semi-logarithmic scale, was used to calculate the Streeter-Phelps temperature s e n s i t i v i t y c o e f f i c i e n t , 9. I t i s apparent that for both systems, the temperature s e n s i t i v i t y varied greatly with sludge age, as well as with the temperature range. The l a t t e r i s Figure 49 - Effect of Sludge Age on Oxygen Uptake Rate @ 20°C Batch Aeration. Figure 50 - Effect of Sludge Age on Oxygen Uptake Rate @ 10°C Batch Aeration Figure 51 - Effect of Sludge Age on Oxygen Uptake Rate @ 5°C Batch Aeration. 20 10 5 TEMPERATURE (°C) Figure 52 - Effect of Temperature on Oxygen Uptake Rate. Continuous Feed Systems. Figure 53 - Effect of Temperature on Oxygen Uptake Rate. Batch Aeration Systems. Figure 54 - Determination of Temperature S e n s i t i v i t y C o e f f i c i e n t s f o r Oxygen Uptake Rate. Continuous Feed Systems. (log R 2 - log R]_) T T2 - T± J 9 = 10 Figure 55 - Determination of Temperature S e n s i t i v i t y Coefficients for Oxygen Uptake Rate. Batch Aeration Systems. i l l u s t r a t e d i n Figures 56 and 57, where the calculated values of 9 for various sludge ages and the two temperature ranges, are plotted for the continuous feed and batch aeration systems, respectively. These figures show quite dramatically the difference i n behaviour of continuous feed digestion and batch digestion, with respect to temperature. The continuous feed systems have a f a i r l y narrow range of temperature s e n s i t i v i t y between the temperature ranges studied, but the temperature s e n s i t i v i t y decreases considerably as sludge age increases; from a high of 1.090, 9 drops as low as 1.022. Except .at sludge ages less than about 17 days, the tempera-ture s e n s i t i v i t y i n the 10-20°C range i s higher than that i n the 5-10°C range. On the other hand, the batch aeration systems show an enormous range i n temperature s e n s i t i v i t y ; from a high of 1.189, 9 drops to 1.000 for these systems. Unlike the continuous feed systems, the batch aeration systems are much more sensitive to temperature i n the 5-10°C range than i n the 10-20°C range, where, for the most part, the temperature s e n s i t i v i t y c o e f f i c i e n t 1.000, i . e . , oxygen uptake rate does not appear to be affected by temperature i n this range. A decrease i n temperature s e n s i t i v i t y with increasing sludge age, as was the case consistently- i n the continuous feed systems, can b explained by gradual acclimatization of the sludge microorganisms to lower temperatures. The behaviour of the batch aeration system however, especially i n the 5-10°C range, where i n i t i a l l y a sharp decrease i n temperature s e n s i t i v i t y occurred, followed by a sharp increase, cannot be readily explained. 40 50 60 SLUDGE AGE (DAYS) 70 Figure 56 - Variation i n Temperature Response with Respect to Oxygen Uptake Rate. Continuous Feed Systems. I I I I I 1 I : I— 0 10 20 30 40 50 60 70 SLUDGE AGE (DAYS) Figure 57 - Variation in Temperature Response with Respect to Oxygen Uptake Rate. Batch Aeration Systems. 175 In previous publications on this work (164, 165, 166, 167) the author had established an average temperature s e n s i t i v i t y c o e f f i c i e n t of 1.058, which appeared to be more or less constant for a l l sludge ages throughout the 5-20°C temperature range. The approach used at that time i n the analysis of results was to combine the continuous feed and batch aeration data as shown i n Figures 58, 59, and 60, i n which l i n e s of best f i t were plotted to the combined sets of data. These curves, i n turn, produced the data for Figure 61, from which the temperature s e n s i t i v i t y c o e f f i c i e n t was calculated. The l i n e s for various sludge ages were judged s u f f i -c i e n t l y close to warrant the use of an average c o e f f i c i e n t , to describe the temperature response of oxygen uptake rate over the f u l l 5°-20°C temperature range. In this case, the average value for 9 was 1.058, as shown by the heavy broken l i n e i n Figure 61. Using the present method of analysis of p l o t t i n g 9 versus sludge age, i t i s apparent that there i s a v a r i a t i o n of 9 with sludge age, and between the two temperature ranges, as shown i n Figure 62. I t appears therefore, that a more exact interpretation of the data, without d i f f e r e n t i a -t i o n between continuous feed and batch aeration, would indicate an average temperature s e n s i t i v i t y , with respect to oxygen uptake rate, which steadily decreases with increasing sludge age; from a high of 1.070 at 10 days to a low of 1.045 at 50 days and over. The l a t t e r i s i l l u s t r a t e d by the broken curve i n Figure 62. Figure 58 - Effect of Sludge Age on Oxygen Uptake Rate @ 20°C-A l l Systems"Combined. Figure 59 - Effect of Sludge Age on Oxygen Uptake Rate @ 10°C. A l l Systems Combined. Figure 60 - Effect of Sludge Age on Oxygen Uptake Rate @ 5°C. A l l Systems Combined. H OC Figure 61 - Determination of Temperature S e n s i t i v i t y Coefficients for Oxygen Uptake Rate. A l l Systems Combined. 10 - 20°C 5 - 10°C _L 10 20 30 40 50 60 SLUDGE AGE (DAYS) 70 Figure 62 - Variation i n Temperature Response with Respect to Oxygen Uptake Rate. A l l Systems Combined. B. DIGESTED SLUDGE CHARACTERISTICS 1. Biochemical Oxygen Demand Tables 24, 25, and 26 contain the BOD, COD and TOC data measured for the digested sludge and supernatant. The COD and TOC data for the digested sludge and the supernatant w i l l be discussed l a t e r i n this chapter. The decision to do BOD measurements on the sludge and supernatant was not made u n t i l late i n the f i r s t run (20°C run). Thus, the BOD data for that run are limited. In addition, the f i r s t set of measurements were made when the continuous feed/automatic decant systems were encountering severe foaming problems. I t was obvious from the individual BOD measurements that these f i r s t measurements were uncharacteristically high, and i t was therefore decided to use only the single set of BOD measurements made at 100 days after start-up for the 20°C run. For the 10°C and 5°C runs, the digested sludge BOD values were judged to have level l e d off s u f f i c i e n t l y to assume steady-state conditions after 60 days from start-up. In addition, measurements made after that time were averaged for each of the units, resulting i n steady-state BOD values for digested sludge for a l l units. A summary of these steady-state values i s contained i n Tables 27 and 28. The relationship between steady-state BOD values and digester sludge age i s shown graphically i n Figures 63, 64, and 65, for the three temperatures studied. Table 24. Sludge and Supernatant Data for 20°C Systems Digested Sludge Supernatant System Days from VSS BOD 5 COD TOC B0D5 COD TOC Start mg/1 mg/1 g/gVSS mg/1 g/gVSS mg/1 g/gVSS mg/1 mg/1 mg/1 F 61 7,520 - - 14,590 1.94 4,115 0.55 - 115 40 69 7,675 - - 20,680 -2^ 69- 3,600 0.47 - 125 45 77 7,475 7,905 1.06 11,590 1.55 4,500 0.60 57 -25# 57 100 6,350 6,608 1.05 9,935 1.56 3,500 0.55 71 115 44 C l 61 11,725 - - 17,470 1.49 5,990 0.51 - 735 ' 225 69 12,000 - - 17,860 1.49 5,860 0.49 - 805 235 77 11,000 6,205 0.57 18,215 1.66 6,400 0.58 278 -2300- ^5-*100 8,550 1,350 0.16 13,615 1.59 5,000 0.58 34 >-9*5- 467 C2 61 10,300 - - 16,130 1.57 5,170 0.50 - 455 145 69 10,800 - - •16,355 1.51 5,285 0.49 - 610 190 77 10,850 4,870 0.45 16,375 1.51 6,100 0.56 108 680 275 *100 10,350 1,950 0.19 14,720 1.42 5,400 0.52 36 595 245 61 7,700 - - 13,630 1.77 4,390 0.57 - 250 90 69 8,150 - - 11,655 1.43 3,995 0.49 - 325 120 77 9,000 7,455 0.83 13,800 1.53 5,400 0.60 104 275 140 *100 8,800 5,580 0.63 13,250 1.51 4,800 0.55 41 230 100 LEGEND FOR TABLES 24 TO 40 * F C B B4 -2300-= Steady-State = Feed Sludge = Continuous Feed./Automatic Decant = Daily Feed/Manual Decant = Batch Aeration = Outlier (value disregarded) ) 1 = low loading ) 2 = intermediate loading ) 3 = high loading Table 24 (cont'd). Sludge and Supernatant Data for 20°C Systems Days from Digested Sludge Supernatant System VSS BOD 5 COD TOC B0D5 COD TOC Start mg/1 mg/1 g/gVSS mg/1 g/gVSS mg/1 g/gVSS mg/1 mg/1 mg/1 B l 61 69 77 *100 7,100 7,600 7,000 6,, 900 2,500 1,855 0.36 0.27 11,905 11,470 13,065 11,040 1.68 1.51 1.87 1.60 4,330 3,925 4,700 3,850 0.61 0.52 0.67 0.56 150 55 575 435 830 380 180 145 175 B2 61 69 77 *100 8,000 8,650 8,800 7,500 1,435 1,910 0.16 0.25 12,670 12,410 13,800 11,590 1.58 1.43 1.56 1.55 4,460 4,250 4,900 4,150 0.56 0.49 0.56 0.55 100 30 470 310 425 480 155 105 215 150 B3 61 69 77 *100 7,900 8,000 7,400 7,300 4,460 4,285 0.60 0.59 12,095 10,905 11,590 10,490 1.53 1.36 1.57 1.44 4,640 3,825 4,500 3,650 0.59 0.48 0.61 0.50 135 145 295 235 275 230 100 85 140 110 B4 61 69 77 100 1,630 1,500 1,350 1,080 170 55 0.13 0.05 2,225 2,145 1,895 1,470 1.37 1.43 1.40 1.36 805 800 640 800 0.49 0.53 0.47 0.74 7.2 140 140 125 140 55 55 51 56 Table 25. Sludge and S upernatant Data for 1Q°C Systems Days Digested Sludge Supernatant System VSS BOD COD TOC B0Dc COD TOC from 5 5 Start mg/1 mg/1 g/gVSS mg/1 g / g v s s mg/1 g / g v s s mg/1 mg/1 mg/1 F 39 8,440 8,300 0.98 12,085 1.43 4,645 0.55 26 98 41 54 7,735 7,950 1.03 - - - - 77 - -65 8,030 9,090 1.13 12,785 1.59 4,420 0.55 61 164 57 81 8,505 8,520 1.00 13,860 1.63 6,045 0.71 53 70 37 86 10,520 10,590 1.01 - - - - 80 - -C l 39 9,000 2,370 0.26 13,865 1.54 4,780 0.53 ±±0- 438 229 54 11,500 3,285 0.29 - - - - 38 - -*65 12,860 2,310 0.18 20,680 1.61 6,580 0.51 24 474 182 *81 13,820 2,784 0.20 21,420 1.55 7,670 0.55 23 545 232 *86 14,210 2,670 0.19 - - - ' - 30 - -C2 39 11,000 6,380 0.58 15,325 1.39 5,180 0.47 57 406 213 54 13,050 5,385 0.41 - - - - 34 - -*65 13,235 1,740 •0^3- 21,430 1.62 7,020 0.53 47 361 161 *81 14,700 5,010 0.34 22,320 1.52 8,530 0.58 36 398 172 *86 15,015 5,600 0.37 - - - - 31 - -C3 39 11,000 7,520 0.68. 15,325 1.39 5,575 0.51 80 283 157 54 12,250 6,270 0.51 - - - - 45 - -*65 12,590 6,375 0.51 19,550 1.55 6,350 0.50 ±41- 243 114 *81 14,100 6,840 0.49 19,980 1.42 7,925 0.56 29 200 82 *86 14,235 7,020 0.49 - - - - 43 - -Table 25 (cont'd). Sludge and Supernatant Data for 10°C Systems Days from Digested Sludge Supernatant System VSS BOD 5 COD TOC B0D5 COD TOC Start mg/1 mg/1 g/gVSS mg/1 g/gVSS mg/1 g/gVSS mg/1 mg/1 mg/1 B l 39 54 *65 *81 *86 8,600 8,500 8,430 6,500 7,255 5,175 4,248 2,976 1,880 2,175 0.60 0.50 0.35 0.29 0.30 12,570 12,970 11,520 1.46 1.54 1.77 4,635 4,465 4,340 0.54 0.53 0.67 -•84 19 22 13 14 247 226 142 146 113 66 B2 39 54 *65 . *81 *86 9,300 8,450 8,055 8,300 9,380 5,820 4,452 4,680 4,596 5,220 0.63 0.53 0.58 . 0.55 0.56 11,030 13,160 12,420 1.19 1.63 1.50 5,350 4,050 4,715 0.58 0.50 0.57 59 29 17 57 21 186 154 126 148 92 58 B3 39 54 *65 *81 *86 9,400 8,580 9,330 11,360 11,275 5,745 5,376 5,580 6,816 7,236 0.61 0.63 0.60 0.60 0.64 15,325 14,665 16,740 1.63 1.57 1.47 5,580 4,635 6,015 0.59 0.50 0.53 25 26 28 16 46 165 113 79 129 74 42 B4 39 54 65 81 86 2,330 2,100 1,805 1,500 1,435 610 429 288 210 200 0.26 0.20 0.16 0.14 0.14 3,370 3,195 1,890 1.45 1.77 1.26 1,560 1,122 1,013 0.67 0.62 0.68 12 17 4 6 6 119 154 103 51 54 45 Table 26. Sludge and Supernatant Data for 5°C Systems Days from Digested Sludge Supernatant System VSS BOD 5 COD TOC BOD5 COD TOC Start mg/1 mg/1 g/gVSS mg/1 g/gVSS mg/1 g/gVSS mg/1 mg/1 mg/1 F 32 7,465 8,250 1.11 12,210 1.64 - - 16 86 -40 7,240 7,455 1.03 11,160 1.54 - - 47 121 -60 8,580 9,010 1.05 9,045 •±-r95- 3,620 0.42 11 51 24 72 9,145 9, 330 1.02 13,985 1.53 5,505 0.60 60 254 93 78 8,045 8,050 1.00 11,590 1.44 4,815 0.60 52 162 61 84 7,780 8,245 1.06 12,275 1.58 4,350 0.56 15 100 41 C l 32 14,000 6,615 0.47 23,390 1.67 - - 50 423 -40 14,545 6,096 0.42 23,580 1.62 - - 58 378 -*60 15,420 5,520 0.36 26,145 1.70 7,930 0.51 ±25- 503 185 *72 14,740 3,756 OT-2-5- 25,025 1.70 8,265 0.56 30 261 105 *78 14,475 5,145 0.35 - - 8,425 0.58 33 282 109 *84 14,490 4,950 0.34 22,135 1.53 7,620 0.53 54 357 141 C 2 32 13,900 7,680 0.55 21,330 1.53 - - 65 370 -40 13,535 6,375 0.47 20,700 1.53 - - 71 275 -*60 13,765 5,550 0.40 20,065 1.46 7,880 . 0.57 95 317 125 *72 13,400 5,325 0.40 22,265 1.66 7,345 0.55 30 202 85 *78 13,275 5,775 0.44 20,610 1.55 7,575 0.57 59 254 99 *84 12,770 5,700 0.45 20,830 1.63 7,115 0.56 56 249 100 32 11,100 7,360 0.66 17,030 1.53 - - 55 229 -40 10,580 6,420 0.61 17,100 1.62 - - 84 234 -*60 11,130 5,625 0.51 16,455 1.48 5,730 0.51 67 233 92 *72 12,010 5,475 0.46 22,630 1.88 6,575 0.55 83 302 116 *78 12,095 6,000 0.50 19,870 1.64 8,120 0.67 99 324 119 *84 12,000 5,025 0.42 17,110 1.43 6,120 0.51 66 277 109 Table 26 (cont'd). Sludge and Supernatant Data for 5°C Systems Days from Digested Sludge Supernatant System VSS BOD 5 COD TOC B0D5 COD TOC Start mg/1 mg/1 g/gVSS mg/1 g/gVSS mg/1 g/gVSS mg/1 mg/1 mg/1 B l 32 10,400 5,520 0.53 15,310 1.47 - - 59 181 -40 10,230 5,880 0.57 16,560 1.62 - - 95 292 -. *60 9,620 3,660 0.38 13,795 1.43 5,555 0.58 23 170 68 *72 9,670 3,600 0.37 15,090 1.56 5,225 0.54 27 193 78 *78 9,595 3,680 0.38 14,350 1.50 6,220 0.65 74 254 98 *84 9,390 3,600 0.38 15,065 1.60 5,125 0.55 67 244 95 B2 32 10,200 6,060 0.59 15,310 1.50 - - 110 243 -40 9,230 5,580 0.60 14,400 1.56 - - 125 241 -*60 9,215 3,660 0.40 13,985 1.52 4,965 0.54 48 218 85 *12 9,430 4,080 0.43 14,905 1.58 4,935 0.52 69 241 97 *78 9,025 4,500 0.50 13,985 1.55 5,245 0.58 120 353 131 *84 9,560 4,356 0.46 13,950 1.46 5,115 0.54 105 273 107 B3 32 12,500 7,695 0.62 17,890 1.43 - - 62 179 -40 11,690 7,020 . 0.60 18,360 1.57 - - 118 238 -*60 11,250 6,150 0.55 18,735 1.67 6,305 0.56 126 286 108 *72 12,185 5,520 0.45 19,320 1.59 6,700 0.55 153 423 159 *78 11,420 5,775 0.51 19,505 1.71 7,055 0.62 161 346 128 *84 13,465 6,450 0.48 20,460 1.52 7,215 0.54 81 234 88 B4 32 6,200 2,356 0.38 9,115 1.47 - - 40 119 - ' 40 5,890 2,179 0.37 9,360 1.59 - 35 193 -60 5,255 1,920 0.36 7,335 1.40 3,015 0.57 24 214 78 72 4,750 1,630 0.34 6,440 1.36 2,895 0.61 10 188 78 78 4,485 1,614 0.36 6,625 1.48 2,720 0.61 9 167 65 84 4,280 1,372 0.32 6,510 1.52 2,745 0.64 5 136 57 Table 27. Summary of Sludge and Supernatant  Data at Steady-State Digested Sludge Supernatant System BOD5 COD TOC B0D 5 COD TOC g/gVSS g/gvss g/gVSS mg/1 mg/1 mg/1 F 1.05 1.68 0.54 64 118 47 C l 0.16 1.56 0.54 34 770 309 C2 0.19 1.50 0.52 36 585 214 20°C C 3 0.63 1.56 0.55 41 270 113 B l 0.27 1.67 0.59 55 555 167 B2 0.25 1.53 0.54 30 421 156 B 3 0.59 1.48 0.55 145 259 109 F 1.03 1.55 0.60 59 111 45 C l 0.19 1.57 0.53 26 486 214 C2 0.35 1.51 0.53 38 388 182 10°C C3 0.50 1.45 0.52 36 242 118 B l 0.31 1.59 0.58 16 205 108 B2 0.56 1.44 0.55 32 155 99 B3 0.61 1.56 0.54 30 119 82 F 1.05 1.55 0.55 34 129 55 C l 0.35 1.64 0.55 39 367 135 C2 0.42 1.56 0.56 60 278 102 5°C C3 0.47 1.60 0.56 79 267 109 B l 0.38 1.53 0.58 48 222 85 B2 0.45 1.53 0.55 86 262 105 B 3 0.50 1.58 0.57 130 284 121 Table 28. BOD of Digested Sludge at Steady-State 20°C 10°C 5°C Mode of Operation S.A. (days) B0D 5 g/gVSS S.A. (days) B0D 5 g / g v s s S.A. (days) B0D 5 . g / g v s s Continuous 69.2 0.16 53.8 0.19 53.5 0.35 Feed Automatic 30.9 0.19 26.7 0.35 21.2 0.42 Decant 10.1 0.63 12.4 0.50 8.3 0.47 Daily 32.6 0.27 32.4 0.31 29.9 0.38 Feed Manual 17.7 0.25 16.7 0.56 13.2 0.45 Decant 7.8 0.59 7.9 0.61 8.5 0.50 Figure 63 - Effect of Sludge Age on BOD @ 20°C I I 1 I I I I I I 1 1 0 10 20 30 40 50 60 70 80 90 100 DIGESTER SLUDGE AGE (DAYS) Figure 64 - Effect of Sludge Age on BOD @ 10°C Figure 65 - Effect of Sludge Age on BOD @ 5°C 193 As can be seen from these curves, the batch data also f i t the curves extremely w e l l , and i t can therefore be concluded that, with respect to mixed-liquor BOD values, there appeared to be no sig n i f i c a n t difference between the response of the steady-state continuous feed systems and the batch systems at any of the temperatures studied. Before evaluating the BOD results further, proper appreciation of the meaning of the mixed-liquor BOD i s necessary. Biochemical oxygen demand i s primarily used as an expression of the biodegradable organic content of a sample, by determining the oxygen equivalent consumed by microorganisms (that are either naturally present i n the test sample, or are introduced to i t ) i n oxidizing the organic material i n the sample. The biochemical oxygen demand of an endo-genously respiring sludge, however, where mainly microbial c e l l s are present, without biodegradable organics i n the surrounding l i q u i d (as i s the case i n a digested sludge sample), represents primarily the endogenous oxygen uptake of these c e l l s over a period of 5 days at 20°C. This value i s , therefore, not t r u l y related to the^total organic content of the c e l l s , but rather to their metabolic state. This concept i s shown cl e a r l y when comparing the 20°C oxygen uptake data for steady-state conditions i n Figure 52 with the steady-state BOD^  data at 20°C shown i n Figure 66. For any p a r t i -cular sludge age, the BOD,, of the 20°C sludge, expressed as mg02/mgVSS 100 8 90 80 70 60 50 40 30 20 10 S.A.= 80 days _S.A.= If) d a ™ 0.10 g o U l rt 0.20 H 0.30 z a 0.40 OQ 0.50 0.60 0.70 0.80 0.90 1.00 20 10 TEMPERATURE (°C) Figure 66 - Effect of Temperature on BOD,. Reduction i n VSS Fraction. Continuous Feed and Batch Aeration Systems. over 5 days, i s approximately equal to the oxygen uptake rate of that sludge, expressed as mgO^/gVSS/hr, when accounting for the conversion factor of 0.12. S i m i l a r l y , the BOD,, of the 10°C and 5°C sludges would represent the oxygen uptake rate values of these sludges when incubated or stored at 20°C. However, as shown i n Figure 66, there i s some discrepancy i n data at the lowest tempera-ture and lower sludge ages tested, which cannot be readily explained The BOD^ of digested sludge, therefore, could afford a means to compare the a c t i v i t i e s of sludges digested at different temperatures r e l a t i v e to a common temperature of 20°C. As even unstabilized sludges hardly ever present odour problems when stored at low' temperatures, i t seems l o g i c a l to establish sludge s t a b i l i t y r e l a t i v e to a temperature at which odour problems generally p r e v a i l . For this reason, and to conform to the standard temperature for the BOD,, test (132) as we l l , 20°C would appear to be a l o g i c a l choice. Odour tests were conducted on stored digested sludges, i n order to attempt to find a correlation between the sludge odour value and the BOD,, value. Table 29 shows the test panel results. The test procedure was held r e l a t i v e l y simple, and i s f e l t to be more accurate for the lower odour values, as no dilutions were used. Figure 67 shows a plot of steady-state, sludge BOD values against odour value. The scatter of data at the higher odour values seems to confirm that the low end of the curve i s the more meaningful one. The curve shows, for example, that for odour values less Table 29. Digested Sludge Odour Value Test Results Source Panel Member Total Average Odour 1 2 3 4 5 Value Value C l 1 0 1 % 0 2h 0.5 C2 2 2 1 1 1 1 1.4 C 1 3 3 2 % 2 ioh 2.1 20°C J B l 1 4 1 0 1 7 1.4 B2 1 1% 2 h -3 8 1.6 B3 3 2 3 2 2 12 2.4 C l 1 1 1 1 Us 5% 1.1 C2 1 2 4 1 2 10 2.0 C ^ 2 3 3 1 3 12 2.4 10°C B l 2 2 2 1 1 8 1.6 B2 3 2% 2 3 3 13% 2.7 B3 4 4 3 4 4% 19% 3.9 C l 2 4 2 1 1 10 2.0 C2 2 5 3 3 3 16 3.2 C i 4 4 2 4 3 17 3.4 5°C 3 B l 2 3 2 2 2 11 2.2 B2 2 3 3 3 3 14 2.8 B3 3 4 2 2 3% 14% 2.9 197 Figure 67 - Relationship Between Digested Sludge Odour Value and Digested Sludge BOD , After Storage at 20°C. 198 than 1, the BOD,, of the digested sludge would need to be less than 0.20 g/gVSS. For the purposes of this research, i t i s assumed that an odour value of less than 1 (one) would be non-objectionable. I t can also be seen from Figure 63 that a sludge age greater than 40 days would be required for digestion at 20°C to reduce the mixed-liquor BOD,, below 0.20 g/gVSS and thus avoid odour problems upon storage. For aerobic digestion at 10°C, a 60-day or higher sludge age would be required to reach that point (Figure 64), whereas at 5°C, even an 80-day sludge age would not reduce the mixed-liquor BOD s u f f i c i e n t l y to prevent odour problems upon storage at elevated temperatures (Figure 65). 2. Chemical Oxygen Demand and Total Organic Carbon The results of the COD and TOC measurements on the various digested sludges are shown i n Tables 24, 25, and 26, and a summary of the average COD and TOC values of the steady-state sludges i s shown i n Table 27. Both COD and TOC values, measured per unit mass of sludge were very constant throughout, for a l l systems and a l l temperatures, including the feed sludge. Whatever small v a r i a t i o n was present i n these parameters, there did not appear to be any pattern with respect to sludge age, type of digester operation, or temperature. The ov e r a l l averages calculated for the two parameters are: COD = 1.54 g/gVSS (std. dev. = 0.12) TOC = 0.56 g/gVSS (std. dev. = 0.06) The fact that these two parameters were f a i r l y constant for the feed sludge, as well as a l l sludges at different stages of digestion, would indicate that the chemical composition of the sludge c e l l remained v i r t u a l l y the same, and that aerobic digestion reduced the t o t a l mass of the sludge c e l l evenly, rather than reducing a sp e c i f i c component of the sludge mass. Using the empirical chemical formula for activated sludge c e l l material proposed by others and described e a r l i e r i n this work, the theoretical chemical oxygen demand would be between 1.98 and 1.42 g/gVSS, depending on whether the end-product of chemical oxidation of organic nitrogen i s ni t r a t e or ammonia. Since the majority of organic nitrogen i n sludge solids i s i n the form of amino groups i n protein, most w i l l be converted to ammonia during chemical oxidation. Only a minor portion of organic nitrogen i n activated sludge would be i n a higher oxidation state, resulting i n a conversion, to n i t r a t e during chemical oxidation. The value of 1.54 found i n this work would, therefore, support the empirical chemical formula C^H^M^ used to characterize sludge mass. The theoretical TOC value for th i s formula i s 0.53, which also compares closely with the results found i n this work. Nitrogen Mineralization, N i t r i f i c a t i o n , and D e n i t r i f i c a t i o n Nitrogen balances were conducted for the various systems and temperatures studied, by measuring Kjeldahl Nitrogen i n the mixed-li q u o r , and Ammonia and Nitrate Nitrogen i n the supernatant. 200 Organic Nitrogen i n the mixed-liquor was arrived at by subtracting Ammonia Nitrogen from Kjeldahl Nitrogen. The results are shown i n Tables 30, 31, and 32 for the 20°C, 10°C, and 5°C runs respectively. A summary of steady-state nitrogen data for the various systems i s shown i n Table 33, and the values shown are averages calculated from the values i n Tables 30, 31, and 32. I t can be noted that the organic nitrogen content for the digested sludges and feed sludge was p r a c t i c a l l y constant, with an o v e r a l l average value of 0.080 gN/gVSS, and a standard deviation of 0.005 gN/gVSS. There did not appear to be any pattern i n the organic nitrogen content of the sludge with respect to sludge age, tempera-ture, or type of digester operation. The organic nitrogen values found i n this research conform closely with results published by others for activated sludge (22, 155). As there i s no apparent difference between the organic nitrogen content of the feed sludge and the digested sludges, the amount of nitrogen converted from the s o l i d phase to the l i q u i d phase i n aerobically digested activated sludge should be equal to.the amount of v o l a t i l e suspended solids destroyed. Because, i n several random measurements, no appreciable amount of organic nitrogen was detected i n the supernatant, i t can be assumed that the destruction of v o l a t i l e suspended s o l i d s , during aerobic digestion of activated sludge, resulted i n almost 100 percent mineralization of organic nitrogen from those s o l i d s , to form ammonia i n solution. 201 Table 30. Nitrogen Data for 20°C Systems System Digested Sludge Supernatant Nitrogen Forms as mg/1 N VSS mg/1 Organic N g/gVSS Nitrogen Forms as mg/1 N Kj eldahl NH3 Organic NH3 N0 3 F 515 1 514 6,300 0.082 1 32 C l 697 35 662 8,500 0.078 35 333 C2 865 40 825 10,300 0.080 40 274 C3 795 35 760 8,800 0.086 35 149 B l 560 30 530 6,800 0.078 30 145 B2 627 35 592 7,400 0.080 35 170 B3 655 30 625 7,100 0.088 30 134 B4 89 25 64 1,100 0.058 25 102 NOTE: 20°C data based pn one single set of measurements at 100 days after start-up. 202. Table 31. Nitrogen Data for 10°C Systems Digested Sludge Supernatant System Days from Nitrogen Forms as mg/1 N VSS Organic N Nitrogen Forms as mg/1 N Start Kj eldahl NH3 Organic mg/1 g/gVSS NH3 N0 3 F 46 708 - 703 7,965 0.088 - 52 58 633 - 628 7,420 0.085 - 39 65 661 - 656 8,030 0.082 - 21 69 - - - - - - 29 80 650 - 645 8,180 0.080 - 110 86 840 5 835 10,520 0.079 5 107 C l 46 913 - 875 10,600 0.083 - 268 58 1022 - 984 12,100 0.081 - 269 65 1008 - 970 12,800 0.076 - 186 69 -. - - - - - 190 80 1014 - 976 13,820 0.071 - 249 86 1084 38 1046 14,210 0.074 38 232 C2 46 983 - 940 12,200 0.077 - 284 58 1120 - 1077 13,200 0.082 - 188 65 1047 - 1004 13,200 0.076 - 173 69 - - - - - - 125 80 1142 - 1099 14,665 0.075 - 220 86 1170 43 1127 15,015 0.075 43 206 -C3 46 952 - 908 11,600 0.078 - 158 58 1047 - 1003 12,150 0.083 - 141 65 1002 - 958 12,500 0.077 - 140 69 - - - - - - 110 80 1053 - 1009 14,100 0.072 - 192 86 1117 44 1073 14,235 0.075 44 204 203 Table 31 (cont'd). Nitrogen Data for 10°C Systems Digested Sludge Supernatant System Days from Start Nitrogen Forms as mg/1 N VSS Organic N Nitrogen Forms as mg/1 N Kjeldahl NH3 Organic mg/1 g/gVSS NH3 N0 3 B l 46 722 - 689 8,400 0.082 - 165 58 767 - 734 8*540 0.086 - 183 65 708 - 675 8,430 0.080 - 151 69 - - - - - - 108 80 689 - 656 8,555 0.077 - 233 86 610 33 577 7,255 0.080 33 172 B2 46 745 - 707 9,400 0.075 - 178 58 725 - 687 8,400 0.082 - 165 65 666 - 628 8,055 0.078 - 96 69 - - - - - - 110 80 888 - 850 11,210 0.076 - 198 86 770 38 732 9,380 0.078 38 163 B3 46 762 - 737 9,100 0.081 - 143 58 759 - 734 9,100 0.081 - 125 65 781 - 756 9,330 0.081 - 107 69 - - - - - - 87 80 994 - 969 12,575 0.077 - 151 86 930 25 905 11,275 0.080 25 127 46 195 - 170 2,310 0.074 - 150 58 190 - 165 2,060 0.080 - 203 65 179 - 154 1,805 0.085 - 110 69 - - - - - - 118 80 132 - 107 1,580 0.068 - 218 86 126 25 101 1,435 0.070 25 168 204, Table 32. Nitrogen Data for 5°C Systems Digested Sludge Supernatant System Days from Nitrogen Forms as mg/1 N VSS Organic N Nitrogen Forms as mg/1 N Start Kj eldahl NH 3 Organic mg/1 g/gVSS NH 3 N0 3 F 32 627 - 626 7,465 0.084 - 141 40 594 - 593 7,240 0.082 - 107 60 504 - 503 6,305 0.080 - 143 72 706 - 705 9,145 0.077 - 118 78 622 - 621 8,045 0.077 - 101 84 610 1 609 7,780 0.078 1 140 C l 32 1145 - 1099 14,000 0.079 - 225 40 1198 - 1152 14,545 0.079 - 223 60 1268 - 1222 15,420 0.079 - 226 72 1224 - 1178 14,740 0.080 - 207 78 1165 - 1119 14,475 0.077 - 246 84 960 46 914 14,490 0.063 46 184 C2 32 1053 - 1033 13,900 0.074 - 185 40 1098 - 1078 13,535 0.080 - 182 60 1142 - 1122 13,765 0.082 - 178 72 1081 - 1061 13,400 0.079 - 162 78 1058 - 1038 13,275 0.078 - 168 84 1014 20 994 12,770 0.078 20 206 C3 32 913 - 905 11,100 0.082 - 148 40 879 - 871 10,580 0.082 - 181 60 916 - 908 11,130 0.082 - 172 72 941 - 933 12,010 0.078 - 188 78 960 - 952 12,095 0.079 - 222 84 904 8 896 12,000 0.075 8 180 205 Table 32 (cont'd). Nitrogen Data for 5°C Systems Digested Sludge Supernatant System Days from Nitrogen Forms as mg/1 N VSS Organic N Nitrogen Forms as mg/1 N Start Kj eldahl NH3 Organic mg/1 g/gVSS NH3 N0 3 B l 32 874 - 860 10,400 0.083 - 175 40 857 - 843 10,320 0.082 - 148 60 790 - 776 9,620 0.081 - 148 72 790 - 776 9,670 0.080 - 168 78 801 - 787 9,595 0.082 - 171 84 770 14 756 9,390 0.081 14 143 B2 32 832 - 829 10,200 0.081 - 168 40 778 - 775 9,230 0.084 - 157 60 756 - 753 9,215 0.082 - 176 72 770 - 767 9,430 0.081 - 136 78 750 - 747 9,025 0.083 - 165 84 745 3 742 9,560 0.078 3 117 B3 32 977 - 976 12,500 0.078 - 146 40 980 - 979 11,690 0.084 - 118 60 944 - 943 11,250 0.084 - 143 72 958 - 957 12,185 0.079 - 137 78 921 - 920 11,420 0.081 - 120 84 1050 1 1049 13,465 0.078 1 143 B4 32 540 - 507 6,200 0.082 - 168 40 538 - 505 5,890 0.086 - 189 60 476 - 443 5,255 0.084 - 187 72 428 - 395 4,750 0.083 - 147 78 409 - 376 4,485 0.084 - 176 84 384 33 351 4,280 0.082 33 163 Table 33. Summary of Nitrogen Data at Steady-State Digested Sludge Supernatant System Nitrogen Forms as mg/1 N VSS Organic N Nitrogen Forms as mg/1 N Kj eldahl NH3 Organic mg/1 g/gVSS NH3 N0 3 F 515 1 514 6,300 0.082 1 32 C l 697 35 662 8,500 0.078 35 333 C2 865 40 825 10,300 0.080 40 274 20°C C3 795 35 760 8,800 0.086 35 149 B l 560 30 530 6,800 0.078 30 145 B2 627 35 592 7,400 0.080 35 170 B3 655 30 625 7,100 0.088 30 134 F 698 5 693 8,350 0.083 5 60 C l 1008 38 970 12,595 0.077 38 250 C2 1092 43 1049 13,625 0.077 43 199 10°C C3 1034 44 990 12,855 0.077 44 158 B l 699 33 666 8,220 0.081 33 169 B2 759 38 721 9,245 0.078 38 152 B 3 . 845 25 820 10,250 0.080 25 123 F 611 1 610 7,625 0.080 1 125 C l 1160 46 1114 14,100 0.079 46 219 C2 1074 20 1054 13,340 0.079 20 180 5°C C3 919 8 911 11,390 0.080 8 182 B l 814 14 800 9,755 0.082 14 159 B2 772 3 769 9,380 0.082 3 153 B3 972 1 971 11,990 0.081 1 135 In performing the nitrogen balance, i t was assumed that, i n the process of n i t r i f i c a t i o n , where ammonia i s converted to n i t r a t e , v i a the intermediary n i t r i t e , no s i g n i f i c a n t amount of n i t r i t e existed i n solution at any time, because n i t r i t e i s reportedly f a i r l y unstable and i s easily oxidized to ni t r a t e (89) . In any case, no correction was made for n i t r i t e i n the n i t r a t e determina-tions; therefore, i f n i t r i t e was present, i t would have been included i n the n i t r a t e measurements. Figure 68 shows a schematic representation of the nitrogen balance i n aerobic digestion, as assumed for the purposes of th i s study. Nitrogen gas l o s t through d e n i t r i f i c a t i o n could not be measured, but was determined as a result of performing the balance, with measurements made of the Kjeldahl, ammonia and ni t r a t e nitrogen of incoming and outgoing sludges and supernatants. D e n i t r i f i c a t i o n can occur only under conditions of near oxygen depletion (22). Such conditions were available i n a l l digestion units used i n this study. In the continuous feed/automatic decant units, near anaerobic conditions most l i k e l y existed at a l l times i n the s e t t l i n g portion of the units, whereas the d a i l y periods of shutting off a i r to the dai l y feed/manual decant, and batch aeration units (which were on a common a i r supply line) no doubt created limited oxygen conditions i n those units. The results of the nitrogen balances for the various systems, calculated as net amounts of ammonia, n i t r a t e , and nitrogen gas produced per gram of v o l a t i l e suspended solids destroyed are shown 1 Nitrogen gas lo s t Feed Sludge _ Organic N (.08g/gVSS) NH -N NO.-N Mineralization (.08gN/gVSS Destroyed) N i t r i f i c a t i o n D e n i t r i f i c a t i o n Aerobic Digester Digested Sludge • Organic N (.08g/gVSS) NH -N NO^ -N Figure 68 - Nitrogen Balance During Aerobic Digestion 209 i n Figure 69, i n the form of a histogram, and i n Figures 70, 71, and 72, as a function of digester sludge age at the three temperatures studied. At 20°C, the amount of nitrogen l o s t , presumably as through d e n i t r i f i c a t i o n , increased with increasing sludge age. At higher sludge ages, the loss of nitrogen was much more pronounced for the intermittent feed system than for the continuous feed system. The amount of nitrogen l o s t varied from about 12 percent, for both systems at 10 days sludge age, to about 30 percent at 70 days for the continuous feed system and about 45 percent at 35 days for the semi-continuous feed system. The net amount of ammonia remaining decreased s l i g h t l y with increasing sludge age for both systems. In the batch aeration system, approximately 50 percent of the organic nitrogen mineralized during aerobic digestion was l o s t after 100 days aeration, with 10 percent remaining as ammonia. The net reduction of n i t r a t e s , as a function of increased sludge age, i s shown i n Figure 73 for both continuous feed systems, as well as the batch aeration system, operated at 20°C. At 10°C, similar trends can be observed as were described for the 20°C run. There appeared to be a lack of d e n i t r i f i c a t i o n at sludge ages less than 10 days for the semi-continuous feed system, and at sludge ages less than 25 days for the continuous feed systems. However, the degree of d e n i t r i f i c a t i o n reached similar proportions to that at 20°C, once the sludge age had increased to ON •00 r-T - r r H -.vO. VO m' ro .Q. AO. ON- •co-l - r o - t - c v r l ool .PH. ro CO 'CO' v o l 00 in vO ir VP ON ro vO C N m _oo_ • -<r -VD-- v o l Temp. (°C) S.A. (Days) C N 00 m ON m C N CONTINUOUS FEED / AUTOMATIC DECANT ON m o C N ON m CO vO CO m oo DAILY FEED / MANUAL DECANT BATCH I AERATION I Figure 69 - Histogram of Nitrogen Forms Expressed i n mg N / g VSS Destroyed. T T O Continuous Feed / Automatic Decant • Daily Feed / Manual Decant • Batch Aeration NO. NO. _L 60 70 80 90 DIGESTER SLUDGE AGE (DAYS) 100 Figure 70 - Nitrogen Balance @ 20°C. Figure 71 - Nitrogen Balance @ 10°C. Continuous Feed / Automatic Decant Daily Feed / Manual Decant Batch Aeration O 60 70 80 90 DIGESTER SLUDGE AGE (DAYS) 1 1 1 O (185.43) 1 1 O l l l l Continuous Feed / Automatic decant • Daily Feed / Manual Decant • Batch Aeration 1 0 20 30 4 0 5 0 6 0 70 80 9 0 DIGESTER SLUDGE AGE (DAYS) Figure 72 - Nitrogen Balance @ 5°C. Figure 73 - Nitrate Nitrogen as a Function of Digester Sludge Age @ 20°C. 35 and 70 days for the semi-continuous and continuous feed systems, respectively. The amount of ammonia decreased with increasing sludge age for both systems, as i t did at 20°C, but the ammonia levels i n the semi-continuous feed systems were consistently lower than those i n the continuous feed systems, unlike the situation at 20°C, where the l e v e l i n both systems was similar at any sludge age. In the batch aeration system, very l i t t l e d e n i t r i f i c a t i o n occurred at 10°C, while n i t r i f i c a t i o n was almost complete at 87 days (about 85 percent of a l l nitrogen mineralized). The l e v e l of nitrates i n the systems, as a function of sludge age, i s shown i n Figure 74 for the 10°C run. At 5°C, the sit u a t i o n changed quite d r a s t i c a l l y , at least for the semi-continuous feed system and batch aeration system. For these systems, d e n i t r i f i c a t i o n appeared to be more advanced than at either of the two higher temperatures, and appeared to be greatest at the lowest sludge age. For the continuous feed system, no d e n i t r i f i c a t i o n occurred at sludge ages less than 20 days, after which the l e v e l of d e n i t r i f i c a t i o n slowly increased with increasing sludge age, always being below the l e v e l of d e n i t r i f i c a t i o n at the two higher temperatures. A very curious result was obtained for the continuous feed system at 8 days sludge age, where the l e v e l of n i trates measured i n s i x independent samples, throughout the steady-state period, greatly exceeded the amount of nitrogen mineralized. This cannot be explained, and even though this result represents a large number of measurements, i t must be considered as being not representative. Figure 74 - Nitrate Nitrogen as a Function of Digester Sludge Age @ 10°C. 217 Ammonia concentrations remaining i n the system at 5°C increased with increasing sludge age, which, again, i s opposite to what happened at 20°C and 10°C. The n i t r a t e l e v e l s , as a function of sludge age, are shown i n Figure 75 for the systems operated at 5°C. It appears from the nitrogen balance results discussed previously, that, of the three digester types studied, the continuous feed, automatic decant systems behaved most closely to generally accepted n i t r i f i c a t i o n - d e n i t r i f i c a t i o n models. In these systems, n i t r i f i -cation, as well as d e n i t r i f i c a t i o n , decreased with decreasing tempera-ture and sludge age. The degree of n i t r i f i c a t i o n , as approximated by the l e v e l of ammonia remaining, was roughly 90 percent at 20°C, 85 percent at 10°C, and 75 percent at 5°C, whereas the degree of d e n i t r i f i c a t i o n and resultant loss of nitrogen from the system reached up to 30 percent at 20°C, 25 percent at 10°C, and up to 15 percent at 5°C. In the daily feed, manual decant systems, very l i t t l e difference i n the degree of n i t r i f i c a t i o n and d e n i t r i f i c a t i o n existed at 20°C and 10°C, with n i t r i f i c a t i o n levels being roughly 85 percent, and d e n i t r i f i c a t i o n levels being as much as 45 percent. At 5°C, these systems demonstrated a markedly higher degree of both n i t r i f i c a t i o n and d e n i t r i f i c a t i o n , up to 100 percent and 80 percent respectively, with such levels decreasing at increased sludge ages. This i s p a r t i c u l a r l y unusual when considering the f a i r l y normal results of the continuous feed systems, with a l l methods of analysis and data O (161.76) 80 70 60 50 40 \-30 20 10 O Continuous Feed / Automatic Decant • Daily Feed / Manual Decant • Batch Aeration 10 20 30 40 50 60 70 80 90 DIGESTER SLUDGE AGE (DAYS) Figure 75 - Nitrate Nitrogen as a Function of Digester Sludge Age @ 5°C. OO interpretation being i d e n t i c a l for both types of systems. A l l temperature runs showed great differences between the levels of d e n i t r i f i c a t i o n of the continuous and semi-continuous feed systems; i t must be concluded, therefore, that this difference i n digester operation must greatly affect the l e v e l of d e n i t r i f i c a t i o n occurring. Possibly, shutting off the a i r for several hours daily i n the semi-continuous digesters provided a better environment for deni-t r i f i c a t i o n than the s e t t l i n g tank i n the continuous digesters. A possible reason for the semi-continuous systems showing a higher degree of n i t r i f i c a t i o n at lower temperatures i s the fact that the nit r a t e content i n the feed sludge increased, from a low of 32 mg/1 N during the 20°C run, to a high of 125 mg/1 N during the 5°C run. This indicates that the feed sludge used i n the lower tempera-ture runs would contain much higher populations of n i t r i f i e r s than that used i n the higher temperature runs. Since feeding occurred i n r e l a t i v e l y large amounts, once d a i l y , i n the semi-continuous feed units, the n i t r i f i e r s were comparatively better able to compete with other types of bacteria i n the low temperature runs than i n the high temperature runs. This would also explain why the degree of n i t r i f i c a t i o n could be higher at the lower sludge ages, as the feed sludge quantities added to the digester are highest at the low sludge age. A reason why both n i t r i f i c a t i o n and d e n i t r i f i c a t i o n i n the semi-continuous feed systems reached increasing levels at decreasing temperatures, may be the fact that the steady-state pH l e v e l increased as temperatures decreased (see Figure 79). The behaviour of the batch aeration systems, with respect to n i t r i f i c a t i o n and d e n i t r i f i c a t i o n , may be explained i n somewhat the same way. I n i t i a l solids concentrations i n the 20°C and 10°C units were almost the same, whereas the solids concentration i n the 5°C unit was almost double; also, the feed sludge used to f i l l the 5°C batch unit probably contained a much higher proportion of n i t r i f i e r s , as indicated by the much higher n i t r a t e concentration i n the sludge used to f i l l the l a t t e r unit. The higher l e v e l of d e n i t r i f i c a t i o n i n the 5°C batch aeration unit, as compared to that at 20°C and 10°C, may be attributed to the higher pH i n that unit. Nevertheless, I t should be noted that the steady-state pH of a l l units was at or below 5.5, and the f i n a l pH of the batch aeration units was at or below 4.0. I t appears, from these r e s u l t s , that n i t r i f i c a t i o n and d e n i t r i f i c a t i o n does proceed s i g n i f i c a n t l y at these pH values, even though optimum pH values have been reported to be i n the 7-9 range (148). The foregoing discussion of n i t r i f i c a t i o n and d e n i t r i f i c a t i o n has been based on the assumption that the unknown term, i n the nitrogen balance calculations, consisted of nitrogen gas released to the atmosphere. As far as the author could determine, t h i s assumption i s generally supported i n the l i t e r a t u r e (22, 90, 148). The re s u l t s , however, p a r t i c u l a r l y at low temperatures, indicate trends contrary to those published by others. As nitrogen measurements i n this research cannot be considered s u f f i c i e n t l y complete, the p o s s i b i l i t y 221 must be recognized that compounds, other than those measured, may have had a s i g n i f i c a n t influence on the nitrogen balance. Ba r r i t (14), showed that n i t r i f i c a t i o n may be inhibited by accumulated CO^ and/or NH^+, or by i n s u f f i c i e n t aeration. He proposed a c y c l i c reaction involving nitrous acid, which would cause a continued drop i n pH, but would not produce additional n i t r a t e s . Nitrous acid was not measured i n the present work, and, i f t h i s c y c l i c reaction i n effect occurs, then this would influence the conclusions reached i n this.work. Although there i s a certain amount of uncertainty about the nitrogen r e s u l t s , i t was f e l t that most of the results are both important and pertinent and can be used to form certain conclusions, subject to the caution expressed e a r l i e r , or may form the basis for further research i n this area. 4. Viable Bacteria i n Digested Sludge Viable bacteria plate counts were conducted under steady-state conditions, with incubation of the agar plates both at the standard temperature of 20°C and at the temperature under investigation, i . e . , 10°C and 5°C. The results of these plate counts are shown i n Tables 34, 35, and 36 for the 20°C, 10°C, and 5°C runs respec-t i v e l y . A summary of the viable bacteria counts at these tempera-tures i s shown i n Table 37, which consists of arithmetic average values of the steady-state measurements shown i n the previous tables. Table 34. Viable Bacteria Counts for 20°C Systems Days Bacteria l VSS mg/1 Bact e r i a l System from Start Count no./ml Count no./mgVSS F 60 7.3xl0 8 7,700 9.5xl0 7 79 2.2xl0 8 9,670 2.3xl0 7 98 2.3xl0 8 5,660 4.1xl0 ? C l 60 1.4x108 11,500 1.2xl0 7 79 2.6xl0 8 11,000 2.4xl0 7 98 4.1xl0 8 8,560 4.8xl0 7 C2 60 1.7xl08 10,200 1.7xl0 7 79 3.5xl0 8 11,030 3.2xl0 7 98 6.1xl0 8 10,900 5.6xl0 7 C3 60 >3.0xl08 7,700 >3.9xl0 7 79 1.9xl0 8 8,700 2.2xl0 7 98 1.4xl0 8 8,200 1.7xl0 7 B l 60 79 1.7xl0 8 5.7xl0 8 7,000 7,500 2.4xl0 7 7.6xl0 7 98 5.4xl0 8 6,900 7.8xl0 7 B2 60 3.0xl08 8,000 3.8xl0 7 79 4.9xl0 8 8,700 5.6xl0 7 98 3.0xl0 8 7,800 3.8xl0 7 B3 60 79 >3.0xl0 8 2.5xl0 8 7,700 7,400 >3.9xl0 7 3.4xl0 7 98 1.4xl0 8 7,400 1.9xl0 7 B4 60 79 7.4xl0 6 4.3xl0 6 1,630 1,300 4.5xl0 6 3.3xl0 6 98 3.6xl0 6 1,100 3.3xl0 6 Table 35. Viable Bacteria Counts  for 10°C Systems Bacteria l Counts System Days from VSS Incubated at 20°C Incubated at 10°C Start mg/1 no./ml no./mgVSS no./ml no./mgVSS F 41 7,725 4.3xl0 8 5.6xl0 7 1.2xl0 8 1.5xl0 7 65 8,030 4.4xl0 8 5.5xl0 7 2.0xl0 8 2.5xl0 7 84 7,600 3.2xl0 8 4.3xl0 7 8.8xl0 7 1.2xl0 ? C l 41 65 9,420 12,860 3.9xl0 8 5.7xl0 8 4.2xl0 ? 4.4xl0 7 3.8xl0 7 2.9xl0 8 4.OxlO6 2.3xl0 7 84 14,100 6.1xl0 8 4.3x107 1.5xl0 8 l . l x l O 7 C2 41 65 11,200 13,235 4.0xl0 8 7.3xl0 8 3.6xl0 7 5.5xl0 7 6.6xl0 7 2.2xl0 8 5.9xl0 6 1.7xl0 7 84 14,950 6.1x108 4.1xl0 7 1.3xl0 8 8.5xl0 6 C3 41 11,350 4.8xl08 4.2xl0 7 4.8xl0 7 4.2xl0 6 65 12,590 5.4xl0 8 4.3xl0 7 1.9xl0 8 1.5xl0 7 84 14,200 3.5xl0 8 2.5xl0 7 8.9xl0 7 6.3xl0 6 . B l 41 65 8,600 8,430 3.0xl0 8 1.4xl0 9 3.5xl0 7 1.7xl0 8 6.5xl0 ? 2.7xl0 8 7.6xl0 6 3.2xl0 7 84 7,350 2.8xl0 8 3.7xl0 7 l.OxlO 8 1.4xl0 7 B2 41 65 9,500 8,055 4.5xl0 8 1.8xl0 8 4.7xl0 7 2.2xl0 7 6.5xl0 7 1.3xl0 8 6.8xl0 6 1.6xl0 7 84 9,500 4.5xl0 8 4.7xl0 7 1.2xl0 8 1.3xl0 7 B3 41 65 9,400 9,330 4.4xl0 8 3.6xl0 8 4.7xl0 7 3.9xl0 7 9.9xl0 7 1.7xl0 8 l . l x l O 7 1.9xl0 7 84 11,500 5.3xl0 8 4.6xl0 7 l . l x l O 8 9.7xl0 6 B4 41 65 2,330 1,805 2.3xl0 7 2.2xl0 7 9.7xl0 6 1.2xl0 7 3.6xl0 6 1.8xl0 7 1.5xl0 6 l.OxlO 7 84 1,460 1.8xl0 7 1.3xl0 7 l . l x l O 7 7.3xl0 6 Table 36. Viable Bacteria Counts  for 5°C Systems System Days from Start VSS mg/1 Bact e r i a l Counts Incubated at 20°C Incubated at 5°C no./ml no./mgVSS no./ml no./mgVSS F 49 72 81 8,160 9,145 8,330 4.5xl0 8 6.3xl0 8 9 1.1x10 5.5xl0 7 6.9xl0 7 1.4xl0 8 l . l x l O 8 1.9xl0 8 2.5xl0 8 1.4xl0 7 2.1xl0 7 3.0xl0 7 C l 49 72 81 14,815 14,740 14,300 1.5xl0 8 3.5xl0 8 5.4xl0 8 l.OxlO 7 2.3xl0 7 3.7xl0 7 4.7xl0 7 7.7xl0 7 1.3xl0 8 3.1xl0 6 5.2xl0 6 9.1xl0 6 C2 49 72 81 13,250 13,400 13,050 3.8xl0 8 5.9xl0 8 7.6xl0 8 2.9xl0 7 4.4xl0 7 5.8xl0 7 6.8xl0 7 1.2xl0 8 2.4xl0 8 5.1xl0 6 9.0xl0 6 1.9xl0 7 C3 49 72 81 10,890 12,010 11,900 4.5xl0 8 8.6xl0 8 9.1xl0 8 4.1xl0 7 7.2xl0 7 7.7xl0 7 9.4xl0 7 1.2xl0 8 2.4xl0 8 8.6xl0 6 9.9xl0 6 2.0xl0 7 B l 49 72 81 9,910 9,670 9,600 2.7xl0 8 4.2xl0 8 7.5xl0 8 2.7xl0 7 4.3xl0 7 7.8xl0 7 4.6xl0 7 1.3xl0 8 1.6xl0 8 4.7xl0 6 1.3xl0 7 1.7xl0 7 B2 49 72 81 9,400 9,430 9,400 5.0xl0 8 7.4xl0 8 8.4xl0 8 5.3xl0 7 7.8xl0 7 9.0xl0 7 7.8xl0 7 1.2xl0 8 1.9xl0 8 8.3xl0 6 1.3xl0 7 2.1xl0 7 B3 49 72 81 12,090 12,185 11,800 4.8xl0 8 7.3xl0 8 1.3xl0 9 3.9xl0 7 6.0xl0 7 l . l x l O 8 1.2xl0 8 2.0xl0 8 2.4xl0 8 9.5xl0 6 1.7xl0 7 2.lxlO 7 B4 49 72 81 5,430 4,750 4,400 l.OxlO 8 4.7xl0 7 2.7xl0 7 1.9xl0 7 9.8xl0 6 6.2xl0 6 1.4xl0 7 5.0xl0 6 l.OxlO 7 2.7xl0 6 l.OxlO 6 2.4xl0 6 Table 37. Summary of Viable Bacteria  at Steady-State Counts Sludge Bac t e r i a l Counts (no. /mgVSS) System Age Incubated Incubated Incubated (Days) at 20°C at 10°C at 5°C F 0 5.3xl0 7 k / C l 69.2 2.8xl07 \ / C2 30.9 3.5x10 20°C 10.1 2.6xl0 7 3 7 B l 32.6 5.9x10 17.7 4.4x10 2 ~7 B3 7.8 3.1x10 F 0 5.1x107 1.7xl0 7 C l 53.8 4.3xl07 1.3xl0 7 C2 26.7 4.4xl07 l.OxlO 7 10°C C3 12.4 3.7xl07 8.4xl0 6 \ 32.4 7.9xl0 7 1.8xl0 7 X B2 16.7 3.9xl07 1.2xl0 7 B3 7.9 4.4xl07 1.3xl0 7 F 0 8.7xl0 7 2.1xl0 7 C l 53.5 2.4xl07 5.8xl0 6 C2 21.2 4.4xl07 l . l x l O 7 5°C C3 8.3 6.3xl07 1.3xl0 7 B l 29.9 4.9xl07 1.2xl0 7 B2 13.2 7.4xl07 1.4xl0 7 B3 8.5 7.lxlO7 1.6xl0 7 These b a c t e r i a l counts are graphically shown as a function of digester sludge age i n Figures 76, 77, and 78. It i s interesting to note that at a l l temperatures, the i n t e r -mittent feed systems showed a consistently higher number of viable bacteria per milligram of VSS than the continuous feed systems, whether the plates were incubated at 20°C or at the temperature of digestion. This may well help to explain the higher l e v e l of d e n i t r i f i c a t i o n which was observed i n the semi-continuous feed systems, as compared to that i n the continuous feed systems. At 20°C and 10°C, the number of viable bacteria per unit mass increased s l i g h t l y with increasing sludge age, after a s l i g h t drop from the number i n the feed sludge; this was the case for the continuous as well as the semi-continuous feed systems. This would indicate that, for those systems, aerobic digestion resulted i n the death of bacteria associated with the v o l a t i l e suspended solids that were destroyed; however, some regrowth and reproduction occurred, probably on account of the d e n i t r i f i e r s . At 5°C there was a dramatic drop i n numbers of viable bacteria per unit mass as sludge age increased for the continuous and semi-continuous feed systems; this indicates that there was a substantial die-off of bacteria associated with the sludge solids remaining. The l a t t e r , however, tends to contradict the observed increased d e n i t r i f i c a t i o n at 5°C. 200 100 90 80 70 60 50 40 30 20 10 9 8 7 6 5 4 3 2 1 T O Continuous Feed/ Automatic Decant • Daily Feed / Manual Decant • Batch Aeration Incubation Temperature: 20°C J I I 1 I I I I I I 20 30 40 50 60 70 80 90 100 DIGESTER SLUDGE AGE (DAYS) Viable Bacteria Counts, 20°C Systems 228 Figure 77 - Viable Bacteria Counts, 10°C Systems 200 100 90 80 70 60 50 40 30 20 10 9 8 7 6 5 4 3 2 1 ( F: 229 ° Continuous Feed / Automatic Decant n Daily Feed / Manual Decant • Batch Aeration Incubation Temperature: 20°C Incubation Temperature: 5°C 50 60 70 80 90 100 DIGESTER SLUDGE AGE (DAYS) Viable Bact e r i a Counts, 5°C Systems 230 The batch aeration systems, i n general, demonstrated a sharp decrease i n numbers of viable bacteria per unit mass, also indicating a substantial die-off of bacteria associated with the sludge solids remaining. Two incubation temperatures were investigated for the two lower temperature runs, with the object of determining whether there may have been a s h i f t i n proportion of psychrophilic bacteria r e l a t i v e to the mesophilic bacteria with increased sludge age, i . e . , whether there would have been acclimatization of the b a c t e r i a l population to the colder temperatures. For the continuous and semi-continuous feed systems, the proportion of viable bacteria able to grow at the temperature of digestion, to those able to grow at 20°C, was p r a c t i c a l l y constant for a l l sludge ages investigated, indicating no appreciable degree of acclimatization of the sludge bacteria. In the batch aeration systems, on the other hand, where longer detention times were possible, the proportion of viable bacteria able to grow at the digestion temperatures of 10°C and 5°C, to those able to grow at 20°C, increased quite dramatically at the longer detention time, indicating a considerable degree of acclima-t i z a t i o n i n the batch aeration systems. 231 5. V o l a t i l e Fraction of Digested Sludge In observing the suspended solids data presented i n Tables 1 to 21, i t i s apparent that the v o l a t i l e f r a c t i o n of the suspended solids decreased somewhat with increasing sludge age during aerobic digestion. As the decrease i n v o l a t i l e content was much less than the percentage v o l a t i l e suspended solids reduction i n the sludge mass, i t i s concluded that a s i g n i f i c a n t amount of fixed suspended solids reduction occurred, as w e l l , during aerobic digestion. This has been recognized by others (19, 105), and has been a t t r i -buted to the fact that, upon c e l l l y s i s , organic compounds from within the c e l l are released into solution. It has been suggested (19) that, because of the destruction of fixed suspended solids as well as v o l a t i l e suspended s o l i d s , t o t a l suspended solids should be used i n any k i n e t i c analysis of solids destruction during aerobic digestion. However, as shown by the results on v o l a t i l e content of sludge mass i n this study, the f r a c -t i o n of fixed suspended solids i s dependent on the physiological state of the sludge mass ( i . e . , the f r a c t i o n of fixed suspended solids becomes greater as sludge age increases), prior to digestion as well as during digestion. I t i s f e l t by this author that the use of v o l a t i l e suspended s o l i d s , i n the k i n e t i c analysis of aerobic digestion, has the inherent advantage of being independent of the physiological state of the sludge, resulting i n a more universally applicable analysis. The reduction of fixed suspended 232 solids can be taken into account once the v o l a t i l e suspended solids reduction has been established, by applying^a factor related to the fra c t i o n of fixed suspended solids expected at the applicable system sludge age. C. SUPERNATANT CHARACTERISTICS Digester supernatant characteristics were measured i n terms of pH, t o t a l dissolved solids (TDS), v o l a t i l e dissolved solids (VDS), COD, TOC, and BOD,, for the steady-state continuous feed systems, as well as for the batch aeration systems at the temperatures studied. Tables 38, 39, and 40 show summaries of the steady-state supernatant data, obtained by averaging ind i v i d u a l measurements during the steady-state periods. They also show indiv i d u a l measurements for the batch aeration systems. The steady-state and batch aeration pH data are shown graphically i n Figure 79, whereas the other supernatant data are shown graphically i n Figures 80, 81, and 82 for the three temperatures studied. The results show a f a i r l y similar system response, with respect to pH, for a l l systems. The feed sludge pH varied between 6.5 and 7.3 during the course of the study. For the continuous feed systems, the pH decreased steadily as the systems progressed towards steady-state conditions. At steady-state, pH decreased with increasing sludge age, with no noticeable difference between the two continuous feed methods. Table 38.. Summary of Steady-State Supernatant Data at 20°C System Sludge TDS VDS COD TOC B0Dc Age 5 (Days) (mg/1) (mg/1) (mg/1) (mg/1) (mg/1) F 0 380 255 118 47 64 C l 69.2 1,848 1,178 770 309 34 C2 30.9 1,308 790 585 214 36 C3 10.1 675 400 270 113 41 B l 32.6 960 623 555 167 55 B2 17.7 850 503 421 156 30 B3 7.8 615 358 259 109 145 61 330 260 140 55 -69 - - 140 55 -Batch 71 - - - - -Aeration 77 - - 125 51 7.2 100 - - 140 56 -101 470 155 - - -Table 39. Summary of Steady-State Supernatant Data at 10°C System Sludge TDS VDS COD TOC B0Dc Age 5 (Days) (mg/1) (mg/1) (mg/1) (mg/1) (mg/1) F 0 380 280 111 45 59 C l 53.8 1,180 708 486 214 26 C2 26.7 825 573 388 182 38 C3 12.4 551 358 242 118 36 B l 32.4 878 618 205 108 16 B2 16.7 793 575 155 99 32 B 3 7.9 586 396 119 82 30 22 350 210 - - -39 - - 119 51 40 650 215 - - -Batch 54 - - - - 17 Aeration 60 450 120 - - -65 - - 154 54 4 81 - - 103 45 6 86 485 345 - - 6 Table 40. Summary of Steady-State Supernatant Data at 5°C System Sludge TDS VDS COD TOC B0Dr Age 5 (Days) (mg/1) (mg/1) (mg/1) (mg/1) (mg/1) F 0 375 315 129 55 34 C l 53.5 908 578 367 135 39 C2 21.2 655 348 278 102 60 C3 8.3 537 340 267 109 79 B l 29.9 442 340 222 85 48 B2 13.2 393 252 262 105 86 B3 8.5 498 298 284 121 130 31 335 245 - - -32 - - 119 - 40 Batch 40 - - 193 - 35 Aeration 60 - - 214 78 24 72 330 165 188 78 10 78 - - 167 65 9 84 525 370 136 57 5 236 1200 1100 1000 g 800 O Continuous Feed / Automatic Decant D Daily Feed / Manual Decant • Batch Aeration 50 60 70 80 DIGESTER SLUDGE AGE (DAYS) Figure 80 - Digester Supernatant Characteristics @ 20°C Figure 82 - Digester Supernatant Characteristics @ 5°C In the batch aeration systems, the pH generally decreased steadily with increased time of digestion. In the 20°C batch system, there appeared to be an i n i t i a l r i s e i n pH, after which there was a steep drop. This has been reported by others, and was also evident i n the continuous feed systems at 20°C (see Figures 25 to 30). It i s apparent from Figure 79 that the minimum pH reached i n a l l systems was lowest at higher digestion temperatures, and was lowest i n the batch aeration systems. Minimum pH values were 4.0, 4.3, and 5.2 for the continuous feed systems at 20°C, 10°C, and 5°C respectively, and were 3.5, 4.0, and 4.2 for the batch aeration systems at 20°C, 10°C, and 5°C respectively. The drop i n pH during aerobic digestion has been attributed to the consumption of a l k a l i n i t y and production of carbonic acid during the n i t r i f i c a t i o n process (22) . D e n i t r i f i c a t i o n would restore part of the a l k a l i n i t y destroyed during n i t r i f i c a t i o n , and result i n p a r t i a l l y o f f s e t t i n g the drop i n pH (22). The remaining supernatant characteristics are shown i n Figures 80, 81, and 82 for digestion temperatures of 20°C, 10°C, and 5°C respectively. The results show a s t r i k i n g difference i n supernatant characteristics of the continuous feed systems and the batch aeration systems, particu-l a r l y at 20°C and 10°C. The continuous feed systems showed increased s o l u b i l i z a t i o n of organic materials with increasing sludge age, as indicated by the v o l a t i l e dissolved solids (VDS), soluble t o t a l organic carbon (TOC), and soluble chemical oxygen demand (COD) data. The batch digesters did not show any appreciable degree of s o l u b i l i z a t i o n of organic materials at any of the temperatures studied. At 10°C and 5°C, there was even some indication that some soluble material disappeared from the supernatant, presumably by assimilation into the c e l l mass. The soluble organics accumulating i n the continuous feed systems were apparently non-biodegradable, as indicated by the soluble BOD,, data. At 20°C, both types of continuous feed systems showed a similar response except i n terms of soluble BOD. The reason for the fluctuating BOD i n the semi-continuous feed system may be attributed to the intermittent feeding procedure. As temperature decreased, the behaviour of the continuous feed system remained approximately the same; however, the semi-continuous feed system demonstrated a lesser degree of s o l u b i l i z a t i o n of organics than the continuous feed system, but s t i l l appreciably higher than the batch system. D. EVALUATION OF FULL-SCALE DATA In order to provide a means of comparison between the results generated on a laboratory scale, three sets of f u l l - s c a l e data have been analyzed. 1. Data collected over an 18-month period at 7 aerobic digester i n s t a l l a t i o n s i n the Province of Ontario, by the Ontario Ministry of the Environment (see References 3 and 4). 2. Data collected over a 13-month period at the Denver, Colo., Metro-polit a n Sewage Treatment Plant (see Reference 38). 3. Data collected over a 13-month period at the Town of Smithers, B. Sewage Treatment Plant. The author spent 10 days at t h i s plant i n early 1975, to set up the monitoring program, and then used the operating data collected by the plant operator (see Reference 128) Summaries of the solids destruction data for the Smithers and Denver plants are shown i n Tables 41 and 42. Solids destruction data for the Ontario plants were too sparse, and have therefore not been used i n this analysis. Figure 83 shows the f u l l - s c a l e solids destruction data plotted against the product of temperature and sludge age. The s o l i d curve represents the laboratory data for this relationship (see Figure 12). At the lower temperatures and sludge ages, the f u l l - s c a l e data conform f a i r l y w e l l with the laboratory curve; however, at higher temperatures and sludge ages, the f u l l - s c a l e data indicate higher VSS reductions. The Denver data represent a combination of higher temperatures (16-28°C) and lower sludge ages, whereas the Smithers data points represent a combination of lower temperatures (5-18°C) and higher sludge ages. Although there i s an appreciable difference between the laboratory and f u l l - s c a l e curves, i t may be s i g n i f i c a n t that the point of deflection i n both curves i s located at approximately the same product value of temperature and sludge age of between 150 and 250, supporting the Table 41. Summary of Smithers STP Digester Operating Data (1975-76) (From Reference 128) Avg. Temp. V o l a t i l e Suspended Solids Sludge Age Period Influent Digester Effluent Destroyed (°C) (kg) (kg) (kg) (kg) (%) (Days) Feb-Mar/75 (54 days) 6.5 9,776 4,969 6,638 3,138 32.1 40.4 April/75 (30 days) 9.8 4,840 5,429 3,020 1,820 37.6 53.9 May/75 (31 days) 12.0 6,863 6,871 3,495 3,367 49.1 61.0 June/75 (30 days) 15.0 5,796 5,152 3,219 2,579 44.5 48.1 July/75 (31 days) 18.5 5,087 5,788 2,549 2,537 49.9 70.4 Aug-Sept/75 (61 days) 17.0 10,462 5,669 5,301 5,161 49.3 65.4 October/75 (31 days) 15.0 5,297 7,384 2,468 2,829 53.4 92.9 November/75 (30 days) 9.0 6,624 6,924 4,149 2,475 37.4 50.0 December/75 (31 days) 8.0 6,778 6,063 2,043 4,735 69.9 92.1 Jan-Feb/76 7.0 9,771 6,085 6,829 2,942 30.1 51.0 Table 42. Summary of Denver STP Digester  Operating Data (1972-73)  (From Reference 38) Period Avg. Temp. (°C) Sludge Age (Days) VSS Destroyed (%) August/72 28.0 5.6 39.8 September/72 28.7 6.9 47.0 October/72 25.1 10.9 47.2 November/72 22.0 13.4 46.2 December/72 18.4 4.3 16.7 January/73 16.4 3.1 11.2 February/73 15.8 2.7 22.4 March/73 17.1 3.7 18.1 April/73 15.9 29.8 41.5 May/73 20.2 18.2 45.8 June/73 24.3 8.6 39.6 July/73 27.6 4.3 19.8 August/73 28.5 3.3 20.2 0 200 400 600 800 1000 1200 1400 1600 1800 2000 TEMPERATURE (°C) X SLUDGE AGE (DAYS) Figure 83 - Combined Effect of Sludge Age and Temperature on VSS Reduction Correlation of Full-Scale Data with Laboratory Data 246 characteristic temperature and sludge age combination for optimum VSS reduction found for the laboratory systems. The author cannot explain with certainty why such a difference i n VSS reductions was found between the laboratory and f u l l - s c a l e r e s u l t s . I t i s doubtful, however, that scale alone i s a s i g n i f i c a n t factor. I t i s the author's experience that solids destruction i n f u l l - s c a l e f a c i l i t i e s may sometimes be overestimated, mainly, because i n most plants, no adequate monitoring exists of the solids leaving the digester i n the supernatant. I t i s not known how well this source of solids was monitored i n the Denver study. In the evaluation of the Smithers data, however, no continuous measurements of this source were available, and the author assumed an average concentration of 400 mg/1 VSS i n the super-natant, based on infrequent grab samples. At a sludge wasting rate of, say 5% of sludge return flow i n the plant, this amounts to a substantial loss of soli d s . An underestimation of this amount could increase the quantity of VSS destroyed considerably. Endogenous decay rates for the Smithers and Denver data are determined (as accurately as possible) i n Figures 84 and 85 respectively, using the same method as for the laboratory data. Both sets of data were s p l i t into two temperature groups. The endogenous decay rates calculated from the plots were then plotted i n Figure 86, which also contains the decay rates obtained i n the laboratory systems (refer to Figure 24 and discussion on p. 135-136). 2.4 -2.2 -2.0 1.8 g° 1.6 1.4 1.2 1.0 M /M = 1 + k . t ' o t d k = 0.0092 d (day" 1) y k = 0.0315 (day ) ^ k d= 0.0107 (day 1) A A Average Temp. 8.0°C A Average Temp. 15.5°C 10 20 30 40 50 60 70 80 DIGESTER SLUDGE AGE (DAYS) 90 Figure 84 - Smithers STP, Determination of Endogenous Decay Rates - VSS Basis 1 1 1 1 I I I L 0 5 10 15 20 25 30 35 DIGESTER SLUDGE AGE (DAYS) Figure 85 - Denver STP, Determination of Endogenous Decay Rates - VSS Basis 0.10 0.05 0.04 0.03 0.02 0.01 0.005 0.004 0.003 0.002 0.001 A A Denver Data (Full-Scale) N \ • Smithers Data (Full-Scale) \ \ ° Continuous Feed / Automatic Decant (Lab) ^ • Daily Feed / Manual Decant (Lab) \ • Batch Aeration (Lab) • I n i t i a l Endogenous Decay Rates Secondary Endogenous Decay Rates A Values for Q = 2 I I \ 25 20 15 TEMPERATURE (°C) 10 Figure 86 - Effect of Temperature on I n i t i a l and Secondary Endogenous Decay Rates. VSS Basis Correlation of Full-Scale Data with Laboratory Data The f u l l - s c a l e results for " i n i t i a l " decay rates, plotted i n Figure 86, conform reasonably well with those generated on a laboratory scale. Both f u l l - s c a l e plants were operated on the once da i l y feed, intermittent decant basis. The "secondary" decay rates for the f u l l - s c a l e and laboratory data show a greater discrepancy; however, as pointed out e a r l i e r , the i n i t i a l decay rates are the s i g n i f i c a n t ones, from an over a l l process ki n e t i c s point of view. Based on the laboratory results and f u l l - s c a l e evaluations, Table 43 i s an attempt to summarize "ranges" within which i n i t i a l k^ values for aerobic digestion of waste activated sludge may be expected to l i e . Table 44 then presents a summary of average temperature s e n s i t i v i t y c o e f f i c i e n t s , 9, based on these re s u l t s . The f u l l - s c a l e results confirm the intermittent feed laboratory r e s u l t s , which indicate increased temperature s e n s i t i v i t y with decreasing temperature. Ful l - s c a l e oxygen uptake rate data from the Denver Study (38) and the Ontario Ministry of the Environment Study (3) are plotted i n Figure 87; for comparison, the laboratory curves for the continuous feed systems are plotted also. These data indicate a higher a c t i v i t y of the digested sludge under f u l l - s c a l e conditions than found i n the laboratory units. This may be due to a higher i n i t i a l a c t i v i t y of the raw sludge i n the f u l l - s c a l e plants, or may be due to the previously discussed overestimate of solids destruction i n f u l l - s c a l e f a c i l i t i e s ; t h is would result i n a proportionally higher calculated digester sludge age. In this respect i t i s interesting to note that the Ontario curves reach p r a c t i c a l l y the same minimum oxygen uptake rate as the laboratory curves at high sludge age. 251 Table 43. I n i t i a l Endogeneous Decay Rate Constants for Waste  Activated Sludge, Daily Feed/Manual Decant Mode  (VSS Basis) Temperature (°C) Maximum Sludge Age (days) I n i t i a l k^, Subject to Maximum Sludge Age Range (days ^) Average (days "*") 25 10.0 0.055 - 0.076 0.066 20 12.5 0.038 - 0.070 0.054 15 16.7 0.027 - 0.044 0.036 10 25.0 0.014 - 0.020 0.017 5 50.0 0.007 - 0.010 0.0085 Table 44. Comparison of Streeter-Phelps Temperature S e n s i t i v i t y  Coefficient, 0, for Endogeneous Decay Rate (VSS Basis) Temperature Range (°c) Daily Feed, Manual Decant Mode Batch Aeration Mode (Lab Scale) Lab Scale (128) Smithers Full-Scale (38) Denver Full-Scale 16.0 - 25.0 8.0 - 15.5 10.0 - 20.0 5.0 - 10.0 1.074 1.156 1.155 1.016 1.120 1.113 i 1 1 1 1 1 1 I i r DIGESTER SLUDGE AGE (DAYS) Figure 87 - Comparison of Full-Scale Oxygen Uptake Rate Data with Laboratory Data 253 V. CONCLUSIONS Based on the results of this experimental work, using waste activated sludge and three temperatures of digestion, the following conclusions can be drawn: A. AEROBIC DIGESTION KINETICS 1. For the temperature range studied, there appears to be an interesting and good correlation between VSS reduction and the product of sludge age and temperature. This relationship indicated that l i t t l e additional VSS reduction can be expected when this product value exceeds approximately 250, giving r i s e to the concept of p r a c t i c a l , optimum VSS reduction and i n i t i a l k-value. Although f u l l - s c a l e results did not confirm absolute VSS reduction values (for possible reasons discussed elsewhere), a similar optimum product value of sludge age and temperature was found for f u l l -scale results. This curve i s considered a valuable tool for design and operation of aerobic digestion f a c i l i t i e s . 2. VSS destruction could be described by accepted k i n e t i c models for endogenous decay; however, two k-values are indicated. The i n i t i a l k-value was always much greater than the secondary k-value. The change i n k-values occurred at a point where the product of tempera-ture and sludge age reaches a value of approximately 250, and i s r e f l e c t i v e of the relationship discussed i n point (1) above. Thus, 254 i n i t i a l k-values, based on VSS, may be used for design purposes, i n conjunction with the k i n e t i c models adopted i n this research, as long as the product of temperature and sludge age i s i n the 0-250 range. This range has been shown to be the range for most practicable VSS reduction. 3. The laboratory results indicated an appreciable difference between batch and continuous feed system k-values and temperature response, urging a cautious approach when interpreting batch data for continuous or semi-continuous feed system design. 4. The temperature s e n s i t i v i t y of the various laboratory systems f l u c -tuated considerably about a value of Q^Q = 2 (9 = 1.072), which has often been used as a "rule of thumb" for b i o l o g i c a l systems. In the 10°C-20°C temperature range, both continuous feed systems had a similar temperature response (9 = 1.074), whereas the batch digestion system had a much greater temperature s e n s i t i v i t y (0 = 1.120). In the 5°C-10°C range, however, the batch system was least affected by temperature (9 = 1.113); i n addition, there was a great d i f f e r -ence i n temperature response between the continuous feed systems (9 = 1.311) and the semi-continuous feed systems (9 = 1.156). I t would therefore appear to be advantageous to operate aerobic digesters i n the batch or semi-continuous mode of feeding at lower temperatures. 255 I t i s thought that the slug method of feeding causes a larger proportion of the fresh sludge organisms to adapt to the unfavourable digester environment, resulting i n greater solids destruction efficiency. 5. As oxygen uptake rate i s d i r e c t l y affected by temperature, and a low oxygen uptake rate at low temperature can s t i l l result i n a higher rate when stored at a higher temperature, t h i s parameter cannot be regarded as a r e l i a b l e indicator of sludge s t a b i l i t y at lower temperatures. I t has been suggested (3) that a s p e c i f i c oxygen uptake rate of less than 1 mg/gVSS/hr would constitute a stable sludge, i . e . , does not develop objectionable odours upon storage. This research found this to be true at 20°C, but not at 10°C and 5°C. Instead, endogenous BOD,, has been proposed as a suitable parameter for determining digested sludge s t a b i l i t y . 6. Temperature s e n s i t i v i t y of oxygen uptake rate was found to vary greatly between temperature ranges and between continuous feed systems and batch aeration systems. Except for batch aeration i n the 5-10°C range, a l l systems showed a decreasing temperature s e n s i t i v i t y with regard to oxygen uptake rate as sludge age increased. This would indicate a possible s h i f t i n sludge organism population from mesophilic to psychrophilic as sludge age increases. 7. Reasonable correlation was shown between the laboratory results and f u l l - s c a l e aerobic digestion data. Reasons for discrepancies 256 between laboratory and f u l l - s c a l e measurements may be the d i f f e r -ence i n raw sludge age, lack of controlled steady-state conditions i n f u l l - s c a l e plants, or underestimation of solids l o s t from f u l l -scale digesters i n the supernatant (which would lead to overestimated solids destruction and sludge age). I t i s f e l t , therefore, that the laboratory results can be extrapolated to f u l l - s c a l e situations with a considerable degree of confidence. B. DIGESTED SLUDGE CHARACTERISTICS 1. Mixed-liquor BOD,, was introduced as a possible means to determine digested sludge s t a b i l i t y , independent of digestion temperature. Based on odour-panel tests, a mixed-liquor BOD,, of 0.20 g/gVSS was put forth as a proposed l i m i t , below which odours would not pose a problem upon prolonged storage of digested sludge at 20°C. Based on this c r i t e r i o n , aerobic digestion at 20°C would require a sludge age greater than 40 days, and digestion at 10°C a sludge age greater than 60 days. For aerobic digestion at 5°C, even an 80-day sludge age would not prevent odours when stored at 20°C. Thus, i f odours are a major concern, system sludge age would have to be s i g n i f i c a n t l y greater than indicated by the optimum value determined from k i n e t i c considerations. 2. There did not appear to be any appreciable difference between the continuous feed systems and the batch aeration systems with respect to mixed-liquor BOD^ r e s u l t s , which further reinforces the usefulness of this parameter to assess aerobic digester performance. Chemical oxygen demand and t o t a l organic carbon measurements conducted on aerobic digester mixed-liquor showed these parameters to be p r a c t i c a l l y constant per unit c e l l mass for a l l systems and sludge ages. The average values for the two parameters were: COD = 1.54 g/gVSS (std. dev. = 0.12) TOC = 0.56 g/gVSS (std. dev. = 0.06) These values support the empirical formula C^H^NO^j frequently used to describe activated sludge c e l l mass. Organic nitrogen measurements conducted on aerobic digester mixed-liquor also showed this parameter to be p r a c t i c a l l y constant, per unit of c e l l mass, for a l l systems and sludge ages. The average value for this parameter was 0.080 gN/gVSS (std. dev. = 0.005). Since the mixed-liquor values of COD, TOC, and Organic Nitrogen were p r a c t i c a l l y constant for a l l systems and sludge ages, i t can be concluded that aerobic digestion does not seem to change the organo-chemical composition of the sludge mass. In the continuous and semi-continuous feed systems, the number of viable bacteria per unit mass remained r e l a t i v e l y constant with increasing sludge age at 20°C and 10°C. At 5°C, these systems, as wel l as the batch aeration systems at a l l temperatures, showed a sharp decrease i n viable bacteria per unit mass. The major s i g n i f cance of this observation i s the apparently different b i o l o g i c a l behaviour of batch digestion as compared to continuous feed diges-t i o n , and the fact that low temperature would appear to have an equalizing effect on the two types of digestion. 7. The v o l a t i l e content of sludge mass decreased somewhat with i n -creasing sludge age during aerobic digestion, indicating physio-l o g i c a l changes i n the c e l l mass as digestion proceeds. Although i t i s recognized that aerobic digestion causes reduction i n fixed suspended solids as wel l as v o l a t i l e suspended s o l i d s , the use of t o t a l suspended solids i n the k i n e t i c analysis of the aerobic digestion process i s not recommended. I t i s f e l t that v o l a t i l e suspended solids present a parameter that i s more independent of the physiological state of the c e l l mass and i s , therefore, a more universally applicable parameter. C. SUPERNATANT CHARACTERISTICS 1. A l l digestion systems experienced a gradual lowering of pH with increasing sludge age. F i n a l batch digestion pH values were generally lower than those of the continuous feed systems, and were lowest at the highest temperature. The low pH did not seem to affect the digestion performance. There appears to be a s t r i k i n g difference i n supernatant character-i s t i c s , as measured by VDS, COD, and TOC, between the continuous feed systems and batch aeration systems. Continuous feed digestion resulted i n s i g n i f i c a n t s o l u b i l i z a t i o n of organic compounds, whereas no s i g n i f i c a n t s o l u b i l i z a t i o n occurred during batch diges-tion. This presents yet another indication that the mechanism of digestion i n continuous feed systems i s substantially different from that i n batch digestion systems. The absence of organic nitrogen i n the supernatants, and the constant organic nitrogen content of the c e l l mass, show that v o l a t i l e solids destruction i n aerobic digestion results i n almost 100 percent mineralization of organic nitrogen, to form ammonia i n solution. The significance of t o t a l mineralization of organic nitrogen asso-ciated with aerobic digestion i s the p o t e n t i a l l y considerable amount of soluble nitrogen returned to the treatment process with the aerobic digester supernatant. Considerable n i t r i f i c a t i o n and d e n i t r i f i c a t i o n occurred i n a l l systems and at a l l temperatures. The continuous feed, automatic decant systems behaved most closely to generally accepted n i t r i f i c a t i o n - d e n i t r i f i c a t i o n models, with n i t r i f i c a t i o n and d e n i t r i f i c a t i o n decreasing with decreasing temperature and sludge age. The d a i l y feed, manual decant systems showed comparable n i t r i f i c a t i o n and d e n i t r i f i c a t i o n levels at 20°C and 10°C (both 260 higher than those of the continuous feed, automatic decant systems), but showed a markedly higher degree of n i t r i f i c a t i o n as wel l as d e n i t r i f i c a t i o n at 5°C. It i s f e l t that the dai l y shut-off of a i r may provide a better environment for d e n i t r i f i c a t i o n i n the semi-continuous digesters than the s e t t l i n g compartment i n the continuous digesters. A possible reason for increased n i t r i f i c a t i o n at lower temperatures i n the semi-continuous digesters i s thought to be the higher pH, as well as the fact that feed sludge used, during the lower temperature runs, contained higher n i t r a t e l e v e l s , and therefore higher levels of n i t r i f i e r s . D. SUGGESTIONS FOR FURTHER RESEARCH As a result of th i s research, the author feels that the following aspects of aerobic sludge digestion require further knowledge, and are suggested as worthwhile future research subjects: 1. As was shown i n this research, the organo-chemical composition of the digested sludge mass, as measured by TOC. COD, and Organic Nitrogen, remained remarkably constant, even when compared to the fresh activated sludge. I t has been observed i n th i s research that aerobic digestion results i n a progressively less biodegradable sludge mass, destruction of t o t a l suspended s o l i d s , s o l u b i l i z a t i o n of organic c e l l materials i n continuous feed systems on the one hand, and no detectable s o l u b i l i z a t i o n of c e l l materials i n batch systems on the other hand. This poses an i n t r i g u i n g question as to the nature of the m i c r o b i o l o g i c a l process occurring during aerobic digestion, and would be a valuable subject of further research. It was shown i n t h i s research that a pH as low as 4.0 did not h a l t the process of endogenous decay, nor did i t seem to impede n i t r i f i -c ation or d e n i t r i f i c a t i o n . No attempt was made i n t h i s research to control pH, nor to monitor the a l k a l i n i t y equilibrium. 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W., "Improvement of the Aerobic Sludge Digestion Process E f f i c i e n c y , " Journal Water P o l l u t i o n  Control Federation, 46, 1, 102 (1974). 125. Smith, A. L., and Greenberg, A. E., "Evaluation of Methods for Determining Suspended Solids i n Waste Water," Journal Water  P o l l u t i o n Control Federation, 35, 7, 940 (1963). 126. Smith, A. R., "Aerobic Digestion Gains Favour," Water and Wastes  Engineering, _8, 2, 24 (1971). 127. Smith, J. E., "Aerobic Digestion at Low Temperatures." Unpublished Master's Thesis, University of Toronto (1970). 128. Smithers, B. C., Town of, Sewage Treatment Plant Operating Records, and Personal Communication, 1972-75. 129. Society of American Bacteriologists, Manual of Microbiological  Methods. New York: McGraw-Hill, 1957. 130. Sorokin, Y. I., and Kadota, H., Eds., Techniques for the Assessment  of Microbial Production and Decomposition i n Fresh Waters. I.B.P. Handbook No. 23. Oxford: Blackwell S c i e n t i f i c Publications, 1972. 131. Squamish, B. C., Corporation of the D i s t r i c t of, Mamquam Sewage Treatment Plant Operating Records, and Personal Communication, 1974-75. 132. Standard Methods for the Examination of Water and Wastewater, 13th ed., American Public Health Association, Inc., New York, 1971. 133. Stanier, R. Y., Doudoroff, M., and Adelberg, E. A., The Microbial  World. 2nd ed., Englewood C l i f f s , N. J.: Prentice H a l l , 1963. 134. Stein, R. M., Jewell, W. J. , Eckenfelder, W. W., J r . , and Adams, C. E., "A Study of Aerobic Sludge Digestion Comparing Pure Oxygen and A i r , " Proceedings 27th In d u s t r i a l Waste Conference, Purdue University (1972). (Reprint). 135. Stein, R. M., Adams, C. E., and Eckenfelder, W. W., J r . , "A P r a c t i c a l Model of Aerobic Sludge S t a b i l i z a t i o n , " Proceedings 3rd  Annual Environmental Engineering and Science Conference, University of L o u i s v i l l e , _3, 781 (1973). 136. Stensel, H. D., Loehr, R. C, and Lawrence, A. 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(October 1975). Van G i l s , H. W., Bacteriology of Activated Sludge. IG-TNO Report No. 32, Research I n s t i t u t e for Public Health Engineering, 1964. Viraraghaven, T., "Digesting Sludge by Aeration," Water and Wastes  Engineering, 3, 9, 86 (.1965). Vrijburg, R., "Betriebserfahrungen und - Kosten der Aeroben und Anaeroben Schlammbehandlung i n den Niederlanden." Paper presented at the 4th International Congress of the I.R.G.R. (Working Group 5 Pretreatment of Sewage Sludge), Basel, Switzerland, 1969. 274 152. Walker, J. D., and Dreier, D. E., "Aerobic Digestion of Sewage Sludge Solids." Paper presented at the 35th Annual Session of the Georgia Water & Pollution Control Association (1966). 153. Walker, J. D., "Aerobic Digestion of the Waste Activated Sludge." Paper presented at the Ohio Water Pollution Control Conference, Cleveland, Ohio (June, 1967). 154. Walker, J. D., "Aerobic Sludge Digestion." Unpublished Mimeo-graphed Copy, 1973. 155. Washington, D. R., and Symons, J. M., "Volatile Sludge Accumula-tions in Activated Sludge Systems," Journal Water Pollution Control  Federation, 34, 8, 767 (1962). 156. Weddle, C. L., and Jenkins, D., "The Vi a b i l i t y and Activity of Activated Sludge," Water Research, .5, 8 621 (1971). 157. Weinberger, L. W., "Nitrogen Metabolism in the Activated Sludge Process." Unpublished Ph.D. Dissertation, Massachusetts Institute of Technology (.1949) . 158. Weston, R. F., and Eckenfelder, W. W., Jr., "Application of Biological Treatment to Industrial Wastes. I. Kinetics and Equilibria of Oxidative Treatment," Sewage and Industrial Wastes,. 27, 7, 802 (1955). 159. Weston, R. F., Inc., Process Design Manual for Upgrading Existing  Wastewater Treatment Plants. Prepared for the U.S. Environmental Protection Agency, under Program No. 17090 GNQ, Contract No. 14-12-933, 1971. 160. Weston, T. J., "The Application of High Purity Oxygen to Aerobic Sludge Digestion," Unpublished Master's Thesis, Michigan Technolo-gical University (.1972) . 161. White, A., Handler, P., and Smith, E. L., Principles of Biochemistry. 4th ed., New York: McGraw-Hill, 1968. 162. Woodley, R. A., "A Study of Aerobic Biochemical Oxidation of Primary Sewage Sludge at Mesophilic and Thermophilic Temperatures." Unpublished Master's Thesis, Purdue University (1961). 163. Wuhrmann, K., " C r i t i c a l Review of Papers on Treatment Processes." International Symposium on Water Pollution Control in Cold  Climates, held at the University of Alaska. Edited by R. S. Murphy and D. Nyquist, 1970, p. 329. 275 PREVIOUS PUBLICATIONS ON THIS RESEARCH 164. Koers, D. A., and Mavinic, D. S., "Aerobic Digestion of Waste Activated Sludge at Low Temperatures," Journal Water P o l l u t i o n  Control Federation, 49, 3, 460 (1977). 165. Koers, D. A., "Aerobic Digestion of Wastewater Sludges". Paper presented at the Technology Transfer Seminar on Sludge Handling and Disposal, sponsored by Fisheries and Environment Canada, Calgary, Alberta (1977). 166. Mavinic, D. S., and Koers, D. A., "Aerobic Sludge Digestion at Cold Temperatures," Canadian Journal of C i v i l Engineering, 4_, 4, 445 (1977). 167. Mavinic, D. S., and Koers, D. A., "Performance and Kinetics of Low-Temperature, Aerobic Sludge Digestion," Journal Water Po l l u t i o n Control Federation, 51, 8, 2088 (1979). 276 APPENDICES '211 APPENDIX A EXPLANATION OF CALCULATIONS REGARDING SOLIDS MASS BALANCE Tables 1 to 18 contain the reduced solids mass balance data for a l l continuous feed systems studied. Figure 8 shows a schematic of the con-tinuous feed system mass balance components. The reduced data i n Tables 1 to 18 were arrived at from the raw data as follows: 1. Feed Sludge Volume This feed rate was set at constant pump rate and was checked once every 2 to 3 days throughout the steady-state period. 2. Feed Sludge TSS and VSS This was determined d a i l y . For each balance period, the solids concentration was averaged. 3. Effluent Sludge Volume This was collected for each balance period. The value shown i n the reduced data i s the t o t a l volume divided by the number of days i n the balance period. 4. Effluent Sludge TSS and VSS This i s the composite concentration i n the t o t a l effluent sludge volume for the balance period. For the 20°C continuous feed/automatic 278.; decant :systems, an allowance was added for the amount of foam and caked solids removed from the digesters during the balance period (see explanation Chapter I I I ) . An allowance i s also included i n these values for samples withdrawn from the digester during the balance period. 5. Digester Sludge TSS and VSS This was measured at the start and end of each balance period. The only parameter i n the solids balance equation that was able to be kept constant, was the hydraulic feed rate. The feed sludge solids con-centration was kept i n as narrow a range as was p r a c t i c a l l y feasible. The other parameters established themselves as the systems allowed, i . e . , the digester solids concentration b u i l t up to a maximum value? related to sludge s e t t l e a b i l i t y and the a b i l i t y of the sludge r e c i r c u l a -t i o n system to retain the solids i n the system. Excess solids were washed out i n the effluent. I t i s apparent that some va r i a t i o n occurred i n the digester sludge solids concentration as well as the effluent solids concentration. I t i s for this reason that values of solids reduction and sludge age for indi v i d u a l balance periods i n any p a r t i -cular system varied somewhat. I t was therefore decided that a more representative value of solids reduction and sludge age would be obtained by conducting the solids balance over the entire steady-state period for each system, resulting i n a single solids reduction and sludge age value for each system studied. 2 7 9 Calculations for t o t a l s o l i d s destroyed and percent s o l i d s destroyed were ca r r i e d i n accordance with the expressions shown i n Figure 8, using values t o t a l l e d for each of the balance periods within the steady-state period for each system. The c a l c u l a t i o n s of system sludge age were c a r r i e d out as follows: (a) Solids contained i n system: This was determined as the average weight of s o l i d s i n the digester during the e n t i r e steady-state period, using measurements of s o l i d s concentration at the beginning and end of each balance period; these were averaged f or each balance period, a f t e r which these averages were weighted by multiplying with the length of the balance period, t o t a l l e d for the e n t i r e steady-state period, and divided by the t o t a l length of the steady-state period. I (b) Solids wasted from system per day: This was determined by subtracting the t o t a l s o l i d s destroyed from the t o t a l s o l i d s fed into the system, over the e n t i r e steady-state period, divided by the t o t a l length of the steady-state period. Sludge age was then calculated as The system loading rate was calculated by d i v i d i n g the average weight of s o l i d s fed to the digester per day, over the e n t i r e steady-state period, by the volume of the digester. APPENDIX B APPLICATION OF RESEARCH DATA - DESIGN EXAMPLE Design Parameters Type of Sewage Treatment Plant: Plant Capacity: Design Population: Type of Sewage: Waste Sludge Production: V o l a t i l e Fraction: Ambient Temperature: Step Aeration, no primary sedimentation 1 US mgd (3,785 m3/day) 10,000 persons Domestic only 0.17 l b TSS/day/cap (0.08 kg TSS/day/cap) 80 percent Average Winter 0°C Average Summer 20°C Aerobic Digester Design 1. Sludge volume to be treated: 10,000 persons x 0.08 kg TSS/day/cap = 800 kg TSS/day Expected underflow concentration from STP c l a r i f i e r i s 10,000 mg/1 TSS = 1% TSS Pump capacity required for digester influent: 800 kg TSS/day x = 80,000 lit r e s / d a y = 0.93 lps V o l a t i l e solids to be treated: 0.80 x 800 = 640 kg VSS/day 281 2. Selected means of digester operation: Continuous feed (daily fill-and-draw). 3. Expected digester temperature: Average Winter: 5°C Average Summer: 20°C 4. Desired VSS reduction: In accordance with findings of this research, the most practicable VSS reduction = 30 percent (see Figure 12). 5. Sludge age required to achieve 30 percent VSS reduction: (a) Kine t i c Approach Conduct batch digestion bench study of waste activated sludge at 20°C i s found to be 0.0402 (see Figure 24). This research has found that the k„ values for batch and d continuous feed digestion are not interchangeable. Using the batch k, value i n the k i n e t i c expression for continuous feed d digestion with recycle, w i l l result i n conservative design. 282 M /M = 1 + k,.t' o t d (T -T ) k J O / k j n = G 2 1 0 = 1.120 (10°-20°C)(Fig. 23) 0 = 1.113 (5°-10°C)(Fig. 23) vd2' " d l k d(20) - ° - 0 4 0 2 k d ( i o ) - ° - 0 1 2 9 k J / c. = 0.0076 d(5) M = 640 kg VSS o M = (1-0.30)640 = 448 kg VSS Sludge Age required at 20°C: t' = 6 4ofo4Q2 1 = 1 0 ' 7 ^ = S l u d S e A § e Sludge Age required at 5°C: , 6 4 0 / 4 4 8-1 c , . , e 1 , . = — o QQ76 = 5 6 . 4 days = Sludge Age (b) Empirical Approach Using the empirical curve obtained i n Figure 12, the following sludge ages are calculated: Sludge Age required at 20°C: Sludge Age = 200/20 = 10.0 days Sludge Age required at 5°C: Sludge Age = 200/5 = 40.0 days 283' I t i s apparent that, unless laboratory bench studies are conducted, using continuous feed digestion models, the k i n e t i c design results i n oversizing of aerobic digesters. Since f u l l - s c a l e plant evaluations have shown that the empirical curve, based on laboratory batch and continuous feed digestion data, predicts VSS reduction values that are somewhat lower than those obtained i n f u l l - s c a l e plants (see Figure 83), the empirical method of aerobic digester design i s recommended for safe design practice. 6. Check sludge age required to prevent odours: This depends on means of digested sludge disposal. If sludge disposal i s to open drying beds, or receives intermediate storage i n open basins on the treatment plant s i t e , prior to f i n a l disposal, odour prevention can be extremely important. If sludge disposal i s rto a d a i l y covered l a n d f i l l , without i n t e r -mediate storage, or i s to a g r i c u l t u r a l land, where i t i s spread i n thin layers, odour prevention may not be as important, and aerobic digester design could be limited to volume reduction only, to reduce ultimate disposal costs. 7. Sludge age calculations for odour prevention: Odours are not expected to be a problem at l i q u i d temperatures below 10°C. Above that temperature, the sludge"stability c r i t e r i o n of 284 0.20 gBOD/gVSS must be met, i n order to prevent odours upon subsequent storage of digested sludge. This would require the following sludge ages: 20°C: 45 days (see Figure 64) 10°C: 60 days (see Figure 65) 8. Digester volume calculations: This can be calculated from the sludge age formula: Weight of VSS i n Digester (W) S.A. Weight of VSS wasted per day from Digester (AW) where: w = VSS(mg/1) x V o l u m e ( c u f t ) x 6 2 m 4 l b i o 6 ,100-%VSS Red.. T T O n / \ A W - ( 100 > X V S S i n f l u e n t Maximum expected VSS concentration i n digester i s 0.80 x 30,000 mg/1 = 24,000 mg/1 (a) Digester volume for odour prevention: W Sludge Age = = 60 i s governing @ 10 C S.A. x Temp. = 60x10 = 600 % VSS Red. = 38 percent (see Figure 12) „ , 60(0.62x0.80x0.17)xlQ6 . . . n , n n Q 1. 3, , Volume = — 0. ,„ , = 3.4 cu ft/cap (0.095 m /cap) 24,000 x 62.4 This volume results i n the following digested sludge volumes to be disposed of: 20°C: S.A. x Temp. = 60x20 = 1200 % VSS Red. = 48 percent (see Figure 12) Total digested sludge volume: -"-"QQ48 x 800 x = 13,870 lit r e s / d a y @ 3% TSS 10°C: S.A. x Temp. = 60x10 = 600 % VSS Red. = 38 percent (see Figure 12) Total digested sludge volume: 1OO— 1 3_QQ x 800 x = 16,530 lit r e s / d a y @ 3% TSS 5°C: S.A. x Temp. = 60x5 = 300 % VSS Red. = 32 percent (see Figure 12) Total digested sludge volume: " 1 0 0 - 1 ° 1 ±Q0 x 800 x Q^QJ = 18,130 litres/day @ 3% TSS Digester volume for optimum VSS reduction: Optimum VSS Reduction = 30 percent @ S.A. x Temp. = 200 (see Figure 12) W Sludge Age = = 40 i s governing @ 5 C ir i 40(0.70x0.80x0.17)xl0 6 _ _ . . ' „, 3 . Volume = — 24,000 x 62.4 = 5 C u f t / c a P ( ° - 0 7 1 ™ / c aP> This volume results i n the following digested sludge volumes to be disposed of: 20°C: S.A. x Temp. = 40x20 = 800 % VSS Red. = 41 percent (see Figure 12) Total digested sludge volume: ^ Q " 4 1 x 800 x = 15,570 litres/day @ 3% TSS 10°C: S.A. x Temp. = 40x10 = 400 % VSS Red. = 34 percent (see Figure 12) Total digested sludge volume: 1 ° ° - 3 4 x 800 x = 17,600 litres/day @ 3% TSS 5°C: % VSS Red. = 30 percent Total digested sludge volume: 1 ° g - 3 0 x 800 x — j = 18,670 l i t r e s / d a y @ 3% TSS Sizing the digester for optimum VSS reduction has to be done for the governing winter temperature (5°C). This results i n a size which i s four times the required size at the summer operating temperature (20°C) and two times the required size at intermediate operating temperatures (10°C). U t i l i z i n g the f u l l volume for aerobic digestion year-round has the advantage of increased solids reduction, and thus reduced digested sludge volumes, during periods of higher operating temperatures. 287-One may also consider a compartmentalized design of the aerobic digester, thus being able to reduce the digester volume i n use at higher operating temperatures. This would result i n reduced a i r requirements for mixing, and would have to be compared against the cost of handling increased digested sludge volumes. 9. Oxygen requirements: Steady-state oxygen requirements for continuous feed aerobic digesters are only a f r a c t i o n of those during start-up of the digesters. Design, however, must allow for oxygen requirements during start-up, as well as aeration requirements for mixing, which are often governing. Biochemical oxygen demand at 0 sludge age i s 1.05 gBOD^/gVSS (see Figures 63, 64, and 65). In addition, n i t r i f i c a t i o n oxygen demand must be met, for a t o t a l b i o l o g i c a l oxygen requirement of 1.60 g02/gVSS. This translates into: 1.60 x 0.8 x 800 = 1024 kg 02/day Volume of a i r required: 1024 0,_ n 3., , . = 3678 m /day of a i r 1.2(0.232) At an oxygen transfer e f f i c i e n c y of 8 percent, this would require an a i r supply of: 0.08(1440) = 3 1 - 9 m 3 / m l n ( 1 1 4 ° S G f m ) For a sludge age of 60 days, the required digester volume i s 3 3.4 cu ft/cap, or 34,000 cu f t (952 m ) t o t a l . The a i r requirement, therefore, would be 1140/34 = 33.5 scfm/1000 cu f t (33.5 m3/min/1000 Air required for mixing has been known to vary between 30 and 50 3 3 m /min/1000 m , depending on tank configuration. VITA AUCTORIS D. Antonie Koers was born i n Haarlem, The Netherlands, on October 10, 1943. From 1950 to 1956 he completed his primary education at the "Floraschool" i n Haarlem, and he attended the "Hogere'Burgerschool - B" i n Haarlem for his secondary education from which he graduated i n 1961 with a Diploma HBS-b, 5-year accelerated course. In 1961 he enrolled at the Delft University of Technology i n De l f t , The Netherlands, i n the C i v i l Engi-neering Department. In 1966 he received his "Kandidaats" Diploma i n C i v i l Engineering from that i n s t i t u t i o n . After a 5-month working v i s i t to B r i t i s h Columbia, Canada, when he worked i n the northern i n t e r i o r of the province for the Department of Fisheries, he returned to Delft to complete his C i v i l Engineering education. In July 1967 he graduated from Delft University of Technology as an "Ingenieur" i n C i v i l Engi-neering, with a major i n hydraulics and water resources, and a minor i n concrete structures. Having already emigrated to Canada i n 1966, he returned i n August 1967 to take up permanent residence i n that country, and enrolled at the University of Toronto with the aid of a University of Toronto Open Fellowship, i n the School of Graduate Studies, as a doctoral candidate i n Sanitary Engineering. After having completed his course work and comprehensive examination, he interrupted his doctoral studies i n 1970 because of the b i r t h of his f i r s t c h i l d . He then accepted an offer of employment with Associated Engineering Services Ltd. i n Vancouver, B. C, where he started work i n May 1970 as a project engineer i n the poll u t i o n control engineering section of that firm. He became a Canadian Ci t i z e n i n June of 1972. In the f a l l of 1972 he transferred his doctoral study credits to the University of B r i t i s h Columbia, and enrolled i n the Faculty of Graduate Studies to return to his doctoral studies i n P o l l u t i o n Control Engi-neering, on a part-time basis, while remaining i n the employ of Associated Engineering Services Ltd. After having completed his research work i n February 1975, he accepted a transfer to the Nanaimo o f f i c e of Associated Engineering Services Ltd. i n May 1975, where he now i s Assistant Manager, i n charge of the company's municipal engineering projects on Vancouver Island. During his Canadian studies, he received a University of Toronto Open Fellowship for two years, and a Central Mortgage & Housing Corporation Fellowship for three years. During the f i r s t three years of his Canadian studies, he was a part-time teaching assistant i n surveying and engineering mechanics for undergraduate students. He i s a registered professional engineer i n The Netherlands and B r i t i s h Columbia, and a member of the Water P o l l u t i o n Control Federation, American Water Works Association, and the B r i t i s h Columbia Water and Waste Association. He i s married and has two children. 

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