f/EPA
United States
Environmental Protection
Agency
Office of Environmental
Engineering and Technology
Washington DC 20460
Research and Development
EPA-600/S2-81-105 July 1981
Project Summary
Evaluation of the Full-Scale
Application of Anaerobic
Sludge Digestion at the
Blue Plains Wastewater
Treatment Facility-
Washington, D.C.
Wilbur Torpey, John Andrews, and Nicholas Mignone
This study investigates the applica-
tion of a new mesophilic-thermophilic
anaerobic sludge digestion process to
the existing mesophilic sludge
digestion system at the District of
Columbia Blue Plains waste water
treatment plant. The study also
evaluates improvements in the exist-
ing mesophilic digestion operation
and possible application of thermo-
philic digestion technology. Detailed
analyses in the full report are designed
to facilitate the use of the approach as
a case study model for other waste-
water treatment facilities considering
the process.
The mesophilic-thermophilic diges-
tion process is a new two-step
concept for treating municipal waste-
water sludges. The first step operates
under mesophilic process conditions
(digestion with anaerobic microorgan-
isms at 90 to 100°F). The second step
operates under thermophilic process
conditions (digestion with anaerobic
microorganisms that thrive at 120 to
130°F). The mesophilic process is the
most commonly used digestion
process. The thermophilic process has
had limited application in this country,
but is used regularly in the U.S.S.R.
The development and application of
the mesophilic-thermophilic process
has been pioneered by the City of New
York under the direction of Mr. Wilbur
Torpey. Full-scale application and
evaluation of its effectiveness has
been undertaken by the Rockaway
Pollution Control plant in New York
City. Results at Rockaway indicate
that the physical characteristics of
mesophilic-thermophilic digested
sludge are changed to the extent that
the economics of dewatering are sig-
nificantly improved. Moreover, the
residual sludge is inert and has met the
time-temperature requirements for
pathogen destruction.
The Rockaway findings resulted in
the desire to investigate the feasibility
of applying the mesophilic-thermo-
philic process to a major wastewater
treatment facility. The Blue Plains
plant was selected because: (1) the in-
fluent wastewater is mainly domestic,
as in the Rockaway influent; (2) it has
anaerobic digesters in operation; (3)
the same activated sludge treatment
process is used; and (4) the sludge
management methodology needed
upgrading for operating and economic
reasons.
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The evaluation at Blue Plains con-
cludes that: (1) a limited expansion of
digester capacity is required to handle
the entire sludge stream; (2) digester
gas would be available for sale to out-
side interests after internal heating
requirements were satisfied; and (3)
the cost of sludge handling could be
reduced by $24 to $31 per million
gallons of influent flow (from
$72/mg to $41 -48/mg). The analysis
also indicates that the improved char-
acteristics of the mesophilic-thermo-
philic digested sludge could reduce
chemical conditioning requirements
so that the cost would be almost $7
less per million gallons of influent flow
than mesophilic digestion and almost
$4 less than thermophilic digestion.
Moreover, the Rockaway results indi-
cate that there may be additional
savings during disposal because the
stabilized material produced would
have in effect been composted.
For any of the three anaerobic
systems to process all the sludge
currently being generated at Blue
Plains, some capital expenditures
would be required to expand or
upgrade existing equipment. A de-
tailed cost analysis was not performed.
However, a unit operation analysis
indicates that the capital cost would
be highest for the mesophilic system
and lowest for the thermophilic
system. The mesophilic-thermophilic
system would lie between the two.
The study concludes that, under the
present circumstances, the thermo-
philic digestion option would require
the least capital expenditure and
would be the most expedient, cost-
effective solution to the sludge
management problem.
This Project Summary was develop-
ed by EPA's Office of Environmental
Engineering and Technology,
Washington. DC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
This study is an engineering evalua-
tion of the application of a new concept
in wastewate engineering to a major
wastewater treatment facility. The con-
cept involves the combination of two
anaerobic digestion processes: the
mesophilic process (anaerobic digestion
operating at temperatures of 90 to
100°F) followed by the thermophilic
process (anaerobic digestion operating
at temperatures of 120 to 130°F) It is
termed the mesophilic-thermophilic
process. Such a system has been
operated at full-scale at the Rockaway
Pollution Control plant in New York City
The existing Blue Plains wastewater
treatment plant in Washington, D.C.
treats approximately 330 mgd and uses
mesophilic anaerobic digestion as a
step in disposal of the sludge produced.
This step has developed operational
constraints and is limited in capacity. As
a result, much of the sludge generated
at Blue Plains is disposed of without
digestion Expansion of some sludge
processing unit operations, now m
progress, will increase capacity, but not
enough to handle the entire sludge
stream as the system now operates.
Finally, related planning and engineer-
ing evaluations now in progress are
considering the long-term sludge dis-
posal options available at Blue Plains,
including anaerobic digestion
Therefore, the purpose of this study is
to identify the relative applicability of
the mesophilic-thermophilic process
compared with other anaerobic diges-
tion processes In particular, the study
evaluates the capabilities of anaerobic
digestion to meet sludge processing
needs at Blue Plains, the operating and
equipment modifications required to
handle the full sludge stream, and the
monetary and energy costs associated
with the systems. Of major concern was
meeting the objective of being able to
identify the sludge digestion process
that could be implemented with no
major construction
Development of the
Conventional (Mesophilic)
Process
Anaerobic digestion, one of the oldest
wastewater treatment processes,
involves the biological conversion of
organic solids to methane and carbon
dioxide. It is a natural process occurring
in such diverse environments as
swamps, stagnant bodies of water, and
stomachs of cows One of its first engi-
neered uses was in the late 19th
century when septic' tanks were used
for wastewater treatment. In the septic
tank, the solids that settle to the bottom
undergo anaerobic decomposition, with
the liquid passing on to a tile drainage
field. Although the solids may be stabil-
ized, the gas which evolves disturbs the
sedimentation process by lifting
particles into the overflow. This can
cause plugging of the tile field, thus
destroying the efficiency of the field and
frequently resulting in malodorous con-
ditions
In the early part of this century, Dr
Karl Imhoff invented a two-story tank to
deal with this septictankdeficiency. The
tank design was such that the gas
generated by anaerobic digestion at the
tank bottom was prevented from rising
to the upper zone where sedimentation
occurred. T he functions of digestion and
sedimentation were thus effectively
separated
A natural evolution of this separation
of functions, which took place in the
1920's, was the construction of sepa-
rate tanks for anaerobic digestion, with
the solids removed m the sedimentation
basin pumped totheanaerobicdigester
This procedure permitted the applica-
tion of heating and artificial mixing to
the anaerobic digester Both heating
and mixing, which began to be applied
m the 1930's and 40's, accelerate the
rate at which the solids are converted to
gas as well as increase the effective
volume of tankage available for diges-
tion Consequently, instead of the three
to six months requirementfor anaerobic
digestion in the Imhoff tank, accelerated
digestion made it possible to complete
the process in one to two months The
obvious result of this functional separa-
tion was a substantial decrease m
required capital cost.
The application of digester mixing
soon made it obvious that mixing and
separation of the supernatant from the
digested sludge were incompatible in
much the same sense that sedimenta-
tion and digestion in the septic tank
were incompatible. This led to a further
separation of functions by the applica-
tion of a two-stage digestion process
where the biological reactions (with
mixing and heating for acceleration)
occur in the first stage and the digested
sludge is transferred to a second stage
for separation of the solids from the
liquid. Two pioneers m the application of
two-stage digestion were A. M. Busell
and A. J Fisher. With a two-stage
system, satisfactory digestion could
consistently be obtained m the first tank
m a nominal retention time of one
month or less.
In the 1950's, NewYorkCityfacedthe
need for expanding the capacity of the
digestion systems of the various plants.
It was recognized that the digestion time
could be substantially decreased if a
significant portion of the water could be
removed from the sludge before it was
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fed to the digester A separate mixed
raw sludge thickener was developed
and placed before digestion This proce-
dure made possible quadrupling the
solids loading rate to the digesters
because the loading rate on the primary
digesters was doubled and the need for
secondary digesters was eliminated
Energy requirements for sludge heating
also were substantially reduced since it
was no longer necessary to heat the
water that was formerly associated with
the sludge
Present State of Practice of
Thermophilic Anaerobic
Sludge Digestion
Thermophilic anaerobic digestion is
very similar to mesophilic anaerobic
digestion except the temperature at
which it operates is 1 20-1 30°F instead
of 90-100°F. It thus takes advantage of
the fact that biochemical reaction rates
can be increased by increasing
temperature It is only natural,
therefore, that conversion of existing
mesophilic digesters to thermophilic
operation should be considered as a
low-cost technique for increasing the
sludge processing capability of
wastewater treatment plants. Full-scale
studies by the Metropolitan Sanitary
District of Greater Chicago, Ontario
Ministry of the Environment, Canada,
and Moscow, USSR have indicated
that the sludge processed per unit
volume of digester capacity could be
doubled by converting from mesophilic
to thermophilic operation
Besides its increased sludge
processing capability, thermophilic
operation also offers two other signifi-
cant advantages over mesophilic opera-
tion improved sludge dewatermg
characteristics and increased destruc-
tion of pathogens.
Garber's work on the vacuum filtra-
tion of thermophilic sludge at the
Hyperion plant in Los Angeles provides
an example of how sludge dewatermg
can be improved by the thermophilic
digestion. He reported a 270 percent
increase in vacuum filter yields with a
48 percent decrease in coagulant
dosage for thermophilic, compared to
mesophilic sludge. Improved solids-
liquid separation is important in land
application of sludge by decreasing the
quantity of wet sludge for disposal and
thus lowering transportation costs
An example of the increased destruc-
tion of pathogens by thermophilic diges-
tion is given by Popova and Bolotma in
their report on the practice of thermo-
philic digestion in Moscow, USSR
They state "The most essential advan-
tage of this process is the sanitary
quality of the thermophilic sludge.
According to the sanitary officials of the
health department, viable eggs of
helminths are absent from such a
sludge " This improvement in sanitary
quality is of special significance in light
of the current trend toward land
disposal of digested sludge
Although mesophilic and thermo-
philic anaerobic digestion are quite
similar in both design and operation,
there are differences which must be
taken into account in adapting meso-
philic digesters to thermophilic opera-
tion Among these are' (1) additional
sludge heating requirements, (2) struc-
tural competency of existing digesters
and piping at the higher temperatures
must be checked, (3) potential odors at
sludge handling areas, (4) closer atten-
tion to temperature control, (5) main-
taining an optimum concentration of
volatile solids in order to operate at a
net energy balance, (6) higher ammonia
levels due to increased protein destruc-
tion, and (7) removal of increased
amounts of moisture from the digester
gas
Development of the
Mesophilic-Thermophilic
Process
In the work at the Rockaway plant in
New York City, a process was developed
to overcome the potential disadvantages
of thermophilic digestion as well as to
improve upon the process This was
accomplished by the use of a two-stage
digestion system, consisting of a meso-
philic stage followed by a thermophilic
stage A part of the thermophilically
digested sludge was also recycled
through the aeration tanks to obtain
additional destruction of organic solids
The advantages of the thermophilic
process are thus retained without the
disadvantages In addition, a substantial
increase in organic solids destruction is
obtained, resulting in improved quality
of the residual digested sludge to the
extent of being comparable to a fully
composted sludge
Application at Rockaway
A full-scale test was conducted for
five months at the Rockaway plant,
which serves a population of 100,000,
to evaluate a new method of reducing
the amount and volume of sludge pro-
duced from the activated sludge process
This method involved using high stabil-
ity thermophilic digestion following
mesophilic digestion and recirculatmg
part of such thermo-digested sludge
directly to and through the secondary
system of the activated sludge process
while the remainder was conducted to a
rethickenmg and elutnation tank
(Figure 1) Operating results (Tables 1-
4) have demonstrated that the volatile
matter normally transported to sea after
meso-digestion was reduced by two-
thirds Moreover, the volume of sludge
produced was lowered by two-thirds
without chemical or mechanical aids
Using a laboratory scale, it was shown
that the residual solids exhibited
improved coagulability after having
undergone thermo-digestion This
change would improve the economics of
all subsequent dewatermg processes
The treatment process performed
without significant adverse effect on
any accepted parameter due to the
continuing recirculation of digested
sludge through the activated sludge
process The concentration of nutrients
and metals in the final effluent were not
affected.
Conclusions
1 It has been found possible to
process the total waste sludge
flow at the Blue Plains plant by
adopting the thermophilic anaero-
bic digestion process to the
present digestion facilities In that
connection, the accumulation of
grit presently in the digester tanks
will not have to be removed.
2. Adoption of the thermophilic
digestion option at Blue Plains
requires the least capital expendi-
ture, would be the most expedient
solution to the sludge manage-
ment problem, and would yield
substantial operational cost
savings. It also offers thepotential
of eliminating the need for
composting because of the
pathogen kill
3 The mesophilic-thermophilic
digestion process would not be
able to handle the entire waste
sludge flow at Blue Plains without
additional capital expenditures for
new digester tanks and separate
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Plant Influent
Plant Effluent
Flow
BODs
SS
20-29 mgd
85-no mg/l
40-90 mg/l
6-8 mg/l
8-15 mg/l
# VM/day
cu ft/day
12,900 #VM from incoming wastewater
2,800 ttVM from digester recycle
1,300 ft VM from thickening overflow
Dilution Water
17.000 ffVM to digestion
2,400
# VM/day
Thickening
and
Elutriation
16,200
# VM/day
9,000
# VM/day
7,200
# VM/day
1,900
#VM/day
Mixed
Gravity
Thickening
Mesophi/ic
Digestion
95°F
Thermophilic
Digestion
121°F
7,700
cu ft/da
7,700
:u ft/day
7.700
cu ft/day
Thickener Overflow
800^ VM/day
Elutriation
Overflow
500 # VM/day
To Final
Disposal
1,650
cu ft/ day
Volatile Matter to Disposal
Before Thermophilic After Thermophilic
System Incorporated System Incorporated
5,700 #, 4,800 cu ft 1,900 #, 1,650 cu ft
HVM/day - Pounds Volatile Material Per Day
Summary
System Removal
Loss VM Mesophilic Digestion
Loss VM Thermophilic Digestion
Loss VM Aerator
Net
12,900 ttVM/day, Inf. to Effl.
7,200 #VM/day, Inf. to Effl.
1.800 # VM/day, Inf. to Effl.
2,000 # VM/day, Inf. to Effl.
1,900 H VM/day to Disposal
Figure 1. Two-stage mesophilic-thermophilic sludge digestion wastewater treatment plant (volatile matter mass balance-daily
rate).
Table 1. Monthly Average Solids
Data, Rockaway, NY Pollu-
tion Control Plant, February -
May 1980
Flow, mgd 25
VSS Captured @ 75% Volatile
Matter, Ib 12.900
Ft aw Thickener Pumping,
cu ft/day 7,700
Volatile Matter Concentration
Flaw Thickener, % 3.4
Mesophilic Digester, % 1.9
Thermophilic Digester, % 1.5
Volatile Matter from
Thickener, Ib 16.200
Volatile Matter Leaving
Mesophilic Digester, Ib 9,000
Thermophilic Digester, Ib 7,200
Ftethickener Elutriator
Underflow, Ib 1,900
Rethickener Elutriator
•Overflow. Ib 500
Table 2.
Monthly Average Nutrient Data, Rockaway, NY Pollution Control Plant,
(mg/l)
Nitrogen
Phosphorus
Total
/VH3 Org. A/03 A/02 Inorg. Total Ortho
July-Dec 1979
(Before Modification)
Jan 1980
(Transition)
Feb-May 1980
(After Modification)
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
10.3
4.5
9.4
0.6
9.4
2.3
10.7
7.0
9.8
3.0
11.0
2.7
0.5
3.7
0.2
6.3
0.4
7.3
0
0.5
0.1
0
0
0
10.8
8.2
9.6
6.9
9.8
9.6
2.3
1.7
2.7
1.8
3.0
2.0
1.9
1.7
1.6
1.5
1.6
1.2
4
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Table 3. Monthly Average Gas
Production, Rockaway, NY
Pollution Control Plant
(cu ft/day)
Mesophilic Thermophilic
Digester Digester
No Recirculation
(Sept-
Dec 1979) 87,600
With
Recirculation
(Feb-
May 1980) 83,900
7,000
14,000
Note: Digester roofs were found to be
leaking after this work was done.
Table 4. Monthly Average Metal
Data, Rockaway, NY Pollu-
tion Control Plant (mg/l)
Cu
Cr
Ni
Zn
Pb
Fe
Cd
Ca
Mg
Hg
Inf.
Eff
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
Inf.
Eff.
July-Dec
1979
.12
.05
.014
008
.03
.02
.15
.17
.036
.009
1.2
.9
.OO04
.OO04
30
32
93
97
OO10
.0009
Feb-May
1980
.088
.027
0047
.002
.0072
.012
.12
.082
.018
.OO35
.72
.24
0018
0015
16
17
58
60
.0005
.0004
heating systems Therefore it is
not recommended at this time,
even though the final product
would satisfy all criteria for stabili-
zation and disinfection compar-
able to effectively operated
composting
4. The amount of grit passing
through the existing Blue Plains
grit removal facilities is substan-
tial This grit is combined with the
primary sludge and both are
pumped to the digestion tanks
The grit accumulates in the diges-
tion tank and reaches equilibrium
when about one-third of the tank
volume is occupied by grit This
grit accumulation has reducedthe
amount of sludge that can be
processed through the existing
digestion tanks by reducing the
efficacy of the internal mixing
process
5 The detailed solids production
analysis prepared for this study
can be incorporated into other
sludge management evaluations
performed by the District of
Columbia.
Recommendations
Based on review of the anaerobic
sludge digestion options and how they
could be adapted to the existing
facilities, the study recommends that
the thermophilic anaerobic digestion
process be implemented on a full-scale
basis at the District of Columbia Blue
Plains plant. This recommendation is
based on a thorough review of the
present state of practice in the U S and
other countries The 30 years of suc-
cessful experience in the City of Los
Angeles Hyperion plant and their
decision to convert the total digestion
system to the thermophilic process was
a consideration in making the
recommendation Another factor was
the successful conversion of the entire
mesophilic digestion system to a higher
temperature operation by the City of
Denver over two and one-ha If years ago.
The Metropolitan Sanitary District
(MSD) of Greater Chicago has also
shown at full scale that the capacity of a
mesophilic digestion tank can be
doubled by converting to thermophilic
operation As a result MSD is planning
to construct six additional digesters
capable of being operated in the
thermophilic range, as opposed to the
old plan of constructing nine additional
mesophilic units
Although the meso-thermophihc
digestion process could be the optimum
solution for other plants, the thermq-
philic process is recommended for Blue
Plains because it could be implemented
with a minimum of time and money
Other significant advantages are (1) in-
creased sludge processing capability,
(2) improved sludge dewatering as to
coagulant demand and yield, and (3) in-
creased destruction of pathogens, all of
which are pertinent to the needs of the
Blue Plains plant
It is especially important to check the
structural competency of the existing
digesters and piping at the thermophilic
temperatures, as well as the tempera-
ture control system prior to start-up
It is recommended that the transition
from mesophilic to thermophilic opera-
tion be implemented as rapidly as
possible in order to alleviate the solids
handling problems in the metropolitan
area. A carefully formulated transition
plan should be prepared so that the
transition be carried out effectively and
with minimum interference with plant
operations
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Wilbur M. Torpey is a consultant, 4923 Hanford Street, Douglaston, NY 11362;
John F. Andrews is with the University of Houston, Houston, TX 77004; and
Nicholas A. Mignone is with Environmental Technology Consultants, P. 0. Box
2550, Springfield, VA 22152.
James Basilico is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of the Full-Scale Application of
Anaerobic Sludge Digestion at the Blue Plains Wastewater Treatment Facility—
Washington, D.C.," (Order No. PB 81-219 123; Cost. $15.50, 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:
Office of Environmental Engineering and Technology
Office of Research and Development
U.S. Environmental Protection Agency
Washington, DC 20460
if U S GOVERNMENT PRINTING OFFICE, 1981 -757-012/7331
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Environmental Protection
Agency
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