Evaluation of the Full-Scale Application
of Anaerobic Sludge Digestion at the
Blue Plains Wastewater Treatment Facility
Washington, D.C.
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
OFFICE OF ENVIRONMENTAL ENGINEERING AND TECHNOLOGY
401 M STREET S.W.
WASHINGTON, D.C.
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FINAL REPORT
Evaluation of the Full-Scale Application
of Anaerobic Sludge Digestion at the
Blue Plains Wastewater Treatment Facility
Washington, D.C.
Submitted to:
Mr. James Basilico
US Environmental Protection Agency
Office of Research & Development
Office of Environmental Engineering & Technology
401 M Street, S.W.
Washington, D.C. 20460
Prepared Under:
USEPA Contract 68-01-5913
Work Assignment 5
WAPORA, Inc.
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DISCLAIMER
This report has been reviewed by the Office of Engineering and Techno-
logy, US Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views
and policies of the US Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or recommendation
for use.
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SUMMARY
This study investigated the application of a new mesophilic-ther-
mophilic anaerobic sludge digestion process to the existing mesophilic
sludge digestion system at the District of Columbia Wastewater Treatment
Facility (WWTP). As part of this study, improvements in the existing
mesophilic digestion operation and possible application of thermophilic
digestion technology were also evaluated. The study was prepared under
Contract No. 68-01-5913 for the US Environmental Protection Agency, with
the cooperation of the city of Washington DC and the Wastewater Treatment
Facility's technical personnel. The analyses are presented in detail to
facilitate the use of the approach as a case study model for other waste-
water treatment facilities considering the application of the process.
The mesophilic-thermophilic digestion process is a new concept in the
treatment of municipal wastewater treatment sludges. It involves a
two-step digestion process, where the first step operates under mesophilic
process conditions, (that is, digestion with anaerobic microorganisms that
thrive at 90 to 100°F) and the second step operates under thermophilic
process conditions (that is, digestion with anaerobic microorganisms that
thrive at 120 to 130°F). The mesophilic process is the most commonly
employed digestion process; the thermophilic process has had limited appli-
cation in this country but is used regularly in the U.S.S.R. Use of the
mesophilic-thermophilic process has been documented only at the Rockaway
facility in New York City.
The development of standard process conditions for anaerobic sludge
digestion is described in this report, including operating conditions and
process effectiveness for the Rockaway treatment facility.
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The development and application of the mesophilic-thermophilic process
has been pioneered by Mr. Wilbur Torpey. A full-scale application and
evaluation of the effectiveness of this dual process approach has been been
undertaken by the Rockaway Pollution Control Plant in New York City. The
results of the Rockaway trials indicate that the physical characteristics
of mesophilic-thermophilic digested sludge are improved and that these can
result in (1) a significant reduction in the dewatering requirements of the
stabilized material, and (2) a more stabilized residual that is nearly
pasteurized and inert. The savings from dewatering improvements appear to
be significant and outweigh any increased costs associated with operating
the two processes rather than either of the conventional single processes.
Moreover, there is the possibility that use of the mesophilic-thermophilic
process may result in an overall reduction in digestion time. This is
because the mesophilic system tends to dampen any fluctuations in waste
sludge characteristics, thus providing a consistent feed to the thermo-
philic system. As a result, the thermophilic system can be designed to
operate with a reduced safety factor for feed sludge variation. Since the
thermophilic system is a higher rate process than the mesophilic, a net
reduction in the overall detention time may be achieved.
These considerations and the success of the Rockaway trial resulted in
the desire to investigate the feasibility of applying the mesophilic-
thermophilic process to a major wastewater treatment facility. The
District of Columbia WWTF was selected because: 1) the influent wastewater
was mainly domestic, as was the Rockaway influent; 2) the facility has
anaerobic digesters in operation; 3) the facility will require modifica-
tions to upgrade the sludge handling facilities; and 4) the staff at the
facility were technically capable and willing to cooperate with the
investigation.
The investigations at the District of Columbia WWTF involved a
detailed review of existing plant operations including the development of
flow and mass balances using existing data developed for the sludge
management area. It was necessary to develop a detailed piping schematic
for the digester area, as this was not available. A tracer study was
performed on the existing twelve anaerobic digesters to determine the
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useful volume. Interviews with plant staff were used to further establish
the existing operating conditions. These sludge operating conditions were
characterized according to the solids generated per million gallons of
influent wastewater flow.
The analytic data were evaluated in conjunction with financial data to
identify the cost of treatment options. Such an evaluation was performed
to determine alternative costs for the following five operating conditions:
1) Current operating practices (January to June 1980), with anaerobic
digestion treatment of a partial sludge stream.
2) Spring 1981, after completion of plant modifications in progress
during the study, with anaerobic digestion treatment of a partial
sludge stream.
3) Mesophilic digestion of all sludge.
4) Thermophilic digestion of all sludge.
5) Mesophilic-thermophilic digestion of all sludge.
The financial analyses were based on a mixed primary-secondary sludge, with
sludge processed by gravity thickening, dissolved air flotation, anaerobic
digestion, elutriation, vacuum filter dewatering, and final disposal
through land application. Undigested sludge in excess of the existing
anaerobic digestion system capacity is limed, dewatered and disposed of
through composting or land trenching.
The engineering and financial evaluations of full-scale application of
the mesophilic-thermophilic anaerobic digestion process concluded that: a
limited expansion of digester capacity is required to handle the entire
sludge stream; there would be digester gas available for sale to outside
interests after internal heating requirements were satisfied; and the cost
of sludge handling could be reduced by $24 to $31 per million gallons of
influent flow. The analysis also indicated that the improved characteris-
111
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tics of the mesophilic-thermophilic digested sludge could reduce chemical
conditioning requirements so 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 indicate that
additional savings may be incurred during disposal because the stabilized
material should be appropriate for use as a soil conditioner.
IV
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RECOMMENDATION
Based on our review of the anaerobic sludge digestion options and how
they could be adapted to the existing facilities, it is our strong recom-
mendation that the thermophilic anaerobic digestion process is the best fit
for the District of Columbia and should be implemented on a full scale
basis. This recommendation is based on our thorough review of the present
state of practice within the United States and other countries. We heavily
weighed the 30 years of successful experience at the City of Los Angeles
Hyperion Plant and their decision to convert the total digestion system to
the thermophilic process. Another factor was the successful conversion,
over 2 1/2 years ago, of the entire mesophilic digestion system to the
therraophilic mode by the city of Denver. The City of Chicago has also
shown that the capacity of a mesophilic digestion tank can be doubled by
converting to thermophilic operation.
The thermophilic process is the option that could be initiated with a
minimum of time and money. Other significant advantages of the process are
(a) increased sludge processing capability, (b) improved sludge dewatering
as to coagulant demand and yield, and (c) increased destruction of patho-
gens, all of which are pertinent to the needs of the District of Columbia
Treatment Facility.
It is especially important that, prior to start up, an independent
engineer check the structural competency of the existing digesters and
piping at the thermophilic temperatures, as well as the temperature control
system.
We strongly recommend and urge that the transition from mesophilic to
thermophilic operation be implemented as rapidly as possible in order to
alleviate the solids handling problems with the metropolitan area. A
carefully formulated plan for the transition should be prepared so that the
transition be carried out with a minimum of interference with plant
operations.
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CONCLUSIONS
1. It has been found possible to process the total waste sludge flow
using the existing facilities for the thermophilic anaerobic digestion
process. In that connection the accumulated grit in the digester
tanks will not have to be removed.
2. Adoption of the thermophilic digestion option requires the least
capital expenditure, would be the most expedient solution to the
sludge management problem and would yield substantial operational cost
savings. It also offers the potential, because of the pathogen kill,
of eliminating the need for composting.
3. The mesophilic-thermophilic digestion process would not be able to
handle the entire waste sludge flow without additional capital expendi-
tures for new digestor tanks and separate heating systems, and as
suchj it was not recommended at this time even though the final
product would satisfy all criteria for stabilization and disinfection
comparable to effectively operated composting.
4. The amount of grit passing through the existing grit removal facili-
ties is substantial. This grit is combined with the primary sludge
and both are pumped to the digestion tanks. The grit accumulates
within the digestion, tank and reaches equilibrium when about 1/3 of
the tank volume is occupied by grit. Accordingly, this grit accumu-
lation has reduced the amount of sludge that can be processed through
the existing digestion tanks by at least 1/3.
5. The detailed solids production analysis prepared for this study can be
incoprorated into other sludge management evaluations performed by the
District of Columbia.
VI
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CONTENTS
Page
Summary i
List of Figures ix
List of Tables xi
Acknowledgment xii
1. Introduction 1
2. Anaerobic Digestion and the Rockaway Story 3
2.1. Development of the conventional (mesophilic) process 3
2.2. Development of the thermophilic process 5
2.3. Development of the mesophilic-thermophilic process 10
2.4. Purpose of the Rockaway tests 11
2.5. Results of the Rockaway tests 12
3. Blue Plains Wastewater Treatment Facility 26
4. Evaluation of Current and Future Primary and Secondary
Sludge Generation 27
4.1 Primary sludge production 27
4.2 Secondary sludge production 28
4.3 Summary 29
5. Evaluation of Existing Sludge Management Operations on
Sludge Processing and Sludge Generation 31
5.1 Gravity thickener operation 31
5.2 Dissolved air flotation thickener operation 32
5.3 Anaerobic digestion operation 35
5.4 Elutriation system operation 47
5.5 Anaerobic sludge dewatering operation 51
5.6 Raw sludge dewatering operation 53
5.7 Final sludge disposal 56
5.8 Sumraa ry 57
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Page
6. Improving Sludge Management Operation Using Anaerobic
Digestion 60
6.1 Existing mesophilic system operation 61
6.2 Evaluation of thermophilic system option 66
6.3 Evaluation of mesophilic-ttiermophilic system option 70
6.4 Summary 72
7. Economic Impact 74
7.1 Operational cost impact 75
7.2 Capital cost impact 79
7.3 Economic impact 81
Appendices
A. January to June, 1980 summary of average monthly operating
data on Blue Plains sludge management operations A-l
B. Analysis of average primary and secondary sludge production
per million gallons of influent flow B-l
C. Analysis of gravity thickener operation C-l
D. Analysis of dissolved air flotation thickener operation D-l
E. Analysis of lithium chloride tracer studies, Blue Plains
digesters E-l
F. Analysis of grit entry into anaerobic digesters per million
gallons of influent flow F-l
G. Anaerobic digestion heat availability and system heal
requirements G-l
H. Anaerobic digestion piping schematic H-l
I. Analysis of anaerobic system volatile matter reduction
and gas production 1-1
J. Analysis of elutriation system operation J-l
K. Analysis of digested sludge dewatering on average sludge
production per million gallons of influent flow K-l
L. Analysis of raw sludge dewatering on average sludge
production per million gallons of influent flow L-l
M. Section 6.1 supporting calculations M-l
N. Section 6.2 supporting calculations N-1
0. Section 6.3 supporting calculations 0-1
P. Thermophilic digestion references for Section 2.2 and 6.2 P-l
viii
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FIGURES
Number Page
2-1 Summary of operating results 20
5-1 Cross-sectional view of spring-operated pressure
relief valve 36
5-2 The effect of secondary sludge on volatile matter
reduction at the Blue Plains anaerobic digestion
facility 41
5-3 The effect of organic loading on volatile matter
reduction at the Blue Plains anaerobic digestion
facility 44
5-4 The effect of hydraulic detention time on volatile
matter reduction at the Blue Plains anaerobic
digestion facility 44
5-5 The effect of feed solids concentration on anaerobic
digestion gas production at the Blue Plains digestion
facility 45
5-6 The effect of volatile matter loading on anaerobic
digestion gas production at the Blue Plains digestion
facility 46
5-7 The effect of elutriation feed solids concentration
on elutriation underflow solids concentration 48
5-8 The effect of digested sludge flow rate on
elutriation underflow solids concentration 49
5-9 The effect of washwater on elutriation underflow
solids concentration 50
IX
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Number Page
5-10 The effect of ferric chloride addition on filter
feed solids concentration 51
5-11 The effect of secondary sludge mass on ferric
chloride addition for dewatering anaerobically-
digested sludge 52
5-12 The effect of feed solid concentration on vacuum
filtration of anaerobically-digested sludge 54
5-13 The effect of feed solids concentration on vacuum
filtration of raw sludge 55
5-14 The effect of secondary sludge on vacuum filtration
of raw sludge 56
7-1 Average cost for chemicals and final disposal
per million gallon of influent flow for the
five conditions given in Table 7-1 77
7-2 Average wet tons of sludge produced per day for
the five conditions given in Table 7-1 78
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TABLES
Number Page
2-1 Rockaway P.C.P. Treatment Efficiency, July 1979 to May 1980 14
2-2 Rockaway P.C.P. Influent-Effluent Nitrogen Concentrations,
July 1979 - May 1980 15
2-3 Rockaway P.C.P. Influent-Effluent Phosphorus Concentrations,
July 1979 - May 1980 16
2-4 Rockaway P.C.P. Influent-Effluent Metals Concentrations,
July 1979 - May 1980 18
2-5 Rockaway P.C.P. Amount and Concentration of Solids Passing
Through System 21
2-6 Rockaway P.C.P. Daily Gas Production 23
2-7 Gas Production cu ft/lb Volatile Solids Reduced 24
2-8 Gas Production and Quality From Anaerobic Digestion of
Several Pure Compounds 24
4-1 Summary of Current and Future Primary and Secondary
Sludge Generation at Blue Plains 30
5-1 Summary of Anaerobic System Heat Requirements 38
5-2 Summary of Addition (Reduction) in Solids Production
due to Various Solids Processing Steps 58
5-3 Summary of Blue Plains Sludge Management Process
Limitations under Current Operation 59
6-1 Expected Primary and Secondary Sludge Quantities
at Current 334 Million Gallon Per Day Influent Flow 60
6-2 Summary of Major Process Considerations in Implementing
Full Anaerobic Digestion of Blue Plains Sludges 73
7-1 Summary of Average Sludge Generation Per Million
Gallons of Influent Flow 76
7-2 Summary of Operating and Capital Cost Requirements
Per Sludge Handling Options 82
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Acknowledgement
This report has been prepared for the U.S. Environmental Protection
Agency, Office of Research and Development, Office of Environmental
Engineering Technology (OEET). The work was inspired by the highly
successful results obtained under the direction of Mr. Wilbur Torpey at the
Rockaway Pollution Control Plant, in New York City. Mr. Torpey served as
the principal consultant on this evaluation for the application of the
mesophilic-thermophilic process. He also contributed the discussion on the
Rockaway test. Mr. James Basilico of OEET, was instrumental in initiating
this study and in obtaining the cooperation of the Blue Plains Wastewater
Treatment Facility staff in performing the analysis. He was also the USEPA
project officer responsible for this study. The detailed analysis of the
sludge management system was performed by Environmental Technology
Consultants, Inc., under the direction of Mr. Nick Mignone. Dr. John
Andrews, University of Houston, assisted in this study as a special con-
sultant on anaerobic digestion, and he also contributed the discussion on
the development of the anaerobic digestion processes. WAPORA, Inc., per-
formed the tracer study on the anaerobic digesters and provided overall
project management and report preparation under the direction of Mr. Robert
France and Mr. Robert Stevens.
Special acknowledgement is due to the District of Columbia management
and the Wastewater Treatment Facility staff. In particular to Mr. Steve
Bennett and Mr. Ed Jones who provided their time and in-depth understanding
of the treatment facility to make the analysis of management alternatives
possible, and to Mr. John Zelinski who spent so much time on helping with
the process piping.
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SECTION 1
INTRODUCTION
This study is an engineering evaluation of the application of a new
concept in wastewater engineering to a major wastewater treatment facility.
The new concept involves the combination of two anaerobic digestion pro-
cesses: the mesophilic process (that is, anaerobic digestion operating at
temperatures from 90 to 100°F), and the thermophilic process (that is,
anaerobic digestion operating at temperatures from 120 to 130°F). In
combination, these represent a new process termed the mesophilic-thermo-
philic process. Such a system has been operated at the Rockaway Pollution
Control Plant in New York City with outstanding operational results.
The existing Blue Plains wastewater treatment plant in Washington,
D.C. treats approximately 330 mgd and uses mesophilic, anaerobic digestion
for sludge treatment. This system has had operational constraints and is
limited in capacity. As a result, a major portion of the total sludge
stream generated at Blue Plains is disposed of without digestion. Expan-
sions of sludge processing unit operations, now is progress, will increase
the system capacity, but not sufficiently to handle the entire sludge
stream as the system now operates. Finally, there are related planning and
engineering evaluations in progress that consider the long-term sludge
disposal options available at Blue Plains, including the complete abandon-
ment of anaerobic digestion in favor of alternate sludge treatment pro-
cesses.
Therefore, the purpose of these evaluations was to identify the rela-
tive merit of the mesophilic-thermophilic process compared with other
anaerobic digestion processes. In particular, the evaluations emphasize
the capabilities of anaerobic digestion to meet sludge processing needs at
Blue Plains, the modifications required to handle the full sludge stream
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(operating and equipment), and the costs associated with the systems (mone-
tary and energy). Existing equipment is used to the maximum in these
evaluations, with the objective to identify an anaerobic sludge digestion
process that can be implemented with no major construction. In this sense,
the study represents an operations evaluation of the Blue Plains facility
for the anaerobic digestion system.
The report is organized to follow logically the analyses performed on
the sludge handling system. Section 2 presents a perspective on anaerobic
digestion and relevant data and discussions on the new mesophilic-thermo-
philic process as applied by Mr. Torpey at Rockaway. Section 3 provides a
background description of the Blue Plains wastewater treatment facility.
Section 4 describes the sludge production at Blue Plains, with detailed
information also provided in Appendixes A and B. An operations evaluation
of the sludge system is included as Section 5, with supporting analyses in
Appendixes C through L. The comparison between anaerobic processes is
performed in Section 6, with supporting calculations and assumptions in
Appendixes M, N, and 0. Financial evaluations are presented in Section 7.
The methods employed in this case study should be generally applicable
to most major wastewater treatment facilities. The application of
anaerobic processes, and in particular the new raesophilic-anaerobic pro-
cess, may be profitably analyzed by following these techniques.
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SECTION 2
ANAEROBIC DIGESTION AND THE ROCKAWAY STORY
The technologies currently used for the anaerobic digestion of sludge
have evolved over the last 100 years as scientific and engineering prin-
ciples have been applied. Tracing this development, there has been a
gradual improvement in performance, both with respect to the stabilization
of the waste material and to the associated cost.
2.1. DEVELOPMENT OF THE CONVENTIONAL (MESOPHILIC) PROCESS
Anaerobic digestion is one of the oldest wastewater treatment pro-
cesses. This process, which involves the biological conversion of organic
solids to methane and carbon dioxide, is a natural process occurring in
such diverse environments as swamps, stagnant bodies of water, and stomachs
of cows. One of its first engineered uses was in the latter part of the
19th century when septic tanks were first 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. The
solids are well stabilized, but unfortunately the gas evolved disturbs the
sedimentation process by lifting particles into the overflow. This can
result in plugging of the tile field, thus destroying the efficiency of the
field, and frequently resulting in malodorous conditions.
This deficiency of the septic tank was recognized and a solution
developed by the famous sanitary engineer, Dr. Karl Imhoff, in the early
part of this century. Dr. Imhoff invented a two-story tank, which now
bears his name. The design of this was such that the gas evolved by
anaerobic digestion in the bottom tank was prevented from entering the
upper tank where sedimentation occurred. The functions of digestion and
sedimentation were thus effectively separated.
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A natural evolution of this separation of functions, which took place
in the 1920's, was the construction of separate tanks for sedimentation and
digestion with the solids removed in the sedimentation basin being pumped
to the anaerobic digester. This decreased the required depth of the tanks
and also permitted the easy application of heating and artificial mixing to
the anaerobic digester. Both heating and mixing, which began to be applied
in the 1930's and AO's, accelerate the rate at which the solids are
digested. They also help to reduce stratification problems in the
digester, thus increasing the effective volume available for digestion.
This increase in effective volume made possible the use of much smaller
digesters. Instead of the three to six months requirement for anaerobic
digestion in the Imhoff tank, it was now possible to accelerate digestion
and complete the process in one to two months. The obvious result of this
was a substantial decrease in required capital cost.
The application of digester mixing soon made it obvious that mixing
and separation of the supernatant from the digested sludge were incompa-
tible in much the same sense that sedimentation and digestion in the septic
tank were incompatible. This led to a further separation of functions by
the application of two-stage digestion where the biological reactions (with
mixing and heating for acceleration) occur in the first stage with the
digested sludge then going to a separate stage for separation of the solids
from the liquid. This second stage also served two other valuable func-
tions, these being the storage of the sludge prior to disposal and as a
source of "seed" sludge for restarting the primary digester in the event of
difficulties with the biological reactions. Two pioneers in the applica-
tion of two-stage digestion were A.M. Buswell and A.J. Fisher. With a
two-stage system, good digestion could consistently be obtained in the
first tank in one month or less.
In the 1950's, Wilbur Torpey was faced with the necessity of expanding
the capacity of the digesters used in the New York City plants. He recog-
nized that the digestion time could be substantially decreased if a portion
of the water could be removed from the sludge prior to its being fed to the
digester. He therefore installed sludge thickeners prior to digestion and
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was able to quadruple the loading of solids to his digesters. This per-
mitted plant expansion to be put off for several years, thus saving the
City of New York a very substantial sum of money. 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.
In addition to his full-scale work on thickening, Torpey conducted
pilot plant studies that established three days as the lower limit of time
at which the process could operate. One would, of course, not normally
operate at three days since it allows no margin of safety and digestion is
incomplete. Also, the volume of Torpey's pilot digester was fully
effective, whereas a portion of the volume of full-scale digesters may be
occupied by grit deposits on the bottom or scum at the top. The lower
limit of time, with which operating engineers in large cities feel
comfortable, is about 15 days.
2.2. PRESENT STATE OF PRACTICE OF THERMOPHILIC ANAEROBIC SLUDGE DIGESTION
Thermophilic anaerobic digestion is very similar to mesophilic
anaerobic digestion except for the temperature at which it is operated,
this being 120-130° instead of the usual 85-95°F. It thus takes advantage
of the well known fact that biological reaction rates can be increased by
increasing temperature. It is only natural, therefore, that conversion of
existing mesophilic digesters to thermophilic operation should be con-
sidered as a low cost technique for increasing the sludge processing cap-
ability of wastewater treatment plants. Full scale studies by the
Metropolitan Sanitary District of Greater Chicago (1), Ontario Minstry of
the Environment, Canada (2), and Moscow, U.S.S.R. (3) all indicated that
they could at least double the amount of sludge that could be processed per
unit volume of digester capacity by converting from mesophilic to thermo-
philic operation. The actual amounts of sludge that can be processed may
be substantially greater than this since the upper limits of the process
have yet to be defined.
In addition to its increased sludge processing capability, thermo-
philic operation also offers two other significant advantages over meso-
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philic operation; these being (a) improved sludge dewatering (4,5) and (b)
increased destruction of pathogens (3,6).
An example of how sludge dewatering can be improved through the use of
thermophilic digestion is afforded by the work of Garber (4) on the vacuum
filtration of thermophilic sludge at the Hyperion plant in Los Angeles. He
reported a 270% increase in vacuum filter yields with a 48% decrease in
coagulant dosage for thermophilic as compared to mesophilic sludge.
Improved solids-liquid separation is of substantial value in land applica-
tion of sludge through decreasing the quantity of wet sludge for disposal
and thus lowering the cost of transport to the disposal site.
An example of the increased destruction of pathogens through the use
of thermophilic digestion is given in the report of Popova and Bolotina (3)
on the practice of thermophilic digestion in Moscow, U.S.S.R. They state
"The most essential advantage 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 the sanitary quality of the sludge is of special signi-
ficance in light of the current trend toward land disposal of digested
sewage sludge.
Although mesophilic and thermophilic anaerobic digestion are quite
similar in both design and operation, there are differences which must be
taken into account in adapting mesophilic digesters to thermophilic opera-
tion. Among these are (a) additional sludge heating requirements, (b)
structural competency of existing digesters and piping at the higher
temperatures, (c) potential odors at sludge handling areas, (d) closer
attention to temperature control, (e) possibly higher concentrations of
dissolved materials in the liquid streams from sludge dewatering
operations, (f) possible ammonia inhibition due to increased protein
destruction, and (g) removal of increased amounts of moisture from the
digester gas.
It should also be pointed out that caution should be exercised in
making the transition from mesophilic to thermophilic operation since the
maximum rate at which this can be accomplished is unknown.
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PROCESS HISTORY
There has been an interest in thermophilic anaerobic digestion since
at least 1930. Buhr and Andrews (7) prepared a review paper of the re-
search aspects and full scale application of the process in 1976 and their
paper should be consulted for details of this earlier work. Since 1976,
there has been a substantial increase of interest in the process. Dif-
ferent organizations have either adopted or are considering adopting the
process to obtain one of the three advantages previously mentioned; these
being (a) increased sludge processing rates, (b) improved sludge dewater-
ing, or (c) increased destruction of pathogens. In additon to these three
reasons for adopting the process, Metro Denver (8) has converted all of its
mesophilic digesters to thermophilic operation in order to compensate for
the loss of capacity when the liquid levels were lowered to alleviate
problems from the existing foaming conditions. A brief summary of the
experiences of some of these organizations is given below.
Los Angeles
The most extensive application of thermophilic anaerobic digestion in
the U.S. has been at the Hyperion plant (340 MOD) in Los Angeles. This
work has been summarized in a recent paper by Garber (9) with more details
of the work being covered in a series of papers by Garber and coworkers
(4,5,6,10). The primary advantage of the process for Los Angeles is the
greatly improved dewatering characteristics of the sludge.
Thermophilic anaerobic digestion was first started in Los Angeles in
1952 and was subsequently extended to one half of their digestion system.
At that time, the digested sludge was vacuum filtered, dried, pelletized,
Sacked and sold as a soil amendment. The half and half mixture of meso-
philic and thermophilic sludge was used since thermophilic sludge has a
lower nitrogen content and could not therefore, alone meet the 2.5%
nitrogen content guarantee for the soil amendment.
In 1957, Los Angeles commenced discharge of liquid digested sludge to
the ocean so dewatering of the sludge was no longer necessary. However,
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they have continued to operate one or two full sized digesters at thermo-
philic temperatures so as to gain experience with operational stability and
to work on the centrifugability and the pathogen content of the thermo-
philic sludge. In 1985, Los Angeles will be required to cease discharge of
digested sludge to the ocean and turn to land disposal. At that time they
plan to operate the entire digestion system at thermophilic temperatures
because they will once again need the improved dewatering characteristics.
The work reported on by Garber and coworkers in 1975 (5) and 1977
(6,10) confirmed their earlier work in 1954 in illustrating that solids
dewatering (both vacuum filtration and centrifugation) was definitely
benefited by thermophilic operation. They also found that the process was
no more difficult to operate than the mesophilic process except that closer
temperature control was required. In a cooperative study with the
Municipal Environmental Laboratory of the Environmental Protection Agency
(11), they also found that thermophilic operation resulted in very sub-
stantial improvements in the destruction of pathogenic bacteria and
viruses. For example, thermophilic digestion consistently reduced the
Salmonella densities to below the detectable limits of the analytical
procedure.
The volatile acids concentration in thermophilic digesters is higher
than that in mesophilic digesters and this was also the case in Los
Angeles. The odor of thermophilic sludge is accordingly also somewhat
higher. Although Garber did not feel that this was at a level which would
be considered obnoxious, he did suggest that care should be taken in the
solids dewatering step to contain these odors. The Los Angeles experiences
also indicate that there are somewhat higher concentrations of dissolved
materials in the liquid streams from dewatering operations.
Moscow, U.S.S.R.
In 1964, Popova and Bolotina (3) reported on the use of thermophilic
digestion at the Kur'yanova plant (260 MGD) in Moscow. Plant scale tests
on thermophilic operation were first made at this plant in 1955 and in 1958
the digestion system was converted to thermophilic operation. The primary
-------
reason for this conversion was the sanitary quality of the thermophilic
sludge. Research at the plant had shown that mesophilic digested sludge
retained up to 20% viable helminth eggs whereas after thermophilic diges-
tion no viable eggs could be found. The sanitary quality of the sludge was
of particular concern since it had become the practice to apply the liquid
sludge directly to agricultural land.
Conversion from mesophilic to thermophilic digestion also permitted a
sharp increase in the capacity of the installation with the digestion time
being shortened from 18 to 9 days.
Chicago
In 1977, the Metropolitan Sanitary District (MSB) of Greater Chicago
converted one of their 12 digesters at the West Southwest plant (1,200 MGD)
to thermophilic operation. The reason for this is that their mesophilic
digesters are currently operating at a 14 day detention time and they
wished to explore the possibility of increasing sludge processing capacity
by operation in the thermophilic range. Although their experimental
program is not completed, they did report on the results of 130 weeks of
operation at the 1980 Conference of the Water Pollution Control Federation
in Las Vegas (1).
Their work did demonstrate that thermophilic digesters could be
operated successfully at a 7 day detention time. Of particular concern to
USD is the energy self-sufficiency of the process since thermophilic diges-
tion does require more heat energy input than the mesophilic process.
However, they did observe increased gas production from the thermophilic
process and believe that this, with the use of a heat exhanger to preheat
the feed sludge 'with the discharged thermophilic sludge, will offset the
increased energy requirements. Just as in Los Angeles, they had no parti-
cular difficulty in operating the process and observed that it could stand
a change in temperature of up to 5°F in 24 hours without adverse effects.
USD is particularly concerned with sludge odor since the digested
sludge is initially lagooned at a location with a high visability. Their
-------
tests indicated only a slightly greater odor intensity from the thermo-
philic sludge, however, the odor was different and there were various
opinions as to whether the'\Hrdef was better or worse. They therefore plan
further investigations of the disposal aspects of the thermophilic sludge.
Ontario Ministry of the Environment, Canada
In 1977, Smart and Boyko (2) of the Ontario Ministry of the
Environment, Canada, reported on the results of their full scale studies on
the thermophilic anaerobic digestion process. They had originally planned
to establish the upper limits at which sludge could be processed by thermo-
philic digestion but due to difficulties with their heating system were
unable to do so. However, they did obtain good overall performance at a
detention time of 7.5 days and stated that the thermophilic digestion
process could be applied in existing plant operations where solids stabi-
lization performance is compromised by organic or hydraulic overloading
conditons.
As a practical matter, they found that standard gas drying equipment
was inadequate for drying the gas produced in the thermophilic digester
since it is at a higher temperature and thus contains more water vapor.
Several modifications of this equipment were recommended for accomplishing
additional moisture removal.
2.3. DEVELOPMENT OF THE MESOPHILIC-THERMOPHILIC PROCESS
Torpey, in his most recent (1979-80) work at the Rockaway plant in New
York City, has found a way to overcome the potential disadvantages of
thermophilic digestion as well as to improve upon the process. He has
accomplished this by again going to two-stage digestion but this time with
the two stages consisting of a mesophilic stage followed by a thermophilic
stage. He also recycles a portion of the thermophilically digested sludge
back through the aeration tanks to obtain additional destruction of organic
solids. The advantages of the thermophilic process are thus retained with
the potential disadvantages being decreased. In addition, a substantial
improvement in organic solids destruction is obtained thus improving the
10
-------
quality of the ultimate product, the digested sludge. The extra gas pro-
duced from the destruction of these solids also helps to offset the higher
heating requirements. However, a disadvantage of the process is that it
does require greater digester capacity (as compared to thermophilic diges-
tion) .
2.4 PURPOSE AND METHODS OF THE ROCKAWAY TESTS
The City of New York, which is under Federal mandate to cease ocean
dumping of sludge derived from the treatment of wastewater, has been
involved in studies of land-based disposal techniques for the past few
years. These studies were initially directed towards determination of the
optimal methods for dewatering digested sludge and identification of the
subsequent steps for ultimate land disposal.
Later the idea was advanced that, as a fundamental and top priority
item in the management program, current plant facilities should be studied
for use in the reduction of the rate of sludge production, both as to
volume and as to volatile matter, from an activated sludge plant. The
method would be based on the exploitation of biochemical mechanisms, namely
by exposing the mesophilically digested solids to the enzyme systems of
thermophilic digestion and activated sludge. A new method of sludge
processing has been formulated to implement this idea. The advantage
gained in this investigation would be reflected commensurately in the
economics of all sludge dewatering and post-dewatering processes which were
previously studied.
The Rockaway Pollution Control Plant (P.C.P.), which is connected to a
population of 100,000 and has a flow of 25 to 30 mgd, was chosen for a full
scale test. The plant uses conventional facilities for the activated
sludge process and the sludge generated undergoes mixed primary and
seconary sludge thickening prior to mesophilic digestion. The digested
sludge is transported to sea. As presently operated, the primary tanks
provide a detention of about 2 hours, the aeration tanks of 3.3 hours with
step feed provisions, and the final tanks of 3 to 5 hours depending on the
number in use. Two 45 ft. diameter thickening tanks are used for mixed
11
-------
sludge thickening and a 1 cu.ft./capita tank is used for the mesophilic
digestion.
As required for this test, the following steps were taken: (1) an
additional 1 cu.ft./capita digestion tank was placed in service to receive
the overflow sludge from the mesophilic digestor and its contents were
heated to 120 - 122°F, the lower limit of the thermophilic digestion range;
(2) piping was installed to carry a portion of the overflow from the
thermophilic digestor directly to a single 45 ft. diameter tank, which was
to be utilized as a rethickening and elutriating tank; (3) piping was
installed to carry the remainder of the flow from the thermophilic digester
into the primary effluent, and thereby, directly into the aerator of the
secondary treatment system; (4) city water was carried to the elutriation
tank. In summary, the elements of this new method of sludge processing
involved (1) subjecting the mesophilically digested sludge to subsequent
thermophilic digestion, (2) circulating a portion of the sludge leaving the
thermophilic digester directly to, and through, the secondary treatment
system, and (3) subjecting the remainder of the sludge leaving the ther-
mophilic digester to a rethickening and elutriation step.
The thermophilic digester was placed in operation in September 1979.
However, it was not until January 15, 1980 that the necessary piping addi-
tions were completed and the recirculaation and rethickening steps of the
new method brought into service. Therefore, although the thermophilic
digester was in operation, test results on treatment plant effectiveness
before January 1980 do not include any effects due to recirculation.
2.5. RESULTS OF THE ROCKAWAY TESTS
2.5.1 Effect of Recirculation on Process Performance (Solids and BOD)
When the full scale test was started, the activated sludge was at a
low sludge density index of 0.6 to 0.7. Microscopic examination revealed a
significant population of bacterial filaments present along with colonies
of stalk ciliates and rotifers. After only a few days of operation, the
digested sludge recirculation caused the sludge density index to rise to
12
-------
1.0 and the bacterial filaments to diminish substantially. Throughout the
entirety of the test following, the sludge density index remained in the
stable range of 1.0 to 1.4.
Since the flow received at the plant is approximately 200 gals/capita/
day, the influent wastewater strength is low, averaging about 100 mg/1 each
of suspended solids and BOD,.. Prior to the test, the suspended solids and
BOD,, in the effluent averaged 14 and 12 mg/1 respectively. The monthly
treatment results for the period from July to December 1979 are presented
in Table 2-1. For comparative purposes, the treatment effectiveness during
the course of the test from January 15 through May 29, 1980, is also
included. These data indicate that no significant effect on treatment
efficiency was experienced as a result of the continuous recirculation of
digested sludge through the aeration system for at least the first three
months. In the latter 2 months, suspended solids in the effluent did
increase by about 3 mg/1 and there was a substantial increase in the flow
rate from about 20 mgd to 27-29 mgd.
2.5.2. Nutrient Removal
To further evaluate whether the recirculation of digested sludge
through the secondary system had an adverse effect on effluent quality, the
data concerning nitrogen and phosphorous are shown in Tables 2-2 and 2-3.
The effluent values are of special interest since the raw wastewater
samples did not contain the recirculating flow. Inspection of the data for
the two periods (1) pre-test and (2) test, shows that the total average
effluent inorganic nitrogen was 9.2 mg/1 and 10.4 mg/1 respectively.
Although data from month to month varies, period averages show that in-
organic nitrogen in the effluent increased 1.2 mg/1 during the test period
over the pre-test period. Conversely, the orgarfic nitrogen decreased 4.3
mg/1. Phosphorus concentrations in the effluent remained essentially
unaffected over the two periods. It appears that the digested sludge
recirculation had a minor effect on the effluent quality with respect to
the nutrients nitrogen and phosphorus.
13
-------
TABLE 2-1. ROCKAWAY P.C.P. TREATMENT EFFICIENCY, JULY 1979 TO MAY 1980
Before Modification
Plant Influent
Month
July 79
Ang
Sept
Oct
Nov
Dec
AVG
Flow
MGD
22
22
23
25
22
21
Suspended Solids BOD,.
mg/1
88
83
93
116
140
125
108
Ibs/day
16,146
15,289
17,839
24,186
25,687
21,892
20,173
mg/1
111
117
90
103
112
124
110
Ibs/day
20,366
21,467
17,264
21,475
20,550
21,717
20,473
Plant Effluent
Suspended Solids
mg/1
12
16
12
18
13
10
14
Ibs/day
2202
2936
2302
3753
2385
1751
2555
BOD
mg/1
13
12
10
12
11
11
12
Ibs/day
2385
2202
1918
2502
2018
1927
2159
After Modification
Jan 80
Fel)
Mar
Apr
Hay
AVG
21
19
23
27
29
86
86
106
94
94
94
15,062
13,628
20,333
21,167
22,734
18,585
91
85
57
49
43
65
15,938
13,469
10,934
11,034
10,400
12,355
8
9
13
15
15
12
1401
1426
2494
3378
3628
2465
8
8
8
6
6
7
1401
1268
1534
1351
1451
1401
-------
TABLE 2-2. ROCKAWAY P.C P. INFLUENT-EFFLUENT NITROGEN CONCENTRATIONS, .JULY 1979 TO HAY 1980
\J\
Before Modification
Ammonia Nitrogen
Flow Influent
Month
July
A..g
Sept
(in
Nov
Dec
AVG
M«D ing/1
22 13.0
22 7.8
23 a
24 78
22 12 5
21 10 4
10.3
Ibs/day
2385
1431
1626
2294
1821
1911
Effluent
"JS/1
1.6
1.8
—
0.6
9.4
9.2
4.5
Ibs/day
294
330
125
1724
1611
817
Organic
Influent
rog/1
5.8
8.4
10.5
8.4
15.1
16.0
10.7
Ibs/day
1064
1541
2014
1751
2771
2802
1991
Nitrogen
Effluent
n.g/1
2.9
3.0
9.6
3.0
8 6
14 8
7.0
His/day
532
550
1841
625
1578
2592
IOSO
Nitrate
Influent
mg/i
0
2.3
0
0.1
0.3
Ibs/day
0
422
0
18
52
Nitrogen
Effluent
•atl l!L-£*z
7.0 1284
S.9 1082
5.2 1084
0.4 73
0.2 35..
3 7 — -b
Total Inorganic Nitrogen
Influent
"6/1
13 0
10 1
7.8
12.8
10.7
10.9
Ibs/day
2385
1851
1626
2348
1874
2017
Effluent
nig/ 1 1 bs/day
10.6 1945
9.7 1180
60 1251
10.3 1890
9.4 1646
9 2 IS82
After Modification
1980
Jan
Feb
Mar
Apr
May
AVG
Analytical
21 94
19 11.6
23 9.6
27 7.0
29 9.4
9 4
results not
1646
1838
1841
1576
2273
1835
available.
0.6C
2.6
3.0
1.0
2.4
2.3
105C
412
575
225
580
448
9.8
10.6
15.6
8.6
9.2
10.8
1716
1680
2992
1937
2225
2110
3.0C
2.2
4.2
3 2
1 2
2,7
525C
349
806
721
290
542
0.2
0.3
0.8
0.3
0.3
0.4
35
48
153
68
73
75
6.3C 1IOJC
8.2 1299
10.0 1918
5.6 1261
5.4 1306
7.3 1446
9.6
11.9
10.4
7.3
9 7
9.8
1681
1886
1995
1645
2346
1911
6 9C I208r
10.8 1711
13 2 2532
1.8 2207
7 8 1887
10 4 2084
Averages not calculated
Transition
month-results not included in
averages.
-------
TABLE 2-3. ROCKAWAY P.C.P. INFLUENT-EFFLUENT PHOSPHORUS CONCENTRATIONS, JULY 1979 TO MAY 1980
Before Modification
Total Phosphorus
Month
July
Ang
Sept
Oct
Nov
Dec
AVG
After
Flow
MGD
79 22
22
23
25
22
21
Modification
Jan 80 21
Feb
Mar
Apr
May
AVG
19
23
27
29
Influent
mg/1
2.3
2.5
2.5
2.0
2.7
1.8
2.3
2.7
2.8
3.9
2.9
2.5
3.0
Ibs/day
422
459
480
417
495
315
431
473
444
748
653
604
584
Effluent
mg/1
1.8
2.1
2.1
1.2
1.6
1.6
1.7
1.8a
1.7
2.0
2.4
2.0
2.0
Ibs/day
330
385
403
250
293
280
323
3153
269
384
540
484
419
Ortho Phosphorus
Influent
mg/1
1.7
1.8
1.1
1.4
1.9
3.5
1.9
1.6
1.8
1.9
1.2
1.3
1.6
Ibs/day
312
330
211
292
349
613
351
280
285
364
270
314
303
Effluent
mg/1
1.8
1.4
1.7
1.2
1.2
3.0
1.7
,5a
.4
0.7
.1
.6
.2
Ibs/day
330
257
326
250
220
525
318
2623
222
134
248
387
248
Transition month results not included in averages.
-------
2.5.3. Heavy Metals Removal
The results of the monthly metal analyses of composite influent and
effluent samples for the pre-test period July to December 1979 and for the
test period January to May 1980 are presented in Table 2-4. Comparison of
the overall averages of these test periods for cadmium and mercury, the two
metals that have been shown to exert a major effect on human physiology,
indicate that there is not a significant difference between the removals.
In fact, the activated sludge process does not seem capable of appreciably
reducing the already low concentration of either of these metals. Also,
mass balance studies of the metal data, excepting cadimum and mercury, have
been generally good. Cadmium and mercury, however, due to their low con-
centrations and to the sensitivity of the testing procedure, had poor mass
balances. Comparative inspection of the data for the remaining heavy
metals, indicates some variable effects of treatment during individual
months, but with no significant changes in the overall averages for the
subject periods.
2.5.4. Effect of Recirculation on Oxygen Requirements
The effect of digested sludge recirculation on air compressor output
over the course of the test could not be determined. The air compressor
was operating at a level to produce more than adequate dissolved oxygen and
its rate could not be lowered before or during the test to attempt to
evaluate any demand changes. A calculated estimate was made based on the
fact that meso-digestion, without the benefit of subsequent thermo-
digestion, destroys 90% of the BOD. present. The estimate indicates that
the BOD5 in the portion of the digested sludge which is continuously recir-
culated, would add less than 5% to the oxygen demand of the primary
effluent.
17
-------
TABLE 2-4. ROCKAVIAY P C.P. 1NFI.UKNT-EFFLUENT METALS CONCENTRATIONS, JULY 1979 TO HAY 1980
00
Copper
Before Modification
luly 1979
Aug
Sept
On
Nov
Dec
AVC
After Modification
J.iii 1980
Feh
Mar
Apr
May
AVG
Before Modification
July 1979
A.ig
Sept
Del
Nnv
Dec
AVG
AfLrr Modification
Jnii 1980
Feh
Mar
Api
May
AVG
Inf
0.11
0 II
0.11
0.13
0. 16
0. 11
0.12
0.09S
0.080
0. 110
0.075
F.ff
0 07
0.05
0.03
0.06
0.05
0.05
0.22
0.0035
0.0400
0.0380
0.088 0.027
Iron
Inf
0.8
0.9
5
.5
.6
.1
.2
0.57
0.65
0.55
0 84
0.8J
0.72
Eff
0.2
1 0
0.7
1.2
2.0
0.1
0.9
1.00
0.16
0.21
0.13
0.46
0 24
Cliromc
Inf
0
0
0
0
0
0
0
0
0
0
0
0
0
.001
.020
012
.009
.011
.OJ4
.014
.007
.0012
.0038
.010
.0038
Eff
0 006
0.015
0.008
0.002
0.007
0 008
0.0012
0.001
0.003
0.005
0.001
.0047 0.002
Cadmi lira
I lit
0.0001
0 0001
0.004
0.0001
0.0010
0.0008
0
0
0
0
0
0
0
.0004
0018
.0005
.0011
.0046
.0011
.0018
Eff
0.0001
0.0001
0.0002
0.0001
0 0015
0.0006
0 0004
0.0005
0.0004
0 0009
0 0029
0.0017
0 0015
Nickel
Inf
0.07
0.02
0.02
0.01
0.02
0.01
0.03
0.015
0.009
0 0042
0.0068
0.0086
F.ff
0.04
0.02
0.01
0.01
0.02
0.02
0.02
0.018
0.021
0.0024
0.014
0.011
0.0072 0.012
Calcium
Inf
41
40
41
29
19
13
10
14
16
14
13
23
16
Eff
36
48
32
41
20
15
32
14
17
15
14
21
17
Zinc
InC
0.11
0.26
0.23
0.09
0.12
0.08
0.15
0.066
0.093
0.090
0.21
0.085
Eff
0 14
0.35
0.16
0.17
0.15
0.08
0.17
0.070
0.086
0.065
0.079
0. 10
0.12 0.082
Magnesium
lnj[
94
107
102
94
85
76
93
70
60
58
54
59
58
Eff
98
112
105
101
88
77
97
72
64
59
55
62
60
Lead
Inf
0.017
0.024
0.110
0.014
0.027
0.023
0.036
0.014
0.049
0.0089
0.0088
0.010
0.018
Mercury
Inf
0.0010
0.0007
0.0007
0.0005
0.0026
0.0005
0.0010
0.0009
O-OOOS
0.0009
0.0003
0.0003
0.0005
Eff
0 006
0.006
0 006
0 008
0 020
0 007
0.009
0.030
0.0024
0.0016
0.0034
0.0064
0.0035
Eff
0.0006
0.0006
0 0002
0 0005
0 0028
0 0009
0 0009
0.0011
O.OOOJ
0.0002
0 0005
0.0004
0 0004
-------
2.5.5. Operating Results
As was previously noted, a 1 cu. ft./capita tank was placed in service
as a thermo-digester at about 121°F. Its contents overflowed by gravity to
either the primary effluent, or to a 45 ft. diameter rethickening and
elutriating tank where city water was added to the influent sludge at a
ratio of about 3 to 1. The rethickened underflow sludge was pumped by a
duplex plunger pump, actuated by time clock, to empty digesters where its
volumetric rate was measured by filling the tanks during the months of
March and April. Such procedures did not involve the use of any man power,
except for periodical blowing back of clogged lines.
The amount and concentration data are presented in Table 2-5. The
volatile matter leaving the meso-digester averaged 9,000 Ibs/day, thus
effecting a reduction of 7,200 Ibs/day (16,200 - 9,000). The thermo-
digester accounted for a further reduction of 1,800 Ibs/day (9,000 -
7,200). Such reductions were effected on the combination of raw primary
solids, activated sludge solids and the recirculating solids (i.e., solids
that had been previously subjected to meso and thermo-digestion).
The conventional activated sludge treatment units and the modifica-
tions made for incorporating the new method of reducing sludge production
are represented in Figure 2-1. Since the economics of sludge disposal is
fundamentally a function of the amount of volatile material to be disposed
of, only the rates of production of volatile matter are shown. Overall, it
can be seen that the reduction of volatile matter by the meso-digester of
7,200 Ibs/day, added to the 1,800 Ibs/day reduction by the thermo-digester,
results in a total of 9,000 Ibs/day. Additionally, the aerator destroyed
2,000 Ibs. V.M./day for a total reduction of 11,000 Ibs.V.M./day. Since
the treatment system was removing a total of 12,900 Ibs.V.M./day, the net
amount requiring disposal was reduced to 1,900 Ibs.V.M./day. Previous data
show the average amount of volatile matter carried to sea from April to
July 1979 (the period just prior to this work) was 5,700 pounds. There-
fore, there was a reduction of two-thirds in the volatile solids requiring
disposal (5700 - 1900 = 2/3). Volume reduction was in the same proportion;
5700
that is, 4,800 cu ft/day to 1,650 cu ft/day, or approximately 2/3.
19
-------
N>
O
*1
S PLANI HiriUlHI _
g flow 20-29 HCO
w BOIk OS-IIOng/l
,0 SS 40 90 «9/l
i lllli'N 7.5-11.5 nq/l
^ lotaf P 2.5-1.0mg/l
CO
1
*
tu
rr
H-
P
oo
U>
P
r»
w
PRIHARY
CIARIFICAIIOII
•
ACIIVAIEO
SLUDGE
REIURH SLUDGE
WASH ACIIVAIED
SLUDGE
FINAL
ClARiriCAIIOII
. 1
4.000 IVH/day
S.IOO cu ft/day
12.900 IVH Iron Incoumlng waslewater
J.300 IVH contributed from thickening over How
17.000 IVH to digestion
S^~^\
/ X
/ ' \ 16.200 ,
/ GnAVIIV ^ IVH/day j
1 liilCKIHinG 1 7.701) *" 1
\ / cu rt/d«y ^
\ /
x^ ^\ /- \
/ \ / \
I/day
1'IAHI IF fl III III
DILUIION UAIER
i
/
2.400 /
TVH/da/
95 F I 7.000*"! 121 r 1 7.700 ' 1
L / cu It/day V /cult/day \
V >/ V X
\
OODc 6-8 my/1
SS D-IS OHJ/I
till, -II 0.6-4.0 mg/l
lolol r 1.7-2.0 nifl/l
^ ~\
X
IIIICKUIIIIG \ 1 .900
AIID \ IVH/Jay
ELIIIRIAIIUII 1 *"'
1 10 FINAI DISPOSAL
/
IIIICKEIIER ovtnriow
BOO IVH/Jay
IIUIRIAIIOri
ovtRnow
500 IVH/day
VOIAIIIE IVUIER 10 DISPOSAL
Oefore therwi|ihlllc
systca Incoipgraled
s./oo r. 4.000 en rt
Alter tlienuo|ililllc
system Incorporated
1.900 I. I.650 cu ft
fVH/ilay - PflllllOS VOIAIIIE HAURIAL FIR DAT
-------
TABLE 2-5. ROCKAWAY P.C.P. AMOUNT AND CONCENTRATION OF SOLIDS PASSING THROUGH SYSTEM
M
t-1
%Conc. V.
Mo.
1980
Jan
Feb
Mar
Apr
May
Avg.
Feb
to
May
Flow
MOD
21
19
23
27
29
25
//VSS
capt.
(75% VM)
10,400
9,500
14,800
13,400
14,200
12,900
Raw
thick.
pump.
cu ft/day
5,900
7,300
8,400
6,500
8,500
7,700
Raw
thick
3.9
3.6
3.3
3.5
3.0
3.4
Meso
dig.
1.6
1.8
1.7
1.9
2.0
1.9
M.
Thermo
dig.
1.1
1.3
1.5
1.5
1.6
1.5
//V.M.
from
thick.
14,500
16,500
17,500
14,500
16,200
16,200
Meso
dig.
6,000
8,200
8,900
8,000
10,800
9,000
//V.M. /day
The rmo
dig
4,100
5,900
7,800
6,400
8,600
7,200
leaving
Rethickencr
lilutriator
Under Over
flow Now
_ _
-
1 ,800 500
2,000 400
600
1,900 500
Note: (1) Calc. of V.M. inventory in digesters after February show the change does not significantly influence
the above data.
(2) Measured volume March 1,600 cu ft/day x 63 x 1.8% = 1,800 ffV.M./rtay
April 1,700 cu ft/day x 63 x 1.9% = 2,000 0V.M./day
-------
The daily amount of gas generated during the course of the thermo-
digestion is shown in Table 2-6. Based on the averages for the period
February to May, the meso-digester produced 83,900 cu ft/day gas, slightly
less than the comparable preceding period without digested sludge recircu-
lation through the aerator. The gas generated by the thermo-digester
increased from an average of 7,000 cu ft/day to 14,000 cu ft/day during
recirculation. Gas production per pound of volatile solids reduced is
presented in Table 2-7, with gas production for several pure compounds
presented in Table 2-8 for comparison.
The gas mixers in both digesters were causing formation of large
solids masses in the digesters with consequent clogging of the overflows.
It was necessary to operate the mixers only a few minutes per day to
alleviate this condition.
Garber (1) in Los Angeles had determined that the thermo-digested
sludge required half the dose of iron coagulant and produced almost four
times the yield on a vacumm filter as meso-digested sludge. Accordingly,
to obtain some estimate of the improvement on coagulability achieved by the
use of the thermo-digestion in this instance, the me so and thermo-digested
sludges were subjected to polymer treatment. It was found on a laboratory
scale, that using a high molecular weight, low charge polymer //2535CH (as
manufactured by American Cynamid) the coagulability improved radically.
Specifically, dosages of up to 4,000 ppm on meso-digested sludge did not
produce an end point, although some flocculation was observed. In con-
trast, the thermo-digested sludge released 73% of the water in 30 minutes
at 1C, at a dose of 2,500 ppm. Thermo-digested sludge, after a 3 to 1
elutriation, required a lesser comparative dose of 1,650 ppm of the same
polymer to release 64% of the water within 30 minutes at 1C.
22
-------
TABLE 2-6. ROCKAWAY P.C.P. DAILY GAS PRODUCTION
Month
September 1979
October
November
December
Mesophilic
digester
cu ft/day
79,200
93,800
90,300
77,200
Thermophilic
digester
cu ft/day
5,300
8,200
6,900
7,500
Average, No
recirculation
January 1980
February
March
April
May
Average, with
recirculation
Feb to May 1980
87,600
79,200
88,000
86,800
76,400
84,300
7,000
8,500
12,700
13,600
11,900
17,700
83,900
14,000
23
-------
TABLE 2-7. GAS PRODUCTION CUBIC FEET/POUND VOLATILE SOLIDS REDUCED
Month
January
February
March
April
May
Mesophilic
digester
9.3
10.6
10.1
11.7
15.6
Thermophilic
digester
4.5
5.5
12.4
7.4
8.4
TABLE 2-8. GAS PRODUCTION AND QUALITY FROM ANAEROBIC DIGESTION OF SEVERAL
PURE COMPOUNDS
Gas production cubic
feet/pound volatile Percent
Material solids reduced content
Crude fibers 13 45-50
Fats 18-23 62-72
Grease 17 68
Protein 12 73
Scum 14-16 70-75
24
-------
SUMMARY
A full scale test was conducted at the Rockaway Plant in New York
City, which serves a population of 100,000, for a period of five months to
evaluate a new method of reducing the amount and volume of sludge produced
from the activated sludge process. This method involved the use of (1)
high stability thermophilic digestion following mesophilic digestion, and
(2) the recirculation of a portion of such thermo-digested sludge directly
to and through the secondary system of the activated sludge process while
the remainder was conducted to a rethickening and elutriation tank.
Operating results have demonstrated that the volatile matter normally
transported to sea after raeso-digestion was reduced by 2/3. Moreover, the
volume of sludge produced was lowered by 2/3 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 dewatering pro-
cesses. The treatment process was performed without significant adverse
effect on any accepted parameter due to the continuing recirculation of
digested sludge through the activated sludge process.
Reference: (1) Buhr, H. 0. and Andrews, J. - The Thermophilic Anaerobic
Digestion Process - Water Research. Vol. II, pp!39-l46,
1977, Permanon Press, Great Britain.
25
-------
SECTION 3
BLUE PLAINS WASTEWATER TREATMENT FACILITY
The District of Columbia WIT is located in the southernmost portion
of Washington, DC, on the east bank of the Potomac River. The treatment
facility receives flow from the District of Columbia, Virginia, and
Maryland. The plant has a tributary area of 725 square miles, with a
design flow rate of 309 mgd and a peak flow rate of 650 mgd. It occupies
an area of more than 200 acres, and is adjoined by the US Naval Research
Laboratory to the north, the Anacostia Freeway to the east, and the Potomac
River to the south and west.
The existing (winter 1980) liquid management treatment scheme consists
of raw wastewater pumping, aerated grit removal, primary clarification,
high rate activated sludge, intermediate clarification with chemical addi-
tion for phosphorus removal, nitrification, final clarification, multi-
media filtration, and chlorine disinfection. Discharge is to the Potomac
River.
The existing (winter 1980) sludge management treatment scheme consists
of the following. All primary sludge is thickened in gravity thickeners;
all waste activated, waste nitrified, and filter backwash sludge is
thickened in dissolved air flotation thickeners. Approximately half of the
total sludge volume is pumped to the existing anaerobic digesters,
elutriated, vacuum filtered, and disposed of on agricultural land. The
other half of the total sludge volume is vacuum filtered with excess lime
and is either landfilled in trenches or composted.
The size and capacities of the existing sludge handling equipment are
discussed in Section 5.
26
-------
SECTION 4
EVALUATION OF CURRENT AND FUTURE PRIMARY AND SECONDARY
SLUDGE GENERATION
The first step in this analysis of ways to improve the existing anaer-
obic digestion operation at Blue Plains involved the evaluation of current
and future sludge production. Current primary and secondary sludge produc-
tion was determined using operation data supplied by Blue Plains personnel
for the months of January through June 1980 (Appendix A, Table A-l).
Future primary and secondary v sludge production was estimated by using
existing data plus assumptions as to the effects on sludge production of
the completion of current on-going plant modifications. All data analyses
relative to sludge production are given in Appendix B.
4.1. PRIMARY SLUDGE PRODUCTION
Primary sludge consists of those solids that settle and are removed in
the primary clarifiers. Currently, these solids are generated from the raw
plant influent after coarse screening and grit removal and the overflow
from the gravity thickening and elutriation tanks. Analysis of the data
indicates that the average dry solids withdrawn from the primary clarifiers
and pumped to the gravity thickeners is 0.348 ± 0.057 dry tons of solids
per million gallons of influent flow (T/MGIF).l The volatile solids con-
tent averages 79.7 ± 2.9 percent or 0.277 ± 0.047 dry tons of volatile
solids per million gallons of influent flow (VT/MGIF).
Primary sludge production and volatile content are not expected to
change by any significant amount within the near future.
1 It is estimated that 0.07 to 0.1 T/MGIF is grit, which is not being
removed in the existing aerated grit chambers.
27
-------
4.2. SECONDARY SLUDGE PRODUCTION
Secondary sludge currently consists of those solids that settle out in
the intermediate clarifiers and are removed from the system as waste
sludge. Currently, these solids come from the biological solids generated
in the high rate, activated sludge system; the suspended solids that did
not settle out in the primary clarifiers; and the solids produced from the
addition of iron for phosphorus removal. Analysis of the data indicates
that the average dry solids wasted from the intermediate clarifiers and
pumped to the dissolved air flotation thickeners is 0.284 ± 0.034 T/MGIF.2
The volatile solids average 65.3 ± 2.1 percent or 0.185 ± 0.022 VT/MGIF.
Secondary sludge production will increase in the near future for three
reasons: increased phosphorus removal, operation of nitrification systems,
and operation of multi-media filters.
4.2.1. Increased Phosphorus Removal
Effluent limitations on phosphorus will need to be reduced from the
current 1.1 mg/1 average to the NPDES permit requirement of 0.53 mg/1.
This will increase chemical sludge production from iron addition by
approximately 0.009 T/MGIF, assuming that the volatile fraction is
negligible.
4.2.2. Nitrification
At a minimum, this system will have to reduce current total Kjeldahl
nitrogen (TKN) levels in the plant effluent from an average of 13.8 mg/1
to 5.3 mg/1. Assuming a biological yield coefficient of 0.1 pounds of
solids produced per pound of TKN reduced, nitrification would generate an
additional 0.004 T/MGIF, of which 70 percent is assumed to be volatile.
2 Approximately 0.045 ± 0.013 T/MGIF is attributed to chemical sludge
generated by iron addition for phosphorus removal.
28
-------
4.2.3. Multi-media Filters
By the end of 1981, the new multi-media polishing filters will be in
operation. It is estimated that these filters will reduce the suspended
solids being discharged from the current 15 mg/1 average to 7.5 mg/1. This
will contribute approximately 0.031 T/MGIF of which 70 percent is assumed
to be volatile.
These three new sources will increase secondary sludge production to
approximately 0.328 + 0.034 tons of dry solids per MG of raw influent flow.
The volatile solids content of the secondary sludge would be reduced to
0.210 ± 0.023 VT/MGIF or 63.8 ± 2.1 percent.
4.3. SUMMARY
Table 4-1 summarizes the information presented in Sections 4.1 and
4.2. Future sludge production should be realized by the fall of 1981.
29
-------
TABLE 4-1. SUMMARY OF CURRENT AND FUTURE PRIMARY AND SECONDARY SLUDGE GENERATION AT BLUE PLAINS
Current sludge generation
Dry tons (pounds) per
mill ion gallons of
Sludge type influent flow
Primary 0.348 i 0.057 (696 ± 114)
Secondary
o High rate system 0.239 i 0.034 ( 478 ± 68)
o Chemical 0.045 ± 0.013 ( 90 i 26)
TOTAL 0.632 1 0.068 (1264 ± 136)
Future sludge generation
Volatile dry tons (pounds) Dry tons (pounds) per
per million gallons of million gallons of
Influent flow influent flow
0.277 ± 0.047 (554 t 94) 0.348 i O.O57
0.185 i 0.022 (370 ± 44) 0.239 + 0.034
0.462 ± 0.051 (924 ± 102) 0.676 ± 0.068
(696 ± 114)
( 478 t 68)
( 108 ± 26)
( 62 )
( 8 )
(1352 + 136)
Volatile dty tons (pounds)
per million gallons of
influent flow
0.277 ± 0.047 (544 ±
0.185 ± 0.022 (370 ±
0.022 ( 44
0.003 ( 6
0.487 i 0.051 (974 1
94)
44)
)
)
102)
OJ
o
-------
SECTION 5
EVALUATION OF EXISTING SLUDGE MANAGEMENT OPERATIONS
ON SLUDGE PROCESSING AND SLUDGE GENERATION
The second step in the analysis of ways to improve the existing anaer-
obic digestion operation at the District of Columbia WWTF was to evaluate
existing sludge management operations. This section evaluates operating
data supplied by Blue Plains personnel for the months of January through
June 1980 (Appendix A). Data analysis, where required, is provided in the
appendix specified in the following discussion.
5.1. GRAVITY THICKENER OPERATION
There are six, 65-foot diameter gravity thickeners. Either primary or
secondary sludge can be pumped to these units, but since the installation
of the dissolved air flotation thickeners, only primary sludge has been
thickened in these tanks. Thickened sludge and/or scum collected from the
tank surface can be pumped either to the anaerobic digestion system or
directly to raw sludge dewatering. Overflow from the thickeners is re-
turned to primary treatment. Operational data for the months of January
through June 1980 is provided in Appendix A, Table A-2. Data analysis is
provided in Appendix C.
Current practice is to pump primary sludge 24 hours per day from the
primary clarifiers to the gravity thickeners at a solids concentration of
less than one percent. In addition, part of the elutriation tank overflow
is continually returned to the gravity thickeners. Dilution water from the
intermediate clarifier overflow is added. Experience has indicated that
the rate of dilution water utilization should be 800 gallons per day per
31
-------
square foot of thickener tank surface. Data analysis indicates that under
this mode of operation:
1. The average thickened solids concentration that can be expected is
7.0 ± 0.5 percent.
2. The average thickened sludge volume per million gallons of plant
influent flow is 1,236 ± 218 gallons.
3. The amount of and volatile content of primary sludge produced per
million gallons of influent flow is altered. After gravity thick-
ening, the average dry solids withdrawn is 0.377 ± 0.060 dry tons
of solids per million gallons of influent flow (T/MGIF).1 The
volatile solids content averages 71.0 ± 5-3 percent or
0.268 ± 0.047 dry tons of volatile solids per million gallons of
influent flow (VT/MGIF).
Future performance of the gravity thickeners could change signifi-
cantly, depending on the following conditions:
1. Change in the amount of grit that is currently getting into the
primary sludge stream.
2. Change in the source and amount of dilution water currently being
used.
3. Change in the quantity of elutriation tank overflow presently
being sent to the gravity thickeners.
4. Change in the type of sludge thickened i.e. thickening a mixture
of waste primary and secondary sludge solids.
5.2. DISSOLVED AIR FLOTATION (DAF) THICKENER OPERATION
There are eighteen, 20-foot wide by 55-foot long (effective length)
DAF thickeners. Either primary or secondary sludge can be pumped to these
units, but currently they are only used for thickening secondary sludge.
Thickened sludge (float), and the heavy solids that settle and are removed
1 It is estimated that 0.07 to 0.1 T/MGIF is grit, that is not being
removed in the existing aerated grit chambers.
32
-------
from the bottom of the tanks, can be pumped either to the anaerobic diges-
tion system or directly to raw sludge dewatering. Subnatant from these
units is returned to the high rate, activated sludge process. Operational
data for the months of January through June 1980 are provided in Appendix
A, Table A-3. Data analysis is provided in Appendix D.
Current practice is to pump waste secondary sludge 24 hours per day
from the secondary system clarifiers at the maximum concentration possible
(7,000 to 9,000 mg/1). Polymer is utilized for the following reasons:
1. Polymer usage is required to float and thicken effectively
secondary waste-activated sludges containing iron salts.
2. Polymer usage maximizes solids recovery, thus minimizing the
recirculation of solids via the subnatant stream that is returned
to the high-rate, activated sludge process.
3. Polymer usage maximizes the solids loading rate that can be
applied per unit of area per unit of tank, thus minimizing the
number of DAF units required to be operated.
4. Polymer usage allows the subnatant stream to be used as a source
of pressurized flow for the DAF thickener. If the subnatant
stream could not be used, plant effluent would have to be utilized
requiring extensive piping and increased operating cost.
Data analysis indicates that under this mode of operation:
1. The average thickened solids concentration that can be expected is
4.1 ± 0.3 percent.
2. The average thickened sludge volume per million gallons of plant
influent flow is 1,644 ± 239 gallons. If secondary solids per
million gallons of influent flow increase by approximately 15
percent (as discussed in Section 4), it is assumed that the thick-
ened sludge volume will also increase proportionately by 15 per-
cent to 1900 ± 239 gallons.
3. Dry polymer usage currently at 7 to 10 pounds per ton of dry
secondary sludge solids is not expected to change in the future.
4. At current average plant influent flows, it is estimated that 7
DAF units are required to be operating. When secondary solids per
million gallons of influent flow increase in the future it is
estimated that 8 DAF units will be required.
33
-------
5.2.1. Removal of Settled Solids
Not all secondary solids are capable of being treated by DAF. Those
solids that are too heavy to float, settle to the bottom of the DAF unit
and are conveyed by a chain and scraper mechanism to a sump located at the
discharge end of each tank. Such solids were planned to be removed from
each sump by use of a telescopic sludge valve; however, there are times
when insufficient static head is available to make the heavy solids flow
upward and out through the top of the telescopic valve. When this occurs,
heavy sludge continues to accumulate in the bottom of the tank until it
affects the performance of the unit, which then has to be taken out of
service.
Although it is beyond the scope of this report to provide detailed
solutions to operating problems, there are two possible methods of dealing
with the problem that should be investigated:
1. Modifying each telescopic sludge valve to an air lift pump. In-
jecting about 3 to 5 cfm of air near the bottom of each valve
would permit the movement of sludge through the telescopic valve.
2. Provide positive sludge removal through the use of pumps discharg-
ing into a common header located in the flotation thickener
basement gallery.
5.2.2. Pressure Regulating Valve
Until recently, pressurized flow for DAF thickeners has been con-
trolled through an air actuator and pneumatic pressure device. This
automatic control system works very well when a high quality air supply
is available and regular operator attention (two to three performance
checks per 8-hour operating shift) is provided. When this actuator mal-
functions, the float quality rapidly deteriorates, polymer usage in-
creases, and consequently more DAF units have to be brought on-line to con-
tinue processing sludge.
3A
-------
This valve has been a continuous source of operating problems.
Recently, the manufacturer of the existing flotation equipment has offered
a new type of spring-operated pressure relief valve in place of the air
actuated system. Figure 5-1 shows a cross-sectional view of the valve as-
sembly.
This spring-operated pressure relief valve relies on an increase in
fluid pressure to push the valve diaphragm against the spring. Since the
spring has a set constant of compression, it permits the diaphragm to open
just far enough to maintain the set pressure and allows flow to pass. This
valve has been tested at one facility for over three years and has per-
formed very well. It is recommended that the WWTF personnel consider use
of this new type valve.
5.2.3. Spare Part Requirements
As a result of the overall financial constraints, spare parts inven-
tory is almost non-existent. In order to keep the required number of DAF
units in operation, several units have been cannibalized" for parts.
Currently, this is not a critical problem because excess capacity for
normal operation is available, but if allowed to continue this could lead
to significant sludge processing problems.
5.3. ANAEROBIC DIGESTION SYSTEM OPERATION
The anaerobic digestion system is made up of several major sub-
systems: digestion tanks, sludge heating, digestion tank sludge mixing,
gas collection and storage, and sludge piping and pumping capabilities. At
the present time only a fraction of the raw sludge is pumped continuously
to the digestion system. Operational data from January 1979 through June
1980 is provided in Appendix A, Table A-4.
35
-------
Ad (usting screw
Designed so spring cannot
be compressed solid.
Leek nut
With set screw to lock
securely in place.
Spring«
Carbon steel or aluminum
pipe. Case and spnng seals
designed to prevent
warping of sonngs or
Interference. Sealed top and
bottom against moisture or
dirt
Adapter bushing
Carbon steel equipped with
grease fitting. "0" Ring seal
in stem.
Spindle (stem)
Stainless steel equipped
with stop collar—acts as a
stop on opening stroke and
takes the load off the
compressor pin on closing
stroke.
Fingerplate
anal compressor
Finger plates, or flngara
cast into the bonnet In
larger valves combine with
the compressor to provide
metal support to the dia-
phragm in all positions.
Sliding stem bonnet
Ductile iron.
Diaphragms are melded
dosed to reduce required
closing forces, give longer
life and provide drop ngnt
closure without stretching
or distortion.
FIGURE 5-1.
Cross-sectional view of spring-operated
pressure relief valve
36
-------
5.3.1. Digestion Tanks
There are 12 earth covered, concrete, digestion tanks. Each tank has
an inside diameter of 84 feet and a theoretical liquid volume of 146,500
cubic feet (1,096,000 gallons). Though no longer usable, internal heating
coils are still located inside each unit. Lithium chloride tracer studies
(Appendix E) indicate that approximately 65 percent of the total actual
volume--!.14 million cubic feet (8.54 million gallons)--is available. The
remaining 35 percent is unusable due to such factors as (1) volume occupied
by scum and grit and (2) inadequate mixing.
Scum—The scum thickness is measured on all tanks on a routine basis.
The findings indicate that the thickness of the layer depends upon the
operation of the digestion tank gas-mixing system (Section 5.3.3). When
the gas-mixing system is not operational, the scum layer builds up rapidly
to a thickness of several feet. When the system is in operation, very
little scum accumulates.
Grit—On a routine basis, digestion system operations personnel probe
the tank bottoms for grit buildup. Grit in the digesters has always been a
problem. Until several years ago, digesters were systematically taken out
of service for cleaning, but this is no longer done. Based on conversa-
tions with long-time plant operators, grit accumulates in each tank until
it reaches a point of equilibrium. Best estimates are that 15 to 20 per-
cent of each tank is occupied by grit. Using these assumptions, calcula-
tions in Appendix F indicate that for each million gallons of influent
flow, 1.4 to 2.0 cubic feet of grit passes through the aerated grit cham-
bers and is removed with the primary sludge.
37
-------
5.3.2. Sludge Heating
There are six, double inlet, double outlet, 6 inch by 8 inch, tube and
tube heat exchangers. The source of hot water is from two steam boilers
and one hot water boiler, each fueled by digester gas with diesel standby.
Originally designed to transfer 3,000,000 BTU per hour per heat ex-
changer (18,000,000 BTU per hour total), current operation is only capable
of 13,000,000 BTU per hour total. Refurbishing of the existing hot water
boiler in 1981 will add another 3,000,000 BTU per hour to the total. In
addition, the current on-going construction contract will tie in the exist-
ing steam boilers with new steam boilers elsewhere on the plant site. These
new steam boilers will have an approximate total capacity of 17,000,000 BTU
per hour, but how much could be used for sludge heating is unknown.
Appendix G gives a detailed analysis of sludge heating capacity. Appendix
G also analyzes system heat requirements for both summer and winter opera-
tion. Table 5-1 summarizes the results.
TABLE 5-1. SUMMARY OF ANAEROBIC SYSTEM HEAT REQUIREMENTS3
For raw sludge addition in BTU's per MGIF
Primary sludge only
Secondary sludge only
Total primary and secondary sludge
Winter
operation
511,808
636,353
1,148,161
Summer
operation
255,904
272.723
528,627
For system radiation heat losses in
BTU's per hour
Roof
Walls
Floor
Sludge piping
Total radiation losses
1,453,112
613,276
546,866
582,400
3,195,654
535,358
285,826
546,866
467,200
1,835,250
Calculations for data are presented in Appendix G.
MGIF - million gallons influent flow.
38
-------
Based on the information presented in Appendix G, there is currently
9,800,000 6TU per hour available for raw sludge addition during the winter
months and 11,100,000 BTU per hour for raw sludge addition during the
summer months. After refurbishing the existing hot water boiler, raw
sludge heating capacity will increase to 12,800,000 BTU per hour and
14,100,000 BTU per hour, respectively.
5.3.3. Digestion Tank Sludge Mixing
Mixing in the anaerobic digestion tanks is primarily done using recir-
culated gas. Three 1,000 gallon per minute recirculation pumps are avail-
able for substitute or supplemental mixing.
All gas-mixing systems are of the type in which digester gas is re-
moved from the tank, compressed, and discharged into the lower part of the
digester through vertical 2-inch pipes (lances) supported from the tank
roof. In some tanks, gas is discharged through only one pipe at a time,
the point of discharge being changed to another discharge pipe either man-
ually or automatically on a timed basis while others discharge through all
points at once.
There are several conditions that prevent the existing gas mixing
systems from adequately mixing each tank.
1. There is a significant amount of grit accumulating in each tank.
Since grit has a specific gravity of 2.65, digestion gas-mixing
systems cannot cope with these accumulations.
2. Since all tanks contain their old internal heating coils, which
completely circle the inside of the tank, the walls act as
barriers to the mixing currents thus allowing material to build up
behind them and reduce useable tank volume.
3. Eight tanks (3, 4, and 7 through 12) were equipped with gas-mixing
systems in 1960 that were intended to serve as scum breakers until
they could be replaced by a permanent gas recirculation system.
39
-------
Since 1978, several of these tanks have had newer systems in-
stalled, but they have been unable to operate continuously because
of the lack of spare parts.
4. As a result of the overall financial constraints, spare parts for
the gas-mixing systems have been difficult to obtain and one to
two gas-mixing systems have been inoperative.
5.3.4. Gas Collection and Storage
Anaerobic digestion produces a gas with an energy value of roughly 600
BTU per cubic foot and the District has the potential of producing two
million cubic feet per day (see Section 5.3.7 for analysis of gas produc-
tion capabilities).
Visual inspection of the existing gas collection and storage system
revealed that there is need for appropriate corrective action at least in
the following instances.
1. Some parts of the gas piping system show extensive corrosion, to
the extent that holes have developed in the piping, with duct tape
wrapping used to prevent leakage.
2. Extensive corrosion on the bottom of the floating gas holder
prevents the unit from being used to its fullest capacity.
Approximately 20 percent of the useable volume has been lost.
3. Some gas safety equipment was inoperative.
4. Where operators must occasionally enter several locations where
digester gas can build up. No gas alarm or portable gas meter has
been provided for operator safety.
5.3.5. Sludge Piping and Pumping Capabilities
The anaerobic digestion system is provided with an extremely flexible
sludge piping and pumping system. Appendix H contains a current sludge
piping schematic that was developed as part of this study. A cursory
review of existing line sizes and pump capabilities indicated that capacity
is adequate to handle all sludge generated.
40
-------
5.3.6. Volatile Matter Reduction
Due to the conversion of some of the biodegradable organic material
(volatile solids) to methane (CH.) and carbon dioxide (CCL), anaerobic
digestion of municipal wastewater sludge decreases the total solids. The
amount of volatile material that can be reduced depends primarily on sludge
type, digestion operating temperature, food to organism ratio, and length
of time the sludge is kept within the digestion tank (sludge residence
time). In addition, at similar digester operating temperatures and sludge
residence times, a greater fraction of primary sludge will digest than
waste activated sludge. Figure 5-2 shows a plot of percent volatile solids
SWMWv
- jivXvivXvX
70
Blue Plains 1980 Sludge
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3 C
ttJ t- £«
QC 4J 60
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«&
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_« J3
*-• ¥:>¥;::»:•
-••••:-x-XvXv:-:-:->:-:-:::v.:.v:::::v.:::::.:.:.:.v.:v:
:-'S: x ifiS*!?
— 'vvX::::
•,56.5+6.5 percent :x:li£;S?;^si5s5S5S
,) volatile solids reduction :'"S?»;?s5:x^
: '"l:r •:•.••
"V53.5+6.5 percent
'_ volatile solids reduction
„. — luinyu* i tion 11 ail siuaye
anaerobically digested
S*>..
il
•
Blue Plains
1981 Sludge
f^ Composition if all sludge
::::;:;>. anaerobical ly digested
SiiHlSSSSSv'i .
llillllllll?:.
•
">:¥x":-:::::¥::::::;:::::::x:<:
::|i|I||||
::^^
**i
• • •' '
X;X;XvX;.p
•;'x-x'>X'X'X'X;X;'x->.
: ,
::::S:Si>£SS:i:SS
'*$M
SSSS?:
tj..
1
1
•X1
10
20
30
40
50
60
70
Percent of Total Sludge Mass Being Digested
That is Secondary Sludge
FIGURE 5-2. The effect of secondary sludge on volatile
matter reduction at the blue plains anaerobic digestion facility2
2See Appendix I for development of data points.
41
-------
reduction versus percent by weight of secondary sludge in the sludge mass
being digested at digestion temperatures of 93 to 95°F and sludge residence
times generally from 17 to 21 days. Each point on Figure 5-2 represents
the average performance over a one-month period.
As the secondary sludge fraction of the total sludge mass increases
(digestion temperature and sludge residence time held relatively constant),
the percent volatile matter reduction decreases as would be expected
(Figure 5-2). The curve drawn in Figure 5-2 is of "eye ball" fit con-
structed under the following assumptions.
o The lack of accuracy involved in performing solids analysis plus
the fact that each data point represented the average of one month
of operating data dictated that a wide band curve incorporating a
range would be required.
o Realization that without data in the region of 0 to 20 percent on
the horizontal axis, the curve would have to lie between 60 to 80
percent volatile matter reduction as the secondary sludge fraction
of the total mass approached zero percent (USEPA, 1979). •
o Realization that without data in the region beyond 52 percent on
the horizontal axis, the curve would have to lie between 30 to 40
percent volatile matter reduction as the secondary sludge fraction
of the total mass approached 100 percent (USEPA, 1979).
Volatile matter reduction as a function of organic loading and hydrau-
lic residence time are given in Figures 5-3 and 5-4, respectively. In
evaluating the information shown in these figures, it must be remembered
that not all sludge goes to anaerobic digestion, in fact not even the same
ratio of primary to secondary sludge is maintained. The amount and ratio
pumped to digestion is presently controlled by the raw and anaerobically
digested sludge dewatering operations.
Organic loading Over the range indicated in Figure 5-3 had little or
no effect on volatile matter reduction. Analysis of the raw data indicates
that at the higher organic loadings and higher volatile matter reductions,
the ratio of primary sludge to secondary sludge was considerably higher
than the normal 50:50 to 60:40 split. These results would be expected
since Figure 5-2 showed that a greater fraction of primary sludge digested
than secondary sludge.
42
-------
Evaluation of the raw data in Figure 5-4 indicates that the four
points beyond 22 days are for time periods in which the primary:secondary
sludge ratio was considerably higher than the normal 50:50 to 60:40 split.
If these data points were removed, the data would indicate no effect of
hydraulic detention time on volatile matter reduction over the range indi-
cated. This would imply that a primary:secondary sludge ratio of 50:50 to
60:40 could operate at less than 16 days hydraulic detention and still
expect 45 to 60 percent volatile matter reduction.
Based on the curve developed in Figure 5-2 and the data presented in
Figures 5-3 and 5-4, the following can be said about volatile matter reduc-
tion using the existing anaerobic digestion system. (Assuming hydraulic
detention times of 16 to 17 days, operating temperatures of 93°F to 95°F,
and organic loadings of 0.12 to 0.14 pounds volatile matter per cubic foot
per day.)3
o Digestion at the current secondary sludge to total sludge mass
generation ratio of 0.43 should provide a 56.5 ±6.5 percent vola-
tile matter reduction. This would give an overall reduction in
solids of 0.256 ± 0.042 T/MGIF.
o Digestion at the future secondary sludge to total sludge mass
generation ratio of 0.465, should provide a 53.5 ± 6.5 percent
volatile matter reduction. This would give an overall reduction in
solids of 0.255 ± 0.042 T/MGIF.
5.3.7. Gas Production
Anaerobic digestion produces a gas with an energy value of approxi-
mately 600 BTU per cubic foot. As indicated in Appendix I,
in order for the existing heat exchangers to operate at their maximum
capacity, the boilers would require 810,000 cubic feet of digester gas per
day. In addition, the District WWTF is negotiating with the Naval Research
and Development Center to sell them excess digester gas at $0.95 per 1,000
cubic feet.
Data analysis for volatile matter reduction is given in Appendix I.
43
-------
70
1-2
ss ->
•- ii
n =1
-5 60
— v
•9 •a
£
50
0.10
O.L2
0.14
0.16
Organic Loading
Pounds Volatile Matter Per Cubic Foot Per Oay
0.18
FIGURE 5-3. The effect of organic loading on volatile matter
reduction at the District of Columbia anaerobic digestion WWTF4
•fi
31
II 3
70
60
-T 50
16 13 20 22
Hydraulic Detention Time - Days
24
FIGURE 5-4. The effect of hydraulic detention time on volatile
matter reduction at the District of Columbia anaerobic digestion WWTF0
See Appendix I for development of data points.
44
-------
Figures 5-5, 5-6, and 5-7 show operating data on gas production per
pound of volatile matter reduced as a function of feed solids concentra-
tion, and volatile matter loading. All data indicate that significant
amounts of gas production can be lost as a result of digestion system pro-
cess stress, possibly caused by either poor mixing or ammonia toxicity.
Gas production per unit of volatile matter reduced declined with
increasing digester feed solids concentration (Figure 5-5). This decline
may be attributed to the existing systems inability to mix the solids at
higher solids concentrations. Insufficient mixing allows concentration
gradients to build up which tend to disrupt the biological process. Two
possible solutions would be (1) increase the mixing intensity or (2)
operate the system at a lower feed solids concentration.
= 3 V
a o se.
55
(^
O
14.0
12.0
10.0
3.0
5.0
6.0
7.0
3.0
Feed Solids Concentration
Percent Solics
FIGURE 5-5. The effect of feed solids concentration on anaerobic
digestion gas production at the District of Columbia digestion WWTF5
5 See Appendix I for development of data points.
45
-------
The decline may also be attributed to ammonia toxicity. Ammonia
nitrogen levels in the digestion tanks vary from 1,200 to 1,500 milligrams
per liter (mg/1). It is known that ammonia nitrogen starts to affect the
digestion process adversely above concentrations around 1,400 to 1,500
mg/1. Therefore, it may be concluded that reduced gas production at the
higher feed solids concentration is a reflection of the higher potential
for ammonia nitrogen toxicity.
The solution to ammonia toxicity is dilution. This dilution would be
accomplished by proper blending of the thickened primary and secondary
sludge. It is desirable to digest as concentrated a sludge as possible and
still maximize gas production. From Figure 5-5, a value of six percent
meets this criterion. To meet the six percent level, the primary sludge
mass should not be more than 65 percent of the total sludge mass pumped to
digestion.
Gas production per unit of volatile matter reduced declined with
increased volatile matter loading (Figure 5-6). The current operational
practice is to maintain a constant liquid volume flow to digestion, but to
vary the primary-secondary sludge mixture.
§£«
li.O-
12.Of-
L
10.0-
8.0
D.-.O
0.12
0.16
Volatile natter Loaemc
Pounds volatile Matze" °er
Jseole Cubic Feet °er Oay
FIGURE 5-6. The effect of volatile matter loading on anaerobic
digestion gas production at the District of Columbia digestion WVTF**
5 See Appendix I for development of data points.
46
-------
Increasing the percentage of primary sludge means more volatile matter
per unit of flow, thereby increasing the volatile matter loading on the
digestion system. Therefore, the higher volatile matter loadings indicated
in Figure 5-6 are due to a high percentage of primary sludge in the incom-
ing flow. This decline may again be due to poor mixing at the higher
feed solids concentration that would develop with the greater percentage
of primary sludge being pumped to the system. It is desirable to maintain
the highest volatile matter loading possible and still maximize gas pro-
duction. From Figure 5-6, a value between 0.12 to 0.14 meets this cri-
terion.
5.4. ELUTRIATION SYSTEM OPERATION
Elutriation (washing) of the anaerobically digested sludge is prac-
ticed to reduce chemical conditioning requirements (reduced operating
cost) in the vacuum filter operation. River water is used as the source
of washwater.
The elutriation system consists of two 35-foot wide by 70-foot long
tanks, each equipped with sludge removal and surface skimming equipment.
The tanks are operated in series. Anaerobically digested sludge is mixed
with wash water before entering the first tank. The sludge that settles to
the bottom is removed and pumped to the second tank where it is again mixed
with wash water. Settled sludge from the second tank is pumped to the
vacuum filter. Part of the elutriation tank overflow flows by gravity to
the primary clarifiers and the remainder is pumped to the gravity
thickeners. Polymer is added to improve solids concentration and capture.
Figures 5-7 through 5-9 show the effect of several variables on the
percent solids concentration of the elutriation underflow pumped to the
vacuum filters. The data presented in these figures are included in
Appendix A, Table A-5 and in Appendix J. As will be demonstrated in
Section 5.5, it is important to maintain this solids concentration as high
as possible.
47
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Figure 5-7 indicates that a feed solids concentration of at least 2.7
percent is required to develop a five percent underflow concentration.
Because there is no digester supernatant, the solids concentration out of
the digesters is controlled by the solids concentration into the digesters.
Assuming an average 53 percent volatile matter reduction (Section 5.3.6),
the feed solids concentration into the digester must be a minimum of 4.5 to
5 percent. This also implies that the primary:secondary sludge mixture has
to be a minimum of 1:6.
The total hydraulic flow to the elutriation system is composed of
digested sludge flow and washwater flow, which currently totals 2.2 to 2.4
4- WO
3
LU
4.0
2.5
3.5
Feed Concentration
Percent Solids
FIGURE 5-7. The effect of elutriation feed solids
concentration on elutriation underflow solids concentration7
7 See Appendix J for development of data points.
48
-------
L.
01 C i-
•O -r- r—
C S- O
=3
•M
C (TJ 4->
O S C
•r- 0) 0)
•M a o
ID S.
•i- o a>
s- t— a.
5.0
4.0
40
50
Digested Sludge Flow
To Elutriation Per Day
Gallons x 104
FIGURE 5-8. The effect of digested sludge flow rate
on elutriation underflow solids concentration7
million gallons per day. Higher total flows are not possible because of
the inability to remove elutriation overflow at higher flow rates. Pre-
liminary analysis of elutriation underflow solids concentration versus
total flow rate showed no relationship. Evaluation of the digested flow
component (Figure 5-8) indicates that elutriation underflow solids concen-
tration is quickly and significantly reduced at digested flow rates over
450,000 to 470,000 gallons per day.
See Appendix J for development of data points,
49
-------
The addition of washwater improves the compaction capabilities of
sludge; however, according to Figure 5-9, washwater usage beyond 4.5 to
4.7 times the incoming digested sludge flow rate is of no value. Because
of the costs to pump washwater from the river and treat it after use, it is
best to minimize the quantity of washwater consumed for elutriation.
J- WO
CD C v
•O-i- i—
C S- O
3 0>
-------
5.5. ANAEROBIC SLUDGE DEWATERING OPERATION
5.5.1. Solids Generation
Currently, anaerobically digested sludge is dewatered on drum-type
rotary vacuum filters after conditioning by elutriation and the addition
of ferric chloride. The addition of ferric chloride adds solids to the
total sludge mass.
The amount of ferric chloride added is governed by several factors:
the operators visual evaluation of cake release from the drum and cake
solids content, both of which will improve with increasing ferric chloride
addition; and management's attempt to keep ferric chloride addition at a
minimum level and still obtain acceptable results. Although existing data
are inconclusive, there are indications that some relationship is exerted
on the ferric chloride addition by the filter feed solids concentration
(Figure 5-10) and by the percent secondary sludge in the total sludge mass
(Figure 5-11).
•a o
••- co
00
0)
9) V
U_ 0.
0)
-(-> C
i— O
(O
3 C
(J O
c
o
5.0
4.0
10
11
12
Ferric Chloride Usage
Tons per day
FIGURE 5-10. The effect of ferric chloride addition
on filter feed solids concentration9
9 Filter operates 24 hours per day. See Appendix K for development of
points.
51
-------
o
11 ««
0)
H 41
*^ jj
5 §
-------
1. At current conditions for the sludge being anaerobically digested,
each million gallons of influent flow generates an additional
0.043 ± 0.009 tons of solids.
2. At future conditions for the sludge being anaerobically digested,
each million gallons of influent flow would generate an additional
0.049 ± 0.009 tons of solids.
5.5.2. Cake Dryness
Final disposal of digested sludge requires truck transport to a final
disposal site. Minimizing the volume of digested sludge produced,
minimizes the transporation cost.
Figure 5-12 indicates that within the range evaluated, increasing feed
solids concentration produced an increasingly drier filter cake. (Note:
Feed solids concentration is the same as that in the elutriation under-
flow.) Therefore, from the standpoint of the dewatering operation, the
digested sludge should be maintained at the highest possible feed solids
concentration.
5.6. RAW SLUDGE DEWATERING OPERATIONS
5.6.1. Solids Generation
Currently, raw sludge is dewatered on cloth belt type, rotary vacuum
filters. Sludge conditioning consists of addition of both ferric chloride
and lime, which adds substantially to the total sludge mass.
No analysis was conducted to evaluate if any relationship existed for
predicting ferric chloride and lime usage as a function of raw sludge
characteristics, (for example, feed solids content, or secondary fraction
of the total sludge mass). Lime is currently added in excess of condition-
ing requirements to maintain a high pH to meet sludge trenching require-
ments. In the near future when trenching is discontinued, lime require-
ments would be expected to decrease by at least 50 percent, which would
affect the amount of ferric chloride utilized.
53
-------
I/)
•O •!->
•r- C
r-
-------
4. In the future, even if caw sludge was not to be trenched but raw
secondary sludge still needed to be dewatered, each million
gallons of influent flow would generate an additional 0.021 ±
0.007 tons of solids from ferric chloride addition and 0.055 ±
0.018 tons from lime addition.
5.6.2. Cake Dryness
As with digested sludge, final disposal of raw sludge requires truck
transport to a final disposal site. Minimizing the volume of raw sludge is
again important to minimizing the transportation cost.
Figure 5-13 indicates that within the range evaluated, increasing feed
solids concentration produced an increasingly drier cake. Therefore from
the standpoint of the dewatering operation, the raw sludge should be main-
tained at the highest possible feed solids concentration. In the future
when lime and ferric chloride additions are reduced, it is believed that
the highest cake solids will still be produced from the highest feed solids
concentration, but that the cake dryness will not be as high as currently
developed for a given feed solids concentration.
7.0
S 5.0
-------
Figure 5-14 indicates that decreasing amounts of secondary solids in
the mixture produce drier dewatered cake. This would be expected since
primary solids are gritty and fibrous, which compact better than gelati-
nous secondary solids.
70
V)
-------
The balance of the dewatered raw sludge is trenched. Hauling and
disposal cost are approximately $35 per wet ton.
5.8. SUMMARY
The discussions of changes in solids production per MGIF caused by the
various processing operations are summarized in Table 5-2. Table 5-3 pre-
sents the various limitations noted for the processing steps.
57
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TABLE 5-2. SUMMARY OF ADDITION (REDUCTION) IN SOLIDS PRODUCTION DUE TO VARIOUS
SOLIDS PROCESSING STEPS
Processing operation
Current sludge generation,
dry tons (pounds)
per million gallons
of influent flow
Future sludge generation
dry tons (pounds)
per million gallons
of influent flow
00
Gravity Thickening
Dissolved air flotation
thickening
Anaerobic digestion
Elutriation
Digested sludge dewatering
o Ferric chloride
Raw sludge dewatering
o Ferric chloride
0 Lime
None
0.001 ( 2 )
0.256 ± 0.042 ((512 ± 84))
None
0.043 ± 0.009 ( 86 1 18)
0.049 ± 0.015 ( 98 ± 30)
0.223 ± 0.042 (446 ± 84)
None
0.002 ( 3 )
0.255 ± 0.042 ((510 ± 84))
None
0.049 ± 0.009 ( 98 ± 18)
0.044 ± 0.015 ( 88 ± 30)
0.119 ± 0.040 (238 ± 80)
-------
TABLE 5-3. SUMMARY OF DISTRICT OF COI.UHUIA'S UwTF SLUDGE NANAGIIMFNT
PROCESS [.IMITATIONS UNDER CURRENT OPERATION
Process
Gravity thickening
Dissolved air flotation
thickening
Limitations
Average thickened primary sludge concentration is 7 0 i 0 5 percent
Average thickened primary sludge volume per MGIF is 1,236 ± 218 gallons
Average primary solids production per MfilF is O.377 i 0 1)60 ton-: (754 * 120
pounds) The average volatile content is 71 ± 5 I percent
Average thickened secondary sludge concentration is 4 I i 0 3 percent
Average current thickened secondary sludge volume per MGIF is 1,644 i 239
gallons; in 1981, it will increase to 1,900 1 239 gallons
Average secondary solids production per MGIF is 0 284 i 0.034 tons
(568 ± 68 pounds) with an average volatile content of 65 percent;
in 1981, it will increase to 0 328 ± 0 034 tons (656 t 68 pounds)
with an average volatile content of 63 9 t 2.1 percent
Anaerobic digestion
o Tankage
Approximately 65 percent of the total volume available or 1,142,458
usahle cubic feet (8,545,587 usable gallons)
Ln
10
o Heating capacity
After meeting system radiation, losses at a 95°F operating temperalnie
available heating capacity is currently 9,800,000 RTU per hour during
the winter and 11,100,000 RTU per hour during the summer In 1981,
this will increase to 12,800,000 and 14,100,000 BTU per hour,
respectively
For maximum gas Maximum feed solids coiiccnliation to digestion uoL to exceed six
production pen en t solids.
Maximum volatile mailer loading not to exceed 0 14 pounds per usable
cubic feet per d«iy
F.lutnalion
Digested sludge
dnwalernig
Digested sludge flow rate to sysli-ra not to exceed 490,000 gallons
per day.
Feed sol ills coiiieiilralion to be a minimnnnf 2 7 penent
Maximum percent take solids achievable on a consistent basis 16 to 17
percent
Raw sludge dew.itering
Maximum current percent lake solids jclnev.ihle on a consistent h.isis
21 to 22 percent In the Fiiliue, tins may dui rease to IV to 2O
pel«enl
-------
SECTION 6
IMPROVING SLUDGE MANAGEMENT OPERATION USING ANAEROBIC DIGESTION
Table 6-1 summarizes expected sludge quantities at the current average
plant influent flow rate of 334 million gallons per day. The purpose of
this study was to evaluate the existing sludge processing operation with
the intent of determining if a new two-stage, mesophilic-thermophilic
anaerobic digestion process could be applied to digest all sludge produc-
tion using the existing equipment in conjunction with a minimal capital
expenditure. If possible, then sludge processing and disposal cost at
Blue Plains would be reduced by more than 50 percent. In addition, the
scope of work included recommendations to improve the current anaerobic
digestion operation and a brief evaluation of the use of thermophilic
digestion. This section of the report will discuss these three objectives
in the following order:
o Existing mesophilic system operation
o Evaluation of thermophilic system option
o Evaluation of mesophilic-thermophilic system option.
TABLE 6-1 EXPECTED PRIMARY AND SECOND-»RV SLUDGE QUANTITIES AT CURREVT 334 ,-ILLIOs GALLO.N PER DA}
INFLUENT FLOW3
Gallons of sludge per ia\ Pounds of sludge per das' \oiaiile pounds sludge pe- o-\
Minimum Average laximum *!inimao Average 'Idxuniift 'linimun Average Id* mi-.
Primary sluage 340,012 412,S2i 485,63c :il.'56 151.836 2»:,916 150,347 ITS 30; 2u7 :»C
Secondary sludge 55-.77. 634,600 71...-20 196.392 2:9.104 2-.l.?;t 12S.691 140^22^ 15- :«.-
TOTALS 394,786 1,047.424 L.200.062 iOS.143 470.940 53J.732 276.OSS 311.03., 3oi a::
Da-.« calculated from Table 5-3
60
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6.1. EXISTING MESOPHILIC SYSTEM OPERATION1
Evaluation of an existing anaerobic digestion process system for the
purpose of improving operations begins at the "end and works towards the
front." This is done to ensure that any individual process constraints,
which normally affect the output of the preceding process, can be identi-
fied and incorporated into the operation of the preceding process. In
addition, the evaluation will be made from the viewpoint that all the
sludge is to be processed through the present system. It is believed that
this is the only impartial way to compare all three digestion processes.
6.1.1. Ultimate Disposal
Ultimate disposal of anaerobically digested sludge is on the land. It
is desirable to have the driest cake possible to minimize the transpor-
tation costs.
6.1.2. Digested Sludge Vacuum Filter Operation
The constraint from ultimate disposal is to produce the driest sludge
cake possible. From the viewpoint of dewatering management, it is impor-
tant to minimize chemical additions (chemical cost and extra solids). In
Section 5.5, the data presented indicated that maximum cake solids and
minimum chemical usage occurred at the highest feed solids concentration.
The data in Section 5.5 also indicated that with proper operation of the
elutriation system, a minimum of five percent feed solids concentration
could be obtained on a regular basis. This would result in an average
16.5 percent cake solids concentration.
Current Operation--
There are four old drum vacuum filters, each with 500 square feet of
filtering area. In 1979 when the secondary to total sludge mass ratio of
the sludge being digested was between 0.4 to 0.5, the average calculated
yield was 2.41 pounds per square foot per hour. Assuming that all four
1 All supporting calculations for Section 6.1 are given in Appendix M.
61
-------
units were operating at the average yield, 115,680 pounds of anaerobically
digested sludge could be dewatared. At an average 16.5 percent cake
solids there would be 390.5 wet tons of sludge per day needing disposal.
At the present, the limited capacity of these facilities is a major
constraint in processing sludge through the digestion system. This will
change shortly with the start-up of six new and larger filters.
Future Operation—
Sometime in early 1981, six new cloth belt vacuum filters will become
operable for anaerobic sludge dewatering and the four existing units will
be abandoned. Each new unit will have 600 square feet of filtering area
and it is assumed the same yield. Under these conditions the digested
sludge dewatering capacity would increase to 208,224 pounds per day. With
these new filters in operation, dewatering capacity will no longer be the
rate-limiting process. In fact, elutriation will be the rate limiting
process and will limit digested solids requiring dewatering to 142,000
pounds per day (479 wet tons to disposal), which would mean only four of
the six filters would need to be operated at any one time.
It should be noted that if all primary and secondary sludge could be
mesophilically digested, 10 of the 15 vacuum filters currently being used to
dewater raw sludge could be used to dewater digested sludge, as piping and
valving exits for such a configuration.
6.1.3. Elutriation
The constraints from the dewatering operation are: (a) to produce a
solids concentration (elutriation underflow solids concentration) of at
least five percent solids; and, (b) the maximum amount of sludge capable of
being dewatered in early 1981 is 208,224 dry pounds per day.
In Section 5.4, the data presented indicated that in order to achieve
a constant five percent solids concentration in the elutriation underflow:
o The flow rate to the elutriation system from the digestion system
should not exceed 490,000 gallons per day.
o The solids concentration of the influent digested sludge stream to
elutriation had to be a minimum of 2.7 percent.
62
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With the start-up of the new digested sludge dewatering operation, the
existing elutriation system would become the next rate-limiting process,
limiting sludge processing to 490,000 gallons per day (40 to 55 percent of
the total sludge flow).
Consideration should be given to using some of the spare
dissolved air flotation tanks as elutriation tanks. Eight of the spare
tanks would provide an additional 880,000 gallons per day elutriation
capacity under the constraints specified. This additional capacity would
allow the elutriation system to process all sludge generated. Appropriate
charges would enable pumping digested sludge and washwater to the flotation
tanks.
6.1.4. Anaerobic Digestion
The constraints from the present elutriation facilities are:
(a) Digested solids concentration from the digesters to be a minimum
of 2.7 percent.
(b) The flow rate from the digestion system cannot exceed 490,000
gallons per day.
(c) The maximum sludge mass not to exceed 208,224 dry pounds per day.
From the viewpoint of digestion management it is important to maximize
flow rate, solids reduction, and gas production within the constraints
stated. In Section 5.3, the data presented indicated the following addi-
tional operating constraints:
(d) Approximately 35 percent of the existing tank volume—1,757,856
cubic feet (13,148,763 gallons)—is unusable, usable tank volume
is 1,142,606 cubic feet (8,546,693 gallons).
(e) Available sludge heating capacity after meeting system radiation
heat loss requirements is 9,800,000 BTU per hour during the
winter and 11,100,000 BTU per hour during the summer. In 1981,
this will increase to 12,800,000 and 14,100,000 BTU per hour.
(f) Maximum gas production achievable on a consistent basis required
that:
63
-------
o Maximum feed solids concentration to digestion not to exceed
six percent solids
o Maximum volatile matter loading not to exceed 0.14 pounds per
usable cubic foot per day
(g) Existing data indicate that hydraulic residence times of 16 days
present no problem over a wide range of feed combinations. Lower
detention times may be possible, but no operating data are avail-
able.
Since all digestion tanks are being mixed and no supernatant is re-
moved, the flow rate out of the digestion tanks should be approximately
equal to the flow into the tanks. The flow rate through the tanks is
governed by one of three constraints: the elutriation system, sludge
heating, and vacuum filtration capacity.
The calculations on sludge heating capabilities indicate that the
existing heat exchangers are capable of heating 683,700 gallons per day to
95°F during the 1980 winter and 1,549,000 gallons per day to 95°F during
the summer of 1981. When the new hot water boiler is put in service during
the summer of 1981, the winter capacity will increase to 893,000 gallons
per day and 1,967,000 gallons per day during the summer.
It should be noted that in order to process all sludge through diges-
tion, an additional 4,400,000 BTU per hour would be required during the
winter months. As explained in Appendix G, 17,000,000 BTU per hour of new
low pressure (9 psi) steam capacity is being interconnected with the exist-
ing sludge heat exchanger steam boilers. If the capacity is available, it
may be possible to place steam lines up to the top of the digestion tanks
and inject the steam directly into the digesters through the roof.
Based on a minimum 16-day hydraulic detention time and 65 percent
usable digester volume, the flow rate would be 534,000 gallons per day. If
the grit problem was solved and all existing tank capacity became avail-
able, then the flow rate would be 821,700 gallons per day.
64
-------
It may be possible to operate this mesophilic system at a lower
hydraulic detention time, but operating data do not currently exist to
verify this.
Both heating capacity and hydraulic detention time flow rate maximiuns
are above the elutriation operation 490,000 gallon per day constraint.
Therefore, total raw sludge flow rate through the existing anaerobic diges-
tion tanks should be and is limited to 490,000 gallons per day under the
present circumstances.
In addition to hydraulic flow rate restrictions on the digestion
tanks, volatile matter loading requirements must also be satisfied. Avail-
able data indicate that the system can be successfully operated at volatile
matter loading ratios of 0.16 to 0.17 pounds volatile matter per usable
cubic foot per day, though gas production seems to deteriorate over 0.14.
Based on 0.16 loading and 65 percent usable digester volume, the total
amount of volatile solids pumped to the system would be 183,000 pounds per
day. If the grit problem was solved and all existing tank capacity became
available, then the amount would be 281,000 pounds per day.
If all the existing digesters were utilizing full capacity, then at
least four more tanks identical to the existing 12 would be required to
meet volatile matter loading requirements.
6.1.5. Primary - Secondary Flow Rates To Digestion
The ratio of thickened primary to thickened secondary sludge volumes
is 0.65:1. At the current time, the ratio being processed through diges-
tion is 1.44:1 (288,000 gallons per day:200,000 gallons per day). As was
discussed in Section 5 of this report, operating at high primary sludge to
total sludge mass ratios leads to digester stress conditions. It is re-
commended that for better overall process operation the primary to secon-
dary flow rate volumes should be altered to 193,000 gallons per day:297,000
gallons per day (0.65:1).
65
-------
6.1.6. Digester Gas Production
Calculations in Appendix M show that at the 490,000 gallon per day
sludge processing rate under the conditions previously discussed, more gas
will be generated than required to meet total sludge heating requirements.
The average daily excess gas production is 658,000 cubic feet per day and
will range from 780,000 cubic feet per day during the summer to 537,000
cubic feet per day during the winter.
6.2. EVALUATION OF THERMOPHILIC SYSTEM OPTION2
Review of thermophilic anaerobic digestion clearly indicates that the
process should be seriously considered for the least cost, short term solu-
tion of the sludge processing and disposal problems of the District of
Columbia's Wastewater Treatment Facility. The three significant advantages
of the process (a) increased sludge processing capability, (b) improved
sludge dewatering, and (c) increased destruction of pathogens, are all
pertinent to the District's situation.
More detailed checks should be made on a number of items prior to
deciding to convert the existing digesters to thermophilic operation.
These include (a) amount and type of additional sludge heating required,
(b) structural competency of existing digesters and piping at thermophilic
temperatures, (c) needed improvements in the temperature control system,
(d) equipment needed to remove the increased amounts of moisture to be
expected from the digester gas, and (e) how to avoid possible inhibition
by ammonia.
2A11 supporting calculations for Section 6.2 are given in Appendix N.
66
-------
6.2.1. Ulitmate Disposal
Thermophilically digested sludge is generated under conditions that
approach disinfection, thus allowing for acceptable final disposal to the
land. As with the current mesophilic operation, it would be important to
minimize transportation cost, therefore, the driest cake possible is
desirable.
6.2.2. Digested Sludge Vacuum Filter Operation
Thermophilically digested sludge exhibits better filterability than
straight mesophilically digested sludge—some times as much as double. In
Appendix N, the analysis used a 25 percent improvement in yield resulting
from thermophilic digestion, a value considered conservative. Calculations
indicate that thermophilic digestion of all Blue Plains sludge would re-
quire the use of 8 of the 21 new vacuum filters for dewatering of the
digested sludge.
Another advantage would be the elimination of the need for sludge
conditioning by elutriation or by added iron salts. Thermophilically
digested sludges can be conditioned using a combination of anionic and
cationic polymers. This change alone would reduce the present amount of
solids to be disposed of by 94 pounds per million gallon of influent flow—
over 16 dry (96 wet) tons per day.
A disadvantage to dewatering this sludge is that the sludge must be
cooled to under 90°F. Calculations in Appendix N indicate that the exist-
ing elutriation tanks might be used as cooling tanks. Potential odor
problems may also be minimized by using the elutriation step because of
this liquid to liquid cooling.
6.2.3. Impact of Increased Heat Requirements
The thermophilic digestion process being evaluated operates at 122°F.
The existing heating capabilities at Blue Plains have been calculated to be
inadequate, with approximately 16.1 x 10 BTU per hour additional heating
67
-------
needed during the winter months. It is suggested that direct steam injec-
tion be considered for supplying the additional heat since this has been
used successfully in Los Angeles, Moscow, and the Canadian study. Struc-
tural competency should be checked by a structural engineer. A control
engineer should be engaged to look at the temperature control system
with a maximum variation of about ±1.5°F being permitted (at 120°F
operation).
6.2.4. Digestion Tanks
Calculations indicate that the existing digestion tanks are capable of
taking the full sludge load in the thermophilic range of operation if they
are cleaned and grit accumulation kept to a minimum.
6.2.5. Volatile Matter Reduction and Gas Production
Analysis indicates that the percentage volatile matter reduction in
the thermophilic system would be about the same as the mesophilic system
but at one half the time. Calculations in Appendix N show that more gas
will be generated than required to meet total sludge heating requirements.
The average daily excess gas production is 840,000 cubic feet per day and
will range from 1,000,000 cubic feet per day during the summer to 600,000
cubic feet per day during the winter.
6.2.6 Transition from Mesophilic to Thermophilic Operation
It would be desirable to make the transition from mesophilic to ther-
mophilic operation as rapidly as possible. However, caution should be
exercised in making this transition since very little information is
available on the maximum rate at which this transition can be effected.
In Garber's early work (4), he indicated that almost six months were needed
to establish the first thermophilic unit as a separate culture. By seeding
and more rapid increases in temperature, he was able to cut this time
to three months.
68
-------
In Garber's later work, he gave more details as to the transition
procedure. This time he increased the temperature from 96°F to 126°F at
the rate of 1°F per day while maintaining the load at approximately 0.1
pound volatile solids per cubic foot per day. This was not successful and
the digester turned "sour." He then reduced the loading to a minimum while
maintaining the temperature at the 126°F level but observed no change in
condition over a four month period. The temperature was then reduced to
120°F and over a three week period, satisfactory digestion commenced.
Because of Garber' experience, personnel in Chicago were more cautious
in raising the temperature. Their procedure was to raise the temperature
at a rate of 1°F per day for only five days and then to wait for two to
three weeks for the digester to stabilize before again increasing the
temperature 1°F per day until a final temperature of 127°F was attained.
During this period the loading on the digester was maintained at about 0.13
pounds volatile solids per cubic foot per day. Several surges (one up to
2,500 mg/liter) were observed during the transition period and it took
approximately one year for the digester to stabilize at consistently low
volatile acid concentrations.
From the above, it can be clearly seen that there is still much to be
learned about the correct procedure for making the transition from
mesophilic to thermophilic digestion. The three variables involved are (1)
rate of change of temperature, (2) rate of change of loading, and (3)
maximum temperature to be attained. Garber's experience indicates that, at
least inititally, the maximum temperature should be limited to 120°F.
In both the Los Angeles and Chicago experiences, the loading on the
digester was maintained at its normal value while the temperature was
gradually increased. An alternate approach would be to stop the loading to
the digester, bring it to the new temperature as rapidly as possible, and
then gradually increase the loading. Limited experience in Atlanta (12)
during the summer of 1980 indicates that the transition might be accom-
plished more rapidly by following the latter procedure.
69
-------
6.3. EVALUATION OF MESOPHI1IC - THERMOPHILIC SYSTEM OPTION3
As noted at the beginning of Chapter 6, the primary purpose of this
study was to evaluate the existing sludge processing operation with the
intent of determining if a new, two-stage, mesophilic-thermophilic anaero-
bic digestion process could be applied to digest all sludge production
using existing equipment and without substantial capital expenditures.
The mesophilic-thermophilic system offers all the advantages of
thermophilic digestion (disinfection, increased volatile matter reduction,
and improved dewatering) and produces an essentially innocuous sludge. In
addition, the system is simple to operate and has built in buffering
capacity to deal with unusual loading conditions. Disadvantages are that
it requires heating of two completely separate digestion processes (one at
95°F, the other at 122°F) and that there is only one operational plant,
handling 100,000 people, in the United States. Both the mesophilic-thermo-
philic and thermophilic processes will require that the sludge be cooled
before being dewatered.
6.3.1. Ultimate Disposal
The sludge from this process is extremely inert and looks very similar
to composted sludge. This quality should allow for disposal to both pri-
vate and public lands.
6.3.2. Digested Sludge Vacuum Filter Operation
The mesophilic-thermophilic process will have the same vacuum filter
operation requirements as for the thermophilic option.
All supporting calculations for Section 6.3 are given in Appendix 0.
70
-------
6.3.3. Impact of Increased and Dual Heat Requirements
The total sludge heating requirements for this process option are the
same as the previous thermophilic option. The major difference is that the
mesophilic digestion tanks are to be maintained at 95°F and the thermo-
philic tanks at 122°F. The suggested method of sludge heating is as
follows:
o For the mesophilic tanks, the existing sludge heat exchangers
would be used and steam (approximately 500,000 BTU per hour)
would be injected into each of the tanks.
o For the thermophilic tanks, all heating would be by direct steam
injection (approximately 5,000,000 BTU per hour per tank).
As was mentioned earlier, calculations indicate that the increased
temperature will have no significant impact on sludge piping and wall
structural integrity.
6.3.4. Digestion Tanks
Calculations indicate that even if all the existing tanks volume was
available, the hydraulic detention time under each process condition would
be inadequate. In order to process all of the currently generated sludge
through this process, four more digesters would be required.
6.3.5. Volatile Matter Reduction and Gas Production
Limited experience indicates that a 50 percent volatile matter
reduction can be expected from digesting the sludge. Calculations in
Appendix 0 show that more gas will be generated than required to meet total
sludge heating requirements. The average daily excess gas production is
1,109,000 cubic feet per day, and will range from 1,384,000 cubic feet per
day during the summer to 833,000 cubic feet during the winter.
71
-------
6.4. SUMMARY
Table 6-2 summarizes the major process considerations in implementing
either of the three anaerobic digestion alternatives to fully process the
maximum sludge quantities given in Table 6-1.
72
-------
TABLE 6-2. SUMMARY OF MAJOR PROCESS CONSIDERATIONS IN IMPLEMENTING FULL ANAEROBIC DIGESTION OF
BLUE PLAIN SLUDGES
Mesophilic
digestion
of all sludge
Additional heat requirements
BTU's x 106
4.4
Thermophilic
digestion
of all sludge
16.1
Mesophilic-thermophilic
digestion
of all sludge
Provisions for better grit removal
Digestion tanks required
yes
18
no
12
yes
16
16.1
Average daily revenues from excess
digestion gas production, dollars
Additional elutriation capacity
Vacuum filters required to be
operational
Maximum wet tons per day for disposal
1298
yes
10
1148
802
no
8
1033
1057
no
8
961
Excess gas production sold to Naval R&D at $.95 per 1,000 cubic feet.
Cake solids - 16.5 percent.
-------
SECTION 7
ECONOMIC IMPACT
Municipal sludge management is a challenging field. It is also an
expensive one. It is not unusual these days for a wastewater treatment
facility to devote over fifty percent of its capital and operational budget
to "managing" the steadily increasing amounts of wastewater sludge.
The Blue Plains Wastewater Treatment Plant is at a crossroads. The
Plant's current wastewater flow generates almost 500,000 pounds per day of
sludge. Several sludge treatment and disposal options are utilized:
o Dewatering of thickened raw sludge followed by land trenching
o Dewatering of thickened raw sludge followed by composting
o Mesophilic anaerobic digestion of thickened raw sludges followed
by dewatering and application to land.
In the near future, the land trenching alternative must be dis-
continued. Composting with its attendant problems and land requirements
is not practical. The third existing disposal scheme—mesophilic
anaerobic digestion—is a variable option.
This section of the report will not answer the question—what should
be done? What it will do is show how economically attractive anaerobic
digestion, in any of three process configurations, can be.
74
-------
7.1. OPERATIONAL COST IMPACT
Table 7-1 presents average sludge generation per million gallons of
influent flow for five different sludge management conditions.
CONDITION I - This is historical information and represents the situation
for the first six months of 1980. During this time period,
approximately 40 percent of the sludge was digested, 40 to
45 percent went to land trenching, and 15 to 20 percent to
composting.
CONDITION II - This is a projection of what the situation will be in late
1981 based on completion of current on-going process
changes. At this time, approximately 40 percent of the
sludge will be digested, the rest will be composted.
CONDITION III- This is a projection of what the situation would be if all
the sludge was processed through a mesophilic anaerobic
digestion system.
CONDITION IV - This is a projection of what the situation would be if all
the sludge was processed through a thermophilic anaerobic
digestion system.
CONDITION V - This is a projection of what the situation would be if all
the sludge was processed through a proposed new process
modification, mesophilic-thermophilic anaerobic digestion.
The economic impact of the numbers shown in Table 7-1 are better
illustrated in Figures 7-1 and 7-2.
In Figure 7-1 a comparison is made of the average cost for chemicals
and final disposal per million gallons of influent flow for the five condi-
tions given in Table 7-1. The 40 percent reduction in cost shown between
the existing 1980 and projected 1981 plan is solely due to eliminating land
trenching and replacing it with composting. The significant cost reduc-
tions shown between the projected 1981 plan and the three anaerobic diges-
75
-------
TABLE 7-1. SUMMARY OF AVERAGE SLUDGE GENERATION PER MILLION GALLONS OF INFLUENT FLOW
-vl
cr>
Based on
January to June 1980
Based on
completion of
current
on-going changes
Dlanning-iall 1981
Sludge Sludge
generated Cost generated Cost
per per per per
MGIF,0 MGIF,
Primary sludge
Secondary sludge
Gravity thickening
Dissolved air flotation
thickening
Anaerobic digestion
Eliilri.ition
pounds
696
568
0
2
dollars
3.83b
MGIF, MGIF,
pounds dollars
696
656
0
3
(512) (5.84)L (510)
0
De.walcring digested sludge
0 Iron salt
0 Polymer
Dewalered raw sludge
0 Iron salt
0 Lime
Disposal digested sludge
Disposal raw sludge
86
98
446
a MGIF = million gallons of
Polymer cost
C Based on 12
1 Based on $0.
Basprl on an
averaged $13.
cubic feet per
0685 per pound
average cost
A
5.B9d
6.71
37. 86*
38. IB1
147. 328
influent flow.
0
98
88
238
4.42
(5.81)
6.71
6.03
20.26
42.86
65.36
Based on
mesophil ic-
Based on Based on tliermopliil ir
mesophilic digestion thermophi lie digestion digestion
of all sludge of all sludge of all sludge
Sludge Sludge
generated Cost generated
per per per
MGIF, MGIF,
pounds dollars
696
656
0
3 4.42
(510) (5.81)
0
98 6.71
42.86
MGIF,
pounds
696
656
0
3
(510)
0
2
Sludge
Cost generated
per per
MGIF, MGIF,
dollars pounds
696
656
0
4.42 1
(5.81) (572)
0
8.00 2
38.50
Cost
per
MGIF,
dollars
4.42
(6.52)
8.00
35.54
50 per ton of dry feed solids.
pound volatile
of this sludge
of $0.0849 per
matter reduced
produced.
pound of this
and $0.95 per 1000 cubic feet.
sludge
produced.
8 Based on a $35 per wet ton composite cost (some composting, some trenching).
Based on a $24 per wet ton composting cost.
-------
118.35
100 -
80 -
5 1
a 2
2 U.
< =
SI
s-
— 1/1
II
§ ri
UJ
o oe
5 £
60 i
40 -
20 -
41.44
EXISTING
1980
CURRENT
PLAN
1981
ALL
HESOPHIL1C
ALL ALL
THERMOPHIUC HESOPHILIC-THERMOPHILIC
Figure 7-1. Average cost for chemicals and final disposal
per million gallons of influent flow for the
five conditions given in table 7-1.
tion alternatives are due to two items. First, in digestion approximately
37 to 40 percent of the raw solids generated are destroyed so that diges-
tion of all the sludge will significantly reduce the mass of solids that
requires disposal. Secondly, the dewatering operation for digested sludge
at Blue Plains does not require lime for conditioning: in the 1981 projec-
tion, the raw sludge dewatering operation will require a significant amount
of lime, which will increase solids by 238 pounds per million gallons of
influent flow. The higher unit cost of mesophilic over the other two
digestion alternatives is mainly because the other two digestion alterna-
tives do not require iron salts in the dewatering operation (equivalent to
96 pounds of solids per million gallons of influent flow). Meso-
thermophilic digestion has a lower overall cost than thermophilic digestion
because of a higher volatile matter reduction.
77
-------
1173
1129
1100
1000 •
I I
§
I
785
1980
RENT
PLAN 1981
ALL
MESOPHILIC
ALL MESOPHUIC-THERMOPHILIC
THERMOPH1LIC DIGESTION
Figure 7-2. Average wet tons of sludge produced per day for
the five conditions given in table 7-1.
Figure 7-2 shows the difference in wet tons generated by the five
different conditions given in Table 7-1. The large reduction in wet tons
is due to two items. First, in the digestion alternatives approximately 37
to 40 percent of the raw solids are destroyed (converted into methane gas,
carbon dioxide, and water); secondly, significantly less chemicals are
required in the dewatering operations. The difference in wet tons between
the three digestion alternatives is due to lower chemical conditioning
requirements for the thermophilic and meso-thermophilic options, and the
higher volatile matter reduction achieved by the meso-thermophilic option.
For every million gallons of flow into the plant, the raw sludge
dewatering option must dispose of 738 more pounds of solids. This is
equivalent to 3432 wet pounds or 1.72 wet tons.
78
-------
Of the three anaerobic digestion alternatives, the mesophilic-ther-
mophilic option is shown to have the lowest operational cost. In addition,
of the three digestion alternatives only the mesophilic-thermophilic option
produces a sludge that is extremely inert, with essentially no pathogens
and little to no odor potential. These benefits should allow the District
of Columbia to develop a strong marketing campaign to allow disposal of
dewatered digested sludge on public and private lands.
7.2. CAPITAL COST IMPACT
In order 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 and upgrade existing equipment. A detailed
cost analysis was not performed. However, the following unit operation
analysis indicates that the capital improvement cost would be the greatest
for the mesophilic system and the least for the thermophilic system. The
mesophilic-thermophilic system would lie between the two.
7.2.1. Improvements in Grit Removal
The analyses in Sections 4 and 5 indicated that 1.4 to 2.0 cubic feet
of grit per million gallons of influent flow is contained in the sludge
stream. Grit is inert, but it reduces the anaerobic digestion process
capacity by occupying tank volume. The calculations of tank volume (in
Section 6) assume minimal grit accumulation, but at the current time
approximately 25 to 30 percent of the existing digestion tank capacity
is occupied by grit. The best options are to increase grit removal before
the influent flow reaches the digesters or to increase digestor capacity
to allow for accumulation. The capital expenditures for grit removal
equipment are approximately equal for all digestion processes.
The cost for the necessary grit removal equipment has not been esti-
mated in this study, but it should be approximately equal for all three
digestion options. However, if grit removal is not provided, then the
digestion capacity would have to be increased by about 25% to allow for
accumulation.
79
-------
7.2.2. Digestion Tanks
Assuming that grit is removed upstream of the digestors, the need for
additional digestion tanks to handle the entire sludge stream is approxi-
mately six for mesophilic, four for the raesophilic-thermophilic, and none
for the thermophilic process. A preliminary analysis indicated that
operating the digesters at the thermophilic temperature would not cause
structural problems.
7.2.3. Sludge Heating Requirements
All three processes would require additional heating capacity to
handle the entire sludge stream. The mesophilic option may be satisfied
by existing in-house heat generation. The other two options would require
auxiliary steam generating equipment to allow direct steam injection
into each digestion tank operating at thermophilic temperatures.
7.2.4. Improvements in Gas Piping
The existing gas collection piping and safety devices need to be
replaced or upgraded for any of the anaerobic digestion processes. The
piping shows corrosion damage in places, and some of the gas protection
equipment is not functional.
7.2.5. Improvements in Mixing
The existing digestor mixing is believed to be deficient for several
reasons. First, the majority of the existing gas-mixing equipment was
originally installed in 1960 on a temporary basis and was not designed as a
complete tank mixing system. Secondly, the existing internal heating coils
allow build up of grit and inert material not capable of being mixed.
Improvements in grit removal, removal of internal heating coils, and an up-
80
-------
grade of the gas-mixing system to current standards would significantly
improve the mixing operation and, hence, improve the tank utilization.
7.2.6. Elutriation Capacity
The mesophilic system would require a 200 percent increase in elutria-
tion tank capacity. The mesophilic-thermophilic and thermophilic processes
both could not require elutriation.
7.3. SUMMARIZED ECONOMIC IMPACT
The operating cost analysis has developed a sludge handling unit cost
for the five options considered in this study. The developed costs are not
necessarily "accurate" (in the sense that all minor cost factors have been
included), but they are consistent and reflect the comparative costs of the
options. The unit costs are presented in Table 7-2, which also includes a
comparative presentation of capital costs for the five options.
A qualitative analysis of the disposal factors also is summarized in
Table 7-2. This analysis is reflected to some extent in the disposal cost
analysis for the current and projected 1981 options, in that disposal costs
of undigested sludge are included. The anaerobic digestion options for the
entire sludge stream will result in a reduced disposal cost. The charac-
teristics of the sludge product as indicated in the disposal factors in
Table 7-2 will determine the eventual disposal cost.
81
-------
TABLE 7-2. SUMMARY OF OPERATING AND CAPITAL COST REQUIREMENTS FOR SLUDGE HANDLING OPTIONS
00
to
Option Operating cost Capital cost items0
1.
2.
3.
4.
5.
($/MGIF) Grit removal Digestion tanks
Current 118.85 NA NA
(January to
June 1980)
Projected 72.12 NA NA
1981
Mesopliilic, 48.18 Yes, even Yes
all sludge
Mesophilie- 41.44 Yes, even Yes
thermopliilic,
all sludge
Thermophilic, 45.41 No No
all sludge
Sludge heating Gas piping Mixing Elutriation
NA NA NA NA
NA NA NA NA
Yes Yes Yes Yes
Yes, + Yes Yes No
Yes, + Yes Yes No
Current and projected options do not include the digestion of all sludge generated.
3
MGIF = million gallons of influent flow.
Yes = a capital expenditure is required or recommended to handle the entire sludge shown.
No = existing equipment is adequate or not required to treat the entire sludge stream
NA = not applicable to the evaluation.
Even = approximately the same capital would be required
Number = the number of digestors required.
+ = a significantly greater capital expense than for the other options is expected.
-------
REFERENCES
U.S. EPA 1979. Process design manual: sludge treatment and disposal.
EPA-625/1-79-001. U.S. Environmental Protection Agency, Cincinnati, Ohio.
83
-------
APPENDIX A
JANUARY TO JUNE 1980
SUMMARY OF AVERAGE MONTHLY OPERATING
DATA ON BLUE PLAINS SLUDGE MANAGEMENT OPERATIONS
The data shown in Appendix A were obtained from operating log sheets and
from conversations held with Mr. Ed Jones, Mr. Steve Bennett, and Mr. Walt
Baily.
The purpose of this analysis was to perform solids mass balances around each
unit process and to segregate the type of solids—primary, waste activated,
lime, ferric chloride—involved in each situation. This is possible at Blue
Plains since extensive process stream testing is conducted on a regular
basis. Analysis of six months of operating data indicates the following:
1. On a total solids basis, mass balance calculations around each unit
process are generally within 5 to 10 percent of closing.
2. Existing flow meters should be maintained and calibrated on a
regular basis. Many of the existing flow meters have been out of
service for some time. Flow calculations are approximated using
either pump strokes, pump curves, or changes in tank liquid levels.
This type of hydraulic data makes for difficult process control and
is believed to be responsible for some of the difficulty in closing
the mass balances around each process.
3. Numerous duplication of data on various log sheets is currently
required. In many cases different data are indicated for situa-
tions in which the data should be identical. It is recommended
that a review of all existing log sheets be conducted, that they be
consolidated where possible, and that better quality control be
maintained over the logging of data.
4. Some of the solids analyses are done by total solids and some by
total suspended solids. Since total solids is equivalent to dis-
solved solids plus suspended solids, a difficulty exists in doing
solids mass balances.
This difficulty arises for several reasons:
o Dissolved solids do not concentrate, but do increase in con-
centration as the sludge moves through the sludge treatment
process.
o Analyses of thickened or dewatered sludges show that the
amount of dissolved solids is almost insignificant compared to
the suspended solids.
A-l
-------
Analyses of clarified liquors, filtrates, or elutriates show
that suspended solids normally comprise only 15 to 50 percent
of the total solids, while dissolved solids normally comprise
the majority.
A-2
-------
TABLE A-l. SUMMARY OF RECENT AVERAGE MONTHLY HISTORICAL DATA ON RAW SLUDGE QUANTITIES
Year
1980
Primary sludge
Total dry sol ids
r Month
) Jan
Fob
Mar
Api
May
Jllll
Vo 1 ume ,
MGD
4 2
3.8
3 7
4.4
4 5
3 0
Percent
0.55
0.72
0.61
0 78
0 62
0.92
Tons .per
day
98
116
96
146
119
117
Total dry
volatile sol ids
Percent
volati le
77
81
78
78
79
85
Tons per
day
75
94
75
114
94
99
Volume,
MOD
2.7
3 1
2.9
3.4
2.8
2.7
Waste activated
Total dry sol idsc
Percent
dry
sol ids
0.91
0.72
0.67
0.73
0.68
0.88
Tons per
day
103
94
82
104
80
101
sludge
Total
volatile
PC remit
volatile
66
68
66
66
62
64
dry
sol ids
Tons per
djy
68
63
r,4
69
65
65
Data obtained from Mr Walt Rally and Blue Plains operating records.
There .ire strong indications that significant amounts of grit are in the primary sludge. Detailed analysis ol volume w.is
not made, but currently is estimated at 300 to 350 cubic feet per day or approximately 30,000 to 35,000 pounds per day
Iron is added for phosphorus removal. The iron - phosphate sludge contribution is between It to 15.5 percent of the total
mass
Density of this material is 8 5 pounds per gallon
Density of this material is 8.39 pounds per gallon.
-------
TABLE A-2. SUMMARY OF RECENT AVERAGE MONTHLY DATA ON GRAVITY Til ICKEN1NO"
Total gravity thickener inflow
Tul.il dry solids0
Year
I'J80
HoiiLh
.Lin
Fell
Mai
A,..
May
Jim
.liil
Vo 1 time ,
MOD
16 0
15. Q
15.7
15 7
15.3
16.0
16.9
mg/l
1600
1915
1600
2350
I960
184-i
Tons
per day
107
127
105
154
125
123
Total
volatile
Percent
volatile
74. /
67 4
69.4
66 5
72.0
69.7
67.6
dry
solids
Tons
per day
74 7
82.2
70.8
99.1
83.5
82.8
Gravity thickener under (low
To La I dry sol ids
Vo 1 nme .
HGI)
328
437
.165
.521
.135
370
Percent
7.3
6.7
6.7
6 9
6 4
7.9
'Inns .
l>er_da/
105
129
IOR
158
120
129
Tnt a 1
vnl.il I In
Pcrrriil
volatile
79
75
70
65
66
71
dry
sol ior ilny
74
97
7f.
101
70
92
. .
Data obtained from Mr. Steve Bennett and Blue Plains operating data. Data jre not avail.ilile on gravity thickener ovetflow
Tnl.il gravity lliirkrncr inflow consiits of flow Irom primary danders, dilution water, approximately 50 percent ol UIK eliitrialiou tank ovcillnw
C Existing d.ila arc* tnnsidered lo be incorrect because they indicate a sludge oass greater th.in the output being disposed ol At this Unit,
tin: niiinlicrs hliown liir tons per day arc based on primary sludge production (Table- A-1) * the suspended solids LontrilmLion of
the dilution w.-iter (0.17 to 1 0 tons per day) + one half of the total solids in the elutriate overflow (Table A-5)
•
Density of this material is 8.B pounds per gallon.
-------
TABLE A-3. SUMMARY OF RECENT AVERAGE MONTHLY DATA OH DISSOLVED AIR FLOTATION THICKENING
Total dissolved air notation
Year
i?8o
Mould
Jan
Nar
A|ir
H.iy
Jim
Volume,
MCI)
2 7
3 1
2 9
3 4
2 8
2.7
Toldl dry
PcrrenL
dry solids
0 91
0 72
0 70
0.71
0 68
0.89
sol ids
Tons
per day
101
91
85
103
79
100
Lliickc-iicr inflow
Total
volatile
Pcrn-iil
vo 1 a 1 1 1 e
66.0
67 7
65 9
66.0
62 0
64.0
dry
sol ids
Tons
per day
6B
63
56
f.R
49
64
Volume,
MCI)
0 665
0.5/0
0.496
0.582
0.448
0.508
Thickened m.ilciial
Total dry
Percj-iil
diy solids
3.7
3.9
4.1
4.2
4 2
4.7
solids
Tons
per day
102 7
92 7
84.8
101 9
78 4
-------
TABLE A-3.
SUMMARY OF RECENT AVERAGE MONTHLY DMA OH DISSOLVED AIR FLOTATION THICKENING0 (continued)
Subnatant Flow
Total dry
suspended solids,
Year Month
1080 Jan
Feb
Hat
Apr
May
Jun
Volume,
HOD
Z 6
2.9
2.2
7 7
3.1
S.5
EB/I
28
21
21
37
'•4
17
Tons
per day
0.3
0.3
0.3
1.2
0.6
0.4
Data obtained from Mr. Steve Bennett and Blue Plains operating data.
-------
TABLE A-4. SUMMARY OF RECENT AVERAGE MONTHLY DATA ON ANAEROBIC DIGESTION
Year
1979
1980
Primary slmlee
Ion th
J a n
Fell
Mar
Apr
Hay
Inn
Jill
A"g
Sept
OiL
Nov
lire
.Ian
Full
Mnr
Apr
May
.Inn
Vi> 1 nine ,
MCI)
0.242
0. 188
0 . 284
0 301
0. 199
0 225
0 194
0.216
0 177
0 U>6
0 177
0 195
0.228
0.227
0.228
0. 196
0 222
0.169
Tula I dry
IVrtenl
dry solids
8 0
8 6
8 1
7.3
9.8
8 8
7 3
7 8
7
69.2
66 9
63.8
63.7
66 3
,lry
sol nls
Tons
JKT d.iy
29 4
2r> 0
IS 4
20 4
IS 4
28 i
28 8
»2 1
17 1
35 1
_ _ — —
18 5
14 0
il 1
1J.S
10 6
27 0
41 1
Data olitJincil from Mr Steve Bennett and Blue Plains operating data.
(continued)
-------
TABLE A-4. SUMMARY OF RKCENT AVERAGE MONTHLY DATA ON ANAEROBIC DIGESTION3 (continued)
Year_
1970
f
CO
1980
Hniith_
.Lin
Krl>
M.II
Apr
H.iy
Jim
In I
AiiR
Sopl
Oct.
Nov
l)er
liin
Fel>
Mar
Ap.
May
Jim
Coinliincil digester input
Vn 1 nine ,
MGD
0 416
0 158
0.384
0 461
0.442
0 431
0 42.)
0 455
0.460
0.441
0.357
0.364
0 503
0 521
0.465
0.464
0 485
Total
Percent
diy solids
7.1
7.0
7.6
6.3
7 4
7 1
6.0
6.4
5.9
5 6
5.4
5.9
5.2
5 2
5.3
5.2
5.8
dry solids
'Inns
per il.iy
123.5
104.3
121.6
122.0
116.0
127.3
105. R
121. 1
113.6
102 6
79.7
90.4
108.3
113.8
103.5
101 6
117.7
Total
volatile
IVrcrnL
volatile
67 6
71 6
71.7
73.0
69.1
67 9
64. 1
64 2
62.1
65. J
70.5
75.8
72.6
68. 4
65.2
65.1
68.5
dry
solids
Tuns
per day
83.5
74 7
87.2
89 1
94.0
86.4
6/ 8
77 7
70.6
67.0
56.2
68.5
78.7
77.8
67.5
66.2
80.6
W.isLc activiitril
sludge
I'tirrcnl
by weight
35
35
19
25
40
35
44
42
52
51
15
23
41
44
46
42
51
t ruction
Price-ill
liy vii 1 Hint:
42
48
26
15
55
48
54
51
62
(>t
45
17
r>r>
56
58
52
f>5
Dal.i nhi nineil From Mr Steve lleimrlt jnd Blue Plains operating data
(cniiLiiiucil)
-------
TABLE A-4. SUMMARY OF RECENT AVERAGE MONTHLY DATA ON ANAEROBIC DIGESTION (continued)
Digested sLmlge
Year Mon III
1979 )7
0 364
0 S03
O.S2I
0 465
0 464
0 485
Total dry
Percent
dry solids
3.3
3 3
3.4
2 9
3 2
3.8
3.8
3.6
3 9
3.6
3.6
2 7
2 3
2.6
2.6
2.8
3.1
3.1
3.1
sol ids
Tons
per day
57.2
49.3
54 4
56.0
59.0
68 3
67.0
68.3
74.8
66 2
47.7
40.2
34.9
54.5
56 5
54 3
60.0
62.5
Total
volali le
Percent
volatile
54 2
54.6
54.2
55.7
55 5
53.3
55.3
54.3
52.3
53.8
56 5
56.6
57.6
55.3
55 1
56.1
55.5
54.8
55.3
dry
solids
Tons
per day
31.0
26.9
29.5
11.2
32.7
36.4
37.1
37 1
39 1
35 6
27.0
22.7
20 1
30 2
31 1
30.5
33.3
34.2
Togo.."
95
96
94
95
96
95
95
95
94
95
95
94
92
91
93
90
9J
95
93
Volatile
acids,
mg/l
168
233
204
158
172
260
207
183
176
166
180
225
142
126
284
338
371
323
329
Alk.C
nig/1
5629
6229
5')56
5313
5868
6488
657H
5742
5238
5209
5099
7272
6784
7624
8403
8914
8328
8366
8140
Nil -N
rag/1
1107
1451
1428
1374
1400
1568
1302
Gns j
produced ,'
cubic feet
xlOOO
912
846
834
999
12)6
1205
R86
749
797
849
808
879
1061
1246
1123
888
906
994
928
Digester gas
co2
rontent ,
percent
32.7
31 4
34.4
31 8
32 6
32.6
12.8
29. 1
33 0
31.9
34.9
35.0
35.5
34.3
35 I
34 6
34.2
34.9
34.8
lias prodiHcd
per pound of
void I lie snl ids
reduced
8 7
8.8
7 2
8 6
9.9
12 (I
14 4
9 I
12 6
11 5
9 6
1 1 0
12 8
12 0
12 0
13 8
10.7
Data obtained from Sieve Bennett and Blue (Mains operating data.
Tempornture measured on recycle sludge before entering heat exchanger.
AIK - alkalinity. Alkalinity is measured on entire solids mass rather than on sulinatant of sludge.
Blue Plains is in the process of signing » contract with Naval Research to sell all excess digester gas for $0.95 per 1000 rnliir Icet.
-------
TABI.K A-5. SUMMARY OF RECENT AVERAGE MONTHLY HISTORICAL DATA ON ELUTRIATION TANK OPERATIONS
Anacrnhic ill (jesters
ToLal dry solids
Year Nonlli
I'JHO J.in
Kcl)
Mm
Apr
May
Illll
Dnld oli I aincd from
A|i|>rox mini <:!y hall
!*• C Data indicate LliaL
Vo 1 lino ,
NCI)
0.397
0.509
o 509
0.666
0.448
0 492
Percent
dry solids
2.6
2.4
2.4
2 7
3 0
3 3
Tons
per
-------
TABLE A-5. SUMMARY OF RECENT AVERAGE MONTHLY HISTORICAL DATA ON ELUTRIAT10N TANK OPERATIONS0 (continued)
Elutriation underflow Lo dewatoring
Elutriation overflow
Total diy solids
Year
19 BO
" Data
b TI..I,
HonLh
Jan
Krl>
Mar
A|>r
Nay
Jun
obtained from Mr.
Volume,
MGU
0.227
0 279
0.309
0 187
0 207
0.241
Steve Bennett and
nrmallv in fsunnnfipl
Percent
dry solids
5.4
4.8
3 6
5.5
5.4
5.2
Blue Plains operating
1 to return to nnmarv
Tons d
per day
54
59
49
46
52
55
records.
treatment bv eravitv
Volume,
MGD
2 44
2.57
2.52
2.56
2.41
2.46
' flow. At the preset
h
Total dry
sol ids
n-B/1
1800
I'J74
1652
1517
12f>4
1208
it tune, only par
Tons
per day
IR 3
21 1
17 4
16 2
12.6
12.4
t of
the overflow flows by gravity to primary treatment The other part is pumped to gravity thickening. Theie are no flow meters
on cither line, and it is not possible to tell how much flow is going to gravity thickening or to primary treatment.
c Suspended solids for the same time period in mg/1 were: Jan-295; Feb-874; Mar-1048; Apr-684; May-479; Jnne-559.
Density of this material is 8 85 pounds per gallon.
-------
TABLF. A-6. SUMMARY OF RECENT AVERAGE MONTHLY DATA ON RAW SLUDGE DEUATEKING°
M
Primary sludge
From gravity thickener
Waste activated sludge from 1,
dissolved air flotation thickeners a
Total dry solids
Year Month
1980 Inn
rcl>
Mar
Apr
May
Jim
a
b
c
-------
TABLE A-6. SUMMARY OF RECENT AVERAGE MONTHLY DATA ON RAW SLUDGE DBWATERJNG (continued)
Uewatered raw sludge off vacuum filler
Yeor
1979
1980
Non Hi
Aug
Sept
Oct
Nov
Dec
Jan
Mar
Apr
May
.Inn
Jill
Wei ions
per day
948
490
910
635
695
Total dry
Pei cenl
dry solids
19 6
20 7
19.9
17.6
17.2
21.6
21.1
21 1
22.6
22 1
sol ids
Tons .
, d
per day
163
106
192
134
157
Tolal
volali Ic
Percent
volatile
44 6
43.3
44 8
48. 1
46.2
41.0
40.9
40 2
42 1
41 4
vacuum filler fillralc
dry
solids Total dry solids
Tons Volumu, Tons ,
per day MOD mg/1 per day
75 0.499
44
78 0.574
54
66 0 440
washwalui
Tolal diy solids
Volume-, 'Ions
MGD nig/I per day
I _ _ .-...— ...--. __.
1-1 a
l*J Da la obtained from Mr. Steve Bennett and Blue Plains operating data.
Data base is extremely limited.
Wasliwater volume is significant but no data are taken on this stream Suspended solids are not believed to be significant
See footnote c on Table A-8.
Analysis based on several samples indicated that total solids were about 500 ing/1 and that suspended solids about 250 mg/l.
llascd on limited data, it is estimated that the total solids in the filtiate average 2 tons per day and can be considered insignificant.
-------
TABLE A-7. SUMMARY OF RECKNT AVERAGE MONTHLY DATA OM DIGESTED SLUDGE DBUATERING*
From clutnation
Vn 1 nine ,
Year Month MCI)
Total (ley
Percent
Dry Solids
solids,
Tons
per day
Dry ferric
chloride added
Tons
per day
I'm Ang
Sept
On
Nov
Her
1980 .Ian 0 227
a
b
c
d
c
Fch 0.279
Mar 0 309
Apr 0.187
May 0 207
.Inn 0.241
Jill
Data obtained from
Data indicate that
5 4
It 8
3 6
5.5
5.4
5.2
54
59
49
46
52
55
Mr. Steve Bennett and
solids contributed by
Density of tins material is 8.83
Sunnier of 1980 Blue Plains pays
pounds
$0.0825
10.2
11.9
10.8
10.0
10.7
10.3
Blue Plains
filtrate are
per gallon.
per pound of
Dewatered digested sludge to disposal
Total
to
dry solids
disposal
Total dry
volatile solids
To disposal
Vacuum liltei filtrate
Total diy
so 1 ids
Dollars. Wet tons Percent Tons Perrent Tons Volume, _ Tons
per day per day dry solids per day volatile per day MGD UIR/ 1 per day
740
864
784
726
777
748
operating data
insignificant
368
361
345
290
289
293
395
337
302
314
328
294
.
compa red
17.8
17.9
16.3
16 1
16.0
15 8
15.6
16.7
16.8
16.9
16.8
65.5
64.6
56.2
46.5
49.9
62.4
52.6
50.4
52.7
55.4
49.4
to total mass of
iron (Fe), which comprises 44
percent of
49.5 32
47 . 7 30
49 6 27
54.3 25
56.3 26
57.2 35
54.1 28
51.6 26
51.9 27
52.5 29
53.8 26
solids included.
.4
.8
9
0
4
.7
.4
.0
.4
.1
.6
0.136
0 205 9600 8.2
0.192 8800 7.0
0.130 12000 6.5
0.144 9200 5.5
0.175 8200 6 0
ferric chloride (Fe CI2).
Suspended solids [or this same time period in ng/1 were: Jan-185; Feb-66; Mar-110; Apr-103; June-270.
-------
TABLE A-8. SUMMARY OF RECENT AVERAGE MONTHLY HISTORICAL DATA ON RAW SLUDGE DISPOSAL
\Jl
Break down of total dry solids to disposal
Year
1980
Month
Jan
Feb
Mar
Apr
Hay
Jim
Jill
Raw sludge
hauled from
plant. D'c
Wet tons
per day
1153
1047
758
1004
862
762
716
Trucking and ,
disposal cost,
Dollars
per day
41,175
37,200
26,362
35,587
30,262
26,512
24,787
Total dry solids,
to disposal ,
Percent
dry solids
as trucked6
14.1
14.5
14.0
19.1
15.5
20.6
Tons
per day
163
152
106
192
134
157
Raw
sludge
solids.
tons
per day
112
106
72
150
93
104
From
lime addition
tons ,
per day
41
36
28
36
35
36
From
ferric chloride addition
tons
per day
11
10
6
6
6
17
Data obtained from Mr. Ed Jones, Mr. Steve Bennett, and Blue Plains operating records.
At the time of this report - Sept. 1980 - dewatered raw sludge was being disposed of as follows:
o Approximately 100 wet tons per day to Beltsville at an approximate trucking cost of $30/wet ton.
o Between ISO to 200 wet tons per day to HT1 for composting at an approximate trucking cost of $30/wet ton. Contract on a
yeai to year basis. Could possibly go up to 400 wet tons per
-------
TAIII.E A-9 SUMMARY OF KKCKNT AVF.RAKK MUNTII1.Y HISTORICAL DATA ON I)IfiKSTKI) SI.UUGK DISPOSAL
Total dry sol ids
lo disposal
Break down of_ |-oLJlJ_|lr.y sojids
Year
1979
1980
HonLli
Ang
Sept
Oct
Nov
Dec
Jan
K.-l.
Mar
A|ir
Mjy
Jim
.liil
Deviate red
digested
slmlgp,
we I Lous
|ici day
168
361
145
290
28V
291
395
337
302
116
128
294
Triu king and .
disposal cost,
do 1 1 a rs
per
4.395
5,925
5,055
4,510
4,710
4,920
4,410
pert nil
dry solids
17.8
17.9 '
16.3
16 1
16 0
15 8
15 6
16.7
16 8
16.9
16 8
tons
pi>r day
65.5
64.6
56 2
46.5
49 9
62.4
52.6
50.4
52.7
55.4
49.4
Digested Sol i
-------
APPENDIX B
ANALYSIS OF AVERAGE PRIMARY AND SECONDARY SLUDGE PRODUCTION
PER MILLION GALLONS OF INFLUENT FLOW
B.I. PRIMARY SLUDGE
TABLE B-l. ANALYSIS OF AVERAGE SLUDGE
PRODUCTION PER MILLION GALLONS OF INFLUENT FLOW3
(January through June 1980)
Primary sludge. Primary sludge
production, 'C volatile solids,
Month T/MGIF0 percent
January
February
March
April
May
June
Average
Standard deviation
98/331
116/325
96/345
146/332
119/327
117/330
= 0.296
= 0.357
= 0.278
= 0.440
= 0.364
= 0.355
0.348
0.057
77
81
78
78
79
85
79.7
2.9
Average dry ton of volatile primary sludge production per MGIF is:
(0.348 ± 0.057) T/MGIF (0.797 ± 0.029) percent volatile
= (0.348)(0.797) ± (0.797) (0.057) + (0.348)2(0.029)2
= 0.277 ± 0.047 VT/MGIF
All data taken from Appendix A, Table A-l.
Sludge as withdrawn from the primary clarifiers and pumped to the
gravity thickeners.
Q
It is estimated that grit equal to 1.5 to 2.0 cubic feet per million
gallons of influent flow at 100 pounds per cubic foot is included with
the primary sludge.
B-l
-------
MGIF = million gallons influent flow.
T/NGIF = dry tons of solids per million gallons influent flow.
VT/MGIF = dry tons of volatile solids per million gallons influent
flow.
B.2. SECONDARY SLUDGE
B.2.1. Current Conditions
TABLE B-2. ANALYSIS OF AVERAGE SECONDARY SLUDGE
PRODUCTION PER MILLION GALLONS OF INFLUENT FLOW3
(January to June 1980)
Secondary sludge Secondary sludge,
production, Volatile solids,
Month T/MGIF percent
January
February
March
April
May
June
Average
Standard deviation
103/331
94/325
82/345
104/332
80/327
101/330
= 0.311
= 0.289
= 0.238
= 0.313
= 0.245
= 0.306
0.284
0.034
66
68
66
66
62
64
65.3
2.1
Average dry tons secondary sludge, volatile solids production per MGIF
is:
(0.284 ± 0.034) T/MGIF (0.653 ± 0.021) percent volatile
= (0.284)(0.653) ± (0.653) (0.034) + (0.284) (0.021)
= 0.185 ± 0.022 VT/MGIF
a All data taken from Appendix A, Table A-l.
Sludge as withdrawn from the secondary clarifiers and pumped to the
dissolved air flotation thickeners.
B.2.2. Future Conditions
Increase Phosphorus Removal —
Effluent limitations on phosphorus will be reduced from the current 1.1
mg/1 average to the new NPDES permit requirement of 0.53 mg/1. This will
be accomplished by increasing the current iron dosage. Past records
indicate 3.8 pounds of chemical sludge produced per pound of P removed by
iron, therefore, every million gallons of influent will generate an
additional:
(1.1 - 0.53)»g/l P (8.34)(1 HC)
B-2
-------
It is assumed there are no volatile solids.
Multi-media Filters —
Within the year the new multi-media polishing filters will be in operation.
It is estimated that these filters will reduce the suspended solids being
discharged from an average of 15 to 7.5 mg/1. Therefore, every million
gallons of influent will generate an additional:
(15.0 - 7.5)mg/l (8.3A)(1 MG) = 0'°31 T/MGIF
It is assumed that 70 percent of the solids will be volatile.
Nitrification--
Within several months the nitrification system will be completely
operational. At a minimum, this system will have to reduce current total
kjeldahl nitrogen (TKN) levels in the plant effluent from an average of
13.8 mg/1 to the NFDES permit level of 5.3 mg/1. Assuming a biological
yield coefficient of 0.1 pounds of solids produced per pound of TKN
reduced, the additional dry solids generated would be:
(13.. - 3.MM/1 (..MXIW • °-0"
It is assumed that 70 percent of the solids will be volatile.
Assuming no change in the standard deviation, the new volatile solids
content would be:
(0.185 + 0 + 0.0313(0.7) + 0.0035(0.7) (100) ,. .
0.284+0.009+0.031+0.004 ~ J Percent
B-3
-------
APPENDIX C
ANALYSIS OF GRAVITY THICKENER OPERATION
There are six 65-foot diameter units. Surface area of each unit is 3,318
square feet. Surface area is 19,908 square feet. All operational data
taken from Appendix A, Table A-l and A-2.
TABLE C-l. AVERAGE MONTHLY HYDRAULIC AND SOLIDS LOADING RATES
AVERAGE MONTHLY HYDRAULIC AND SOLIDS LOADING RATES
(January through June 1980)
Month
Hydraulic loading rate,
gallons per day per square
foot of tank surface
Solids loading rate,
gallons per day per square
foot of tank surface
January
February
March
April
May
June
804
799
789
789
769
804
10.7
12.8
10.5
15.5
12.6
12.4
TABLE C-2. AVERAGE MONTHLY THICKENING PERFORMANCE
(January through June 1980)
Month
Thickened sludge volume,
gallons thickened sludge
per million gallons of
plant influent
Thickened sludge
concentration,
percent
solids
January
February
March
April
May
June
Average
Standard deviation
328,000/331 =
437,000/325 =
365,000/345 =
521,000/332 =
435,000/327 =
370,000/330 =
991
1,345
1,058
1,569
1,330
1,121
1,236
218
7.3
6.7
6.7
6.9
6.4
7.9
7.0
0.5
C-l
-------
No relationship between hydraulic loading rate or solids loading rate
versus thickened sludge concentration could be established.
TABLE C-3. ANALYSIS OF EFFECT OF GRAVITY THICKENING ON AVERAGE PRIMARY
SLUDGE PRODUCTION PER MILLION GALLONS OF INFLUENT FLOW
(January through June 1980)
sludge. Thickened sludge
production, ' volatile solids,3
Month T/MGIF percent
January
February
March
April
May
June
Average
Standard
105/331
129/325
108/345
158/332
120/327
129/330
deviation
= 0.317
= 0.397
= 0.313
= 0.476
= 0.367
= 0.391
0.377
0.060
79
75
70
65
66
71
71
5.3
Average VT/MGIF as removed from the gravity thickener.
(0.377 ± 0.060) T/MGIF (0.71 ± 0.053) percent volatile
22 22
= (0.377K0.71) ± (0.71) (0.060) + (0.377) (0.053)
= 0.268 ± 0.047 VT/MGIF
Sludge as withdrawn from the gravity thickeners.
It is estimated that grit equal to 1.5 to 2.0 cubic feet per million
gallons of influent flow at 100 pounds per cubic foot is included with
the primary sludge.
MGIF = million gallons influent flow.
T/MGIF = dry ton of solids per million gallons influent flow.
VT/MGIF = dry ton of volatile solids per million gallons influent
flow.
C-2
-------
APPENDIX D
ANALYSIS OF DISSOLVED AIR FLOTATION (DAF) THICKENER OPERATION
There are 18, 20-foot wide by 55-foot long (effective length) units.
Effective thickening surface area of each unit is 1,100 square feet. Total
effective surface area is 19,800 square feet. These units operate
continuously at 30 to 34 pounds of secondary solids per day per square foot
of effective thickening area. Polymer is required to operate at this
loading rate. Polymer usage is 7 to 10 pounds of dry polymer per dry ton
of feed solids. All operational data taken from Appendix A, Table A-l and
A-3.
TABLE D-l. AVERAGE MONTHLY THICKENING PERFORMANCE
(January through June 1980)
Thickened sludge volume, Thickened sludge
gallons thickened sludge concentration,
per million gallons of percent
Month plant influent solids
January
February
March
April
May
June
Average
Standard
665,000/331 =
570,000/325 =
496,000/345 =
582,000/332 =
448,000/327 =
508,000/330 =
deviation
2,009
1,754
1,438
1,753
1,370
1,539
1,644
239
3.7
3.9
4.1
4.2
4.2
4.7
4.1
0.3
Since float concentration is not expected to change in the future, the
additional secondary solids generated in the future (Appendix B) will
increase thickened sludge volume by approximately the same amount or 15
percent (from 1,644 ± 239 to 1,900 ± 239 gallons per million gallons of
influent flow).
At 30 pounds of dry secondary solids per day per square foot of effective
thickening area, each DAF can thicken:
30 pounds 1100 square feet _ 33,000 pounds
day-square foot x unit day-unit
D-l
-------
In the future, each million gallons of influent flow will contribute
0.328 ± 0.034 tons (656 ± 68 pounds). Assuming a value of 656 + 68 or 724
pounds per million gallons of influent and an average daily flow of
334 million gallons per day (MOD), then the minimum number of DAF
thickeners required to be operating is:
1 unit 724 pounds 334 MOD _ g -t
33,000 pounds X 1 MGD X 1 "
D-2
-------
APPENDIX E
ANALYSIS OF LITHIUM CHLORIDE (LiCl) TRACER STUDIES
BLUE PLAINS DIGESTERS
E.I. MECHANICAL DATA
Inside diameter of each digester - 84 feet
Cone depth - 13 feet
Average cylindrical height - 22.1 feet
Theoretical tank volume
(42) (3.1416K22.1 + 13/3) = 146,488 cubic feet
= 1,096,000 gallons
Theoretical weight of digester fluid - 9,140,000 pounds
E.2. TEST PROCEDURE
Each day three digesters were taken off line and 6 pounds (Ibs) of
LiCl added in approximately four equal fractions at manholes spaced about
25 feet from the center of the digester. On digester No. 1, two of the
manholes could not be entered, so one-fourth of the LiCl was added at a
manhole located on a line between the two manholes that could not be used,
and one-fourth at a location adjacent to the wall of the digester. On
digester No. 9, one manhole could not be entered, so one-fourth of the LiCl
was added through a manhole at the center of the digester instead. Mixing
was accomplished by means of the regular gas-mixing system and with a
recycle of approximately 1,000 gallons per minute (GPM) by means of a pump
for one hour before the sampling time. Samples were taken before the
addition of the LiCl (0 time) and at 3, 6, and 8 hours after the addition.
Gas compressors were not available on digesters 9 and 10, so mixing in
these digesters was accomplished by means of the 1,000 GPM recycle pump
running continuously during the 8-hour test period.
Samples were analyzed for lithium by means of an atomic adsorption
instrument, using a graphite furnace. The available volume was calculated
from the difference in concentration between the 0-hour and 8-hour samples
resulting from the addition of the 6 Ib dose of LiCl.
E.3. RESULTS
The results of the tracer studies are presented in Table E-l.
E-l
-------
E.4. DISCUSSION
It is apparent that mixing was a problem, as indicated by the fact
that in digesters 3, 4, 6, 9, and 11, the 8-hour and 6-hour concentrations
differed by more than 10 percent, indicating that equilibrium may not have
been reached.
TABLE E-l. LITHIUM TRACER STUDY RESULTS
Lithium
Digester 0-hr
1
2
3
4
5
6
7
8
9
10
11
12
0.00
0.16
0.29
0.52
0.00
0.16
0.39
0.46
0.03
0.19
0.47
0.07
3-hr
0.95
1.15
2.11
4.15
1.50
1.16
2.37
2.17
14.13
1.09
18.20
7.15
Concentrations,
6-hr
0.59
1.13
1.64
1.82
1.20
1.14
1.57
1.00
3.00
0.96
1.50
1.73
mg/1
8-hr 8-hr - 0-hr
0.55
1.09
1.32
0.84
1.20
1.45
1.43
1.09
1.37
0.98
1.12
1.59
0.55
0.93
1.69
0.32
1.20
1.29
1.04
0.63
1.34
0.79
0.65
1.52
Volume ,
gallons
x 1000
1,310
775
699
2,251
600
558
693
1,143
538
912
1,108
474
Usable
volume,
percent
119
71
64
205
55
51
63
104
49
83
101
43
The value for digester No. 1 is also suspect. As discussed above, a
portion of the LiCl was added near the tank wall of the digester, rather
than at the more central addition points used in the other digesters. The
very low concentration of LiCl found in digester No. 1 would indicate that
mixing was not adequate in the area near the tank wall, and that a portion
of the LiCl was not recovered.
The distribution of recycle to the digesters appears to be good, based
on the 0-time readings for digesters 4, 8, and 11, the last three tested.
The content of LiCl in the digesters before addition of LiCl was 0.52,
carried to the digesters by the recycle from the other digesters.
The usable-volume values obtained in these tests exclude portions of
the tank volume occupied by grit and also those areas where mixing was so
poor that the LiCl never reached them. The results of digester No. 1
indicate that this may be a factor in the areas of the tanks near the
walls. It is also of note that 4 of the 9 gas-mixing tubes in the No. 1
digester were inoperative. It should also be noted that a scum layer,
which may range from 0 to 12 inches, will also be excluded from the usable
volume.
Lithium chloride appears to be quite satisfactory as a tracer material
for this test. More reliable results would be obtained if the digesters
E-2
-------
were kept off-line for a longer period to allow more complete mixing. A
test where one slug of LiCl was added to the recycle line would allow a
determination of the recycle distribution to all tanks, and a determination
of usable volume for the entire system, but a test of this type would also
have problems in distinguishing between areas occupied by grit and areas of
very poor mixing; to make this determination, longer-term off-line tests
are probably required.
The system was simulated by computer analysis, using an average usable
digester volume of 700,000 gallons. The values predicted by the simulation
agreed very well with the analyzed values through the end of the third day
of testing, but the analyzed values were about 20 percent low during the
fourth day. It is not known what problems could have caused this dis-
crepancy during the last tests, those of digesters 4, 8, and 11.
E.5. COMPUTER SIMULATION
A computer program was developed to simulate the behavior of lithium
chloride during the test period. For the purposes of the simulation, the
digesters were divided into four groups of three digesters each, in
accordance with the test procedure. Each group was assumed to have a total
volume of 2.1 million gallons (700,000 gallons per digester) and a recycle
rate of 1,500 gallons per minute, (500 GPM per digester). The flow of
fresh sludge into the group and treated sludge out of the tanks was assumed
to be 1/15 of the total volume per day. The program calculated the esti-
mated LiCl concentrations each half hour; a subroutine was set up to cal-
culate the changes in concentration that would take place because of the
removal of tank contents by the recycle, the addition of recycle sludge
that was composed of mixed sludge from all four groups, the removal of
treated sludge from the system, and the addition of fresh sludge. The main
program in turn applied this subroutine to each group of digesters for each
half-hour time period, noting the change in concentration of each group of
the combined recycle, taking into account the additions of LiCl to a dif-
ferent group each day, and the fact that one group was off-line for an
8-hour period each day. Results were printed out from 9:00 AM (time of
LiCl addition and removal of the group from line) and 5:00 PM (the time
that the group was placed back on-line).
The results of the simulation are presented in Table E-2. It can be
seen that there was good correlation between the concentrations predicted
by the model and the field results until Sunday at 5:00 PM.
Both the 9:00 AM and the 5:00 PM results on Monday, the last day of
the test, indicated that actual results were substantially lower than those
predicted by the model. We are unable to explain the apparent loss of LiCl
from the system indicated by the 5:00 PM, Monday recycle sample. In order
to make the computer simulation agree with the field results, it was
necessary to postulate a withdrawal of treated sludge and replacement with
fresh sludge of 3,300 GPM starting at 5:00 PM on Sunday and continuing
through 5:00 PM on Monday or some other addition of several million gallons
of fresh sludge to the system. There are no such upsets reported from the
plant. One of the digesters did overflow, but this accident took place the
next day. The weight of LiCl left (23 Ibs) from the original 100 Ib drum
E-3
-------
TABLE E-2. MODEL SIMULATION OF DIGESTER ANALYSIS
Lithium concentrations, mg/1
Day Time
Friday 5 PM
Saturday 9 AM
Saturday 5 PM
Sunday 9 AM
Sunday 5 PM
Monday 9 AM
Monday 5 PM
Digester
1 2
i'°
-------
Ibs of LiCl in the system, as compared to a total weight of 56 Ibs pre-
dicted by the model. Various explanations are possible:
o There was a large influx of fresh sludge to the system, flushing
out the LiCl. As discussed earlier, this would have had to have
been on the order of several million gallons, and no such upset
occurred.
o A mistake was made in the addition of LiCl. As discussed above,
a check of the remaining LiCl indicates that the proper amount
was added.
o The average volumes of the digesters are nearer to the theoreti-
cal 1,096,000 gallons than the assumed 700,000 gallons. This
does not appear reasonable, in that if this assumption is made,
all of the analyses for the first three days would have been in
poor agreement with the model, and such an assumption does not
fit with the individual test results.
o The missing 12 pounds of LiCl, which cannot be accounted for by
the analyses, may be concentrated in areas of poor mixing so that
the tracer, or portion of it, never entered the system.
E.6. COMPARISON OF RESULTS WITH CANADIAN TRACER STUDIES
It is of interest to compare results with those obtained by J. Smart
(1978) (An Assessment of the Mixing Performance of Several Digesters Using
Tracer Response Techniques, Research Publication No. 72, Dec. 1978, Ontario
Ministry of the Environment.) He used fluoride as a tracer on 10 anaerobic
digesters ranging from 165,900 to 1,690,000 gallons in size, using both
mechanical and gas mixing with mixing power ranging from 0.02 to 0.25
HP/1,000 gallons.
Smart found mixing to be a problem, with an average of 55 percent
usable volume, as compared to 65 percent found in our study. He found that
the tracer concentration leveled out in 4 to 6 hours, which agrees well
with our finding that in most cases the tracer leveled out in 4 to 8 hours.
He could find no correlation between the amount of dead space and the size,
age, condition, applied mixing power, or type of mixing used in the
digester.
E.7 CONCLUSIONS
The results are consistent with an average usable volume of about 65
percent of theoretical, with the dead space consisting of the grit layer,
the scum layer, and areas where there is little or no mixing.
A tracer study where the tracer is added to the recycle line, thus
feeding all 12 digesters at once, will help to resolve some of the ques-
tions about short-circuting and total hydraulic detention time.
E-5
-------
APPENDIX F
ANALYSIS OF GRIT ENTRY INTO ANAEROBIC DIGESTERS PER
MILLION GALLONS OF PLANT INTLUENT FLOW
Based on discussions with anaerobic system operating personnel:
o It takes 2 to 3 years for a digester to accumulate grit to a point
of equilibrium, that is grit no longer accumulates but passes
through the digestion tanks.
o Grit in the tank was estimated to occupy 15 to 20 percent of the
total tank volume. Since each tank has a theoretical volume of
146,469 cubic feet, then grit occupies 21,970 to 29,294 cubic feet
of tank volume.
Assuming three years for grit build-up within a tank to reach equilibrium
then:
21 970 cubic feet off grit = 2Q
(365 day/year) (3 years)
or
29,294 cubxc feet of grit cub-c d
(365 days/year) (3 years) & *
Approximately half the daily sludge production goes to anaerobic digestion
or the plant influent flow equivalent of 167 million gallons per day (MGD).
There are 12 digestion tanks and it is assumed that sludge is pumped
uniformly to all of them, therefore each tank receives raw sluge generated
from:
12?tanks = 13'9 MGD °f plant influent flow
At 20 cubic feet of grit per day per tank, each MGD of plant influent flow
is contributing:
20 cu^c feet 8rit per tank = :.4 cubic feet per HGD
13.9 MGD per tank r
At 100 pounds per cubic foot, 140 pounds or 0.07 tons of dry solids per
million gallons of plant influent flow (T/MGIF) is being contributed.
At 26.8 cubic feet it is 2.0 cubic feet per MGD, which at 100 pounds per
cubic foot is 200 pounds or 0.1 T/MGIF.
F-l
-------
APPENDIX G
ANAEROBIC DIGESTION HEAT AVAILABILITY AND SYSTEM HEAT REQUIREMENTS
G.I. DIGESTER HEATING CAPABILITIES
There are six double inlet, double outlet, 6 inch by 8 inch, tube and
tube heat exchangers each with a heated exterior sludge tube surface of 360
square feet. Each unit is specified as originally having a rated output
capacity of 3,000,000 BTU per hour. The original design criteria could not
be found, but knowing how the heat exchanger supplier would have designed
the units, it is believed that the 3,000,000 BTU per hour output was predi-
cated on the following:
o Sludge at each inlet (there are 12) would be at an average tempera-
ture of 80°F at a flow rate of 350 gallons per minute (gpm).
o Sludge at each outlet (there are 12) would be at an average tem-
perature of 95°F at a flow rate of 350 gpm.
o Hot water at each inlet (there are 12) would be at 180°F at a flow
rate of 200 gpm.
o Hot water at each outlet (there are 12) would be at 160°F at a flow
rate of 200 gpm.
The source of hot water was to be provided by two, 250 horsepower, low
pressure steam boilers; one, 150 horsepower, hot water boiler; and engine
jacket cooling water.
Through the years since the sludge heating system was first conceived
and installed, changes have taken place that have altered the heating
capabilities as follows:
o Sludge at each inlet averages 92 to 95°F at a flow rate of 500 gpm.
The higher than expected sludge temperature decreases the expected
thermal gradient and therefore decreases heat transfer capabili-
ties. However, the higher flow rate tends to increase heat trans-
fer capabilities.
The original heat exchanger design would have been predicated on
two standard industry assumptions:
- Sludge flow through a 6-inch sludge tube would be approximately
350 gpm.
G-l
-------
- Sludge flow into the heat exchanger would have consisted of 2
parts digested, recirculated sludge at 95°F and 1 part raw sludge
at 50°F (assumed winter conditions).
At Blue Plains raw sludge is added after the heat exchangers rather
than before. The addition is downstream because of a historical
problem with heat exchanger plugging due to rags in the primary
sludge. The required screening of plant influent, to eliminate
rags in the raw sludge has only recently been accomplished.
Even if all the available raw sludge (approximately 600 gpm) was
added upstream, the average inlet temperature would be reduced by
4°F during the winter and by 2°F during the summer. This small
decrease is due to the high digested sludge recycle flow, which is
10 times the raw sludge flow instead of the 2 times originally
envisioned.
It is not known why the recycle flow rate was increased to 500 gpm
per sludge inlet. It has a beneficial effect on the heat exchanger
in that the high velocity (5.7 feet per second) through the sludge
tubes prevents tube fouling.*
o Hot water at each inlet is at 195°F. The higher hot water tempera-
ture tends to increase the thermal gradient, therefore increasing
heat transfer capabilities.
Plant operating experience indicated that the hot water being
utilized could be increased to 195°F without causing any scaling
problems within the heat exchanger.*
o At the present time the source of hot water is provided by two, 250
horsepower, low pressure steam boilers and one, 150 horsepower, hot
water boiler. Engine jacket cooling water is no longer available.
The 150 horsepower hot water boiler is in poor condition and is
scheduled to be replaced within the next couple of years. Assuming
an overall efficiency of 80 percent for the steam boilers and heat
exchangers, about 13,200,000 BTU per hour are available. The new
hot water boiler would add an additional 3,000,000 BTU per hour.
In addition, the current construction contract calls for linking
together the steam boilers in the heat exchanger building with the
three 150 horsepower, low pressure steam boilers in the power house
and the two 100 horsepower, low pressure steam boilers in the new
grit chamber structure being built. These changes would produce an
additional 17,000,000 BTU per hour availability, but how much could
be used for sludge heating is not known at this time.
G.2. DIGESTION SYSTEM HEAT REQUIREMENTS
Digestion system heat requirements consist of two components: heat
required for raw sludge addition and heat required for radiation losses.
* Inspection of one unit during this study showed no scaling on either
sludge or hot water side of 6 inch sludge tube.
G-2
-------
G.2.1. Heat Required For Raw Sludge Addition
The amount of heat required for raw sludge addition per million
gallons of plant influent flow can be calculated as follows:
(gallons of sludge/MGIF)(Density)(T2 - T^
where:
gallons of sludge/MGIF = gallons of raw sludge per million gallons of plant
influent flow
density = density of the liquid sludge stream, pounds per
gallon
T_ = temperature to which the raw sludge stream is to
be raised, °F
TI = expected temperature of the raw sludge stream, °F
G.2.2. Heat Required For Primary Sludge Addition Only
From Appendix C, the primary sludge volume expected per MGIF is
1236 ± 218 gallons. The highest value or 1454 gallons is used for the
following calculations. Density of primary sludge stream is 8.8 pounds per
gallon. Sludge temperature to be raised to 95°F. During the winter
season, the coldest primary sludge temperature is 55°F. During summer
operation, the primary sludge temperature is 75°F.
Winter operation:
1454 gallons 8.8 pounds (95 - 55)°F
MGIF X gallon X 1
= 511,808 BTU's per MGIF
Summer operation:
1454 gallons 8.8 pounds (95 - 75)°F
MGIF X gallon X 1
= 255,904 BTU's per MGIF
G.2.3. Heat Required For Secondary Sludge Addition Only
From Appendix D, the future secondary sludge volume expected per MGIF
is 1900 ± 239 gallons. The highest value or 2139 gallons is used for the
following calculations. Density of secondary sludge stream is 8.5 pounds
per gallon. Sludge temperature to be raised to 95°F. During the winter
G-3
-------
season, the coldest secondary sludge temperature expected is 60°F. During
summer operation, the secondary sludge temperature expected is 80°F.
Winter operation:
2139 gallons 8.5 pounds (95 - 60)°F
MGIF X gallon X I
= 636,353 BTU's per MGIF
Summer operation:
2139 gallons 8.5 pounds (95 - 80)°F
MGIF X gallon X 1
= 272,723 BTU's per MGIF
G.2.4. Heat Required For All Sludge Addition
The heat input required for all sludge per MGIF is calculated by
totaling the primary and secondary requirements as follows:
Winter Summer
operation, operation,
BTU's per MGIF BTU's per MGIF
Primary sludge 511,808 255,904
Secondary sludge 636,353 272.723
TOTAL 1,148,161 528,627
G.2.5. Heat Required For Conductive/Convective Losses
The amount of heat required for conductive and convective losses from
the digestor equipment can be calculated as follows:
(U)(Area)(T3 - T^)
where:
i
U = -5=
^ = Conductance for a certain thickness of material
BTU
hour-square foot-°F
x. = Thickness of material in inches
k. = Thermal conductivity of material
BTU - inch
hour-square foot-°F
G-4
-------
A = Area of material normal to direction of heat flow in square feet
T_ = Temperature of inside boundary, °F
T, = Temperature of outside boundary, °F
Figure G-l shows a cross sectional view of one of the 12 existing anaerobic
digestion tanks. In the following analysis the assumptions made are:
Earth Cover
T
24"
_L
*'-l*
.»? Concrete Roof 10.25"
Moving, Heated, VJater
Saturated Air Space
..
,. -.1 . • ,
Liquid at 35°F
>
•V '.
'
Concrete Tank Wall
Concrete Floor
FIGURE G-l. Cross sectional view of existing anaerobic digesters
G-5
-------
1. The liquid contents of the tank are being mixed and maintained
at an inside boundry temperature of 95°F.
2. The average (several weeks) coldest outside temperature (winter
operation) is 0°F.
3. The average (several weeks) warmest outside temperature (summer
operation) is 60°F.
4. All earth is considered to be wet (worst condition).
5. The average temperature of the soil underneath the digestion
tanks is 40°F.
G.2.6. Heat Loss Through Roof
T_ is 95°F; T^ for winter is 0°F and for summer is 60°F; area normal to
heat flow is:
CTT)(one internal tank diameter)2 (12 tanks)
4
= (JT)(84)2 (12) = ^
4
U can be calculated as follows:
o liquid-air space interface, 1/C = 0
o air space (as indicated), 1/C = 0.5
o air space - concrete interface, 1/C = 0.33
10 25
o concrete 10.25 inches thick, x/k = —^ = 0.854
o concrete - earth interface, 1/C =1.0
24
o wet earth 24 inches thick, X/T6= — = 1-5
U =
o earth - atmosphere interface, 1/C = 0.167
1
(0 + 0.50 + 0.33 + 1.00 + 0.167) + (0.854 + 1.50)
1 _ 0.23 BTU
4.35 ~ hour-square foot-°F
Winter operation:
(0.23 BTU)(66,501 square feet)(95-0)°F = BTU
hour-square foot-°F ' ' *
G-6
-------
Summer operation:
(0.23 BTU)(66,501 square feet)(95-60)°F _
hour-square foot-°F
G.2.7. Heat Loss Through Walls
= 535,333 BTU per hour
T« is 95°; T, for winter is 0°F and for summer is 60°F; area normal to heat
flow is:
(TT)(one mean tank diameter) (12 tanks) (vertical wall height)
= (TT)(84 + ^)(12)(22) = 71,154 square feet.
All vertical surface is not exposed to the outside. As shown in
Figure G-2, the grouping of the existing digestion tanks only allows part
of each tank to be exposed to the outside environment, the rest is exposed
to each other. The wide dark lines in Figure G-2 indicate the part of the
circumference (59 percent) that is considered exposed to the outside. The
other 41 percent is exposed to a protected environment with an average year
round temperature of 70°F.
FIGURE G-2. Plan view of existing anaerobic digestion tanks.
G-7
-------
U can be calculated as follows:
o liquid - concrete interface, 1/C = 0
21 5
o concrete 21.5 inches thick, x/k = —=--j- =1.79
o concrete - earth interface, 1/C = 1.0
o wet earth 72 inches thick, x/k = || = 4.5*
U ~
o earth - atmosphere interface, 1/C = 0.167
1
(0 + 1.00 + 0.167) + (1.79 + 4.5)
1 0.13 BTU
7.46 hour-square feet-°F
Winter operation:
(0.13 BTU)(71.154 square feet)[(0.59)(95-0)°F + (0.41)(95-70)°F]
hour-square foot-°F
= 613,276 BTU per hour
Summer operation:
(0.13 BTU)(71.154 square feet)1(0.59)(95-60)°F + (0.41)(95-70)°F]
hour-square foot-°F
= 285,826 BTU per hour
— £OJ,O£U O1U pel. 11UUI.
G.2.8. Heat Loss Through Floor
T,. is 95°F; T, is 40°F for both winter and summer; area normal to heat flow
is:
,__w one mean w one mean . vertical height, ,.., ,.__.-*
[III! II . ,. T f. Jll£ UanKS 1
^ yvtank radius 'vtank radius of cone
= 90,391 square feet
* In reality, the factor is larger since the earth cover is becoming
thicker. In calculating heat loss, the effective protection of an earth
cover is never used as more than 10 feet; therefore the maximum value
is 7.5.
G-8
-------
U can be calculated as follows:
o liquid - concrete interface, 1/C = 0
12
o concrete 12 inches thick, x/k = -r = 1.0
o concrete - earth interface, 1/C = 1.0
120
o wet earth 120 inches thick*, x/k = -rr =7.5
U =
(0 + 1.0) + (1.0 + 7.5)
1 0.11 BTU
9.5 hour-square foot-°F
Winter and summer operation:
(0.11 BTU)(90,391 square feet) (95-40)°F = ^ ^
hour-square foot-°F
G.2.9. Heat Losses Through Sludge Piping
All sludge piping involved in transporting sludge from the sludge heat
exchangers to the digesters and from the digesters back to the heat ex-
changers is made of steel pipe of various diameters. It is estimated that
approximately 600 linear feet (6,000 square feet of surface area) of this
pipe is buried in shallow concrete galleries located between the digestion
tanks and heat exchanger complexes. The U factor for this pipe was esti-
mated at 0.32 BTU per hour-square foot-°F. It is estimated that another
3,600 linear feet (10,000 square feet of surface area) is located in
several building structures, where the temperature stays about 70°F all
year long. The U factor for this pipe is estimated at 1.6 BTU per hour-
square foot-°F.
Winter operation:
(0.32 BTU)(6000 sq. feet)(95-0)°F + (1.6 BTU)(10000 sq. feet)(95-70)°F
hour-square foot - °F hour-square foot-°F
= 182,400 + 400,000 = 582,400 BTU per hour
Summer operation:
(0.32 BTU)(6000 sq. feet)(95-60)°F + (1.6 BTU)(10000 sq. feet)(95-70)°F
hour-square foot-°F hour-square foot-°F
= 67,200 + 400,000 = 467,200 BTU per hour
In calculating heat loss, the effective protection of an earth cover is
never used as more than 10 feet.
G-9
-------
G.2.10. Summary of Heat Requirements
Winter Summer
operation operation
For raw sludge addition in BTU's per MGIF:
o Primary sludge only 511,808 255,904
o Secondary sludge only 636,353 272,723
TOTAL PRIMARY & SECONDARY SLUDGE 1,148,161 528,627
For system conductive/convective heat loss in BTU's
per hour:
o Roof 1,453,112 535,358
o Walls 613,276 285,826
o Floor 546,866 546,866
o Sludge piping 582,400 467,200
TOTAL HEAT LOSSES 3,195,654 1,835,250
G-10
-------
APPENDIX H
ANAEROBIC DIGESTION PIPING SCHEMATIC
In order to perform this study an up-to-date piping schematic of the
anaerobic digestion system was required. Enclosed in this appendix are
the following three drawings:
Drawing 117-1-1 Existing Piping Schematic Digesters 1-8
Drawing 117-1-2 Existing Piping Schematic Digesters 9-12
Drawing 117-1-3 Existing Exterior Solids Processing Piping
H-l
-------
AVAILABLE
DIGITALLY
-------
APPENDIX I
ANALYSIS OF ANAEROBIC SYSTEM
VOLATILE MATTER REDUCTION AND GAS PRODUCTION3
I.I. VOLATILE MATTER REDUCTION
Table 1-1 lists historical volatile matter reduction and several possible
related variables. Data are plotted in Figures 5-2, 5-3, and 5-4 in Section
5.3.6 of this report.
TABLE 1-1. LISTING OF AVERAGE VOLATILE MATTER REDUCTION AND SEVERAL RELATED VARIABLES
Year
1979
1980
Month
January
February
. March
April
May
June
July
August
September
October
Noveober
December
January
February
March
April
May
June
Average volatile.
matter reduction,
percent
62.9
64.0
66.2
65.0
65.2
57.9
45.3
52.2
. 44.6
46.9
—
59.6
70.7
61.6
60.0
54.8
49.7
57.6
Secondary sludge
fraction,
percent by weight
35
35
19
25
40
35
44
42
52
51
—
35
23
41
44
46
42
53
Volatile matter
loading
pounds volatile
matter per
usable cubicc
foot per day
0.15
0.13
0.15
0.16
0.17
0.15
0.12
0.14
0.12
0.12
—
0.10
0.12
0.14
0.14
0.12
0.12
0.14
Hydraulic
detention
time .
days*
20.5
23.9
22.3
18.5
19.3
19.8
20.2
18.8
18.6
19.4
26.9
23.9
23.6
17.0
16.4
18.4
18.4
17.6
a All data taken from Appendix A, Table A-4.
b Calculated as follows:
avg tons volatile solids in - avg tons volatile solids out
avg tons volatile solids in X
c Based on the information in Appendix E, it was assumed that only 65
percent of the total 1,757,628 cubic feet of digestion tank volume
was usuable.
d Based on the information in Appendix E, it was assumed that only
65 percent of the total 13,152,000 gallons of digestion tank was
usuable.
1-1
-------
1.2. ANALYSIS OF ANAEROBIC DIGESTION ON AVERAGE VOLATILE MATTER REDUCTION
PER MILLION GALLONS OF INFLUENT FLOW*
1.2.1. Current Conditions - All Sludge To Digestion
Primary sludge to anaerobic digestion:
0.377 ± 0.060 T/MGIF** 0.268 ± 0.047 VT/MGIF
Secondary sludge to anaerobic digestion:
0.284 ± 0.034 T/MGIF 0.185 ± 0.023 VT/MGIF
Summation of primary and secondary T/MGIF and VT/MGIF:
(0.377 + 0.284) ± (0.060) + (0.034) = 0.661 ± 0.069 T/MGIF
2 2
(0.268 + 0.185) ± (0.047) + (0.023) = 0.453+0.052 VT/MGIF
Ratio of secondary sludge to total sludge mass:
(0.284) T (0.661) = 0.43
From Figure 5-2, Section 5.3.6, the average volatile solids reduction
expected is 56.5 ± 6.5 percent or:
(0.453)(0.565) ± (0.565) (0.052) + (0.453) (0.065)
=0.256 ± 0.042 VT/MGIF
The VT/MGIF reduction would be equivalent to the T/MGIF reduced for
the total sludge mass.
1.2.2. Future Conditions - All Sludge To Digestion
Primary sludge to anaerobic digestion:
0.377 ± 0.060 T/MGIF 0.268 ± 0.047 VT/MGIF
Secondary sludge to anaerobic digestion:
0.328 ± 0.034 T/MGIF 0.209 ± 0.023 VT/MGIF
* Data taken from Appendix B, Tables B-l through B-3.
** T/MGIF = dry ton of solids per million gallons influent flow.
VT/MGIF = dry ton of volatile solids per million gallons influent
flow.
1-2
-------
Summation of primary and secondary T/MGIF and VT/MGIF:
(0.377 + 0.328) ± (0.060) + (0.034) = 0.705 ± 0.069 T/MGIF
2 2
(0.268 + 0.209) ± (0.047) + (0.023) = 0.477 ± 0.052 VT/MGIF
Ratio of secondary sludge to total sludge mass:
(0.328) -=- (0.705) = 0.465
From Figure 5-2, Section 5.3.6, the average volatile solids reduction
expected is 53.5 ± 6.5 percent or:
(0.477)(0.535) ± (0.535)2 (0.052)2 + (0.477)2 (0.065)2
= 0.255 ± 0.042 VT/MGIF
The VT/MGIF reduction would be equivalent to the T/MGIF reduced for
the total sludge mass.
1-3
-------
1.3. GAS PRODUCTION
Table 1-2 lists historical gas production and several possible related
variables. Data are plotted in Figures 5-5, 5-6, and 5-7 in Section 5.3.7
of the report.
TABLE 1-2. LISTING OF AVERAGE GAS PRODUCTION AND SEVERAL RELATED VARIABLES
Gas production,
cubic feet
per pound of
volatile natter
\ear Month
1979 January
Februarv
.larch
April
Hay
June
July
August
Septenaer
October
N'ovemoer
December
1980 January
February
ttarcb
April
May
June
reduced
8.7
8.S
7.2
8.6
9.9
12.0
14.4
9 2
12 6
13.5
«
9.6
11.0
12 8
12.0
U.O
13.8
10 7
Volatile matter
Fsed to digestion.
solids,
percent
7.1
7 0
7 6
6.3
7 4
7.1
6.0
6.4
5 9
5.6
..
5.4
5.9
5.2
5.2
5.3
5.2
5.S
tons
per day
83.5
J4.7
87.2
89.1
94.0
86.4
67 8
77.7
'0.6
67.0
..
56.2
o8.5
78.7
77.8
67. S
66.2
3C.6
Volatile matter
loading.
pounds volatile Secondary
natter per sludge fraction.
usable cubic
feet per day3
0.15
0.13
0 15
0 16
0.17
0.15
0.12
0.14
0.12
0.12
..
0.10
0.12
0.14
0.14
0 12
0 12
0.14
percent by
weight
35
35
19
25
40
35
44
42
52
51
..
35
:3
to»»
to
42
53
Hydraulic
detention
time,..
days "
20 S
23.9
:2.3
18 5
19 3
19 8
20.:
18 S
-.8.6
19 4
26 9
23.9
23.o
17.0
16 •
IB.i
18 -
17 0
3 Based on the information in Appendix E, it was assumed that only 65
percent of the total 1,757,628 cubic feet of digestion tank volume was
usuable.
Based on the information in Appendix E, it was assumed that only 65
percent of the total 13,152,000 gallons of digestion tank volume was
usuable.
NOTE: The existing hot water source for the heat exchangers are two steam
and one hot water boiler (Appendix G). To allow the boilers to
operate at maximum output, approximately 20,250,000 BTU per hour of
energy is required. Assuming digester gas at 600 BTU's per cubic
foot, then the amount of digester gas needed is:
20,250,000 BTU per hour required
600 BTU per cubic foot
= 33,750 cubic feet per hour
= 810,000 cubic feet per day
1-4
-------
APPENDIX J
ANALYSIS OF ELUTRIATION SYSTEM OPERATION
Table J-l lists historical 1980 elutriation underflow to dewatering solids
concentration and several possible related variables. Data are plotted in
Figures 5-8 through 5-10 in Section 5.4 of this report. All data taken
from Appendix A, Table A-5.
TABLE J-l. LISTING OF ELUTRIATION UNDERFLOW TO DEWATERING SOLIDS
CONCENTRATION AND SEVERAL POSSIBLE RELATED VARIABLES
Elutciatioo. Total dry Solids loading
underflow to solids to Volume of rate, Ratio of
dewacenag solids elutriation, digested sludge feed solids pounds per tasn water flow
concentration, cons CO elutriacion, concentration, day per h Co digested
Month percent solids per day MOD* percent solids square toot sludge flow
January
Feoruarv
Marci
April
May
June
5 i
i 3
3 6
5 5
3.i
5.2
45
53
S3
54
SB
70
0 397
0.509
0.509
0 4o6
0.448
0 492
2.6
2.4
2.4
2.7
3.0
3-3
36.7
43.2
43.3
44.1
47.4
57 I
5.7
4 6
4 b
4 9
4 3
i.5
a MGD - million gallons per day.
b There are two elutriation tanks each 35-feet wide by 70-feet long
operated in series; therefore the first tank receives the entire solids
load. Loading rate calculated as follows:
(tons dry solids to elutriation per day) (2000 pounds per ton)
(35) (70) square feet of tank
J-l
-------
APPENDIX K
ANALYSIS OF DIGESTED SLUDGE DEWATERING ON AVERAGE
SLUDGE PRODUCTION PER MILLION GALLONS OF INFLUENT FLOW*
TABLE K-l.
Ferric chloride usage
Month
January
February
March
April
May
June
Average
Standard
Tons per
day
10.2
11 9
10.8
10. 0
10.7
10. 1
deviation
Tons per day of dry
solids to filter
10.2/54 = 0.189
11.9/59 s 0.202
10.8/49 = 0 220
10 0/46 = 0.217
10.7/52 = 0.206
10.3/55 - 0.187
0.204
0.014
Feed solids
concentration.
percent
5.4
4.3
3.6
5.5
5.4
5.2
ftetatered cake
percent
solids
16 0
15 8
15.6
16.7
16.6
16 9
16.2
0 6
Secondary sludge
fraction,
percent by weight
23
41
44
46
42
53
The amount of solids contributed by ferric chloride per million
gallons of influent flow is estimated below.
The average amount of ferric chloride added per ton of dry solids
pumped to the vacuum filters is 0.204 ± 0.014 tons. It is assumed that all
iron solids (44 percent of the ferric chloride) and 16.3 ± 0.5 percent of
the chloride solids remain in the sludge (the remainder of the chlorides
leave with the filtrate). Therefore, the amount of solids contributed by
ferric chloride would be:
(0.44 + [0.163 ± 0.006][0.56])(dry weight ferric chloride added)
= (0.53 ± 0.003)(dry weight ferric chloride added)
= (0.53 ± 0.003)(0.204 ± 0.014)(dry solids to dewatering)
= (0.108 ± 0.007)(dry solids to dewatering)
For current conditions, sludge to digestion, each million gallons of
influent flow would contribute:
.063)2 + (0.04)
(0.661 - 0.256) ± s/XO.<
= 0.405 ± 0.075 dry tons to dewatering after anaerobic digestion.
All data taken from Appendix A, Table A-7 or Appendix I.
K-l
-------
The amount of solids contributed by ferric chloride addition would be:
(0.108 ± 0.007)(0.405 ± 0.075)
= (0.108X0.405) ± \A0.405)2(0.007)2 + (0.108)2(0.075)
= 0.043+0.009 T/MGIF*
For future conditions, sludge to digestion, each million gallons of
influent flow would contribute:
(0.705 - 0.255) ± xA°-063)2 + (0.04)2
= 0.450 ± 0.075 dry tons to dewatering after anaerobic
digestion.
The amount of solids contributed by ferric chloride addition
would be:
(0.108 ± 0.007)(0.450 ± 0.075)
= (0.108)(0.450) + \/0.450)2(0.007)2 + (0.108)2(0.075)2
= 0.049 ± 0.009 T/MGIF
For current and future conditions with only primary sludge
to digestion, each million gallons of influent flow would
contribute:
/i f
(0.377 - 0.188) ± V(0.055) + (0.035)
= 0.189 ± 0.065 dry tons to dewatering after anaerobic
digestion.
The amount of solids contributed by ferric chloride addition
would be:
(0.108 ± 0.007)(0.189 ± 0.066)
(0.108)(0.189) ± \/(0.189)2(0.007)2 + (0.108)2(0.065)2
= 0.020 ± 0.007 T/MGIF
For future conditions with only secondary sludge to digestion,
each million gallons of influent flow would contribute:
/ 2 r
(0.328 - 0.073) ± V(0.031) + (0.015)
= 0.255 ± 0.034 dry tons to dewatering after anaerobic
digestion.
* T/MGIF = Dry ton of solids per million gallons of influent flow.
K-2
-------
The amount of solids contributed by ferric chloride addition
would be:
(0.108 ± 0.007)(0.255 ± 0.034)
(0.108)(0.255) ± V/(0.255)2(0.007)2 + (0.108)2(0.065)2
0.028 ± 0.010 T/MGIF
K-3
-------
APPENDIX L
ANALYSIS OF RAW SLUDGE DEWATERING ON
AVERAGE SLUDGE PRODUCTION PER MILLION GALLONS OF INFLUENT FLOW*
TABLE L-l.
r«rrtc chloride usite
.Innch
faniuirv
February
March
April
•lav
June
Weraie
Standard
Tons
per
Tom per ton of dry
Tena per
d»» solids to filter dav
20
17
II
II
II
12
3
a
3
.3
3
3
20 5/114
17 8/1 10
11 5/73
II 3/134
II 5/96
12.5/108
deviation
0 180
0 162
0 133
0.073
0.12(1
0 116
0 134
0 031
55
48
38
i3
47
48
Line usage
Tons p«r ton
if dry
solids to filter
55/114
48/110
38/7S
18/134
47/96
48/108
0
a
a
0
0
!
0
0
482
4)6
307
312
490
_44i
443
071
feed soklda Oraatere-1 cake.
concentration, percent
percent solids
44 17
: 17
) :i
a 21
5 21
6 ZZ_
:o
2
2
9
6
1
1
_6
Z
2
Secondary sludge
fraction.
percent by 'eight
72
44
47
36
38
33
The amount of solids currently contributed by lime addition per
million gallons of influent flow is estimated below.
The average amount of lime (CaO) added per ton of dry solids pumped to
the vacuum filters is 0.445 t 0.065 tons. It is assumed that all calcium
solids (71.5 percent of the CaO) remain in the sludge and that there are 5
percent inerts associated with the high grade CaO utilized. Therefore, the
amount of solids contributed by lime addition would be:
(0.71 + 0.05)(dry weight CaO added)
= (0.76)(0.445 ± 0.071)(dry solids to dewatering)
= (0.338 ± 0.054)(dry solids to dewatering)
For current conditions, raw sludge to dewatering, the amount of solids
contributed by CaO addition per million gallons of influent flow would be:
(0.338 ± 0.054)(0.661 ± 0.063)
= (0.338)(0.661) ± (0.661) (0.054) + (0.338)2 (0.063)2
= 0.223+0.042 T/MGIF**
* All data taken from Appendix A, Table A-6
** T/MGIF = Dry ton of solids per million gallons of influent flow.
L-l
-------
In the future, CaO requirements per ton of dry solids pumped to the
vacuum filter are expected to drop by at least 50 percent. This reduction
is due to the current addition of lime in excess of conditioning require-
ments so that a high pH can be developed to meet land trenching require-
ments. In the near future, trenching will not be utilized and lime
requirements will decrease to that required to enhance dewatering opera-
tions only. This 50 percent reduction would change the amount of solids
contributed by lime addition from [(0.338 ± 0.054)(dry solids to
dewatering)] to [(0.169 ± 0.054)(dry solids to dewatering)].
For future conditions, if raw sludge to dewatering, the amount of
solids contributed by CaO addition per million gallons of influent flow
would be:
(0.169 ± 0.054X0.705 ± 0.063)
(0.169)(0.705) ± vAO.705)2 (0.054)2 + (0.169)2 (0.063)2
= 0.119 ± 0.040 T/MGIF
For future conditions, if only primary sludge to dewatering, the
amount of solids contributed by CaO addition per million gallons of
influent flow would be:
(0.169 ± 0.054)(0.377 ± 0.055)
(0.169)(0.377) ± \A0.377)2 (0.054)2 + (0.169)2 (0.055)2
0.064 ± 0.022 T/MGIF
For future conditions, if only secondary sludge to dewatering, the
amount of solids contributed by CaO addition per million gallons of
influent flow would be:
(0.169 ± 0.054)(0.328 ± 0.031)
(0.169)(0.328) ± x/(0.328)2 (0.054)2 + (0.169)2 (0.031)2
0.055+0.018 T/MGIF
The amount of solids currently contributed by ferric chloride per
million gallons of influent flow is estimated below.
The average amount of ferric chloride added per ton of dry solids
pumped to the vacuum filters is 0.134 ± 0.038 tons. It is assumed that all
iron solids (44 percent of the ferric chloride) and 20.2 ± 2.0 percent of
the chloride solids remain in the sludge (the remainder of the chlorides
L-2
-------
leaves with the filtrate). Therefore, the amount-of solids contributed by
ferric chloride would be:
(0.44 + [0.202 ± 0.02][0.56](dry weight ferric chloride added)
= (0.55 ± 0.011)(dry weight ferric chloride added)
= (0.55 ± 0.011)(0.134 + 0.038)(dry solids to dewatering)
= (0.074 ± 0.021)(dry solids to dewatering)
For current conditions, raw sludge to dewatering, the amount of solids
contributed by ferric chloride addition per million gallons of influent
flow would be:
(0.074 ± 0.021)(0.611 ± 0.063)
(0.074)(0.66l) ± \A0.66l)2 (0.021)2 + (0.074)2 (0.063)2
= 0.049+0.015 T/MGIF
In the future, ferric chloride requirements per ton of dry solids
pumped to the vacuum filter are expected to drop at least 15 percent. This
reduction is due to the significant reduction in lime requirement brought
about by elimination of the trenching operation. This 15 percent reduction
would change the amount of solids contributed by ferric chloride addition
from [(0.074 ± 0.021)(dry solids to dewatering)] to [(0.063 ± 0.021)(dry
solids to dewatering)].
For future conditions, if raw sludge to dewatering, the amount of
solids contributed by ferric chloride addition per million gallons of
influent flow would be:
(0.063 ± 0.021)(0.705 ± 0.063)
(0.063)(0.705) ± V(0.705)2 (0.021)2 + (0.063)2 (0.063)2
= 0.044+0.015 T/MGIF
For future conditions, if only primary sludge to dewatering, the
amount of solids contributed by ferric chloride addition per million
gallons of influent flow would be:
(0.063 ± 0.021)(0.377 ± 0.055)
(0.063X0.377) + \A0.377)2 (0.021)2 + (0.063)2 (0.055)2
0.024 ± 0.009 T/MGIF
L-3
-------
For future conditions, if only secondary sludge to dewatering, the
amount of solids contributed by ferric chloride addition per million
gallons of influent flow would be:
(0.063 ± 0.020(0.328 ± 0.031)
(0.063X0.328) ± v/0.328)2 (0.021)2 + (0.063)2 (0.031)2
0.021 ± 0.007 T/MGIF
L-4
-------
APPENDIX M
SECTION 6.1 SUPPORTING CALCULATIONS
M.I. DIGESTED SLUDGE VACUUM FILTER OPERATION
M.I.I. Current Operation
There are four existing units each having 500 square feet of filtering
area. Their yield depends upon the ratio of secondary and primary sludge.
In 1974 when the mixture was 40 to 50 percent secondary, the yield averaged
2.41 pounds per square foot per hour.
2.41 pounds 500 square feet 4 units 24 hours
square foot-hour X unit x 1 x day
= 115,680 pounds per day
Assuming an average feed solids concentration of five percent and a
density of 8.85 pounds per gallon, the volume of digested sludge capable of
being dewatered is:
115,680 pounds 1 gallon
x
day 8.85 pounds 0.05
= 261,423 gallons per day
Assuming an average cake solids of 16.5 percent, the number of wet
tons to be hauled away including the ferric chloride (FeCl_) conditioner
would be:
t 115,680 pounds sludge + 13,180 pounds Fed- , x 1 ton x 1
day day J 2000 pounds 0.165
= 390.5 wet tons per day
M.I.2. Future Operation
If six new filter units (600 square feet filtering area each with the same
yield of 2.41 pounds per square foot per hour) are placed into operation, then:
2.41 pounds 600 square feet 6 units 24 hours
square foot-hour unit 1 x day
= 208,224 pounds per day
M-l
-------
Assuming same feed solids concentration and density as before, the
volume of sludge capable of being dewatered is:
208.224 pounds 1 gallons „ 1
X
day 8.85 pounds 0.05
= 470,563 gallons per day
Assuming an average cake solids of 16.5 percent, the number of wet
tons to be hauled away including the ferric chloride (FeCl_) conditioner
would be:
r 208.224 pounds sludge + 23,725 pounds Fed. , x 1 ton x 1
day day J 2000 pounds 0.165
= 703 wet tons per day
If the maximum sludge production generated at 334 million gallons per
day influent flow was mesophilically digested under the conditions de-
scribed in Section M.5 of this Appendix, there would be 340,050 pounds per
day to dewater. The maximum number of filters required would be:
340,050 pounds 1 filter - day
day 34,704 pounds
= 9.8, or 10 filters required
Assuming an average cake solids at 16.5 percent, the maximum number of
wet tons to be hauled away including the ferric chloride (FeCl_) condi-
tioner would be:
340,050 pounds sludge + 38,744 pounds Fed, x 1 ton x 1
day day 2000 pounds 0.165
= 1,148 wet tons per day maximum
M.2. ELUTRIATION OPERATION
In order to generate 115,680 pounds of solids per day in the elutriate
underflow at a maximum hydraulic flow rate of 490,000 gallons per day and a
density of 8.64 pounds per gallon, the incoming digested solids concentra-
tion would have to be:
115,680 pounds 1 gallon 1 day 100
day X 8.64 pounds X 490,000 gallons X 1
= 2.7 percent solids
For any flow less than 490,000 gallons per day, the solids concentra-
tion would have to be proportionately higher.
M-2
-------
In order to generate 208,224 pounds of solids per day under the same
stated conditions, the incoming digested solids concentration would have to
be:
208,224 pounds 1 gallon 1 day 100
day x 8.64 pounds x 490,000 gallons x 1
= 4.9 percent solids
M.3. HEAT EXCHANGER OPERATION*
Available heating capacity limits the amount of sludge that can be
hydraulically processed. The maximum flow rate possible based solely on
sludge heating capacity is:
1980 winter operation
(9,800,000 BTU per hour) (24 hours)
(8.6 pounds per gallon)(95 - 55°F) (day)
= 683,721 gallons per day
1981 summer operation
(11.100,000 BTU per hour) (24 hours)
(8.6 pounds per gallon)(95 - 75°F) (day)
= 1,548,837 gallons per day
1981 winter operation
(12,800,000 BTU per hour) (24 hours)
(8.6 pounds per gallon)(95 - 55°F) (day)
= 893,023 gallons per day
1982 summer operation
(14.100.000 BTU per hour) (24 hours)
(8.6 pounds per gallon)(95 - 75°F) (day)
= 1,967,442 gallons per day
M.4. DIGESTION TANK OPERATION
The available data indicate that the system can be operated success-
fully at an average hydraulic residence time of 16 days. The maximum flow
rate possible based solely on usable tank capacity is:
* See Appendix G for discussion of calculation performed.
M-3
-------
8,545,581 gallons of usable tank volume
16 days
= 534,099 gallons per day
If the digestion tanks were cleaned and grit accumulation kept to a
minimum, the maximum flow rate possible would increase to:
13,147,055 gallons of usable tank volume
16 days
= 821,691 gallons per day
If the maximum sludge production generated at 334 million gallons per
day influent flow rate was mesophilically digested at a minimum hydraulic
detention time of 16 days, and if the existing digesters were clean of grit
and grit accumulation was kept to a minimum in the system, the maximum
number of digestion tanks required (all the same dimensions) would be:
1,200.062 gallons 16 days 1 digester
day 1 x 1,096,000 gallon capacity
= 17.5, or 18 tanks required; six new tanks would be required to meet
this condition.
Operating data at Blue Plains indicate that the system can be operated
successfully at volatile matter loading ratios averaging 0.16 to 0.17,
though gas production seems to deteriorate over 0.14. The maximum volatile
matter loading based on 0.16 pounds per usable cubic foot per day would be:
1.142,458 cubic feet of usable tank volume 0.16 pound volatile matter
1 usable cubic foot
= 182,793 pounds volatile matter per day
If the digestion tanks were cleaned and grit accumulation was kept to
a minimum, the maximum volatile matter loading based on 0.16 pounds per
usable cubic foot per day would be:
1,757,628 cubic feet of usable tank volume 0.16 pound volatile matter
1 usable cubic foot
= 281,220 pounds volatile matter per day
If the maximum sludge production generated at 334 million gallons per
day influent flow rate was mesophilically digested at a maximum volatile
matter loading of 0.16 pounds per cubic foot per day, and if the existing
digesters were clean of grit and grit accumulation was kept to a minimum in
the system, the maximum number of digestion tanks required (all the same
dimensions) would be:
362,022 pounds volatile matter 1 cubic foot - day 1 tank
day 0 16 pounds volatile matter x 146,469 cabic feet
= 15.4, or 16 tanks required; hydraulic flow rate governs.
M-4
-------
M.S. GALLONS OF PRIMARY AND SECONDARY SLUDGE PUMPED TO DIGESTION
If the existing digestion system was processing all the Blue Plains
sludge, the ratio of thickened secondary sludge volume to thickened primary
sludge volume would be 1.54:1. Using this ratio for allocating the 490,00
gallons gives:
Gallons
per day
192,913
297,087
490,000
Pounds
per day
118,834
103,683
222,517
Volatile Pounds
per day
84,373
66.254
150,627
Primary sludge
Secondary sludge
TOTAL
The following constraints apply to the above calculations:
o Total flow to digestion system to be no greater than 490,000
gallons per day - ok
o Minimum ratio of secondary sludge mass to total sludge mass to be
0.35
103,683 pounds secondary sludge per day ,_
222,517 pounds total sludge per day = ' " °K
o Concentration of feed solids into digestion tanks not to exceed six
percent
(222,517 pounds total sludge per day)(100) .,
(8.6 pounds per gallon) (490,000 gallons per day) ~ *"** " OK
o Concentration of solids into elutriation system from digestion
tanks to exceed 2.7 percent
From Figure 5-2, at a secondary:total mass ratio of 0.46 an
average volatile matter reduction of 53.5 percent is expected.
1(222.517 pounds of sludge per day) - 0.535(150.627 pounds volatile sludge per dap]'. 100'
(3.50 pounds per jalion)(490,000 gallons per day)
"= 3.4 percent - ok
o Sludge mass from digestion tank not to exceed 115,680 pounds per
day using the existing drum filters for dewatering and 208,234
pounds per day using the six new belt type filters for dewatering.
222,517 pounds of sludge per day - (0.535)(150,627 pounds volatile sludga per day}
= 141,932 pounds per day - not ok for current operation of drum
filters; ok for future operation with new vacuum filters.
M-5
-------
M.6. GAS PRODUCTION
M.6.1. 490,000 Gallon Per Day Sludge Processing Rate
Under the constraints imposed in developing the 490,000 gallon per day
flow rate, gas production is expected to range from 12 to 14 cubic feet per
pound of volatile matter reduced.
Volatile matter reduced:
(0.535) (150, 627) = 80,585 pounds per day
80,585 pounds 12 cubic feet „,_ no. , . - t ,
1 day - x pound - = 967'020 cubic feet per day
80,585 pounds 14 cubic feet . ,-„ inn , . , . ,
' day - X - pTSnd - = 1.128,190 cubic feet per day
Gas consumption to meet winter and summer sludge heating requirements
are:
Winter operation:
690.000 gallons 8.6 pounds (95-55)"F 24 hours 5.195.656 3TU radiation loss
day gallon * 1 day x -
600 BTU 0.8 heat efficiency factor
cubic foot of gas 1
= 510,949 cubic feet per day
Summer operation:
690.000 gallons 8.6 pounds (95-75)T t 24 hours 1,835.250 3T1' radiation loss
day gallon x 1 day x hour
600 BTU 0 8 heat efficiency factor
cubic fooc of gas 1
= 267,346 cubic feet per day
Average expected excess:
(967.Q20 + 1,128.190) cubic feet _ (510,949 + 267,346) cubic feet
2 day " 2 day
= 658,458 cubic feet per day available to be sold
M.6.2. Maximum Gallon Per Day Sludge Processing Rate
Again, gas production is expected to range from 12 to 14 cubic feet
per pound of volatile matter reduced.
M-6
-------
Volatile matter reduced:
(0.535)(319,030) = 170,681 pounds per day
170.681 pounds 12 cubic feet . ... ._, , . , ,.
- - x pound - = 2>OA8>172 cublc feet P"
170,681 pounds 14 cubic feet „ ,on _0/ , . ,
- - X pound - = 2>389>534 cublc feet
Gas consumption to meet winter and summer sludge heating requirements
(based on 18 digestion tanks) would be:
Winter operation:
1.200.062 gallons 8.6 pounds (95-5STT . 2i hours 5.272.829 BTU radiation loss
- d^ - X galloD x 1 day hour
600 BTU 0.8 beat efficiency factor
cubic feet, of gas
= 1,123,686 cubic feet per day
Summer operation:
1 200.062 gallons 8.6 pounds (95-75)°F 26 hours 3.C2S.162 BTU radiation loss
- day * gallon x 1 day hour
600 STU0.8 heat efficiency factor
cubic feet of gas 1
= 581,430 cubic feet per day
Average expected excess:
(2.048,172 + 2,389,534) cubic feet _ (1,123,686 + 581,430) cubic feet
2 day " 2 day
= 1,366,295 cubic feet per day available to be sold
M-7
-------
APPENDIX N
SECTION 6.2 SUPPORTING CALCULATIONS
N.I. DIGESTED SLUDGE VACUUM FILTER OPERATION
Each vacuum filter has 600 square feet of filtering area. Assuming a
minimum yield for this sludge, thermophilically digested, of 3.0 pounds per
hour per square foot, each filter would be able to dewater:
3.0 pounds 600 square feet 24 hours
square foot-hour filter day
= 43,200 pounds per day
As indicated elsewhere in this Appendix, if the maximum sludge produc-
tion generated at 334 million gallons per day influent flow rate was
thermophilically digested, there would be 340,050 pounds per day to de-
water. The maximum number of filters required would be:
340,050 pounds 1 filter-day
day x 43,200 pounds
= 7.9, or 8 filters required
Assuming an average cake solids of 16.5 percent, the maximum number of
wet tons to be hauled away including the polymer conditioner would be:
340,050 pounds sludge + 680 pounds polymer solids x 1 ton x 1
day day 2000 pounds 0.165
= 1,033 wet tons per day maximum
N.2. ELUTRIATION (COOLING) OPERATION
Sludge elutriation is not required, instead, the elutriation tanks
would be utilized as sludge cooling tanks. The system would still be
operated as a two-stage series, co-current, washing operation but under
different washing rates. The maximum hydraulic flow through digestion at
334 million gallon per day influent flow rate is 1,200,062 gallons per day.
The temperature of digested sludge is 122°F. Cooling water on a 1:1 flow
basis would be mixed with digested sludge before entering the elutriation
tanks. The warmest cooling water is assumed to be 60°F. In the first
stage, 1,200,062 gallons of sludge at 122°F is mixed with 600,031 gallons
of wash water at 60°F. Under these conditions, the minimum sludge dis-
charge temperature would be:
(1.200.062 gallons)(122°F) + (600,031 gallons)(60°F) _ 0
(1,200,062 + 600,031) gallons ~ J
If we assume that the sludge concentrates to 75 percent of its ori-
ginal volume, is pumped to the second stage, and blended with 600,031
gallons of wash water at 60°F, the minimum sludge temperature discharge to
dewatering would be:
N-l
-------
(1,200.062 gallons)(0.75)(101.3°F) + (600,031 gallons)(60°F) .„.
[(1,200,062)(0.75) + 600,031] gallons ~ 8^tt *
NOTE: It may be economically attractive to build a tube and tube heat
exchanger for raw sludge going to digestion and digested sludge leaving
digestion.
N.3. EFFECT ON SYSTEM HEATING CAPABILITIES DUE TO INCREASE IN SLUDGE
OPERATING TEMPERATURE"
N.3.1. Heat Required for Primary Sludge Addition Only
Winter operation:
1454 gallons 8.8 pounds (122 - 55)°F
A X ^"^^^^^^^^—
MGIF** gallon 1
= 857,278 BTU's per MGIF
Summer operation:
1454 gallons 8.8 pounds (122 -
MGIF x gallon x 1
75)°F
= 601,374 BTU's per MGIF
N.3.2. Heat Required for Secondary Sludge Addition
Winter operation:
2139 gallons 8.5 pounds (122 - 60)°F
MGIF X gallon X 1
= 1,127,253 BTU's per MGIF
Summer operation:
2139 gallons 8.5 pounds (122 - 80)°F
MGIF x gallon X 1
= 763,623 BTU's per MGIF
* Assumptions and calculations are as per Appendix G except digestion
operating temperature at 122°F rather than 95°F.
** MGIF - million gallons influent flow.
N-2
-------
N.3.3. Heat Required for Conductive/Convective Losses
Roof - winter operation:
(0.23 BTU)(66,501 square feet)(122 - 0)°F _ „
- hour:squarj foot.oF - - 1,866,018 BTU per hour
Roof - summer operation:
' ° per hour
Walls - winter operation:
(0.13 BTU) (71. 154 square feet) [ (0.59) (122-0)°F + (0.4l)(122-70)°F3
hour-square foot-°F
= 863,027 BTU per hour
Walls - summer operation:
(0.13 BTU) (71. 154 square feet) [ (0.59) (122-60)°F + (0.4l)(122-70)°F]
hour-square foot-°F
= 535,576 BTU per hour
Floor - summer and winter operation:
(0.11 BTU) (90.391 square feet)(122-40)°F _ ft
hour:squar^ f00t-oF - - 815,327 BTU per hour
Sludge piping - winter operation:
(0.32 BTU) (6000 square ft)(122-0)°F + (1.6 BTU)(10,000 square ft) (122-70)°F
hour-square foot-°F hour-square foot-°F
= 234,240 + 832,000 = 1,066,240 BTU per hour
Sludge piping - summer operation:
(0.32 BTU) (6000 square ft) (122-60)°F + (1.6 BTU) (10000 square ft) (122-70)°F
hour-square foot-°F hour-square foot-°F
= 119,040 + 832,000 = 951,040 BTU per hour
N.3.4. Summary of Heat Requirements
Winter Summer
operation operation
For raw sludge addition in BTU's per MGIF
Primary sludge only 857,278 601,374
Secondary sludge only 1,127,253 763,623
TOTAL PRIMARY & SECONDARY SLUDGE 1,984,531 1,364,997
N-3
-------
Winter
operation
1,866,018
863,027
815,327
1,066,240
Summer
operation
948,304
535,576
815,327
951,040
For system heat loss in BTU's per hour
0 Roof
0 Walls
0 Floor
0 Sludge piping
TOTAL HEAT LOSSES 4,610,612 3,250,247
Based on a 334 million gallon per day (MGD) of influent flow, the
total digestion sludge heating requirements would be:
Winter operation:
(1.984,531 BTU's)(334 MGIF) + (4,610.612 BTU)(24 hours)
(MGIF) (day) (MGIF) (day)
= 7/8 x 108 BTU's per day (32.2 x 106 BTU per hour)
Summer operation:
(1.364,997 BTU's)(334 MGIF) + (3,250,247 BTU)(24 hours)
(MGIF) (day) (hour) (day)
= 5.3 x 108 BTU's per day (22.2 x 106 BTU per hour)
N.4. EFFECT ON SLUDGE PIPING DUE TO INCREASE IN SLUDGE OPERATING
TEMPERATURE
All sludge piping involved in transporting sludge from the sludge heat
exchangers to the digesters and from the digesters back to the heat ex-
changers is made of steel, located in pipe galleries protected from outside
weather conditions, and not ridgidly anchored at the ends. Any increase in
sludge operating temperature will cause a corresponding increase in pipe
expansion. Linear expansion would be of primary concern at this installa-
tion.
The additional linear expansion of the sludge piping due to increasing
the sludge temperature from the existing operating temperature to a higher
operating temperature can be calculated using the equation.
L = (o)(L)(AT)
where
L = the change in length of the piping system being considered,
inches.
a = the coefficient of linear expansion, for steel it is
78 x 10" inches per foot per °F
N-4
-------
L = length of piping being considered, feet
A T = increase in temperature, °F
The sludge piping involved currently operates at a temperature of 90°F
to 93°F. Operating at a maximum sludge temperature of 130°F, the maximum
increased temperature, T, would be 40°F.
Maximum linear expansion would occur on the longest pipe segment. The
longest pipe segment is 400 feet long and is located on the north side of
the digestion facility where it carries heated sludge to the anaerobic
digesters.
The maximum increased linear expansion, L, is calculated to be
78 x 10"6 inches x 400 feet x 40°F
foot - °F
or 1.25 inches
Based on calculations for expansion of all other pipe segments and
best engineering judgement, the increase in temperature and the resultant
pipe expansion will not adversely effect the sludge piping involved.
N.5. EFFECT ON ANAEROBIC DIGESTION TANKS DUE TO INCREASE IN SLUDGE
OPERATING TEMPERATURE
Increasing the sludge temperature within the existing concrete diges-
tion tanks will create increased stresses on the outer face of the diges-
tion tank walls. The amount of stress developed in the concrete and the
amount of reinforcing steel required needs to be calculated and compared to
the existing situation to make sure a structural failure would not occur.
Figures N-l and N-2 show the situation that would occur on the exist-
ing structure assuming that the maximum sludge temperature is 130°F and
the coldest ambient air temperature is 0°F.
TQ - DIGESTER TEMP.
ASSUME 130°F
HORIZONTAL STEEL
REINFORCEMENT
*8 99" EF •
72
TA ' AMBIENT AIR TEMP.
ASSUME 0°F
SOIL
FIGURE N-l. Cross section of existing
anaerobic digestion tank wall.
-•— t = WALL THICKNESS
=• 21.5"
N-5
-------
V 130°F
DIGESTER
CONCRETE
WALL -
t«ZI.S
SOIL
TEMPERATURE
GRADIENT
72"
AIR
TA- 0°F
FIGURE N2. Assumed temperature gradient through digester wall and soil.
As indicated in Figures N-l and N-2, the existing tank walls are 21.5
inches thick with #8 rebar placed every 9 inches. In addition, the entire
structure is buried; the minimum soil cover on the side walls being 72
inches.
The stress in the outer concrete wall is calculated on the following
two pages.
Temperature difference through the wall, T, can be found by the fol-
lowing equations.
T = (Tn - TJ t/K
and
where:
1/K = 1/S + t/K + t./K
T. =
K =
K- =
t =
t. =
S =
1
digester temperature (°F)
ambient air temp (°F)
coefficient of conductivity for concrete
coefficient of conductivity for soil
thickness of concrete (inches)
thickness of soil (inches)
outside surface coefficient
21.5
T = (130-0)
12
= 22°F
1
6
21.5 . 72
~12~ ~~8
N-6
-------
18. T*
in Cylindrical Tank Wall* *
Tanks containing lioc liquids ire subiect co
cefflpennire stresses. Assume chat the temperature is
Ti in the inner face, T- at the oucer face, ma that the
cempenruK decreases uniformly from inner to outer
face. Ti - T, being denoted as T Fig J7 shows a seg-
ment of i tank wall in two positions, one before and
one after a uniform increase in temperature. Tie anginal
length of the are of the will has been increased, but an
increase chat is uniform throughout will not create
JUT stresses as long as the nng is supposed to be free
tod unrestrained at its edges. It is the cemperanue
differential onlv, T, which creates stresses.
Aftor
no. 37
Toe >*frr fibers being noner cend :o expand
note than the outer fibers, so if the segment is cut loose
from the adjacent portions of the wail. Point A m
Fig 38 will move to iV. 3 will move to B', and section
X ourjiOQ
rlB, which, represents the itreuless condition due to 4
uniform temperature cnange throughout, will move to
i new position A'B' Actually tne movements cram A
co A' and 3 to B' ire prevented since the circle must
remain i circle, ind irresses will be created caac ire
proportional co che horizontal distances between A3
mo A'B'
It is dear that AA' - SB' - movement due to a
temperature change of \%T or wnen > is tne coefficient
of expansion, chat
AA' • SB' » Yit X • per unit length of arc.
and
4.~.TX *
\fy i
In i homogeneous section, the moment ,V( re-
quired co produce an angle change * in an element of
unit length mav be written as
M - Elt
Eliminating 9 gives
.. El X T X i
m • —^———
i
The stresses in che extreme nben created bv M are
/-^X--M£XrX,
The stress distribution across the cross section
is as indicated in Fig. 38. The stresses are numerically
equal at the two faces but have opposite signs. Note
chat che equation applies to uncracked sections onlv,
and that this procedure of stress calculation is co be
considered merely as a method bv which the problem
can be approached. The variables £ and I in che equa-
tions are uncertain quantities. E mav vary from
1,300.000 up to 4,300.000 p.s.i.. and / may also varv
considerably because of deviations tram che assumption
of linear relation between stress and strain. Finally, if
che concrete cracks, M can ao longer be set equal co
Eli, nor / equal to -j X — As a result, the equation
/ - yftt is to be regarded as merely indicative rather
chan formally correct.
The value of « may be taken as 0.000006, and for
che purpose of this problem choose £ • 1.300,000 p.s.i.
Then £ X i - 9 and/ - 4.jr
The value of T a the difference between tempera-
tures in che two surfaces of che concrete which mav be
computed from che temperature of cne stored liquid
and che outside air.
HO. If
a circular Concrete Tanks Wlthouc Preacreaaing.
5420 Old Orchard Road, Statue, 111. 60076.
Portland Generic Association.
N-7
-------
M'hen (he ilow of he-ic n uniform from the inside
co the outside of che wall section in Ftp. )9. the tem-
perature difference, T - 7"i - Ti. a smaller than che
difference. T, — T.. becweea che inside liquid and che
ouoide air Standard cextbooki give
T - (T, - T.)
in which
i
4- !
/ i ^ i,
• coefficient of conductivity of stone or
jravel coocrete - U B.C. it. per hour per
sq.ft. per deg. F. per to. of chicknai
ii - coefficient of conductivity of insulating
material
i • ctuckneu of concrete in in.
'i - thickness of insulating material
; - outside surface confident • 6 B.t.u. per
hour per sq.ft. per deg. F
Assuming an anuuulated wall
6 12
Consider a "«fc with wall thickness / • 10 m.
which holds a liquid with a temperanire T, - 120 deg
F while che temperature of che ouoide air 7". - 30
deg. F Then
10
, (120 - 30^^-75 deg. F
and
/ - 4 ST - 4 5 X 75 - 37J p 1.1.
The stress of / • 373 p.s.i. is tension in che out*
side and compression in the inside face. If ue uniformly
distributed nag tension due to load in che tank is. sav,
300 p.s.i.. che combined stress will be:
Ouoide fiber: 300 -r 375 - «75 p.s.i. (tension)
Inside aber: 300 - 375 - -75 p.s.i. (compression)
In realicv, coo much significance should not be
attached to che temperature stress computed from che
equation derived. The stress equation is developed from
che strain equation. AA' - >$ Ti, based on the'assump-
tion chat stress is proportional to strain. This assump-
tion is rather inaccurate for the case under ducussioa.
The inaccuracy mav be reclined co some extent by using
i relatively low value for £<, such as £, - 1,300.000
p 1.1., whica is used in this section. An even lower
value may be iiuuned.
Ai computed in che example, a temperature
differential of 75 deg r. jives a stress at 375 p.s.i. m
che extreme fiber. This is probaoiy more than che con-
crete can cake in addition co che regular nng tension
strew without cracking on che colder surface. The
temperature stress may be reduced bv means of insu-
lation. which serves co decrease the temperature dif-
ferential. or additional horizontal reinforcement mav be
provided dose to che colder surface. A procedure will
be illustrated for determination of tempera cure steel.
Ic is not based awn a rigorous mathematical analysis
but will be helpful as a guide and as an aid to engineer-
ing judgment.
It u proposed to base che design on the moment
derived in this section. M. - EJTi/i, in which che
value of £< is taken aa 1. JOO.000 p.s.i. If / is taken for a
secaoa 1 ft. high. I equal* r*. and M is the moment
per ft. Then
M - 1,300,000 X r1 X T X O.OOOOOfi - 9rT
ui.lb. per ft. in which
f m thickness of wall in in.
r • temperature differencial in deg. F.
The area of horizontal steel at the colder face
computed as for a cracker! section is
M
"3552
For example, assume t - 15 m., T • 75 deg. F ,
and d m 13 in., which gives
This area is in addition co che tegular nng steel.
N-8
-------
The stress, f, on the concrete can be determined by the equation:
f = 4.5 T = 4.5 (22°F) = 99 psi
The maximum stress on the wall should be less than 300 psi as dictated
by common engineering practice. Since the temperature stress is only 99
psi, the concrete will not crack.
The amount of reinforcing steel required to resist the temperature
stress can be calculated from the following equation:
A = 9t2T
17,500d
where A,, = cross sectional area of steel per foot of wall - square
inches (sq. in.)
d = effective concrete thickness (21.5" - 2(3") = 15.5")
A_ = 9(21.5)2(22) = 0.34 sq. in.
17,500 (15.5)
The maximum temperature stress occurs in the outer wall face. The
amount of steel in that face is one //8 bar every 9 inches. Since a #8 bar
equals 1 inch diameter or 0.79 sq. in., there is 0.79 x 1.33 or 1.05 sq.
in. of steel in the outer face. This is more than is required to resist
the temperature stress, therefore, the wall will not crack.
N.6. DIGESTION TANK OPERATION
If the existing digestion tanks were cleaned and grit accumulation
kept to a minimum, the following hydraulic and volatile matter loading
conditions would prevail.
N.6.1. Hydraulic Residence Time
Maximum 13,147,055 gallons of capacity
894,786 gallons of sludge per day
= 14.7 days
Average 13,147,055 gallons of capacity
8 1,047,424 gallons of sludge per day
= 12.6 days
M. . 13,147,055 gallons of capacity
ninimum 1,200,062 gallons of sludge per day
= 11.0 days
N-9
-------
N.6.2. Volatile Matter Loading
M. . 276,038 pounds volatile matter per day
Minimum *—___r.-0 r-.—•=—-—- *\ *•
1,757,628 cubic feet of capacity
= 0.157 pounds volatile matter per cubic
foot per day
. 319,030 pounds volatile matter per day
age 1,757,628 cubic feet of capacity
= 0.182 pounds volatile matter per cubic
foot per day
M . 362,022 pounds volatile matter per day
1,757,628 cubic feet of capacity
= 0.206 pounds volatile matter per cubic
foot per day
N.7. GAS PRODUCTION
Gas production is expected to range from 12 to 14 cubic feet per pound
of volatile matter reduced. Volatile matter reduction is expected to be
53.5 percent (same as mesophilic system but with a shorter hydraulic deten-
tion period).
Volatile matter reduced
(0.535X319,030) = 170,681 pounds per day
170,681 pounds x 12 cubic feet „ rt/0 -,„ , . c . ,
L—-,—*- -; = 2,048.172 cubic feet per day
day pound ' ' * i
170,681 pounds x 14 cubic feet = 4 cubic
day pound ' ' v 3
Gas consumption to meet winter and summer sludge heating requirements
(based on 12 digestion tanks) would be:
Winter operation:
0
7.8 xlO BTU x 1 cubic foot gas x 1 BTU delivered
day 600 BTU 0.8 BTU utilized
= 1,625,000 cubic feet per day
N-10
-------
Summer operation:
c
5.4 xlO BTU x 1 cubic foot gas x 1 BTU delivered
day 600 BTU 0.8 BTU utilized
= 1,125,000 cubic feet per day
Average expected excess:
(2,048,172 + 2,389.534) cubic feet _ (1.625,000 + 1.125.QQQ) cubic feet
2 day 2 day .
= 843,853 cubic feet per day available to be sold
N-ll
-------
APPENDIX 0
SECTION 6.3 SUPPORTING CALCULATIONS
0.1. DIGESTED SLUDGE VACUUM FILTER OPERATION
Each vacuum filter has 600 square feet of filtering area. Assuming a
minimum yield for this sludge, mesophilically and thermophilically
digested, of 3.0 pounds per hour per square foot, each filter would be able
to dewater:
3.0 pounds 600 square feet 24 hours
square foot-hour filter day
= 43,200 pounds per day
As indicated elsewhere in this Appendix, if the maximum sludge produc-
tion generated at 334 million gallons per day influent flow rate was
mesophilically-thermophilically digested there would be 316,519 pounds per
day to dewater. The maximum number of filters required would be:
316,519 pounds 1 filter-day
day x 43,200 pounds
= 7.3, or 8 filters required
Assuming an average cake solids of 16.5 percent, the maximum number of
wet tons to be hauled away including the polymer conditioner would be:
{"316,519 pound sludge 640 pounds polymer solids'! 1 ton 1
day day J x 2000 pounds X 0.165
= 961 wet tons per day maximum
0.2. EFFECT ON SYSTEMS HEATING CAPABILITIES DUE TO DUAL HEATING SYSTEM
REQUIREMENTS
0.2.1. Mesophilic System Heat Requirements
The following calculations and numbers are derived and discussed in
Appendix G.
0-1
-------
Winter operation:
r 1,148. 161 BTU 334 MGIF •> 3,195.654 ,BTU
L * J
MGIF* 24 hours hour
= 19,174,228 BTUs per hour
Summer operation:
r 528. 627 BTU 334 MGIFi 1.835.250 BTU
L MGIF x 24 hours J hour
= 9,191,976 BTUs per hour
0.2.2. Thermophilic System Heat Requirements
Heat required for sludge both winter and summer:
1,200.062 gallons 8.64 pounds (122-95)°F 1 day
day x gallon X 1 x 24 hours
= 11,664,603 BTU per hour
Heat required for conductive/convective losses:
Roof - winter operation:
(0.23 BTU) (33.205 square feet)(122-0)°F _ ner
hour-square foot-°F - ~ 931'732 BTU Per hour
Roof - summer operation:
' «» « hour
Walls - winter operation:
(0.13 BTU)(35.577 square feet) [(0.59)(122-0)°F + (0.4l)(122-70)°F]
hour-square foot-°F
= 431,513 BTU per hour
Walls - summer operation:
(0.13 BTU)(35.577 square feet) [(0.59)(122-60)°F + (0.41) (122-70)°F]
hour-square foot-°F
= 267,788 BTU per hour
" MGIF = million gallons influent flow.
0-2
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Floor - summer and winter operation:
(0.11 BTU)(45,195 square feet)(122-40)°F
hour-square foot-°F
= 407,659 BTU per hour
Sludge piping heat losses in thermophilic section are minimal.
Total hourly winter heat requirements:
11.664,603 BTU
hour
1.772.186 BTU 13,436,789 BTU
hour
hour
0.3. DIGESTION TANK OPERATION
At the present time, the existing tank layout lends itself to using
digesters 1 through 8 as the mesophilic units and digesters 9 through 12 as
the thermophilic units. If all the existing digestion tanks were cleaned
and grit accumulation kept to a minimum, the following hydraulic and
volatile matter loading conditions would prevail.
0.3.1. Hydraulic Residence Time
MesoohiHc System
Olcesters 1-3
Thennopnlllc System
Olqestara 3-12
Maximum
Average
Minimum
8.768,000 gallons of capacity
.394,786 gallons of sluge per day
» 9.3 days
8.763.000 gallons of capacity
1,047,424 gallons of sluge per day
• 8.2 days
8.768.000 gallons of capacity
1,200,062 gallons of sluge per day
• 7.2 days
a.384.Oflfi gallons cf capacity
394,786 gallons of sludge per day
• 4.9 days
4.384.000 gallons of capacity
1,047,424 gallons of sludqe per day
- 4.1 days
4.384.000 gallons of capacity
1,200,062 gallons of jludqe oer day
• 3.5 days
Hydraulic residence times are too short for both a mesophilic and a
thermophilic system. If two digesters of same size were added to both
processes, the new hydraulic residence times would be:
Maximum
Average
Minimum
Mesophilic
system
10 tanks
12.2
10.5
9.1
Thermophilic
system
6 tanks
7.3
6.3
5.5
0-3
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0.3.2. Volatile Matter Loading
Volatile matter loading based on 10 mesophilic tanks only:
. 276,038 pounds volatile matter per day
ninimum 1,464,690 cubic feet of capacity
= 0.19 pounds volatile matter per cubic
foot per day
319,030 pounds volatile matter per day
Average 1,464,690 cubic feet of capacity
= 0.22 pounds volatile matter per
cubic foot per day
362,022 pounds volatile matter per day
tlaximum 1,464,690 cubic feet of capacity
= 0.25 pounds volatile matter per
cubic foot per day
0.4. GAS PRODUCTION
Gas production is expected to range from 12 to 14 cubic feet per pound
of volatile matter reduced. The volatile matter reduction should be a
minimum of 60 percent.
Volatile matter reduced:
(0.60)(319,030) = 191,418 pounds per day
191,418 pounds 12 cubic feet = fi cubi{. d
day pound ' ' r
191,418 pounds 14 cubic feet ^ic f^ d
day pound
Gas consumption to meet winter and summer sludge heating requirements
(based on 16 digestion tanks) would be:
Winter operation:
7.9 x 10 BTU x 1 cubic foot gas x 1 BTU delivered
day 600 BTU 0.8 BTU utilized
= 1,645,833 cubic feet per day
0-4
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Summer operation:
5.3 x 108 BTU x 1 cubic foot gas x 1 BTU delivered
day 600 BTU 0.8 BTU utilized
= 1,104,167 cubic feet per day
Average expected excess:
(2,297.016 + 2,679,852) cubic feet (1.654.833 + 1.104,167) cubic feet
2 day 2 day
= 1,108,934 cubic feet per day available to be sold.
0-5
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APPENDIX P
THERMOPHILIC DIGESTION REFERENCES FOR SECTION 2.2 AND 6.2
1. Rimkus, R.R., J.M. Ryan and E.J. Cook, "Full Scale Thermophilic Diges-
tion at the West-Southwest Sewage Treatment Works," Paper
presented at the Annual Water Pollution Control Federation
Conference, Las Vegas, October, 1980.
2. Smart, J. and B.I. Boyko, Full Scale Studies on the Thermophilic
Anaerobic Digestion Process, Report No. 59, Ontario Ministry of
the Environment, Toronto (1977).
3. Popova, N.M. and O.T. Bolotina, "The Present State of Purification of
Town Sewage and the Trend in Research Work in the City of
Moscow," Advances in Water Pollution Research, Vol. 2 (W.W.
Eckenfelder, ed.). MacMillan Co., New York (1964).
4. Garber, W.F., "Plant Scale Studies of Thermophilic Digestion at Los
Angeles," Sewage and Industrial Wastes, 26, 1202-1216 (1954).
5. Garber, W.F., G.T. Ohara, J.E. Colbaugh and S.K. Raksit, "Thermophilic
Digestion at the Hyperion Treatment Plant," Journal Water
Pollution Control Federation. 47, 950-961 (1975).
6. Garber, W.F., "Certain Aspects of Anaerobic Digestion of Wastewater
Solids in the Thermophilic Range at the Hyperion Treatment
Plant," Progress in Water Technology, 8, No. 6, 401-406 (1977).
7. Buhr, H.O. and J.F. Andrews, "Review Paper: The Thermophilic
Anaerobic Digestion Process," Water Research, 11, 129-143 (1977).
8. Puntenney, J.L., Personal communication, January, 1981.
9. Garber, W.F., "Thirty Years of Experience with Thermophilic Anaerobic
Digestion at the Hyperion Treatment Plant in Los Angeles,
California," Unpublished manuscript.
10. Garber, W.F., G.T. Ohara, S.K. Raksit and D.R. Olson, "Studies of
Dewatering Anaerobically Digested Wastewater Solids at the
Hyperion Treatment Plant," Progress in Water Technology, 8, No.
6, 371-378 (1977).
11. Stern, G. and J.B. Farrell, "Sludge Disinfection Techniques," Proceed-
ings National Conference on Composting of Municipal Residues and
Sludges, Information Transfer, Inc., Maryland (1977).
12. Karr, P.R., Personal communication, December, 1980.
13. Heukelekian, H. and A. J. Kaplovsky, "Effect of Change of Temperature
on Thermophilic Digestion," Sewage Works Journal, 20, 806-816
(1948).
P-l
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