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1.
THE EFFECT OF THERMOPHILIC HEAT TREATMENT
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ON THE ANAEROBIC DIGESTIBILI1Y OF PRIMARY SLUDGE
by
Hilton Izzett, BSc (Civ Brig) <Eape Town
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A thesis submitted in partial fulfillment
of the requirements. for the degree of Master
,, ' ' • : ~. 1 + "
of Science at the University of Cape Town
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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.
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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
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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
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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.
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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.
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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.
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TABLE OF CONTENTS
Page
SYNOPSIS 1
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ACKNOWLEDGEMENTS IV
TABLE OF CONTENTS v
LIST OF FIGURES viii
LIST OF TABLES xiv
LIST OF SYMBOLS xv
CHAPTER 1: INTRODUCTION 1
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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
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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
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APPENDIX A: EXPERIMENTAL APPARATUS ~KING UP
THE DIGESTION SYSTEM I
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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
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Description:.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
