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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