EPA-600/2-75-049
November 1975
Environmental Protection Technology Series
RAW SEWAGE COAGULATION AND
AEROBIC SLUDGE DIGESTION
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development,
U.S. Environmental Protection Agency, have been grouped into
five series. These five broad categories were established to
facilitate further development and application of environmental
technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in
related fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental degradation from point and non-
point sources of pollution. This work provides the new or improved
technology required for the control and treatment of pollution
sources to meet environmental quality standards.
This document is available to the public through the National
Technical Information Service, Springfield, Virginia 22161.
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EPA-600/2-75-049
November 1975
RAW SEWAGE COAGULATION AND
AEROBIC SLUDGE DIGESTION
by
Richard H. Jones
T. A. Burnszytnsky
John D. Crane
Environmental Science and Engineering, Inc.
Gainesville, Florida 32604
Grant No. 11010FAC
Project Officer
Edmond P. Lomasney
U.S. Environmental Protection Agency
Region IV
Atlanta, Georgia 30309
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection
Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
ii
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ABSTRACT
Laboratory and full-scale studies were conducted at the Hollywood,
Florida, sewage treatment plant to determine the efficiency of
chemical coagulation for treatment of raw sewage. Various polyelec-
trolytes were investigated in laboratory tests and several cationic
polyelectrolytes were chosen for field study. A full-scale primary
clarifier was converted to a chemical coagulation reactor and clarifier.
Polyelectrolyte addition was evaluated at various dosages and mixing
speeds in order to achieve maximum solids separation. Unit design
deficiencies were noted and evaluated.
The full scale clarifier was unable to duplicate the high treatment
efficiencies achieved in the laboratory tests, probably due to a
lack of adequate mixing and higher concentration of soluble BOD.
A comparative study of aerobic digestion of primary municipal sewage
sludge was also performed at the Hollywood plant. Detention time
and loading rate were deliberately varied along with the naturally
varying temperature, sludge qualities, seasonal flow, evaporation,
and precipitation. Primary and digested sludges were analyzed for
reductions in chemical oxygen demand (COD), biochemical oxygen demand
(BOD), total solids, total volatile solids, suspended and dissolved
solids, alkalinity, pH and oxygen uptake rate. A batch digestion test
was compared to the continuous feed tests. Operating conditions were
optimized for maximum performance based on behavior of digested sludge
on sand beds in terms of filterability, drainability, lack of odor,
and nutrient content. Sludges were successfully digested with as
little as ten days hydraulic detention. Process monitoring parameters
of pH, oxygen uptake rate, and alkalinity were studied and evaluated as
being moderately effective. Note was taken of operating difficulties
such as foaming, poor sludge metering, and equipment deficiencies. A
process design including tank and aerator sizing, and equipment and
operating costs was developed for the existing Hollywood plant based
on actual operating data. A storm water infiltration model, with an
elimination of periodic flow fluctuations, was used to analyze the
effects of rainfall on sewage treatment plant inflow.
It was concluded that aerobic sludge digestion provides an excellent
method of stabilizing sludge prior to final disposal. An oxygen
uptake rate of up to 1.8 mg of 02/(gm T.S.)(hr) was observed for
digestion tests with sludge ages greater than 20 days. Larger up-
take rates were observed for digestion tests of 15 days or less
detention. Sludge alkalinity was reduced to below 200 mg/1 after
15 days and the pH stabilized at near 7.0. The solids content of
m
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the digested sludge was between 4 and 6 percent and further gravity
thickening was not practical. Digestion can be conducted in either
a batch or continuous flow made with a recommended detention time
of 20 days.
This report was submitted in fulfillment of project number 11010FAC
under the partial sponsorship of the Office of Research and Development,
Environmental Protection Agency.
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TABLE OF CONTENTS
Section Page No,
I CONCLUSIONS "I
II RECOMMENDATIONS 3
HI INTRODUCTION 5
IV BACKGROUND INFORMATION 13
V PROCEDURES 34
VI STUDY RESULTS 43
VII DISCUSSION OF RESULTS 86
VIII REFERENCES 1°0
IX APPENDICES 102
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FIGURES
No.
1 Aerial View of Hollywood Sewage Treatment Plant
2 Floating Mechanical Aerator For Aerobic Digester
3 Modified Primary Clarifier with Clariflocculator
4 Effect of Various Dosages of Cat Floe on Removal
of BOD and Suspended Solids in Sewage from the
Effluent of the Gainesville Grit Chamber 14
5 Effect of Various Dosages of Cat Floe on Removal
of BOD and Suspended Solids in Sewage from the
Effluent of a Gainesville Primary Settling Tank 15
6 Effect of Various Dosages of Cat Floe on Removal
of BOD and Suspended Solids in Sewage from the
Effluent of the University of Florida Grit Chamber 17
7 Effect of Various Dosages of Cat Floe on Removal
of BOD and Suspended Solids in Sewage from the
Effluent of a University of Florida Primary
Settling Tank 18
8 Effect of Various Dosages of Dow C-31 on Removal
of BOD and Suspended Solids in Sewage from the
Effluent of the University of Florida Grit Chamber 20
9 Effect of Various Dosages of Primafloc C-7 on
Removal of BOD and Suspended Solids in Sewage from
the Effluent of the University of Florida Grit
Chamber 21
10 Effect of Revolutions of Mixing on Removal of BOD
in Sewage from the Effluent of the University of
Florida Grit Chamber. Initial BOD 120 mg/1, Mixing
Rate 20 rpm, Polymer Dosage 2.8 mg/1, Electrophoretic
Mobility 0.0 [u/(sec)(v)(cm)] 22
11 Effect of Revolutions of Mixing on Removal of
Suspended Solids in Sewage from the Effluent of the
University of Florida Grit Chamber. Initial Suspended
Solids 116 mg/1, Mixing Rate 20 rpm, Polymer Dosage
2.8 mg/1, Electrophoretic Mobility - 0.0 [u/(sec)(v)
(cm)] 23
vi
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FIGURES (Continued)
No. Page
12 Alkalinity and Volatile Solids Reduction with
Detention Time 30
13 Solids, Alkalinity and pH vs. Detention Time 31
14 Increase in Nitrate - Nitrite Nitrogen During
Aerobic Digestion of a Mixture of Raw and Waste
Activated Sludges 32
15 Liquid Fouled Digester Level Indicator 41
16 Foaming on Aerobic Digester 50
17 Q£ Uptake Rate, 2-stage Digestion, 43+ Day Detention 54
18 02 Uptake Rate, 2-stage Digestion, 43+ Day Detention 55
19 0£ Uptake Rate (mg/l/hr), 24-30 Day Digestion 59
20 02 Uptake Rate [mg/(gm T.S.)(hr)], 24-30 Day Digestion 60
21 02 Uptake Rate [mg/0 )(hr)]t 22 Day Digestion 67
22 02 Uptake Rate [mg/(gm T.S.)(hr)], 22 Day Digestion 68
23 02 Uptake Rate [mg/(gm T.S.)(hr)], 22 Day Digestion 69
24 02 Uptake Rate [mg/(l)(hr)], 15 Day Digestion 72
25 02 Uptake Rate [mg/{gmT.S.)(hr)], 15 Day Digestion 73
26 Biological Degradation with Time, Batch Digestion Test 75
27 Alkalinity and pH vs. Time, Batch Digestion Test 77
28 Percent Removal of Solids Versus Detention Time,
Pilot Plant Studies 91
29 A Double Bed of Dried, Well Cracked Aerobic Sludge 92
30 Reduction of Sludge Constituents by Lagooning Outdoor
Samples 97
31 A Well Mixed Aerobic Digester 98
Vll
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TABLES
No. Page
1 Chemical Analyses of Coagulation Unit 43
2 Laboratory Jar Tests 44
3 Full Scale Coagulation Test 44
4 Coagulation of Raw Sewage with Cat Floe 48
5 Average Constituent Reductions Through Two Stage
Digestion 52
6 Average Constituent Reductions At 23-29 Days
Digestion 57
7 Average Constituent Reductions At 29 Days Digestion 62
8 Average Constituent Reductions At 22 Days Digestion 65
9 Average Constituent Reductions At 15 Days Digestion 70
10 Chemical Analyses of Aerobic Digestion Batch Test 76
11 Average Constituent Reductions At 20 Days Digestion 79
12 Average Constituent Reductions At 14 Days Digestion 81
13 Average Constituent Reductions At 10 Days Digestion 83
14 Average Constituent Reductions At 5 Days Digestion 85
15 Comparison of Average Constituent Reductions 87
16 Chemical Analyses of Lagooned Sludges 93
vm
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ACKNOWLEDGEMENTS
The funding and support for this project provided by the City Council
of the City of Hollywood, Florida,is sincerely appreciated. Mr. Wallace
Venrick, City Engineer and Mr. Marshall Bergacker, City Engineer, sup-
plied labor from their departments to accomplish the many tasks of this
project. During the project, the entire treatment plant and particularly
the aerobic digesters were operated most ably by Mr. Bud Calhoun, Mr.
Ernie Atkins, and their staff of plant personnel.
The operation of the pilot plant, laboratory analyses and program direc-
tion were provided by the staff of Environmental Science and Engineering,
Inc., who also compiled the Progress and Final Project Reports. The
aerobic digestion studies were directed by Dr. R. H. Jones who was
assisted in the various duties by Messrs. M. K. Hamlin, R. G. Maxwell,
and the entire laboratory staff. The final report was prepared by
Messrs. T. A. Burnszytnsky and J. D. Crane.
The support given the project by the Office of Research and Development,
Environmental Protection Agency, and the understanding help provided by
the Project Officer, Mr. Ed Lomasney, are very sincerely appreciated.
IX
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SECTION I
CONCLUSIONS
RAW SEWAGE COAGULATION
1. Both laboratory jar tests and laboratory flow-through systems
have shown that certain polyelectrolytes are highly effective for
raw sewage coagulation. This effect can be explained by the exis-
tence of low concentrations of soluble BOD and high concentrations
of colloidal BOD in the wastewater tested.
2. The Dorr-Oliver clariflocculator installed in an existing clarifier
at the Hollywood sewage treatment plant was unable to duplicate
the high treatment efficiencies achieved in laboratory jar tests.
This was probably due to a lack of adequate mixing.
3. The most efficient coagulation of sewage was found at or near an
electrophoretic mobility of 0.0 u/(sec)(v)(cm). However, signi-
ficant reductions in BOD and suspended solids were found over a
wide range of electrophoretic mobility values.
AEROBIC SLUDGE DIGESTION
1. Aerobic sludge digestion provides an excellent method of stabilizing
primary municipal sewage sludge prior to ultimate disposal. It was
indicated that waste sludge from an aerobic digester may contain 40
percent less COD, 80 percent less BOD, 11 percent less total solids,
and 26 percent less volatile solids concentrations than undigested
primary sludge. Actual reductions are higher, but evaporative
losses approaching 25 percent reconcentrate the digested sludge.
2. Aerobically digested sludge will dry at a high rate on a sand bed
with a good underdrain system. Properly digested sludge may be removed
from the bed after four weeks of drying. The sludge should have no
objectionable odor during the drying process. The depth of sludge
applied to the sand bed was approximately 12 inches.
3. Aerobically digested sludges, dried on a sand bed, have little mineral
fertilizer value. Phosphorus values were less than 0.4 percent and
Kjeldahl nitrogen less than 3.7 percent by dry weight. Nitrate plus
nitrite nitrogen were found at less than 0.8 mg/gm of dry sludge.
4. Aerobically digested sludge for all detention times experienced
substantial reductions of waste constituents upon lagooning. Total
mass reductions of COD, BOD, total solids, and volatile solids were
between 30 and 50 percent within a 70-day lagooning period. Consti-
tuent reductions approached zero at 70 days lagooning of batch samples.
A potential problem with lagooning of aerobically digested sludge is
the possibility of odors.
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5. The specific resistance values of aerobically digested sludges were
from 25 to 182 times greater than undigested primary sludge. It
would therefore appear that aerobically digested sludges are more
amenable to mechanical dewatering than anaerobically digested sludges.
6. Digested primary municipal sewage sludge was not amenable to further
gravity thickening. The total solids concentrations of the aerobic
digester were usually between 40,000 and 60,000 mg/1.
7. Process monitoring parameters could be used to detect gross changes
in sludge quality. An oxygen uptake rate up to 1.8 mg of 02/(gmT.S.)(hr)
characterized, digestion tests with sludge ages greater than 20 days.
Uptake rates of greater than 1.8 mg of 02/(gmT.S.)(hr) occurred with
digestion tests of 15 days or less detention. Alkalinity of aerobi-
cally digested sludge was greatly reduced from that of primary sludge.
Sludges digested for 15 days or longer had average alkalinities be-
low 200 mg/1. There was a gradual and small rise in the pH of di-
gested sludges from 5.9 to near 7.0 within the digestion periods of
this program. Of the three parameters just mentioned, none provide
an accurate picture of sludge quality that would relate to behavior
on a sand bed or upon filtration without further correlation and
testing.
8. Visual parameters of aid to the plant operator are the color and
sludge thickness in the digester.
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SECTION II
RECOMMENDATIONS
RAW SEWAGE COAGULATION
Further research on coagulation of sewage with polyelectrolytes should
only be conducted in treatment systems especially designed to provide
adequate mixing and settling conditions with high concentrations of
colloidal BOD.
Control systems need to be found which can automatically regulate
polyelectrolyte addition to provide optimum coagulation during wide
fluctuations of wastewater flow and composition normally found at
sewage treatment plants.
New polyelectrolytes should be developed which will provide effective
sewage coagulation at a minimum cost, as existing polymer costs are
quite high.
AEROBIC SLUDGE DIGESTION
It is recognized that there is a growing trend in this country to
provide at least secondary treatment for municipal wastewaters. When
primary waste sludges alone, or even combined with waste activated
sludges, are to be stabilized prior to final disposal, aerobic diges-
tion should be given serious consideration as a prime alternative.
Specific design recommendations are presented in Appendix B,
The operation and maintenance of an aerobic digester is relatively
simple and trouble free. Still needing to be determined are rapid
physical or chemical tests which would indicate the degree of digestion
a sludge has undergone and which could be accurately related to future
behavior of the sludge in drying and final disposal. Until such tests
are available, design parameters will need to be conservative and allow
for overdigestion to produce a given stabilization. These tests should
be developed with full- scale aerobic digesters under the rigors and
changing conditions of daily operation.
Mechanical equipment should be improved for use with aerobic digesters.
Currently, wind driven spray from mechanical mixers coats equipment
and surrounding areas. Future designs should include provisions for
spray containment.
Accurate determinations of volume change in a large scale digester
cannot readily be made. Fouling of measuring devices by detritus
or foam blankets precludes the use of most inexpensive devices avail-
able. Similarly, sludge transfer mechanisms, particularly under gravity
flow, need reliable metering equipment. Such equipment could greatly
improve an operator's control over his plant and additionally could be
the tools needed to find process monitoring parameters.
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In view of the trends toward secondary treatment of municipal wastewaters,
it is strongly recommended that a study similar to the one described
herein be performed for primary and waste activated or trickling filter
sludges combined with an appropriate representation of the various waste
activated and trickling filter sludge available.
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SECTION III
INTRODUCTION
This research program involving a study of raw sewage coagulation and
aerobic sludge digestion originated in 1968 when, as a result of the
increasing number of ocean outfalls and the lack of knowledge concerning
their effects, the Environmental Protection Agency funded a grant to the
city of Hollywood, Florida, to demonstrate new sewage treatment methods.
Environmental Science and Engineering, Inc., of Gainesville, Florida, was
contracted to perform the necessary research work.
The study of raw sewage coagulation and aerobic sludge digestion
was part of the overall project which also dealt with a study of diffu-
sion from the Hollywood ocean outfall and a biological study of the
effects of the outfall on the environment. The results of the outfall
studies have been published in a separate report.
Financial support for the research effort necessary to accomplish this
project was partially provided by an EPA Research and Development Grant
(No. 57 (RO-01-68) of $300,000 which is equivalent to 50 percent or
less of eligible project costs. The City of Hollywood provided the bal-
ance of the project funding.
THE HOLLYWOOD SEWAGE TREATMENT PLANT
The City of Hollywood Sewage Treatment Plant in Hollywood, Florida,
serves a principally residential region with very little industry.
The Hollywood region is one of rapid growth and ever expanding pop-
ulation. The City of Hollywood and adjoining communities are situated
on coastal land bordered on the east by the Atlantic Ocean. West of
the city at distances of 10 to 15 miles from the coast lie extensive
freshwater swamps.
The city of Hollywood draws its water from local aquifers. Increasing
population demands on these aquifers have led to salt water intrusion
along the coast and newer wellfields are being utilized farther inland.
The salt water intrusion into the ground water table at the coast has
been noticed in the seepage to the city's sewage system. High chloride
levels of 3,500 mg/1 are common in raw sewage entering the treatment
plant. At some locations on the coast, very old and leaking sewer
systems experience direct seawater infiltration and thus substantially
increase the salinity of the total sewage flow to the sewage treatment
plant. A further discussion of sewer line infiltration is contained in
Appendix A.
The average plant flow of raw sewage in 1971 was 13.6 million gallons
per day. Typical influent concentrations of constituents were bio-
chemical oxygen demand at 130 mg/1, chemical oxygen demand at 340 mg/1,
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suspended solids at 110 mg/1, and chloride ion at 3,500 mg/1. These
values are indicative of a relatively weak sewage generated by resi-
dential communities with little industrial input. The topography of
the Hollywood area is flat with a low elevation. Sewer flows are at
minimum velocities and sewage has often arrived at the plant in an
anaerobic state due to long residence times in the sewers. There
being no storm sewers in the Hollywood area, storm water and sea water
infiltration is collected by the sanitary sewer system. This additional
water provides a noticeable dilution of normal sewage constituent con-
centrations.
At the time of the study the wastewater treatment plant (shown in
Figure 1) served solely as a primary solids separation plant. Plant
flow was 13.6 MGD with a capacity of 36 MGD. Raw wastewater was
initially screened before flowing to grit collectors. The grit col-
lectors could remove a minimum of 95 percent of TOO mesh grit having
a specific gravity of 2.65. The plant was equipped with two grit collec-
tors each capable of handling a maximum flow of 40 MGD.
The de-gritted wastewater flowed to a below grade influent pumping
station. This pumping station provided the lift necessary to feed
the primary clarifiers which were located entirely above ground level.
Flow from the pumping station was distributed to the various clarifiers
by an elevated distribution box.
The physical plant had nine primary clarifiers; however, only six of
these were in use during the research project. The primary clarifiers
varied in diameter from 65 feet to 120 feet and in depth from 9 ft.
7 in. to 11 ft. 5 in. The efficiencies of the primary clarifiers in
removing solids are presented with the data for each specific test run.
Solids were wasted from the primary clarifiers1 sludge well by manual
control. Sludge would be pumped from the clarifier to the digester until
the operators noticed a lessening in solids from the clarifier. At that
time the pumps would be stopped with a relatively light liquid present
in the pipes leading to the clarifiers. This light liquid would some-
what dilute the sludge in the next pumping cycle. Progressive cavity
sludge pumps from the primary clarifiers were equipped with time clocks
and during the program were calibrated against a measured liquid flow.
Sludge digestion facilities consisted of three 75 ft. diameter aerobic
digesters with an operating depth of 12.0 feet and individual capacities
of 396,345 gallons. Based on normal design criteria for primary sludge
of three cubic feet per capita, the aerobic digesters could handle a
design population of 53,000. This criterion will be re-evaluated in a
later section. Each digester was equipped with a 100 HP floating me-
chanical aerator capable of providing 3.5 pounds of oxygen per horse-
power hour as shown in Figure 2. Based on the results of this study,
design recommendations for aerobic sludge digestion are presented in
Appendix B.
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FIGURE 1: AERIAL VIEW OF HOLLYWOOD SEWAGE TREATMENT PLANT
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FIGURE 2: FLOATING MECHANICAL AERATOR FOR AEROBIC DIGESTER
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Digested sludge was either wasted onto sludge drying beds or dewatered
in two experimental centrifuges. Ultimate sludge disposal was in land-
fill areas.
Septic tank waste was received at a concrete unloading station with a
receiving channel equipped with 45° bar screen. The septic tank sta-
tion's wet well had a liquid capacity of 11,800 gallons and a pump-
out capacity of 10,230 gallons. The septic tank raw sludge pump trans-
ferred the screened septic tank raw sludge to Digester No. 3 during
most of the program except for the last two tests with the permanent
plant digesters.
The sewage treatment plant effluent was discharged by gravity flow
through a 9,700 foot ocean outfall. The diffusion of the effluent
and its biological effects were also subjects of this study and are
covered in a separate report.
The primary wastewater treatment plant had recently undergone conversion
from a secondary trickling filter plant of lower capacity. At the start
of the research effort,conversion of all equipment and the laboratory
had not yet been completed. This occasionally resulted in discontinuous
operating conditions for the aerobic digesters and a lack of analytical
data in the early part of the program. With time, difficulties in
plant operations were eliminated and both plant performance and opera-
ting data improved.
OBJECTIVES
Raw Sewage Coagulation
Coagulation of raw sewage with polyelectrolytes was one objective of
this research and demonstration project. The City of Hollywood sewage
treatment plant consisted of only primary sedimentation with final
discharge through an ocean outfall. The use of polyelectrolytes to
coagulate raw sewage within the primary clarifiers held the potential
of significantly increasing treatment efficiency with minimum capital
investment. In order to demonstrate the coagulation of raw sewage with
polyelectrolytes within a primary clarifier, the necessary modifications
were made and equipment installed in an existing 2.0 MGD clarifier, as
shown in Figure 3. Results of laboratory and plant scale tests are re-
ported in Section VI.
Aerobic Sludge Digestion
The basic objectives were to study the operation of an aerobic digester
processing primary sewage sludge and to provide operating and design
data applicable to future aerobic digesters in the treatment of muni-
cipal wastewaters. The specific objectives in this project were greatly
influenced by the limitation of the project facilities. The City of
Hollywood had recently converted an old trickling filter sewage treat-
ment plant to a primary sedimentation plant with an ocean outfall for
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CENTERLINE OF MACHINE
NEW CHECKERED PLATE
(ALUM.) EL. 19.77
EXISTING RAIL CUT
S FITTED TO SUIT
n
FLOCCULATOR TANK
MOLDED FIBERGLASS
EXISTING CENTER
OLUMN tfl
CUT EXISTING SKIMMER
POST HERE
Figure 3. Modified Primary Clarifier with Clariflocculator
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the effluent. Some construction work was still proceeding at the
inception of this program. Limitations, therefore, had to be im-
posed on the scope and depth of project objectives in keeping with
new, often partially functioning equipment, and a plant staff bur-
dened with the necessity of establishing a properly functioning
facility.
The first objective during the project was to get the primary plant
and the aerobic digesters operating. During the initial months
of the study so much effort went into this task that process control
and laboratory analyses were minimally conducted. Valuable informa-
tion was obtained on general plant operation, equipment performance,
and suitability.
The second objective was to experimentally determine the optimum
digester operating conditions for maximum performance. Once an
operating routine had been established and the equipment shakedown
completed, a series of tests was conducted to verify results achieved
by previous researchers in pilot and laboratory scale. Detention time
was deliberately varied to determine a minimum economical period for
sludge stabilization. Digester loading could not be varied except
by detention period because of the already thick sludge being pumped
from the primary clarifiers.
A third objective was to obtain a basic analytical characterization
of the aerobic digestion process. Primary sludge constituents of
total solids, volatile solids, suspended solids, pH, chemical oxygen
demand, biochemical oxygen demand, alkalinity, and chloride were
measured, tabulated, and compared with the resulting sludges in the
aerobic digesters. This information provided invaluable design data.
It was also helpful in defining the best method of ultimate sludge
disposal.
The fourth objective was to provide process control parameters through
the analyses and evaluation of sludge constituents. Previous research-
ers had reported changes in all constituents, particularly alkalinity
and pH with increased digestion. This project measured these changes
in full-scale digesters under actual operating conditions in order to
evaluate their suitability as control tools for the plant operator.
A fifth objective was to evaluate stabilized sludge qualities, both
physical and chemical, and to relate them to aerobic digester
operating conditions. Sludges were measured for settleability,
filterability, odor, drying characteristics on sand beds, and final
nutrient content. Such information would enable an evaluation of the
final end product of digestion, the stable sludge, with respect to
further study steps.
The sixth objective was to provide realistic plant scale operating
costs for the aerobic digestion process. Operating economics were
determined with a high degree of reliability.
11
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A specially tailored computer program was used to determine the
effects of rainfall through sewer line infiltration on total plant
flow. Two years of operating data provided a basis upon which a
computer separated regular daily and seasonal variations from
fluctuations due to storm water infiltration. The results are
presented in Appendix A.
12
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SECTION IV
BACKGROUND INFORMATION
Raw Sewage Coagulation
Domestic sewage is composed of a wide variety of both dissolved and
suspended organic and inorganic matter. In the past the popularity
of chemical treatment of sewage has undergone several cycles of rising
and waning. As a sole process for treatment of sewage, chemical treat-
ment possesses the inherent limitation of being less effective for the
removal of soluble organic matter than biological processes.
The most frequently used coagulants, salts of iron and aluminum, due
to their ability to remove phosphates, are finding ever wider appli-
cation. The use of synthetic organic polyelectrolytes for wastewater
coagulation has been advocated by manufacturers for a number of years.
However, little laboratory data and even less full scale data are
available on the subject.
Laboratory tests indicated that several cattonic polyelectrolytes
are capable of effectively coagulating both raw and settled domestic
sewage. Figure 4 and Table Cl in the Appendix show the effects of
various dosages of Calgon Cat Floe on removal of BOD and suspended
solids from the effluent of the Gainesville, Florida, Sewage Treat-
ment Plant grit chamber. Figure 4 shows that initial BOD concentra-
tions could be reduced from 152 mg/1 to 19 mg/1, a removal efficiency
of 86 percent. Suspended solids were reduced from 130 mg/1 to 10 mg/1
for a removal efficiency of 92 percent.
Also shown in Figure 4 is the change in electrophoretic mobility with
polymer dosage. Electrophoretic mobilities were measured by the use
of a Zeta-Meter (Zeta-Meter, Inc.). Initial electrophoretic mobility
values of colloidal particles were found to average 1.7 u/(sec)(v)(cm)
and these values were increased with increasing dosages of the cationic
polymer.
Figure 5 and Table C2 in the Appendix show the effects of various dos-
ages of Cat Floe on removal of BOD and suspended solids from the effluent
of a primary settling tank. Initial BOD concentrations were reduced from
170 mg/1 to 33 mg/1 for a reduction efficiency of 80 percent. Initial
suspended solids concentrations were reduced from 80 mg/1 to 17 mg/1
for a reduction efficiency of 79 percent. Electrophoretic mobility
values were changed from 1.8 to + 0.3 u/(sec)(v)(cm).
A comparison of Figures 4 and 5 shows several interesting facts. In
Figure 4, BOD and suspended solids removal efficiencies were quite high
at low polymer dosages as compared to Figure 5. This is due to the fact
13
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14
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Fig. 5 Effect of Various Dosages of Cat Ploc on
Removal of BOD and Suspended Solids in Sewage
from the Effluent of a Gainesville Primary
Settling Tank.
15
-------
that Figure 4 shows results of coagulation of unsettled raw sewage and
Figure 5 shows results of settled sewage. A significant reduction of
BOD and suspended solids would have been realized without polymer ad-
dition to raw sewage due to sedimentation alone,while little or no
reduction would have been realized in the primary settled sample without
polymer addition. Polymer dosages cannot be compared because the exact
chemical composition of each sample was not determined and varied widely.
In general, the optimum dosage of polymer was that dosage which changed
the electrophoretic mobility of domestic sewage from approximately - 1.8
u/(sec)(v)(cm) to approximately 0.0 u/(sec)(v)(cm). The addition of
cationic polymer dosages in quantities required to increase the electro-
phoretic mobility to greater than 0.0 u/(sec)(v)(cm) generally caused a
reduction in treatment efficiency as would be expected due to the repulsion
of like charges of the colloidal particles.
The high reduction of BOD was due to the high reductions in suspended
and colloidal BOD and not to removal of soluble BOD. Laboratory analyses
on filtered and unfiltered samples showed that the soluble BOD of the
Gainesville sewage was exceedingly low which resulted in high percentage
removal of BOD through polyelectrolyte coagulation. Each of the samples
tested were grab samples and, therefore, do not necessarily represent
the "typical" sewage at the Gainesville sewage treatment plant.
Figure 6 and Table C3 in the Appendix show the effect of various dosages
of Cat Floe on removal of BOD and suspended solids in sewage from the
effluent of the University of Florida grit chamber. Initial BOD concen-
trations were reduced from 120 mg/1 to 30 mg/1 for a reduction of 75
percent. Initial suspended solids concentrations were reduced from 95
mg/1 to 0.0 mg/1 for a reduction of 100 percent.
Although electrophoretic mobility values were increased from - 1.7 to
+ 1.1 u/(sec)(v)(cm), there was little reduction in BOD removal effi-
ciencies at the higher mobility levels. Suspended solids removal ef-
ficiencies remained highest at approximately 0.0 mobility values, and
decreased as mobility values increased above 0.0 u/(sec)(v)(cm).
Figure 7 and Table C4 in the Appendix show the effect of various dosages
of Cat Floe on removal of BOD and suspended solids in sewage from the
effluent of a University of Florida primary settling tank. BOD concen-
trations were reduced from 50 mg/1 to 5 mg/1 for a removal efficiency
of 90 percent. Suspended solids concentrations were reduced from 31
mg/1 to 2.0 mg/1 for a removal efficiency of 94 percent.
Electrophoretic values were increased from-1.9 to + 1.2 u/(sec)(v)(cm).
The highest efficiencies for removal of BOD and suspended solids oc-
curred at electrophoretic mobilities of approximately 0.0 u/(sec)(v)(cm).
Results shown in Figures 4 through 7 indicate the high level of treat-
ment efficiency which can be realized by proper coagulation of domestic
16
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Polymer Dosage (mg/1)
0
9 10 11 12
BOD
Suspended
Solids
2345678
Polymer Dosage (mg/1)
9 10 11 12
Fig. 6 Effect of Various Dosages of Cat Floe on
Removal of BOD and Suspended Solids in Sewage
from the Effluent of the University of Florida
Grit Chamber.
17
-------
4J r-i
A3 *—*
* > g
o V.H.
jC .rl^
ft .H >
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Fig. 7 Effect of Various Dosages of Cat Floe on
Removal of BOD and Suspended Solids in Sewage from
tho Effluent of a University of Florida Primary
Settling Tank.
18
-------
sewage with the cationic polyelectrolyte Cat Floe. Figure 8 and Table
C5 in the Appendix show the effect of various concentrations of Dow
C-31 (Dow Chemical Company). Figure 9 and Table C6 in the Appendix
show the effect of various concentrations of Primafloc C-7 (Rohm
and Haas Company) on removal of BOD and suspended solids and changing
mobility values from negative to positive values. In general, re-
moval efficiencies were greatest at mobility values of 0.0 u/(sec)
(v)(cm).
Perhaps the most important factor in the use of polyelectrolytes for
coagulation of sewage is mixing or energy input. A distinction can
be made between the terms coagulation and flocculation in order to
describe more accurately the basic mechanisms of polymer colloid
interactions. Coagulation is defined as being a general kinetic
process which obeys the simple Smoluchowski equation. It is brought
about by neutralization of the repulsive potential of the electrical
double layer, allowing the forces of attraction between particles
to bring them together. Flocculation, on the other hand, is visual-
ized as a completely different mechanism whereby colloidal particles
are bound together by a bridging mechanism in which adsorption plays
the major role.
Assuming that an adequate polymer dosage has been introduced to
reduce the repulsive potential of the electrical double layer,
i.e., increase the electrophoretic mobility from a negative value
to near 0.0 u/(sec)(y)(cm), then it is essential to provide adequate
mixing so that individual colloidal particles may come together to
form larger particles which will settle from suspension.
Figure 10 shows the effect of revolutions of mixing in a jar test
machine on removal of BOD from the effluent of the University
of Florida grit chamber. The initial BOD was 120 mg/1, mixing
rate 20 rpm, polymer dosage 2.8 mg/1, and electrophoretic mobility 0.0
u(sec)(v)(cm). Figure 11 shows the effect of mixing in the same
jar test for the removal of suspended solids. These data are tab-
ulated in Table C7 in the Appendix. Results of these tests simply
show that with increased mixing or energy input up to some maximum
value, removal efficiencies for BOD and suspended solids increased.
Excess mixing led to the limitation of particle size due to sheer
forces.
The above laboratory results demonstrated that a domestic sewage with
a relatively low concentration of soluble BOD coagulated with cationic
polyelectrolytes in a properly designed and operated system with ade-
quate mixing and settliqg.
Effective use of cationic polyelectrolytes for sewage coagulation has
been limited by their low efficiency when attempts have been made
to use them in existing sewage plants, the desire being to upgrade
the efficiency of an existing treatment system at a minimum capital
expenditure. These attempts have almost always resulted in failure
19
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u
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180
160
140
120
100
80
60
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9 12 15 18 21 24 27 30 33 36
Polymer Dosage (mg/1)
BOD
Suspended
Solids
0 3 6 9 12 15 18 21 24 27 30 33 36
Polymer Dosage (mg/1)
Effect of Various Dosages of Primafloc C-7 on
Removal of BOD and Suspended Solids in Sewage
from the Effluent of the University of Florida
Grit Chamber.
21
-------
75
60
~ 45
\
Cn
E
Residual BOD
M U)
O (J) O
— — _
— _
— .
- — c
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- Cat Floe N7o
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^— ^
\^,
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3*2 °*5 12 5 10 20 50
Paddle Revolutions (Hundreds)
Fig. 10. Effect of Revolutions of Mixing on Removal of BOD from the
Effluent of the University of Florida Grit Chamber.
-------
ro
OJ
Residual Suspended Solids
(ir.g/1)
H NJ CO ,£» CT
o M *• a\ co c
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0.2 0.5 1 2 5 10 20 50
Paddle Revolutions (Hundreds)
Fig. 11 . Effect of Revolutions of Mixing on Removal of Suspended Solids
from the Effluent of the University of Florida Grit Chamber.
-------
due to lack of facilities to provide proper mixing conditions required
for coagulation. The inefficiency of cationic polyelectrolytes for
removal of soluble BOD, and their high costs, have made it uneconomical
to design a new sewage treatment plant which would provide proper con-
ditions for effective coagulation.
The use of polyelectrolytes for treatment of domestic sewage in existing
plants has been primarily limited to anionic and nonionic polymers
whose principal mechanism is flocculation. Flocculation, in which
adsorption plays the major role, does not require a high level of
mixing for effective coagulation. Flocculation is usually effective
for removal of large particles, such as fibers, and ineffective for
colloidal removal. Therefore, in sewage treatment plants, efficiency
in BOD and suspended solids removal is usually much lower than that
which could be achieved if adequate mixing were provided with the use
of cationic polymers.
Aerobic Sludge Digestion
A considerable amount of research effort has been expended in converting
soluble and fine suspended matter into insoluble and readily settleable
solids which may be removed by standard physical separation processes.
While the subsequent stabilization or elimination of collected sludges
from the wastewater treatment systems has long been a neglected field,
anerobic sludge digestion is not an intrinsically new or unique pro-
cess. Basic bio-oxidation lagoons convert soluble and some insoluble
organic matter into carbon dioxide, water, and cell matter. In the
extended aeration process, retention times of the wastes are suffi-
ciently long that initial organic solids and newly created activated
sludge solids are biologically converted under aeration to carbon
dioxide and water, although in practice the extended aeration process
experiences a slow buildup of inert matter that must eventually be
wasted.
In the aerobic sludge digestion process and in the preceding examples,
the underlying principle of sludge digestion is the auto-oxidation of
biological material. When deprived of a food source, microorganisms
undergo endogenous respiration in a continuing process of digestion
that produces carbon dioxide, water, and fewer cells than at the start
of digestion. Eventually the self destruction of biological matter
reaches an irreducible minimum and the sludge is said to be stabilized.
A stable sludge has a minimum of organic or volatile solids but re-
tains many of its original inorganic nutrients. Stable sludges are
relatively free of odor, are more acceptable to final disposal on
land or in the ocean, and often have characteristics of easier handlea-
bility.
Aerobic digestion differs from other aeration processes in that
primary settled solids and waste activated solids, separated from
the main wastewater stream, are concentrated and aerated in a separ-
ate reactor. Advantages of the aerobic sludge digestion process
include the smaller volumes of liquid that need to undergo the ex-
tended aeration process, and the reduced capital resulting from
24
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those smaller volumes. Other advantages claimed for the aerobic
digestion process are ease and simplicity of operation, the highly
stable nature of the end product, and improvements in the ease of
handling the digested sludges.
In order to provide reasonable comparisons between various batches of
data on the aerobic digestion of sludges, a common language with com-
mon sets of parameters which may be transferred from one data group
to another needs to be employed. Previous work in this field has
been scattered in scope and has resulted in inconsistent terminology.
Differing waste characteristics have complicated most comparisons.
The data on sludge characteristics at least provide performance goals
as well as some basis for comparison against other processes.
Eckenfelder1, studying aerobic digestion of domestic sewage activated
sludge in a batch process at 25°C, achieved a reduction in chemical
oxygen demand of 48 percent in 7 days; a 48.7 percent reduction in
total suspended solids; and a 38.3 percent reduction in total sus-
pended solids. Carpenter and Blosser? achieved a 14.5 percent reduction
in volatile solids after 30 days batch digestion at 30°C with waste
activated boardmill sludges.
Jaworski, Lawton, Rohlich3 performed continuous feed studies on a mix-
ture of primary and waste activated domestic sludges. At 20°C they
reported a 44 percent volatile solids reduction with a 30 day hydraulic
detention time and 46 percent volatile solids reduction after 60 days
retention time. The loading rate during the test varied with detention
period due to a fixed reactor volume and feed concentration. More than
600 mg/1 of nitrate nitrogen was reported after 60 days retention time.
Malina and Burton4 recorded a 43 percent reduction in volatile solids
in a continuous feed process treating primary domestic sludge. The
hydraulic retention time of 15 days was achieved with a sludge loading
rate of 0.14 Ib/(ft3)(day) at 35°C.
Nature of Waste
The performance of an aerobic digester depends on several independent
variables, of which the designer has little control. The nature of
the original waste processed in the sewage treatment plant is paramount
in a determination of sludge treatability. A 12 percent reduction in
volatile solids with aerobic digestion of activated sludge from domestic
wastes at 20°C and 12 days aeration was observed by Carpenter and Blosser2.
Also at 20°C, they found 7 percent reductions in 27 days and 9 percent
reductions in 19 days with activated sludge from boardmill wastes and
de-inking wastes, respectively. A laboratory study by BarnhartS demon-
strated markedly differing volatile solids reductions for several types
of wastes. Reductions of volatiles achieved at 8 days aeration were 29
percent with mixed pulp and paper waste; 42 percent with biochemical
wastes; 46 percent with domestic sewage; and 60 percent with textile and
domestic wastes.
25
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Sludge Age
Sludge age has been reported to be a good indicator of expected volatile
solids reductions for any particular waste, but researchers have been
inconsistent in defining their terminology. Work done by Norman^ at the
University of Wisconsin described a semi-logarithmic correlation between
percent volatile solids reduction and sludge age. Increasing sludge
age caused increased reductions. Norman defined sludge age to be
the ratio of the weight of volatile solids in the digester to the
weight of volatile solids added daily. In essence his report gave a
mathematical fit to the statement that with increased treatment time,
the rate of volatile solids reduction decreases exponentially.
The common definition of sludge age relates to the theoretical detention
time of any particle of solid matter in the activated sludge process.
Loehr7 indicated that sludge with a high incoming sludge age will ex-
perience a lower reduction in volatile solids than a sludge with a low
age. This was attributed to a previous partial oxidation of the older
sludge in the activated sludge system. An application of this obser-
vation may be made in explaining the differences in oxygen demand between
raw settled sludge and aerated activated sludge. The fresh raw sludge
must first undergo synthesis of biomass and this raises oxygen require-
ments in the aerobic digester, according to Loehr, by a factor of six.
Detention Time
Most researchers agree that most of the volatile solids reduction in an
aerobic digester occurs within the first 15 days. A typical set of data8
relating volatile solids reductions to detention time at 20°C reads as
follows: 10 percent reduction at 2 days, 24 percent at 5 days, 41 per-
cent at 10 days, 43 percent at 15 days, and 46 percent at 60 days.
Doubling digester volume from 15 days detention to 30 days detention
would substantially increase capital costs while providing, in this case,
approximately a 1 percent decrease in volatile solids content.
Very tentative research work conducted by Bruemmer^ on the use of oxygen
enriched air for sludge digestion indicated that sludge stabilization
could occur in a shorter period of time using oxygen. However, the data
indicate eventual equal levels of BOD stabilization.
In batch tests conducted on municipal waste activated sludge, Reynolds
achieved better than 50 percent reduction in volatile solids in less than
6 days aeration. This represented stabilization of most of the biode-
gradable material present.
Kehr further substantiated the more rapid stabilization of waste
activated sludges by reporting that the oxygen uptake rate was minimal
after 5 days digestion in a batch test. Dryden, e£. al_. ,12 reported
a 25 percent per day reduction in volatile suspended solids during the
aerobic digestion of pharmaceutical waste activated sludge.
26
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Reaction Temperature
Another factor involved very heavily in aerobic digester performance is
the temperature at which reaction takes place. For long detention periods
of approximately 60 days and for short detention periods of less that 5
days, temperature differences do not appear to affect volatile solids
reductions3. Reactions may be considered essentially complete at 60 days
for any temperature between 15°C and 35°C, while reaction rates below 5
days detention are so rapid as to be unaffected by temperature. At in-
termediate detention times, typical volatile solids removals reported
for several temperatures were 45 percent at 35°C, 41 percent at 20°C,
and 32 percent at 15°C, all at 8 days3; and 11,4 percent at 20°C and
13.7 percent at 30°C over a period of 25 days2. From this data it would
appear that increased temperatures are extremely beneficial to the effi-
ciency of the aerobic digestion process, but it has been reported/ that
high temperatures on the order of 60°C are not as effective in improving
aerobic digestion of sludges. An optimum temperature would appear to
lie near 30 to 35°C with the exact temperature used in an actual plant
depending on the economics of artificially heating the aerobic digester.
Loading Rate
The final design parameter which affects aerobic digester performance is
the loading rate or rate of application of fresh feed per unit volume of
digester. The loading rate is not an independent variable in that it will
be determined by incoming volatile solids concentration in the sludge
liquor and also by the design retention period which fixes the aerobic
digester tank volume. Most investigators have used a fixed loading rate
determined by volumetric detention time and thus have reported optimum
loading rates on the order of 0.10 IDS volatile solids/(ft3 of digester)
(day) which roughly corresponds to 12 to 15 days retention time. In the
study by Malina and Burton4, loading rates were varied from 0.10 to 0.14
Ibs/(ft3)(day) with a 15 day retention period. At 35°C the higher loading
rate was associated with a 43.2 percent volatile solids reduction as com-
pared to 33.3 percent at the lower rate. Further comparable data is
lacking of effects of loading rates on aerobic digester performance, but
an upper limit would eventually be reached due to mixing and oxygen transfer
limitations. The exact loading rate would still be beyond the direct con-
trol of the plant designer or operator due to the previously mentioned
reasons.
Oxygen Uptake
Reliable data on oxygen uptake rates are generally unavailable for
aerobic digestion of sludges. Research work to date has included
oxygen supply rates to fixed volume containers, but with little mention
of oxygen transfer methods or efficiencies. Various uptake rates have
been measured for endogenous respiration of sludge with values ranging
from 2 to 10 mg 02/(hr)(gm VSS) depending on the nature and age of the
sludge. Since a 400 percent variation in oxygen uptake rate will make
a considerable difference in the sizing of aeration equipment_for the
digester, it is necessary to measure specific rates for individual
sludges prior to design.
27
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Sludge Dewatering
Previous investigators have not reported methods used for digested
sludge thickening or discussed difficulties encountered with such
operations. Brief discussions have been presented of supernatant
decanting or centrifugation with no reliable supporting figures.
Research has usually been conducted with a 2 to 5 percent concentra-
tion of sludge transported directly to a dewatering process. Irgens
and Halvorsonl3 did report, however, that aerobically digested raw
sludge settled poorly but that settleability increased to excellent
when waste activated sludge was added to the digestion process.
Liquid drainage from aerobically digested sludge is different from
that obtained with anaerobically digested sludge. Randall and Koch14
reported that aerobically digested waste activated sludge demonstrated
more rapid drainage in a sand bed with a higher percentage of drainable
water. Drainage and drying were both improved when dissolved oxygen
levels were kept above 1.0 ppm and by increased retention time. Dreier
and ObmaS agreed with Lawton and Normanl5 that increased digestion
periods were desirable for improved liquid - solids separation and
suggested a minimum retention period of 10 days. It has also been
reportedl6 that aerobic treatment of an anaerobically digested sludge
greatly improves the filterability of the product sludge.
Supernatant
The supernatant obtained from either decantation or centrifugation of
aerobically digested sludge is of sufficiently fine quality that basic
plant performance is not upset when the supernatant is reintroduced to
the headworks. Biological oxygen demand values of supernatant have
been consistently reported to be less than 100 mg/1 -- with a single
high value of 240 mg/1 found by Vararaghavan17. Chemical oxygen demand
values have ranged below 700 mg/1. Malina and Burton^ achieved reduc-
tions of 70 to 82 percent in COD using a continuous feed apparatus on
primary sludge with a 15 day retention period. Research in batch oper-
ations has been conducted on waste activated sludge and combinations of
waste activated sludge and primary waste sludge for varying detention
times (10, 15, and 60 days). As a result, direct comparison between
specific values becomes difficult and almost meaningless in view of the
fact that reductions depend so heavily on the original nature and source
of the raw sewage.
Process Monitors
Certain effluent parameters, in addition to BOD, COD, and solids, which
may be of use in monitoring process conditions are alkalinity, pH, and
nitrate content of the supernatant or mixed sludge. Values for these
parameters have been duplicated in batch studies by various researchers
with some supporting evidence on the applicability of these values to
continuous feed processes.
28
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Researchers have consistently reported gradual or no decline in alka-
linity with retention times up to 15 days, and more rapid declines in
alkalinity thereafter up to 60 days. This has been attributed to re-
moval of C02 and subsequent buffering capacity during the aeration
process. Inasmuch as the greater majority of endogenous respiration
with its concomitant release of C02 occurs during the first 15 days
of retention time, minimal reduction in alkalinity is expected during
that period. As an example, Dreier and Obma^ reported alkalinities of
510 mg/1 at starting, 560 mg/1 after 15 days, and 81 mg/1 after 30 days
of digestion.
Figure 12 taken from Viraraghewanl?, shows the close relationship be-
tween alkalinity reduction and volatile solids reduction at various
detention times. Figure 13 shows a similar comparison from a report by
Dreier and Obma^.
In batch aerobic digestion of waste activated or primary sludges, the
pH of the system undergoes an increase during the period of most active
digestion and thereafter the pH decreases to values in the vicinity of
5.0 after approximately 60 days. The pH increase is presently not
explained, but the subsequent decrease, occurring after major digestion
is completed, may be attributed to two causes: the ever increasing
rise in nitrate ion reported by most researchers to occur after most of
the volatile solids have been destroyed, and the decreasing alkalinity
with resultant loss of buffering capacity, again occurring after the
major portion of endogenous respiration has transpired. Figures 12 and
13 clearly show more thana coincidental relationship between reduction
of volatile solids and sludge pH.
In their continuous feed digestion studies on primary sludges, Malina
and Burton^ maintained a retention period of 15 days, which has been
reported to be nearly optimum for major digestion to be completed.
Their mixed sludge pH was 8.0 compared to a feed pH of 6.2. This result
negates the fears of other researchers who have felt that the pH of 5.0
obtained after 60 days digestion was too low for effective aerobic
digestion. Most treatment units would be operated at shorter retention
periods than 60 days and concomitantly higher pH levels.
Nitrate and nitrite forms of nitrogen increase in concentration with
progressive detention times as ammonia and organic forms decrease.
The increased quantities of nitrates offset the losses in volatile acids
due to aerobic digestion and account for decreases in sludge pH with
extended digestion periods. Figure 14 taken from a report by Dreier
and Obma8. demonstrates the increasing concentrations of nitrate and
nitrite ions with time and, in comparison with Figure 13,relates these
concentrations to decreasing pH and alkalinity.
Full-scale Performance
One of the few investigations concerning full-scale aerobic digestion
was conducted by Ahlberg and Boyko'8. The aerobic digestion processes
29
-------
o
(J
TJ
QJ
c
i
Q_
Detention Time in Days
Figure 12. Alkalinity and Volatile Solids
Reduction with Detention Time
30
-------
Q.
Q.
9
7 .
5
4
1000 -.
600 -
Aerobically Digested Sludge
Effluent
20
i r
10 20 30
40
ra
15
;± 200 -
i i
4.0 T
3.0-
to
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2.0-
1.0
\
20°
50
60
10 20 30 40 50 60
Total
=Solids
15°
15° Volatile
- — —^^—^^—^^—-Solids
10
20 30 40
Detention Time-Days
50
60
Figure 13. Solids, Alkalinity and pH
vs. Detention Time
31
-------
1000 -
Aerobically Digested
Sludge Effluent
a.
i
oo
o
2.5 .
1.5
.5
15
10
Figure 14.
20
20
30
40
50
60
Detention Time-Days
Increase in Nitrate-Nitrite Nitrogen
During Aerobic Digestion of a Mixture
of Raw and Waste Activated Sludges
32
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at 7 treatment plants in Ontario were evaluated. All 7 processes
utilized diffused air.
It was concluded that the aerobic digestion process is capable of
producing a stable sludge and a supernatant low in organics. Nutrient
return from the digesters represented less than 5 percent of the
total plant nutrients.
Operational problems included poor settleability in some cases due to
low DO levels and high solids concentrations. Foaming at high tempera-
tures and icing at low temperatures were observed to occur.
33
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SECTION V
PROCEDURES
RAW SEWAGE COAGULATION
Laboratory Selection of Polyelectrolytes
It was necessary to select effective polyelectrolyte coagulants
for full scale testing at the City of Hollywood. This was accomplished
by sending letters to manufacturers requesting polymer samples to be
evaluated. As a result, seventy-eight samples representing cationic,
anionic, and.nan.iniitc polymers were received. A list of those polymers
evaluated may be found in Appendix D.
Evaluation of polymers was done by the standard jar test procedure us-
ing raw sewage from the Hollywood treatment plant. Rapid mixing was
performed at a speed of 100 rpm for 20 minutes, followed by slow mix-
ing at 20 rpm for 20 minutes, then 30 minutes of settling. Efficiency
of each polymer for coagulating raw sewage was determined by measuring
the change in light transmission with a Klett Photometer for various
dosages. The original intent was to use a Zeta Meter to determine the
electrophoreticjnobility during each jar test so that polymer dosages
could be properly controlled. However, attempts at using the Zeta
Meter proved unsuccessful, as the high ionic concentrations of the
Hollywood sewage caused turbulence in the cell. The high ionic con-
centrations resulted from sea water infiltration into sewer lines
which caused chloride concentrations to approach 7,000 mg/1.
Extensive laboratory coagulation studies indicated that the following 14
polymers gave the best results under laboratory conditions:
1. Betz Poly Floe 1175
2. Betz Poly Floe 1100
3. Calgon Cat Floe
4. Calgon Coagulant Aid #25
5. Dow Purifloc C-31
6. Garratt - Callahan 72A
7. Garratt - Callahan 73
8. Garratt - Callahan 78S
9. Ionic Chemical NC - 720
10. Nalco 607
11. Nalco 610
12. National Natron 86
13. Rohm and Haas C-7
14. Herco Floe 828
34
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Sufficient quantities of the six most highly effective polymers were
purchased to conduct full scale plant coagulation tests; these included:
1. Betz Poly Floe 1175
2. Betz Poly Floe 1100
3. Calgon Cat Floe
4. Calgon Coagulant Aid #25
5. Rohm and Haas C-7
6. Herco Floe 828
Coagulation Unit
An existing 65 foot diameter clarifier was modified in an attempt to
provide the necessary mixing and settling for effective coagulation.
Laboratory tests had shown that coagulation could be achieved by elim-
inating long term rapid mixing, and utilizing only slow mixing followed
by sedimentation. The Dorr-Oliver clariflocculator was selected as
being the most suitable for installation in an existing clarifier.
Figure 3 presented a detailed drawing of the modified clarifier. A fiber-
glass baffle, 35 feet in diameter and 6.0 feet deep, was installed as
shown. Agitation was supplied by three 2-H.P. variable speed mixers.
At a nominal flow rate of 1.0 MGD, the theoretical detention time in
the slow mix section was approximately one hour. The lack of bottom
in the fiberglass baffle allowed direct flow from the slow mix section
to the sedimentation section of the clariflocculator. Special mixing
blades were utilized in an attempt to prevent mixing outside of the
baffled area. The hydraulic surface loading rate on the sedimentation
section at 1.0 MGD flow was 420 gal/(ft2)(day).
Polyelectrolyte feed solutions were made in one of two 1,000 gallons
tanks. Agitation was provided by a 1.0 H.P. Lightning Mixer. Dilutions
of 1.0, 0.5, or 0.25 percent were utilized as feed solutions, depending
upon the required dosage of a particular polymer to be fed. Polymer
solutions were fed by a variable capacity Wallace and Tiernan positive
displacement pump directly into the influent line to the clariflocculator.
A sample of the polyelectrolyte solution was used to conduct a labora-
tory jar test to determine the optimum polymer dosage. Flow through the
clariflocculator was regulated to 1.0 MGD and recorded on a Hershey
Sparling flow recorder.
After start up, the system was allowed to stabilize for several hours
before automatic sampling equipment was used for collecting composite
samples of the influent raw sewage and coagulated and settled effluent.
Each test normally was conducted for a minimum of 24 hours. The com-
posited samples were analyzed for BOD, COD, and suspended solids.
35
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AEROBIC SLUDGE DIGESTION
Operating Conditions
The operating conditions of the aerobic digesters were varied to obtain
the best possible performance from those units. The variable parameters
at the operator's control were digester volume, controlled by depths
varying between 8 feet and 12 feet, and digester loading rates which
could be applied to 1, 2, or 3 digesters.
A pilot scale digester was employed to perform tests at loading rates
that could not be achieved in the full-scale digester. The digester
was converted from a 480 gallon activated sludge aeration tank. Due to
excessive liquid splashing from the compressed air aerators, the max-
imum volume at which the digester could be operated was 350 gallons. The
pilot scale digester was fed manually on a daily basis.
The test conditions which were imposed during the research program
were partially chosen by trial-and-error in that each test improved
upon the deficiencies of the previous test. Certain parameters
such as detention time and feeding were anticipated at the start of
the program, but could not be defined until the capabilities of the
basic plant facilities were studied and evaluated. The following
is a list of the test conditions:
1) Test 1 was conducted with two digesters operating in series
with each digester having a minimum retention time of 21 days. Sludge
feed flow was somewhat variable in that new primary settling tanks were
coming on-and off-stream while plant operation was being stabilized.
The nominal retention time in the two digesters was, therefore, 43+ days.
2) Test2 was conducted in a single digester with a varying
detention period between 23 and 30 days. Since solids concentration
in. the digester was maximized, thickening by decanting was not
practicable. Feed sludge was added daily and waste sludge was removed
as necessary.
3) Test 3 was similar to Test 2 in that a nominal 29 day deten-
tion period was used in the aerobic digester. This test came about
because equipment failure forced a change in digesters during Test 2.
The practice of partially filling a digester with groundwater during
startup in order to float the mechanical aerator so disrupted equilibrium
test conditions that an entirely separate designation was given to Test 3.
Feed sludge was added daily and waste sludge was removed as necessary.
4) Test 4 was a continuous feed aerobic digestion test with an
average detention time of 22 days. During Test 4 all plant sludge
flow, including septic tank wastes, was added to the digester. The
digester was operated to its maximum hydraulic capacity and waste sludge
was withdrawn approximately once per week.
36
-------
5) Test 5 was a continuous feed aerobic digestion test with an
average detention period of 15 days. In order to achieve this test
condition, all primary plant sludges and septic tank sludges were
added to a single digester with an 8 foot sidewall depth operating
at minimal hydraulic capacity.
6) Test 6 was a single batch feed digestion test conducted
in a 55 gallon drum. This test provided a correlation between the
changes in wastewater constituents with increased degree of digestion
of a single sludge mass.
7) Test 7 was a 20 day digestion test with a carefully controlled,
constant volume daily feed of primary sludge. This test, conducted
in the 350 gallon pilot plant, provided a basis of comparison between
performance of the pilot plant and the full scale digester.
8) Test 8 was a 14 day digestion test with a carefully controlled,
constant volume daily feed of primary sludge. This test, similar to
Test 7, provided a basis of comparison between the pilot plant and the
full-scale digester.
9) Test 9 was a 10 day digestion test with a carefully controlled,
constant volume daily feed of primary sludge. This test, conducted with
the 350 gallon pilot plant, provided a retention period unobtainable in
the full-scale digesters due to time and equipment limitation.
10) Test 10, a 5 day retention digestion test with a carefully
controlled, constant volume daily feed of primary sludge, was conducted
with the 350 gallon pilot plant and provided a retention period un-
obtainable in the full-scale digesters.
Special studies were carried out on the physical behavior of digested
sludges. Sand bed drying of digested sludges was accomplished on the
existing standard size sand bed at the treatment plant. Waste sludges
from the full scale digester were used to flood a full sand bed to a
depth of 17 inches in order to determine whether a greater depth of
sludge would dry adequately on the bed. The smaller quantity of sludges
available from the pilot scale digester was dried in a restricted sand
bed. This was accomplished by sinking a bottomless 55 gallon drum to a
depth of six inches into a standard sand bed. Waste sludge would be
contained in that drum during the period of drying. All sludges were
kept on the sand beds until they had dried, cracked, and were easily
handleable by shovel or pitch fork.
Simulated lagooning studies were also carried out with the aerobically
digested sludges. Four gallon plastic waste cans were filled with
sludge at the end of a digestion test and placed in an open area ex-
posed to the natural elements. Four liter containers were also filled
with the same sludge and placed in the air conditioned laboratory for
comparative purposes. The sludges were then observed over a period of
time and finally chemically analyzed at the end of the test program.
37
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Sampling and Analyses
Sludge samples for chemical analyses were normally collected three
times per week. Six single samples were composited over a 24 hour
period because, in most of the waste streams, automatic sampling
equipment could not handle the solids content.
Refrigeration was the standard means of sample preservation for all
analyses except nutrient content. Basic analyses were conducted
at the test site during the day of the sampling. Nutrient analyses
were performed in the Gainesville laboratories of Environmental
Science and Engineering, Inc.
Samples to be analyzed for nutrient content were preserved with mercuric
chloride and sulfuric acid in addition to being refrigerated. The nutrient
samples were then shipped by bus to Gainesville. It was inevitable that
some samples were delayed in shipment and arrived both warm and in an
anaerobic state.
With the exception of the automated analyses discussed below, analyses
for waste constituents and qualities were conducted in accordance with
the 12th and 13th Editions of Standard Methods for the Examination of
Water and Wastewater. published jointly by the American Public Health
Association, the American Water Works Association, and the Water Pollu-
tion Control Federation.
Nitrate and nitrite nitrogen and total ortho-phosphorous phosphate
analyses were performed on the Technicon Auto-Analyzer. The automated
procedure for nitrate plus nitrite uses hydrogen sulfate and a copper
catalyst to reduce nitrates to nitrites. The nitrites are then treated
with sulfanilamides under acidic condition to produce a diazo compound
which reacts with N-l naphthylethylenediamine di-hydrochloride to form
a soluble dye. The samples are prefiltered with a coarse filter to
prevent clogging of capillary tubes in the Auto-Analyzer and diluted to
stay within the working range of the method. Control tests have indi-
cated salinity to not be an interfering factor. The analysis for nitrate
nitrogen involves reaction of the nitrates with brucine sulfate and sul-
fanilic acid in an acidic solution to form a measurable colored complex.
The samples for nitrate ion are also prefiltered by a coarse filter and
diluted to the working range of the Auto-Analyzer. Total phosphate is
measured by digesting polyphosphate forms with ammonium persulfate in
an autoclave. Then analyses proceed as for orthophosphorous. Orthophos-
phorous reacts with ammonium molybdate to form molybdophosphoric acid
which is subsequently reduced with aminonaphtholsulfonic acid to a
molybdenum blue complex. Filtration of the raw sample is practiced only
after any digestion in the autoclave.
Specific Resistance
The specific resistance of the digested sludges was measured during
the study by means of the Buchner Funnel technique. In this procedure
38
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specific sludge volume is placed in the funnel, a cake allowed to form
on the filter by gravity flow, and a vacuum of 20 in. Hg applied to
the cake. The volume of filtrate collected per unit time is recorded,
forming the basis for further calculation.
Carman, as discussed by Coackley^, has shown that filtration in the
case of compressible filter cakes is:
dl = PA2
de u(rcV + RmA)
(1)
V = volume of filtrate, ml
a = time, sec
p = pressure, inches Hg
A = filtration area, cm2
u = filtrate viscosity, poises
r = specific resistance, sec2/gm
c = weight of solids/unit volume of filtrate, gm/ml
Rm = initial resistance of a unit area of filtering surface
The specific resistance, r, is numerically equal to the pressure
difference required to produce a unit rate of filtrate flow of unit
viscosity through a unit weight of cake.
Integration of Equation 1 yields:
9 = MIL
2PA
or
• = urc,
V 2PAZ
PA
uRm
(2)
i.e.,
i = bV + a
If e_ is plotted against V, a straight line of slope b is obtained where
V
b = urc
To calculate the specific resistance, it is necessary to obtain
adequate data to plot -8-against V so that b may be measured. Once
slope b has been determined and the other variables are known, the
specific resistance may be calculated from the equation:
r = 2bPA2
uc
39
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Data Reduction
During the course of the program, voluminous amounts of data were
collected on waste flows, constituent concentrations, and sludge
qualities. Presentation of massive repetitive data and assimilation
of this data would be time consuming and of marginal value. There-
fore, in most instances, average data figures are used and special
note is taken of unusual operating conditions.
With the exception of a brief period during the beginning of this pro-
gram, when laboratory facilities were not yet completed, detailed
analyses were conducted of most of the waste streams on a three times
per week basis. These analyses permitted the definition of a "settl-
ing-in" period for each test and a stabilized operating period there-
after. In the presentation of the data, average results from each
test were used. The averaged data covers only the stabilized operatT
ing condition of the digesters. Noticeable trends or specific oddi-
ties in daily data are mentioned in the presentation.
While daily feed to the digesters was measured'by means of time clocks
on calibrated progressive cavity sludge pumps, there was no accurate
way to measure volumes of the gravity flow of sludge wasted from the
digester. An attempt was made to measure volumetric changes within the
digester itself in order to relate such changes to evaporation, rainfall,
and sludge wasting, first by using a float mechanism as pictured in Figure
15. However, this mechanism became rapidly fouled with coarse solids and
sludge and became unuseable. The second attempt at measuring wasted sludge
volume involved the installation of verticle sight tubes at the sides of
the digesters. These sight tubes were filled with clear water, could be
blown out with each use, and were to give an indication of water level
within the digester. In a special experiment the sight tubes were found
to have a minimum precision of greater than one inch of water level.
Since a one inch change in the digester represented 2,780 gallons of
sludge or approximately 15 percent of the daily feed to the digesters,
the sight tubes were discarded as too inaccurate. Fouling of the inside
of the digester surface from spray and foam prevented accurate measurement
of digester liquid level directly. No other satisfactory method of sludge
volume measurement was provided in this program.
A special computer program was used to analyze the daily sewage
flow data and compare them with daily rainfall statistics. This pro-
gram was able to separate surge flows due to stormwater infiltration
from daily and seasonal variations in domestic wastewater discharge.
With the aid of this program, any municipality providing detailed daily
plant flow records and detailed rainfall information could accurately
estimate the amount of storm water infiltration in both sanitary and
combined sewer systems. A discussion of the program is presented in
Appendix A.
40
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FIGURE 15: FOULED DIGESTER LIQUID LEVEL INDICATOR
41
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SECTION VI
STUDY RESULTS
RAH SEWAGE COAGULATION
Initial Tests
Prior to full scale tests, a laboratory jar test was conducted to
determine optimum polymer dosage. In July 1970, the first full
scale raw sewage coagulation experiments were conducted. Flow
through the coagulation unit was limited to 1.0 MGD and an initial
dosage of 5 mg/1 of Cat Floe was utilized. It was immediately
obvious that excessive mixing velocities caused any settled sludge to
be resuspended and remixed with the incoming sewage. Therefore,
little or no coagulation treatment efficiency was realized (see
Table 1).
Two steps were taken to alleviate the mixing problem, one was to
raise the mixing blades to a point where their zone of influence
did not include the settled sludge and the other was to order new
gears for the mixing equipment in order to reduce its rpm range.
Before the new gears were obtained.another series of coagulation
tests were conducted with Cat Floe and Calgon Coagulant Aid #25.
In all of these tests it was found that mixing velocities were
still too high and that sludge was being resuspended from the
bottom. Toward the end of 1970 the mixing equipment had been
modified as recommended by the Dorr Oliver Research Department.
Mixing blades had been set at optimum height and the new gears in-
stalled to lower the rpm of the mixing equipment.
Laboratory coagulation tests conducted at the Hollywood sewage plant
showed that the sewage could be effectively coagulated with a variety
of cationic polymers; however, the Dorr-Oliver Clariflocculator was
unable to duplicate the laboratory results before or after modifications
were made to the coagulation unit. Typical results of laboratory coag-
ulation of the Hollywood sewage are shown in Table 2. Rohm and Haas C-7
cationic polymer at a dosage of 20 mg/1 was used to obtain the maximum
BOD reduction of 70 percent. The maximum COD reduction of 69 percent
occurred at a polymer dosage of 40 mg/1 and the maximum suspended solids
reduction of 99 percent occurred at a polymer dosage of 50 mg/1.
Full scale plant tests at a flow of 1.0 MGD were conducted with the
coagulation unit. Tests were conducted over several 24 hour periods
42
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TABLE 1
CHEMICAL ANALYSES OF COAGULATION UNIT
Date
Flow
(mgd)
Polymer
BOD COD Sus. Solids
INF EFF INF EFF INF EF.F
(mg/1) (mg/1) (mg/1) (mg/1) (mg/1) (mg/1)
6/ 9/70 0.89
7/12/70 1.0
7/13/70 1.0
7/14/70 1.0
7/15/70 1.0
7/21/70 2.2
None
None
Cat Floe
10 mg/1
Cat Floe
5 mg/1
Cat Floe
5 mg/1
None
98
83
124
76
85
95
75
103
70
53
186 124
193 152
109 101
170 145
164 148
145 52
165
81
65
69
41
40
77 55
43
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TABLE 2
LABORATORY JAR TESTS
POLYMER* DOSE BOD
JAR (mg/1) (mg/1)
Mixed 0 119
Settled 0 86
2 10 79
3 20 36
4 30 42
5 40 42
6 50 46
* Rohm and Haas C-7
TABLE 3
FULL SCALE COAGULATION TEST
POLYMER* DOSE COD
TEST (mg/1) (mg/1)
Influent 0 274
Effluent before test 0 145
Effluent after test 30 118
COD
(mg/1 )
414
261
225
171
207
126
135
BOD
(mg/1)
116
no
108
SS
(mg/1)
186
94
64
14
8
4
2
SS
(mg/1)
112
28
19
*Rohm and Haas C-7
44
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with composite samples being collected from both the influent and
effluent. Typical results of these tests are shown in Table 3.
Results show that the BOD was reduced by only 7.0 percent, the COD
by 57 percent, and the suspended solids by 83 percent. The actual
increase over plain sedimentation was 2.0 percent for BOD, 10.0
percent for COD, and 9.0 percent for suspended solids. These treat-
ment efficiencies were far below that achieved in the laboratory.
Continuous Flow Laboratory Studies
Since full scale coagulation studies at Hollywood could not duplicate
laboratory results, further laboratory studies were conducted to
determine how the Hollywood plant facilities could best be modified.
The tests were done primarily to determine if poor results of polymer
coagulation on a plant scale basis, when compared to laboratory tests,
were mainly due to poor or improper mixing conditions. A laboratory
scale coagulation-flocculation-clarification unit with a one gallon
per hour capacity was assembled. Primary settled sewage was obtained
at the University of Florida sewage treatment plant in Gainesville,
Florida. A reservoir of this feed water was maintained in a large
plastic container. From there, a submerged centrifugal pump delivered
the feed water at 63 ml/min to a 400 ml rapid mix plastic chamber.
Polymer addition was employed to coagulate the sewage solids. Calgon
Cat-Floe was added because of previously successful experience with
this product. A positive displacement diaphragm pump delivered 123
ml/hr of the polymer solution to the rapid mix chamber. The energy to
the rapid mix was provided at first by a single air pump and later by
two air pumps. Water from the rapid mix overflowed into a 1/2 gallon
slow mix chamber where a flat paddle type stirrer rotated at 20 rpm.
Retention time was approximately 6.3 minutes in the rapid mix and 30
minutes in the slow mix. A siphon tube was used to gently transport
the flocculated mixture to the 5 gallon sedimentation basin where
finished effluent was collected as overflow.
At startup, all containers except the basic feed reservoir were filled
with tap water. The primary sewage gradually displaced the tap water
and, as it had been preserved for several hours by refrigeration, the
primary sewage was several degrees colder. The temperature difference
manifested itself in the clarifier where the coagulation sewage displaced
the tap water in plug flow from the bottom to the top.
A standard jar test apparatus was used to find the optimum dosage of
polymer for the sewage in question. Results were as follows:
Polymer Dosage (mg/1) Light Absorbance on
Klett Photometer
0 100
Tap Water 66
2 86
4 80
6 82
8 86
10 89
45
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Since the best results were obtained at levels of 4 and 6 mg/1, a
dosage of 5 mg/1 was chosen for the continuous feed test. It should
be noted, however, that the heaviest, most readily settled floe oc-
curred at 8 mg/1 of polymer. The following is a summary of the
results of the continuous feed experiment using 5 mg/1 of Cat-Floe
as a coagulant:
Time
Klett Reading on Effluent
9:00 a.m. Feed = 95, Tap = 63
9:30
10:00
10:30
11:00
11:30
12:00 p.m.
12:30 p.m.
1:00
1:30
2:00
2:30
3:00
3:30
4:00
65.0
63.0
65.0
63.0
63.5
73.0
73.5
73.0
Comments
Start Feed
Floe forming in slow mix
Distinct floe in clarifier
No change
Added second air supply
to rapid mix
Greater floe size in
slow mix
Large floe in slow mix
No change
No change
No change
No change
No change
Tap water displaced in
clarifier
Good separating floe
No change
Floe formed by increasing energy input to the rapid mix was notice-
ably larger. In all cases, this floe was relatively light and easily
disturbed by water currents. Results of the laboratory study showed
that polyelectrolytes could coagulate raw sewage on a continuous flow
basis. The three principal requisites of good operation were 1) high
energy input to the rapid mix to obtain a good dispersal of polymer
and the formation of pinpoint floe; 2) adequate energy input and
detention time in the slow mix to build up floe; and 3) provision of
adequate detention time and quiescent conditions in the final clar-
ifier to provide effective liquid-solid separation.
The deficiencies in the Hollywood treatment plant were determined to
be a lack of high energy input for rapid mix before entering the Dorr-
Oliver clariflocculator. Provisions were made to modify the system
so that approximately 15 minutes of rapid mixing could be provided
to the sewage before it entered the coagulation unit. Polymer
addition was moved to a point upstream of an existing distribution box.
46
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Modification of the distribution box consisted of the addition of an
air header and compressor which hopefully would provide the energy
input and detention time required for rapid mixing prior to discharge
into the existing slow mix unit.
Final Coagulation
Further full scale coagulation tests were conducted after modifications
were made to provide rapid mix prior to the Dorr-Oliver clariflocculator.
The results of several 24-hour tests using Cat-Floe at various coagulant
dosages are shown in Table 4. Little if any improvement in treatment
efficiency was experienced. Several other experiments were conducted
with other polymers and a wide range of polymer dosages, but the re-
sults were similar to previous tests with essentially no increase in
treatment efficiencies. Due to the inability of the full scale unit
to duplicate laboratory results, coagulation studies were terminated.
Since the existing facility could not be modified to provide the mix-
ing conditions necessary to achieve efficient raw sewage coagulation,
further coagulation studies with the existing facilities would have
been unproductive.
AEROBIC SLUDGE DIGESTION
The operating rationale at the Hollywood Sewage Treatment Plant was to
construct an aerobic digester feeding and aerating schedule that utilized
existing equipment and personnel from one test to another. First the
most conservative operating factors were chosen,then, by trial and error,
the operating factors were modified to achieve optimum digester perform-
ance. Occasionally equipment would fail during a test and the test would
have to be restarted. The end result of varying test conditions and
digesters was a collection of independent trials providing parametric
comparisons of the effects on aerobic digestion.
The ultimate standard of performance for the aerobic digestion process
is a stabilized, odor free,and manageable waste sludge. In this project,
waste sludge quality was determined by its behavior on a sand drying
bed and by the filterability of the sludge. These parameters were not
measured for all tests. The original tests in the program were to
establish a minimum theoretical detention time for sludge digestion
based on analyses of chemical constituents. As lower detention times
were approached, sludge quality was also included in the observations
of the program. Certain chemical parameters, such as pH and alkalinity,
were monitored and compared with process performance.
TEST 1 — 43+ DAY DIGESTION
The first trial conducted at the Hollywood Sewage Treatment Plant was
begun before all construction work at the plant had been completed.
47
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TABLE 4
COAGULATION OF RAW SEWAGE WITH CAT FLOC
Test No.
1
2
3
4
Raw Sewage
Polymer Dose
(mg/1 )
1
3
5
10
-
BOD
(mg/1)
124
125
134
147
130
Supernatant
COD
(mg/1)
384
360
261
310
374
SS
(mg/1)
88
66
84
80
80
48
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Therefore, necessary changes in flows due to equipment malfunction,
equipment installation, and an occasional error provided variable
detention periods for this test. The basic concept at that time was
to treat the primary settled sludge by two-stage series digestion.
The digesters were prepared for operation by filling them two-thirds
full of groundwater and then adding sludge on a daily basis. Digester
1 commenced operation on March 24, 1970, and Digester 2 was filled two-
thirds with groundwater on April 1, 1970. This particular test was
terminated on January 6, 1971. During much of this period measurements
of liquid volumes to the digesters were based on changes in the liquid
levels in the digesters. These values were often difficult to determine
because of heavy surface foam and fouling of indicator devices. The
flow data commencing on November 1, 1970,were determined by reading
time clocks mounted on the progressive cavity pumps feeding the digesters.
The time clocks were calibrated against measured pump outputs.
During the test period, feed to the digesters varied from a low of 5,000
gallons per day to a high of 35,000 gallons per day with volumes equal
to the raw feed being transferred from Digester 1 to Digester 2 on a
daily basis. Hydraulic detention in both Digesters 1 and 2 varied from
a low of 22.2 days to a high of 208 days.
The detention time in Digester 2 was considered to be nominal because
during the first part of the test (until November 1, 1970) decanting
for sludge thickening was necessary because of the initial two-thirds
groundwater charge. Decanting provides for a sludge residence time
longer than simple hydraulic detention calculations can estimate.
Measurements of decanted volumes were too sporadically recorded to
permit accurate determinations of residence time. This, combined with
natural solids reduction due to digestion, made estimations of exact
sludge residence time in Digester 2 extremely difficult.
Some flow changes to the digesters were deliberately initiated to alter
conditions within the digester. A criterion of overloading in Digester 1
was the measure of dissolved oxygen in the mixed sludge. When the D.O.
fell below 1.0 ppm, fresh feed sludge would be diverted to Digester 2
until the D.O. was increased in Digester 1. This condition happened sev-
eral times during the test and was not alleviated until it was recognized
that the floating mechanical aerators were not producing their rated horse-
power. Adjustment of the aerators by the manufacturers terminated most
difficulties of low dissolved oxygen levels in the digesters.
A second factor which forced flow variations was foaming. Heavy and thick
layers of foam, as illustrated in Figure 16, decreased aerator
efficiency in the digester and temporarily forced a lower overall digester
feed flow to reduce the loading. This was done by wasting undigested
primary sludge.
49
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en
o
\
FIGURE 16: FOAMING ON AEROBIC DIGESTER
-------
Feed Sludge Quality. The samples collected for subsequent analyses
were of practical necessity: 6 individual samples composited over a
24-hour period, 3 times weekly. The feed qualities were analyzed from
the start of the program but, due to the length of this particular test
period and the two month period during which no analyses occurred, the
average feed sludge conditions presented in Table 5 are for the period
of stable conditions of the digester only. Influent BOD varied from
8,000 to 16,000 mg/1 in November and December of 1970. The values for
COD, total solids, and total volatile solids varied from 38,000 to 78,000
mg/1, 44,000 to 81,000 mg/1, and 35,000 to 55,000 mg/1, respectively.
These values are indicative of a heavy or well concentrated sludge being
withdrawn from the primary sedimentation basins. The COD to BOD average
ratio was 5.2:1 and the total solids to total volatile solids average
ratio was 1.4:1.
Digester Mixed Liquor. During the first day of operation, each digester
was filled two-thirds full of groundwater in order to utilize the
floating aerators. Daily feeding with primary sludge gradually dis-
placed the groundwater, particularly in Digester 2 where decanting was
practiced until steady state operation was achieved. From initiation
of the analyses, a gradual increase in the sludge liquor constituent
concentrations, as the original groundwater was being displaced in the
digester, could be detected. Total solids reached a 4 percent con-
centration in September, 1970. Since a 4 percent solids concentration
in the aerobic digester was considered the minimum achievable with
primary sludge, this value was chosen as the point of stabilization
of the digester. At the termination of this particular test, the
solids concentration had crept to 5.7 percent. The average values
for digester constituents are presented in Table 5.
From November 9 through January 6, there were 5 days out of 59 when
the dissolved oxygen level in Digester 1 dropped below 1.0 mg/1 and
the feeding of primary sludge directly to Digester 2 was necessitated.
The average dissolved oxygen level in Digester 1 was 4.5 mg/1 and
in Digester 2 was 5.1 mg/1.
The temperature of a mechanically aerated digester is to some extent
dependent on ambient weather conditions. Temperatures in Digester 1
varied between 10°C and 24°C for an average of 17.6°C, while the temp-
eratures in Digester 2 varied from 9°C to 21°C for an average of 16.6°C.
The pH in both digesters only varied from a low of 6.6 to a high of 8.0.
The average pH for each digester was 7.4.
51
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TABLE 5
AVERAGE CONSTITUENT REDUCTIONS THROUGH TWO STAGE DIGESTION
(mg/1)
Total
Total Volatile
Liquor COD BOD Solids Solids
Primary Sludge 61,086 11,786 58,060 41,076
Digester 1
Mixed Liquor 40,460 2,202 47,619 27,843
Digester 2
Mixed Liquor 33,332 926 48,511 25,510
Total Percent
Reduction in 1 34 81 18 32
Total Percent
Reduction in 2 11 11 -2 6
Total Percent
Reduction 45 92 16 38
52
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Chloride levels in the digesters were relatively high due both to the
initial filling of the digesters with chloride bearing groundwater and
to sewer line infiltration by brackish and ocean waters. The average
chloride level in Digester 1 was 6,800 mg/1 as Cl'and in Digester 2 was
7,800 mg/1 as Cl".
The total solids content of Digester 1 averaged 47,619 mg/1 for an 18
percent reduction over the feed sludge. The total solids increased in
Digester 2 to an average of 48,511 mg/1 for a net reduction of 16 per-
cent over the feed sludge. It is felt that the differences between
digesters are insignificant and may possibly be due to difficulties in
obtaining a representative sample. Evaporation in Digester 2 could in-
crease concentrations of constituents more rapidly than digestion removes
them. There was an overall reduction of 38 percent in total volatile
solids through the two digesters with Digester 1 accounting for 32 per-
cent at 27,843 mg/1 average total volatile solids. Digester 2 achieved
a further 6 percent reduction to 25,510 mg/1 average total volatile solids.
There was a reduction in suspended solids between the two digesters from
36,029 mg/1 to 34,513 mg/1. Volatile suspended solids experienced a
similar magnitude reduction from 24,734 mg/1 in Digester 1 to 20,703
mg/1 in Digester 2.
Digester 1 provided a 34 percent reduction in chemical oxygen demand to
46,460 mg/1. Digester 2 further reduced the COD by an overall 11 per-
cent to an average 33,332 mg/1. The 45 percent total reduction in COD
is substantial and compares well with the total reduction in BOD of 92
percent, keeping in mind that increased biodegradation usually increases
the COD to BOD ratio. The COD to BOD ratio was 36:1. The BOD of the
mixed sludge in Digester 1 averaged 2,202 mg/1 and in Digester 2 averaged
926 mg/1.
Oxygen Uptake. A common indicator of biological activity is the oxygen
uptake rate of the mixed liquor. Besides providing comparative values
of low or high activities, the uptake rate furnishes a measure of the
aeration equipment necessary for an aerobic digester at any temperature.
Figure 17 relates the volumetric oxygen uptake rates measured during
November and December, 1970, against temperature. Figure 18 relates
the oxygen uptake rates for the same period per weight 9f total solids
in the digesters as a function of temperature. This eliminates varia-
tion in uptake due to fluctuating solids and biomass in the digesters.
Too few data points are available for an exact relationship between up-
take and temperature; however, an upward trend in oxygen uptake with
53
-------
140 -
120 -
100 -
80 -
OJ
fO
-(->
Q.
CM 60
O
40 -
20 _
Digester No. 1
Digester No. 2
T~
28
12
14
16 18 20 22
Temperature (°C)
T~
24
26
Figure 17. 02 Uptake Rate, 2-Stage Digestion
43+ Day Detention
30
54
-------
2.8-
2.4-
2.0-
Jl 1.6
CD
I—I
O)
tO
-P
O-
3
CM
O
1.2.-
0.8
0.4_
_____ Digester No. 1
.— Digester No. 2
10 12
~T
14
16 18 20 22
Temperature (°C)
T"
24
26
T"
28
30
Figure 18. Op Uptake Rate, 2-Stage Digestion,
43+ Day Detention
55
-------
increasing temperature is noted. Oxygen uptake in Digester 1 ranged
from 47 to 86 mg/(l)(hr), and in Digester 2 from 72 to 76 mg/(l)(hr).
Presented on a mass basis, the oxygen uptake rates were between 12 and
20 mg/(gm T.S.)(hr) in Digester 1 and 18 and 21 mg/(gm T.S.)(hr) in
Digester 2.
Primary Plant Performance. During the period of this test, the raw
sewage to the treatment plant contained an average of 122 mg/1 of BOD,
166 mg/1 of COD, and 122 mg/1 of suspended solids. Analytical diffi-
culties during this period may have given low values for chemical oxygen
demand. After primary separation, the plant effluent contained 112 mg/1
of BOD, 150 mg/1 of COD, and 75 mg/1 of suspended solids.
TEST 2 — 23 TO 29 DAY DIGESTION. On January 7, 1971, the two stage
digestion test was terminated. The second stage digester was taken
out of operation and subsequently, all wastesludge from Digester 1
was discharged to either sand beds or, more often, to the ocean out-
fall. The second major test in the program consisted of primary
sludge going to Digester 1 only. The second test commenced on
January 7, 1971, and terminated on May 6, 1971. Because a full
digester was used at the start of this test, stable operating condi-
tions may also be assumed to have started on January 7, 1971. During
the first two months of the test, septic tank sludge was added to
Digester 1 because other facilities for septic tank sludges were
temporarily out of order. After March 1, 1971, septic tank sludge
was no longer added to Digester 1. The average flow to Digester 1
during the first two months of aeration was 17,170 gpd for a 23 day
theoretical detention period. For the subsequent two months the
average flow as 13,760 gpd for a theoretical detention period of
29 days. Daily flow variations of primary sludge were small, ranging
from 13,420 gpd to 19,970 gpd in the first two months, and from 8,390
gpd to 21,350 gpd with the majority of the flows falling between
9,490 gpd and 17,790 gpd in the second two months. Decanting was
not practiced at any time.
Feed Sludge Quality. The feed sludge quality during this test did not
change appreciably from the previous test. Table 6 presents averaged
primary sludge parameters. These values have not been adjusted for the
change in quality due to addition of septic tank sludge during a two
month period. Total averaged COD, BOD, total solids, and total volatile
solids values were 65,700 mg/1, 16,240 mg/1, 60,036 mg/1, and 44,905
mg/1, respectively. The septic tank sludge qualities had much lower con-
'
rn . 33,685 mg/1, and 21,439
for COD, BOD, total solids, and total volatile solids, respectively.
56
-------
TABLE 6
AVERAGE CONSTITUENT REDUCTIONS AT 23-29 DAYS DIGESTION
Constituents, (mg/1)
01
— 1
Liquor
Raw Sludge
Mixed
Liquor
% Reduction
COD
65,700
53,800
m
BOD
16,240
4,910
70%
Total
Solids
60,036
58,334
3%
Total
Volatile
Solids
44,905
39,111
13%
Volatile
Suspended Suspended
Solids Solids
-
49,351 34,854
_ _
Chloride
-
6,666
_
6.5
Dissolved
Oxygen
2.5
Temp.
°C
20
-------
As explained below, effects from septic tank sludge additions
are minor.
Digester Mixed Liquor. The digester in this test had been full
from a previous test at a 30 day theoretical detention condition.
Since feed water quality and test conditions did not change
significantly from the first test to the second, it was decided
to continue with a full digester. Therefore, stabilized condi-
tions and analytical data commenced on the first day of the test.
Table 6 shows that only BOD was substantially reduced to a mixed
liquor value of 4,910 mg/1. Other comparable reductions were
COD to 53,800 mg/1, total solids to 58,334 mg/1, and total volatile
solids to 39,111 mg/1. The total solids concentration is high and
approaches the practical limit of solids content in an aerobic
digester from mixing, thickening, and oxygen transfer considera-
tions. A good portion of the total solids in this digester (greater
than 1 percent) are soluble salts from seawater intrusion to the
sewer system.
Suspended solids averaged 49,351 mg/1, which was close to the total
solids content. In view of the high level of chloride ion in the
digester (6,666 mg/1), it is apparent that the majority of the true
sewage waste constituents are in suspended solids. This is further
substantiated by the volatile suspended solids level of 34,854
mg/1, which was close to the total volatile solids level.
The average temperature in the digester during the test was 20° C
with a range from 11°C to 25°C. The average pH was 6.5 with a low
of 6.2 and a high of 7.0. Dissolved oxygen levels varied from
0.3 to 7.8 mg/1 but on most days fell between 0.5 and 5.0 mg/1
with an average of 2.5 mg/1.
Oxygen Uptake. The oxygen uptake rate of the aerobic digester
varied slightly as shown in Figure 19. Figure 20 correlates
uptake rate to total solids. The highest rate recorded was 1.8
mg/(gm T.S.)(hr) and the lowest was 1.1 mg/(gm T.S.) (hr) with a
noticeable increase with increasing temperatures.
m
Primary Plant Performance. During the period of this test, the
Hollywood Sewage Treatment Plant was receiving a raw sewage with
an average of 128 mg/1 of BOD, 340 mg/1 of COD, and 109 mg/1 of
suspended solids. The average plant effluent contained 119 mg/1
of BOD, 314 mg/1 of COD, and 84 mg/1 of suspended solids.
58
-------
140 -
120 -
TOO -
s-
0)
_*:
(O
4->
o-
80 -
60 -J
40 -
20 -
16
10 12
14
18 20 2Z
Temperature (°C)
~S
Figure 19. 02 Uptake Rate (mg/l/hr)
24-30 Day Digestion
30
59
-------
2.8-
2.4 _
2.0 -
1.6 _
en
OJ
(C
4->
a.
CJ
o
0.8 _
0.4 -
1 j , j j r
10 12 14 16 18 20 22
Temperature (°C)
nr
24
26 28
30
Figure 20. 0? Uptake Rate [mg/(gm T.S.)(hr}],
24-30 Day Digestion
60
-------
TEST 3 — 39 DAYS DIGESTION. A mechnical failure in Digester 1
forced a continuation of the previous test in a new digester.
Changing digesters involved partially filling the new digester
with groundwater to float the mechnical aerators. Thus, test
conditions were so disrupted that the following test must be con-
sidered independently. In this particular case, some mixed liquor
from Digester 1 had been pumped to Digester 2 with the groundwater.
Thus, Stabilization was achieved more quickly.
How Data. Sludge feed to the digester for Test 3 commenced on
May 7, 1971, and terminated on July 31, 1971. Average feed to
the digester was 13,570 gpd with a daily low of 8,300 gpd and a
high of 18,970 gpd. The great majority of flows fell between
11,100 gpd and 15,420 gpd. The average detention period was 29
days. Whenever maximum capacity was reached, 2.5 feet of mixed
sludge was wasted from the digester. This occurred approximately
once per week. Decanting was not considered practical due to the
density of the mixed sludge and to jar test results which indicated
poor settleability.
Feed Sludge Quality. Table 7 gives the average values for analyses
completed on the feed sludge to the digester. During this test
the average COD to BOD ratio was 4.8:1. The total solids varied
widely from 45,380 mg/1 to 74,860 mg/1, averaging 6 percent solids.
Suspended solids averaged 50,593 mg/1, thus indicating that a
significant portion of the total solids could be attributable to
the "salt" content of the incoming sewage. The total volatile solids
concentration varied from 35,068 to 57,058 mg/1 and constituted a
large portion of the total solids. Volatile suspended solids
analyses indicated that most of the volatile matter collected in
primary separation was insoluble. The alkalinity of the incoming
sludge was a relatively high 1,293 mg/1, while the average pH
was 5.9.
Digester Mixed Liquor. Averaged values of mixed sludge constituent
concentrations are also presented in Table 7, with the percentage
reduction obtained for each constituent. Dissolved oxygen con-
centration of the mixed sludge averaged 1.6 mg/1 during the test.
While the dissolved oxygen dropped as low as 0.3 mg/1, the di-
gester never became anaerobic. The average digester temperature
was 26°C and ranged from 23 to 27°C. The pH hardly varied during
this test from the average value of 6.5, which represented an
increase over the feed sludge pH of 5.9. Reductions in COD and
BOD were 42 percent and 83 percent, respectively. The COD to BOD
ratio changed from 4.8:1 in the feed to 16:1 in the digested
sludge. Total suspended solids and volatile solids concentrations
61
-------
TABLE 7
AVERAGE CONSTITUENT REDUCTIONS AT 29 DAYS DIGESTION
Constituents, (mg/1)
CT, Liquor
no
Primary
Sludge
Mixed
Liquor
Percentage
Reduction
COD
65,851
37,952
42
BOD Solids
13,841 60,780
2,368 47,580
83 22
Total
Volatile
Solids
46,718
28,340
39
Suspended
Solids
50,593
35,637
30
Volatile
Suspended
Solids Alkalinity Chloride _pH
42,492 1,293 5,250 5.9
25,259 - 6,268 6.5
41 - -19
Dissolved Temp.
Oxygen °C
-
1.6 26
-------
varied little during the test, although significant reductions of
solids were obtained when compared to feed study values.
Chloride content in the digesters varied from a high of 7,250 mg/1
three weeks after the beginning of this test to a low of 5,650 mg/1
at the termination.
Primary Plant Performance. Collected data indicated that the per-
formance of the primary treatment plant was marginal at best. In-
fluent suspended solids of 103 mg/1 were reduced to an effluent of
72 mg/1. Incoming BOD of 124 mg/1 was a low value for typical muni-
cipal wastewater; however, the Hollywood sewage system serves very
few industries and heavy groundwater and ocean water infiltration
dilutes the sewage flow. Primary plant effluent contained an
average of 113 mg/1 of BOD. Influent COD averaged 288 mg/1 while
the effluent contained 311 mg/1. The apparent increases in COD values
may be due to sampling deficiencies in that the influent and effluent
samples were collected concurrently, and there exists a time lag of
several hours between influent and effluent points. The effluent
samples could include a high COD waste that would be missed by the
influent sampler.
TEST 4 — 22 DAY DETENTION. An analysis of the data from Test 3
indicated that the digester was not necessarily being utilized to
its fullest capacity. Therefore, all primary sludges which had been
diverted to another experimental digester, including septic tank
sludges, were now added to the digester treating the main body of
the waste. The digester was still maintained up to maximum liquid
capacity of 12.0 feet and the.theoretical detention time of sludges
in the digester was 22 days.
The 22 day digestion test commenced on August 1 and terminated on
November 4, 1971, when equipment failure occurred. The average
flow to the digester during this test was 18,459 gallons per day
with a single low of 9,490 gallons per day and a high of 27,220
gallons per day. The majority of the daily flow values fell between
23,330 and 14,230 gallons per day. Septic tank sludges were added
intermittently. The monthly totals were averaged with the accumu-
lated daily flows to obtain an overall daily average. Digested
mixed liquor was wasted approximately once weekly.
Feed Sludge Quality. Test 4 commenced with a full digester from
a previous test that had been operating at a 29 day digestion period.
The basic change in the new test was the increase in feed sludge
flow and the addition of septic tank sludges. Careful review of
the individual sample analyses of the digester mixed liquor in the
22 day retention test produced no noticeable changes with time in
any of the monitoring parameters except alkalinity, which gradually
increased. For these reasons, stabilized test conditions more
arbitrarily assumed to start with the first day of sampling and all
data from that period was included in the averages.
63
-------
The feed sludge characteristics presented in Table 8A are indica-
tive only of the primary sewage sludge and do not include septic
tank sludges. Separate analyses were not conducted on septic
tank sludges during the test. Previous tests had indicated that
a reduction in primary sludge constituent concentrations of ap-
proximately 2 percent would result from the addition of septic
tank sludges. Table 8B presents the expected feed sludge charac-
teristics based on a theoretical 2 percent change due to septic
tank sludges. A comparison of Tables 8A and 8B, shows little
difference in the overall removal percentages for the various
applicable categories. Since the corrections for septic tank sludge
qualities were principally an arithmetic exercise in extrapolation
from earlier tests and the data on primary sludges have a much more
concrete basis,and because the differences in the final outcome
are small, the following discussion pertains to the main flow of
primary sludge only.
The chemical oxygen demand of the primary sludge varied between
31,000 mg/1 and 84,000 mg/1 with an average of 51,260 mg/1. The
biological oxygen demand averaged 12,030 mg/1 with a low of 4,500
mg/1 and a high of 22,500 mg/1. The COD to BOD ratio was 4.3:1.
Variations in total, suspended, and volatile suspended solids were
from 24,572 to 61,746, 19,032 to 53,388, and 28,817 to 31,478 mg/1,
respectively.
The alkalinity of the feed sludge was relatively high at 1,060
mg/1. This value was not due to intrusion into the sewer system
of seawater, which has an alkalinity on the order of 100 mg/1.
The chloride level in the feed sludge remained comparatively high
at 4,535 mg/1. The pH of the feed sludge varied between 5.9 and
6.4.
Digester Mixed Liquor. The digester maintained relatively stable
during the period of the test. Tables 8A and 8B present the
average constituent concentration and values in the digester and
the percentage reductions from the feed source. Since the addi-
tion of septic tank sludges into the calculation creates at most
one percentage point difference in removal efficiencies, the dis-
cussion will include the primary sewage sludge only.
Chemical oxygen demand was reduced by 46 percent while BOD ex-
perienced a greater reduction of 84 percent. The resulting COD
to BOD ratio for the digested sludge was 14:1. Total solids re-
ductions were not as great as that of suspended solids. Total
volatile solids, the principal measure of digestion completeness,
were reduced by 36 percent. The volatile suspended solids declined
by 41 percent. The change in alkalinity from 1,060 mg/1 to 176
mg/1 was quite substantial. The chloride content changed little
with digestion.
64
-------
TABLE 8
AVERAGE CONSTITUENT REDUCTIONS AT 22 DAYS DIGESTION
cn
TABLE 8A
Average Constituent Reductions at 22 Days Digestion Based On Primary Sludge Analyses
Constituents (mg/1)
Liquor
Primary
Sludge
Mixed
Liquor
% Re-
duction
Total
COD BOD Solids
51,260 12,030 44,818
27,701 1,940 36,211
46 84 19
Total
Volatile
Solids
31 ,478
20,200
36
Volatile
Suspended Suspended Dissolved Temp.
Solids Solids Alkalinity Chloride pH Oxygen °C
35,463 28,817 1,060 4,535 6.1
25,466 16,940 176 4,320 6.9 3.5 26
28 41 83 - -13 -
TABLE 8B
Average Constituent Reductions at 22 Days Digestion Based on Primary Sludge Analyses
Adjusted for Septic Tank Sludges
Liquor
Feed
Sludge
Mixed
Liquor
% Re-
duction
Total
COD BOD Solids
50,235 11,790 43,922
27,701 1,940 36,211
45 84 18
Total
Volatile
Solids
3C 848
20,200
35
Constituents (mg/1)
Volatile
Suspended Suspended Dissolved Temp.
Solids Solids Alkalinity Chloride pH Oxygen °C
34,754 28,240 - -
25,466 16,940 176 4,320 6.9 3.5 26
27 40 ....
-------
Dissolved oxygen was maintained at a high average of 3.5 mg/1
and never dropped below 1 mg/1. Temperature in the digester
ranged from 22°C to 28°C during the test.
Nutrients. Primary feed sludge was also analyzed to determine
its nutrient content prior to entering the digester. During the
test period, primary sludge contained an average of 408 mg/1
Kjeldahl nitrogen, 0.4 mg/1 nitrate nitrogen, and 233 mg/1 total
phosphorus. The BOD to nitrogen ratio was 101:1 and the BOD to
phosphorus ratio was 220:1. Corresponding nutrient levels in the
digester liquor were 1,434 mg/1 Kjeldahl nitrogen, 1.2 mg/1 nitrate
nitrogen, and 215 mg/1 total phoshporus.
Oxygen Uptake. Temperatures during the test varied from 22°C to
28°C. Figure 21 indicates only slightly increasing oxygen uptake
rates with increasing temperature. Similarly, Figure 22, which
expresses uptake rate per mass of solids, also does not present
a definitive relationship. The oxygen uptake rate per mass of solids
plotted against time in Figure 23 reveals a high utilization rate of
approximately 28 mg/(gm)(hr) during the first 30 days of the test
and a low rate of approximately 17 mg/(gm)(hr) during the last 30
days of the test.
Primary Plant Treatment The primary sewage treatment plant performed
better than average during this test period. Plant operation was
improved and equipment failures became less frequent. Suspended
solids removal in the primary clarifiers and grit chambers averaged
34 percent, chemical oxygen demand was reduced by 13 percent, and
dropped by 16 percent.
TEST 5—15 DAY DETENTION This test was conducted immediately
following the 22 day test. All primary and septic tank sludges
were added to a single digester which operated at a reduced volume
of 330,000 gallons and had an average hydraulic detention period
of 14.7 days with an average daily feed rate of 22,525 gallons.
This test commenced on November 8, 1971, and terminated March 15,
1972.
Feed Sludge Quality Feed sludge analyses were conducted 3 days
weekly on a composite of 6 samples per day. The septic tank
sludges were not included in the sampling; however, it has been
shown that their contribution is negligible and may be omitted.
Table 9 presents the averaged daily feed sludge analyses of
samples taken during this test. The chemical oxygen demand varied
between 27,400 and 113,000 mg/1. The BOD ranged from 5,400 to
26,900 mg/1, and the COD to BOD ratio was 4:1.
66
-------
no.
ioo_
©
o
©
s-
-C
90.
80 _
©
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0
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I—1
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70-.
60 _
50
©
©
©
©
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22
T
23
Figure 21.
T"
24
T"
25
Temperature (°C)
nr
26
02 Uptake Rate [mg/(l)(hr)],
22 Day Digestion
T
27
67
-------
3.3-
3.0-
2.7-
©
oo
l-^
E
^2.4-
en
i i
o>
(0
Q_
CM
O
2.1-
©
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1.8.
1.5-
22
©
©
24 25
Temperature (°C)
Q
0
26
Figure 22. 02 Uptake Rate [mg/(gm T.S.)(hr)]
22 Day Digestion
27
68
-------
Does not include first
28 days of test
Time (days)
Figure 23. Oo Uptake Rate [mg/(mg T.S.)(hr)]
22 Day Digestion
69
-------
TABLE 9
AVERAGE CONSTITUENT REDUCTIONS AT 15 DAYS DIGESTION
Constituents, (mg/1)
Total Volatile
Total Volatile Suspended Suspended
Liquor
O
Primary
Sludge
Mixed
Liquor
Percentage
Reduction
COD
63,842
45,567
29
BOD
15,994
5,068
68
Solids
48,916
45,959
6
Solids
36,025
31,745
12
Solids
39,536
38,677
2
Solids
33,110
28,312
14
Alkalinity Chloride pH Oxygen °C.
1,156 5.7
170 4,277 6.4 2.1 23
85 . ...
-------
Total solids ranged from 27,554 mg/1 to 65,284 mg/1, while sus-
pended solids fell between 21,244 mg/1 and 55,034 mg/1. Volatile
solids ranged between a low of 18,314 mg/1 and a high of 51,418
mg/1. Volatile suspended solids varied from 12,014 mg/1 to 40,923
mg/1.
Digester Mixed Liquor To permit stabilization of the digester
without unduly affecting performance results, the first 4 weeks
of data were discarded. Values included in the averages for
digester constituent concentrations in Table 9 were taken only
after the digester total solids had risen to 4 percent and appeared
to have stabilized. The COD of the feed sludge was reduced by 29
percent and the BOD was reduced by 68 percent. The COD to BOD ratio
changed to 9:1. Total solids decreased by only 6 percent but this
is offset somewhat by evaporation losses in the digester. Suspended
solids decreased by only 2 percent. Total volatile solids were
reduced by 12 percent while volatile suspended solids experienced
a 14 percent reduction. Alkalinity surprisingly decreased by 85
percent from 1,156 mg/1 to 170 mg/1, and the pH increased to 6.4
from 5.7. Dissolved oxygen was maintained at an average of 2.1
mg/1 and never dropped lower than 0.7 mg/1. The temperature in
the digester varied from 19°C to 26°C.
Oxygen Uptake Rate The oxygen uptake rate was measured at regular
intervals during this test and the resulting data are presented
in Figures 24 and 25 as a function of temperature. It may be seen
that there is little correlation of oxygen uptake rate to tempera-
ture. The oxygen uptake rate varied from 86 mg/ (l)(hr) [1.82 mg/
(gm)(hr)] to 140 mg/(l) during the initial two weeks of the stabil-
ized test period, but subsequently the rates remained consistently
near to 1.0 mg/(l)(hr).
SHudge Dewatering Aerobically digested sludge from the 15 day de-
tention test was placed on a sand bed on January 19, 1972, at a
depth of 17 inches. By April 7, 1972, there were 2.75 inches of
dried cake on the sand bed. In the interim period the sand bed
was exposed to a cumulative total of 6.7 inches of rainfall. During
the first four weeks of drying there was a disagreeable odor in the
immediate vicinity of the sand bed. Subsequently, no odor was no-
ticed from the drying sludge, except for an earth odor after periods
of rainfall. An analysis of the dried sludge from the sand bed re-
vealed the concentrations of phosphorus, Kjeldahl nitrogen, and
NOX nitrogen to be 3.9 mg/gm, 33.4 mg/gm, and 0.023 mg/gm, re-
spectively.
In measuring the filterability of the aerobically digested sludge,
specific resistance was determined by means of the Buchner Funnel
test. The specific resistance of sludge digested for a 15 day de-
tention period was 5.5 x 108 secVgm.
71
-------
140 -
120 -
100 -
0 G0
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8
0
r- 80 -
cn
O)
-M
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o
60 -
40 .
20 -
10
I
12
I
14
16
Figure 24.
i
18
20
i
22
T
24
i
26
Temperature (°C)
Q£ Uptake Rate [mg/(l)(hr)]t
15 Day Digestion
28
30
72
-------
3.0-
2.8-
o
2.6-
©
O
0
0
2.4-
i
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rd
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O.'
•P 9 9'
. 1.1
CSJ
O
2.0-
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1.8-
10 12
14
r
16
T
18
r
20
22
24
Temperature (°C)
r
26
28 30
Figure 25,
02 Uptake Rate [mg/(gm T.S.)(hr)]s
15 Day Digestion
73
-------
Plant Performance Much of the original equipment shakedown had been
completed by the time of this test and the primary plant performance
had noticeably stabilized. The influent suspended solids of 110 mg/1
had been reduced by 35 percent; the biochemical oxygen demand of 141
mg/1 had been reduced by 16 percent; and, the chemical oxygen demand
reduced 14 percent from 416 mg/1.
TEST 6 — BATCH DIGESTION On August 4, 1971, a 55 gallon aerobic di-
gester was filled with sludge directly from the primary clarifier and
subjected solely to aeration for a period of 23 days. Measurements
were taken of the sludge constituents to provide a basis of compari-
son between batch and continuous aerobic digestion. Table 10 presents
the analytical results. There was a definite decrease with time of
all the sludge constituents except for chloride which varied about
the 4,500 mg/1 concentration and was not expected to change. The
dissolved oxygen was supplied by an air compressor of fixed output.
Dissolved oxygen levels fell from 7.0 mg/1, near the beginning of the
test, to 0.5 mg/1 during the time of maximum decrease in total vola-
tile solids. By the 23rd day of digestion the oxygen content of the
batch had risen to 7.8 mg/1.
All comparisons of sludge constituents hinge on the reduction of
volatile solids with time of digestion. Figure 26 presents the re-
duction in total volatile solids and in biochemical oxygen demand
with time. The steepest declines occurred between the 2nd and the
10th day and between the 12th and 14th day of digestion. Relatively
stable conditions persisted after the 14th day. A maximum reduction
of 63 percent was achieved for total volatile solids after 23 days
of aeration. The rapid slowdown in the reduction rates for volatile
solids and biochemical oxygen demand after the 14th day of aeration
indicates that little biodegradable material remained.
Total and suspended solids also experienced maximum decreases during
the first 14 days of aeration. Table 10 shows that after 14 days
aeration, most constituent concentrations remained stable with
erratic fluctuations that could be attributed solely to the diffi-
culties of obtaining representative sludge samples for analyses.
The oxygen uptake rate of the digesting sludge varied from a high of
114 mg/1 at the beginning to an average of 17 mg/l(hr) during the last
11 days of the test. The oxygen uptake rate measured 50 mg/(l)(hr) at
the 14th day of operation and fell to 14 mg/(l)(hr) two days later. In
terms of oxygen uptake per mass of solids, during the first 14 days of
operation the rate varied from 4.5 to 3.1 mg 02/(gm T.S.)(hr).
Figure 27 depicts the change in alkalinity and pH with respect to time
during the batch digestion test. The pH experienced a gradual increase
from 6.5 to 8.0 over 23 days. Alkalinity was gradually reduced from
700 mg/1 to 240 mg/1, with the majority of the reduction occurring
during the first 12 days of operation.
74
-------
16,000
-5,000
en
14,000-
s-
cu
10
-o
o
CO
to
o
12,000-
10,000-
8,000_
6,000
20 22 24
Time, Days
Figure 26. Biological Degradation with Time,
Batch Digestion Test
-------
CT>
TABLE 10
CHEMICAL ANALYSES OF AEROBIC DIGESTION BATCH TEST
DATE
8/04/71
8/06/71
8/09/71
8/11/71
8/13/71
8/16/71
8/18/71
8/20/71
8/23/71
8/25/71
8/27/71
D.O.
(mg/1)
7.0
3.4
1.9
0.5
4.9
3.4
5.5
7.6
7.9
7.8
TEMP
°C
-
27
28
25
26
27
27
27
27
27
26
M
6.5
7.4
7.3
7.2
7.4
7.4
7.5
7.7
8.0
7.9
8.0
Cl"
(mg/1)
4,950
4,250
-
4,450
-
-
3,850
4,100
-
5,100
4,050
ALK
(mg/1)
700
400
850
550
560
340
420
340
300
320
240
BOD
(mg/1)
4,800
3,500
1,900
2,150
1,425
675
465
465
420
470
335
COD
(mg/1 )
23,000
14,000
17,100
13,700
10,350
10,300
8,000
8,660
9,150
9,230
7,280
TS
(mg/1)
25,378
21 ,888
23,610
20,754
19,098
16,282
14,578
15,774
17,598 .
18,774
15,042
TVS
(mg/1)
16,074
13,784
13,940
11,792
9,064
8,296
5,932
7,034
7,536
8,646
5,984
SS
(mg/1)
12,702
12,760
11,466
9,342
8,812
6,934
5,504
6,672
8,114
7,576
5,670
VSS
(mg/1)
11,726
11,068
10,760
8,980
6,932
5,836
3,796
5,134
4,336
7,238
3,812
02*
UPTAKE
114
54
70
74
-
50
14
14
28
12
18
02**
UPTAKE
4.5
2.5
3.0
3.6
-
3.1
1.0
0.9
1.6
0.6
1.2
Uptake expressed in mg/(liter)(hour).
Uptake expressed in mg/(gm T.S.)(hour).
-------
8
Time, Days
Figure 27. Alkalinity and pH vs. Time
Batch Digestion Test
-------
TEST 7 -- 20 DAY DETENTION A small pilot plant was used to establish
test conditions and obtain data that could not be scheduled into the
program with the large digester. While there were 3 digesters available
to the program, there was sufficient feed to operate only one large
digester. A series of tests were conducted to provide correlation
between the large and small digesters and to approximate full digestion
at 20 days retention. The pilot scale digester, a converted activated
sludge plant with a 480 gallon aeration compartment, experienced such
vigorous aeration and splashing that the unit could only be operated
at a 350 gallon volume. The aerobic digester was fed 17.5 gallons of
primary sludge per day. The test lasted 30 days.
Feed Sludge Quality The primary sludge fed to the pilot scale digester
was of the same source and quality as fed to the full-scale digester at
15 day digestion. Septic tank wastes were not included. A common set
of constituent analyses served both digesters. Table 11 presents the
averaged feed sludge qualities for the 20 day digestion test.
The chemical oxygen demand varied from 46,000 mg/1 to 113,000 mg/1
while the biochemical oxygen demand ranged between 8,400 mg/1 and
26,900 mg/1. The COD to BOD ratio of the feed sludge was 4.1:1.
Both total and volatile solids concentrations deviated little from
their averages. Suspended solids varied between 3 and 5 percent.
Volatile suspended solids also stayed near the average of 36,300
mg/1 and indicated that the bulk of organic matter was in suspended
form. Alkalinity of the primary sludge was 1,200 mg/1. The pH
showed little variation from the average of 5.6.
Digester Mixed Liquor Total test duration was one month; however,
the first 10 days of data were discarded to allow for stablization
of digester operation. Table 11 presents the averaged digester
mixed liquor constituent concentrations and the percentage reduc-
tion in those concentrations from the feed sludge.
Both chemical oxygen demand and biochemical oxygen demand were
significantly reduced while total solids were only slightly de-
creased. Si nee no decanting of supernatant was practiced during the
test, solids would be expected to increase with losses of water in
evaporation and spray. Therefore, it appears that reductions in
solids due to digestion just matched the reduction of water in the
digester. Suspended solids increased slightly. Total volatile solids
averaged a 15 percent decrease while volatile suspended solids were
reduced by 17 percent.
The original alkalinity was decreased by 84 percent in the mixed
liquor to a concentration of 190 mg/1, although the pH increased
slightly to 6.5 mg/1.
78
-------
TABLE 11
AVERAGE CONSTITUENT REDUCTIONS AT 20 DAYS DIGESTION
Constituents, (mg/1)
10
Liquor
Primary
Sludge
Mixed
Liquor
Percentage
Reduction
COD
74,050
38,340
48
BOD
17,880
3,740
79
Total
Solids
51 .475
51,178
1
Total
Volatile
Solids
39,070
33,134
15
Suspended
Solids
42,505
42,841
-1
Volatile
Suspended Dissolved Temp.
Solids Alkalinity pH Oxygen °C
36,300 1,200 5.6
30,024 190 6.5 4.0 24
17 84
-------
Neither dissolved oxygen nor temperature varied significantly
during the test.
Oxygen Uptake Rate During the stabilized portion of the test the
oxygen uptake rate averaged 84 mg/(l)(hr) or 1.6 mg/(gm T.S.)(hr).
Lack of sufficient data prevented a correlation between digester
temperature and the oxygen uptake rate of the mixed sludge.
Sludge Dewatering An evaluation of the practicality of any sludge
treatment process must include information on sludge dewatering
and final disposal. The Buchner Funnel test was performed on
aerobically digested sludge withdrawn from the digester during the
last day of operation. The calculated specific resistance found
by this test was 1.7 x 109 (sec)2/gm.
An experiment in sand bed drying of the digested sludge was also
conducted. An initial sludge depth of 12 inches was placed over
a 3.1 (ft)2 section of a large sand bed. Within the first week
of drying the depth dropped to 2 inches but little volume change
was observed in the subsequent weeks of the test. Initial surface
cracking appeared on the sludge cake within the second week, and
at the end of 4 weeks the cake was sufficiently dry for removal
from the sand bed. There was no objectionable odor from the sand
bed at any time during the test. Rainfall during the period totaled
2.6 inches. The dried sludge contained 4 mg/gm of total phosphorus,
37 mg/gm of Kjeldahl nitrogen, and 0.016 mg/gm of NOX nitrogen.
Primary Plant Performance Tests conducted with the pilot scale
digester were concurrent with the last test in the full-scale
digester, i.e. between November 8, 1971, and March 15, 1972.
Data for comparable primary plant performance may, therefore, be
found in the presentation for Test 5.
TEST 8—14 DAY DIGESTION The pilot plant was also used to obtain
data on a 14 day digestion test for correlation with the full-scale
digester in this operating range. The aerobic digester was main-
tained at a 350 gallon volume and was fed 25 gallons of primary
sludge daily. Prior to the feeding, 25 gallons of complete mixed
sludge were withdrawn to waste. The test lasted 31 days from
September 22 through November 22, 1971. The digester had pre-
viously been filled with sludge for a "warm up" with a 25 day de-
tention period.
Feed Sludge Quality Averaged results of the feed sludge analyses
obtained from the samples collected for the concurrent test in
the full-scale digester are presented in Table 12. The daily
chemical oxygen demand varied from 27,800 mg/1 to 46,000 mg/1.
The biochemical oxygen demand gradually increased from 4,500
mg/1 to 9,450 mg/1. The COD to BOD ratio was 4.7:1. Total solids
fell between 24,572 mg/1 and 37,234 mg/1, averaging 29,130 mg/1.
Suspended solids changed little. Total volatile solids were about
80
-------
TABLE 12
AVERAGE CONSTITUENT REDUCTIONS AT 14 DAYS DIGESTION
Constituents, (mg/1)
00
Liquor
Primary
Sludge
Mixed
Liquor
Percentage
Reduction
COD
35,660
15,905
55
BOD
7,630
1,080
86
Total
Solids
29,130
25,300
13
Total
Volatile
Solids
19,197
13,209
31
Suspended
Solids
22,572
14,733
35
Volatile
Suspended
Solids
16,560
10,294
38
Alkalinity
732
250
66
Dissolved Temp.
£H Oxygen "C
6.2
7.1
4.3
27
-------
half of the total solids with a daily average of 19, 192 mg/1.
Volatile suspended solids accounted for the bulk of the total
solids, with an average concentration of 16,560 mg/1.
Alkalinity of the feed sludge was lower than for most other tests
and the pH of the sludge ranged from 6.1 to 6.4.
Digester Mixed Liquor Since the digester had been previously
used, it was decided to average performance data over the entire
period of the test. Table 12 provides averaged daily mixed
liquor constituent concentrations and percentage reductions from
the feed sludge.
Oxygen Uptake Rate The oxygen uptake rate of the digester mixed
liquor ranged from 34 to 86 mg/(l)(hr) or 1.35 to 3.45 mg/(gm T.S.)
(hr). The temperature variation between 25°C and 28°C was too small
to provide a meaningful variation in the oxygen uptake rate. The
rate was probably more strongly influenced by the nature of the
sludge fed for any day. The average uptake rate was 52 mg/(l)(hr)
or 2.1 mg/(gm T.S.)(hr).
TEST 9 -- 10 DAY DIGESTION Test 9 was conducted with a 10 day
digestion period in an effort to ascertain the minimum acceptable
digestion time for the primary sludge at the City of Hollywood.
The 350 gallon pilot scale aeration tank was fed 35 gallons per day
of primary sludge while a similar amount of mixed liquor was withdrawn
prior to each feeding. The test was conducted for a period of 30
days following a 10 day stabilization period. Prior to the test
the digester had been used for a 5 day retention test and contents
were not removed between tests.
Feed Sludge Quality Both the pilot and plant scale digesters were
fed the same sludge. Averaged daily analyses are presented in
Table 13. Chemical oxygen demand varied from 40,200 mg/1 to 85,700
mg/1, and biochemical oxygen demand varied from 10,650 mg/1 to
23,400 mg/1. The COD to BOD ratio was 3.8:1.
Total solids varied by almost 100 percent between 33,916 mg/1 and
60,756 mg/1. Suspended solids constituted a large part of the
total solids with a large range between 24,926 mg/1 and 50,586
mg/1. Volatile solids ranged from 22,508 mg/1 to 46,228 mg/1,
while volatile suspended solids ranged from 20,860 mg/1 to 43,158
mg/1.
Digester Mixed Liquor The mixed liquor constituents were rela-
tively unchanged. The averages of the mixed liquor analyses and
the percentage reduction of the various constituents are presented
in Table 13. Chemical oxygen demand was reduced by 54 percent and
biochemical oxygen demand was reduced by 84 percent. The COD to
BOD ratio became 11:1.
82
-------
TABLE 13
AVERAGE CONSTITUENT REDUCTIONS AT 10 DAYS DIGESTION
Constituents, (mg/1)
Total Volatile
Total Volatile Suspended Suspended Dissolved Temp.
m Liquor COD BOD Solids Solids Solids Solids Alkalinity pH Oxygen oc
CO
Primary
Sludge 52.147 13,690 45,679 32,399 35.816 29,505 1,100 5.9
Mixed
Liquor 24,025 2,188 33,846 20,533 24,822 17,054 240 6.7 3.9 25
Percentage
Reduction 54 84 26 37 31 42 78
-------
Oxygen Uptake Rate The oxygen uptake rate varied between 48 mg/
(l)(hr) [1.6 mg/(gmT.S.)(hr)] and 76 mg/(l)(hr) [2.5 mg/(gm T.S.)
(hr)]. The lower oxygen uptake rates occurring toward the end of
the test indicated some relationship of the uptake rates with
varying strength or age of sludge. This relationship nullifies
the value of temperature-uptake comparisons.
Sludge Dewatering When placed on a sand bed, the digested sludge
dewatered rapidly from an initial depth of 12 inches to a final
depth of 2 inches within one week. At the end of 4 weeks the sludge
could be removed from the bed. There was a disagreeable foul odor
from the drying sludge during the 4 weeks; however, rainfall on the
sand bed totaled 9 inches over the drying period.
A Buchner Funnel test showed a specific resistance of 6.08 x 108
sec2/gm in the digested sludge.
Dried sludge was removed from the sand bed and analyzed for nutrient
content. The following concentrations were measured: 1.2 mg/gm of
phosphorus; 17.8 mg/gm of Kjeldahl nitrogen; and 0.065 mg/gm of NOX
nitrogen.
TEST 10 -- 5 DAY DIGESTION Test 10 had the shortest detention time
of all tests conducted in the program. The 350 gallon pilot scale
digester was fed 70 gallons of primary sludge per day. The same
quantity of mixed liquor was removed on a daily basis. This test
was conducted for 36 days.
Feed Sludge Quality Table 14 presents a tabulation of averaged
daily values for constituents of the feed sludge. The reduction
in chemical oxygen demand was 40 percent and the biochemical oxygen
demand fell by 80 percent. The COD to BOD ratio increased to 12:1.
Oxygen Uptake Rate The average oxygen uptake rate was 86 mg/(l)
(hr) or 2.0 mg/(gm T.S.)(hr). The uptake rate was considerably
lower in the beginning when the digester still contained sludge
from the 20 day detention test.
Sludge Dewatering Sludge dried as well on a sand bed as in
the lO day digestion test. A 12-inch initial depth of sludge
dropped to two inches within one week of drying. Within 4 weeks
of drying the sludge was well dried and could be removed from the
sand bed. Cumulative rainfall was 5.4 inches. During the first
2 weeks there was a noticeable foul odor from the cake, but by
the end of the 4th week the odor had disappeared. Using a digested
sludge sample containing 2.7 percent total solids, the measured
specific resistance was 2.32 x 10° sec^/gm. When dried sludge was
removed from the sand bed, a sample was retained for nutrient
analyses. The weight concentrations of nutrients in the dried
sludge were 1.21 mg/gm, 16.1 mg/gm, and 0.08 mg/gm for total
phosphorus, Kjeldahl nitrogen, and NO^ nitrogen, respectively.
84
-------
CO
en
TABLE 14
AVERAGE CONSTITUENT REDUCTIONS AT 5 DAYS DIGESTION
CONSTITUENTS (mg/1)
Liquor
Primary
Sludge
Mixed
Liquor
Percentage
Reduction
COD
64,500
38,680
40
BOD
16,050
3,213
80
Total
Solids
50,670
42,420
16
Volatile
Solids
37,564
27,522
27
Suspended
Solids
40,709
33,441
18
Suspended
Solids
34,305
24,159
30
Alkalinity
1,100
208
81
Dissolved Temp.
£H Oxygen °C
5.6
6.5 3.2 26
-------
SECTION VII
DISCUSSION OF RESULTS
CONSTITUENT REDUCTIONS Average feed sludge total solids varied,
with one exception, in a narrow range between 45,000 mg/1 and
61,000 mg/1. Other constituents varied similarly, and except
for the 14 day test which had low feed sludge constituent con-
centrations, all tests were begun with a similar feed.
There was no discernible relationship between initial sludge con-
stituent concentrations and percentage reductions although there
was some relationship between initial and final sludge constituent
concentrations. Therefore, a basis of percentage reductions was
used for data comparison to make the information of the project
more universally applicable.
The averaged constituent reductions for each test are presented
in Table 15. In comparison with the total mass of available data,
it appears that the results of the 23 to 29 day digestion test are
atypical, but such performance may be expected at times in a digester.
Reductions in chemical oxygen demand ranged from 17 to 55 percent.
No direct correlation could be made between COD reduction and di-
gestion period. The high reduction of 55 percent appeared in a
14 day digestion test, while the next to lowest reduction of 29
percent occurred during the 15 day digestion test. The reduc-
tion in BOD also could not be correlated with the length of the
digestion period. It was reduced by a low 68 percent in the 15
day digestion test, by 86 percent in the 14 day digestion test,
and by 92 percent in the 43+ day digestion test.
Difficulties also resulted when an attempt was made to analyze
the solids data for a relationship between percent reductions
and aeration period. Total solids were reduced from 1 percent
in the 20 day digestion tests to 26 percent in the 10 day digestion
test. Volatile solids reductions also fluctuated widely between
12 percent at the 15 day digestion test and 39 percent at the 29
day digestion test. Suspended solids decreases varied from minus
1 percent to 35 percent while volatile suspended solids reductions
fell between 14 and 42 percent.
It appears from these data that an exact correlation between consti-
tuent reduction and period of digestion cannot be made. However, in
Table 10, where the results of the batch digestion test are presented,
it is obvious that there is a gradual reduction in constituent con-
centrations with increased digestion time. A clue to the discre-
pancies in the data, and perhaps even in the uniqueness of the 23
to 29 day digestion test, lies in the differences between the
total volatile solids reductions.
86
-------
00
TABLE 15
COMPARISON OF AVERAGE CONSTITUENT REDUCTIONS
Concentration Reduction, %
Digestion
Period
(days)
43+
23 - 29
29
22
20
15
14
10
5
COD
45
18
42
46
48
29
55
54
40
BOD
92
70
83
84
79
68
86
84
80
Total
Solids
16
3
22
19
1
6
13
26
16
Volatile
Solids
38
13
39
36
15
12
31
37
27
Suspended
Solids
30
28
-1
2
35
31
18
Volatile
Suspended
—
41
41
17
14
38
42
30
-------
During the 20 day digestion test there was a 1 percent reduc-
tion in total solids and a 1 percent increase in suspended solids.
This compares to a 15 percent reduction in volatile solids and a
17 percent decrease in volatile suspended solids. A major factor
that could explain such differences in reductions is evaporation
of the digester water, which during a 5, 10, or 40 day period can
substantially concentrate the remaining sludge constituents. This
could even cause an increase in suspended solids. Varying evapora-
tion rates on sunny, cloudy, or rainy days could affect minor dif-
ferences in digested sludge concentration and thereby confuse com-
parisons between reductions .and digestion periods. Actual reduc-
tions of constituent concentrations were, therefore,probably
substantially greater than is presented in the data. Estimated
evaporation rates of 20 to 25 percent during a 20 day detention
test would reconcentrate remaining sludge constituents and decrease
percentage reductions. The lack of readily available sludge meter-
ing equipment has been mentioned previously as the reason for
comparing constituent concentration rather than total mass changes.
Furthermore, the parameters measured in this study are essentially
the same ones measured by other treatment plant operators who must
relate data to final sludge quality under the same environmental
factors encountered in this study, i.e., evaporation, rainfall,
and temperature changes.
Other factors also influence differences in the reduction between
total solids and volatile solids. Oxidation and digestion of
volatile matter are accompanied by the creation of an inert re-
sidue which is measured as total solids.
In Test 6, where a single batch of sludge was digested over an ex-
tended period, digestion and reduction of constituents were essen-
tially completed within a period of 14 days. The aerobic digestion
batch test was conducted under the same conditions of liquid evapor-
ation as were the tests in the pilot and full-scale digesters.
Proceeding under the assumptions that essentially all digestion would be
completed by the 14th day of aeration and that subsequent aeration would
not significantly change sludge concentrations, the average reductions
for all tests between 14 and 43 days digestions were averaged together.
The average effluent from an aerobic digester treating primary sewage sludge
at and above a minimum hydraulic retention period of 14 days would be 40
percent lower in chemical oxygen demand, 80 percent lower in biochemical
oxygen demand, 11 percent lower in total solids, 26 percent lower in volatile
solids, 19 percent lower in suspended solids, and 30 percent lower in volatile
suspended solids than undigested primary sludge. These averages reflect
seasonal changes in population loading, rainfall, and temperature due to
the extended test periods involved.
The excessively long detention periods of the 43+ day digestion test were
unnecessary in achieving acceptable digested primary sludge. Two-stage
digestion as performed in that test was unnecessary except to achieve
the long detention period. Two-stage digestion is employed at times in
sewage treatment plants that also aerobically digest waste activated sludges.
-------
These sludges frequently have poor settling characteristics and would re-
quire large digester volumes for adequate retention periods. A short
period of digestion in the first stage of approximately 5 days permits
sludge thickening in the second digester which, with decanting or a separate
thickening facility, may be constructed at a reduced volume to achieve the
same loading rate. The primary digested sludges encountered in this pro-
gram did not thicken in settling tests and became anaerobic within several
hours of the start of such tests. For these reasons, thickening was not
practiced in a digester except during some start up periods where there
had been an initial charge of groundwater to the digester.
As stated previously, daily feed to the full scale digesters was measured
by means of time clocks on the sludge pumps, but there was no accurate
way of measuring wasted sludge volumes. Attempts were made to measure
volumetric changes within the digester, but were found to be unsatisfactory.
Therefore, with the lack of reliable information concerning sludge discharge
and rainfall/evaporation effects on the full scale digesters, an attempt
was made to develop a mass balance for the pilot plant aerobic digestion
studies.
The pilot plant studies consisted of four tests conducted at detention
times of 5, 10, 14, and 20 days with a constant reacter volume of 350 gal.
A cover was placed over the unit to reduce liquid loss by evaporation and
spray. Measurements were made of total solids (TS), total volitile solids
(TVS), suspended solids (SS), and volitile suspended soilds (VSS) in the
sludge feed and in the reacter.
It would be expected that the reduction of each of the parameters would
increase with detention time. In Figure 28 each of the parameters is plotted
against detention time, and an increasing reduction is indeed indicated through
a detention time of 10 days; however, after 10 days the percent reduction
decreases. This would indicate that a concentration effect occured which
must be attributed to evaporation loss. It can be assumed that if evaporation
could be taken into account and negated, the curves in Figure 28 would have
steeper slopes and would not develop negative slopes.
Obviously, since the reactor volume was maintained at 350 gallons in all
cases, either the volume of feed or the volume of sludge removal (or both)
had to be varied accordingly to compensate for evaporation loss. However,
the records indicate merely that 350/t gall were removed and added each
day for each test. Unfortunately, it was not anticipated at that point in
time that more accurate volume measurements would be desirable.
SLUDGE DEUATERING Sludges resulting from the aerobic digestion of primary
sludge were tested and analyzed to determine minimum acceptable diges-
tion times for a stable, odor free waste. Digested sludges were vacuum
filtered and the rate of water removal gave an indication of dewatering
characteristics. Digested sludges were also placed on sand beds where
drying times and odor characteristics were noted. The combined data
was used to evaluate a minimum digestion retention time. Additional
data was collected on simulated lagooning of single batches of aerobically
digested sludge.
89
-------
Filterabili'ty Filterability of digested and undigested sludges was
measured to determine which sludges would be more suitable for mechani-
cal dewatering devices. Specific resistance, a quantity calculated from
measurements of the liquid volume passing through a filter cake, was used
as the comparative index for the various sludges. The specific resis-
tance is numerically equal to the pressure difference required to pro-
duce a unit rate of filtrate flow of unit viscosity through a unit
weight of cake. Calculated specific resistances for the 20, 15, 1(L
and 5 day digestion tests were 1.7 x 109 (sec)2/gm, 5.6 x 108 (sec) /gm,
and 2.3 x 10° (sec)2/gm, respectively. The values span one order of
magnitude and are substantially less than the specific resistance of
primary undigested sludge which was measured to have a specific resis-
tance of 4.2 x 1010 (sec)2/gm. The greatest reduction in specific re-
sistance was by a factor of 182 in the 5 day digestion test and the
least measured reduction was by a factor of 25 in the 20 day digestion
test.
Studies performed by Jones20 on the filterability of anaerobically
digested sludges provide interesting comparison of data. Anaerobically
digested sludge was measured to have a specific resistance of 2.05
x 10'° (sec)2/gtn which is half of the specific resistance of the
primary sludges in this study and substantially more than the specific
resistances of the aerobically digested sludges. Only with the aid of
polymers was Jones able to lower the specific resistance of anaerobi-
cally digested sludges to values between 1.53 x 108 (sec)2/gm and
3.69 x 107 (sec)2/gm.
It would appear that aerobically digested primary sludges are much more
amenable to dewatering by filtration than either primary sludges or
anaerobically digested primary and waste activated sludges, and that shorter
aerobic digestion periods produce sludges with lower specific resis-
tances than do longer periods.
Sand Bed Drying Sand bed drying is one of the most commonly used
methods of sludge dewatering due to its simplicity and ease of main-
tenance. Sand bed tests were conducted on waste sludges from 20, 15,
10, and 5 day digestion tests. Sludge from the 15 day test in the
full-scale digester was placed to a depth of 17 inches on a Targe
sand bed. Sludges from the 20, 10, and 5 day digestion tests, con-
ducted in the pilot scale digester, were placed in opened bottom,
55 gallon drums, set at least six inches into a standard, underdrained
sand bed. The digester sludge from the 15 day test took approximately
11 weeks to dry to a well cracked cake. The sludge from the 15 day
test, however, had experienced relatively low removal of constituents
during digestion. Objectionable odors noticed in the immediate vi-
cinity of the sand bed might be attributable to inadequate digestion.
90
-------
40
30
20
10
CD
O
n>
O>
O.
o
?40
30
20
10
0
TVS
10
Detention Time
15
20
Figure 28. Percent Removal of Solids Versus Detention Time,
Pilot Plant Studies
-------
ro
FIGURE 29: A DOUBLE BED OF DRIED, WELL CRACKED AEROBIC SLUDGE
-------
TABLE 16
CHEMICAL ANALYSES OF LA600NED SLUDGES
Constituents, grams
co
Digester
Retention
5 Day
10 Day
15 Day
20 Day
Sample
Outdoor
Indoor
Outdoor
Indoor
Outdoor
Indoor
Outdoor
Indoor
Days
Lagooned
0
70
Decrease
0
70
Decrease
0
51
Decrease
• 0
51
Decrease
0
70
Decrease
0
70
Decrease
0
33
Decrease
0
33
Decrease
Volume
(liter)
15.1
7.6
3.5
1.9
15.1
10.4
3.5
2.5
15.1
7.6
3.5
2.1
15.1
16.1
3.5
2.7
COD
625.3
367.2
41 %
144.6
91.8
37 %
590.5
356.0
40 %
136.5
114.0
16 %
635.9
418.7
34 %
147.0
115.0
22 %
620.8
569.5
8 %
143.5
134.7
6 %
BOD
60.6
31.8
48 %
14.0
8.0
43 %
40.1
41.4
-3%
9.3
10.2
-10 %
68.1
36.3
47 %
15.8
10.3
34 %
49.6
31.4
37 %
11.5
9.5
17 %
Total
Solids
663.3
413.1
38 %
153.3
107.7
30 %
613.5
438.3
29 %
141.8
127.8
10 %
702.3
443.5
37 %
162.3
124.3
23 %
814.7
643.4
21 %
188.3
166.0
12 %
Volatile
Solids
429.2
230.2
46 %
99.2
58.6
41 %
402.4
250.8
38 %
93.0
73.6
21 %
477.0
261.9
45 %
110.3
73.0
34 %
513.2
366.5
29 %
118.6
93.4
21 %
Suspended
Solids
536.4
329.0
39 %
124.0
96.2
22 %
506.1
333.7
34 %
117.0
104.5
11 %
607.7
356.9
41 %
140.5
107.9
23 %
672.4
502.1
25 %
155.4
140.1
10 %
Dissolved
Solids
144.5
121.0
16 %
33.5
30.9
8 %
146.9
125.3
15 %
34.0
32.2
5 %
167.2
92.9
44 %
38.6
25.0
35 *
190.1
136.6
28 %
43.9
34.5
22 %
Volatile
Dissolved
46.3
20.6
56 %
10.7
5.2
51 %
40.1
23.7
41 %
9.3
3.8
59 %
45.6
20.0
56 %
10.5
5.0
53 %
54.9
25.4
54 %
12.7
3.5
73 %
-------
The sand bed drying of pilot plant sludges was performed in a restricted
sand bed area due to the small volumes of sludge available from the
digester. The sludges from the 20, 10, and 5 day digestion tests de-
watered rapidly and could be removed from the sand beds within 4 weeks.
Of these three, only the 5 day digestion period sludge exhibited odors
in the immediate vicinity of the sand bed and these odors disappeared
by the 4th week. In view of the fact that the filterability of the 15
day digestion sludge was similar to that of the other sludges, it is
probable that water retention in the 15 day sludge was due to a poor
sand bed underdrain system. When allowed to drain rapidly 1n a vertical
and somewhat horizontal direction as in the 55 gallon sand beds, the
aerobically digested sludges dewatered very well with as little as a
10 day digestion period.
Chemical analyses of the nutrient value of aerobically digested, sand
bed dried sludges indicated very little mineral fertilizer content.
Analyses were conducted on the 5, 10, 15, and 20 day digestion sludges.
The Kjeldahl nitrogen varied between 16.1 and 36.8 mg/gm of dried sludge.
The nitrate forms of nitrogen were from 0.016 to 0.08 mg of N/gm of dry
sludge. With the exception of the nitrate and nitrite forms, increased
sludge digestion periods resulted in dried sludges of greater nutrient
content.
Figure 29 shows a well dried, odor free sludge that was obtained by
spreading a double layer of wet sludge on a sand bed.
LAGOONING OF DIGESTED SLUDGES Due to the identical physical behavior
of all sludges tested under simulated lagooning, the data from all the
lagooning tests have been compiled in this section. At the end of the
5, 10, 14, and 20 day digestion tests, the digested sludges were placed both
in containers located outdoors and in others 1n the laboratory. Obser-
vations were made of the sludge condition with passing time and analyses
were conducted on the samples at the end of the lagooning tests.
Table 16 presents detailed chemical analyses of the digested lagooned
sludges. Lagooning time for each sample varied due to decreasing
available time at the end of the program. In all samples except the
20 day digestion outdoor sample, there was a total decrease in liguid
volume with a slight corresponding Increase in most constituent con-
centrations. The net effect of lagooning was a reduction in almost
all absolute quantities of constituents. Reductions were generally
greater for the 5 day digested sludges than for the 10, 15, and 20 day
sludges. This is partially due to the longer lagooning period of 70
days on the 5 day test, but also to the fact that 5 day digested
sludge has a greater content of unstabilized organic material than do
the other sludges.
94
-------
During the lagooning tests common physical behavior was noted for all
the samples, both outdoor and indoor. During the first several days,
an inversion resulted in all sludge solids rising to the surface
where they hardened and formed a crusty cake. At the very surface of
the cake, aerobic decomposition occurred, giving a tan to light brown
color to the cake. However, below the surface the sludge cake turned
black and within a week, gases producing obnoxious odors broke through
and persisted throughout the remainder of the tests. After approxim-
ately one week the solids at the bottom of the sludge cake began to fall
to the bottom of the container. These solids were light and no compac-
tion was noted at the bottom of the container. Eventually the entire
volume of the liquid under the sludge cake was occupied by the light
solids. After 45 days of lagooning the surface sludge cake broke up
into discrete particles. A considerable amount of obnoxious odor was
noted at this time. The sludge at the end of the test was a very black,
odorous liquor with finely dispersed solids. Several species of flies
were continually attracted to the samples during the test. The flies
laid viable eggs and produced-many larvae.
The simulated lagooning of aerobically digested sludge indicated
that a better reduction of sludge constituents occurred in the out-
door containers than in the laboratory containers. In all prob-
ability, external factors such as wind and rain achieved a degree
of mixing in the outdoor containers that was absent in the labora-
tory. Warmer daytime temperatures also occurred in the outdoor
samples than in the air conditioned, laboratory samples and provided
a stimulus for greater biological activity.
Varying lagoon detention periods confuse a comparison of sludge changes
from the lagooning of 20, 15, 10, and 5 day sludges. The two samples
of the 15 and 5 day digestion sludges were lagooned for an equal period
of time. Table 16 shows little difference in reduction of 5 day sludge
constituents and 15 day sludge constituents over a 70 day lagooning
period. The comparative reduction for the 5 and 15 day sludge outdoor
test are: 41 percent vs. 34 percent of chemical oxygen demand; 48 per-
cent vs. 47 percent of biochemical oxygen demand; 38'percent vs. 37
percent of total solids; 46 percent vs. 45 percent of volatile solids;
39 percent vs. 41 percent of suspended solids; 16 percent vs. 44 percent
of dissolved solids; and 56 percent vs. 56 percent of volatile dissolved
solids. Slightly greater differences were noted 1n the indoor lagooned
samples for the two retention period tests.
The outdoor lagoon samples of all the various test periods contained
similar initial quantities and concentrations of sludge constituents.
In view of the fact that these sludges were in an advanced stage of
auto-oxidation and probably had similar predominant organisms and
chemical compound forms, the data from the 5, 10, 15, and 20 day digestion
tests were combined for an analysis of constituent change upon sludge
lagooning. The percentage change 1n absolute Quantities of constituents
with lagooning time may, therefore, be compared for the various sludges.
95
-------
Figure 30 shows these changes plotted as a function of time. It appears
that the rate of change in organic constituents represented by chemical
oxygen demand, biochemical oxygen demand, and volatile solids , tapers to
a low value after approximately 70 days of lagooning.
As previously mentioned, lagooning of aerobically digested sludges was
accompanied by a surface seal of floating sludge cake while anaerobic
decomposition occurred below the sludge cake. Escaping gases from this
anaerobic digestion produced noxious*odors. It is possible that in a
large scale lagoon there would be adequate wind Induced turbulence to
prevent such a cake from forming and to provide substantial mixing within
the lagoon. The possibility exists that such a lagoon would not become
anaerobic and/or would not release strong objectionable odors.
MONITORING PARAMETERS Much previous research, particularly that conducted
on laboratory aerobic digestion of single sludge batches, has raised the
question of finding a process parameter, the monitoring of which would
provide the plant operator with information of the efficiency or degree
of digestion of the sludge. Process parameters mentioned for possible
consideration were pH, alkalinity, and oxygen uptake rate. These para-
meters may be relatively quickly obtained while others such as reduction
of chemical oxygen demand or total volatile solids require more time
and elaborate procedures.
This study has monitored pH, alkalinity, and oxygen uptake rates for most
of the digestion tests including a reference batch test. It was found
that there were definite changes in these indicators between digested
and undigested samples. Alkalinity of primary sludge averaged 1,092 mg/1.
Sludges digested for 15 days and longer had alkalinities less than 200
mg/1. Individual sample differences did not permit greater differentia-
tion in the alkalinities of samples at specific digestion periods. The
batch digestion test provided a gradual 1f slightly erratic decrease in
alkalinity from 700 mg/1 in the primary sludge to 340 mg/1 at 14 days
digestion and 240 mg/1 at 24 days digestion. The greatest decrease
in alkalinity occurred during the first 14 days of digestion; however,
a specific quantitative endpoint associated with a specific degree of
digestion could not be chosen.
The pH of all samples increased during digestion. In the batch digestion
test the pH increased most rapidly after the first 14 days of digestion
and during the first 2 days of digestion when there was a temporary drop
in alkalinity. It would appear that there was an initial air stripping
of carbon dioxide from the sludge with a concomitant rise in pH, followed
by increased biological activity which released additional carbon dioxide,
increased alkalinity, and sliqhtlv depressed pH, until the 12th dav of
aeration. The initial pH of most sludges was low and averaged 5.9.
The pH of the continuously fed aerobically digested sludges varied
between 6.4 and 7.1; however, no correlation could be made between
period of digestion and waste sludge pH.
96
-------
80 „
n COD
A BOD
O T.S.
« T.V.S.
60 .
c
o
•r"
-P
O
-o
Q-
40.
20.
0-
-10.
—\—
20
40
—r~
60
~80~
Poo
Time (day)
Figure 30,
Reduction of Sludge Constituents by
Lagooning Outdoor Samples
97
-------
FIGURE 31: A WELL MIXED AEROBIC DIGESTER
98
-------
Oxygen uptake rate of a digesting sludge reflects the degree of biological
activity taking place within the sludge. In the batch test, the oxygen
uptake rate was 4.5 mg/(gm T.S.)(hr) on the first day of aeration and fell
to 1.2 mg/(gm T.S.)(hr) by the 24th day of aeration with an intermediate
low of 0.6 mg/(gm T.S.j(hr) at the 22nd day of aeration. Prior to the
14th day of the test all oxygen uptake rates measured above 2.5 mg/
(gm T.S.)(hr) and after the 14th day fell below 1.6 mg/(gm T.S.)(hr).
Between the 14th and 24th day of the test there was constant variation
in the oxygen uptake rate which was not reflective of the degree of
digestion or the period of sludge digestion. In the continuously fed
digestion tests the oxygen uptake rate varied between 1.3 and 2.35
mg/(gm T.S.)(hr). While uptake rates of tests of 20 day digestion and
longer were below 1.8 mg of 02/(gm T.S.)(hr) or greater, there was
again no exact comparison between the oxygen uptake rate and the de-
gree or period of sludge digestion.
In the continuously fed digestion tests the oxygen uptake rate varied
between 1.3 and 2.35 mg/(gm T.S.)(hr). While uptake rates of tests of
20 day digestion and longer were below 1.8 mg of 02/(gm T.S.)(hr) and
of tests of 15 days and less were 1.8 mg of Oo/tgm T.S.)(hr) or greater,
there was again no exact comparison between trie oxygen uptake rate and
the degree or period of sludge digestion.
From the evidence collected in this program it appears that pH, alkalinity,
and oxygen uptake rate may provide gross differentiation between raw
sludge, partially digested sludge, and completely digested sludge. These
parameters cannot provide a fine distinction on the degree of digestion
of any sludge, nor could they be related to final sludge filterability
or behavior on a sand bed. An evaluation of these parameters and their
quantitative values should be performed for each separate aerobic sludge
digestion installation.
Certain visual and physical parameters were noted that could help an
operator controlling an aerobic digester. The first visual aid recognized
was foaming at the surface of the digester. This occurred in the tests of
greater than 22 day sludge detention. The foaming was associated with an
underloaded digester; however, it gave an appearance of overloading. This
was because the surface foam, which was several feet thick, impeded oxygen
transfer to the sludge by the mechanical mixer. The sludge, becoming an-
aerobic black, and foul smelling, gave the impression that the digester was
too overloaded for its oxygen transfer capabilities. In contrast to Figure
16 which depicts a foaming digester, Figure 30 presents a properly functioning,
well mixed aerobic digester.
Well aerated, aerobic sludges had a light brown color. Anaerobic and
near-anaerobic sludges were from dark brown to black in color.
A relative observation of a "too-thick" sludge was usually accompanied by
a darkening of the sludge near the digester bottom. This indicates that
the sludge is too heavy for proper mixing and should be wasted to a sand
bed.
99
-------
SECTION VIII
REFERENCES
1. Eckenfelder, W. W., "Studies on the Oxidation Kinetics of Biological
Sludges," Sewage and Ind. Wastes, 28., 1956.
2. Carpenter, W. L. and Blosser, R. 0., "Aerobic Decomposition of
Secondary Papermill Sludges," Proc. 17th Ind. Waste Conf.. Purdue
University, 1962.
3. Jaworski, N., Lawton, G. W., and Rohlich, G. A., "Aerobic Sludge
Digestion," Advances in Biological Waste Treatment, ed. by W. W.
Eckenfelder and Brother Joseph McCabe, New York, MacMillan Co.,
1963.
4. Malina, J. F., Jr., and Burton, H. N., "Aerobic Stabilization of
Primary Wastewater Sludge," Proc. 19th Ind. Waste Conf., Purdue
University, 1964.
5. Barnhart, E. L., "Application of Aerobic Digestion to Industrial
Waste Treatment," Proc. 16th Ind. Waste Conf., Purdue University,
1961.
6. Norman, J. D., "Aerobic Digestion of Waste Activated Sludge,"
Master's Thesis, Unviersity of Wisconsin, Madison, 1961.
7. Loehr, R. C., "Aerobic Digestion—Factors Affecting Design,"
Water and Sewage Works, Reference No. R-169, 1965.
8. Dreier, D. E., and Obma, C. A., "Aerobic Digestion of Solids,"
Walker Process Equipment Co., Bulletin No. 26-5-18194, 1963.
9. Bruemmer, J. H., "Use of Oxygen in Sludge Stabilization," Proc. 21st
Ind. Waste Conf., Purdue University, 1966.
10. Reynolds, T. D., "Aerobic Digestion of Waste Activated Sludge,"
Water and Sewage Works. 114. 2, February 1967.
11. Kehr, D., "Aerobic Sludge Stabilization in Sewage Treatment Plants,"
Paper I1-8, Third Intl. Conf. on Water Pollution Research, Munich,
Germany, 1966.
12. Dryden, F. E., Barrett, P. A., and Kissinger, J. C., "The Evaluation
and Design of Biological Treatment Facilities for Pharmaceutical
Wastes," BiologicalL Treatmentof Sewage and Industrial Wastes,
1, Reinhold, New York, 1956.
13. Irgens, R. L., and Halvorson, "Removal of Plant Nutrients by Means
of Aerobic Stabilization of Sludge," Applied Microbiology. 13, 3,
May, 1965.
100
-------
14. Randall, C. W., and Koch, C. T., "Dewatering Characteristics of
Aerobically Digested Sludge," J. Hater Pollution Control Federation,
£L, May 1969.
15. Lawton, G. W., and Norman, J. D., "Aerobic Sludge Digestion Studies,"
J. Water Pollution Control Federation, 36> April 1964.
16. Coackley, P., "Research on Sewage Sludge Carried Out in the C. E.
Department of University College, London," J. Institute of Sewage
Purification. 1955.
17. Vararaghavan, V., "Digesting Sludge by Aeration," Water Works and
Wastes Engr., 2_, 9, September 1965.
18. Ahlberg, N. R., and Boyko, B. I., "Evaluation and Design of Aerobic
Digesters," J. Water Pollution Control Federation. 44_, April 1972.
19. Coackley, P., and Jones, B. R. S., "Sludge Dewatering and Specific
Resistance," Sewage and Industrial Wastes, 28, 1956.
20. Jones, R. H., "Liquid-Solids Separation in Domestic Waste with a
Cationic Polyelectrolyte," Doctoral Dissertation, University of
Florida (December 1966).
101
-------
SECTION IX APPENDICES
APPENDIX A
SEWER LINE INFILTRATION
Rainfall and sewer flow were collected on a daily basis during the
two year experimental program. This information was fed to a computer
where system modeling using discrete transform techniques provided a
relatively accurate model of rainfall effects on sewer flow.
The basic model may be expressed as
n A n = p-0.184n -1 + 0 77 P + 0
°-avg + Qstorm e 4storm ' "'" v gavg
where Qavg = mean sewage flow
0 *. = predicted storm flow for day of rain
xstorm
Qstorm "^ = measurec' storm flow for preceding day
and P = precipitation for the day of rain.
This equation is specifically fitted to the City of Hollywood sewage
system with its attendant unique conditions of lag time, rainfall fre-
quency, beach front infiltration and general community composition.
Careful filtering of the raw data to remove noise and periodic sewage
flow variations has produced the constant 0.77 which, when multiplied
against precipitation in inches, will result in the daily storm water
flow in MGD in the sewage system.
For dry weather flow of 13.6, a 3 inch rainfall would increase
the one day flow to 15.9 MGD. This added flow increment of 2.3 MGD
then decays at a daily rate of e""-^84. As an example of the accuracy
of this program a portion of the actual rainfall vs. flow data has
been graphically presented in Figure Al. The computer model was then
applied to the rainfall and same mean plant flow over a similar period
of time. Figure A2 presents the resulting predicted combined sewage
flow, less the periodic fluctuations and noise not associated with
storm flow. The close match in profiles and magnitude of the sewage
flow during the same time period substantiates the validity of the
mathematical model.
102
-------
25-
= 20
'•s
J 15~
M —
* 10-
I 5-,
0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
Time (Days)
FIGURE Al: Actual Daily Sewage Flow
o
co
25
o 20
• 15 -J
«3
5 J
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
Time (Days)
FIGURE A2: Computer Predicted Sewage Flow
-------
APPENDIX B
AEROBIC DIGESTION
System Outline Based on the data collected in this report, an aerobic
sludge digestion system would be recommended for serious consideration
in any sewage treatment system. Aerobic sludge digestion is a simple
process and its component equipment parts would include piping leading
to and away from the digester, a plain circular tank as the digester, and
a floating mechanical aerator. Special circumstances may dictate the use
of rectangular tanks with compressed air aeration. The digester tank
should be sized based on hydraulic retention period of the primary sludge.
Information gathered in this program indicates a recommended design re-
tention period of 20 days.. The BOD loading of the digester would then
be a function of the influent sludge concentration. Only the oxygen
transfer requirements should be affected by the BOD loading for primary
municipal sludge. A high loading would relate to a high uptake rate,
and thusly affect the aerator requirements. The principal design con-
sideration is hydraulic retention time.
The peak oxygen uptake rate for the 20 and 22 day digestion tests was
measured at 112 mg of ty 0)(nr)' In the 396,000 gallon aeration tank
at Hollywood this is equivalent to a demand of 370 Ibs. of oxygen per
hour. The 100 Hp mechanical aerator, used at Hollywood, was rated by
the manufacturer to supply 350 Ibs. of oxygen per hour. This aerator
apparently sufficed because of the average oxygen concentration in the
digester during the 22 day test was 3.5 mg/1 and at no time did the
digester become anaerobic.
System Economics The average wastewater flow at the City of Hollywood
Sewage Treatment Plant was 13.6 MGD. The required digester aeration
volume for this flow was found to be 396,000 gallons. The following items
are assumed:
(1) Site is normal—no high water table or unsuitable soil to
be removed,
(2) Digester tank base slab to be placed on existing ground
elevation,
(3) Only fine grading involved,
(4) Machine excavation required for footings, etc.
Using this data, a capital cost in May 1972 dollars was calculated for
the installation of an aerobic digester at an existing sewage treatment
facility. The following items are included in the complete digester:
1) Excavation (5) Curing
2) Forms (6) Reinforcing steel
(3) Concrete (7) Aluminum ladders and rails
(4) Finishing, float (8) 3 hand hoists
104
-------
(9) Piping and valves
10) Finishing screens
11) Waterstop
12
13
14
Aerator, 100 Hp, floating
Electrical controls
Sludge pump
Total cost for installation of this aerobic digester, including contractor's
equipment, overhead and profit, is estimated at $130,000. This is approxi-
mately one dollar per hundred gallons of main plant flow.
The estimated life of the fixed digester hardware is 30 years, while the
aerator and sludge pump have a 15 year estimated life. The total cost of
capital equipment over a 30 year period would be $163,189. The yearly
cost of this sum amortized over a 30 year period at 7.5 percent interest
compounded annually would be $13,817.41. This is equivalent to a cost
of 0.28 cents per 1,000 gallons of raw sewage.
Maintenance costs may be assumed at 10 percent of the yearly cost of the
aerobic digester. This adds 0.03 cents per 1,000 gallons for a combined
capital and maintenance cost of 0.31 cents per 1,000 gallons.
The power requirements for the 100 Hp aerator are 60 kilowatts per hour
at a power cost of 0.117 cents per 1,000 gallons. The power requirements
for the sludge transfer pump are relatively negligible. The total capital
and operating costs exclusive of labor and land for the aerobic digestion
process should be less than 0.43 cents per 1,000 gallons at the City of
Hollywood. This is equivalent to 0.32 cents per gallon of waste sludge
at 4.5 percent total solids concentration.
105
-------
APPENDIX C
EFFECTS OF VARIOUS DOSAGES
OF POLYELECTROLYTES ON
BOD AND SUSPENDED SOILDS REMOVAL
TABLE Cl
EFFECT OF VARIOUS DOSAGES OF CAT FLOC ON REMOVAL
OF BOD AND SUSPENDED SOLIDS IN SEWAGE FROM THE
EFFLUENT OF THE GAINESVILLE GRIT CHAMBER
Polymer
Dosage
mg/1
0.0*
0.5
0.9
1.9
2.8
3.7
4.7
5.6
7.5
9.4
11.2
Suspended
Solids
rng/1
130
18
13
22
20
13
13
12
7
10
11
BOD
mg/1
152
61
58
50
37
26
24
21
19
18
19
Final
PH
7.27
7.32
7.35
7.36
7.36
7.63
7.61
7.59
7.55
7.68
7.47
Electro-
phoretic
Mobility
y/(sec)(v)(cm)
-1.7
-1.4
-1.3
-1.2
-1.2
-1.1
-1.0
-0.9
-0.7
0.3
0.0
*Effect of 0.0 polymer dose from raw sample, not a blank.
106
-------
TABLE C2
EFFECT OF VARIOUS DOSAGES OF CAT FLOC ON REMOVAL
OF BOD AND SUSPENDED SOLIDS IN SEWAGE FROM THE
EFFLUENT OF A GAINESVILLE
PRIMARY SETTLING TANK
Polymer
Dosage
mg/1
0.0*
0.5
1.0
3.0
5.0
7.0
10.0
12.0
15.0
Suspended
Solids
mg/1
80
50
45
53
34
36
28
33
17
BOD
mg/1
170
165
135
85
68
55
33
33
35
Final
PH
7.42
7.49
7.49
7.47
7.50
7.58
7.62
7.60
7.62
Electro-
phoretic
Mobility
y/(sec)(v)(cm)
-1.8
-1.6
-1.4
-1.2
-0.9
-0.7
-0.2
+0.1
+0.3
*Effect of 0.0 polymer dose from raw sample, not a blank.
107
-------
TABLE C3
EFFECT OF VARIOUS DOSAGES OF CAT FLOC ON REMOVAL
OF BOD AND SUSPENDED SOLIDS IN SEWAGE FROM THE
EFFLUENT OF THE UNIVERSITY OF FLORIDA
GRIT CHAMBER
Polymer
Dosage
mg/1
0.0*
0.5
0.9
1.9
2.8
3.7
4.7
5.6
6.6
7.5
9.4
11.2
Suspended
Solids
mg/1
95
25
25
20
3
10
0
7
• •
14
8
14
BOD Final
mg/1 pH
120 8.50
89
71
51
41
35
30
30
• • • •
34
31
31
Electro-
phoretic
Mobility
u/(sec)(v)(cm)
-1.7
-1.6
-1.4
-1.3
-0.9
-0.5
-0.1
+0.3
• •
+0.7
+0.9
+1.1
*Effect of 0.0 polymer dose from raw sample»not a blank.
108
-------
TABLE C4
EFFECT OF VARIOUS DOSAGES OF CAT FLOC ON REMOVAL
OF BOD AND SUSPENDED SOLIDS IN SEWAGE FROM THE
EFFLUENT OF A UNIVERSITY OF FLORIDA
PRIMARY SETTLING TANK
Polymer
Dosage
mg/1
0.0*
0.4
0.8
1.7
2.5
3.3
4.2
5.0
5.8
6.7
8.4
10.0
Suspended
Solids
mg/1
31
26
26
13
2
13
17
14
15
13
12
10
BOD Final
mg/1 pH
50 7.60
34
21
17
9
5
6
7
5
9
10
10
Electro-
phoretic
Mobility
p/(sec)(v)(cm)
-1.9
-1.5
-1.0
-0.9
-0.1
0.0
+0.2
+0.9
+0.8
+1.1
+1.3
+1.2
*Effect of 0.0 polymer dose from raw sample, not a blank
109
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TABLE C5
EFFECT OF VARIOUS DOSAGES OF DOW C-31 ON REMOVAL OF BOD
AND SUSPENDED SOLIDS IN SEWAGE FROM THE EFFLUENT OF
THE UNIVERSITY OF FLORIDA GRIT CHAMBER
Polymer
Dosage
mg/1
0.0*
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
Suspended
Solids
mg/1
106
30
15
2
3
2
5
5
17
BOD
mg/1
142
89
66
54
49
49
42
37
35
Final
pH
8.20
8.28
8.48
8.41
8.43
8.42
8.41
8.43
8.43
Electro-
phoretic
Mobility
y/(sec)(v)(cm)
-1.7
-1.6
-1.3
-1.2
-0.7
-0.2
+0.1
+0.3
+0.5
*Effect of 0.0 polymer dose from raw sample, not a blank.
110
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TABLE C6
EFFECT OF VARIOUS DOSAGES OF PRIMAFLOC C-7 ON REMOVAL OF BOD
AND SUSPENDED SOLIDS IN SEWAGE FROM THE EFFLUENT OF
THE UNIVERSITY OF FLORIDA GRIT CHAMBER
Polymer
Dosage
mg/1
0.0*
5.0
10.0
13.0
16.0
20.0
25.0
30.0
35.0
Suspended
Solids
mg/1
106
34
13
3
5
6
10
9
18
BOD
mg/1
142
89
66
58
58
57
57
50
50
Final
pH
8.20
8.25
8.25
8.24
8.18
8.10
8.06
7.96
7.92
Electro-
phoretic
Mobi 1 i ty
p/(sec)(v)(cm)
-1.7
-1.5
-1.3
-0.9
-0.5
-0.1
+0.1
+0.4
+0.9
*Effect of 0.0 polymer dose from raw sample, not a blank.
Ill
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TABLE C7
EFFECT OF REVOLUTIONS OF MIXING ON REMOVAL OF BOD AND
SUSPENDED SOLIDS IN SEWAGE FROM THE EFFLUENT OF THE
UNIVERSITY OF FLORIDA GRIT CHAMBER;
POLYMER USED: CAT FLOC
Mixing Rate
No. of
Revolutions
0*
20
50
100
200
500
1,000
2,000
5,000
20 RPM
Suspended
Solids
mg/1
116
57
43
41
35
25
17
10
6
BOD
mg/1
120
75
73
71
66
54
50
44
36
Polymer Dosage 2.8 mg/1
Electrophoretic
Final Mobility
pH y/(sec)(v)(cm)
8.29 -1.9
. . 0.0
. . 0.0
. . 0.0
. . 0.0
. . 0.0
. . 0.0
. . 0.0
. . 0.0
*Raw sample with no polymer added not a blank.
112
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APPENDIX D
LIST OF POLYMERS
TABLE Dl
LIST OF POLYELECTROLYTES EVALUATED
COMPANY
Calgon Corporation
P. 0. Box 1346
Pittsburg, PA 15230
Allstate Chemical Company
Box 3040
Euclid, Ohio 44117
National Starch and Chemical
1700 West Front Street
Plainfield, N.J. 07063
Corp.
Garratt-Callahan
111 Rollins Road
Millbral, California
94031
The Dow Chemical Company
Abbott Road Center
Midland, Michigan 48640
POLYMER
Cat Floe Coagulant
Aid 25
All-Flok #16
National Natron 86
National Resyn 3285
72 A
73
74
74 B
74 D
74 E
78 C
78 F
78 G
78 H
78 I
78 J
78 S
78 CF
78 FH
76
76 A
76 C
Purifloc C-31
Purifloc C-32
N-ll
N-12
N-17
A-21
A-22
113
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TABLE Dl
LIST OF POLYELECTROLYTES EVALUATED
(Continued)
COMPANY
Kelco Company
8225 Aero Drive
San Diego, California
92123
Nalco Chemical Company
6216 W. 66th Place
Chicago, Illinois 60638
Betz Laboratories, Inc.
Gillingham and Worth Streets
Philadelphia, Pa-
Ionic Chemical Company
Dearborn Chemical Division
W. R. Grace & Company
Merchandise Mart Plaza
Chicago, Illinois 60654
POLYMER
KNW-10433-49
2N-8551-29
KL-3859-49
675
610
635
607
603
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
Poly Floe
NI-701
NC-720
NI-700
NA-710
Aquafloc 415
Aquafloc 412
Aquafloc 410
1100-A
1100
1120
1130
1120-A
1130-A
1150-L
1160-L
4D
1160
1150
1175
1170
114
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TABLE Dl
LIST OF POLYELECTROLYTES EVALUATED
(Continued)
COMPANY
The Mogal Corporation
20600 Chagrin Boulevard
Cleveland, Ohio 44122
Rohm and Haas Company
Independence Mall West
Philadelphia, Pa. 19105
Dubois Chemical Company
Stein, Hall, and Company, Inc.
605 Third Avenue
New York, N.Y. 10016
POLYMER
Mogal Co-091
Mogal Co-983
Mogal Co-984
C-7
C-3
A-10
C-5
C-6
540
545
532
M-295
115
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
i. REPORT NO.
EPA-600/2-75-049
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
November 1975 (Issuing Date)
RAW SEWAGE COAGULATION AND AEROBIC SLUDGE DIGESTION
6. PERFORMING ORGANIZATION CODE
7. AUTHOR1SI
Richard H. Jones, T. A. Burnszytnsky,
and John D. Crane
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
City of Hollywood, Hollywood, Florida 33022
Through subcontract with
Environmental Science & Engineering, Inc.
P.O. Box 13454, University Station
Gainesville, Florida 32604
10. PROGRAM ELEMENT NO.
1BC611
11010FAC
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati. Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Laboratory and full-scale studies were conducted at the Hollywood, Florida, sewage
treatment plant to determine the efficiency of chemical coagulation for treatment
of raw sewage and aerobic digestion of primary sewage sludge.
While various polyelectrolytes produced high treatment efficiencies in the laboratory,
these efficiencies could not be achieved in full-scale tests due to inadequate mixing
and higher soluble BOD concentrations.
Sludges were successfully digested aerobically with as little as ten days detention.
An oxygen uptake rate of up to 1.8 gm Op/Cgm T.S.)(hr) was observed for sludge ages
greater than 20 days. The recommended detention time of 20 days produced a solids
content between 4 and 6 percent.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
*Sludge digestion
Waste treatment
Waste water
*Sewage treatment
Sludge disposal
*Aerobic processes
*Sludge drying
*Wastewater treatment
*Sewage coagulation
Sludge treatment
Sewer infiltration
Municipal sewage
*Aerobic digestion
13B
8. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
126
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
116
OUSGPO: 1976 — 6S7-695/5340 Region 5-11
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