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DISCLAIMER
The information in this document has been funded wholly or in part by
the United States Environmental Protection Agency under assistance agreement
*CR811022 to Lehigh University. It has been subjected to the Agency's peer
and administrative review and has been approved for publication as an EPA
document. Mention of trade names or commerical products does not constitute
endorsement or recommendation for use.
ii
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FOREWORD
Today's rapidly developing and changing technologies and industrial
products and practices frequently carry with them the increased generation of
materials that, if improperly dealt with, can threaten both public health and
the environment. The U.S. Environmental Protection Agency is charged by Congress
with protecting the Nation's land, air, and water resources. Under a mandate
of national environmental laws, the Agency strives to formulate and implement
actions leading to a compatible balance between human activities and the ability
of natural systems to support and nurture life. These laws direct the EPA to
perform research to define our environmental problems, measure the impacts, and
search for solutions.
The Risk Reduction Engineering Laboratory is responsible for planning,
implementing, and managing of research, development, and demonstration programs
to provide an authoritative, defensible engineering basis in support of the
policies, programs, and regulations of the EPA with respect to drinking water,
wastewater, pesticides, toxic substances, solid and hazardous wastes, and
Superfund-related activities. This publication is one of the products of that
research and provides a vital communication link between the researcher and the
user community.
One of the major procedures for stabilization of municipal wastewater
sludge is anaerobic digestion. Thermophilic digestion is of interest because
it insures pasteurization of sludge. This report deals with the conversion of
mesophilic digesters to thermophilic conditions and evaluates the potential
effect of sudden temperature decreases on thermophilic digester performance.
E. Timothy Oppelt, Acting Director
Risk Reduction Engineering Laboratory
111
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ABSTRACT
As part of a larger study on the comparison between mesophilic and
thermophilic anaerobic digestion, a study of the operation of anaerobic systems
under temperature transition was conducted. Systems seeded with domestic sewage
sludge, but subsequently fed a chemically defined complex medium, were operated
at 20- and 30-day detention times at 35 C. The temperature was raised to 55°C
at rates varying from 0.25°C to 2.5°C per day in duplicate, parallel units.
Irrespective of temperature rise rate, as soon as the temperature exceeded 45°C
methane production shut down. The units were held at 45°C until recovery
occurred. Once recovery of methane production ability occurred, transition to
55°C took place with little incident except for minor difficulty at 51°C. Data
analysis indicated that rate of temperature rise had little effect on the total
time required to obtain stable operation at 55°C; detention time had a minor
effect with longer detention times yielding superior results.
A temperature drop study was conducted from 55°C to as low as 47.5°C. No
adverse effect was observed until the temperature was reduced to less than 50°C.
Washout of methane bacteria seemed to occur when the temperature was suddenly
dropped below 50°C and the system detention time was less than 20 days.
Comparison of operation at 55°C vs. 35°C under steady-state at detention
times ranging from 7.5 days to 30 days indicated that based on effluent volatile
acids level, mesophilic operation was superior and that this superiority was
greater at the lower detention times.
The organisms which function under thermophilic conditions appear to be
present in mesophilic sludge but are not active at low temperature. When
thermophilic conditions are brought about, the thermophilic organisms will
multiply to an adequate level in several weeks.
This report was submitted in fulfillment of Grant No. CR811022 by Lehigh
University under the sponsorship of the U.S. Environmental Protection Agency.
This report covers a period from October, 1984 to April, 1986 and work was
completed as of March 31, 1988.
iv
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CONTENTS
Disclaimer ii
Foreword iii
Abstract iv
Figures vi
Tables ix
1. Introduction 1
2. Conclusions 2
3. Recommendations 3
4. Experimental Plan and Details 4
5. Results 10
6. Discussions 61
References 64
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FIGURES
Number Page
1 Batch Daily Feed Digester System 6
2 Gas production from Unit 9A (control): 30 day
detention time 11
3 Gas production from Unit 9B (control): 30 day
detention time 12
4 Temperature and volatile acids concentration vs. time
for Unit 9A (control): 30 day detention time 13
5 Temperature and volatile acids concentration vs. time
for Unit 9B (control): 30 day detention time 14
6 Gas production from Unit 10A (control): 20 day
detention time 15
7 Gas production in Unit 10B (control): 20 day detention
time 16
8 Temperature and volatile acids concentration vs. time
for Unit 10A (control): 20 day detention time 17
9 Temperature and volatile acids concentration vs. time
for Unit 10B (control): 20 day detention time 18
10 Gas production in Unit 3A (5°C increment): 30 day
detention time 19
11 Gas production in Unit 3B (5°C increment): 30 day
dentention time 20
12 Temperature and volatile acids concentration vs. time
for Unit 3A (5°C increment): 30 day detention time 21
13 Temperature and volatile acids concentration vs. time
for Unit 3B (5°C increment): 30 day detention time 22
14 Gas production from Unit 6A (5°C increment): 20 day
detention time 24
vi
-------
15 Gas production from Unit 6B (5°C increment): 20 day
detention time 25
16 Temperature and volatile acids concentration vs. time
for Unit 6A (5°C increment): 20 day detention time 26
17 Temperature and volatile acids concentration vs. time
for Unit 6B (5°C increment): 20 day detention time 27
18 Gas production from Unit 5A (3.5°C increment): 30 day
detention time 28
19 Gas production from Unit 5B (3.5°C increment): 30 day
detention time 29
20 Temperature and volatile acids concentration vs. time
for Unit 5A (3.5°C increment): 30 day detention time 30
21 Temperature and volatile acids concentration vs. time
for Unit 5B (3.5°C increment): 30 day detention time 31
22 Gas production from Unit 7A (3.5°C increment): 20 day
detention time 33
23 Gas production from Unit 7B (3.5°C increment): 20 day
detention time 34
24 Temperature and volatile acids concentration vs. time
for Unit 7A (3.5°C increment): 20 day detention time 35
25 Temperature and volatile acids concentration vs. time
for Unit 7B (3.5°C increment): 20 day detention time 36
26 Gas production from Unit 1A (2°C increment): 30 day
detention time 37
27 Gas production from Unit IB (2°C increment): 30 day
detention time 38
28 Temperature and volatile acids concentration vs. time
for Unit 1A (2°C increment): 30 day detention time 39
29 Temperature and volatile acids concentration vs. time
for Unit IB (2°C increment): 30 day detention time 40
30 Gas production from Unit 4A (2°C increment): 20 day
detention time 41
31 Gas production from Unit 4B (2°C increment): 20 day
detention time 42
vii
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32 Temperature and volatile acids concentration vs. time
for Unit 4A (2°C increment): 20 day detention time 43
33 Temperature and volatile acids concentration vs. time
for Unit 4B (2°C increment): 20 day detention time 44
34 Gas production from Unit 2A (1°C increment): 30 day
detention time 45
35 Gas production from Unit 2B (1°C increment): 20 day
detention time 46
36 Temperature and volatile acids concentration vs. time
for Unit 2A (2°C increment): 30 day detention time 47
37 Temperature and volatile acids concentration vs. time
for Unit 2B (1°C increment): 30 day detention time 48
38 Gas production from Unit 8A (1°C increment): 20 day
detention time 50
39 Gas production from Unit 8B (1°C increment): 20 day
detention time 51
40 Temperature and volatile acids concentration vs. time
for Unit 8A (1°C increment): 20 day detention time 52
41 Temperature and volatile acids concentration vs. time
for Unit 8B (1°C increment): 20 day detention time 53
42 Volatile acids concentration vs. time: 30 day detention
time 54
43 Volatile acids concentration vs. time: 20 day detention
time 55
44 Mean volatile acids concentration vs. time for units
subjected to temperature drops from 55°C to 55, 52.5,
50, and 45.5°C 57
45 Mean volatile acids concentration vs. time for units
subjected to temperature drops from 55°C to 55, 52.5,
50, and 47.5°C 58
46 Comparison of mean volatile acids concentration in
units at 30 day and 30 day detention times subjected to
temperature drops from 55°C to 47.5°C 59
viii
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TABLES
Number Page
1 Identification of Units in Temperature Rise Study 5
2 Feed Used in Study 7
3 Steady State Performance of Thermophilic and
Mesophilic Anaerobic Treatment Systems 60
4 Summary of Performance During Temperature Transition
from Mesophilic to Thermophilic Conditions 62
ix
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SECTION 1
INTRODUCTION
The purpose of this study was to conduct a comparative evaluation of
the performance of anaerobic digestion systems under different temperature
regimes. The temperature regimes chosen were those most commonly used in
field installations (i.e. mesophilic 35°C and thermophilic 50-55°C).
Evaluation of performance was in terms of a number of parameters including:
stability of operation, degree of waste stabilization, dewaterability of
digested sludge and odor.
The work has been divided into two phases. The first, which is
reported on here, deals primarily with the operation of anaerobic digestion
systems under situations of temperature transition. The second phase will
deal with differences between system performance at steady state in the two
temperature regimes.
The primary reason for conducting temperature transition studies
revolves around the question of how to start up a thermophilic digestion
system. The most efficient procedure is to seed with sludge from an
operational thermophilic system. However, this material may not be readily
available. In such a case it will be necessary to convert a mesophilic
anaerobic digestion system to a thermophilic system. Because methane
forming microorganisms are known to be quite sensitive to all environmental
conditions, it" was felt that a rapid change in temperature may not yield
positive results. On the other hand, if a very low rate of adjustment of
temperature is used, the anaerobic treatment system may be out of operation
for a long period of time. Consequently, an evaluation was made here into
the effect of temperature rise rate when converting from mesophilic to
thermophilic operation.
Another question which was addressed in this study was the effect of
temperature decreases on the performance of thermophilic systems. It is
generally considered that thermophilic systems are more sensitive than
mesophilic systems. Thus, small reductions in temperature may have severe
consequences on thermophilic system performance. This part of Phase I of
the study thus, incorporated experiments in which the temperature was
intentionally and rapidly reduced to determine the magnitude of the effect
on the thermophilic anaerobic systems. Finally, limited steady-state data
on system performance were collected at 55°C and 35°C to determine if
thermophilic anaerobic digestion is superior in performance to the
conventional temperature 35°C. Much more extensive steady-state data were
collected during Phase II and is reported on in the Phase II Report.
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SECTION 2
CONCLUSIONS
1) Race of temperature increase had little effect on the time required to
reach stable operation at 55°C during conversion from mesophilic to
thermophilic conditions.
2) The transition from mesophilic to thermophilic conditions occurs at
45°C as operation is interrupted (methane production halted) for
several weeks when this temperature is reached.
3) Eventual recovery occurs indicating that thermophilic organisms exist
in mesophilic sludge but are dormant.
4) It takes several days after reaching 45°C for the retardation to be
fully manifest.
5) At longer detention time operation temperature transition effects are
not as severe.
6) When the temperature is rapidly reduced from 55°C no effect on
operation is manifest until the temperature is reduced below 50°C and
the detention time below 20 dayc.
7) The temperature drop effect is probably not a reduction in the ability
of the methane bacteria to process substrate but in the ability to
reproduce.
8) Steady-state operation at 35°C was slightly superior to operation at
55°C at long detention times and clearly superior at short detention
times.
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SECTION 3
RECOMMENDATIONS
1) The data collected here gave some indication of a second retardation
temperature slightly above 51°C, this should be investigated.
2) These studies should be repeated with a feed of sewage sludge to
observe the effect of continuous reseeding from organisms in raw
sludge.
3) The washout phenomenon observed when a temperature drop took place
should be investigated.
4) Storage of thennophilic sludge at cold temperature (approx. 5°C) should
be investigated as a method of saving thennophilic sludge for later use
as seed for starting thermophilic digesters.
5) The data collected here indicated best performance of anaerobic
digestion may be- at temperatures in the range 40°C to 45°C. This
should be studied in depth.
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SECTION 4
EXPERIMENTAL PLAN AND DETAILS
Phase I contained four separate periods listed as A through D below:
A. Temperature Rise Rate Study
B. Steady State at 55°C and 35°C at 20 & 30 day detention times.
C. Temperature Drop Study
D. Steady State at 55°C & 35°C at detention times in the range from
20 to 7.5 days.
In all studies an artificial feed was used and feed and withdrawal was
on a periodic batch basis. In all studies monitoring of the units was based
on pH, alkalinity, volatile acids, gas volume and gas composition. Specific
details of A follow, and specific details of B, C & D will be given as
necessary when the results of these studies are presented.
TEMPERATURE RISE RATE
The major goal of Phase I of the study was to determine the effect of
rate of temperature change on system performance during conversion from
mesophilic conditions to thermophilic conditions. The basic plan used was
to set up a series of bench scale units at 35°C, operate them to a steady
state and then raise the temperature at various rates while maintaining a
normal feeding pattern as long as possible.
A series of i liter bench scale anaerobic digestion systems were set up
using digested sewage sludge from the anaerobic digester at the Allentown,
PA Sewage Treatment Plant. The liquid volume used in each digester was 750
ml. Each unit was connected to a cylindrical gas collection tube which had
been calibrated Jn increments of 10 ml. An acidified saturated salt
solution was used as the confining fluid to trap the gas evolved. The
confining fluid was held in a reservoir connected to the bottom of the gat
collection tube. A diagram of this experimental arrangement is given Lrt
Figure 1. Throughout this study these units were operated on a batch feed
and withdrawal basis. At each feeding interval a small portion of the
contents of the unit were first withdrawn and then replaced by a like amount
of fresh feed. The volume fed and withdrawn was a function of the hydraulic
detention time at which the unit was being operated. Just prior to
withdrawal, and also just after feeding, each unit was manually shaken for
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approximately 10-15 seconds to insure complete dispersion of the contents.
Thus these units were operated at identical solids and hydraulic retention
times (SRT-HRT).
A total of 20 units were set up, ten at each detention time used. In
addition, units were operated in pairs. This provided for four sets of
units at each detention time in which the temperature would be raised plus
one pair as a control. The control units were maintained at 35°C. The
units were numbered 1 through 10 with duplicates identified as A & B. The
four rates of temperature rise used were 1°C, 2°C, 3.5°C and 5°C per feeding
period (the term feeding period will be defined below). Table 1 lists the
numbering system used and the conditions of operation for each unit.
TABLE 1. IDENTIFICATION OF UNITS IN TEMPERATURE RISE STUDY
Identification Detention Time Temperature Rise Rate
1A-1B
2A-2B
3A-3B
4A-4B
5A-5B
6A-6B
7A-7B
8A-8B
9A-9B
10A-10P,
30 days
30 days
30 days
20 days
30 days
20 days
20 days
20 days
30 days
20 days
2°C
1°C
5°C
2°C
3.5°C
5°C
3.5°C
1°C
0°C
0°C
All of the units were set up in a temperature controlled room
maintained at 35+ 0.1°C. The units in which the temperature was to be
raised were in addition immersed in temperature controlled water baths.
Each consisted of a 10 gallon glass aquarium insulated with sheets of
polystyrene on all vertical sides. Beads of polystyrene were placed on top
of the water surface to act as insulation. Variable setting aquarium
heaters were used to raise the temperature above amibient levels in the room
(35°C). The water in the bath was kept in circulation by bubbling a small
quantity of air continuously into the water. Prior to actually conducting
the experiments detailed below, the settings on the thermostats were
manually calibrated. Mercury thermometers were inserted into 1 liter
bottles containing 750 ml of water. These were placed in the water bath and
used to calibrate heater setting; and thereafter for temperature
measurement. The thermometer units were kept in the water baths throughout
the study to monitor the temperatures. Eight water baths were used. Each
contained a pair of bench scale anaerobic units and the temperature
measurement bottle.
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Figure 1. Batch daily feed digester system. Black triangles indicate fluid levels.
glass
pressure
indicating
tube
withdraw! tube
gas
collection
one liter
digesters
airline
pressure
reducing
valve
_J
T
tuoes
V
V
r
n
?r valve
20 liter
reservoir
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As indicated above, these units were originally started with digested
sludge. It had been planned to use raw sludge from the Allentown Sewage
Treatment Plant as the feed during these tests. The initial detention times
chosen were 7.5 and 15 days. After several weeks it became apparent that
good operation at these conditions (35°C, 7.5 & 15 day detention time) was
not achieved. This was based on high levels of volatile acids in the units
(1000 mg/1). Consequently the detention times were increased to 20 and 30
days. Improvement was observed (the volatile acids fell to about 500 mg/1)
but operation was not satisfactory because of the high volatile acids level.
It was decided that an inhibitory substance or a nutritional deficiency was
manifest in the Allentown Treatment Plant. Consequently it was decided to
use an artificial feed in these studies. The feed chosen was a commercial
product "Carnation Instant Breakfast" suspended in whole milk. It was
chosen because when mixed with whole milk in the recommended proportion (1
envelope per 8 fluid ounces of milk) it had a fat, carbohydrate and protein
content similar to that of raw sludge. In addition, it contained most of
the organic and inorganic nutrients required by microorganisms. The feed
was, in addition, supplemented with several inorganic materials for which
methane fermenting organisms have a high demand, i.e., Fe, Co, Ni. A
complete analysis of the feed is given in Table 2. Within a few weeks of
this change in feed, the volatile acid level in the digester effluents was
consistently in the range of 100 to 200 mg/1, and stable gas production was
observed. The units were continued in operation for several months to
insure washout of the original seed material.
TABLE 2. FEED USED IN STUDY
Na2HP04 25 mg/1
Carnation Instant Breakfast 6.5 g/1
NH4HC03 1.9 g/1
CoCl2 1 mg/1 as Co
FeCl2 150 mg/1
Ammonium Molybdate 0.02 mg/1
MnCl2 0.4 mg/1
Boric Acid 0.01 mg/1
Sodium Tungstate 0.02 mg/1
NaCl 0.2 g/1
MgCl2 0.1 g/1
Nickel Acetate 2 mg/1
K2S04 0.2 g/1
Milk 66.6 ml
Yeast Extract 10 mg/1
Bethlehem Tap H20 1 liter
After the washout period was complete (>3 HRT's), the temperature
change period was initiated. The volume of feed required for the detention
times used corresponds to 25 ml per day at a 30 day HRT and 37.5 ml per day
at a 20 day HRT. It was felt that these volumes were too small to be fed
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accurately. Thus during this study the 30 day HRT unit was fed 50 ml once
every 2 days and the 20 day HRT units were fed 75 ml once every 2 days. It
was indicated above that nominal temperature changes per feeding period were
to be used. Thus a feeding period represents the period of 2 days after a
feeding event.
In conducting the temperature rise study the following procedure was
used. At the beginning of a feeding period the heater in the water bath of
the appropriate units was adjusted based on the prior calibration and the
normal once per 2 day withdrawal and feed took place. Periodically over the
next 2 days the temperature in the bath was checked to insure that it was
not above or below the new set point. At the end of the 2 day period a
comparison was made between the gas production from the last 2 days and that
from the control units (which were maintained at 35°C). If gas production
was close to that of the control unit the temperature was again raised and
the unit was given a normal feeding. If gas production was significantly
less than in the control unit the temperature was not raised and the units
were not fed. The units were not fed again until gas production data
indicated that most of the previous feed had been converted to methane and
carbon dioxide. The temperature was not raised again until most or all of
the feed added during the last feeding was consumed in the standard 2 day
period. It should be appreciated that judgement of the experimenter was
used in determining when to feed and when to raise the temperature. No firm
quantifiable rules were employed. As each run progressed and experience was
gained the decisions on feeding and temperature change were modified.
Each time a withdrawal was made from a unit the digester mixed liquor
was analyzed for pH, alkalinity and volatile acids. Daily gas production
was determined and at the end of each feeding period gas analysis for
methane and carbon dioxide was conducted. Alkalinity, pH and volatile acids
were monitored by the procedures given in Standard Methods(l) (volatile
acids by distillation); gas analysis was conducted using a Fisher Gas
Partioner(2).
STUDIES B, C, AND D
Over a three month time period the temperature of each unit except the
controls was raised to 55°C. At that time it was decided to continue
operation of these units at 55°C & 35°C respectively to establish a
comparison between operation at these temperatures with this feed and at
these 2 detention times.
Subsequent to this steady state period, a study was conducted in which
the temperature was suddenly dropped by various amounts while normal
operation i.e., feed and withdrawal were maintained. This study determined
the effect that a sudden loss of heat supply could have on the operation of
a thennophilic digester.
Only some of the 16 units being maintained at 55°C were used in the
temperature drop study. The remainder were used in a study in which the
detention times of the 55°C and the 35°C units were reduced down to as low
as 7.5 days. The purpose of this study was again to obtain a comparison
-------
between operation of units In parallel at the two temperatures but at much
lower detention times.
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SECTION 5
RESULTS
TRANSITION TO THERMOPHILIC TEMPERATURES
The results of this study are presented in the 40 figures which follow.
Some of these figures give gas production as a function of time and
temperature, while others provide plots of digester volatile acids versus
time and temperature. The results are discussed below grouped primarily
according to rate of temperature rise and secondarily according to detention
time. In the plots of gas production each vertical bar represents one
feeding period. During periods of good operation a feeding period
represents 2 days so the bar has 2 sections (unmarked and crosshatched) .
The unmarked section is gas production during the first day after feeding.
The crosshatched section represents the gas production on the second day
after feeding. When retarded operation occurred each bar had more than 2
sections as it required more than 2 days to consume the feed. Each section
of the bar, either unmarked or crosshatched represents one day. Thus, if a
vertical bar has 6 sections it means it required 6 days to consume a
standard feeding. For the first 20 days (10 feeding periods) of this study
no temperature rise took place. These data give an indication of normal
operation at 35°C prior to initiation of the temperature rise.
Rate of Temperature Rise - 0°C - 30 Day Detention Time
Figures 2-5 present the performance of the 30 day detention time
controls during the 100 days of this study (units 9A and 9B). Gas
production was 584 + 37 ml for 9A and 574 + 36 ml for 9B. Approximately 90%
of the gas was produced during the first day. The abnormally high
production of gas on day 80-82 for 9A was due to a minor leak which occurred
on that day. Volatile acids averaged <200 mg/1, with occasional excursions
near 400 mg/1.
Rate of Temperature Rise - 0°-C - 20 Day Detention Time
Figures 6-9 present the performance of the 20 day detention time
controls (units 10A & 10B). Results obtained were similar to those for the
30 day controls. Gas production was consistently good and volatile acids
low; <200 mg/1. However, one unusual event which occurred was that during
the period day 26-28 unit 10A was killed when the acid-salt confining fluid
accidentally gained entrance to the unit. A new unit was immediately set up
using previous days effluent from units 10A & 10B which had been stored in
the refrigerator at 4°C. It can be seen that this new unit started up
almost instantaneously. This phenomenon of being able to start an anaerobic
10
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Figure 2. Gas production from unit 9A (control): 30 day detention time.
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Figure 4. Temperature and volatile acids concentration versus
time for unit 9A (control): 30 day detention time.
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Figure 6. Gas production from unit 10A (control): 20 day
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Figure 7.
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20
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Time in days
80
100
17
-------
Figure 9. Temperature and volatile acids concentration versus
time for unit 10B (control): 20 day detention time.
-i 60
en
E
12
10
8
6
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3
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u
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Figure 10. Gas production in unit 3A (5°C increment)
30 day detention time.
700 -n
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Figure 11,
Gas production in unit 3B (5°C increment)
30 day detention time.
7°°-|35
I
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Figure 12. Temperature and volatile acids concentration versus
time for unit 3A (5°C increment): 30 day detention time.
55C
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80
100
21
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Figure 13. Temperature and volatile acids concentration versus
time for unit 3B (5°C increment): 30 day detention time,
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22
-------
digestion unit rapidly from digester effluent or mixed liquor stored at 4°C
was observed several other times during this study. The average gas
production from unit 10B was 837 ±45 ml. About 15% of the gas production
took place on the second day of the feeding period. Data for 10A were not
averaged because of its replacement as detailed above. The ratio of gas
production for the 20 day detention time to that for the 30 day units was
1.45 which is close to the theoretical value of 1.5. As indicated above
volatile acids were always low except for an excursion to 500 mg/1 in unit
10B on day 54.
Rate of Temperature Rise - 5°C - 30 Day Detention Time
Figures 10-13 present the data for the 5°C rate of rise. It can be
seen that the units (3A & 3B) exhibited similar performance. At 40°C and
45°C, performance was as good or even better than the control units (35°C) .
However the transition from 45°C to 50°C created an adverse situation both
with respect to the acid formers and methane formers. The effect was not
immediately apparent in the first feeding period after the transition except
in the slight shift in proportion of gas production on the first and second
days (90% first day at 45°C, 80% first day 50°C). However, the units
essentially shut down during the second feeding period after the temperature
reached 50°C. It was not until day 58 (a period of 30 days) that these
units could function at the normal 2 day feeding period. Even then the
proportion of gas production during the first day was low. By day 68 the
units exhibited nearly normal operation and low volatile acids. On day 69-
70 the temperature was raised to the target of 55°C. Over the next 30 days
the normal 2 day feeding period could be maintained but the volatile acids
gradually increased (almost linearly) from 200 mg/1 to 800 mg/1.
Rate of Temperature Rise - 5°C- 20 Day Detention Time
Figures 14-17 depict the results for these conditions. Similar results
to those obtained at 30 day detention time were observed. Until 45°C no
adverse effect was noted. A small drop in gas production and concomitant
rise in volatile acids occurred during the first feeding period after 50°C
was reached. During the second feeding period a significant shutdown
occurred. Both methane and acid formers were affected but the effect on the
former was much greater. It was not for another 40 days (day 26 - day 66)
that the normal 2 day feeding period could be resumed, and 10 more days were
required until it was felt that it was safe to raise the temperature to
55°C. The shift to 55°C was not as uneventful as at the longer detention
time. Some retardation which resulted in modest increases in volatile acids
occurred. To keep the volatile acids under control a 4 day feed period was
used during day 90-94. Even so volatile acids were quite high, 1100 mg/1 in
unit 6A while they were low, 450 mg/1 in unit 6B. A short temperature
excursion to almost 60°C due to a malfunction of the heater in the 6A-6B
water bath may be to blame for the poorer performance of these units.
Rate of Temperature Rise - 3.5°C - 30 Day Detention Time
As illustrated in figures 18-21 the major adverse effect was manifest
when the transition from 45.5°C to 49°C occurred. As before in the first
23
-------
Figure 14. Gas production from unit 6A (5°C
increment): 20 day detention time.
45
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Time in days
24
-------
Figure 15. Gas production from unit 6B (5°C
increment): 20 day detention time.
— y L/U-
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82 86 90 $6 300
25
-------
Figure 16. Temperature and volatile acids concentration versus
time for unit 6A (5°C increment): 20 day detention time.
20
40 60
Time in days
80
100
-------
Figure 17. Temperature and volatile adds concentration versus
time for unit 6B (5°C increment): 20 day detention time.
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Time in days
80
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27
-------
Figure 18. Gas production from unit 5A (3.5 C
increment): 30 day detention time.
055 ml
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Time in days
-------
Figure 19. Gas production from unit 5B (3.5 C
increment): 30 day detention time.
N3
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78 82 86 SO $*• 58
-------
Figure 20. Temperature and volatile acids concentration versus
time for unit 5A (3.5°C increment): 30 day detention time.
•I2r
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80
100
30
-------
Figure 21. Temperature and volatile acids concentration versus
time for unit 5B (3.5°C Increment): 30 day detention time.
52.55
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80
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31
-------
feeding period after the transition to 49°C, only minor difficulty was
observed but during the second feeding the full effect of the higher
temperature was observed. It was 24 days before the normal 2 day feeding
pattern could be resumed. Recovery was excellent as the transition to
52.5°C was made in 4 days and that to 55°C in another 4 days. At 55°C some
retardation was observed as volatile acids started to rise. The volatile
acids returned to normal in unit 5A (200 mg/1) but became elevated (700
mg/1) in unit 5B.
Rate of Temperature Rise - 3.5°C - 20 Day Detention Time
Figures 22-25 illustrate a pattern similar to that observed at the
longer detention time. All was satisfactory until the transition to 49°C
from 45.5°C. In this case the initial result of the change was more severe
than at the 30 day detention period. It took 30 days until the normal
feeding pattern could resume and 12 more days until a transition to 52.5°C
could take place. In another 2 days a transition to 55°C took place, but
this may have been premature. A rapid rise in volatile acids occurred which
was brought under control by going through a single 4 day feeding period.
Eventually the normal feeding of once per 2 days could be continually used,
and volatile acid levels dropped to the range 200 to 400 mg/1. An
interesting observation is that at 55°C the second day gas volume was close
to 25% of the total versus about 15% at 35°C.
Rate of Temperature Rise - 2°C - 30 Day Detention Time
Figure 26-29 illustrates the familiar pattern except that the use of a
slower rate of temperature rise allows finer delineation of the temperature
effect. Under these conditions the initial adverse effect is noted at the
transition fr 45°C to 47°C. Again the full effect is delayed for at least
one feeding p^ -id. It took only 14 days till the normal 2 day feed period
could be reinstated, however, 16 more days passed before the transition to
49°C occurred. In retrospect this was excessive as volatile acid levels
were normal 8 days earlier. However, at this stage the investigators had
become sensitized to being optimistic about the ability of these systems to
avoid adverse effects of temperature change. After 6 days at 49°C the
temperature was pushed to 55°C in the next 6 days. Volatile acids rose and
gas production fell temporarily, but were soon reasonably satisfactory
without the need to depart from the 2 day feed schedule.
Rate of Temperature Rise - 2°C - 20 Day Detention Time
With these units, much more severe retardation was observed (Figures
30-33) than at the 30 day detention time. It was decided not to stop at
47°C but to go on to 49°C. This caused a 36 day period in which the 2 day
feed cycle could not be used and a total period of 40 days before another
temperature increase (to 51°C) was attempted. At 53°C some minor
retardation occurred and 55°C was reached in 6 more days. However, it was
necessary to twice use a 4 day feed cycle before the units operated normally
at 55°C. In this situation the A unit did not do as well as the B unit.
Volatile acids in A were 800 mg/1 while they were 300 mg/1 in B after steady
operation at 55°C was achieved.
32
-------
Figure 22. Gas production from unit 7A (3.5°C
increment): 20, day detention time.
J.UUU -
900 .
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Time in days
33
-------
Figure 23.
Gas production from unit 7B (3.5°C
increment): 20 day detention time.
45.5
1000 -•
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Figure 24. Temperature and volatile adds concentration versus
time for unit 7A (3.5°C Increment): 20 day detention time.
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80
100
35
-------
Figure 25. Temperature and volatile acids concentration versus
time for unit 7B (3.5°C increment): 20 day detention time.
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36
-------
Figure 26. Gas production from unit 1A (2°C increment): 30 day
detention time.
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Figure 27. Gas production from unit IB (2°C increment)
30 day detention time.
CO
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-------
Figure 28. Temperature and volatile acids concentration versus
time for unit 1A (2°C increment): 30 day detention time.
40 60
Time in days
80
100
39
-------
Figure 29. Temperature and volatile acids concentration versus
time for unit IB (2°C increment): 30 day detention time.
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Figure 33. Temperature and volatile acids concentration versus
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40 60
Time in days
80
100
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-------
Figure 34. Gas production from unit
detention time.
2A
(1°C
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Figure 36. Temperature and volatile acids concentration versus
time for unit 2A (1°C increment): 30 day detention time.
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-------
Figure 37. Temperature and volatile acids concentration versus
time for unit 2B (1°C increment): 30 day detention time.
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Rate of Temperature Rise - 1°C - 30 Day Detention Time
Figures 34-37 illustrate system performance under the lowest rate of
temperature rise and loading stress used. Some initial effect was observed
at 46°C (note change in proportion of gas between first and second day) but
it was decided to go up to 47°C. At this temperature, retardation started
but in only 10 days a return to the 2 day feed pattern was possible and in 6
more the temperature was increased to 48°C. The system was fed normally but
held at 48°C for six days and then pushed to 53°C. The volatile acids rose
sharply and increased during the rise to 53°C; but this was overcome by
holding for 6 days at 53°C. After 84 days the temperature reached 55°C
where satisfactory operation at the normal feeding interval was attained.
Again it should be noted that gas production in the second day is a higher
proportion of the total at 55°C than at 35°C.
Rate of Temperature Rise - 1°C - 20 Day Detention Time
The data presented in figures 38-41 indicate that at this detention
time temperature effects were worse than at the 30 day detention time. Here
the fault cannot be excessive zeal in pushing the temperature rise as it was
for the 2°C units. As with the 30 day detention time units, retardation was
observed at 46°C. Here it took 28 days until the normal feed pattern could
be used and 4 more days till a rise to 47°C was tried. The temperature was
quickly raised 51°C but this was accompanied by a rapid rise in volatile
acids so the unit was held at 51°C for 4 rather than 2 days. A rapid rise
to 54°C was again accompanied by a volatile acid rise. A feeding was
skipped at 54°C leading to a significant fall in volatile acids. After 6
days at 54°C the unit went to 55°C and satisfactory operation seemed to be
manifest.
STEADY STATE AT 55°C AND 35°C AT 20 & 30 DAY DETENTION TIMES
Once all the units discussed above had reached 55°C and exhibited
satisfactory operation, it was decided to maintain these systems at the
conditions reached at the end of the temperature rise rate study, and
observe their operation over an extended period of time. A plot of the
effluent volatile acids for the next ninety days is given in Figures 42 and
43. It can be seen that initially there was some disparity in the
performance of the units. Over the first 30 to 40 days of operation some
units exhibited quite good performance (Units 2A and 5A at 30-day detention
time and Units 6B, 7A, and 7B at 20-day detention time). The other units
exhibited an increase in volatile acids to approximately 1200 mg/1. In
addition, it was noted that the gas produced in these units was rather
odorous.
It was apparent that some thermophilic organisms were present in Units
2A, 5A, 6B, 7A and 7B that were not present in the other units. In order to
get all of the units operating equally, it was decided to seed the poorly
operating units with organisms from the units showing good operation.
Consequently, on 45th day for the 20-day detention time units and the 46th
day for the 30-day detention time units extra effluent was withdrawn from
49
-------
Figure 38. Gas production from unit 8A (1°C increment):
20 day detention time.
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Figure 39. Gas production from unit 8B (1°C increment):
20 day detention time.
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51
-------
Figure 40. Temperature and volatile acids concentration versus
time for unit 8A (1°C increment): 20 day detention time.
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40 60
Time in days
80
52
-------
Figure 41. Temperature and volatile acids concentration versus
time for unit 8B (1°C increment): 20 day detention time.
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Figure 42. Volatile acids concentration versus time: 30 day detention time.
1400 r
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A ^ reseed all units
------ unit 1A & IB
.......... unit 2A & 2B
unit 3A & 3B
---- unit 5A & 5B
' ' I I 1 1 1 1 1 1 1 '
10 20 30 40 50 60
Time in days
70 80
90
-------
Figure 43. Volatile acids concentration versus time: 20 day detention time.
Ln
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unit 6A & 6B
unit 7A & 7B
unit 8A & 8B
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Time in days
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-------
all units, pooled and anaerobically Inserted back into each unit. The data
in Figures 42 and 43 clearly illustrate the efficacy of this strategy.
Within 10 to 15 days the volatile acid level in the effluent from all units
was between 200 and 400 mg/1. For the remainder of the time of operation
satisfactory performance was obtained from all units. In addition, the
disagreeable odor disappeared from the gas.
Analysis of these data do not indicate any particular pattern with
respect to units which retained satisfactory organisms in the transition to
55°C and those which did not. For example, in the case of Unit 2A and 2B
(duplicates) one unit exhibited excellent operation, while the other
exhibited marginal operation after the transition to 55°C was achieved. The
same situation was manifest for Units 5A and 5B and Units 6A and 6B. Rate
of temperature rise was not a factor in determination of units which
achieved the proper population at 55°C; nor was detention time.
TEMPERATURE DROP STUDY
One of the concerns expressed with respect to utilization of
thermophilic digestion systems is the sensitivity which they may exhibit to
temperature changes. Since these units operate at temperatures much higher
than mesophilic digesters it is feared that a small temperature drop will
have much more of an adverse effect than it would on the lower temperature
systems. To determine the effect of a temperature drop on thermophilic
systems several of the 55°C units were subjected to a temperature drop. The
temperature was dropped from 55°C to the new setpoint over night and then
maintained at the new setpoint. The normal feed and withdrawal pattern was
maintained. The performance of these systems was then monitored for the
next 25 days. Units with detention times of both 20 and 30 days were used.
Temperatures were reduced to: 52.5°C, 50°C, and 47.5°C. Controls were run
with units maintained at 55°C and 35°C.
The results at a 30-day detention time are given in Figure 44. These
data indicate that effluent volatile acids remain low in all units and were
similiar to the control values. There was some early tendency for the
volatile acids to increase in the unit whose temperature was reduced to
47.5°C. However, this trend was soon reversed. The data for the 20-day
detention time units, given in Figure 45, exhibits a similar situation;
except that both the 50°C and 47.5°C units exhibited an early tendency for
volatile acids to rise. The trend was reversed for the 50°C unit, but not
for the 47.5°C unit. Figure 46 is a replot of the data from the 20-day and
30-day detention time units in which the temperature was dropped to 47.5°C.
This plot clearly shows the significant difference which resulted. The
pattern of rise in volatile acids exhibited by the 20-day detention time
unit is typical of a wash-out phenomenon. That is, the temperature
reduction does not interfere with the microorganisms ability to degrade the
substrate, but for some reason their ability to reproduce is impaired.
Thus, the biomass in the system gradually decreases, and the effluent
volatile acids gradually increase. To be sure no direct measurement of
biomass was made so this is a speculation based on failure mode. It is
56
-------
Figure 44.
Mean volatile acids concentration versus time for
units subjected to temperature drops from 55°C to
55, 52.5, 50, and 47.5°C. Units at 35°C are con-
trols. 30 day detention time.
300
day of /,
temperature drops / .
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57
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Figure 45.
800
700
u 600
500
° 400
AJ
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Mean volatile acids concentration versus time for
units subjected to temperature drops from 55°C to
55, 52.5, 50, and 47.5°C. Units at 35°C are con-
trols. 20 day detebtion time.
55°
•52.5°
47.5°
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day of
temperature drops
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58
-------
Figure 46. Comparison of mean volatile acids concentration in
units at 20 day and 30 day detention times subjected
to temperature drop from 55°C to 47.5°C.
800
700
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30 day detention time
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Time in days from temperature drop
-------
Possible that the 30-day detention time unit would also have gone into a
tallure mode if the run had been continued for a longer period of time.
Despite the problem noted above, when the temperature was dropped to
47.5 C, these data indicate that operational problems from failure of the
heating system of a thermophilic digester should not be a major concern.
The only adverse situation which was manifest took several weeks to develop.
It is unlikely that the heating system of a digester could not be repaired
in a few days. In addition, the rate of temperature drop utilized here was
extreme compared to that which would occur in the field. Field digestion
systems t. re massive tanks with high heat capacity. It is unlikely that the
digestion temperature would drop by more than 1°C per day even if a complete
failure of the heating system occurred in the middle of the winter.
COMPARISON OF MESOPHILIC AND THERMOPHILIC OPERATION
The units which had operated .t 55°C and which were not used for a
temperature drop study were used to evaluate the operation of anaerobic
treatment systems at 55°C vs. 35°C for detention periods below 20 days.
Detention periods used here were 15, 10 and 7.5 days. The performance at 20
and 30 day detention time was similar based on effluent volatile acids;
although the mesophilic units had slightly lower volatile acids levels. It
was expected that if there was a difference between mesophilic and
thermophilic operation it would be most apparent at the lower detention
times. Units were converted to the lower detention time and then operated
at the new detention times for a period equal to at least 3 detention times.
All units eventually exhibited steady-state operation in which the effluent
volatile acids stayed relatively constant within a narrow range. These data
are presented in Table 3 and include all of the hydraulic detention times in
this study (7.5, 10, 15, 20, 30 days). These data show that as the
detention time was decreased the disparity in performance between the
mesophilic and thermophilic units increased. At 10 day detention time or
lower the mesophilic units clearly exhibited superior performance.
TABLE 3. STEADY STATE PERFORMANCE OF THERMOPHILIC AND MESOPHILIC
ANAEROBIC TREATMENT SYSTEMS
HRT Effluent Volatile Acids mg/1
55°C 35°C
30
20
15
10
7.5
100-150
150-250
200-300
300-600
1000-1200
50-100
75-100
100-150
100-200
200-300
60
-------
SECTION 6
DISCUSSIONS
TRANSITION TO THERMOPHILIC TEMPERATURES
The major question explored in this part of the research effort was
what is the best procedure for converting a mesophilic anaerobic digester to
a thermophilic unit? Table 4 presents a summary of pertinent data which can
be used to address this question. For each temperature rise rate and
detention time the following data are presented: time in days to first
reach 55°C, time in days to achieve stable operation at 55°C, time in days
to the first temperature inhibition, time in days to be able to return to a
normal 2 day feed pattern after inhibition, time in days to the next
temperature change after the first temperature inhibition and the number of
2 day periods during which feeding took place until 55°C was achieved. The
figures presented in the table except for the first and last columns
represent days after the first change from 35°C. The following major points
can be deduced from this table and from Figures 2-41.
1. The slower the temperature rise the longer it takes to initially
reach 55°C.
2. The rate of temperature rise has only a minor effect on the time
to reach stability at 55°C.
3. Irrespective of the rate of temperature rise, a major retardation
occurs when ever the temperature exceeds 45°C.
4. The effect of this retardation is less at low temperature rise
rates.
5. At high temperature rise rates once acclimation to the
thermophilic conditions occur, few problems are manifest.
6. At low temperature rise rates small periods of acclimation are
needed as the unit proceeds from 45°C to 55°C.
7. At low temperature rise rates the unit can be fed a high
percentage of time during the transition procedure.
8. At high temperature rise rates the unit can be fed only a small
percentage of time during the transition period.
9. There seems to be a zone of minor retardation in the 50-55°C
range.
61
-------
10. Temperature changes adversely affect the methane bacteria to a
much greater extent than the acid formers.
11. Temperature effects are not Instantaneous. During the first feed
period after 45°C is exceeded almost normal operation occurs.
Retardation is manifest during the next feeding period.
12. Temperature effects are magnified at lower detention time.
TABLE 4. SUMMARY OF PERFORMANCE DURING TEMPERATURE TRANSITION
FROM MESOPHILIC TO THERMOPHILIC CONDITIONS
Temp.
Change -
Detention
Time
5-30
5-20
3.5-30
3.5-20
2-30
2-20
1-30
1-20
Time to
Reach
55°C
52
56
54
48
54
60
64
74
Time to
Reach
Stability
at 55°C
52
72
54
56
62
68
64
74
Time to
First Major
Inhibition
6
6
10
8
14
12
24
22
Time to
Resume 2 day
Feed Pattern
38
48
44
34
26
48
36
48
Time Till
Next Temp.
Change
52
56
48
46
42
52
42
50
# of
Feeding
Periods
to 55°C
13
13
13
14
23
21
30
27
Overall it does not seem that the rate of temperature rise affects the
speed of conversion to thermophilic conditions. A decision on the rate to
use will depend on another major factor, i.e., what can be done with the raw
sludge during the conversion process. If there is no alternate sludge
handling procedure the slow rate of rise should be used, as for the most
part, feeding of the digester can continue. If an alternate sludge handling
procedure exists the rapid rise method would achieve operation at 55°C in a
shorter period of time. Since operation is satisfactory between 35°C and
45°C, perhaps a combination procedure would be best; a rapid rise to 45°C
followed by a slow rise to 55°C.
In this study it was observed that once the temperature increased above
45°C the units suffered retardation. The retardation was more dramatic when
the temperature increment was high than when modest increments were used.The
5°C increment units virtually shut down for several weeks and then started
to recover. The units with lower temperature increments did not exhibit
such a complete shutdown, rather they suffered some degree of temporary
retardation. All of the units eventually completely recovered and
demonstrated the ability to operate satisfactorily at temperatures in the
range of 45°C to 55°C. It is probable that the units contain organisms
which are thermophilic, but which do not function at temperatures below
62
-------
. Once the temperature enters a range that these organisms can function
n. they begin to function. The amount of time required to build the
population of thermophiles to the level required for optimum performance was
the several weeks noted above. This time period is consistent with the
known doubling time of methane bacteria which is in the range of 3 to 4
days. Since most of the original seed sludge had been washed out of these
units prior to starting the rise-rate study, and was replaced with
mesophilic organisms which could reproduce on the substrate used, it is
probable that very few thermophilic organisms were present when tlie
temperature initially reached thermophilic levels. Thus, if domestic sewage
sludge had been the feed transition to thermophilic conditions would have
been easier because the units would have been continually reseeded with
dormant thermophilic microorganisms. It is unlikely that the artificial
substrate actually used here contained thermophiles because the Carnation
Instant Breakfast is radiation sterilized by the manufacturer, and the milk
used is pasturized at elevated temperatures (much higher than 55°C). Thus,
the presence of thermophilic methane bacteria in the feed is unlikely.
STABILITY OF OPERATION AT 55°C
After the temperature transition to 55°C was achieved the units were
maintained at this temperature for the next 3 months. It was observed that
some of the units exhibited good operation while others exhibited marginal
operation (volatile acids of approximately 1,000). It was found that some
of the microflora required for optimum operation at 55°C did not survive the
transition. Seeding from units which had this microflora restored optimum
operation. Thus, while it is apparent that thermophilic organisms exist in
anaerobic treatment systems which are operating at mesophilic conditions,
some may have difficulty surviving the transition. Thus, some seed from a
thermophilic digester exhibiting good operation may be necessary in order to
insure trouble free transition to thermophilic conditions.
TEMPERATURE DROP STUDIES
The data collected indicate that a temporary temperature reduction to
as low as 47.5°C will not have an adverse effect on a thermophilic system
which has been operating at 55°C. A temperature drop to as low as 50°C will
not have an adverse effect even if this temperature drop is permanent. The
magnitude of the effect observed when the temperature drops below 50°C will
depend upon the detention time of the system. Because the adverse effect
noted appeared to be related primarily to washout rather than an inability
of the organisms to degrade the substrate, the adverse effect is magnified
at lower detention times.
COMPARISON OF MESOPHILIC VS. THERMOPHILIC OPERATION
At long detention times it was found that mesophilic and thermophilic
anaerobic digestion systems exhibited similar operation; although the
mesophilic systems were slightly superior. Once the detention time was
reduced below 10 days significant performance advantage accrued to the
mesophilic system.
63
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REFERENCES
1. Standard Methods for the Examination of Water and Wastewater APHA,
AWWA, WPCF, 15th Ed. (1985).
2. Fisher Scientific Co., Catalogue (1986), Model #1200, p. 250.
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