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Anoxic-Aerobic Digestion of Waste Activated Sludge A

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Anoxic-Aerobic Digestion of Waste Activated Sludge A ANOXIC-AEROBIC DIGESTION OF WASTE ACTIVATED SLUDGE A LAB SCALE COMPARISON TO AEROBIC DIGESTION WITH AND WITHOUT LIME ADDITION By CHRISTOPHER JAY JENKINS B.A.Sc.., The University of British Columbia, 1986 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Civil Engineering) ' We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA July 1988 Christopher Jay Jenkins, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of civil Engineering The University of British Columbia Vancouver, Canada Date July 18, 1988 DE-6 (2/88) ABSTRACT A lab-scale study of anoxic-aerobic digestion of waste activated sludge was performed, using 6 litre digesters, and operated in a semi - continuous (fed- once-a-day) manner with solids retention times (SRTs) of 20, 15 and 10 days and mixed-liquor temperatures of 20 °C and 10 °C. Raw sludge was obtained from a pilot-scale biological phosphorus removal facility operating at U.B.C. Fresh sludge was obtained daily and digested by three different digestion modes: anoxic-aerobic, aerobic with lime addition and aerobic. Two aerobic control digesters were run in parallel with the anoxic-aerobic digesters. One of the aerobic digesters received a daily dose of lime slurry. All three digesters were operated under identical conditions (except for the cycling of air supply to the anoxic-aerobic digesters) so that direct comparison could be made between the three digestion modes. Comparisons were made on the basis of five main parameters related to: (1) digestion kinetics, (2) digested sludge characteristics, (3) supernatant quality, (4) ORP monitoring, and (5) an overall rating system. Percent volatile suspended solids (VSS) reduction was used as one performance variable. Despite using only 42 percent of the air required by the two controls, anoxic-aerobic digestion showed comparable percent VSS reductions. All three digestion modes showed increased solids reduction with increasing SRT and temperature. There was a linear relationship between percent TVSS and the product of SRT and temperature. All three digestion modes had a propensity to retain their percent nitrogen and phosphorus within their solids. However, with respect to retaining phosphorus, the aerobic controls were the least effective. Anoxic-aerobic digestion maintained neutral mixed-liquor pH (MLpH) throughout. Lime controls were maintained at MLpH close to neutral. Aerobic digestion, in general, resulted in MLpH levels below 5.0, however, there were periods when the MLpH of the aerobic digesters varied widely between 4.2 and 6.8. Supernatant quality was superior for the anoxic-aerobic digesters. Due to the incorporation of non-aerated periods, there was almost 100 percent denitrification of nitrates produced during the aerated time. This nitrification-denitrification resulted in very low soluble nitrogen levels in the effluent, as well as considerable removal of nitrogen gas. Neither of the controls showed this ability. The lime and aerobic controls produced high levels of effluent nitrates, as well as occasional measurements of ammonia and nitrite. Phosphorus levels were lowest for the lime control and anoxic-aerobic digesters. Presumably, due to reduced pH levels, the soluble phosphorus levels from the aerobic digesters were 2 to 3 times those in the lime or anoxic-aerobic digesters. Alkalinity was conserved in the anoxic-aerobic digesters as well as the lime control. However, the purely aerobic digesters consumed alkalinity until very little buffering capacity remained. Oxidation-reduction potential (ORP) was used as a means of monitoring the anoxic-aerobic digesters on a real time basis. ORP was particularly useful during the non-aerated periods, due to the fact that, at those times, dissolved oxygen was undetectable. Characteristic real time ORP profiles were revealed. Slope changes correlated well with events of theoretical and engineering interest; the' disappearance of ammonia and nitrates, as well as the (dis)appearance of detectable dissolved oxygen, could be predicted by these slope changes. As a result of the findings, ORP may prove to be an ideal parameter for the control of the anoxic-aerobic digestion process. Finally, an overall rating system was developed. The results of this study suggest that, for the digestion of waste activated sludge, anoxic-aerobic digestion out-performed both lime-control and conventional digestion modes. iv TABLE OF CONTENTS PAGE ABSTRACT ii LIST OF TABLES vii LIST OF FIGURES ix ACKNOWLEDGEMENTS xii I INTRODUCTION 1 II LITERATURE REVIEW 4 A - Kinetics of Aerobic Digestion 4 B - Influence of Temperature on Digestion Kinetics 7 C - ORP Monitoring 9 D - Previous Research in Aerobic Digestion 11 E - Previous Research in Anoxic-Aerobic Sludge Digestion 22 F - Need for Future Research 23 III MATERIALS AND METHODS 25 A - Experimental Apparatus 25 B - Experimental Design 27 C - General Procedures 30 D - Analytical Procedures 32 IV RESULTS AND DISCUSSION 38 A - Digestion Kinetics 39 1. Solids Reduction 39 2. Endogenous Decay Coefficients 49 B - Digested Sludge Characteristics 64 1. Daily TSS and TVSS Levels 64 2. Daily Mixed-Liquor pH Levels 74 3. Nitrogen Balance 81 4. Phosphorus Balance 85 5. Alkalinity Consumption and Production 91 6. Chemical Oxygen Demand 93 7. Qualitative Observations 95 C - Supernatant Characteristics 97 1. Nitrogen 97 2. Phosphorus 113 3. COD 124 4. Alkalinity 126 D - Monitoring Results 130 E - Overall Rating System 135 V CONCLUSIONS 138 A - Digestion Kinetics 138 B - Digested Sludge Characteristics 139 C - Supernatant Characteristics 140 D - Monitoring Results 142 E - Overall Rating System 143 F - Future Research 143 v TABLE OF CONTENTS (continued) PAGE REFERENCES 144 APPENDICES 150 A - Solids Data 151 B - Duplicate Solids Determinations 177 C - Nitrogen Data 185 D - Phosphorus Data 198 E - Real Time ORP Profiles 211 BIOGRAPHICAL INFORMATION 223 vi LIST OF TABLES TABLE PAGE 1. Summary of Percent TVSS Reduction Based on an Overall Mass Balance 42 2. Summary of Percent Reduction Based on Consecutive 1 Balance Periods 43 3. Median Endogenous Decay Coefficent Based on TVSS and TSS Measurements 54 4. Summary of Daily TSS and TVSS Levels for 20 day SRT 65 5. Summary of Daily TSS and TVSS Levels for 15 day SRT 66 6. Summary of Daily TSS and TVSS Levels for 10 day SRT 67 7. Average Nitrogen Levels Within Raw and Digested Sludge 83 8. Nitrogen Removal Data 86 9. Average Phosphorus Levels Within Raw and Digested Sludge 89 10. Phosphorus Balance 90 11. Average Alkalinity Levels Within Raw and Digested Sludge 92 12. Average COD Levels Within Raw and Digested Sludge 94 13. Average Supernatant Phosphorus Levels 114 14. Average Supernatant COD Levels 125 15. Summary of Nitrification and Denitrification Rates for Anoxic-aerobic Digesters 134 16. Summary of Rating Points for Comparison of Three Digestion Modes 137 A.l. Solids Data for 20 day SRT at 20°C 152 A.2. Solids Data for 20 day SRT at 10°C 158 A.3. Solids Data for 15 day SRT at 20°C 164 A.4. Solids Data for 15 day SRT at 10°C 168 A.5. Solids Data for 10 day SRT at 20°C 171 A. 6. Solids Data for 10 day SRT at 10°C 174 B. l. Multiple (n=9) Solids Determinations on a Single Sample for November 20, 1987 178 B.2. Multiple (n=9) Solids Determinations on a Single Sample for February 4, 1988 178 B.3.Daily Duplicate Solids Determinations for Run #1 179 B.4. Daily Duplicate Solids Determinations for Run #2 ^ 181 B.5. Daily Duplicate Solids Determinations for Run #3 183 vii LIST OF TABLES (continued) TABLE PAGE C.l. Nitrogen Data for 20 day SRT at 20°C 186 C.2. Nitrogen Data for 20 day SRT at 10°C 188 C.3. Nitrogen Data for 15 day SRT at 20°C 190 C.4. Nitrogen Data for 15 day SRT at 10°C 192 C.5. Nitrogen Data for 10 day SRT at 20°C 194 C. 6. Nitrogen Data for 10 day SRT at 10°C 196 D. l. Phosphorus Data for 20 day SRT at 20°C 199 D.2. Phosphorus Data for 20 day SRT at 10°C 201 D.3. Phosphorus Data for 15 day SRT at 20°C 203 D.4. Phosphorus Data for 15 day SRT at 10°C 205 D.5. Phosphorus Data for 10 day SRT at 20°C 207 D.6. Phosphorus Data for 10 day SRT at 10°C 209 viii \ LIST OF FIGURES FIGURE PAGE 1. Schematic of Side-A Digesters 26 2. Schematic of Automated Monitoring Equipment 28 3. Solids Mass Balance in Digesters 37 4. Performance Curve Based on Percent TVSS Reduction at Various Combinations of SRT and Temperature 44 5. Percent TVSS Reduction Versus SRT at 20°C and 10°C 45 6. Daily Endogenous Decay Coefficients for 20 day SRT at 20°C 52 7. Endogenous Decay Coefficients for 20 day SRT at 20°C - TVSS Basis 55 8. Endogenous Decay Coefficients for 20 day SRT at 10°C - TVSS Basis 56 9. Endogenous Decay Coefficients for 15 day SRT at 20°C - TVSS Basis 57 10. Endogenous Decay Coefficients for 15 day SRT at 10°C - TVSS Basis 58 11. Endogenous Decay Coefficients for 10 day SRT at 20°C - TVSS Basis 59 12.
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