United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-88/072 June 1989
&EPA Project Summary
Reductions of Enteric
Microorganisms During Aerobic
Sludge Digestion: Comparison of
Conventional and Autoheated
Digesters
John H. Martin, Jr.
The objectives of this investigation
were to: (1) determine the seasonal
variations in sludge stabilization and
reductions in the densities of
indicator organisms, Salmonella ssp.,
and enteroviruses that occur with
conventional aerobic digestion in
cold climates, and (2) demonstrate
that both sludge stabilization and
reductions in the densities of these
microorganisms can be improved by
simple modifications that increase
process temperature. Two 32 m3
aerobic digesters located at a small
municipal wastewater treatment plant
were operated continuously over a
period of 20 mo to obtain the data
necessary to satisfy these objec-
tives. One digester was a conven-
tional digester while the other was
designed to minimize heat losses
and thus, facilitate autoheating.
This Project Summary was devel-
oped by EPA's Risk Reduction
Engineering Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report
ordering information at back).
Introduction
Land application is one of the more
commonly utilized methods for the
ultimate disposal of municipal wastewater
treatment (sewage) sludges in the United
States. This is particularly true for small
and medium size municipal wastewater
treatment plants provided that concen-
trations of heavy metals and toxic organic
compounds are at or below established
acceptable levels. The land application of
these "clean" sludges is not without risk,
however, due to the possible presence of
pathogenic organisms and the potential
for direct or indirect public contact.
In recognition of the potential for
disease transmission related to the land
application of sewage sludges, the U.S.
Environmental Protection Agency
(USEPA) was required by Subtitle D of
the Resource Conservation and Recovery
Act (PL94-580) and by Section 405(d) of
the Clean Water Act of 1977 (PL95-217)
to establish criteria for the control of
infectious diseases in the land application
of sewage sludges and septic tank
pumpings. Such criteria were promul-
gated by the USEPA in 1979 as part of 40
CFR 257, "Criteria for Classification of
Solid Waste Disposal Facilities and
Practices" (U.S. Code of Federal Regu-
lations, 1979).
The interim final criteria contained in
Section 257.3-6 of 40 CFR 257 specify
minimum operating parameters for
processes used to stabilize sewage
sludges and septage prior to surface
application or incorporation of these
materials into the soil. For aerobic
digestion to be acceptable as a "Process
to Significantly Reduce Pathogens," the
following operating parameters are re-
quired. The process must be conducted
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by agitating sludge with air or oxygen to
maintain aerobic conditions at residence
times ranging from 60 days at 15°C to
40 days at 20°C, with a volatile solids
reduction of at least 38%.
With the exception of anaerobic
digestion, aerobic digestion is probably
the most widely used process of the
designated processes to significantly
reduce pathogens in the United States.
Both ease of operation and relatively low
capital costs have made aerobic
digestion of sewage sludges particularly
attractive for small municipal wastewater
treatment plants such as those that are
common in rural areas. However, several
disadvantages also are associated with
the use of aerobic digestion for sewage
sludge stabilization. The principal
disadvantages are high energy costs and
the fact that process performance is
significantly influenced by climate. Due
to relatively long residence times, nor-
mally a minimum of 10 to 15 days, and
the use of open tanks, mixed liquor
temperatures can vary by as much as
25°C between summer and winter
operation in northern climates.
As with all biological waste treatment
processes, the performance of the
aerobic sludge digestion process is
temperature dependent. As temperature
decreases, the rate of microbial activity
and thus the rate of oxidation of bio-
degradable organics, which translates
into the rate of stabilization, is reduced..
For example, an empirical relationship
that has been developed suggests that it
is necessary to increase the solids
resistance time (SRT) from 22.5 days at
25 °C to 45 days at 10°C to realize a
40% reduction in the concentration of
volatile solids. At 5°C, a 90-day SRT
appears to be necessary.
Available evidence suggests that
temperature not only affects the rate of
sludge stabilization but also the rates of
inactivation of pathogens and indicator
organisms. Little information has been
available, however, concerning the
effectiveness of aerobic sludge digestion
in reducing the densities of these
microorganisms, particularly at psy-
chrophilic and mesophilic temperatures.
Materials and Methods
This investigation was performed
under a cooperative agreement between
Cornell University and the Risk Reduc-
tion Engineering Laboratory (RREL) of
the USEPA. The research was conducted
at the Trumansburg, NY, wastewater
treatment plant and in the Waste
Management Laboratory, Department of
Agricultural Engineering, New York State
College of Agriculture and Life Sciences,
Cornell University. The climate of this
area made the Trumansburg wastewater
treatment plant a very appropriate site for
this study. Average monthly air temper-
atures range from a low of -5.6°C in
January and February to a high of 20 °C
in July. During the period from mid-
December through mid-March, daily
minimum temperatures below -12°C
are not uncommon and temperatures as
low as -23°C to -29°C can occur.
Wastewater Treatment Plant
Details
The Trumansburg wastewater treat-
ment plant, which was designed for an
average flow of 946 m3/day, employs the
conventional activated sludge process
without primary clarification to provide
secondary treatment for the Village's
wastewater. Waste activated sludge is
thickened without chemical conditioning
using a gravity thickener and then is
stabilized using conventional aerobic
digestion. Following stabilization, Tru-
mansburg sludge is lagooned and
ultimately disposed of by spreading on
agricultural land by a private contractor.
Investigative Facilities
To provide the facilities necessary to
satisfy the objectives of this investigation,
two 32-m3 aerobic digesters were
added to the Trumansburg wastewater
treatment plant. One digester was
designed to minimize heat losses and
thus facilitate autoheating while the
second was designed to be a
conventional aerobic digestion unit. Two
vertical 3.66-m-diameter by 3.96-m-
high tanks fabricated from 6.35-mm
mild carbon steel plate were used. One
tank, the autoheated digester, was a
closed tank with a 0.91-m manhole
located in the top of the tank to permit
access for aerator installation and
removal. The second of these two tanks,
which was used as the conventional
digester, originally was an open top tank.
Both digesters were insulated with a
7.6-cm coating of 32-kg/m3 density
urethane foam. Following 17 mo of
operation, an insulated cover was added
to the conventional digester.
In the autoheated digester, a Framco*
submersible, self-aspirating aeration
unit was used. This aeration unit has a
tap water oxygen transfer efficiency of
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use
approximately 22% as compared to ^
to 8% for coarse bubble diffuse
commonly used for conventional aerol
sludge digestion. Thus, effluent gas hi
losses were minimized. Original
Chicago Pump Discfuser coarse bubl
diffusers were installed in the ccnvc
tional digester. After 17 mo of operati
and in conjunction with the addition of
insulated cover to this digester, t
coarse bubble diffusers were replac
with Wyss Flex-A-Tube fine bubt
diffusers in an attempt to increa
oxygen transfer efficiency and redu
diffuser fouling problems.
Data Collection
In order to develop the data ba
necessary to satisfy the objectives of t
investigation, the two previous
described 32.2-,m3 aerobic digest*
were operated continuously from July ;
1985, through March 30, 1987. Duri
this period, the autoheated digester vi
operated at residence times of 10,
and 20 days while the residence time
the conventional digester was ht
constant at 20 days. Operation of I
autoheated digester at a residence tii
of 5 days also was attempted but v
terminated before steady-state con
tions were established since the oxyr:
transfer capacity of the Framco aera
was found to be inadequate to satisfy
exerted oxygen demand at this resider
time. The frequency of digested slue
withdrawals and raw sludge additions v
daily. The draw and fill mode of operati
was selected to eliminate the possibi
of effluent characteristics being inl
enced by short-circuiting of raw slue
additions. Thus, a minimum of 24 hr
treatment always was ensured.
Throughout the period July 29, 19
through March 30, 1987, raw a
digested sludge samples were routin
collected and analyzed for physical i
chemical parameters including tc
solids, total volatile solids, chemi
oxygen demand, total Kjeldahl nitrog
ammonia nitrogen, pH, and temperati
Determination of these physical a
chemical parameters was performed
project personnel at the Corn
University Department of Agriculti
Engineering's Game Farm Road Wa
Management Laboratory on the day
sample collection. Raw sludge samp
were generally collected on Monda
Wednesdays, and Fridays. The
samples were collected during the d;
transfer of raw sludge from a n
chanically-mixed feed tank to the 1
digesters. Digested sludge samples w
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generally collected on Tuesdays and
Thursdays. These samples were mixed
liquor samples taken prior to effluent
'withdrawal and subsequent raw sludge
addition.
During periods of steady-state oper-
ation, raw sludge samples collected on
Mondays and Wednesdays and digested
sludge samples collected on Tuesdays
and Thursdays also were analyzed to
determine the densities of the total
coliform, fecal coliform, and fecal
streptococcus groups of indicator
organisms and the enterovirus group of
viruses. These samples also were
analyzed to determine densities of
Salmonella spp. through May 13, 1986.
At this time, Salmonella enumerations
were discontinued as no meaningful data
were being obtained. During one period
of steady-state data collection, April 9
through May 13, 1986, raw and digested
sludge samples also were analyzed to
determine the densities and viability of
Ascaris and Toxocara ova. Again, no
meaningful data were obtained and these
analyses were terminated. During per-
iods of steady-state data collections, a
minimum of seven sets of raw and
digested sludge samples were collected
and analyzed for the microbiological
parameters noted above.
Enumerations of the densities of the
total coliform, fecal coliform, and fecal
streptococcus groups of indicator
organisms and Salmonella spp. were
conducted under the direction of Dr.
Sang J. Shin, Director of Bacteriology at
the New York State Diagnostic
Laboratory, Cornell University College of
Veterinary Medicine. Enterovirus enum-
erations were performed by the Envi-
ronmental Monitoring Support Labora-
tory, USEPA. Enumerations of the enteric
parasites, Ascaris and Toxocara, also
were performed at the New York State
Diagnostic Laboratory under the direction
of Dr. Richard H. Jacobson.
Raw sludge and mixed liquor
temperatures in both digesters were
routinely measured. Raw sludge tem-
peratures were measured and recorded
daily during raw sludge additions using a
digital thermometer while a continuous
record of mixed liquor temperatures was
provided by a dual recording ther-
mometer. In addition, a continuous
record of ambient temperatures at the
field site was obtained using a recording
thermograph located in a standard
weather instrument shelter. Mean daily
mixed liquor and ambient temperatures
were calculated by taking the average of
the minimum and maximum tempera-
tures recorded in a 24-hr period be-
ginning at 0800 hr.
Additional data routinely collected and
recorded, generally on Mondays,
Wednesdays, and Fridays, included
mixed liquor dissolved oxygen concen-
trations and airflow rates. Mixed liquor
dissolved oxygen concentrations were
measured approximately 24 hr after the
previous raw sludge addition using a
Clark-type polarographic oxygen probe
with a temperature compensation. In-
line rotameters were used to measure
airflows to both digesters. Air temper-
ature and pressure also was measured
and recorded to permit calculation of
airflows-under standard conditions.
Thus, constant operating conditions were
ensured.
Results and Discussion
Because of limited space, emphasis in
this summary will be on microbial
reductions. Please see the complete
report cited at the end of this summary
for results related to sludge stabilization
and other details.
There were five periods of steady-
state operation of the autoheated diges-
ter during which data to characterize
performance with respect to mixed liquor
temperature, reductions in the densities
of the three groups of indicator
organisms and the enterovirus group of
viruses, and sludge stabilization were
obtained. The dates of these five periods
of autoheated digester steady-state
operation are noted in Table 1. Also
noted in Table 1 are the dates of
concurrent data collection to characterize
the performance of the conventional di-
gester using the same parameters that
were used for the autoheated digester.
The additional period of data collection
for the conventional digester, July 28
through September 3, 1986, without
concurrent data collection for the auto-
heated digester was because of an
atypical period of excessive autoheated
digester foaming making the collection of
representative samples impossible. The
factor or factors responsible for this
atypical period of excessive autoheated
digester foaming remain unclear. Thus,
there was a total of 11 rather than 12
periods of steady-state operation as
planned. The results obtained during the
steady-state operation of the two
digesters are summarized and discussed
below.
Temperature
With respect to mixed liquor
temperatures, both digesters performed
as anticipated. In the conventional
digester, daily mean mixed liquor
temperatures ranged from 5"C with some
surface ice formation during extended
periods of cold weather to 28 °C during
summer months. Monthly means of
mixed liquor temperatures ranged from
8°C to 26°C and varied seasonally and
linearly with ambient air and influent
sludge temperatures.
Mixed liquor temperatures for each of
the five periods of autoheated digester
steady-state operation (Table 1) are
summarized and compared with ambient
air and.influent sludge temperatures in
Table 2. Autoheated and conventional
digester mixed liquor temperatures
during periods of concurrent steady-
Jable 1. Periods of Steady-State Operation Used to
Characterize Autoheated and Conventional
Digester Performance
Residence Time, Days
Dates
Autoheated
Conventional
Hov 6-Dec. 5. 1985
Jan 6-Jan. 30. 1986
Apr 9-May 13. 1986
July 28-Sept. 3. 1986
Sept. 15-Oct. 9, 1986
Feb. 2-Mar. 26, 1987
20
10
15
-
10
15
20
20
20
20
20
20
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Table 2. Summary of Ambient Air, Influent Sludge, and Autoheated Digester Mixed Liquor
Temperatures During Periods of Steady-State Operation
Temperature, °C*
Dates
Nov. 6-Dec. 5, 1985
Jan. 6- Jan. 30, 1986
Apr. 9-May 13, 1986
Sept. 15-Oct. 9, 1986
Feb. 2-Mar. 26, 1987
Residence
Time, Days
20
10
15
10
15
Ambient
Air
3.9 ±4.6
-4.3+7.0
9.3 ±5.8
13.9±4.4
-3.8 ±7.2
Influent
Sludge
12.8 ±1.4
6.8*0.7
12.2 ±1.4
17.4±1.0
7.7 ±1.0
Autoheated
Digester
Mixed Liquor
38.2 ±0.9
31.0 ±1.2
39.8 ±2.4
37.5 ±1.3
29.0 ±1.5
"Mean ± standard deviation.
Table 3. Comparison of Autoheated and Conventional Digester
Mixed Liquor Temperatures During Concurrent Periods
of Steady-State Operation.
Mixed Liquor Temperature, °C*
Dates Autoheated
Nov. 6-Dec. 5, 1985 38.2 ±0.9
Jan. 6-Jan. 30, 1986 31.0 ±1.2
Apr. 9-May 13, 1986 39.8 ±2.4
Sept. 15-Oct. 4, 1986 37.5 ±1.3
Feb. 2-Mar. 26, 1987 29.0 ±1.5
Conventional
14.6 ±2.1
8.0 ±1.6
17.5 ±2.3
21.7±2.0
23.7t±1 1
"Mean ± standard deviation.
fConventional digester with insulated cover-
state operation are compared in Table 3.
As shown in these tables, the design of
the autoheated digester provided sub-
stantially higher mixed liquor tempera-
tures as compared to both ambient air
and influent sludge temperatures and
also to mixed liquor temperatures in the
open conventional digester.
Microbial Reductions
The observed reductions in the
densities of the three groups of indicator
organisms and the enterovirus group of
viruses for both the conventional and
autoheated digesters during the 11
periods of steady-state operation are
summarized in Table 4. From these data,
it can be seen that total conforms
generally were the most easily destroyed
group of indicator organisms while fecal
streptococci were the most resistant to
destruction. Interestingly, reductions of
enteroviruses were comparable to the
total coliforms in some situations but
comparable to fecal streptococci in
others instances.
From the data summarized in Table 4,
it also can be seen that both residence
time and temperature appear to be
important factors in reducing the density
of each of the three groups of indicator
organisms, whereas, residence time
appears to be of lesser importance as
compared to temperature with respect to
reductions in enterovirus densities. This
apparent dependence of reductions in
the densities of the three groups of
indicator organisms on both residence
time and temperature becomes even
more obvious when the reductions
summarized in Table 4 are first grouped
by residence time and then ordered with
respect to temperature (Table 5).
Interestingly, the same pattern of
increasing reductions with increases in
temperature for each residence time also
applies to the enteroviruses.
This apparent dependence of the
reductions in the densities of these four
groups of microorganisms on be
residence time and temperatu
suggested that it might be possible
use the Arrhenius equation (Equation
to describe the temperature dependen
of these reductions mathematically if t
nature of these reactions could
characterized.
k = A exp (- u/RT)
(1
where: k = the temperature depende
reaction rate coefficient
A = constant
u = the temperature character
tic
R = the universal gas constant
T = the absolute temperature
If one assumes that the Arrheni
equation does describe the temperatu
dependence of a reaction, the linearizi
form of the Arrhenius equation (Equatd
2) can be used to determine the nature
that reaction.
Ln(k)
,.-(1)
R\T/
Ln(A)
A plot of the natural logarithms of tl
temperature dependent reaction ra
coefficients versus the reciprocals
absolute temperature should yield
straight line if the assumed nature of tl
reaction is correct.
Initially, it was assumed that tl
observed reductions in the densities
each of the three groups of indicat
organisms and the enterovirus group
viruses could be characterized as eith
zero-order (Equation 3) or first-ord
(Equation 4) relationships.
where: ko =
81 =
e =
the zero-order reactic
rate coefficient, CFU
PFU per 100 ml pi
day.
geometric mean infli
ent microorganis
density, CFU or PF
per 100mL.
geometric mean efflue
microorganism densit
CFU or PFU per 1C
mL.
residence time, days
-------
1"
where:
(4)
= the first-order reaction rate
coefficient, days'1
There was no evidence of strong or
even moderately strong linear rela-
tionships, however, when the natural
logarithms of the calculated zero-order
and first-order reaction rate coefficients
total coliform, fecal coliform, and fecal
streptococcus groups of indicator orga-
nisms and the enterovirus group of
viruses without suspect values deter-
mined statistically to be outliers were
plotted versus the reciprocals of absolute
temperature (see complete report), the
anticipated linear relationships (Table 6)
were realized. The strengths of these
linear relationships are indicated by the
linear regression correlation coefficients
noted.
Unfortunately, this review yielded only
two sets of suitable data. Both of these
studies were comparable to the study
being described and discussed in this
project summary in that both mixed
liquor residence time and temperature
were variables with similar ranges of
values. Data were available, however,
only for the fecal coliform and fecal
streptococcus groups of indicator orga-
nisms. In spite of these limitations, both
of these data sets were of value in testing
Table 4. Summary of Observed Reductions in the Densities of Indicator Organisms and Enteroviruses During Autoheated and
Conventional Aerobic Digestion
Reduction LogJO Basis
Digester
Autoheated
Conventional
Residence Time,
Days
10
10
15
15
20
20
20
20
20
20
20
Mixed Liquor
Temperature, °C*
31.1 ±1.2
37.5 ±1.3
29.0*1.5
39.8 ±2.4
38.2*0.9
8.0 + 1. 6
14.6±2.1
17.5 ±2.3
27.7*2.0
23.7 ±1.1
25.6*7.8
Total
Coliforms
0.84
0.90
7.44
2.20
2.55
0.68
7.27
7.70
0.69
2.77
7.43
Fecal
Coliforms
7.04
7.70
7.32
7.58
2.42
0.64
7.07
7.38
7.78
7.74
0.56
Fecal
Streptococci
0.60
0.82
0.80
7.23
7.60
0.33
7.07
7.77
7.00
7.42
0.72
Enteroviruses
7.08
2.43
7.03
2.33*
3.76*
0.72
0.95
0.98
0.85
7.06
7.28
"Mean * standard deviation
for each of the four groups of micro-
organisms were plotted versus the
reciprocals of absolute temperature. This
failure of both the zero-order and first-
order Arrhenius type models to describe
the temperature dependence of the
microbial reductions observed in this
study led to the formulation of a simple
empirical rate equation (Equation 5) as
an alternative.
k =
(5)
where: k = the empirical reaction rate
coefficient, Log TO reduction
per day.
S, = influent microorganism
density, Logic CPU or PFU
per 100 mL
Se = effluent microorganism
density, Log10 CPU or PFU
per 100mL
6 = residence time, days
When the natural logarithms of the
empirical reaction rate coefficients for the
Table 5. Reductions in the Geometric Mean Densities of Indicator Organisms, and
Enteroviruses Grouped by Residence Time and Then Ordered with Respect to
Temperature
Log 10 Reductions
Residence
Time, Days
10
15
20
Mixed Liquor
Temperature, "C
31.1
37.5
29.0
39.8
8.0
74.6
77.5
27.7
23.7
25.6
38.2
Total
Coliforms"
0.84
0.90
7.44
2.20
0.68
7.27
7.70
0.69
2./7
7.43
2.55
Fecal
Coliforms"
1.04
1.10
1.32
1.58
0.64
1.01
1.38
1.18
1.74
0.56
2.42
Fecal
Streptococci*
0.60
0.82
0.80
1.23
0.33
1.07
1.17
1.00
1.42
0.72
7.60
Enterovirusest
1.08
2.43
1.03
2.33
0.72
0.95
0.98
0.85
1.06
7.28
3.76
"Colony-forming units per 100 mL basis.
•fPlaque-forming units per 100 mL basis.
In order to locate the data necessary to
test the validity of these empirical
Arrhenius type relationships, a review of
the pertinent literature was conducted.
the validity of this empirical modeling
approach. There was a reasonable
degree of agreement between the
empirical reaction rate coefficients pre-
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dieted by the regression equations
summarized in Table 6 for both fecal
coliforms and fecal streptococci and the
values calculated from the data obtained
from the two other studies.
methodology for determining the resi-
dence time necessary for a given mixed
liquor temperature that will provide a
desired reduction in the densities of the
three groups of indicator organisms and
Table 6. Summary of Linear Regression and Correlation Analyses Results for
the Empirical Arrhenius Models
Empirical Model
Total conforms
Fecal conforms
Fecal streptococci
Enteroviruses
Regression Equation
ink = 7.0662 - 2854.3056 X
Ln k = 8.2924 - 3244.2881 X
Ln K = 3.9504 - 2020.1446 X
Ln k = 13.9923 - 4950.6759 X
Correlation
Coefficient
0.85
0.94
0.87
0.90
From the results of this model valida-
tion process, it can be at least tentatively
concluded that the relationships between
residence time and temperature and
reductions in the densities of fecal
coliforms and fecal streptococci can be
best described by the empirical Arrhen-
ius type models developed as part of this
study. Also, this tentative conclusion can
be extended by inference for total
coliforms since fecal conforms are a
component of the total coliform group.
Unfortunately, the only evidence avail-
able to test the validity of the empirical
Arrhenius model for enteroviruses is the
correlation coefficient associated with the
regression analysis. As noted in Table 6,
the correlation coefficient for the
empirical Arrhenius models for entero-
viruses was 0.90. Thus, it also appears,
at least for this study, that the empirical
Arrhenius model provided a reasonable
description of the observed relationship
between mixed liquor temperature and
residence time and reduction of entero-
virus density during aerobic sludge
digestion.
The objective of this mathematical
modeling exercise was to develop a
the enterovirus group of viruses during
aerobic sludge digestion. Using the
empirical model regression equations
(Table 6), it was found that even a
modest 10°C increase in mixed liquor
temperature generally results in a sig-
nificant reduction in required residence
time and, thus, aeration basin volume. It
is interesting that the residence times of
60 days at 15°C to 40 days at 20°C
specified for aerobic digestion in Section
257.3-6 of 40 CFR 257 (U.S. Code of
Federal Regulations, 1979) have pre-
dicted reductions of at least log 10 in the
densities of the indicator organisms and
enteroviruses.
Conclusions
The objectives of this investigation
were attained. Seasonal variations in the
performance of conventional aerobic
sludge digestion in cold climates were
characterized. Also, it was demonstrated
that process performance could be
substantially improved by simple modi-
fications to increase mixed liquor
temperatures. From the experimental
results obtained in this study, it also was
possible to delineate mathematical rela-
tionships between residence time and
mixed liquor temperature and reductions
in the densities of the total coliform, fecal
coliform, and fecal streptococcus groups
of indicator organisms and the entero-
virus group of viruses. In addition, it was
possible to describe mathematical rela-
tionships between residence time and
mixed liquor temperature and reductions
in total volatile solids (TVS) and chemical
oxygen demand (COD) concentrations
using results obtained in this study in
combination with results reported by
other investigators (see complete report
for details).
From the results of this study, it can be
concluded that use of aerobic digestion
as a process to significantly reduce
pathogens, assuming two log-irj reduc-
tions in the densities of coliform
organisms and a 38% reduction in total
volatile solids concentration as require-
ments, is technically feasible at ambient
air temperatures at or below freezing.
The long residence times required at the
low mixed liquor temperatures typical
during winter months in northern climates
makes operation at these temperatures
impractical, however. Yet, it is possible to
use aerobic digestion as a process to
significantly reduce pathogens in these
climates by reducing heat losses through
the use of closed reactors with insulated
covers singularly or in combination with
high efficiency aeration units. Because of
the relative simplicity of these modi-
fications, they are applicable not only to
new but also existing facilities. In
addition, it appears that the use of
autoheated aerobic digestion for sewage
sludge stabilization can be a cost
effective alternative to conventional
digestion particularly in cold climates
(see complete report for details).
•The full report was submitted in ful-
fillment of Cooperative Agreement No.
CR-811776 by Cornell University under
sponsorship of the U.S. Environmental
Protection Agency.
U. S. GOVERNMENT PRINTING OFFICE: 1989/648-013/07013
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John H. Martin, Jr., is with Cornell University, Ithaca, NY 14853.
Gerald Stern was the EPA Project Officer (see below for present contact).
The complete report, entitled "Reductions of Enteric Microorganisms During
Aerobic Sludge Digestion: Comparison of Conventional and Autoheated
Digestion," (Order No. PB 89-138 846/AS; Cost: $21.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The present EPA Project Officer Harry E. Bostian can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Official Business
Penalty for Private Use $300
EPA/600/S2-88/072
0000329 PS
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