xvEPA
United States
Environmental Protection
Agency
Municipal Environmental Resear
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-8M71 Oct 1981
Project Summary
Engineering and Economic
Assessment of Autoheated
Thermophilic Aerobic
Digestion with Air Aeration
P. W. Keohan, P. J. Connelly, and A. B Prince
Engineering and economic analyses
were made of test results obtained by
W.J. Jewell. R.M. Kabrick, and J.A.
Spada in experiments sponsored by
the U.S. Environmental Protection
Agency (EPA) on a modified sludge
stabilization process termed auto-
heated aerobic thermophilic digestion
(ATAD) with air aeration. The ATAD
process tests had been conducted in
1979 at the Binghamton-Johnson
City Sewage Treatment Plant in
Binghamton, New York.
In this study, the Jewell et al. tests
results were analyzed for system
kinetics, heat balance, aerator transfer
efficiency, pathogen destruction, de-
waterability, and heavy metal interac-
tions. Additionally, economic viability
of ATAD was examined using very
conservative criteria in application to
facilities of 1-. 10-, and 100-mgd
capacity.
ATAD was found to be a feasible
process, readily interpretable by con-
ventional measures of system effi-
ciency and effectiveness, and a
potentially economical sludge-diges-
tion process at smaller size plants.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory. Cincin-
nati. OH 45268, to announce key
findings of the research project which
is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
Aerobic digestion is a simple unit
process for stabilizing sludge A major
disadvantage of this process, however,
is that its efficiency is greatly reduced
during periods of cold weather."Treat-
ment plant operators then must either
provide very long detention times (e.g ,
up to two months to achieve a 40-
percent reduction in volatile solids) or
accept poorer reduction in volatile
solids.
During the past decade, several
researchers have investigated the
possibility of controlling heat losses in
aerobic digestion so as to conserve the
energy generated by microorganisms as
they degrade organic material When
this heat energy is conserved, it
becomes possible for the digester
system to maintain its operating tem-
peratures within the thermophilic range
(45°C or higher) despite severe winter
conditions. As a result, detention times
can be cut to less than a week The first
successful large-scaleapplication of the
ATAD process with air aeration (as
opposed to aeration with high-purity
oxygen) was completed by Jewell et al
in EPA Project Number R804636 (1 979)
entitled, "Autoheated Aerobic Thermo-
philic Digestion with Air Aeration "
The purpose of this study was to
analyze the results obtained by Jewell
et al. and to determine whether the
process could be competitive with other
stabilization alternatives.
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Basis of Analysis
Jewell et a/, conducted tests of the
ATAD process over an 18-month period,
using a blend of primary and waste
activated sludge that had been gravity-
thickened to an average 4.5-percent
total solids Testing was conducted in
batch-scale reactors (which were used
in tests to estimate the biodegradability
of sludge), in semi-continuous and con-
tinuous bench-scale reactors, and in a
28-m3 (1,000-cubic-foot) full-scale
reactors. The batch- and bench-scale
apparatus were maintained in a 50°C
water bath, while the totally enclosed
and insulated full-scale reactor was
self-heated. Steady-state data were
collected from 30 full-scale tests and 21
bench-scale tests and were grouped by
hydraulic retention time (HRT) and
averaged within the group.
Analysis of Test Results
Data from Jewell et al on system
kinetics generally agree with those of
other researchers. The rate of change in
biodegradable sludge components is
dependent on the concentration of
biodegradable organics, as approximated
in the first-order equation
RS = -KS
where Rs = rate of change of components
S = concentration of biodegrad-
able organics
K = reaction rate coefficient
Test results obtained by Jewell et al.
show S for the following components to
be as follows:
Biodegradable
Portion (%)
46 4 - 76.5
22.9 - 62.0
42.2 -71.7
57.0 - 85.0
Component
COD
Total solid
Total volatile
solids
Total Kjeldahl
nitrogen
The wide variation observed in these
data (which are consistent with those of
other researchers) can cause some
uncertainty in design. The reaction rate
coefficient K (using COD as a measure
of biodegradable organics) is seen to
vary with temperature as illustrated in
Figure 1, which was compiled from
several sources of data.
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Temperature of Liquid in Aerobic Digester, °C
Figure 1 • Reaction rate coefficient versus aerobic digester liquid temperatures.
80
Figure 2 illustrates the components of
heat entering and leaving an aerobic
digester. There are three sources of
heat loss: loss to surroundings by
convective radiation, loss through
exiting of the moisture-laden exhaust
gas (both through evaporation and loss
of sensible heat), and loss through the
exiting of digested sludge from the
system. Design of an ATAD system
requires that an overall heat balance be
developed in order to calculate operating
temperatures. Jewell et al. fit their data
to satisfy a simplified, empirical heat-
balance relationships as follows:
HL = a(TR-TA)bDLCpL
where HL =heat loss coefficient,
cal/mVhr
TR = reactor temperature, °C
TA = ambient air temperature,
°C
DL = liquid density, gm/ml
CPL = specific heat capacity,
cal/gm - °C
a = intercept coefficient
b = slope coefficient
This relationship does not apply to
ATAD reactors in general, however, as
in fact the only heat loss that is related
to the difference between reactor and
ambient air temperatures is the heat
loss to the surroundings. Loss of heat by
exiting gases depends on total air flow,
relative humidity of the exhaust gas,
and/or exhaust gas temperature. Loss
of heat with effluent sludge depends on
the volume and temperature of sludge
leaving the system.
Of the three sources of heat loss, the
more important losses are via the
exhaust gas and the effluent sludge.
Convection losses from the vessel and
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pipeline are significant, but of less
importance; these can be controlled by
insulation and covering of the digester
The two major sources of heat loss can
be controlled, respectively, by (1) pro-
viding efficient aerators (eg., of 15
percent efficiency or better) to reduce
air flow through the reactor, and (2)
thickening the sludge fed to the reactor
in order to reduce the total quantity of
material to be processed (thickening of
sludge to 3-percent solids or greater has
been recommended by several re-
searchers). Jewell et al. obtained poor
dewatering of sludge processed by
ATAD in the full-scale reactor; the
researchers observed a substantial
increase m capillary suction time (CST).
It is possible that the high rotational
speeds required by the self-aspirating
aerator used in the full-scale digester
caused a deflocculation of the sludge
that would affect dewatering character-
istics. Deflocculation would not be
expected with a submerged turbine
aerator (which also would provide high
aeration efficiency), so in the economic
study described below it was assumed
that a submerged turbine aerator would
be used
Analysis of Process Economics
The costs of the ATAD process were
compared with those of aerobic digestion
at ambient temperatures and of meso-
philic anaerobic digestion. The com-
parison was conservative, in that it was
assumed that influent sludge would be
at 3-percent solids. As discussed above,
this level is the minimum acceptable for
efficient ATAD operation.
Costs were examined in detail for a
3,800-mVd (1 -mgd) plant and compared
also for plants of 38,000-m3/d (10-mgd)
and 380,000-mVd (100-mgd) capacity
Estimated sludge production from the 1 -
mgd plant is 0.9 dry tonne per day; a
peaking factor of 50% was assumed.
Economic criteria used in the comparison
were as follows:
Capital cost base
Capital cost
amortization
Salary
Electricity
3,140 (ENR April
1980)
20 years, 7%
(equipment)
40 years, 7%
(structures)
$10 per hour
(including benefits)
$0.06 per kWh
Heat With
Sludge Input
Heat With
Gas Input
Biological Heat
Production (HB)
Heat Loss With
Gas (Hs + HM)
Heat Loss With
Sludge (HE)
Heat Loss to
Surrounding (f/
HB
Hs
Figure 2. Heat balance components for aerobic digestion.
Facilities were sized to achieve a 40-
percent reduction of total volatile solids,
on sludge assumed to have a 70-per-
cent volatile fraction of which 70
percent was biodegradable. The desired
reduction in volatile solids would
require detention of 7.5 days, assuming
aerator efficiency of 15 percent.
Unit costs for ATAD were found to be
$ 160 per tonne for the 3,800-mVd (1 -
mgd) plant, this being derived from total
capital costs of $385,000 and total
annual costs of $53,000. Major oper-
ating expenses were in powerand labor.
Unit costs for ATAD for the 38,000-
mVd (10-mgd) and 380,000-mVd
(100-mgd) facilities were found to be
$90 per tonne and $80 per tonne,
respectively.
At the 1 -mgd facility, ATAD costs are
substantially lower than those projected,
using the same criteria, for ambient
aerobic digestion ($260 per tonne) and
for mesophihc anaerobic digestion
($220 per tonne). As shown in Table 1,
however, the low net power costs for
anaerobic digestion at large-scale
facilities—where recovery of methane
digester gas is sufficient to power the
digestion process virtually free of
charge—tend to make anaerobic diges-
tion the system of choice at the plants of
10- and 100-mgd capacity.
In summary, the ATAD process costs
for a 3,800 mVd (1 -mgd) plant are only
about 62 percent of those of ambient
aerobic digestion and 73 percent of
those of anaerobic digestion, on a unit-
cost basis.
Conclusions and
Recommendations
Autoheated thermophilic aerobic
digestion with air aeration is feasible on
a thickened municipal sludge. The
process' favorable economics at plants
of 3,800 mVd (1-mgd) capacity merit
serious consideration in planning for
design of sludge processing facilities at
such plants.
Some problems observed in the tests
conducted by Jewell era/.—specifically,
the less-than-complete mixing of digester
contents and the poor dewaterability of
the digested sludge—might be solved by
use of submerged-turbine aerators or
other systems with efficient oxygen
transfer characteristics. Testing is
recommended.
In general, the characteristics of
sludge processed by ATAD to a 40-
percent reduction in volatile solids
should be similar to those of sludge
processed by ambient aerobic digestion
or mesophilic anaerobic digestion
Jewell et al. observed better reductions
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Table 1. Economic Comparison of ATAD, Aerobic Digestion and Anaerobic
Digestion Treatment Systems
Plant
Size
Sludge Digestion
Process
Capital
Cost
Annual Amortized
Operating Annual Unit Cost
Cost Cost (per tonne)
3,800-m3/d
d-mgd)
38,000-m3/d
(10-mgd)
380,000-m3/d
(100-mgd)
ATAD
Aerobic Digestion
Anaerobic Digestion
ATAD
Aerobic Digestion
Anaerobic Digestion
ATAD
Aerobic Digestion
Anaerobic Digestion
385,000 21,500 53,000 $160
550,000 42,000 88,000 260
700,000 14.000 73,000 220
1,200.000 200,000 290,000 90
2,500,000 910,000 930,000 20O
1.700,000 42,000 190,000 55
6,200,000 1.200.000 2,500.000 80
14,000,000 9,000,000 10,000,000 180
9,400,000 310,000 1,100,000 35
P. W Keohan, P. J. Connelly, and A. B. Prince are with Camp, Dresser, and
McKee, Inc., One Center Plaza, Boston, MA 02108.
Roland Villiers is the EPA Project Officer (see below)
The complete report, entitled "Engineering and Economic Assessment of A uto-
heated Thermophilic Aerobic Digestion with Air Aeration," (Order No
PB 82-102 310; Cost: $6.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U. S. Environmental Protection Agency
Cincinnati, OH 45268
of pathogens in their test ATAD facilities
than in the full-scale anaerobic digesters
at the wastewater treatment plant
where their tests were conducted. This
is probably due to the fact that addition
of sludge to the test units was stopped
for 12 to 24 hours before the sludge was
sampled for pathogens. This practice is
typical at smaller plants; at larger
plants, where continuous feed of sludge
is the practice, pathogen reduction
would probably be less effective.
It is possible that use of heat ex-
changers (pre-warming influent sludge
with excess heat from effluent sludge)
and series operation of ATAD equipment
would further improve process effi-
ciency. There are not at present suffi-
cient data to predict the results of
instituting either measure. At plants
where pure oxygen is readily available
and being used for other purposes, such
as supply for pure-oxygen activated
sludge process, the potential increased
efficiency possible through oxygen
aeration of ATAD units, and the con-
comitant 25-percent reduction in digester
volume requirements, make this process
option worth considering.
\ U S GOVERNMENT PRINTING OFFICE, 1981 — 559-017/7386
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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