United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-142 Sept. 1984
4>EPA Project Summary
Demonstration of Thermophilic
Aerobic-Anaerobic Digestion at
Hagerstown, Maryland
Oscar W. Haas
A thermophilic aerobic-anaerobic
digestion system with a nominal
secondary sludge capacity of 16,400
gallons per day was designed and
constructed at the Hagerstown,
Maryland, Wastewater Treatment Plant.
This project establishes the process
performance of the dual digestion
system in a full-scale design. The
system included a short (approximately
1 day) retention time aerobic digester
followed by a high-rate anaerobic
digester. Thickened, air-activated
sludge was autothermally heated by the
aerobic oxidation of organic substrates
in the first step and then fed to the
anaerobic second step, where the sta-
bilization process was completed with
the formation, of methane gas. Data
were collected to evaluate the system's
performance regarding volatile solids
destruction, oxygen consumption,
power draw, heat production, and
process stability. Analysis of pathogens
and indicator organisms were also
made to determine the effectiveness of
the aerobic step to inactivate
pathogenic bacteria, viruses, and para-
sites.
Thermophilic temperatures (greater
than 45°C) were rapidly achieved upon
start-up of the dual digestion system
and were maintained in the aerobic
reactor at a hydraulic retention time of
approximately 1 day. The high shear
aeration device demonstrated suffic-
ient oxygen transfer capacity to achieve
and maintain these high temperatures
at reasonable power densities in the
aerobic reactor. The system responded
well to variations in feed flow and solids
concentration as well as to operational
upsets. Analyses were performed that
illustrate the ability of the dual digestion
system to achieve significant
reductions in the level of pathogenic
organisms in sewage sludge. Finally,
over the course of some 20 weeks of
operation, the dual digestion system
proved itself to be an effective sludge
stabilization process, achieving an
overall volatile solids reduction of 41.6
percent, with weekly averages in the 24
to 58 percent range.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory. Cincinnati, OH,
to announce key findings of the
research project that is fully document-
ed in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The proper treatment and disposal of
wastewater sludges has become a
problem of increasing concern (and
expense) throughout the world. At
present, the two most widely practiced
biological processes for sludge stabiliza-
tion are anaerobic and aerobic digestion.
Anaerobic digestion, long a mainstay of
wastewater treatment plant design, is a
low-rate process typically operating with
hydraulic retention times in the 15- to 30-
day range. An advantage of the anaerobic
process is that it requires little mechani-
cal energy input to maintain operation,
and in fact it produces a combustible
methane gas. On the other hand, aerobic
digestion, a relative newcomer, is a faster
and more stable process but it is very
energy intensive. The dual digestion
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system, by combining a short-retention-
time, autothermal, aerobic first step with
a high-rate anaerobic second step,
provides a novel approach to the biological
stabilization of sludge that incorporates
the advantages of each traditional
method and minimizes their drawbacks.
Process Description
The dual digestion system designed for
the Hagerstown, Maryland, Wastewater
Treatment Plant consists of a short-
retention-time, autothermal, aerobic
digester followed in series by an existing
anaerobic digestion step. The primary
purpose of this process is to stabilize and
pasteurize the thickened waste second-
ary sludge produced by the air-activated
sludge plant through the reduction of
volatile matter in the sludge and the
production of methane in the second
step. The pasteurized residue can then be
trucked for safe disposal on land. Figure 1
presents a process flow schematic for the
dual digestion system.
The first step of the dual digestion
system consists of a high-rate
autothermal aerobic digester contained
in an insulated, covered, concrete tank of
cylindrical geometry (Figure2). The tank is
11.5 feet in inside diameter with a
maximum sludge sidewater depth of 24
feet and a minimum freeboard height of 2
feet. An adjustable external telescoping
valve and overflow box maintains the
liquid height in the aerobic digester at a
depth of between 20 and 24 feet. During
this demonstration program, the liquid
level was maintained at 20 feet, creating
a reactor volume of 16,400 gallons which
includes the volume of the drainage well
at the bottom of the tank. The aerobic
reactor can be maintained at
thermophilic temperatures (greater than
45°C) through the conservation of heat
generated by biological oxidation of
degradable organic matter in the waste
sludge and does not require the use of an
external heat source. High-purity oxygen
is introduced into the aerobic reactor by
means of static course bubble diffusers
located at the bottom of the tank. Oxygen
transfer from the gas to the liquid phase is
enhanced by means of a rotating high-
shear device that breaks up the oxygen
gas bubbles as they rise through the
sludge. The use of high-purity oxygen, in
addition to enhancing the oxygen transfer
rate, minimizes the volume of water-
saturated gas (mainly carbon dioxide and
oxygen) vented to the atmosphere. This
low gas volume minimizes latent and
sensible heat losses from the aerobic step
and allows the desired operating
temperature to be maintained more
efficiently. The major mode of
temperature control in the first step is the
adjustment of the oxygen feed rate and
thus the extent of heat-producing
biological oxidation that occurs. Typically,
only 5 to 15 percent of the volatile solids
content of the sludge needs to be j|
metabolized to produce the required
quantity of heat, depending on the
desired operating temperature,
environmental heat losses, and influent
sludge characteristics. The latter was a
crucial factor during the Hagerstown
program because of persistently low feed
solids concentration (<4%TS). This factor
necessitated the occasional use of sludge
flow adjustment as another mode of
temperature control when feed solids
concentrations dropped below critical
levels. By decreasing the sludge flow
rate, the amount of heat lost with the
aerobic sludge effluent was reduced.
Despite the undesirability and inconveni-
ence of low sludge concentrations, the
ability to control the process was
effectively demonstrated.
The second step of the dual digestion
system consists of a high-rate anaerobic
digester that receives the heated and
partially digested sludge from the
autothermal aerobic pretreatment step.
The anaerobic reactor completes the
stabilization process by further breaking
down volatile matter into carbon dioxide
and methane gas. For purposes of this
investigation, an existing fixed-cover
anaerobic digester with a single 50-foot-
diameter tank and a sidewater depth of
25 feet (nominal capacity of 396,550
gallons) was used. To keep anaerobic
hydraulic retention times within a
reasonable range and yet maintain a gas
Anaerobic
Vent Gas
Influent
Sludge
Shear
Blades
Anaerobic
Pump
Effluent*
Sludge
Aerobic Step
Anaerobic Step
Figure 1. Dual digestion system process flow schematic.
2
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Figure 2. Liquid oxygen storage tank (left) and first step aerobic reactor.
seal over the mid-depth influent sludge
pipe, the reactor was operated at slightly
more than half of its total volume. The
reactor cover was sealed to prevent gas
leakage at this reduced liquid level. A 7.5
horsepower pump was installed for
sludge wasting and recirculation mixing.
Process Observations
The Hagerstown dual digestion
demonstration plant was started on June
11, 1980, with the aerobic reactor being
filled to the 20-foot level (approximately
16,400 gallons) with thickened
secondary sludge. Beginning June 13, a
secondary sludge feed was initiated at a
rate sufficient to maintain a 1 -day aerobic
retention time. As can be seen from Figure
3, by the end of the seventh day of
operation, the aerobic digester tempera-
ture had risen to the thermophilic region
(greater than 45°C). By the ninth day the
temperature had reached 55°C without
the aid of external heating. At that point,
the oxygen feed rate was lowered to
Stabilize the sludge temperature and
increase oxygen utilization.
Thermophilic temperatures were
maintained in the aerobic step despite a
wide range of process and operating
conditions. Table 1 summarizes phase
average values of key parameters for the
20 weeks of continuous operation docu-
mented in this report. In addition to the
data listed, information was also obtained
concerning heat and oxygen balances
around the aerobic reactor, kinetic rate
constants, high-shear power draw, pas-
teurization performance (bacteria,
viruses, and parasites), and process
stability and flexibility.
Conclusions
The Hagerstown dual digestion system
has successfully demonstrated that
thermophilic temperatures may be
autothermally achieved and maintained
in a full-scale system at relatively short
(0.95 to 2.25 days) aerobic retention
times. Despite wide variations in sludge
feed flow rates, solids concentrations,
and degradability, control of aerobic
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80
70
60
50
I
40
30
20
10
Start Tank Fill, QQ~16 cfm. Purge On
False LEL Alarm, Mixer+O2 Off
4
End Tank Fill. Mixer Started ~ 32 rpm. Purge Off
Mixer On+Off for Strain Gaging. kW meter
Begin Sludge Feed, R.T. ~ 1 Day
QG ~ 22 cfm. Mixer Off for Short Periods
Mixer+Oi Restarted
QG Reduced to 17.5 dm
Aerobic Reactor
11
12
13
14
15
16 17
June 1980
18
19
20
21
22
Figure 3. Process start-up.
reactor temperature was maintained
through simple adjustment of the oxygen
feed flow rates and hydraulic retention
times.
Volatile solids removal rates averaged
27.2 percent across the aerobic step and
41.6 percent over the entire system.
These values agree well with historical,
mesophilic, anaerobic digester perform-
ance at much longer (20 to 50 days)
retention time. Oxygen consumption
ratios in the range of 1.6 to 1.8 pounds
oxygen per pound of volatile solids
removed were found by mass balances
around the aerobic step. The aerobic heat
of reaction was found to be a function of
the extent of volatile solids removal and
varied in the 3000 to 9500 Btu per pound
of volatile solids removed. These values
are consistent with expected results for
operation on waste secondary air-activa-
ted sludge at relatively low influent solids
concentrations (1.9 to 2.9 percent volatile
solids). The results confirm design model
predictions and allow extrapolations to
higher solids concentrations with confi-
dence.
The high-shear aeration device
successfully demonstrated sufficient
oxygen transfer capacity to achieve and
maintain thermophilic temperatures in
the aerobic step at short hydraulic reten-
tion times and reasonable power
densities (0.5 to 1.1 shaft horsepower per
1000 gallons). The capability of the device
for efficient oxygenation of thickened,
waste-activated sludge was also proved.
Oxygen use exceeded 65 percent of the
feed gas flow when the shear device was
operated at 32.2 rpm. Furthermore, the
data suggested that the efficiency of the
high-shear device would be enhanced at
higher influent sludge concentrations.
Solids and temperature profiles indicated
that the contents of the aerobic reactor
were reasonably well mixed under all
conditions.
Preliminary information obtained from
an aerobic digestor temporarily used as
the second process step indicates that
good overall volatile solids destruction,
process stability, and acceptable
methane purities are possible with the
dual digestion system. Total alkalinity and
volatile acid concentrations for the
anaerobic step averaged 3559 and 265
mg/L, respectively, during the course of
the test program.
Data collected on bacteria, virus, and
parasite kills demonstrate the ability of
the dual digestion system to reduce
significantly (by several orders of magni-
tude) the levels of human pathogens
when operated at thermophilic tempera-
tures.
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Table 1. Test Program Process Conditions
Parameter Phase Average
Range
Aerobic retention time, days
Anaerobic retention time, days
Temperature, °C
Feed
Aerobic
Anaerobic
Ambient
Solids concentration, %
TS
VS
Volatile solids removal, %
Aerobic
Overall
Oxygen flow rate, CFM NTP
Gas purities, %
Aerobic vent O2
Anaerobic vent CHt
CO,
02
1.38
19.9
23.8
50.5
41.7
21.5
3.02
2.41
27.2
41.6
19.2
68.9
49.1
33.1
1.6
0.6 - 8.8
9.6 -34.0
16.8 -28.3
42.3 - 60.0
37.5 -47.0
5.0 - 40.0
1.50- 4.11
1.23 - 3.33
15
24
-51
-57
10.3 -33.4
31.5 -91.5
35.5 - 62.5
24.2 -49.5
0 - 4.5
Feed
Aerobic
Anaerobic
6.00
7.09
7.30
5.5 - 6.9
6.6 - 7.7
6.7 - 7.65
Recommendations
The performance of the dual digestion
system under operating conditions closer
to typical design values should be
investigated further. In particular, the
long-term operations at a 1 -day aerobic,
8-day anaerobic retention time with high
influent solids concentrations (greater
than 3 percent volatile solids) is
necessary to demonstrate the system's
advantages completely.
The full report was submitted in
fulfillment of Grant No. S805823-01 by
Union Carbide Corporation under the
sponsorship of the U.S. Environmental
Protection Agency.
Oscar Haas is with Union Carbide Corporation, Tonawanda, NY 14150.
B. Vincent Salotto is the EPA Project Officer (see below).
The complete report, entitled "Demonstration of Thermophilic A erobic-A naerobic
Digestion at Hagerstown, Maryland. "(Order No. PB 84-238 252; Cost: $13.00,
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
, US OOVERNMENT PHIKTINO OFFICE. 1M4- 759-102/10707
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Environmental Protection
Agency
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Information
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