United States
Environmental Protection
Agency
Municipal Environmental Researc
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-82-023 May 1982
Project Summary
Autohealed, Aerobic
Therqtfo^hilic Digestion with
.
William J.d&tyeJI? Randolph M. Kabrick, and James A. Spada
This 2-year study developed a new
sludge treatment process capable of
rapid stabilization, pasteurization,
and heavy metal removals from dilute
sewage sludge. A full-scale system
(28.4 m3 reactor) demonstrated that
simple self-aspirating aerators that
used ambient air could achieve high
oxygen transfer efficiencies and
thereby allow conservation of the heat
of oxidation to achieve autoheating to
high temperatures. A one-stage
digestion system using a continuous
feed of primary and waste-activated
sludge (3 to 6 percent total solids)
resulted in autoheated reactor
temperatures ranging from 45° to
65° C, even when air temperatures
were 20° C and sludge temperatures
were 0°C.
The relationship between process
variables and the autoheating pheno-
mena were examined at full-scale and
bench-scale levels. The process varia-
bles included organic loading rate and
dissolved oxygen concentration. It
was observed that intermediate load-
ing rates (12 to 15 kg TS/m3-reactor-
day) and low dissolved oxygen
residuals (< 1 ppm) allowed maximum
temperature development.
Two different aerators were tested
and were found to achieve oxygen
transfer efficiencies exceeding 20
percent at reactor temperatures that
often exceeded 60°C. Operational
problems associated with these aera-
tors as well as with the other equip-
ment on the thermophilic digestion
facility were identified and examined.
The potential of the autoheated
thermophilic digester to inactivate
pathogens was investigated. Virus
inactivation was 100 percent in most
cases, with bacterial and parasite indi-
cator counts less than those found in
the effluent from the full-scale, meso-
philic anaerobic digester.
The dewaterability of the auto-
heated, thermophilic digester effluent
deteriorated at all loading conditions
studied. The aerobic, thermophilic-
digestion process appears to increase
the solubility of various heavy metals
such as cadmium.
A computer model was developed
from the full-scale data for predicting
the reactor temperature under given
loading conditions.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory,
Cincinnati. OH. to announce key find-
ings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Sewage sludge and other waste
organics should be used for beneficial
purposes whenever such practices are
safe and cost effective. Implementation
of this policy usually results in the appli-
cation of sludge to agricultural land for
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soil property improvement or for crop
fertilization. If the land application rates
of sewage sludge were limited by the
plan nitrogen requirements, sewage
sludge would provide the required plant
nutrients for millions of acres of crop-
land. Present sludge management
technology, however, often cannot
guarantee public health protection or
cost effective solutions. This report
summarizes the results of a full-scale
investigation of a simple sludge treat-
ment system that has the potential of
providing a stabilized, pasteurized sew-
age sludge at costs less than present
aerobic processing systems.
Approximately 20 percent of the sew-
age sludges produced in the United
States are used in agriculture, and this
practice is expected to increase as
ocean dumping and other disposal prac-
tices are terminated. Of course, alterna-
tive methods are known that stabilize
and pasteurize sewage sludge, but few
if any can achieve the high level of treat-
ment required at a low cost without sig-
nificant disadvantages.
Among the options available for a
more effective sludge treatment
approach is thermophilic biological
treatment. Both anaerobic and aerobic
treatment processes can operate in the
thermophilic temperature zone of 43° to
70°C. Autoheated aerobic digestion dis-
cussed here refers to a process that
uses the metabolic heat of oxidation of
the organics to increase the tempera-
ture of the aerating slurry.
The theoretical energy input required
to increase sludge temperatures to the
thermophilic range would be about 40
Kcal/L of sludge processed and, at
today's energy prices, would cost about
$10/million gallons of sewage flow, or
about $20/ton of dry sludge. If the ther-
mal energy generated in aerobic diges-
tion is conserved, a surprisingly small
amount of substance needs to be bio-
logically oxidized to reach the ther-
mophlic range with no requirement for
externally generated heat, and conse-
quently no fuel cost. For a sludge con-
taining 2 per cent volatile solids,
oxidizing 50 percent of the volatile sol-
ids would produce 50 Kcal/L. Theoreti-
cally, this would provide the energy
necessary to heat the cold sludge from
10° up to 60°C, assuming no heat
losses. Thus, it would appear that aero-
bic treatment would have potential in
this area. The challenge, therefore, is to
manage the heat generated by micro-
bial oxidation, primarily by controlling
heat loss in the exiting vapor and liquid
streams to maximize the autoheating
capabilities of aerobic digestion.
The potential advantages of high-
temperature, aerobic sludge treatment
are thought to include:
— increased rates of oxidation, thus
resulting in smaller digester vol-
ume require-merrts-; increased
quantity of stabilized organics;
— destruction of most pathogenic
bacteria, viruses, and parasites;
— significantly lower oxygen require-
ments because of the elimination
of nitrification over ambient-
temperature aerobic digestion;
— increased ease of liquid-solid sepa-
ration; and
— destruction of weed seeds.
If these advantages were applied to
sludge treatment with a little or no cost
increase over conventional processes,
it would represent a significant
improvement in sludge treatment
technology.
Problem Magnitude
One of the objectives of the Water
Pollution Control Act Amendments in
PL 92-500 is to provide all domestic
wastewaters with a minimum of secon-
dary treatment. For every 10,000 peo-
ple, about 38 mVday (10,000
gallons/day) of sewage sludge will be
generated (at about 4 percent dry solids
concentration) with the application of
secondary treatment. When this objec-
tive is achieved, the quantity of domes-
tic sewage sludge that must be
disposed of will be greater than 0.8 mil-
lion mVday. If this material is to be dis-
tributed on private land, used for food
production, or used indiscriminately by
homeowners, the public health aspects
of the material must be a prime
concern.
Long-term anaerobic digestion at
35°C has been reported to control the
majority of human pathogens effec-
tively, but some may survive for long
periods. In smaller, less well-operated
sewage facilities, the sludge may not
receive as effective treatment as in the
larger plants with efficient anaerobic
digesters. Also, because of the com-
plexities of the anaerobic digestion pro-
cess, many small town sewage facilities
use the simpler aerobic digestion pro-
cess. It is known that many groups of
pathogens can survive the ambient
temperature treatment received in
many small aerobic digestion facilities.
Thus, it is clear that a simple process
that would be capableof providing more
effective sludge treatment and patho-
gen control at a cost not exceeding
existing sludge treatment cost should
have a high priority in new technology
development. This study was under-
taken to determine the prospects of
adapting existing technology and
knowledge to produce a new simple
process capable of yielding a pasteu-
rized, stabilized sludge without a large
energy input.
Goals and Objectives
The general goal of this 2-year study
was to demonstrate that a simple, auto-
heated aerobic-digestion process using
full-scale equipment, shown to be
effective with animal wastes, could be
used to treat sewage sludge. Specific
objectives of this study were to:
(1) demonstrate the feasibility of
achieving autoheating to
temperatures exceeding 43°C
with typical primary and second-
ary waste-activated sludge using
a simple air aeration system;
(2) estimate the practical operating
problems of a full-scale system
over an extended period of opera-
tion;
(3) develop the relationships be-
tween the sludge autoheating
characteristics and the volumet-
ric and organic loading rates,
thereby enabling the predic-tion
of the re-actor temperature;
(4) define the aeration
requirements and heat balance
needed to achieve autoheating
with air aeration;
(5) measure the high-temperature
treatment impact on the dewater-
ability of the sludge; and
(6) determine the effectiveness of
the autoheated aerobic system on
the destruction of major groups of
pathogens and compare these re-
sults with the existing anaerobic
digestion sys-tem at the sew-age
treatment facility.
Materials and Methods
Initial investigations of the auto-
heated, aerobic thermophilic-digestion
process using agricultural wastes were
performed at Cornell University with
bench-scale and pilot-scale reactors.
Subsequently, a commercially available
full-scale system was installed at Cor-
nell University's Animal Science Teach-
ing and Research Center, where the
system's performance was studied
using a dairy waste substrate over a
2-year period. Because of this success-
ful demonstration of the autoheating
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concept, a single-stage digestion facil-
ity was designed and constructed at the
Binghamton-Johnson City Sewage
Treatment Plant in Binghamton, NY,
where this study took place over a 2-
year period. The Binghamton Sewage
Treatment Plant is a modern, 1.5 X 10s
mVd (40-mgd), activated sludge plant
that was constructed in 1960.
The full-scale system was operated
under conditions that would result in
significant biodegradation but would
reflect the advantages of high-
temperature loading rates, with the
majority of operation at less than 5-day
hydraulic retention time. One set of
conditions was maintained constant for
periods up to 60 days to provide time to
evaluate practical operation and main-
tenance problems as well as to measure
the impact of sludge composition varia-
tions on the process.
Process Design and
Construction
The system was designed to enable
measurement of mass balances for flow
and energy by providing large, com-
pletely mixed and insulated feed and
effluent tanks, each one capable of stor-
ing a volume approximately equal to
one hydraulic retention period. The
reactor, a cylindrical tank, was 3.7 m in
diameter and 4.3 m high. Equipment
was first delivered to the site on March
21,1977, and the unit was operational
by May 18, 1977.
Aeration and mixing in the reactor
were accomplished by one of two aera-
tors tested. During most of the study,
the DeLaval Separator Company Centri-
rator* (240 volt, 3 phase, 3.7 kW, 1750
rev/min) was used. This aerator was
suspended in the liquid by four styro-
foam blocks set equidistant from each
other. Such a flotation system ensured
that optimum immersion depth was
maintained for the impeller. This aera-
tor was of the self-aspirating type —
that is, the vacuum created at the center
of the aerator impeller draws air down
the hollow air intake tube and draws
liquid up from the center of the tank,
thereby providing aeration of the liquid
at the impeller. The other aerator stud-
ied was an LFE Corporation Midland-
Frings (240 volt, 3 phase, 5.2 kW, 1750
rev/min) self-aspirating aerator. This
aerator sat on a tripod 30 cm from the
reactor bottom, with the impeller spin-
ning against a stator plate to draw air
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
down the intake tube and move the liq-
uid mixture to the reactor perimeter.
Note that on the Midland-Frings aera-
tor, the impeller was encased between
two stator plates, whereas the Centri-
rater impeller hung free in the liquid.
The effluent tank was identical to the
influent tank with the exception that it
was set approximately 1.5 m into the
ground to facilitate gravity overflow
from the reactor and thereby provide
the required effluent volume (28.4 m3).
An entire, retention period's effluent
was collected before it was sampled
and analyzed. This enabled collection of
sa mples that were representative of the
entire test period and minimized varia-
tions during the test period. The gravity
overflow system consisted of a 15-cm
diameter black steel pipe, 2.0 m in
length, and set at a 40-degree angle to
the reactor wall, with one-half of the
pipe length extending from the reactor
and connecting to a 30-cm diameter
polyvinyl chloride (PVC) pipe. The PVC
pipe acted asa trough to carry liquid and
foam from the overflow pipe to the efflu-
ent tank. Mixing of the effluent tank for
sampling purposes and subsequent
wasting was accomplished by a centrif-
ugal pump.
Continuously and semicontinuously
fed laboratory-scale reactors were
operated in conjunction with the full-
scale reactor under similar conditions.
The temperature in the reactors was
maintained by water baths operated at
55°C. Hydraulic retention times (HRT) of
2 to 7 days were studied. The reactor
volumes varied from 15 to 21 L
A series of 14 long-term batch biode-
gradability studies was conducted
using 3-L reactors maintained at 50°C.
These studies were intended to mea-
sure the maximum biodegradable frac-
tion of the wastewater sludge and to
monitor the variability of the sludge as
related to the time of the year.
Pathogen Sampling
and Analyses
During most of the steady-state con-
ditions, samples were collected for
virus enumeration, bacterial analyses
(coliforms and enteric pathogens), and
parasites (viable and nonviable ova).
Several laboratories were contracted to
perform these analyses. Their complete
methodology for analysis is presented
in the Project Report. Typically, samples
were collected, aseptically transferred
to sterile containers, packed in dry ice
(virus samples) or cold packs (patho-
genic bacteria and parasites), and
shipped via air freight to several
laboratories.
Results and Conclusions
This 2-year demonstration focused
on operating a full-sca'le system for
sludge treatment for populations
exceeding 5,000 people. Mixtures of
thickened, waste-activated sludge and
primary sludge were autoheated to
temperatures in excess of 43°C for all
conditions tested. Self-aspirating air
aerators and well insulated tanks were
used with a full-scale reactor volume of
28.4 m3 during 1.5 years of operation.
Autoheated slurry temperatures nor-
mally exceeded 50°C and reached a
maximum of 65°C.
These results indicate that a simple
aerobic digestion system can achieve
autoheating to thermophilic tempera-
tures with typical sewage sludges (50
percent biodegradable total volatile sol-
ids (TVS) or greater) at concentrations
greater than 2 percent total solids under
cold weather conditions. Use of heat
exchangers would enable even more
dilute sludges to be autoheated to high
temperatures.
Start-Up
Thermophilic sewage sludge aerobic
reactors are easily started and will
achieve temperatures in excess of 40°C
in approximately 10 days when sludge
temperatures are as low as 0°C and air
temperatures are -10°C.
Kinetics, Oxygen Transfer.
and Dewaterability
Maximum autoheated temperatures
were obtained at daily loading rates of
between 3 and 10 kg biodegradable
COD (BCOD) per m3, with the maximum
organic removal rate (6.5 kg BCOD/m3)
obtained at a daily loading rate of 8 kg
BCOD/m3 for the Midland-Frings aera-
tor. Approximately 75 percent of the
biodegradable organics were oxidized
at this loading rate. This corresponded
to a 5-day HRT at total solids concentra-
tions of approximately 5 percent.
The efficient conservation of the heat
of organic oxidation was achieved using
two kinds of self-aspirating aerators
that were shown to be capable of
achieving greater than 20 percent oxy-
gen transfer efficiency. At high loading
rates, oxygen transfer efficiencies
exceeded 23 percent with air aeration
for both aerators tested with organic
slurries up to 5 percent total solids at
temperatures exceeding 50°C. Normal
oxygen transfer analysis using compar-
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ative tap water testing does not apply to
these kinds of aeration systems.
The dewaterability of the treated
sludge deteriorated significantly at all
HRT's tested (3 to 11 days).
The conservation of nitrogen through
the system was observed at all condi-
tions tested and was a function of high
temperature inhibition of nitrification
and minimal volatilization of ammonia.
Pathogen Control
The thermophilic aerobic digester
exhibited a high degree of pathogen
control with respect to bacteria, viruses,
and parasites. Complete inactivation,
that is, below the limits of detection, of
Salmonella sp. and total plaque-
forming units (viruses) was observed for
all but one test date. Parasite numbers
(viable ova) were significantly reduced
by the aerobic thermophilic system, but
not completely.
Practical Considerations
Practical operational and mainte-
nance problems were found to be min-
imal and a function of the temporary
nature of the digestion facility.
Daily operational requirements of the
aerobic thermophilic system are
minimal.
Process Design Criteria
and Suggested System
Major requirements for autoheated
aerobic digestion of sewage sludge
include a minimum biodegradable
organic concentration, an insulated
reactor, and the use of efficient aerators
such as the two tested in this study. The
potential for autoheating can be pre-
dicted using energy and mass balances.
The following equation was developed
for the prediction of the autoheated
reactor temperature:
PTp = HRT KT BCODe R . TQ 0407
DL CPL
(TH - TA)12765] HRT + To
where:
PTR = predicted reactor
temperature, °C
HRT = hydraulic retention time, day
KT = reaction rate coefficient,
day"1
BCODe =biodegradable COD in the
effluent, g/L
R = heat released during micro-
bial respiration, Kcal, g COD
DL = density of the liquid
sludge, g/L
CPL = specific heat capacity of
the liquid sludge, cal/g-°C
TR = reactor temperature, °C
TA = ambient temperature, °C
To = feed sludge temperature, °C
The reaction rate coefficient, which is
needed for solution of this equation,
was found to be a strong function of
temperature. For the conditions of
these experiments and within the
temperature range of 47° to 62°C, KT is
approximated by the following
relationship:
KT = (0.022) 1.076 V20
The best practical system for maxi-
mum organic stabilization and min-
imum aeration requirement includes
using a bottom-mounted, self-
aspirating aerator, a daily organic load-
ing rate of around 15 kg of BCOD/m3,
and an aerator input of 0.18 kW/m3 of
reactor.
The autoheated, air-aerated-sewage-
sludge digestion system would appear
to be more economical than other aero-
bic or anaerobic digestion facilities at
equal solids conversion efficiencies.
Design Procedure
Based upon the information deve-
loped by this study, a full-scale, thermo-
philic aerobic-digestion facility can be
designed for the organic stablization
and pasteurization of municipal sewage
sludges and other waste organic slur-
ries. The following approach is recom-
mended for the design of a thermophilic
aerobic digestion facility:
1. The organic slurry that is to be di-
gested should be characterized with
respect to total chemical oxygen
demand (TCOD), fraction that is biode-
gradable, and solids concentration. The
TCOD value of the slurry would give an
indication of the autoheating potential
of the slurry since approximately 3.5
Kcal are released when 1 g of TCOD is
oxidized. A review of the history of the
slurry, that is, its age, source, and other
available characteristics would yield an
.approximate range of expected
'biodegradability.
2. The reactor(s) should be sized
according to the desired level of organic
treatment efficiency and operating
temperature, both of which are deter-
mined by the organic loading to the
reactor. A loading rate of 12 to 15 kg
TS/m3 reactor-day was found to allow
maximum temperature development
(55° to 62°C) and maximum organic
removal rate.
3. The reactor should be shaped to
provide efficient mixing and should be
covered and insulated. The insulation
selected should provide maximum re-
sistance to heat loss, specifically, the
insulation should have a thermal
conductivity of about 0.019 cal/sec • cm
•°Candbe applied at a thickness to yield
anR>25.
4. Self-aspirating aerator(s) of the
type studied here should be installed in
the reactor(s) to provide approximately
0.15 to 0.20 kW/m3 of reactor. This rate
of aeration was found to provide effi-
cient mixing and oxygen transfer. No
other aerator should be substituted
unless it is shown to be capable of oxy-
gen transfer efficiencies exceeding 20
percent in sewage sludge at 60°C.
5. Foam produced in the reactor(s)
should be controlled with mechanical
foam cutter(s) of the type studied here,
and applied at the rate of 0.1 kW/m2
surface (slurry) area. A backup foam
cutter(s) should be available on an auto-
matic activation basis for control of
excess foam such as occurs during
changes in loading conditions.
Recommendations
Information on the autoheated,
aerobic-digestion process should be
made available to engineering firms
involved in the design of sludge treat-
ment facilities where the ultimate dis-
posal method would be land
application. Effluent from the auto-
heated process was shown to be well
stabilized and pasteurized, thereby min-
imizing the public health risks asso-
ciated with land application of this type
of sludge.
The full report was submitted in ful-
fillment of Grant No. R804636-01 by
Cornell University under the sponsor-
ship of the U.S. Environmental Protec-
tion Agency.
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William J. Jewell, Randolph M. Kabrick, and James A. Spada are with Cornell
University. Ithaca, NY 14853.
B. Vincent Salotto is the EPA Project Officer (see below).
The complete report, entitled"A utoheated. Aerobic Thermophilic Digestion with
Air Aeration," (Order No. PB82-196908; Cost: $27.00, subjectto change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
1982—559-092/3409
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Environmental Protection
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