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
Agency
Center for Environmental Research
Information
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