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The effect of Thermophilic heat treatment on the Anaerobic digestibility of primary sludge PDF

159 Pages·2016·5.11 MB·English
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, . !_, ' " 'r! . 1. THE EFFECT OF THERMOPHILIC HEAT TREATMENT ,-. ON THE ANAEROBIC DIGESTIBILI1Y OF PRIMARY SLUDGE by Hilton Izzett, BSc (Civ Brig) <Eape Town · i ,. I A thesis submitted in partial fulfillment of the requirements. for the degree of Master ,, ' ' • : ~. 1 + " of Science at the University of Cape Town i Department of Civil Engineering University of Cape Town The University of Cape Town has been given the right to reproduce this thesis in whole or in part. Copyright is held by the author. .:I The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. DECLARATIOBNYC ANDIDATE IH,i lBtaoryrnI zzheetrted,be yc lare thtahtti hse ;sm.iy oc: wn w orak,n tdh iahtta s nobte esnu b·. fo ...:.ra d egreaeta noyt huenirv ersity Septembe, 1992 SYNOPSIS In a time when the world is becoming more environmentally conscious, and is looking for simple, efficient, economical, and environmentally friendly solutions to sewage and sludge treatment, the dual digestion system presents itself as an attractive alternative to other sludge treatment systems. The dual digestion system comprises an autoheated thermophilic (55-65°C) aerobic first stage and a mesophilic (37°C) anaerobic second stage. Past research into the dual digestion system has given rise, inter alia, to the following claims (de Villiers, et al, 1991): a) sludge disinfection and stabilisation occur in one process - disinfection .in the thermophilic aerobic first stage and stabilisation in the anaerobic second stage. b) the stability of the anaerobic stage is considerably 'improved by the increase in H2C03 * alkalinity and pH in the aerobic stage. c) in the aerobic stage the sludge is aerobically or thermally pretreated (conditioned) making it more readily digestible under anaerobic conditions, thereby allowing significantly reduced retention times from 25 t~ 30 days for normal digestion, to 8 to 10 days. Messenger (1991), in a full scale investigation at the Potsdam Wastewater Treatment Plant in Milnerton, Cape Town, verified claims (a) and (b), but was unable to verify claim ( c) above. It was the intention in this investigation to verify this claim at full scale using the existing dual digestion plant at Milnerton. However, after 9 months of starting up the plant, the fiberglass aerobic tank failed structurally along one of its seams. This failure was so extensive that the plant could not be started up again and the research project was terminated. The failure of the aerobic reactor necessitated continuing this thesis investigation in the laboratory. With regard to claim (c) above, no conclusion could be reached by past research as to whether the conditioning of the sludge was caused simply by the heating of the sludge or by the biological action of the thermophilic bacteria on the sludge. Therefore, it was the initial intention of the laboratory investigation to operate 3 anaerobic units using 3 different feed sludges viz: 1) autotheral thermophilic aerobic sludge, 2) sludge heat treated to thermophilic temperatures, and 3) primary sludge as a control. By comparing the anaerobic performance of feed sludge types (1), (2) and (3) above, with progressively reduced retention times, it was i hoped to establish which feed type displayed superior anaerobic digestibility ie. which feed type could be treated at the lowest anaerobic retention time. The difference in digestibility between feed types (1) and (2) would indicate whether the heating only, or the aerobic biological heating, conditioned the sludge. However, difficulties were experienced in starting up and operating an autothermal thermophilic aerobic reactor at laboratory scale and this part of the investigation had to be abandoned. Consequently, experimentation continued using only feed sludge types (2) and (3) above. The objectives of the investigation were modified since it would no longer be possible to establish the difference in anaerobic digestibility between the heat treated sludge and the thermophilic aerobic sludge. The objectives of this investigation , therefore, became to: (1) determine whether or not the exposure of the sludge to thermophilic temperatures caused the conditioning of the sludge (2) determine how the performance of the anaerobic digester fed heat treated primary sludge compared with a digester fed untreated primary sludge In order to fulfill the objectives above, four 14 litre anaerobic digesters were operated using the two different feed types (feed types (2) and (3) above). The heat treatment of the primary sludge took place in stainless steel reactors that were operated at a retention time of 1.5 days and a temperature of 65°C. These parameters were selected to simulate the heating conditions in the aerobic reactor that was operated as part of the dual digestion system at Milnerton. The anaerobic digesters were all operated at a temperature of 37°C while the retention times were progressively reduced until they failed. The purpose of this was to establish the minimum retention for the anaerobic digestion of the two feed types mentioned above and, therefore, to determine if there was any difference in digestibility between the two sludge types. From these results it would be possible to determine if the heat treatment process had any conditioning effects on the primary sludge that enhanced anaerobic digestion. The results from the experiments described above gave rise to the following conclusions: (1) As indicated by measurements and calculations of the following parameters: a) percent volatile solids (VS) removed ii b) percent COD removed c) gas produced per VS and COD removed d) digester gas composition (% C02 and CJ4) e) pH * f) H2C03 alkalinity g) Short chain fatty acid (SCFA) concentration h) SCFA :alk ratios the digestibilities of the primary sludge and the heat treated sludge showed no significant difference. (2) There was no noticeable conditioning of the primary sludge by the heat treatment process insofar as digestibility is concerned. (3) The digesters fed primary and heat treated sludge were both operated under stable conditions at a retention times of 20, 15, 12, 10 and 7 days, but failed at a retention time of 4 days. --5 .. Past research at a bench scale (Drnevich and Matsch, 1978) has shown that the anaerobic stage of a dual digestion system can be operated at a retention time as low as 3 days. This thesis investigation has shown that a normal digester fed primary sludge can be operated a retention time as low as 7 days. Therefore, it is possible that claim (c ) above, regarding digester retention times, takes into account a safety factor so that the given retention times are relevant to full scale digestion under conditions that are not ideal. From this thesis investigation it is clear that thermophilic heat treatment of primary sludge does not improve the digestibility of the sludge under anaerobic conditions. In other words, the heating process does not condition the sludge. This result indicates that in a dual digestion system, the heat generated in the aerobic stage is not responsible for the conditioning effect that the aerobic stage has on the sludge. It is most probable that that the bacteriological action on the sludge is responsible for any conditioning. However, the effects of the aerobic conditioning on the anaerobic digestion process are still uncertain and more research on this system is required to conclusively verify some of the claims about it. iii ACKNOWLEDGEMENTS I wish to express my sincere thanks and appreciation to: Professor G A Ekama, my supervisor, for his encouragement during trying times. His guidance in the preparation of this thesis has been invaluable. Taliep Lakay, the laboratory technician, for his good humour and hard work that made life in the laboratory more bearable. Professor G vR Marais for his interest in my research and his never ending supply of advice and anecdotes. Dr MC Wentzel for his excellent administrative skills that kept the laboratory well stocked and in order. My bursary sponsors; Stewart Scott Incorporated, for allowing me the extra time required to complete this thesis. iv· TABLE OF CONTENTS Page SYNOPSIS 1 . ACKNOWLEDGEMENTS IV TABLE OF CONTENTS v LIST OF FIGURES viii LIST OF TABLES xiv LIST OF SYMBOLS xv CHAPTER 1: INTRODUCTION 1 ".} :. CHAPTER 2: OVERVIEW OF EXPERIMENTAL INVESTIGATION 6 2.1 Introduction 6 2.2 Operation of the mesophilic anaerobic digesters and the heat treated sludge reactors 6 2.3 Tests conducted on the influent and effluent sludges 10 2.4 Monitoring digester stability 11 2.5 Description of operational changes made to digesters Dl, D2, D3, and D4 13 2.5.1 Description of operational changes made to digester Dl 14 2.5.2 Description of operational changes made to digester D2 17 2.5.3 Description of operational changes made to digester D3 19 2.5.4 Description of operational changes made to digester D4 19 v CHAPTER3: PRESENTATION, EVALUATION AND l)ISCUSSION OF THE RESULTS FROM THE INVESTIGATION 23 3.1 Introduction 23 3.2 Summary of experimental data collected from digesters Dl, D2, D3 and D4 23 3.3 Evaluation and discussion of the volatile solids removal data 28 3.4 Evaluation and discussion of the COD removal data 30 3.5 Evaluation and discssion of the gas production data 32 3.6 ·Evaluation and discussion of the gas production per VS and COD removed data 32 3.7 Evaluation and discussion of the gas composition data 36 3.8 Evaluation and discussion of the pH data 36 3.9 Evaluation and discussion of the H2C03 * alkalinity data 39 3.10 Evaluation and discussion of the SCPA data 41 3.11 Evaluation and discussion of the SCPA :alk data 41 3.12 Discussion on the failure of digesters Dl and D3 44 3.13 Comparision of results from this investigation with results from other investigations on anaerobic digestion of sewage sludge 52 CHAPTER 4: CONCLUSIONS 58 4.1 Objectives of investigation 58 4.2 Experimental procedure 58 4.3 Conclusions 59 4.4 Discussion 60 REFERENCES 61 vi APPENDIX A: EXPERIMENTAL APPARATUS ~KING UP THE DIGESTION SYSTEM I / APPENDIXB: PROCEDURES FOR THE OPERATION, SAMPLING AND TESTING OF THE DIGESTION SYSTEMS VI APPENDIXC: TABLES AND GRAPHS OF THE DAY-TO-DAY MEASURED AND CALCULATED PARAMETERS FOR DIGESTERS Dl, D2, D3 ANDD4 XIII vii

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.in the thermophilic aerobic first stage and stabilisation in the anaerobic second stage in the aerobic stage the sludge is aerobically or thermally pretreated ·1.lk
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