Water Pollution Control Research Series
A STUDY OF SLUDGE HANDLING
AND DISPOSAL
V>S. DEPARTMENT OF THE INTERIOR
Federal Water Pollution Control Administration
PUBLICATION WP-20-4
-------
A STUDY OF SLUDGE HANDLING AND DISPOSAL
By
R. S. Burd
This Study was supported in
part by Grant No. PH 86-66-32
while Mr. Burd was with the
Dow Chemical Company.
U. S. DEPARTMENT OF THE INTERIOR
Federal Water Pollution Control Administration
Office of Research and Development
May 1968
-------
Ill
In its assigned function as the Nation's principal natural
resource agency, the United States Department of the Interior bears
a special obligation to ensure that our expendable resources are
conserved, that renewable resources are managed to produce optimum
yields, and that all resources contribute their full measure to the
progress, prosperity, and security of America — now and in the future.
The HATER POLLUTION CONTROL RESEARCH SERIES of reports was
established to describe the results of research studies of water
pollution. This SERIES provides a central source of information
on the intramural research activities of the Federal Water Pollution
Control Administration in the U. S. Department of the Interior and
on the research program's cooperative and contractual activities
with Federal, State, and local agencies, research institutions, and
industrial organizations.
Reports in this Series will be distributed to requesters as
supplies permit. Requests should be sent to the Publications Office,
Ohio Basin Region, Federal Water Pollution Control Administration,
Cincinnati, Ohio 15226.
Water Pollution Control Research Series Publication No. WP-20-H
-------
Foreword
Sludge handling and disposal have often been called the most
troublesome aspect of water and wastewater treatment. If this is
true, it is unfortunate that researchers and design engineers
have in recent years neglected sludge handling in favor of the
more glamorous problems associated with the liquid portion of
wastewaters.
In anticipation of a renewed search for new sludge handling
techniques, the Dow Chemical Company and the Federal Water Pollution
Control Administration agreed that the status of the sludge handling
art should be reviewed. Researchers and design engineers could
then use the comprehensive report as a basic reference for new
exploration. Many sludge handling techniques have been tried in
the past and many different techniques are in use today. But, until
this report was prepared, no accurate and complete documentation of
sludge handling processes was available.
The Dow Chemical Company has been studying the many facets of sludge
handling for years, partly as an adjunct to sludge disposal from its
own waste treatment facilities and partly because the company has
been developing commercial products for sludge conditioning. This
latter has resulted in sludge handling investigations at hundreds
of waste treatment facilities from coast to coast. The investigations
enabled the Company to propose valid new approaches and to summarize
the status of past and present activity.
This report is considered to be complete, accurate, and of practical
value to many people. A very thorough literature survey was followed
by numerous field interviews which accomplished two objectives:
1) they provided a check on the accuracy of published data and 2) they
provided the most up-to-date information.
Some technical details have been omitted because they exceeded the
study's objectives. The bibliography includes more than 450 references
containing additional details on any specific unit process. Detailed
theory of sludge handling unit processes can be secured from these
technical papers. Nevertheless, this report contains as much detail
as was practicable, without its becoming too bulky for use as a reference
document.
-------
VI
Water plant sludge handling is discussed in less detail than waste-
water sludge because little has been written about the former and
it has not been as troublesome as wastewater sludge handling.
The report suggests items for further study as well as describing
the current state of the art. It is offered to researchers, design
engineers, equipment manufacturers, owner-operators of treatment
facilities, and regulatory personnel as a basic and practical guide
to sludge handling and disposal. The Federal Water Pollution Control
Administration has accepted the report in fulfillment of the Dow
Chemical Company contract obligation.
Robert S. Burd
Sanitary Engineer
Federal Water Pollution Control
Administration
-------
Vll
Sludge Handling and Disposal Report
Key Words
Sludge handling
Sludge disposal
Sewage treatment
Water treatment
Industrial waste treatment
Wastewater treatment
Sludge
Combustion
Composting
Concentration
Conditioning
Dewatering
Digestion
Disinfection
Drying
Elutriation
Incineration
Lagooning
Land disposal
Ocean disposal
Odor control
Pipeline transportation
Recovery
Thickening
Utilization
-------
Vlll
ACKNOWLEDGEMENTS
In addition to information from the literature, the author
gratefully acknowledges the assistance of contributors represent-
ing the organizations listed below.
Allegheny County Fa., Sanitary Authority
Bartlett-Snow-Pacific, Inc.
Black and Veatch Consulting Engineers
California State Health Department
Camden, New Jersey, Sewage Treatment Plant
Chicago Metropolitan Sanitary District
Consoer and Townsend Consulting Engineers
Dorr-01iver, Inc.
Dow Chemical Company
East Bay Municipal Utility District (Calif.)
East Cliff-Capitola Sanitary District (Calif.)
Eimco Corporation
Hayward, Calif., Sewage Treatment Plant
Hershey Estates, Pa.
Kirkham, Michael and Assoc., Consulting Engineers
Los Angeles County Sanitary District
Los Angeles Hyperion Sewage Treatment Plant
Middlesex County, New Jersey, Sewerage Authority
Midland, Michigan, Sewage Treatment Plant
Milwaukee Sewerage Commission
National Canners Association
National Council for Stream Improvement
New York City Dept. of Water Pollution Control
Nichols Engineering and Research Corp.
University of North Carolina, Dept. of Sanitary Engineering
Quirk, Lawler and Matusky Consulting Engineers
San Diego Utilities Dept.
S. San Francisco Sewage Treatment Plant
Stanford University
Tillo Products, Inc.
U. S. Public Health Service
Walker Process Equipment, Inc.
Washington D. C. Dept. of Sanitary Engineering
Water and Sewage Works Magazine
Westchester County, New York, Dept. of Public Works
Roy F. Weston, Inc., Consulting Engineers
Whitman, Requardt and Assoc. Consulting Engineers
-------
IX
CONTENTS
Page
ABSTRACT xii
1. INTRODUCTION 1
2. SUMMARY AND CONCLUSIONS 3
3. SCREENING, DEGRITTING, AND SKIMMING ... 7
H. CLARIFICATION 11
A. Sedimentation 11
B. Flotation 21
5. SLUDGE THICKENING 22
A. Gravity 23
B. Flotation 36
C. Centrifugation 53
6. SLUDGE BLENDING 60
7. SLUDGE DIGESTION 61
A. Anaerobic 61
B. Aerobic 77
8. SLUDGE ELUTRIATION 89
9.. LAGOONING - LANDFILLING 102
10. LAND DISPOSAL OF LIQUID SLUDGE 109
11. PIPELINE TRANSPORTATION 117
12. OCEAN DISPOSAL - DILUTION 120
13. UNDERGROUND DISPOSAL 128
-------
X
14. SLUDGE DEWATERING 130
A. Vacuum Filtration 130
B. Pressure Filtration and Miscellaneous
Processes 160
C. Centrifugation 165
D. Sand Bed Drying 184
E. Screening 207
15. SLUDGE CONDITIONING AND DEWATERING
UNUSUAL PROCESSES 210
A. Freezing 210
B. Heat 213
C. Solvent Extraction 216
D. Electric 216
E. Ultrasonic 219
F. Bacteria 219
16. COMPOSTING 220
17. DISPOSAL OF DRIED SLUDGE AS A FERTILIZER
OR SOIL CONDITIONER 231
18. NON-FERTILIZER BY-PRODUCT RECOVERY . . . 242
19. SLUDGE COMBUSTION 247
A. Multiple Hearth 250
B. Flash Drying-Incineration 258
C. Fluidized Bed 259
D. Atomized Spraying 269
E. Wet Oxidation 275
-------
XI
F. Burning with Refuse and Miscellaneous
Techniques 289
G. Summary 294
20. PYROLYSIS 296
21. HEAT DRYING 297
22. SLUDGE ODOR CONTROL AND DISINFECTION . . 304
A. Odor Control 304
B. Disinfection 308
23. WATER PLANT SLUDGE DISPOSAL 311
24. SUMMARY OF SLUDGE HANDLING AND DISPOSAL
ECONOMICS 317
25. FUTURE APPROACHES . . . 322
REFERENCES
-------
• •
Xll
SLUDGE HANDLING AND DISPOSAL
Abstract
This report discusses in detail the broad subject of water and
wastewater sludge handling and disposal. Sludge handling and
disposal procedures are reviewed and evaluated by discussing
methods, materials and equipment used today and in the past. Thus,
the report provides an information base and suggestions for new
approaches to the sludge treatment art for use by researchers,
design engineers, and operators of treatment facilities.
The material is presented in the same sequence as solids processing
steps used at treatment plants. The text begins with the grit
chamber and ends with ultimate sludge disposal.
A major conclusion from the report is: additional support should
be given to the research and development of better ways to treat
the solid portion of wastewaters, after separation from the liquid.
Eight other major conclusions of the report are: (1) Standardized
accounting and reporting procedures are needed. (2) Sludge handling
and disposal should be integrated into the total wastewater treatment
system. (3) Wastewater sludge disposal could be considered as a
part of total solids-disposal system that includes refuse and other
solid wastes. (4) Incineration is a promising ultimate disposal
technique. (5) Mechanical dewatering systems are replacing more
primitive dewatering systems. (6) There is a trend to ocean disposal
of sludge by coastal or near-coastal cities. (7) Raw sludge handling
is becoming more popular. (8) The cost of ultimate sludge disposal
for most installations ranges from $5 to $55 per ton of dry solids.
-------
-1-
1. INTRODUCTION
In January, 1930, the editors of the Sewage Works Journal quoted
the following passage from Charles Rann Kennedy1s — "The Servant
In The House:"
"That's what I come 'ere to tak abaht — my job.
P'r'aps you'll think as it ain't tasty subjec, before
a lot o'nice, clean, respectable people as never 'ad anythin1
worse on their fingers than a bit o1 lawn dirt, playin1
crokey; but some one 'as to see to the drains, some one 'as
to clear up the muck of the world; I'm the one, an I'm
'ere to tell you abaht it."
Probably since wastewater plants were first constructed, cleaning
up "the muck of the world," or sludge handling and disposal, has
been considered as the most troublesome phase of sewage and industrial
waste treatment. Because more efficient wastewater treatment plants
are constructed and operated to produce more difficult-to-handle
sludges, this phase of water pollution control is becoming an
increasingly difficult problem. The problem is complicated also by
rising volumes of sludge from domestic and industrial sources
coupled with reduced land availability and lessening public tolerance
of air and water pollution. This situation has narrowed the choice
of acceptable disposal practices in many locations.
Many people have made the statement that sludge handling and disposal
is the most difficult part of wastewater treatment but it is well to
remember that it is often the most costly. This is of particular
consequence when considering that the total gallons of sludge produced
is frequently less than one percent of the total gallons of wastewater
collected and treated.
Sludge may be defined as: a semi-liquid waste having a total solids
concentration of at least 2500 ppm. It flows, it can be pumped; and
it exhibits hindered settling characteristics in gravity settling basins.
Sludge handling and disposal includes: (1) collection of the sludge,
(2) transportation of the sludge, (3) processing the sludge to convert
it to a form suitable for disposal, and (4) final disposal of the sludge.
It has been stated that final disposal is accomplished only when the
material has been entirely removed from the treatment plant in a manner
that is sanitary, permanent, and satisfactory to all parties concerned.
For this report, the above definition of sludge is expanded to include
grit and screenings. These are discussed because they are an integral
part of the total solids handling and disposal process at waste treatment
plants.
-------
-2-
Water plant sludge as well as wastewater sludge is discussed
because it too is becoming increasingly difficult and costly to
dispose of in a satisfactory manner.
Most of the discussions concerning various unit processes emphasize
sewage sludge handling and disposal because the technical literature
contains comparatively little information about industrial wastewater
sludge. However, the unit processes and equipment are usually the
same for both sludge types; thus, the information is generally
applicable in all circumstances.
This study critically reviewed and evaluated water treatment and
waste treatment sludge handling procedures. It discussed the methods,
materials and equipment in use today as well as those that have been
tried and abandoned in the past. The review of the art followed the
sequence of solids processing established at waste treatment plants,
starting with the grit chamber and ending with ultimate sludge
disposal. A discussion of theory, important parameters, performance
and cost data, degree of success and areas of possible improvements
was included for most unit operations.
Information used in the report was collected from many literature
references plus interviews with consulting engineers, equipment
manufacturers, regulatory agency personnel and operators of water and
waste treatment facilities.
The basic purpose in writing this report was to provide a comprehensive
study of sludge handling and disposal to serve as a review of the
known art for researchers investigating improvements in the art. It also
includes some suggestions for new approaches to consider in the future.
Design engineers, operators of water and waste treatment plants, and
regulatory agency personnel will hopefully find the report to be a
useful general reference on sludge treatment.
-------
-3-
2. SUMMARY AND CONCLUSIONS
Preventing water pollution by removing solids from sewage and
industrial waste is the primary purpose of waste treatment. The
effluent, sludge and gases obtained as by-products of the treatment
process must be disposed of efficiently, at a reasonable cost, and
without risking public health and good will. By and large, researchers
and design engineers have neglected sludge disposal in favor of the
more glamorous problems associated with advanced waste treatment of
the wastewater effluent. Likewise, the problem of gas has often been
ignored.
The specific objectives of sludge handling and disposal are:
1. To decompose organic matter to a relatively stable
material.
2. To reduce sludge volumes by removing liquids.
3. To destroy or control pathogens.
4. To use by-products of the process to minimize the
overall cost of operation.
Which process is selected to accomplish the above objectives depends
on the following:
1. Character of the sludge; raw, digested, or industrial.
2. Land availability.
3. Suitability of sludge for disposal by dilution.
4. Local possibilities for using sludge as a soil
conditioner or fertilizer.
5. Climate.
6. Capital and operating costs.
7. Size and type of wastewater treatment plant.
8. Proximity of the plant to residential areas and local
air pollution control regulations.
-------
_*»_
The objectives, and processes used to accomplish them, are given
different emphasis depending on whether the sludge source is
industrial or municipal. Important factors responsible for the
difference in outlook are: (1) industrial sludge may be mostly
inorganic, (2) industrial and municipal sludges can have vastly
different handling characteristics, (3) industry has a greater
interest in unconventional disposal methods, and (4) industry is
more insistent about low costs.
Sludge must be considered a liability to any waste treatment plant;
there is no known technique for making a profit on its collection
and treatment. Often the case in any decision, the method of sludge
handling and disposal selected is usually the one that is most
economical yet acceptable to all parties concerned.
The following general observations can be made from a review of all
sludge handling and disposal processes in use today:
1. Anaerobic digestion followed by sand bed dewatering is the
most common method of handling sludge at sewage treatment
plants. The obvious reasons for its popularity are
simplicity and low cost. Few large cities dewater sludge
on drying beds.
2. Lagooning is the most common method that industry uses
to dispose of waste sludge.
3. For coastal cities, anaerobic digestion followed by
pipeline transportation to the ocean or land reclamation
areas is by far the cheapest method of sewage sludge
disposal.
4. For many near-coastal cities with navigational access
to the ocean, digestion followed by barging is the most
economical method of sludge disposal.
5. Marketing dried waste sludge has been generally a failure.
Heat drying of sewage sludge is, therefore, rarely given
serious consideration by consulting engineers.
6. Sludge treatment presents many operational problems
involving odors, inefficient solids capture, constant
supervision and general lack of scientific controls.
7. Almost all of the methods of sludge handling and disposal
now used were known in 1930(^9).
-------
-5-
Certain trends are noticeable in the field:
1. Mechanical dewatering of sludge is being adopted
by increasing numbers of cities and^ industries due
to increasing land and labor costs.' This acceptance
includes cities having populations formerly considered
too small for mechanical dewatering techniques.
2. Sludge incineration is considered to be the process
with the brightest future. Its popularity will
continue to increase at a rapid rate as other disposal
techniques become unacceptable. Incineration is being
accepted at small as well as large installations.
3. Raw sludge incineration is replacing anaerobic digestion
at medium and large size treatment plants.
4. Barging of digested sludge to the ocean is being
considered and adopted by more and more cities near
coastal areas.
5. The use of centrifuges in place of vacuum filters is
growing.
6. The overseas popularity of composting sewage and
industrial waste sludge is declining.
7. Land disposal of liquid digested sludge is increasingly
popular at small sewage treatment plants.
8. Design engineers and plant operators are giving less
consideration to sludge elutriation and heat drying
sludge.
9. Competition is increasing among equipment and chemical
suppliers. As a result, equipment design and chemical
activity is being improved continuously. During the past
four years, there has been a steady substitution of polymeric
flocculants for inorganic types in sludge conditioning
processes.
10. Sludge volumes are rising and becoming more difficult
to dewater.
Despite changes and developments in sludge disposal procedures,
insufficient attention has been paid to the problem. Sludge handling
and disposal deserves more attention for many reasons. First, it is
-------
-6-
a costly operation. Often it represents 25 to 50 percent of the
total capital and operating cost of a wastewater treatment plant.
Second, it is the most annoying phase of waste treatment for the
plant operator. The process presents him with many problems that
he must solve with inadequate tools. Unfortunately, efficiency
of sludge handling and disposal depends on the ingenuity of the
plant operator. He has done a remarkable job, but the substitution
of some science for the operator's art is long overdue. Finally,
the problem is growing; as indicated by McCarty's estimate, the
volume of waste sludge will increase 60 to 70 percent within the
next 15 years (61).
Sludge handling and disposal should be an integral part of the
total waste treatment process. The effectiveness of the waste
treatment system and, therefore, the quality of the receiving water
is influenced by the efficiency of sludge handling and disposal
processes. Unless these are of the highest efficiency, filtrates,
centrates, elutriates and particularly digester supernatant liquors
overload these units with fine solids, upon return to clarification
and biological treatment units; this lowers the overall plant treat-
ment efficiency. This fact should be considered in plant designs.
A study indicated that only 4 to 9 percent of the nitrogen in raw
sewage sludge is removed by the sludge digestion process(65). The
remaining 91 to 96 percent is returned to the treatment plant as
supernatant liquor and passes through, often unchanged, to the
receiving water. Fertilization of receiving waters by nitrogen and
phosphates is one of the major water pollution problems.
Air pollution may be caused by any number of sludge handling processes
including incineration, heat drying, lagooning, sand bed dewatering,
and raw sludge thickening. In this case, the waste treatment objective
of maintaining good will is in jeopardy.
There is no doubt that new approaches to the problem of sludge disposal
are needed. Suggestions for new approaches are discussed in detail at
the end of each section in this report and in the final chapter.
Additional research into the practical aspects of sludge treatment
should be encouraged immediately.
-------
-7-
3. SCREENING, DEGRITTING. AND SKIMMING
General - Grit and screenings are waste solids that must be
disposed of at wastewater treatment plants along with skimmings and
other solids. Fortunately their volume is very small so disposal
is not as complicated as that for other solids collected in the
treatment processes.
Screenings are materials in the raw wastewater that are caught on
screens having openings usually 1/2 inch to 2 inches. The screens,
placed at the head of the treatment plant, remove materials such as
rags, sticks, and garbage. Grit can be described as small inorganic
solids that are removed from the wastewater after screening. Examples
of grit are sand, silt, gravel, ashes, and coffee grounds. Skimmings
consist of all types of flotable material which rises in sedimentation
tanks.
While small in volume, it is desirable to remove grit, screenings,
and skimmings because these solids cause the following operational
problems: (1) they plug, wear out, and break pumps and other mechanical
equipment; (2) they occupy space needlessly in treatment units,
particularly digesters; (3) they are difficult, to remove from treat-
ment units such as digesters and sedimentation basins; (4) they can
clog pipes and solids dewatering equipment; and (5) they can produce
odors and interfere with digestion.
Screenings Disposal - The quantity of screenings captured in a treat-
ment plant is 0.5 to 6.0 cu. ft. per million gallons of sewage for
screen openings of 1/2 to 2 inches and 5.0 to 3.0 cu. ft. for openings
of 3/32 to 3/4 inches^ '. Screenings have a moisture content of about
85 to 95 percent and an organic content of 50 to 80 percent(S). A
sanitary means of disposal is required due to the high organic content.
Therefore, these materials are usually buried. Sometimes they are
incinerated or ground by hammennill-type shredders into small particles
and added to sewage for later removal in sedimentation basins.
Burial after draining for about one day is the most common means of
screenings disposal. The solids are placed in a hole or trench and
covered with at least 6 inches of dirt. Lime and odor-masking chemicals
are sometimes used to prevent nuisance problems such as odor development
and insect breeding.
Incineration is possible in a separate unit, in a skimmings incinerator,
a refuse incinerator, or a dewatered sludge incinerator. The screenings
-------
-8.
moisture content before incineration should be reduced to about
60 to 65 percent by drainage, pressing, or dewatering in a
centrifuge. One pound of screened solids has a Btu value of 1400
to 3500(1).
Grit Disposal - Grit is removed at almost all sewage treatment
plants even though the wastewater collection system is separated
so theoretically street washings will not be a part of the wastes to
be treated.
There are two approaches to grit removal: one advocates grit
collection units at the head of the treatment plant; the second
advocates the use of hydrocyclones to remove grit from the settled
solids in the primary sedimentation basins. At the present time,
hydrocyclones are installed in only a few waste treatment plants,
primarily because they are relatively new, but there is considerable
data proving their great efficiency.
Heavy inert particles or grit are selectively deposited in units,
installed at the head of treatment plants, by velocity control in
simple gravity settling structures or by air flotation-classification
of the inerts and lighter organics in aeration tanks. Aerated grit
chambers have the disadvantage of being a source of odors, so they
are not recommended when septic wastewater is expected unless the
unit is completely covered to capture gases and thereby reduce odors.
Generally, grit collection units are designed to remove particles
having a specific gravity of 2.65 and diameters down to 0.2 mm. The
quantity of grit collected normally varies from 1 to 12 cu. ft. per
million gallons with an average of 4(7). Specific quantities removed
depend on many parameters including topography in the wastewater
collection area, the surface cover, size of sewers and whether they are
separate or combined, the intensity of rain storms, and the design of
the grit removal system. The moisture content of grit varies from
14 to 34 percent. Grit is often washed after collection to reduce the
organic concentration which may be as much as 50 percent of the total
solids.
The nature and quantity of the grit influences the method of ultimate
disposal. Because there is often a high concentration of organics,
burial is the most common disposal technique. Burial reduces the
chance of developing odor, insect, and rodent problems. If solids
separation is very efficient and if less than 15 percent volatile solids
are included in the grit, it can be disposed of as fill without
nuisance. Well-washed grit has been used on sludge drying beds, as a
-------
- 9 -
cover for screenings, and as a surfacing material for walks and
roadways. A few sewage treatment plants have incinerated grit along
with dewatered sludge. Being largely inorganic, most of the grit
solids are ultimately discharged with the incinerator ash.
Skimmings Disposal - The volume of scum or skimmings collected from
"sedimentation Basins or separate skimming tank normally varies from
0.1 to 7 cu. ft. per million gallons of sewage. Wide variations are
possible due to industrial discharges to the sewerage system. Skim-
mings normally have a moisture content of 60 to 90 percent and a
volatile solids concentration of 90 to 95 percent. Because skimmings
are collected as floating material, they include high concentrations
of grease and fibrous trash. The heat value can vary from 8,000 to
18,000 Btu per pound.
Skimmings are usually disposed of in one of four ways: (1) buried;
(2) pumped to digesters; (3) dewatered by mechanical equipment; or
(4) incinerated. Burial is simple but requires immediate covering
and concern for nuisance problems. Disposal to digesters is very
common, particularly with completely mixed units. Without thorough
digester mixing, skimmings may form a scum layer which leads to
operational problems. Dewatering requires careful control to avoid
media plugging. Vacuum filter dewatering normally requires prior
mixing with other more easily drained materials. Skimmings, however,
could be added to a vacuum filter after a sludge precoat has been
formed.
Burning skimmings in incinerators is becoming more popular as the volume
of material increases with the increased use of garbage grinders.
Separate incineration of skimmings at the source (skimming tank or
sedimentation basin) is recommended by some people because it eliminates
operational problems associated with pumping grease to a distant
incinerator. However, incineration of this highly volatile and high
Btu value material can be a problem due to the development of high
temperatures. Most conventional incinerators are not constructed to
withstand the very high temperatures (in excess of 2,000°F) that can
result from burning skimmings without other lower Btu value solids. In
addition to incinerator damage from high temperatures, flashing and
odors are two problems that may develop from the burning of skimmings,
if proper design and operational procedures are not adopted. Tempera-
tures can be reduced with water sprays. Flashing can be minimized in
multiple hearth furnaces by a parallel flow of skimmings and hot gases.
The most common incineration technique is to burn the skimmings in
the same furnace used for burning vacuum filter or centrifuge cake
solids. Detroit, for example, has for years successfully burned grit
-------
-10-
and skimmings in a multiple hearth furnace used basically for
incinerating filter cake solids(15). Before incineration, skimmings
should be settled, the liquid decanted, and the solids ground to a
small size.
A number of investigators have considered grease recovery from sewage
treatment plant skimmings. This is a popular money saving scheme
used in the wool-scouring and food processing industry. However,
recovery of grease is not practical in the sewage treatment business
because the volume is small and the grease is too contaminated with
other materials. Purifying the grease would be too expensive and
FDA approval is doubtful. Further comments are included in the By-
Product Recovery chapter of this report.
Summary - The volume of screenings, grit, and skimmings collected at
waste treatment plants is fairly small, but proper disposal is
important because they are the most objectionable materials processed.
Problems involving odors, insects, rodents, and unsightliness can
develop if these solids are not correctly handled. Burial, grinding
with discharge to raw wastewater, and digestion with adequate mixing
have been satisfactory methods of disposal. In the future, incinera-
tion will be more popular. However, incinerator design should be
improved if skimmings and screenings are to be burned without the
addition of dewatered sludge.
-------
-11-
4. CLARIFICATION
Introduction. - The water and wastewater solids, requiring treatment
and disposal first must be collected in some kind of basin or screen-
ing mechanism. At sewage treatment plants, most of the solids are
separated in primary sedimentation basins from the liquid transporting
them. In addition to raw waste, disintegrated screenings and secondary
sludges from the biological stage of the treatment process may be
settled in primary sedimentation basins. The secondary or biological
sludges may also be captured in final or secondary sedimentation
tanks. In some cases where the sewage is weak and the solids are
primarily organic, sedimentation before the activated sludge process
has been eliminated in the design of treatment plants. Flotation
rather than sedimentation is often prescribed for removing certain
industrial wastewater solids, but it is rarely used for clarifying
raw sewage.
This section discusses the theory and operation of clarification units
in regards to the production of sludge most susceptible to dewatering
and ultimate disposal.
A. Sedimentation
General - The design and operation of sedimentation basins have
emphasized B.O.D. and suspended solids removal rather than the produc-
tion of the thickest and freshest sludge possible. Sludge thickening
is usually considered to be the "second function" of settling tanks'"''.
Most investigators agree it is desirable to produce the freshest and
most concentrated sludge possible for the following reasons: (1) it
saves pumping and digester capacity, (2) it reduces the heat require-
ment for digesters, and (3) it saves chemicals and operating time when
dewatering solids. Since securing the highest suspended solids removal
efficiency is not necessarily incompatible with securing the thickest
underflow solids, more attention should be given to the sludge character-
istics, particularly at secondary treatment plants.
There are two schools of thought recommending different ways of achiev-
ing fresh and concentrated sludge. One advocates the Densludge process.
Its proponents claim that the use of sedimentation basins for sludge
thickening as well as for wastewater clarification sacrifices the basin
efficiency for B.O.D. removal. The Densludge system involves the
continuous pumping of low volumes of a dilute relatively fresh sludge
from sedimentation basins to separate thickening tanks. Better
efficiencies in primary sedimentation tanks are said to be possible
when the sludge is pumped continuously at a low rate. This is because
-------
-12-
of the minimized changes in hydraulic flow and the elimination of
sludge storage. Also eliminated is the accompanying possibility of
solids escaping and septic conditions developing. Use of a separate
thickener allows better control of the sludge;' its operation has little
effect on the clarification step. The Densludge process is discussed
in greater detail in the chapter on Thickening in this report.
Most wastewater treatment plants are designed in accordance with the
second school of thought, which holds that thickening as well as
clarification should be attempted in sedimentation basins. As stated
above, the usual basin design depends on optimizing the removal of
B.O.D. and suspended solids from the wastewater. Designs are available
however, that attempt to produce above average sludge thickening by
including special equipment. One manufacturer accomplishes this by
placing a large circular sludge hopper, one-third to one-half the
diameter of the circular sedimentation tank, at the bottom of the tank.
Theoretically, clarification occurs in the upper part of the tank and
sludge thickening in the bottom. The hopper is provided with pickets
for slow agitation of the sludge and piping for adding well aerated
plant effluent or chlorine to the sludge blanket to prevent septicity.
Another design uses a helix-screw mechanism in the sludge hopper of
rectangular sedimentation basins. This device provides a kneading and
squeezing action that collects and compacts the sludge. Both of the
sedimentation tank modifications thicken the sludge to a greater degree
than is possible with conventional basins. As expected, they make the
tanks more expensive to construct.
Theory and Design - Sedimentation is defined as: the removal of
suspended particles heavier than water by gravitational settling^ '.
Many of the recommended parameters for efficient suspended solids
settling will also benefit sludge thickening. These parameters include
the basin depth and shape, sludge detention period, type of baffling
utilized, and operating conditions. Consulting engineers and equipment
manufacturers generally agree that the design standards recommended in
the "10 States' Standards" are valid for sewage solids removal in
primary and secondary clarifiers. Industrial wastewater clarification,
because of the great variations in the solids characteristics, requires
laboratory or pilot plant evaluations to determine the best hydraulic
and solids loading parameters.
Many people, including Camp, Hazen, Gummings, and Kynch, have discussed
methods and theory of designing continuous flow clarification units.
(Fill-and-draw units are not discussed in this report because they are
not common, particularly in sewage treatment.) Mancini reviewed the
procedures for clarifier design. He concluded that batch settling tests
-------
-13-
can be used in designing clarifiers used to capture solids having
hindered settling characteristics (including many industrial wastes)
if the test conditions were related to the hydraulic performance of
the particular full-scale unit under consideration'^2)^ The batch
settling test develops a settling curve that can be analyzed to
determine the surface area required for clarification and the surface
area required to concentrate the settled solids to a particular
concentration. These tests define the settling characteristics
which are affected by properties of the solids such as particle size,
distribution, density, concentration and agglomeration'^^).
Sedimentation is generally described as incorporating three steps:
clarification, zone settling, and compression^). This report is
most concerned about the compression of solids after they pass through
the clarification and settling zones. When the solids first reach
the bottom of the sedimentation tank, they form a blanket having a
high percentage of void spaces. However, in time and with additional
settling of solid particles, the blanket becomes compacted due to the
hydrostatic pressure of its own weight. During compression, water is
squeezed from the compacting sludge mass. In effect, therefore,
compression is the opposite of the preceding two steps inasmuch as a
liquid is being removed from solids rather than-solids from a liquid.
Figure 4.1 shows the different settling zones for biological sludges.
Factors that are thought to influence the concentration of the settled
sludge in sedimentation basins include: (1) settleable solids character-
istics - their density, shape, flocculant structure, viscosity, per-
centage of volatiles, and electrostatic charge; (2) the solids concentra-
tion in the original suspension; (3) the depth and surface area of the
sludge blanket; (4) the sludge detention time, and (5) structural
modifications of the sludge blanket by pressure, vibrations, and
mechanical action(97). The first two factors are independent of the
design of the sedimentation tank, but the next three are dependent
upon its shape and other design features.
-------
Plgure 4.1
Clear Water Zone
Hindered Settling
A < Constant Composition
1 V«f(C)
Transition Zone
Variable Composition
Compression Zone
TIME
CYUNDER
Schematic representation of Bottling zones.
(Reprinted by permission JWPCF, Vol. 29, No. 10.
p. 115, Oct. 1957) —
-------
-15-
Many investigators agree that for a given detention time, a shallow
compression zone will produce a greater underflow concentration than a
deep one. As the depth is increased, the detention time must be
increased to maintain the same solids concentration. With long
detention periods there is the danger that organic sludges will
become septic, causing odors and bulking, which in turn reduce the
solids concentration. The decrease in compaction rate which accompanies
increased sludge depth occurs because the displaced water has to pass
upward through diminishing void spaces against increasing resistance
to liquid flow. Water displacement can usually be enhanced by the
hydraulic movement of the sludge blanket and by the action of the
sludge collection mechanism breaking up the arched settled sludge.
The degree of improvement from gentle agitation of the sludge
blanket depends on the type of solids being settled.
Sludge collection hopper designs can have a significant effect on
the underflow solids concentration. Inadequate hopper capacity can
result in thin sludge because the operator is forced to withdraw
sludge frequently to prevent it from accumulating outside the hoppers
and becoming septic. Steeply sloped hopper sides (2:1) will aid in
sludge concentration because sludge arching will be reduced. Sludge
withdrawal pipes should be at least 8 inches in diameter and have a
minimum of obstructing supports. These factors will decrease the
incidence of clogged hoppers and arching sludge. A sludge depth of
at least 18 inches in the hopper is recommended.
Settled sludge compaction is influenced by the following sedimenta-
tion basin currents: (1) eddy currents created by the inertia of
influent and effluent flows, (2) wind-induced currents, and (3) density
currents caused by wastewater temperature differences including that
between the sludge blanket and the clarified overhead water(7). These
currents and secondary currents established by their action can cause
bottom scour of deposited sludge. Solids compaction will be reduced
due to resuspension. The overall plant treatment efficiency may be
reduced by the subsequent carryover of sludge to the weirs. The
sedimentation basin inlet and outlet must be adequately designed to
minimize the creation of currents. Effluent weirs should be adequate
in length, and inlets should distribute the wastewater broadly over
the entire cross-sectional area of the tank with minimum inlet
velocities (96). Baffling the area in front of the inlet openings will
help to reduce velocities and to distribute the flow broadly. Camp
thought sedimentation tanks should only be deep enough to prevent
scour and should be long and narrow to minimize the effects of various
currents'*''. Others may disagree but in any case, settling tanks
should be able to accomplish a reasonable degree of sludge thickening
as well as to separate settleable and floatable solids from liquids.
-------
-16-
Operations - The operation of sedimentation basins is obviously
closely related to the operation of other treatment plant processes.
To facilitate subsequent sludge handling s-teps, it is usually
desirable to deliver a thick, fresh sludge from the sedimentation
basins. However, because the production of a thick sludge may
require a long detention time, this goal may not always be
compatible with the goal of achieving a fresh sludge. Some
compromise in the goals may, therefore, be required. Good sedimen-
tation tank operation starts with: (1) equalizing the flow between
parallel tanks, (2) preventing density currents by using baffles
and adjusting effluent weirs to give a uniform distribution, and
(3) setting the wet well pumping controls to minimize surging in
the basinsw).
Sludge collection and withdrawal parameters to be considered are:
(1) depth of the sludge blanket, (2) operation of sludge collection
mechanisms, (3) sludge pumping schedule, and (4) operation and
maintenance of sludge pumps(8). Most settling tanks are operated
with blanket depths between 2 and 6 feet deep. This level seems to
give a reasonable sludge concentration without storing the sludge
so long that extreme anaerobic conditions develop. Many operators
of waste treatment plants successfully use the blanket depth as a
measure of solids concentration as if the two were directly propor-
tional. Sludge withdrawal is then controlled by the blanket depth.
Compaction of settled sludge is usually thought to be enhanced by
agitation, particularly for hydrous colloidal precipitates, but some
people consider the degree of agitation introduced by the solids
collection mechanism to be insignificant. To prevent resuspension of
the sludge, the collection mechanism should be operated at 2 feet per
minute or less. It normally is in operation when sludge is being
pumped in order to prevent liquid from being "pulled through" the
sludge blanket. Camp and others believe the sludge should be
collected and moved in the same direction as the flow through the
basin for maximum effectiveness(90).
Sludge withdrawal techniques have an important effect on concentration
and freshness; prompt removal of heavy sludge is the goal. Sludge is
usually removed by pumping or hydrostatic pressure. Biological sludges
are sometimes removed by suction devices near their point of deposition
if freshness is important. Industry and small sewage treatment plants
often use the hydrostatic pressure method of disposal even though it
has the disadvantage of displacing significant quantities of water with
the sludge.
The question, "What is the best type of pump for sludge?" will produce
answers indicating that almost any kind is suitable to someone.
-------
-17-
Obviously, a pump that doesn't clog readily is desirable.
Variable speed pumps that can deliver a small volume of sludge
may be desirable.
Pumping sludge is the most common withdrawal technique and the
particular schedule used determines the solids concentration and
freshness. In general, the most successful operations have
adopted a schedule having frequent but short pumping periods at
low rates such as 25 gpm. The schedules normally are determined
by experience because of the varying solids load received at the
treatment plant. Once a pattern is established, the pumping
schedule is programmed on a time clock or set manually. At small
waste treatment plants sludge is pumped about one to three times
each day and at large plants, hourly.
The sludge "quality" is usually controlled by visual means through
sight glasses or with sampling valves located between the sludge
pump and the digester or other points of disposal. Other indica-
tions of sludge thickness are obtained by reading pump discharge
pressures, checking the torque developed on the settling tank
scraper mechanism, noting the sludge blanket depth, or by radio-
active density (or mass) analyzer readings.
Not all radioactive density meter installations have been successful
but the one at the Los Angeles County Sanitation District has been
given credit for contributing to smooth treatment plant operations.
Garrison describes this completely automated sludge pumping control
system as having two basic components: non-clog sludge pumps and
radioactive density meters(l°5). At the District's Main Treatment
Plant the average solids concentration has been increased from 3.5
percent before automation to 6 percent after automation. The
increased consistency in digester performance obtained after automa-
tion has been attributed in part to the increase in solids concentra-
tion. The density meter is preset at 6 percent and electrical control
equipment permits sludge pumping only when the concentration is near
that figure. One meter can control sludge pumping from six sedimenta-
tion tanks(18). Visual observations were unsuccessful because of
human error and because changes in sludge were noticed too late. The
use of timers to control pumping was unsatisfactory because the rate
of sludge accumulation was not uniform. Current-sensitive relays
on motors to measure the change of power required at different sludge
concentrations also produced erratic results in comparison with
density meter controlCIS),
Reefer evaluated radioactive density meters at Baltimore and found
them to be very accurate over a range of sludges from 0.1 to 6.6
-------
-18.
percent total solids^95). Gerson Chanin reported that the
concentration of primary sludge significantly increased in the
East Bay Municipal Utility District plant after substituting
the density meter for visual control of sludge pumping. He
reported that calibration of the meter by digester supernatant
has maintained a uniform operation. This fact is probably
significant when reviewing the performance of radioactive density
meters. Frequent maintenance and calibration are probably
necessary for a successful system.
Any technique that will insure the pumping of thick sludge is
desirable because subsequent sludge handling costs and problems
are reduced. A minimum amount of water is pumped; digester
operations are improved because less heat and space is required;
less supernatant liquor is produced; and sludge dewatering
process costs are reduced. The radioactive meter may not, however,
be the ultimate control device; perhaps ultrasonics or some other
technique will prove to be even more successful.
Performance - The volume, concentration and general characteristics
of the sludge produced by sedimentation will be affected by a
number of factors including the following(70):
1. Characteristics of the raw wastewater from which
the sludge is derived.
2. The type of secondary treatment given the wastewater
and whether secondary sludges are handled separately
or returned for resettling with the primary sludge.
3. The design and operation of the sedimentation tanks.
4. Whether chemical or mechanical aids to settling are
used.
Characteristics of sewage and industrial wastewater vary tremendously,
so it is natural to expect the sludges to vary likewise. After the
wastewater is in the sewer, there is not much that can be done about
changing the basic nature of the sludge. Biological processes
produce large volumes of dilute flocculent sludges that are difficult
to dewater. In many cases trickling filter humus and waste-activated
sludge are returned to the head of the treatment plant where they
are resettled with the primary sludge. The uniform mixture of the
two types of sludge may facilitate ultimate sludge dewatering and
final disposal. Fair and Geyer report the expected sludge concentra-
tions for separate and mixed (primary and secondary) sludges as
described in Table 4.1<7).
-------
-19-
Table 4.1
Type
Raw Sludge
Plain sedimentation
Trickling filter
Trickling filter, mixed
Activated
Activated, thickened
Activated, mixed
Digested Sludge
Plain sedimentation
Trickling filter
Activated
Activated, mixed
Solids Cone.
2.5
5
3
0.5
1
4
10
10%
2
6
5%
10%
6%
1%
2%
5%
- 15%
- 3%
8%
With biological sludges in particular, the possible underflow
concentration that can be achieved is limited. The sludge in
the settling basin will consume the available oxygen and deteriorate
into an anaerobic condition if the detention period is too long.
This could result in gasification and floating of the sludge to
the surface of the tank.
Pre-aeration of sewage prior to primary sedimentation is a fairly
common technique. It offers the advantages of separating grease
and grit from the other sewage solids; it degasifies the sewage;
it freshens the sludge and improves its settle-ability and degree
of compaction. Pre-aeration is particularly useful if the raw
wastewater is septic(4, 81). Mechanical flocculation also may
improve the settle-ability and subsequent compaction of sludge by
encouraging particle agglomeration.
Chemicals have been applied to raw wastewaters for many years with
varying degrees of success. Alum, lime, and iron salts have been
used to achieve intermediate levels of treatment but the effect on
sludge handling has been to complicate the process. Large volumes
of sludge are produced due to the additional solids removed from the
wastewater plus the large quantities of added chemicals. Lime also
produces an unfavorable pH and alkalinity for sludge digestion.
Chlorine treatment of wastewater, including mixed liquor solids,
has improved subsequent sludge handling procedures by reducing
septicity, allowing better grease separation, and reducing the bound
water and sludge volume index (SVI) in bulky activated sludge. A
-------
-20-
more concentrated settled sludge and improved digester operation
are two resultant advantages. Use of 10 ppm chlorine reduced the
SVI from 177 to 120 and the bound water in the sludge by 47 percent(87)
Copper sulfate has been used in place of chlorine at a dosage of 5 to
8 ppm(93).
Polyelectrolytes are being successfully employed in applications
that produce thicker underflow solids concentrations with subsequent
improvements in dewatering and digester operations. Crowe and
Johnson reported on the use of a low dose of anionic polymer at
Battle Creek, Michigan, to capture additional suspended solids in
the primary clarifiers(102). The use of the chemical also increased
the concentration of raw sewage solids from 4.3 to 8.0 percent,
which in turn increased the effective capacity of the sludge holding
tanks and digesters. The thicker sludge permitted a smoother de-
watering operation at reduced chemical costs and vacuum filter
operating time.
The results provide an excellent illustration of the operational
improvements that can be obtained through increased sludge solids
concentrations. Because the cost of treating secondary sludge is
much greater than that for primary sludge, it is an advantage to
increase also the ratio of primary to secondary sludge. A significant
increase is possible with chemical treatment of the raw wastewater.
-------
-21-
B. Flotation
General - Flotation-clarification processes are in use at numerous
industrial waste treatment plants and a lesser number of sewage
treatment plants. Three methods of flotation, using rising air
bubbles to increase the buoyancy of solid particles, are most
commonly used: (1) dispersed air flotation where bubbles are
generated by introducing air through a revolving impeller or
porous media, (2) dissolved air-pressure flotation where air is
put in solution under elevated pressures and later released at
atmospheric pressure, and (3) dissolved air-vacuum flotation which
applies a vacuum to wastewater aerated at atmospheric pressure^ '.
As expected, sludge "skimmed" from raw wastewater flotation units
contains large quantities of grease, oils, and other low-specific-
gravity materials. Little data has been reported describing the
performance of these units when used for dilute wastewater clarifi-
cation, but it can be assumed that the floatable material collected
would be difficult to handle. The solids concentration of the
sludge (or float) would be affected to an unknown degree by the
air-to-solids ratio used by the detention time in a floated state,
and whether or not chemicals are added. In the food industry the
floated material is often collected and sold as a by-product of
waste treatment.
A detailed discussion of dissolved air flotation as it applies to
separate sludge thickening is presented in the Thickening chapter of
this report.
-------
-22-
5. SLUDGE THICKENING
Introduction - Thickening or concentration can be defined as
tne process of removing water from sludge after its initial
separation from water and wastewater. The basic objective of
thickening is to reduce the volume of liquid sludge to be handled
in subsequent sludge disposal processes (65) ^
Sludge thickening provides the following advantages:
1. Improves digester operation and costs because space
is conserved, the heating requirement is decreased,
the detention period of existing units is increased,
less supernatant liquor is produced, a higher solids
loading to the digester per cubic foot is possible, and
the microorganisms active in the digestion process are
more efficient.
2. Reduces the sludge volume and, therefore, the costs
of sludge pumping and ultimate disposal to the ocean
or land.
3. Reduces the cost of chemical conditioning prior to
sludge dewatering because of increased solids concentra-
tions.
4. Eliminates water where it usually is the easiest to do
so, ahead of digestion and dewatering.
5. Smooths-out fluctuations in sludge quantity and quality.
6. Generally reduces treatment costs due to savings, such
as in the physical plant size, labor, and power.
Thickening used to be considered an art, but today there are
proven techniques that elevate the process to an engineering science
if not an exact science.
The degree of concentration that can be expected from various
thickening processes depends on several variables. Certainly the
method of wastewater treatment is very important as is the initial
composition of the raw wastes. The difference between biological
floes and raw primary sewage provides a good example of the varia-
tions that can result from different treatment methods -- biological
floes are bulky and concentrate to a lesser extent than raw primary
sludge. The initial concentration of the sludge to be thickened,
the density of the particles, their size and shape, the temperature
and age of the sludge, and the ratio of organics to inorganics are
also important factors in the final sludge concentration produced.
Thickening in separate units can produce a more concentrated sludge
than thicl-ening in the initial wastewater clarification units.
-------
_23.
Sludge concentration becomes the primary objective tfhile overhead
clarity assumes a secondary role; this situation is the reverse of
sedimentation-clarification.
The simplest method of thickening is gravity settling without the
use of mechanical or chemical aids. In the search for methods
producing higher concentrations than is possible with simple gravity
settling, other techniques have been evaluated and adopted. These
include: (1) biological and dissolved air flotation, (2) centrifuga-
tion, and (3) chemical conditioning.
A. Gravrty
General - Thickening by gravity is the most common concentration
process in use at wastewater treatment plants. It is simple and
inexpensive, but it does not produce as highly concentrated sludges
as other thickening processes. Gravity thickening is essentially
a sedimentation process similar to that which occurs in all settling
tanks. But, in comparison with the initial waste clarification
stage the thickening action is relatively slow(65). The operation
of gravity thickening tanks has generally been satisfactory but
improvements in the degree of solids concentration are always desira-
ble.
Theory, Parameters, and Design - Gravity thickening usually exhibits
the "hindered" settling phenomenon due to the relatively concentrated
nature of the sewage and industrial wastewater solids. According
to Mancini this hindered settling phenomenon is influenced by the
particle size distribution, density, concentration and agglomeration
as well as the hydraulic conditions in the settling basins(92). Most
investigators recognize four basic zones in a gravity thickening
system: (1) a clarification zone at the top containing the relatively
clear supernatant liquid, (2) a settling zone characterized by a
constant rate of solids settling, (3) a compression zone characterized
by a decreasing solids-settling rate, and (4) a compaction zone where
the settling rate is very low(6> 92). Figure 5.1 illustrates these
zones and labels them respectively as A, B, C, and D\°).
In the settling zone the particles are settling under hindered
conditions but their concentration ramains the same. The settling
rate in this zone can be used to determine the area required for waste-
water clarification. In the compression zone the solids concentration
increases as the entrained water is forced upward through void spaces.
The solids settling rate decreases as the resistance to relative motion
-------
-24-
Figure 5*1
Thickening Zones
IV
Reprinted by permission from Chemical Engineers'Handbook
lJ.« H. Perry Ed.) by A. Anable, copyright 1950, New York,
McGraw-Hill Book Company, IncT)
-------
-25-
between the solid and liquid phase increases (^2). An anaiysis
of a plot of this zone can also be used to obtain thickener
design parameters. The compaction zone is the last zone; it is
defined as an area supported by the solids below them^^.
Numerous studies by Kynch, Talmadge, Fitch and others have
revealed that continuous thickening units for suspensions with
hindered settling characteristics can be designed on the basis of
data collected during batch settling tests("> 100> 112)^ A
solid-liquid interface settling curve developed from laboratory
tests can be used to determine the surface area required for
clarification and the surface area required to thicken the sludge
to a particular solids concentration. Mancini believes, however,
that the batch test has only a limited usefulness and pilot plant
studies should, therefore, be considered in order to develop a
more reliable design criteria(92). ^ typical curve and how it can
be used to design thickening units has been described by Mancini(92).
Figure 5.II shows one of his typical curves. As reported by some
engineers, a comparison of actual operating results with the batch
settling lab tests has often indicated that the plant-scale unit
performs better than anticipated(H2) m
The degree to which waste sludges can be thickened depends on many
factors; among the most important are the type of sludge being
thickened and its volatile solids concentration. Figure 5.Ill by
Budd, shows the relationship between the underflow solids concentra-
tion and the percentage of volatile solids for primary sludge and
a mixture of primary plus biofilter sludge(23). Bulky biological
sludge, particularly that from the activated sludge process, will not
concentrate to the same degree as raw primary sludge. The degree of
biological treatment and the ratio of primary to secondary (biological)
sludge will affect the ultimate solids concentration obtained by
thickening. It is obvious from the data shown in Figure 5.Ill that
higher solids concentrations are attained with a decrease in the sludge
volatile content.
Rudolfs and Logan investigated the importance of initial solids
concentration and temperature on sludge thickening(34). As shown in
Figure 5.IV, the percentage increase in settled solids concentration
is much greater with low initial feed solids concentrations than with
a high concentration thickener feed. The importance of temperature is
also significant, particularly with "aged" sludge. Compaction is
greatest at 37°C, regardless of the initial feed solids concentration;
higher and lower temperatures resulted in less compaction.
-------
-26-
Figure 5•II
CHARACTERISTIC SETTLING CURVE
FOR A SLURRY HAVING HINDERED
SETTLING CHARACTERISTICS
K> 20 30 40
SETTLING TIME (Min)
(Reprinted by permission Schools of Engineering, Purdue University)
-------
-27-
Figure 5.Ill
LJ
Q
Z
18
16
14-
z
£ 12
y
f 10
I
8
P
«fr
TOTAL SOLIDS
VS.
70 VOLATILE
50 40
Relationship of total solida to percent volatile fiolids.
90 80 70 60
% VOLATILE SOLIDS
(Reprinted by permission Southern Municipal and Industrial
Waste Conference, p. 166, 1959)
-------
-28-
Figure 5.IV
1000
900
KM
z
o
p
< so
K
u
8
u
i
\20»e.
01 234 S «
INITIAL CONCENTRATION - PER CENT
Effect of temperature and initial concentration on the percentage Increase in fresh
solid* concentration (after 06 hours).
(Reprinted by permission from Vol. 15, No. 5,
Sept. 1943, Sewage Works Journal) -*
-------
-29-
In general, laboratory pilot-plant and full-scale investigations
of the importance of initial solids concentration have determined
that optimum results are achieved when a feed solids concentra-
tion is between 0.5 and 1.0 percent(117> H8). Within this range
the settling rate is rapid, and the sludge compaction and overhead
clarity are optimized.
New York City achieves good underflow solids concentrations by
thickening mixtures of dilute raw primary and activated sludge as
part of a process called the Densludge system. This system
depends on a dilute sludge mixture which is normally achieved by
pumping underflow from both primary and secondary clarifiers
continuously at a ratio of about 8 parts secondary sludge to 1
part primary. The solids concentration of the feed solids mixture
at New York is normally a little less than one percent(H^). New
York recently reported further increases in the average sludge
compaction by the technique of adding digested sludge to the raw
sludge thickener feed. Without dilution, the solid-liquid separa-
tion process is much slower due to interference between the particles.
Most investigators believe the depth of the sludge blanket in a
thickener is an important parameter as regards the ultimate solids
concentration. However, contrary to what might be expected, it
is generally agreed that underflow solids concentrations are
independent of sludge blanket depths greater than 3 feet (H7> H8)t
Comings determined for a constant detention that the underflow solids
concentration decreased as the depth of the compression zone in-
creased(99). At depths greater than 3 feet, there is apparently a
significant increase in the resistance to the flow of water from
the sludge blanket. Also, as the sludge accumulates in the deeper
blankets it often becomes septic, producing entrained gas and a
bulky sludge which doesn't compact well.
Sludge blanket depth and detention time are naturally closely inter-
related. Most observers agree that increased detention of the solids
in the sludge blanket results in increased solids concentration, up
to a point. For maximum compaction, 24 hours has been suggested as
the time required(H8). Comings reports the underflow solids concen-
tration increases as the detention time increases in a compression
zone of constant depth(99). A compromise must, therefore, be made
between detention time and the sludge blanket depth parameter
described in the preceding paragraph. It has been suggested that
a specific sludge concentration is attained in the shortest time by
operating with a shallow compression zone depth'99'. Greater depths
require greater detention times, but where organic sludges are
involved a greater septicity will occur.
-------
-30-
Gentle agitation of the sludge blanket is generally thought to
facilitate compaction, but the degree of compaction depends on
the type of sludge. Hydrous colloidal precipitates and certain
metallurgical pulps particularly benefit from agitation(lOl).
It has been claimed that the efficiency of a thickening tank can
be improved from 15 to 20 percent by attaching vertical steel
members or "pickets" to the sludge collection mechanismsC?0).
These "pickets" move through the sludge blanket and create
passages for entrained water and gas to reach the surface, as
well as aid particle agglomeration. Mancini presented data
demonstrating that gentle mixing of activated sludge greatly
increased the settling rate of the sludge. Schroepfer and Ziemke
also reported a dramatic increase in the solids concentration of
anaerobic sludge by the use of pickets (wickets)(36). Laboratory
and pilot-plant tests will indicate to what degree agitation aids
thickening.
Chemicals and inert weighting agents can influence the degree
of sludge thickening and its freshness. Rudolfs investigated
many different additives to many different sewage sludgesC3^).
He observed that alum and ferric salts did not significantly
increase sludge concentrations after 24 hours compaction. Sulfuric
acid improved the compaction of sludge, but the required dose (600
- 1000 ppm) is economically impractical. Lime dosages of 250 to
500 ppm increased significantly sludge compaction.
Rudolfs also added iron oxides, diatomaceous earth, and fly ash to
sludge to promote compaction, but the effects were insignificant at
reasonable dosages(33).
Chlorine can be used to prevent sludge septicity and gasification
which interferes with optimum solids concentration of organic materials,
Decomposition of unstable sludges during thickening process can
produce gas which adheres to the solid particles, changes their
density, and often buoys them to the liquid surface(H). A chlorine
residual of 0.5 to 1.0 ppm in the thickening tank overhead prevents
this problem. Overdosing must be avoided because excessive chlorine
may disperse biological sludges(5).
The successful use of organic polyelectrolytes as aids to sludge
compaction has been demonstrated by a number of investigators.
Anionic, cationic, and nonionic polymers are known primarily to
increase the sludge settling rates, the overhead clarity, and the
allowable tank loadings, but often they also increase the settled
-------
-31-
solids concentration. Higher dosages produce higher degrees
of compaction. Returning filtrate containing some residual
polymer or inorganic flocculent from the vacuum filter operation
to the thickening tank may produce some beneficial effects.
Another parameter important to sludge thickening processes is
the Sludge Volume Index (SVI). This number indicates potential
settling and compaction characteristics of the sludge solids.
A high index indicates a bulking sludge that is difficult to
dewater and compact ^36'.
The thickener solids loading rate affects the degree of sludge
compaction. It is related to the Sludge Volume Index discussed
above and the volatile solids concentration; a high SVI or volatile
matter concentration requires a low solids loading rate. For
sewage sludges the following loading rates are generally recom-
mended :
Primary sludge 22 Ibs./sq.ft./day
Primary + trickling filter sludge 15 Ibs./sq.ft./day
Primary + waste activated sludge 8-12 Ibs./sq.ft./day
Waste activated sludge 4 Ibs./sq.ft./day
Loadings for industrial sludges vary greatly, so laboratory and
pilot-plant tests are recommended to determine the exact design
parameters. Eckenfelder and O'Connor present two examples of the
variability of industrial sludge loadings (5); (1) an anaerobic
packing house sludge with an SVI of 50 to 100 was loaded at 50-85
pounds per day per square foot of thickened area and (2) an anaerobic
dairy sludge with an SVI of 100 to 300 was thickened at loadings of
25 to 35 pounds per day per square foot.
For best results, the following points should be considered in the
design of wastewater sludge thickening tanks:
1. At secondary waste treatment plants, thickening of
mixed sludges (primary and secondary) should be considered.
Secondary sludges normally release their water slowly,
but mixtures of secondary and primary and/or digested
sludge seem to respond well to thickening.
-------
-32-
2. Thickening tanks generally should be deep (15
feet), be of circular design, have inlet facilities
that dissipate the entrance velocities, and have a
single sludge outlet pipe with short suction con-
3. The liquid displacement period in thickeners is of
secondary importance for all sludges, but a
minimum detention time of 6 hours and a hydraulic
overflow rate of 400 to 800 gallons per square
foot per day are recommended.
The quality of the overhead liquid removed from the sludge
solids is important in any thickening operation because this
liquid is usually returned to the treatment processes. Generally,
the overhead quality is similar to that of raw sewage, 150
to 300 mg/1 suspended solids and a Biochemical Oxygen Demand
(B.O.D.) of about 200 mg/1. A well-operated thickener should
have a minimum of anaerobic decomposition and a solids capture
exceeding 90 percent; using these guides the overflow returned
to the primary clarifiers or the aeration tank of an activated
sludge system should not present an operational problem.
Performance - Gravity thickening of waste activated and mixed
sludges was recently evaluated in an experimental picket thickener
at the Chicago Sanitary District. The following conclusions
resulted(56):
1. Picket thickeners can be successful. Table 5.1
shows that thickeners with pickets increased the
solids concentrations from 33 to 100 percent over the
concentrations obtained with standard gravity
thickening tanks. The advantage of including primary
sludge and the decrease in concentration with increas-
ing SVI are also demonstrated.
2. Wet air oxidation ash used as a weighting agent did
not improve thickening.
3. Treating the thickner feed with a cationic poly-
electrolyte permitted solids loadings two to four
times greater than those possible with untreated
feed. However, no increase in sludge compaction was
noticed until the polymer dosage was increased beyond
10 pounds per ton of dry solids.
-------
-33-
Table 5.1
Average Results of Experimental Picket Thickner
Feed
Type of
Sludge
Activated
(SVI - 74)
Activated
(SVI - 97)
50% Activated
Preliminary
Solids
(7.)
1.06
0.87
1.10
Loading
(psfd)*
21
20
20
*Pounds per square foot per day
Thickened Solids (%)
Picket
Thickener
3.0
2.8
4.4
Standard
Thickener
1.8
1.4
3.3
Reefer successfully investigated Densludge thickening at Baltimore;
raw primary, waste activated, and trickling filter sludge was concen-
trated to as much, as 8.5 percent solids(HO). The optimum operating
criteria were as follows: (1) the mixed sludge was diluted with plant
effluent to a concentration of 1400 to 3200 mg/1, (2) the detention
time was 1.5 hours, (3) surface loading was 790 gallons per square
foot per day, and (4) the suspended solids loading was 14 to 14.5
pounds per square foot per day. This performance was a 45 percent
improvement over conventional plant thickening.
Beaumont, Texas, reported very good thickening performance^^).
Normal operations produced a thickened primary and trickling filter
sludge having 8.7 percent solids. Controlled operations where the
dissolved oxygen (D.O.), settleable solids, and sludge blanket level
were closely watched, produced a concentration of 11.5 percent. The
thickening tank was kept in an aerobic state by adjusting the amount
of filter humus and well-aerated plant effluent pumped to the tank.
Loading rates between 2 and 14 pounds per square foot per day produced
a consistent sludge compaction. The thickener overflow, averaging 130
ppm B.O.D. and 98 ppm suspended solids, was discharged to the secondary
trickling filters with no effect on B.O.D. removal.
The Cleveland Southerly Sewage Treatment Plant thickens waste activated
sludge in deep Dortmund tanks from a feed of 1.72 percent to 2.7 percent.
Ninety-eight percent solids capture was attained by batch operation
with a detention period of 7.5 hours'2^'. At Cleveland Easterly,
standard final settling tanks have been operated on a continuous feed
basis for six years. They increased the average concentration of waste
-------
.34 _
activated sludge from 1.64 to 2.49 percent. Overflow from the
thickening tanks contained a very low (26 mg/1) suspended solids
concentration(27).
Torpey presented the results of thickening mixed sludges at the
Bowery Bay Sewage Treatment plant over a 19 month period (107).
He reported that average solids concentrations of 11.2 percent for
primary and modified activated sludge were achieved with less than
24 hours' detention. The concentration attained with a mixture
of primary and conventional activated sludge was 6 percent total
solids. Torpey recently disclosed that much greater concentrations
are possible by adding digested sludge to the mixture of raw sludges.
Several New York City sewage treatment plants have used chlorine
in thickening tanks during the summer months. As quoted by Wirts,
Donaldson considered chlorine as a useful tool to improve the
thickener operation during summer months or when the settling
characteristics of the sludge were below averageC27).
At Hagerstown, Maryland, lime used in conjunction with sludge
aeration increased activated sludge thickening from a solids con-
centration of 0.6 to 3.5 percent (201). The dose was 180 pounds
per ton. In laboratory and pilot plant studies, Caron and Carpenter
demonstrated a polyelectrolyte dosage of 1.2 pounds per ton could
increase boardmill sludge thickening rates by 55 percent. A 2.4
pound per ton dose increased the rate for deinking sludge by 140
percent(194). The tank overhead was clearer than a control but the
solids concentration was unchanged.
Economics - A good economic case can be made for gravity thickening
because it offers the chance of reducing sludge volumes by one-half.
The reduced volume of sludge in turn results in reduced costs of diges-
tion and sludge dewatering. Beaumont, Texas, reports savings of
$175,000 in plant construction costs by use of thickeners which
allowed reduced digester capacity (from 510,000 cubic feet to 240,000
cubic feet)^*-™'. In addition thickening solved some digester operation
problems, such as the production of excessive supernatant liquor.
Burlington, North Carolina, saved an estimated 11.3 percent on the
construction cost of a sewage treatment plant by incorporating thicken-
ing and high rate digestion(H6).
Specific costs of thickening equipment and annual costs per ton of
sludge handled vary widely depending on local conditions. In comparison
with thickening activated sludge by pressurized air flotation, the
initial costs incurred with gravity thickening are greater but the
operation costs less(65). At large plants, maintenance and operating
costs for gravity thickening are about $2 per ton of dry solids. In
general, total annual costs (capital and operating) vary between $1.50
and $5.00 per ton of dry solids.
-------
-35-
Summary - Gravity thickening of waste sludges has been success-
fully practiced for many years. Its basic purpose is to reduce
sludge volumes; this in turn allows reductions in the costs of
sludge digestion and dewatering. The capacity of the digesters
is increased, their heat requirement is reduced, and they operate
more efficiently. The cost of dewatering sludge in mechanical
equipment, particularly vacuum filters, is less because production
rates rise with increased feed solids concentrations and chemical
costs are reduced.. Since thickening reduces the volume of digester
supernatant liquor, the overall treatment plant efficiency is
improved because less solids (contained in the supernatant liquor)
are returned to other treatment processes. Liquid which would
be returned as supernatant liquor is returned as thickener
overflow, a much more desirable condition(H6).
While gravity thickening has some important advantages, it also
has some possible disadvantages. First, thickening tanks must be
well operated or odors will develop; septic-bulky sludge will
form, resist compaction, and float to the surface of the tanks,
and the entire treatment plant efficiency could be impaired by the
recycle of thickener overflow containing a high concentration of
B.O.D. and suspended solids. Anaerobic conditions can be
minimized by the addition of aerated effluent, dilute biological
sludges, or chlorine, and by proper sludge pumping procedures.
Degasifying the feed sludge by vacuum has been proposed as an
efficient method of enhancing solid-liquid separation and sludge
compaction. Another disadvantage is the capital cost of gravity
thickening units. It is substantial but often justifiable because
of other process cost savings. Air flotation units may be cheaper,
but they have other disadvantages that often preclude their use,
particularly with non-activated sludges.
Considerable "art" is still involved in the operation of gravity
thickening tanks. Careful observation and reporting by operators
is needed to insure the best possible operation. Additional
improvements in the thickening "art" or "science" are needed.
Mancini suggested that new research is needed to develop a bench-
scale continuous-flow thickening test that will lead to reliable
design parameters for full-scale units(92). Also, increased
compaction of solids is usually a desirable goal. Perhaps mechanical
dewatering before incineration could be eliminated if a higher
degree of thickening than is possible today were achieved. Improved
designs of mechanical facilities should be considered. Combining
various sludges or tank overflows with or without settling aids has
proven to be a successful technique. Torpey's discussions of mixed
sludge thickening, including the addition of digested sludge to
sludge have indicated increased solids concentrations are possible
-------
-36-
The control of odor in raw sludge thickening operations
deserves more attention because this is a major problem at
many wastewater treatment plants.
Gravity thickening definitely has a future in the handling of
wastewater solids. It particularly offers a good way to thicken
mixed sludges at a low operating cost.
B. Flotation Thickening
General - Flotation thickening units are becoming increasingly
popular at sewage treatment plants, especially for handling waste
activated sludges. With activated sludge they have the advantage
over gravity thickening tanks of offering higher solids concentra-
tions and lower initial cost for the equipment.
Four methods of flotation, using rising gas bubbles to increase
the buoyance of solid particles, are used in the waste treatment
field: (1) dispersed air flotation where bubbles are generated
by introducing air through a revolving impeller or porous media,
(2) dissolved air-pressure flotation where air is put in solution
under elevated pressures and later released at atmospheric pressure,
(3) dissolved air-vacuum flotation which applies a vacuum to waste-
water aerated at atmospheric pressure, and (4) biological flotation
where the gases formed by natural biological activity are used to
float solids. Only dissolved air-pressure flotation and natural
biological flotation will be discussed in this section on thicken-
ing. The two other processes generally are more applicable to
wastewater clarification because significant increases in sludge
solids concentrations are difficult to achieve.
Dissolved Air-Pressure Flotation
Theory - The objective of flotation-thickening is to attach a
minute air bubble to suspended solids and cause the solids to
separate fron the water in an upward direction. This is due to
the fact that the solid particles have a specific gravity lower
than water when the bubble is attached.
Dissolved air flotation depends on the formation of small diameter
bubbles resulting from air released from solution after being
pressurized to 40 to 60 psi. Since the solubility of air increases
with pressure, substantial quantities of air can be dissolved(121).
In current flotation practice, two general approaches to pressuriza-
tion are used: (1) air charging and pressurization of recycled
-------
-37-
clarified effluent or some other flow used for dilution, with
subsequent addition to the feed sludge; and (2) air charging
and pressurization of the combined dilution liquid and feed
Air in excess of the decreased solubility, resulting from the
release of the pressurized flow into a chamber at near atmos-
pheric pressures, comes out of solution to form the minute air
bubbles (average diameter 80 microns). Sludge solids are floated
by the air bubbles that attach themselves to and are enmeshed in
the floe particles (5). Rich observed that entrapment of rising
air bubbles in the floe particle is the predominant action with
flocculent materials such as activated sludge. Contact of bubble
and particle by adhesion has more significance with non- flocculent
solidsC6). The degree of adhesion depends on surface properties
of the solids. Ettelt states that complete attachment of all the
air bubbles is theoretically possible but is not achieved(12l) .
When released into the separation area of the thickening tank,
the buoyed solids rise at a rate similar to that shown in Figure
5.V^ '. Hindered conditions analogous to gravity sedimentation
occur and can be called hindered separation or flotation. The
upward moving particles form a sludge blanket having a solids con-
centration gradient. Figure 5. VI by Katz and Geinopolos shows
a typical gradient plotted for waste activated sludge(65a). The
degree of sludge thickening depends on the compressional force and
the surface active properties of the solids resisting compression(121)
Parameters - The primary variables for flotation thickening are:
(1) pressure, (2) recycle ratio, (3) feed solids concentration,
(4) detention period, (5) air-to-solids ratio, (6) type and quality
of sludge, (7) solids and hydraulic loading rates, and (8) use of
chemical aids.
Air pressure used in flotation is important because it determines
air saturation, size of the air bubbles formed, and it influences the
degree of solids concentration and the subnatant (separated water)
quality. In general, increased pressure, or air, produces greater
float (solids) concentrations and a lower effluent suspended solids
concentration. There is an upper limit, however, because too much
air breaks-up fragile floes. Mayo recommended 0.03 cubic feet per
square foot of tank surface area per minute(124).
The recycle ratio and feed solids concentration are inter-related.
Additional recycle of clarified effluent does two things: First, it
-------
Figure 5.V
30
41
HI
CO
cr
u
8
o:
u
25
20
15
10
(Reprinted by permission
Schools of Engineering,
Purdue University)
I I ' I I T
RATE OF RISE OF FLOATED SOLIDS INTERFACE
t» 0.27 cm/sec
LABORATORY CELL
Cs=0.3l%
A/SS0-006
Sl=84
CD
50
100 150 200
TIME (sec.)
250
300 350
-------
-39-
Figure 5.VI
4.6
4.4
4.2
4.0
3.8
3.6
*
w 3.4
O
V)
S 3.0
Su
li.
Ill
* 2*
° 2.4
O
»~ 2.3
»-
5 20
O
8 "
to
a
AVERAGE SOL
3 K> » e
\
I
\
\
\
\
\
«
\
<
TEST CONDITIONS:
Activated Sludge U
Initial Solids Conce
Flotation Detention
X
>^^
1 —
1
.
l*^"*1
J
sed
ntrati
Time
> »,
jn«=l
= IHc
i
38%
ur
DISTANCE BELOW SURFACE OF FLOATED SLUDGE-ln.
—Solids concentration gradient in the floated sludge blanket
(in. X 2.54 = cm).
(Reprinted by permission from Vol. 36, No. 4, p. 409,
April 1964, J. Water Pollution Control Federation)
-------
-HO-
allows a larger quantity of air to be dissolved because there
is more liquid and second, it dilutes the feed sludge. Dilution
reduces the effect of particle interference on the rate of separa-
tion. At the Chicago Sanitary District, 40 percent recycle
proved to be optimum(121). Katz presented the data shown in Figure
5.VII depicting greater rise rates with increasing recycle ratios(122)
Experience has also proven that dilution of the feed sludge to a
lower concentration increases the concentration of the floated
solids(H9).
The concentration of sludge increases and the effluent suspended
solids decrease as the sludge blanket detention period increases'-''.
Figure 5.VIII shows data by Katz that emphasizes the importance of
thickening timeO22). jn plant tests there was a rapid increase
in solids concentration with time up to 3 hours. Beyond 3 hours
no additional thickening was observed.
As indicated in Figure 5.IX the air-to-solids ratio is a parameter
that influences the sludge rise rate. Increasing concentrations
of air/solids causes increases in floated solids production(121).
Eventually with unlimited use of air, a ratio can be reached where
no further increase in concentration would be possible. Ettelt
reported the effluent (subnatant) solids concentration was not
dependent on the air-to-solids ratio except for very low air input
rates or very high solids loading rates^121). Mayo suggested that
0.02 pounds of air per pound of solids was an effective ratio(124).
Similar to gravity sedimentation, the type and quality of sludge to
be floated affects the unit performance. Flotation thickening is,
as stated before, most applicable to activated sludges but higher
float concentrations can be achieved by combining primary with
activated sludge. Equal or greater concentrations may be achieved
by combining sludges in gravity thickening units. A high sludge
Volume Index (SVI), representing a bulky sludge, results in poor
thickener performance. Figure 5.X describes the importance of this
parameter as it applies to the flotation of activated sludge at the
Chicago Sanitary District (121).
Unit loading rates naturally affect the performance of flotation
thickening units. Figure 5.XI shows a typical example of relating
unit loadings, solids production, and floated solids recovery at the
Chicago Sanitary District^121'. In general, higher loadings impair
the performance of thickening units.
-------
-41-
Pigure 5.VII
Figure 5.VIII
20
i.a
1.6
tn 12
at _
• E in
o* |C
UJ ~
i °e
O.6
0.4
02
/
/ \
/
'
1
/
/
/
>
THE EFFECT OF RECYCLE
3ATIO ON THE RATE OF RISE
OF THE INTERFACE
(1% Activated Sluda*)
FIGURE 1.
X> 200 300 400
RECYCLE RATIO
(%)
LABORATORY
TESTS
O
PLANT TESTS
THE EFFECT OF THICKENING
TIME ON SOLIDS CONCENTRATION
OF ACTIVATED SLUDGE
FIGURE
THICKENING TIME (houri)
(Reprinted by permission Public Works, p. 114,
December 1958)
-------
-U2-
Figure 5-IX
-x 13
- ~ 12
> .020
t-
o
Lvl
LJ
JL
o
.0!5
o
«*•
>
i
to
p
O
O
cr
Q.
§ 10
o
Q 9
L'J
£-
.010 L 7
I
I
FLOTATION UNIT
IXLET DESIGN WO. 6
AVERAGE LOADING OF 22TONS/D3!
OVERFLOW RATE OF 0.214 GPM/f tB
SI = 35
FLOATED
SOLIDS
I
I
I
MAXir/JUC.1
'AIR INPUT—
[EQUAL TO
65 7- OF
I SATURATION
,G3«F Q
'CO PSIO _
I
I I
0 .005 .010 .015 O20
LSS OF AIR PER LB OF SOLIDS (A/S)
Effect of air content on floated solids production
and effective terminal velocity at constant overflow
rate.
(Reprinted by permission Schools of Engineering, Purdue University)
-------
-43-
Figure 3*X
§?
DC
O
LJ
I I I I I
FLOTATION UNIT
INLET DESIGN NO. S
AVERAGE LOADING OF 26.8 TONS/DAY
RECYCLE OF 60%
A/S B 0.015
1
40 50 60 70 80 90
SLUDGE VOLUME INDEX (SI)
100
Effect of Sludge Volume Index on floated solids production,
(Reprinted by permission Schools of Engineering, Purdue University)
-------
Pigure 5 .XI
13
12
10
60
50
S
o
o
LU
IT
h 30
(/>
o
_J
o
to
h 020
LJ
1
^ 10
T 1 -T- 1
FLOTATION UNIT
INLET DESIGN NO. 6
AVERAGE FLOATED SOLIDS OF 4.0%
Sl=83
PER CENT SOLIDS
RECOVERY.
SOLIDS
PRODUCTION
I
I
^ 20 25
UNIT LOADING (TONS/DAY)
Effect of loading on floated solids production and recovery.
(Reprinted by permission Schools of Engineering, Purdue University)
-------
-45-
Many different chemicals have been used in various air flotation
systems. Industrial processes, particularly in the mining industry,
use frothers and collectors to lower interfacial tension, and they
use promoters to improve the effect of collectors(6). In the waste
treatment field, flocculating chemicals have agglomerated solids
into stable floes that promote increases in the terminal velocity and
facilitate capture of gas bubbles. The overall effect is to increase
the allowable solids loadings, increase the percentage of floated
solids, and increase the clarity of the effluent. Cationic poly-
electrolytes have been the most successful chemical used in sewage
sludge thickening.
Design - Solids loading is the design parameter governing the sludge
surface area of the flotation thickening unit(65a). This and other
design parameters are evaluated in pilot plants by the two major
manufacturers of air flotation equipment. Scale-up to full scale
units does not always correlate accurately because of flow turbulence
in the pilot plants. Laboratory tests usually precede pilot plant
tests for the purpose of determining recycle ratios, chemical treat-
ment dosages, and the susceptibility of sludge thickening by dissolved
air flotation.
s
The principal components in a flotation system are: (1) a pump to
increase pressures for greater air solubility, (2) a retention tank
where air and liquid are mixed under pressure for 1 to 2 minutes,
(3) a pressure release valve, and (4) the flotation unit containing a
quiescent zone and a sludge withdrawal mechanism. Tank inlet design
is one of the most critical features of the flotation unit. A design
that encourages reduced turbulence and provides greater bubble-solid
confinement with maximum adhesion efficiency is desirable v"1/.
Turbulence can cause a separation of bubbles from the solids resulting
in particle settling rather than flotation. At the Chicago Sanitary
District the use of cylinders to reduce inlet velocities and to disperse
the floes vertically was successful' *.
Performance - A pilot plant comparison of gravity thickening versus air
flotation thickening was described by Katz and Geinopolos(65a). Mixtures
of primary and activated sewage sludge were thickened to an average of
4.5 percent in the gravity unit and 6.0 to 6.5 percent in the flotation
unit. The gravity unit was loaded at 8 pounds per square foot per day
and the flotation unit with 20 to 30 pounds per square foot per day.
Extensive flotation thickening evaluations at the Chicago Sanitary
District resulted in the following conclusions(121):
1. Waste activated sludge can be thickened to a higher
solids concentration by flotation (47.) than by gravity
settling (2%).
-------
2. Maximum floated solids production and solids
recovery was obtained at a loading of 13.5 pounds
per square foot per day.
3. Optimum combined, floated plus settled, sludge
thickening resulted in a solids concentration of
3 percent and a total solids capture of 92.5 percent.
4. The use of a cationic polyelectrolyte at a dosage of
20 pounds per ton doubled the floated tonnage, increased
solids capture to 99.6 percent, and increased the solids
concentration to 3.9 percent. Figure 5.XII shows the
effect of the flocculent dosage over a wide range.
The importance of the air content has also been demonstrated at
Chicago. Table 5.2 presents data showing the performance at 50
percent air saturation and 75 to 82 percent saturation'"':
Table 5.2
50% Saturation 75 to 82% Saturation
0.42 Floated to settled 0.67
sludge ratio
11.4 Solids loading in 13.8
Ibs./sq.ft./day
3.2 Floated solids 3.5
concentration
Braithwaite discussed the successful use of polymeric flocculents
as an aid to flotation thickening of waste activated sludge at
Warren, Michigan(i2°). In general, polymers permitted consistent
and reliable treatment and produced thickening results that would
be impossible to achieve without polymers. A summary of the
Warren data is as follows:
Without With
Chemical Aids Chemicals Aids
Solids removal rate 0.9 3.34
(Ibs./sq.ft./hr.)
Solids concentration (%) 4.1 6.2
-------
-U7-
Figure 5.XII
Effect of Catlonic Flocculant on
Solids Production
20
g 18
(5
O
1 14
cc
CL
CO
9 12
O
CO
a
-------
-48-
The data indicated a 73 percent reduction in theoretical flotation
unit surface area is possible with the use of chemicals.
Rudolfs investigated many chemicals as aids to flotation thickening
of activated sludge*33'. He found that calcium hypochloride was
particularly effective; a dosage of 16 pounds per 1,000 gallons of
sludge (364 Ibs./ton) increased the solids concentration from 1.05
to 3.75 percent after 6 hours compaction. Other chemicals investi-
gated by Rudolfs included chlorine, sulfuric acid, carbon dioxide,
and nitrogen. He was looking for a material that would flocculate
and dehydrate solids as well as to allow gases to form and rise to
the surface.
Economics - Flotation thickening is a relatively new process
installed at very few sewage treatment plants. As a result the
literature contains very few references regarding costs. However,
an estimate of the operating cost without the use of chemicals is
$4 to $5 per ton of solids. Because chemicals could cost another
$5 to $6 per ton, the operating costs using chemicals would be
double the non-chemical cost. In general, it is accepted that the
initial cost of flotation is lower than gravity thickening but the
operating cost is higher. The operating cost of flotation units
should be less than the operating costs of centrifuges unless
chemicals must be used. One leading manufacturer of flotation
units specifically recommends the use of chemicals. In general,
the total annual cost of air flotation thickening varies from $6 to
$15 per ton of dry solids.
Any evaluation of thickening should consider the economy resulting
from the production of a thicker sludge. This will include the
conventional items associated with a thicker sludge such as lower
digestion costs and lower vacuum filtration costs. At the Chicago
Sanitary District air flotation thickening has a tremendous
influence on sludge digestion costs(56). As illustrated in
Figure 5.XIII, a 3 percent sludge has a digestion disposal cost of
about $24 per ton while a 4.5 percent sludge cost less than
$19 per ton of dry solids.
Summary - The need for an effective process to thicken waste
activated sludge has long been recognized and is now available.
While dissolved air flotation units were first applied to clarifica-
tion operations, they have proven their value for thickening
activated sludge to a concentration higher than is possible in
gravity thickening tanks. These units can also be used for thicken-
-------
28
26
24
o
CO
20
o 18
16
O
o
14
12
-'49-
Figure 5.XIII
PRESENT
100% A
I
OPERATION
TIVATED
PRESENT OPERATION
PRELIMINARY-
85V. ACTUATED
34567
FEED SOLIDS CONCENTRATION (%)
200
180
160
140
120
100
80
60
40
8
o
a.
H
zf
CD
-------
-50-
ing mixed primary and activated sludge, but they do not do this job
as effectively as gravity thickening units unless the percentage of
primary sludge is very low.
Flotation thickening has the disadvantage of having many parameters
that can affect its performance. As a result inconsistent thicken-
ing may occur by changes in such factors as sludge particle size,
sludge volatile concentration, sludge age, and quality of recycle
water. The use of flocculents as recommended by one manufacturer
can correct this problem but at cost that could double the total
operating cost.
Basically, flotation processes are not as simple, consistent, and
economical as other thickening processes. As a result its use will
be limited generally to what it does best - thickening waste
activated sludge or low specific gravity, non-activated industrial
waste solids. Even then, unless activated sludge is disposed of
separately, gravity thickening could be more efficient because the
sludges are usually blended before dewatering in mechanical equip-
ment. Blending might as well be performed in gravity thickening tanks.
Biological Flotation
General - The flotation of organic sludges after extended storage in
a tank (without the aid of heat) is a well-known phenomenon. Improve-
ment on this natural flotation technique by controlling heat and
detention is best illustrated by the Laboon Process(131). This
process, as designed for the Allegheny County Sanitary Authority
(Pittsburgh, Pa.) consists of the following steps:
1. Disintegration of the raw primary sewage sludge
(5-10% solids) to prevent clogging of pumps and
equipment in subsequent sludge handling processes.
2. Heating of the sludge to 35°F in heat exchangers
operated at 15 psi.
3. Concentration of the sludge by biological means for
5 days. Escaping gases buoy and compact the sludge
in the concentration tanks.
H. Separation of subnatant liquor on the fifth day and
pumping the concentrated sludge to the Incineration
Building for subsequent incineration (discussed
further in the Combustion chapter of this report).
-------
-51-
Parameters important to biological flotation include: (1) sludge
temperature, (2) detention period, (3) type of sludge and its
volatile content, and (4) sludge pH. Laboratory studies using
numerous sludges indicated that 35°C was the optimum temperature
and 5 days was the optimum detention period^129). Activated
sludge could not be concentrated because it would not float.
Some mixtures of primary and activated sludge could be concentrated
to some degree by flotation, but settling occurred during extended
detention periods. High sludge pH inhibited bacterial action re-
sulting in insufficient gas to cause solids flotation.
Performance - A natural biological flotation pilot plant was
operated by Laboon for one year. Raw primary sludge was con-
centrated from 4 percent to 20 percent solids by his process.
This sludge could be dewatered on vacuum filters without prior
chemical treatment and it could be digested^129). The concentration
tank subnatant (separated liquid) had an average B.O.D. of 2970 ppm
and suspended solids of 4000 ppm. It was estimated upon recycle to
the head of the treatment plant that this subnatant liquid would
increase the total sewage flow by less than 7 ppm B.O.D. and
suspended solids by less than 10 ppm. No solids build-up from the
subnatant recycle was expected'23;, Actual plant scale operation
of the Laboon Process at the Allegheny County Treatment Plant
approximates the pilot plant performance. An average sludge concen-
tration of 18 percent is produced from a feed sludge of 10.7 percent.
Ashland, Ohio, thickens sludge to 15 percent by biological flotation
without heat. This enabled them to incinerate their sludge in a
multiple hearth furnace with the use of supplemental fuel^129^. The
Laboon Process is being used to thicken raw primary and waste
activated sludge at Charlotte, North Carolina^66). Raw sludge con-
taining considerable industrial wastes is concentrated to 10 to m
percent solids before digestion.
Summary - Natural biological flotation is being successfully used to
concentrate raw sludge at a few sewage treatment plants. At
Pittsburgh it makes possible the goal of sludge incineration without
prior mechanical dewatering. What the overall sludge handling costs
are at this plant has not been reported in the literature. Therefore,
an estimate of the suitability of the Laboon Process in comparison
with other systems cannot be made precisely. The thickening process
itself probably is fairly expensive because of the sludge heating,
the>lengthy detention period, and the need to blend sludges from the
various concentration tanks in order to insure a uniform feed to the
-------
-52-
incinerators. Another disadvantage associated with any raw sludge
thickening process is the possibility of odor development. Enclosed
units with a system to draw gases off into a deodorizing chamber are
required.
Biological flotation as a sludge concentration technique seems limited
unless improvements are made in the process. Secondary sludges
apparently do not respond well and on the surface the cost appears
high. Odors must be closely controlled. But, it has demonstrated the
advantage of eliminating the need for a mechanical dewatering step
ahead of sludge incineration. Perhaps with the use of chemical
additives and/or waste heat from incineration units, the process can
become less expensive and more efficient.
-------
-53-
C. Centrifugat ion
General - The use of centrifuges to thicken and dewater sludge
is not a new development. For many years this equipment has been
successfully used by the process industries. Centrifuges were
first evaluated for waste treatment applications nearly 40 years
ago, but only recently were they installed for regular use at
wastewater treatment plants.
This section discusses only performance data for the thickening
application of centrifuges. The dewatering chapter of this report
discusses centrifugation theory, parameters, and economics.
Oewatering can be distinguished from thickening by assuming that
it is the dehydration stage where solids are not fluid and can't be
pumped. Thickened solids are fluid and can be pumped.
Performance - One of the first reports of sludge thickening by
centrifugation describes an evaluation at Sioux Falls, South
Dakota'23, 125 J^ They investigated a system where activated
sewage sludge containing 0.5 to 0.8 percent solids was passed over
a vibrating 65-mesh wire-cloth screen and then thickened in a
centrifuge. Solids discharged from the bowl-type centrifuge had
an average concentration of 5 to 7 percent. The centrate (separated
liquid), containing 250 ppm solids, was recycled to the primary
clarifers with no impairment in overall treatment plant efficiency.
The total annual cost of the centrifuge operation was estimated to
be $4.37 per ton in 195>*. Power costs accounted for 20 percent of
this total and labor, 26.5 percent.
Centrifuge thickening of waste activated sludge was extensively
studied at the Chicago Sanitary District. Ettelt and Kennedy
reported that both disc-type and solid-bowl centrifuges were
evaluated^56). The disc-type machine concentrated the activated
sludge to about 7.0 percent at 6,000 rpm, but operational problems
made its use impractical. Clogging of the sludge discharge nozzles
required repeated maintenance. Vibrating screens could not remove
a sufficient amount of oversized solids to prevent clogging; rotary
screens were more effective, but at low flow-through rates. The
centrifuge captured 87 to 97 percent of the feed solids at a feed
rate of 3,600 gallons per hour(gph).
Two solid-bowl centrifuges were also evaluated, using Chicago
activated sludge; one was a concurrent machine, the other a counter-
current^56^. Using activated sludge alone, the concurrent machine
-------
-5M-
thickened sludge to about 7 to 7.5 percent. The countercurrent
machine thickened the sludge to about 6.6 to 7.0 percent. A 1:1
mixture of preliminary and activated sludge was thickened to 9.8
percent. By lowering the liquid level in the centrifuge, activated
sludge was thickened to 16 percent, but the solids capture was
very poor. Figure 5. XIV shows the decrease in solids capture with
increasing degrees of thickening resulting from lowering the liquid
level.
The use of cat ionic polyelectrolytes significantly improved the
centrifuge performance as indicated by Figures 5. XV and 5.XVl(56).
The use of these flocculents allowed significant increases in cake
production and solids concentration. It is interesting to note the
effect of sludge quality (SVI) and the blending of preliminary/
activated sludge on the centrifuge performance. As shown in
Figures 5. XV and 5. XVI, much higher production rates and solids
concentrations are possible with a mixed preliminary and activated
sludge. Also a sludge having a high SVI (indicating a low quality
material) cannot be thickened as rapidly or to as high a solids
concentration as a good-quality sludge can.
Solid bowl centrifuges operate with lower gravitational forces than
disc centrifuges. While gravitational force is not the only factor
in determining centrate clarity, the solid bowl machine does not
usually produce a high quality centrate, in part due to the lower
force. The solid bowl machines were evaluated at speeds between
and 2,700 rpm.
The Yonkers Sewage Treatment Plant of Westchester County, New York,
has a very successful centrifuge thickening operation. They
thicken digested, primary sludge prior to ocean barging. In the
two year period 1961-1962, the sludge barge averaged 79 trips each
year. During 196U-1965, after centrifuge thickening was adopted,
the barge trips per year averaged 32.5'^27)>
At Yonkers, gravity settling is used to thicken a dilute primary
sludge (0.35%) to about 8 percent before digestion. The average
digested-sludge solids-concentration fed into the centrifuge is U
to 5 percent. Thickening to 10 or 11 percent is done in the
centrifuge; it could do more but at a higher level the sludge would
not flow to the barge(127). it has been observed that a high-
volatile sludge does not thicken as well as a low-volatile sludge.
-------
100
-55-
Figure 5.XIV
8 10 12 14
SOLIDS CONCENTRATION (%)
16
DECREASE IN RECOVERY WITH
CORRESPONDING INCREASE IN SOLIDS
CONCENTRATION FROM LOWERING LIQUID
LEVEL IN CENTRIFUGE. ACTIVATED (SVI«9I)
(Reprinted by permission from Vol. 3§, No. 2, p. 253,
February 1966, J. Water Pollution Control Federation)
-------
S
-------
-57-
Figure 5.XVI
50% P
A<
c/ACTI
3ELIMINAR
:TIVATEO
/AT ED
SVI « 69
10 20 30 40 50
POLYMER DOSAGE ( LB PER TON )
EFFECT OF CHEMICAL ADDITION ON
SOLIDS CONCENTRATION BY CONCURRENT
CENTRIFUGE AT 90% RECOVERY.
(Reprinted by permission from Vol. 38, No. 2, p. 253, Feb. 1966,
J. Water Pollution Control Federation)
-------
-58-
The machine centrate, at Yonkers, is returned to the pre-aeration
tank for mixing with raw sewage. No operational problems were
caused by this recycle of centrate. Polymeric flocculents were
tested; they increased the solids capture from 76.2 to about 93
percent. Chemicals are not being used because of the added cost
and because centrate solids resettle in the primary clarifiers.
The total capital cost of the Yonkers1 centrifuge and the building
in which it is contained was $1U1,900^127^. Annual costs are
about $15,500; the operating cost is about 39 percent of this total.
Sixty man-hours per week are assigned to the centrifuge. On a per
ton basis the annual cost for centrifuge thickening is about $3 to
$4 per ton because nearly 5,000 tons of sludge are barged to the
In 1961, The Dow Chemical Company evaluated centrifuge thickening
of waste activated sludge produced from treating an integrated
chemical plant wastewater^17 ) . A solid-bowl machine thickened
a feed from 2.5 to 6 percent. Machine parameters used were:
average speed, 2300 rpm and pool depth, 2-1/8 inches. About 2
pounds of dry solids were produced per minute with a solids capture
of 85 to 97 percent.
Summary - Centrifugation has definitely proven its ability to
thicken a variety of wastewater sludges. With recent improvements
in machine design, this process will become more popular. The
annual operating cost is about $3 to $8 per ton, if chemicals are
not required. The thickening of waste activated sludge by itself
can probably be done with less cost in air flotation units, unless
chemicals are required in the flotation unit operation but not the
centrifuge. Chemicals could add $3 to $10 per ton to the operating
cost of centrifuges.
The Yonkers operation appears to be a good application for the use
of centrifuges to thicken digested sludge in conjunction with
gravity thickening of raw primary sludge. At one location, the
centrifuge used to dewater digested sludge is also used to
thicken the raw sludge fed to the digester. Unless space and odors
are a problem, however, raw primary and mixed sludges are most
efficiently thickened in gravity settling tanks.
-------
-59-
Centrifuges are a compact, simple, flexible, self-contained unit,
and the capital cost is relatively low. They have the disadvantages
of high maintenance and power costs and often a poor, solids-capture
efficiency if chemicals are not used. Overall, the advantages out-
weigh the disadvantages, so they will be installed with increasing
frequency. The dewatering chapter of this report contains a detailed
discussion of centrifuges.
-------
-60-
6. SLUDGE BLENDING
Most physical, chemical, and biological processes proceed more
efficiently if the input material is of a uniform nature. This
is true for typical waste treatment processes such as sludge
thickening, sludge dewatering, sludge digestion, and sludge
incineration. For this reason, blending becomes important at
secondary treatment plants where two vastly different sludges are
produced.
Blending may be simply achieved by recycling secondary sludges
to primary clarifiers where the sludge resettles and is mixed
with the primary wastewater solids. The mixing of different
sludges in pipelines, feeding thickening tanks allows adequate
blending particularly if the sludges are dilute. The sludge
collectors and picket thickening devices further aid blending
in gravity tanks. Digesters designed for complete mixing can
uniformly blend the contained sludges.
Blending sludges to produce a uniform mixture is particularly
important ahead of incinerators, mechanical dewatering equipment,
and digesters having inefficient mixing systems. For example,
processing a uniform sludge can significantly improve the
economics and performance of vacuum filter operations. In
situations where sludge characteristics vary, the usual operating
procedure is to overdose the sludge with chemicals to insure
satisfactory mechanical performance. Overdosing wastes chemicals.
But, improperly conditioned sludge leads to low filter yields, high
cake moisture, and excessive labor for supervision of filtration
and maintenance of blinded filter media. A blended sludge permits
the more efficient use of chemicals because it has a more uniform
and predictable flocculent demand than unblended material.
Different sludges are usually blended before the mechanical dewater-
ing, incineration, and digestion steps by air agitation or by
vigorous mechanical mixing in storage tanks. Both methods have the
disadvantage of liberating entrained gases from the sludge. These
gases cause air pollution problems unless the tanks are covered and
the gases are exhausted to a deodorization process. Air agitation
could be the superior technique because it freshens sludge and there
is abundant data which shows that freshening lowers filtration costs.
Overly vigorous mixing must be avoided otherwise the sludges to be
dewatered may be deflocculated; this in turn increases the cost of
dewatering.
Design engineers should be aware of the importance of thoroughly
blending sludges ahead of certain unit processes.
-------
-61-
7. SLUDGE DIGESTION
A. Anaerobic Digestion
General
Design engineers do not express uniform opinions on the merits of
anaerobic digestion. Because there are significant advantages and
disadvantages to this process, its application should be thoroughly
evaluated for each individual situation.
Digestion essentially competes with incineration and mechanical
dewatering. If a sludge is to be incinerated, it appears reason-
able to eliminate expensive digesters because digestion lowers the
Btu value of each pound of solids. At small wastewater treatment
plants, landfilling of mechanically dewatered raw sludge is often
feasible. It may be competitive with digestion followed by
lagooning or liquid land disposal of the end product.
The major justification for digestion is that it stabilizes raw
sludge and makes it more acceptable for final disposal. Other
arguments for digestion, such as volume reduction and the production
of usable gas, are insignificant in comparison with the conversion
of noxious raw material, including fats, proteins, cellulose, and
pathogenic organisms, into a more acceptable product. This important
justification has made anaerobic digestion the most common method of
processing organic sludges (53), There is a trend to systems
designed around raw sludge handling but digestion will continue to
be popular, particularly at small sewage treatment plants and in
large coastal cities; digestion permits inexpensive land and ocean
disposal at these locations.
The development of high rate digestion brought about significantly
improved process economics. However, digester operating problems
continue to plague most waste treatment plants, so additional process
improvements are desirable. This chapter discusses anaerobic
digestion from the standpoint of how it affects ultimate sludge
disposal.
Theory and Objectives - Anaerobic digestion can be defined as the
decomposition of organic matter in the absence of free oxygen.
The decomposition is accompanied by gasification, liquefaction,
stabilization, colloidal structure breakdown, and release of
moisture^7). Digestion occurs in a mixed culture of microorganisms
where particular species are most active in different stages such
as acidification and gasification. In the digestion process, de-
composition is not complete; the products of intermediate metabolism
include organic acids, ammonia, methane, hydrogen sulfide, carbon
dioxide, and carbonates. A 60 to 75 percent reduction in volatile
-------
-62-
solids is commonly achieved by anaerobic digestion, depending on the
initial volatile solids content of the sludge.
The objectives of anaerobic digestion have been defined by various
people to include the following:
1. Nuisance prevention by decomposing organic solids
to a more acceptable stable form.
2. Sludge volume reduction by converting organic solids
to gases and liquid.
3. Volume reduction by concentrating the remaining solids
into a dense sludge.
4. Sludge storage to accommodate fluctuations in wastewater
flows and to permit flexibility in subsequent dewatering
operations.
5. Homogenizing sludge solids to facilitate subsequent
handling steps.
6. Production of useful by-products such as gas and soil
conditioner.
7. Reduction of pathogenic organisms.
8. Production of more easily dewatered solids.
Not all of these objectives are met in an individual anaerobic
digestion system. For example, sludge concentration and the
production of a more easily dewatered material are frequently not
accomplished. "Bound" water in the sludge is often not released
when the microorganisms attack and break down the complex molecular
structure of the solids. High-rate digested sludge, having much
entrained gas, is particularly resistant to solid-liquid separation.
Design Parameters and Operations - Anaerobic digestion is influenced
by the following factors:(1) nature of sludge solids and their
volatile content, (2) detention period in the digester, (3) tempera-
ture, (4) degree of digester mixing, (5) concentration of the feed
sludge, (6) chemical additives, and (7) solids loading rate. These
parameters affect ultimate sludge disposal processes in that a well-
designed and operated digester produces a material that can be
handled more easily and cheaply. A well-digested sludge has a low
content (HO to 50%) of volatile material and dewaters reasonably
well on sand beds or in mechanical equipment.
-------
-63-
Since anaerobic digestion is a biological process, it is obvious
that toxic materials in large quantities should be excluded from
the digester. Examples of potentially toxic materials are heavy
metals and cyanide. High concentrations of floating material
such as skimmings, and industrial greases, and oils can be trouble-
some if the material in the digester is not completely mixed
during the digestion process. Scum layers formed by low-specific-
gravity solids can inhibit digestion and the free discharge of gas.
Generally, the higher the sludge volatile content is, the more
efficient digestion becomes. Because digesters are designed on
the basis of volatile pounds per cubic foot, this factor will
influence the digester capacity.
A factor used to judge the quality of digested sludge is its
volatile solids content. The percent of volatile solids remaining
in the digested sludge is a function of the detention time in the
digester(I36). Figure 7.1 illustrates two facts: (1) as the length
of time allowed for decomposing raw sludge increases, the reduction
in volatile matter increases and (2) larger percentage reductions
in volatiles occur in raw sludges having higher initial volatile
concentrations(H).
The rate of sludge digestion varies greatly with temperature; low
temperatures result in low digestion rates. Figure 7.II shows the
effect of temperature on the time required to reduce volatile solids
in raw sludge to a desirable 40 to t5 percent range^D. Garber
determined that temperature also affects other sludge character-
istics <1U°). He evaluated sludge digestion at 85°F, 100°F, and 120°F.
As expected, different microorganisms predominated at each tempera-
ture. However, other changes were noted at 120°F that affected
sludge dewatering and disposal: (1) the average particle size was
larger, (2) the proteinaceous material was more completely digested,
and (3) the sludge had less total nitrogen, and the nitrogen
present was in a different form. In addition, the methane content
of the digester gas was higher at 120°F than 85°F.
Digester contents should be well mixed, especially for high-rate
digestion. Adequate mixing keeps microorganisms functioning at
peak efficiency because they are in continuous contact with their
food supply. In addition, mixing keeps the concentration of bio-
logical end products uniform and prevents scum accumulation. Mixing
is done mechanically or by gas recirculation. About four techniques
are available for each basic type of mixing; most people agree that
all are technically satisfactory. Gas mixing has an operational
advantage; it minimizes specialized equipment requirements.
-------
Figure 7-1
Figure 7-II
-64-
10 CO 90 40 50 60
DETENTION W DIGESTER (doyt)
70
Reduction In volatile matter for varloui sludges and
digestion timei.
rto
122
•M
t
WK>4
I
§ a6
50
(
£.
^
-/
^
X
\.
^^
^•^
^-^
u » «> SO 40 50 60 70
TME OF DKiCSTCN (Ooy.)
Digctdon tim«-t«mp«r»tur« reUtion.hip.
(Reprinted by permission from Manual of Practice No. 11,
pp. 43 and 44, J. Water Pollution Control Federation)
-------
-65-
In general, attempts are made to digest as thick a raw sludge as
possible. A maximum feed concentration is considered desirable
because: (1) it conserves heat due to the minimum amount of water
present, (2) it prevents dilution of the digester buffering
capacity, (3) it encourages microorganism efficiency because their
food supply is concentrated, CO it increases detention periods,
(5) it minimizes the supernatant volume returned to other treat-
ment plant processes, and (6) it promotes the efficiency of sub-
sequent dewatering steps. There is, however, a sludge concentra-
tion limit for satisfactory digestion. Viscosity and biological
limitations are thought to occur at approximately the following
maximum desirable concentrations (assuming good grit removal):
1. Raw primary sludge 10-16%
2. Raw primary + biofilter sludge 8%
3. Raw primary + waste activated sludge 6%
Chemicals are usually added to sludge digestion tanks when there
are operational problems. Historically, lime has been used when
digestion is poor and acid conditions exist. Each digester may
have its own critical pH which often is just 6.5 to 7.0. A recent
innovation involving chemicals is the addition of anionic or
cationic polyelectrolytes to digesters to improve supernatant
quality. These organic materials are added as flocculents to sludge
transfer lines between primary and secondary digesters. Flocculation
of the solids promotes liquid separation in the secondary digesters.
The digester solids loading rate is an important parameter affecting
unit performance. The microorganisms in a digester are relatively
easy to upset, so they must be "fed" at acceptable rates. An
optimum procedure would be a low uniform rate of sludge addition.
Increasing the loading rates of digesters and decreasing the detention
time of the sludge — a technique referred to as high-rate digestion
— represents a recent advance in solids handling and disposal tech-
niques. The following loading and design criteria reveal why high-
rate digestion has been accepted so enthusiastically (157, 38, 151):
-------
-66-
1. Solids loading (pounds volatile solids/cu. ft./day)
Standard rate High rate
0.04 to 0.1 0.15 to O.i+O
2. Design criteria (cu. ft. of digester space/capita)
Standard rate High rate
a) primary sludge 2 to 3 1-1/3 to 2
b) primary sludge H to 5 2-2/3 to 3-1/3
+
biofilter sludge
c) primary sludge 4 to 6 2-2/3 to 4
+
activated sludge
3. Digestion period required (days)
Standard rate High rate
39 (avg.) mTs (avg. from
actual plant data)
Ten days is considered a practical minimum detention period for
high-rate digestion, 6 days a theoretical minimum^153;> Adequate
mixing is the key to successful high-rate digestion because the
digester volume is fully used for biological activity. Good contact
of microorganisms and feed solids occurs. The homogenization of
sludge, grease, and grit reduces build-up of unwanted scum and
deposition of grit. The Densludge Digestion System developed at
New York carries the process further, providing, in addition to
continuous effective mixing, the following features^47):
1. Thickening of the digested feed sludge.
2. Feeding of the sludge on a substantially continuous
basis.
3. Degritting of the sludge to a point where no grit
settles out in the digester to occupy valuable
space.
Degritting is done in hydrocyclone units (see the Grit Section of
this report) and thickening, in gravity sedimentation tanks. The
Densludge process usually has no supernatant liquor because the
solid-liquid separation is accomplished in the thickening tanks.
-------
-67-
Certain layout and structural features affect the performance of
anaerobic sludge digestion. Two-stage digestion was considered
an important advancement in the art. In the first stage, at least
two-thirds of the decomposition occurs with an accompanying high
gas production. Biological activity is too intense to permit
solid-liquid separation with subsequent compaction of the digested
sludge. These steps must, therefore, be accomplished in a second
tank (stage). Heating and mixing facilities are often not incor-
porated in the second stage, on the assumption that they may hinder
separation and compaction.
Unfortunately, high-rate digested sludge resists compaction and
effective separation of liquids. As a result, the supernatant
liquor has a high concentration of solids. This situation usually
requires either: (1) removal of the supernatant liquid as thickener
overflow before digestion, as recommended by the Densludge pro-
ponent s^22)t or (2) elutriation-thickening of the digested sludge
before dewatering. Few investigators have explored the difference
in dewatering characteristics between standard rate and high-rate
digested sludges. MacLaren and others caution that the end-
product of high-rate digestion may be difficult to dewater(53).
Digestion tanks should be designed so that they can be easily
emptied to remove deposited grit or to make mechanical repair
Some recent digester designs have incorporated hopper bottoms with
flushing nozzles to facilitate the removal of grit without the
need to use sludge as a carrier.
Facilities to use digester gas are often designed into waste treat-
ment plants. The methane fuel value is commonly used to heat
buildings, including the digester, and for plant power production^7).
If the hydrogen sulfide concentration is high, the gas must be
scrubbed. The gas is also used to mix the contents of the digester.
In general, digestion tanks should be designed and operated for
optimum liquefaction and gasification while converting the sludge
to a form acceptable for disposal. Variations in ultimate disposal
techniques, however, may dictate differences in design and opera-
tional procedures ("7 ).
Performance - Much of the recent digester performance data compares
high-rate and standard-rate digestion. Suhr and Brown made interest-
ing comparisons at two Westchester County, New York, sewage treat-
ment plants^1>l7). Their data showed the following:
-------
-68-
High Rate Standard Rate
(Yonkers Joint Plant) (New Rochelle)
Design capacity 0.4 cu.ft./capita 3.0+ cu.ft./capita
Digestion period 16 days 30 days
Volatile solids 64.5% 65.9%
reduction
Digested sludge 3.5 to 4% 6 to 9%
solids
As the data indicates, high-rate digestion can produce the same
volatile solids reduction as standard rate with much smaller
digester capacity. However, the water does not separate as easily
from the high-rate sludge as it does the standard rate. The
Yonkers plant eventually added a centrifuge to thicken high-rate
sludge prior to barging.
Rankin also compared high-rate with standard rate digestion
performance(22).
Volatile Solids Reduction (%)
Detention Time High Rate Standard Rate
(Days) (74.5% Volatile Sludge) (75.0% Volatile Sludge)
10 54.5
15 57.0 49.0
20 59.0 51.0
25 - 53.0
30 - 55.0
40 - 59.0
Standard-rate digestion required twice as long to reach 59 percent
volatile solids reduction as the high-rate did.
Torpey (a pioneer in high-rate digestion and other processes) deter-
mined at New York that high-rate digestion could be done in 25 per-
cent of the digester capacity normally required with conventional
digestion' 1J*2). For 56.9 percent volatile matter reduction, this
meant 0.5 cubic feet of digester capacity per capita rather than
2.1 cubic feet and 31 days detention rather than 54 days. Pre-thick-
ening of modified aerated sludge to 10.2 percent removed the water
normally separated as supernatant liquid in conventional units.
Thickening followed by digestion in a single high-rate digester
resulted in a digested sludge volume equal to that produced by four
standard-rate digesters.
-------
-69-
Whichever digestion process is used the concentration of digested
sludge is (as shown in Figure 7.Ill) a function of its volatile
content. The data represent similar volumes of the two sludges.
The 1950-1951 data in Figure 7.Ill represents standard rate
digestion*142).
Garber reported some very interesting data on high temperature
digestion at the Los Angeles Hyperion Sewage Treatment Plant'I1*0/.
As mentioned in the design parameters portion of this chapter,
Garber investigated temperatures of 85°F, 100°F, and 120°F to
produce a sludge more conducive to subsequent dewatering. The
study showed little change in sludge characteristics between 85°F
and 100°F. In either case the material was well digested but
difficult to elutriate and vacuum filter. Supernatant separation
was adequate only after very long detention periods. An attempt
was made to thicken a 4-percent digested, elutriated sludge in one
digestion tank, but after 25 days, the resulting 7 percent sludge
could not be vacuum filtered.
Operations carried out at 120°F, however, resulted in significant
improvements in sludge handling characteristics. First, there
was good solid-liquid separation in the secondary digester which,
in turn, improved vacuum filtration. This could improve overall
treatment plant performance where supernatant liquid is returned
to other plant processes. The solids concentration of the primary
digested sludge increased from about 3.64 to t.85 percent. Second,
the improvement in vacuum filtration performance was unprecedented.
The average filter yield increased from 1.7 to 6.3 pounds per
square foot per hour. At the same time the ferric chloride
flocculent demand was reduced from 6.5 to 3.1 percent. It was
suggested that this improvement was due in part to less sludge
"fines" (small particles) and the less gelatinous nature of the
solids. Vacuum filter media plugging became less of a problem.
A wet screen analysis showed that the percentage of particles
passing a 200 mesh screen decreased from 80 to 65 percent.
The thermophilic (120°F) cultures were established in 30 days
with seeding; once established the culture was quite stable and
resistant to upsets. Gas produced from high temperature digestion
had a higher-than-normal methane content; the protein in the
sludge was more completely digested; and the sludge nitrogen
content was less than normal. Doubling the solids loading of the
digester had little effect except to reduce the gas production
and volatile solids reduction a small amount.
-------
-70-
Figure 7.III
s •
-t
4
LEOCND
K - PRIMARY ft MODIFIED AERATION. I9SO-W8I
O - HIOM-RATE OIOESTIOM, l»62
40
46
SO
VOLATILE -»
66
•0
Relation of concentration of digested sludge to volatile content; primary and
modified aeration cludge.
(Reprinted by permission from Vol. 26, No. 4, p. 485,
April 1954, J. Water Pollution Control Federation)
-------
-71-
The problem of poor solid-liquid separation of anaerobically
digested sludge was discussed by Steffen wnd Lemen in relation
to meat packing wastesd^). At Albert Lea, Minnesota, a 20-inch
vacuum is applied to remove entrained gases from digester liquor
prior to separation of solids and liquid. This technique was
successful and allowed a 390 percent increase in the digester
loading because the separated solids are returned to the digester
as seed. A moderate dose of cationic polyelectrolyte further
increased the solid-liquid separation and resultant digester
efficiency^3).
Economics - Digesters are expensive, about $2.00 to $2.50 per
cubic foot(initial capital cost)but in many instances they are
easily justified because they convert obnoxious raw sludge to a
form suitable for disposal by relatively inexpensive methods.
For example, most small sewage-treatment plants digest sludge
because it permits sand-bed drying, lagooning or liquid land
disposal. Most large coastal cities digest sludge because nuisance
problems associated with ocean disposal are minimized. High-rate
digestion systems have significantly improved the economics of
this process for moderate to large facilities, but small plants
usually are still designed on a standard rate basis because of
the need for storage through winter months and other factors.
Any evaluation of competitive sludge handling processes should
consider all factors involved in the waste treatment system. On
this basis, digestion has certain disadvantages because it often
results in a poor-quality supernatant liquor which causes problems
when returned to other treatment units; it generates sludge "fines"
that increase dewatering costs; and it probably is the major
operational headache at most sewage treatment plants. A dollar
and cents value can be attached to these digestion problems.
Some specific case histories of favorable digestion economics have
been reported in the literature. Lynam and co-workers discussed
the cost of new digestion systems at the Chicago Sanitary
District(I58). Using a 3.5 percent sludge, Chicago's cost of
handling activated sludge by digestion and lagooning is $22 per
ton. Because sludges of other concentrations thicken and digest
differently, their disposal costs naturally vary a great deal as
shown in the following summary:
-------
-72-
Total Solids in Feed
£l H Hi I± M ll
Feed tons 17,820 26,800 35,850 1+5,000 54,000 72,400
per year
Lagoon
area in 108 162 216 271 325 435
acres
Total
annual
cost per $32.32 $24.03 $20.06 $17.51 $15.89 $13.77
ton
The figures include capital and operating costs for sludge thick-
ening by air flotation, digestion, pumping, and lagooning. Sludge
thickening and digestion accounts for about 53 percent of total
annual cost. Capital costs are based on 40-year amortization at
4% interest.
A comparison of the costs of high-rate and standard rate digestion
systems in two cities has been reported by Westchester County,
New Yorkd1*7). The capital cost at Yonkers for high-rate digesters,
two thickening tanks, and two storage tanks totaled $2.18 per
capita. At New Rochelle, the capital cost of four standard-rate
digesters was $5.86 per capita.
Estrada summarized actual cost differences between high-rate and
standard-rate digestion systems. He stated that considerable savings
are possible with high-rate designs due to the smaller size and de-
creased operational problems(154),
MacLaren estimated the cost in Canada for anaerobic sludge digestion
using the following criteria: (1) raw sludge volatiles of 70 percent,
(2) 30 days detention time, and (3) volatiles in the digested sludge
of 45 percent(53). For cities between 1,000 and 100,000 population,
annual capital charges are $9 to $14 per ton; operating costs are
$2 to $4 per ton of sludge treated. The total annual cost for
digestion alone is $11 to $18 per ton.
A general review of the literature indicated the cost of anaerobic
digestion by itself should be $5 to $18 per ton. Total overall
sludge handling costs for digestion and raw sludge systems are
discussed further in the Economics chapter of this report.
-------
-73-
Summary - In the majority of sewage treatment plants designed in
the past, anaerobic digestion has been included as a primary part
of sludge handling and disposal. The major justification for this
popularity is the fact that nuisance-producing materials are made
amenable to further disposal steps^15''. Digestion stabilizes
organic solids; odors are reduced; grease and other flotables can
be assimilated and digested; and pathogenic organisms seem to die
within 7 to 10 days of the start of digestion<17» 336).
The two other most frequently mentioned reasons for anaerobic
digestion are gas production and sludge volume reduction. However,
many local utility rates are so low that it is uneconomical to
install equipment to develop power from the digester gas^152).
Alternately, gas production may be sold to local industries as is
being done at some West Coast sewage treatment plants^150). Volume
reduction has been over-emphasized because, except for that result-
ing from gas production, it occurs only because of the formation of
supernatant liquid. Supernatant production is one of the many dis-
advantages of digestion. It imposes a high B.O.D. and solids load
on other treatment plant processes and the effluent receiving water.
Host engineers do not make allowances for recycled supernatant liquid
when they design clarifiers and biological units. These units are,
therefore, often exposed to a build-up of the fine supernatant
solids. As a result, treatment plant costs are higher than expected
and overall treatment efficiencies lower.
In addition to supernatant production, the other major disadvantages
of anaerobic digestion are: (1) the loss of nitrogen to receiving
streams, (2) the cost, (3) the creation of many operational problems,
and (*») the complication of sludge dewatering steps.
Nitrogen and phosphates remain in the supernatant liquid, eventually
fertilizing the receiving water. This ever-increasing fertilization
is a major problem in the water pollution control field. Where
sludge is dried and sold as fertilizer, the decreased nitrogen con-
tent reduces the value of the material. Digestion tanks are big,
expensive, and do not represent ultimate disposal. For these reasons
they are often omitted from the designs of many wastewater treatment
plants.
In regard to operational problems, digestion (and digesters) are
usually the operator's biggest headache, they require a lot of
attention. For example: digesters get "sick"; they foam; they re-
quire liming; grit removal is a major operation; gas production
falls off; odor problems arise because the sludge is incompletely
digested; scum blankets form; mechanical mixers corrode; and sludge
does not thicken.
-------
-7U-
Digested sludge is often dewatered mechanically. This step may be
preceded by elutriation to remove the undesirable by-products of
digestion that interfere with efficient dewatering (see the
Elutriation chapter). The resultant elutriate which is recycled
to the primary sedimentation basins, often contains a high concen-
tration of fine solids. These solids present the same problem as
supernatant solids. The two together (elutriate and supernatant
solids) create a major B.O.D. and solids load onto the other unit
processes. Some investigators claim that digestion alters the
solids water binding characteristics and makes dewatering easier
and less costly. On the other hand, digestion increases the
concentration of fine particles and the specific resistance to
filtration*i3'(see the Vacuum Filtration chapter for typical data).
In summary, the ultimate sludge disposal technique plus the size
and location of the waste treatment plant greatly influence the
decision of whether to use anaerobic digestion. Certainly small
plants and coastal plants will continue to digest because it pre-
pares the organic sludges for cheap final disposal. Many plants
not in these two categories must weigh local conditions and the
many technical factors involved with digestion. Mechanical dewater-
ing and incineration of raw sludge is becoming more popular, even
at small plants, in part due to design improvements. However, odor
control is a problem when handling raw sludge; this fact again
emphasizes the major justification for digestion — nuisance pre-
vention by sludge stabilization. High-rate digestion has improved
the economics of digestion considerably but it introduced other
problems such as the production of poor quality supernatant liquid
and a sludge that may be difficult to dewater.
Because anaerobic digestion will continue to be the most common
method of processing sludge for many years, new research is needed
to make the process more efficient. Additional analytical tools
would be desirable to control the process more accurately.
Measurements such as pH, volatile acids, bicarbonate alkalinity,
and gas production indicate the general state of digestion, but
may not give warning of failure in the system. Frequent analyses
and scientific interpretation of the basic data may allow an early
prediction of trouble, but better techniques are still needed.
Continuous analyses for methane, carbon dioxide, hydrogen sulfide,
and ammonia by gas chromatography could aid in controlling digestion
if the data were interpreted scientifically in conjunction with the
other basic data. Other measurable parameters have been suggested;
these include dioxyribonucleic acid (DNAjdSO)^ viscosity, and the
protein-carbohydrate-fat ratio. A simple test to indicate needed
-------
-75-
operational changes before a digester is fouled with poorly digested
sludge would be extremely valuable.
The design of a digestion system that is more efficient and stable
would be a great improvement. High-rate digestion with its related
vigorous-uniform mixing and sometimes concentrated feed is an
improvement but further improvements are desirable. A better defini-
tion of loading criteria including frequency of digester feed, would
be useful. Perhaps different biological cultures in different
digestion stages should be developed. Addition of enzymes and bio-
catalysts has been briefly explored but not in sufficient detail.
The addition of nutrients to the digester has been suggested to
increase the tank efficiency(157). Spohr examined the use of an
electric current to keep microorganisms in the low growth phase as a
means of more efficient digestiond^B). Garber and others concluded
that the advantages of thermophilic digestion far outweighed the
disadvantages and so it appears further research is justifiedd^O).
Tailoring the anaerobic digestion process to produce a sludge which
is easily dewatered in mechanical equipment or on sand drying beds
should be considered. This may require operating at uncommon tempera-
tures and detention periods as well as placing the analytical emphasis
on solids characterization.
Because digester supernatant liquid often reduces overall treatment
plant efficiences, improved techniques for its handling are desirable.
Pre-thickening of raw sludge (or raw sludge mixed with digested
sludge) ahead of the digester may be the most acceptable technique
because the supernatant liquor would be completely replaced by less
troublesome thickener overflow. Handling supernatant separately
rather than returning it to other treatment plant processes can be
done satisfactorily by a number of methods. Dewatering on sand dry-
ing beds, particularly after chemical conditioning, is an efficient
method of dewatering prior to ultimate land disposal (see the Sand
Bed Drying chapter). Supernatant liquid can be stabilized by aera-
tion, copper sulfate, or chlorine and then discharged to lagoons or
spread on land as a liquid fertilizer. Dewatering of supernatant
liquor in centrifuges prior to land disposal has been successful.
Treating supernatant with lime, separately or in combination with
elutriation basin overflow, is an interesting possibility because a
saleable fertilizer would result due to the nitrogen and phosphates
in supernatant liquids.
Other miscellaneous improvements that would be beneficial to digestion
operation include: (1) more efficient removal of the grit in digestion
-------
-76-
tanks, (2) tanks designed to facilitate cleaning, and (3) techniques
for degasifying sludge (such as vacuum techniques) to allow increased
solid-liquid separation in subsequent units.
Since the conversion to biodegradable (LAS-type) detergents, digester
problems have reportedly increased. A number of sewage treatment
plant operators believe these new detergents cause sludge to resist
digestion at normal efficiencies. Research to investigate this
situation is needed.
Without doubt, much more research in anaerobic digestion is justified.
Heavy metals toxicity has been extensively studied; if the same
amount of effort were directed at solving the problems discussed in
the preceding paragraphs, the operation of treatment plants would be
substantially improved.
-------
-77-
B. Aerobic Digestion
General - Aerobic digestion is not commonly practiced at sewage treat-
ment plants. However, in recent years, due to a rapid increase of
"extended aeration" treatment plants, aerobic digestion of sludges has
been receiving increased attention.
The major advantage of aerobic digestion is that it produces a
biologically stable end product suitable for subsequent treatment in
a variety of processes. Volatile solids reductions similar to
anaerobic digestion are possible with short detention periods.
Aerobic digestion followed by lagoons is often considered by industry
to be the most suitable technique for solids disposal.
This section emphasizes aerobic digestion as it applies to a separate
treatment unit after solids-liquid separation steps.
Theory - Aerobic digestion has been described as a process where micro-
organisms, "Obtain energy by auto-digestion of the cell protoplasm and
the biologically degradable organic matter in the sludge cells is
oxidized to carbon dioxide, water, and ammonia"^67")t Tne ammonia is
further converted to nitrates as aerobic digestion proceeds. After a
while the oxygen uptake rate levels-off, and a final material is pro-
duced that consists of inorganics and volatile solids that resist
further biological destruction. Loehr stated that oxidation in
aerobic digestion systems includes the direct oxidation of biodegrad-
able matter by organisms plus endogenous respiration or the oxidation
of microbial cellular material^168'. He concluded that endogenous
respiration is the predominant metabolic reaction in aerobic digestion.
Parameters - Some parameters affecting the aerobic digestion process
are: (1) rate of sludge oxidation, (2) sludge temperature, (3) system
oxygen requirements, («O sludge loading rate, (5) sludge age, and
(6) sludge solids characteristics.
Barnhart reported that volatile solids in a variety of domestic and
industrial sludges were substantially reduced after ten days aeration
(16H). Figure 7.IV shows that the volatile solids reductions for
all sludges except the mixed pulp and paper waste were quite accept-
able with increasing detention times.
Viraraghavan's laboratory studies using sewage sludge showed that
an almost straight-line relationship existed between volatile solids
-------
-7R-
O Blo-chMlcal
4
««W»y.
A Kixcd Dcwwctlc Sawmga
O T>uctil« * Do-..tic Ha.ta
I • *V • Solids reduction by aerobic digestion.
(Reprinted by permission Schools of Engineering,
Purdue University)
-------
-79-
reduction and detention time up to 15 days. Figure 7.V reveals
little further reduction of volatiles after 15
The volatile solids reduction in aerobic digestion approaches a
limit with increased detention periods; the exact limit depends
on the characteristics of waste fed to the system. Substantial
data from activated sludge systems indicates that a volatile
solids reduction limit of 40 to 60 percent can be expected when
treating domestic and industrial sludges*16**).
Laboratory research on aerobic digestion using a mixture of raw
primary and waste activated sludge was sponsored by Walker Process.
Figure 7. VI shows the effect of detention time and temperature on
volatile solids reduction as determined by this study(167)-
Volatile solids reduction increases sharply as the detention time
is extended to 12 days. Beyond that time only a moderate increase
was noticed. The curves show increased removal with higher sludge
temperature of from 15°C to 35°C.
In general, the data developed from the Walker Process studies re-
vealed the following effects of temperature*167); (1) at a
detention period of 60 days, temperature has no effect since
digestion was complete at all temperatures, (2) a minor tempera-
ture effect was noticed at the very short detention period of
5 days, and (3) the 10 and 30 day detention periods were notice-
ably influenced by temperature. Higher temperatures produced
greater volatile solids reductions.
The investigations of temperature by Reyes and Kruse have been
reported by Leohr C 168 ) . After aerobic digestion for 20 days, they
achieved sewage sludge volatile solids reductions of 25 to 35 per-
cent at 8°C and 67 percent at 60°C. One report concluded that
excessive sludge temperatures can be detrimental to subsequent
handling steps. Aerobic digestion of primary sludge was reported
to be more efficient at 35°C (mesophilic) than at 52°C (therroo-
philic)*168). Thermophilic oxidation produced a greater reduction
in ether solubles than mesophilic, but the oxidized sludge did not
settle and the sludge after drying had a fiberous character.
Loehr reported that sludge oxidation rates vary depending on the
sludge microbial population, the characteristics of the raw waste,
the sludge age, and the sludge temperature*16**). Old sludges have
been partially oxidized before aerobic digestion, therefore, the
volatile solids reduction is less than that of fresh sludge. The
-------
-80-
Figure 7.V
50
et
ui
B.
S
, •-
0 5 10 15 20 25 30
DETENTION TIME IN DAYS
Volatile solids reduction—effect of detention time.
(Reprinted by permission Water and Wastes Engineering)
Figure 7.VI
0.25
% Reduction ' I Volotil* Soli
VDiNG-"%jft ioy-Volatile Sc
24 32 40
DETENTION TIME- Days
(Reprinted by permission Walker Process Equipment, Inc.)
-------
-81-
work of Burton and Malina on loading rates was also reviewed by
Loehr^168). Their data revealed an increase in volatile solids
reduction at higher loadings. Volatile solids were reduced 43
percent at a loading of 0.14 pounds volatile solids per day per
cubic foot, and 34 percent at a loading of 0.10 pounds per day
per cubic foot.
Figure 7. VII shows typical oxygen utilization curves for aerobic
digestion* 164). The initial rate is high but it rapidly decreases
after the first day. Barnhart suggested that the utilization rate
is a function of the material stored in the cell at the start of
aerobic digestion^164). After 1 to 2 days the rate of oxygen
utilization decreases very slowly.
One theory applied to small extended -aeration treatment plants was
that prolonged aeration eventually oxidizes the sludge solids to
carbon dioxide and water, and no net sludge accumulation occurs.
But, it has been observed that 20 to 25 percent of the biological
solids produced is relatively immune to bio-oxidation and, there-
fore, accumulates in the
Numerous investigations of aerobic, digested-sludge drainage
characteristics have been made. In general, the data indicated
that sludge aeration for at least 10 days is required for good
drainage. After 5 days aeration, the drained sludge was of poorer
quality than undigested sludge, but after 10 days aeration, the
drained sludge was better than undigested
Design - Aerobic digestion has been applied mostly to various
forms of activated sludge treatment, usually "total oxidation"
or contact stabilization plants. However, aerobic digestion is
suitable for many types of municipal and industrial wastewater
sludges, including trickling filter humus as well as waste
activated sludges. Information on design criteria is not abundant,
but the technical literature contains figures that can be used as
limits. Any design for an aerobic digestion system should include:
an estimate of the quantity of sludge to be produced, the oxygen
requirements, the unit detention time, the efficiency desired, and
the solids loading rate<168>. The following limits, relating to
the above estimates were discussed in the literature:
-------
-82-
Figure 7-VII
2468
Tine of Aeration (days)
Tine of Aeration (days)
Oxygen utilization during aerobic digestion.
(Reprinted by permission Schools of Engineering,
Purdue University)
-------
-83-
Solids Loading Rates*167):
1. Primary sludge plus waste activated sludge
0.20 pounds total suspended solids (T.S.S.) per
capita per day.
2. Primary sludge plus trickling filter humus
a) high rate filters 0.19 Ibs. T.S.S./capita/day
b) standard rate filters 0.17 Ibs. T.S.S./capita/day
Air Requirement(167)«
15-20 cfm per 1,000 cubic feet of digester capacity is adequate.
The air supplied must keep the solids in suspension; this require-
ment may exceed the sludge oxidation requirement. Loehr recommended
a liquid velocity in the aeration tank of 0.5 fps to keep all the
biological solids in suspension*168). A dissolved oxygen concen-
tration of 1 to 2 ppm should be maintained in the aerobic digestion
tanks.
Power Requirement*167):
About 10 BMP (Brake Horse Power) per 10,000 population equivalent
is an estimate of power required for aeration.
Detention Time*167):
1. Waste activated sludge only, after sludge thickening.
10-15 days volumetric displacement time.
If sludge temperatures are much less than 60°F, more
capacity should be provided.
2. Primary sludge mixed with waste activated or trickling
filter humus.
20 days displacement time in moderate climates.
Two-stage digestion was evaluated and the conclusion was made
that it offered no advantages over one-stage aerobic digestion.
Tank Design* J-67);
Aerobic digestion tanks are normally not covered or heated,
therefore, they are much cheaper to construct than covered,
insulated, and heated anaerobic digestion tanks. In fact,
an aerobic digestion tank can be considered to be a large
-------
-84-
open aeration tank. Similar to conventional aeration tanks,
the aerobic digesters may be designed for spiral roll or
cross roll aeration using diffused air equipment. The system
should have sufficient flexibility to allow sludge thickening
by providing supernatant decanting facilities.
More detailed design considerations are discussed by Loehr(168).
He made the very significant observation that in secondary waste
treatment plants, the most economical sludge disposal system could
be one where the primary and secondary sludge is handled separately.
This could mean anaerobic digestion of primary sludge and aerobic
digestion of waste activated sludge. Loehr reported that the
oxygen supply should be increased almost nine times when aerobically
digesting primary with activated sludge, as opposed to waste
activated alone. This means a minimum air supply of 90 cfm per
1,000 cubic feet of aeration capacity. Sludge segregation should
certainly be considered.
Loehr cautioned that the aerobic digester should be designed so the
required degree of oxidation occurs during colder months(3.68). Be-
cause aerobic digestion tanks are normally not covered or heated,
the minimum volatile solids reduction occurs in the winter.
Performance - One of the most important aerobic digestion case
histories describes sewage sludge handling at the OSO plant in
Corpus Christi, Texas(!&'). Waste activated sludge, formerly
recycled to the primary sedimentation basins, upset many unit
processes: the primary tanks were odorous and inefficient due to
the high solids load; the aeration tanks were overloaded because
sludge did not settle well in the primary tanks; anaerobic
digestion capacity was limited due to the inclusion of waste
activated sludge; and the sludge mechanical dewatering and drying
processes were costly and odorous as a result of combining waste
activated with primary sludge.
After substituting an aerobic digestion system for the waste
activated sludge in place of recycling the following operating
improvements were noted: (1) the efficiency of the primary tanks
increased and less activated sludge solids were produced, (2) odors
from primary sedimentation basins and thickening tanks were
eliminated, (3) anaerobic digestion began operating smoothly on
primary sludge alone, and (4) the sludge drying time was greatly
reduced. The aerobic digester, operating with a 10 day detention
period, produced a stable sludge that was used on the treatment
plant lawns with no noticeable odor. Decanted supernatant liquor
had an average B.O.D. of 10 ppm.
-------
-85-
Dreier reviewed aerobic digestion performance at Batavia,
Illinois(161» 167). This city aerobically digested a mixture
of raw primary and waste activated sewage sludge prior to vacuum
filtration. Sludge fed to the digester averaged 4.75 percent
solids and that withdrawn for filtration, about 2.77 percent.
The operation included sufficient air to maintain aerobic conditions
and a decanting arrangement to thicken the sludge. Chemical costs
per ton of solids filtered at Batavia increased slightly above the
cost for undigested sludge, but the total economics of the system
improved due to sludge volume and labor cost reductions.
The sanitary district at Rockford, Illinois, evaluated aerobic
digestion of mixed primary and trickling filter humus(161, 167).
After 30 days detention, the sludge was placed in a lagoon in an
odor-free condition. Because anaerobic digesters were already in
existence at Rockford, they considered the possibility of following
anaerobic digestion with aerobic digestion to assure an odor-free
lagoon operation.
Aerobically digested sewage sludge from a typical contact
stabilization process treatment plant was reported to have the
following characteristicsC167):
pH 5.6
Alkalinity 283.0 ppm
Total solids cone, by decanting 2.76%
Volatile solids reduction 32.0% (48.8% volatiles
initially)
Nitrate-nitrogen 48.0 ppm
Ammonia nitrogen 1.75 ppm
B.O.D. of the supernatant liquor 16.0 ppm
ORP (oxidation Reduction Potential) 740.0 MV
Small "package plants" produce an aerobically digested sludge that is
discharged to lagoons, sand drying beds, and receiving streams
usually without causing nuisance problems. Many references indicated
rapid dewatering on sand beds was achieved after the sludge had been
digested for at least 10 days.
Carpenter and Blosser investigated aerobic digestion of waste
activated papermill sludges (boardmill and deinking)ll63).
-------
-86-
They arrived at the following conclusions from their study:
1. Little volatile solids reduction occurs after
27 days digestion.
2. There is a small increase in volatile solids reduction
with the addition of nitrogen and phosphorus to the
sludge.
3. The rate of solids decomposition is doubled with a
10°C rise in the temperature.
4. System oxygen requirements are:
a) before digestion, 9-12 ppm per hour per 1,000 ppm
volatile solids
b) after one day aerobic digestion, U-7 ppm per hour
per 1,000 ppm volatile solids
c) after two days to completion, 2.5-6 ppro per hour
per 1,000 ppm volatile solids.
5. Sludge aerated for long periods floated unless
degasified by a slight vacuum.
6. Aerobically digested sludge did not dewater as well as
raw sludge. Mechanical breakdown of the sludge floe
may be responsible for the unsatisfactory dewatering
characteristics.
Barnhart reported on the stabilization of thickened industrial
sludges with prolonged aeration^161*). He observed that subse-
quent solid-liquid separation steps such as thickening and vacuum
filtration could proceed normally. The volatile solids reductions
were similar to anerobic digestion so long as the temperatures
exceeded 20°C. Below this point, solids reduction rates decreased
rapidly. Other experimenters have agreed that low temperatures are
not acceptable. With the proper temperature, 15 days detention
time was acceptable in all cases studied.
Economics - Cost information for aerobic digestion systems is not
plentiful because the process is relatively new. In general,
however, the power cost for a treatment plant with large quantities
of sludge, and therefore a need for large volumes of air, is a
major disadvantage to the process. One engineer estimated that the
horsepower requirement in a biological treatment plant would be
doubled after adopting aerobic digestion. This is not a major con-
cern for small treatment plants, but it certainly would be for a
large facility.
-------
-87-
Hhen compared with anaerobic digestion, the capital cost for
aerobic digestion is much lower because the tanks required are
smaller and less costly to construct per cubic foot. Duquesne,
Pennsylvania, for example, constructed an aerobic digestion
facility at a cost significantly less than that possible with
anaerobic digestion^1»0)t They produce a stable odorless sludge
after IH-days aeration of a gravity thickened material.
Summary - Many municipal and industrial wastewater treatment
plants practice aerobic digestion. Technical data from these
operations are not plentiful which, unfortunately, encourages
the idea that aerobic digestion is a new, unproven technique for
solids handling. But, aerobic digestion has some important
advantages that justify attention by researchers and design
engineers.
The advantages most often claimed for aerobic digestion are:
1. A humus-like, biologically stable end product is
produced.
2. The stable end product has no odors, therefore, simple
land disposal, such as in lagoons, is feasible.
3. When compared with anaerobic digestion and other schemes,
capital costs for an aerobic system are low.
1. Aerobically digested sludge usually has good dewatering
characteristics. When applied to sand drying beds, it
drains well and redries quickly if rained upon.
5. Volatile solids reduction equal to anaerobic digestion
is possible with aerobic systems.
6. Supernatant liquors from aerobic digestion have a lower
B.O.D. than those from anaerobic digestion. Most tests
indicated that B.O.D. would be less than 100 ppm. This
advantage is important because the efficiency of many
treatment plants is reduced as a result of recycling high
B.O.D. supernatant liquors.
7. There are fewer operational problems with aerobic
digestion than with the more complex anaerobic form
because the system is more stable. As a result, less
skillful labor can be used to operate the facility.
8, In comparison with anaerobic digestion, more of the sludge
basic fertilizer values are recovered.
-------
-88-
The major disadvantage associated with aerobic digestion is, as
mentioned earlier, high power costs. This factor is responsible
for the high operating costs in comparison with anaerobic digestion.
At small waste treatment plants, the power costs may not be
significant but they certainly would be at large plants. Some
investigators have observed that aerobically digested sludge does
not always settle well in subsequent thickening processes. This
situation leads to a thickening tank decant having a high solids
concentration.
Some sludges do not dewater easily by vacuum filtration after being
digested aerobically(163). Two other minor disadvantages are the
lack of methane gas production and the variable solids reduction
efficiency with varying temperature changes.
Aerobic digestion may be particularly suitable for industrial sludge
treatment and to sludge at small, activated-sludge plants. The
industrial community apparently favors aerobic digestion because of
the low capital investment and simple operation. In industry,
mechanical aerators are often used in inexpensive open tanks followed
by lagoons. While there is a difference in emphasis at municipal
waste treatment plants as regards costs, it seems logical that
aerobic digestion should be further evaluated, particularly for
activated sludge facilities.
The experience at Corpus Christi is significant because a difficult
problem was solved by handling secondary sludge separately from the
primary sludge. Corpus Christi's problem was certainly not unique.
Biological sludges often upset unit process operations and lower
the overall treatment plant efficiency. Separating this sludge, as
they did at Corpus Christi, and digesting it aerobically could be a
good solution to a difficult problem. It permits more efficient and
economical operation of primary sedimentation basins, anaerobic
digestion tanks, and vacuum filters because the gelatinous biological
sludge is removed from the system. Usually it is more economical to
combine sludges prior to dewatering and disposal; but, where difficult
sludges are encountered, separate treatment may be desirable.
Certainly more technical information on aerobic digestion is needed
for a proper evaluation of the process. Acquiring additional data
from existing systems would be a desirable first step. A consider-
able amount of new research in the process and in engineering design
should be accomplished to improve existing technology. Very little
information is currently available concerning loading rates, air
requirements, rate of sludge oxidation, the effects of varying sludge
characteristics, sludge amenability to subsequent handling and dis-
posal steps,and cost-performance. Aerobic digestion will not be
routinely included in sludge treatment evaluations by consulting
engineers until more data is collected and disseminated.
-------
-89-
8. ELUTRIATION
General - Elutriation can be defined as a washing operation which
removes sludge constituents that interfere with thickening and
dewatering processes. This unit operation is usually associated
with vacuum filtration of digested sewage sludge. Elutriation
reduces the flocculent demand of a sludge by improving the physical
and biochemical quality of its solid and liquid components. In
addition to reducing the chemical conditioning required prior to
filtration, elutriation has also been proven to be a useful sludge
thickening device at numerous sewage treatment plants. Other
"secondary" applications of elutriation include washing-out toxic
materials that inhibit sludge digestion or other biological processes,
and the treatment of dirty digester supernatant liquor^177). Toxic
materials should be removed from wastewater before discharge to a
sewerage system but, if not, removal is possible by elutriation^5).
Elutriation of digester supernatant liquor permits the capture of
the solids and return of relatively clear supernatant liquor to
primary sedimentation. The captured solids are stored and dewatered
with the normal plant sludge(177).
Unfortunately, elutriation has not been a completely successful
process at all locations. A major operational problem has developed
in many plants due to poor solids capture in elutriation basins.
The high concentration of fine solids in the recycled elutriate at
these plants overloads other processes, thereby decreasing the
overall plant efficiency.
This chapter discusses elutriation as part of the following subject
headings: (1) sludge alteration, (2) process design and operation,
(3) chemical reduction, (U) sludge thickening, (5) chemical aids,
(6) economics, and (7) evaluation and summary of the process.
Sludge Alteration - Digestion substantially reduces the organic
fraction in sludge solids while increasing biochemical products
in the liquid fraction. Ammonium bicarbonate is the most
important of these biochemical materials. It is quite common to
have the bicarbonates present in the free water of raw sludge
increased at least sixty times during digestion of the solids*177'.
The flocculent demand exerted by a sludge to be dewatered consists
of a solids demand and a liquid demand best expressed by the
alkalinity.
Center defined elutriated sludge as one, "That has had the alka-
linity of its biochemically fouled water reduced by dilution.
sedimentation, and decantation in water of lower alkalinity11^21'.
-------
-90-
The washing operation removes excessive concentrations of soluble
ammonium and other compounds while fractionating the solids
allowing effective settling of coarse particles. It also washes
out adsorbed gas bubbles developed in the digestion process.
Fine and colloidal solids are not recovered. This is an advan-
tage as regards vacuum filtration because fines consume a
disapportionate amount of flocculents and clog the filter media.
However, recycled elutriate having an excessive concentration of
fines overloads other plant processes.
Elutriation produces a sludge with fairly uniform flocculating
and dewatering characteristics. The bulk of the fines and
alkalinity are removed along with the nitrogen compounds and
entrained gas.
Process Design and Operation - Three common methods are used in
elutriation operations^).Figure 8.1 illustrates the three
procedures. The simplest method is the single stage with a single
contact between the solids and liquids. Multistage cocurrent
contact in one or more basins and multistage countercurrent contact
are the other two methods commonly used. "Fresh water" is used in
all stages of the cocurrent method. In a two-tank countercurrent
system, fresh water is added only to the second stage washing.
Second stage elutriate, or overhead water, provides the necessary
wash in the first stage.
Elutriation operations may be continuous in one or more units,
intermittent in one unit, or they may be operated on a batch
basis. Progressive designers and operators usually reject fill-
and-draw batch operations in wastewater treatment but this
technique permits a high solids capture. After the basin is
allowed for thorough subsidence and concentration of the washed
solids. The relatively clear elutriate is then decanted and the
sludge is pumped to the next solids handling process.
Wash water for elutriation may be plant effluent or water from
nearby streams or wells. Ratios of wash water to sludge usually
fall within the range of 2:1 to 12:1. The most common is 2:1 or
3:1. Factors affecting the operating ratio include sludge
alkalinity, desired alkalinity of the washed sludge, wash water
alkalinity, the sludge handling process following elutriation,
and the availability of water and elutriation basin capacity.
Center used the following equations to calculate the alkalinity,
solid and liquid flocculent demand, of elutriated sludge^21);
-------
-91-
Figure 8.1
Singto
vc
(a)
<«
*» r
ft
Stag* *<
1
ri/wvo
ya
V,
n
i Stag*
^ 2
Jv.
f -
M
v.
f
*"-*r Stag*
n
,J,
(ft)
n
V,
*»-!
Flow diagram! illuatrating rarioua arrangement employed in dia-
peraed-oontaot leaching operation!, (a) Single contact (6) Multiatage oocurrent
contact (c) Multiitage countorourrent contact
(Reprinted by permission from Unit Operations of Sanitary
Engineering, by L. 0. Rich, Copyright 19bl, New York,
Jonn Wiley and Sons, Inc.)
-------
-92-
D •+• RW
(1) E = R + ^ for single-state elutriation,
D -4- W /?R+1 V* 1 7
(2) E = * " for n-st»8° elutriation, and
2-f.p ^
'
R* + R + 1 two mixing and settling tanks.
-f.
p. - D + (W) (R ' for countercurrent elutriation in
~
E = alkalinity of the elutriated sludge
D = alkalinity of the unelutriated sludge
R = volume ratio of the wash water to the sludge to be
elutriated
W = alkalinity of the wash water
n = number of times sludge is washed
Sludge and wash water can be mixed in pipelines feeding the
elutriation basins, in separate mixing chambers, or in the basin
itself. The primary mixing requirement is to provide intimate
contact of the water and sludge. Usually 20 to 30 seconds of
vigorous mixing is adequate.
Elutriation basin overflow rate and solids detention time are
critical design parameters if maximum solids capture and sludge
solids concentration are required. Experience at New York indicated
that overflow rates were less than UOO gpd per square foot^^l).
Because poor solids capture is such a common problem, rates nearer
200 gpd per square foot seem more reasonable. MacLaren(53) recommended
solids loading rates of 8 to 10 pounds of dry solids per square foot
per day, but others believe 10 to 15 pounds per day per square foot
is satisfactory. The optimum rates for any specific location depend
on the sludge characteristics and subsequent dewatering processes.
Because elutriation is normally followed by vacuum filtration of the
sludge, maximum solids concentration is a worthwhile goal in the
design of elutriation basins. Sparr reported that 12 hours should
be the absolute minimum detention time but 2H hours is preferable^170).
Data from New York confirmed Sparr's statement (171). The New York
data were based on detention of the solids in the sludge blanket, not
the liquid detention period. Sludge blanket depths at New York were
limited to less than 3 feet; this appears to be a good operational
procedure for most sludge handling operations.
-------
-93-
Genter believed that the concentration of volatile solids in the
sludge to be elutriated was an important design consideration.
The New York data by Torpey and Lang illustrated this point (17D.
Figure 8.II shows significantly lower solids concentrations being
attained with increasing percentages of volatile solids. Lower
solids loading rates were necessary to compensate for the increased
volatiles.
The design of elutriation systems is very important. Hany of these
systems operate inefficiently and thereby decrease overall plant
performance. The effective separation of the sludge particles and
the water used to wash the sludge is difficult'66'. To accomplish
a good separation, tanks must be loaded at low rates,inlet velocities
must be slow, tanks should be baffled and effluent weir loading
rates must be small. Akron, Ohio, secured significant increases in
elutriation efficiency by: (1) lengthening the tanks U5 percent,
(2) installing a longer overflow weir, and (3) adding scum collectors,
These changes were responsible for a reduction in the concentration
of elutriate solids from one percent to one-tenth percent'178^. A
maximum weir loading of 5,000 gallons per foot per day is often
recommended.
Chemical Reduction - The primary justification for the elutriation
process is to remove digested sludge constituents that inhibit
chemical flocculation before sludge dewatering by vacuum filtration.
Elutriation appears to be the simplest method of reducing the
flocculent demand exerted by the liquid portion of the sludge.
Various authors have stated that elutriation eliminates the need for
lime completely and reduces the ferric chloride dosage by 50 to 80
percent. Often, however, when reviewing performance records the
requirement for lime was not completely eliminated by elutriation
and the ferric chloride dosage was reduced less than 50 percent.
Center's diagrams in Figure 8.Ill show the relative flocculent
demand by the solid and liquid portion of the sludge for different
types of sludges: (a) primary, (b) primary and filter humus, and
(c) primary and waste activated^21). According to Center's data,
digestion lowers the solids demand and elutriation lowers the
liquid demand.
Figure 8.IV shows the relative dose of ferric chloride for varying
degrees of elutriation at Washington, D. C.^175\ Obviously,
elutriation effectively reduces the requirement for ferric chloride.
McNamee conducted some tests to prove that bicarbonates are
responsible for the high ferric chloride dosages associated with
-------
-94-
Figure 8.II
VI
1°
6 6 7 8 0
LOADING RATE - ti»./IO.»T./o*v
10
Relation of loading rate to concentration for various per cent
volatile aludges.
(Reprinted by permission from Vol. 24, No. 7, p. 8l9»
July 1952, Sewage and Industrial Wastes)
-------
-95-
Flgure 8.Ill
Quantity of ferric chloride required for vacuum filtration of various
domestic sludges.
(Reprinted by permission from Vol. 28, No. 7, p. .836,
July 1956, Sewage and Industrial Wastes)
-------
-96-
Figure 8.IV
NOT ELUTRIATED
J I VOL WATER
3 3 VOL. WATER.
5 VOL. WATER
15 VOL. WATER
Q 63 VOL WATER.
Relative doses of ferric chloride for varying degrees of elutriation.
(Reprinted by permission from Vol. II. No. 9, p. 766,
September 1939* Sewage Works Journal)
-------
-97-
digested sludge^175). After filtration tests on washed sludges to
which ammonium salts and bicarbonates were added, he concluded that
the amount of ferric chloride needed to condition sludge is related
to the bicarbonate concentration and is influenced very little by
the ammonium ion. McNamee and others believe lime as a sludge con-
ditioning agent is of little value in dewatering elutriated sludge.
Elutriation is particularly valuable where the digested sludge
contains some waste activated sludge. This sludge, if unelutri-
ated, has an extremely high flocculent demand. At the Los Angeles
Hyperion sewage plant, the digested primary and waste-activated
sludge had to be elutriated before normal operation. The chemical
cost without elutriation was $20 per ton and with elutriation, $4
per tond80). No one questioned the ability of the elutriation
process to decrease the requirement for chemicals. There is, however,
the question of the importance of washing out "fine" solids to
reduce the chemical demand. Many people believe that the chemical
demand is reduced primarily because there are less small sludge
particles to condition. These uncaptured particles often end-up in
the treatment plant effluent^180*.
Sludge Thickening - While the elutriation process was developed for
chemical reduction in vacuum filtration, it is also an effective
sludge thickening technique. Elutriation is successful as a thicken-
ing process because it washes out gas bubbles formed during sludge
digestion, thereby lessening the buoyant effect on the solids. It
also washes out the fines and colloidal material that interfere with
sludge concentration^65^. In most sludge thickening operations, it
is advantageous to dilute sludge particles in order to promote
settling and greater solids compaction. Also, the detrimental
effects of septicity on concentration are reduced when sludge is
diluted with well aerated liquid.
Torpey and Lang concluded that single-stage elutriation was as
effective in sludge concentration as a secondary digester having
twelve times the volume of an elutriation basin*171'. At New York,
digested primary sludge was thickened from 2 to 3 percent solids
to 3.9 to 7.4 percent solids. The exact figure depended on the
sludge volatile content and the basin loading rate.
Intermediate or interstage elutriation has been described by Torpey,
Lang, and Kennedy(65b, 1/1). This process involved the elutriation
of sludge being transferred from primary to secondary digesters.
The advantages were described as follows:
-------
-98-
1. It increased the effective capacity of primary
and secondary digesters.
2. It reduced the B.O.D. of digester supernatant liquor,
therefore, easing disposal problems.
3. It simplified digester operation.
•». It reduced the chance of digester foaming.
5. It reduced the volume of final sludge thereby
saving sludge bed and lagooning space.
At New York, interstage elutriation of digested primary and modified
aeration sludge reduced the sludge volume by 30 percent, compared
to conventional primary-secondary digestion(176). A U.9 percent
primary digester sludge was washed 4:1; the result was a secondary
digester sludge with an average solids concentration of 6.4 percent.
There was a loss of digester gas production equal to 10 percent of
the normal volume. The New York officials concluded that a 20
percent reduction in sludge volume would justify interstage elutri-
ation .
The San Francisco Richmond-Sunset plant thickened primary digester
supernatant solids from 1.5 to 4 - 5 percent in elutriation basins
before the solids were pumped to secondary digesters(65b)< This
technique, called interstage elutriation, increased the final solids
concentration by about one-third.
A number of plants that elutriate secondary digester sludge have
reported substantial increases in solids concentration. For example,
the operating records from the Washington, D. C. sewage treatment
plant showed an average increase from 5.7 to 8.9 percent.
Kennedy advised engineers to give special consideration to the
design of elutriation basins if sludge thickening is to be a primary
goaltSSb'. Sludge collection equipment and the storage hopper areas
of tanks should have extra capacity. Also, the problem of pumping,
thickened, elutriated sludge needs special attention, particularly
for sludges exceeding a solids concentration of 8 percent.
Chemical Aids - Elutriation solves the high-cost problem of vacuum
filtration, but often causes another due to elutriate solids recycle.
Recycling uncaptured elutriate solids can overload aeration
facilities at activated sludge plants to the point where the overall
plant performance is seriously affected. One way to solve this
problem is to enlarge the physical facilities so that the solids and
hydraulic loadings in the elutriation basins are reduced. Another
way to increase the solids capture, thereby improving the elutriate,
is to chemically flocculate the fines. The normal procedure would
-------
-99-
be to add a flocculant to the wash water or sludge-wash water
mixture. A less effective method is to return filtrate containing
some residual flocculent from vacuum filtration.
The following data from one large activated sludge plant describes
possible advantages from the use of an anionic polymeric flocculent
(dose of 1.6 pounds per ton) to treat elutriation basin
Sludge Type - digested primary and activated
Before After
Flocculent Use Flocculent Use
Elutriated Susp. Solids 3,835 mg/1 365 mg/1
Solids Capture 65.1% 95.3%
Underflow Solids Cone. 3.5% 4.3%
Goodman has also reported improved elutriation basin performance
after conditioning basin feed with polymers^203). Without chemicals,
the zone settling rate was 5 inches per hour. With a chemical dose
of 3.5 pounds per ton, the rate was 10 inches per hour and at 7
pounds per ton, 17 inches per hour.
Economics - Many observers agree with Center that the economy of
dewatering digested sludge by vacuum filtration is enhanced by sludge
elutriation^172). There is no doubt that it reduces the cost of
chemicals. HacLaren, however, pointed out that elutriation basins
significantly increased capital costs; he estimated a cost of $2 per
ton of dry solids based on a 30 year amortization at 5%(53). Average
operating costs were estimated to be 75 cents per ton of dry solids.
The total annual cost is, therefore, a little less than $3 per ton.
Yet, data from many sources showed that chemical cost reductions
resulting from elutriation substantially exceeded $2 per ton. Con-
sidering that lime is usually completely eliminated and ferric
chloride is reduced from 50 to 80 percent, it seems easy to justify
the economics of elutriation. In addition to the direct chemical
cost reduction resulting from the elimination of lime and some
ferric chloride, there are the added benefits of less equipment
maintenance, improved incinerator performance, descreased quantity
of incinerator ash to be disposed, elimination of ammonia odors,
and less chemical handling. In terms of dollars these added
benefits can be very important. But, any economic evaluation must
also consider the added cost of treating recycled elutriate.
-------
-100-
Evaluation and Summary - The elutriation process has been proven to
be a useful tool for reducing the cost of dewatering digested sludge
by vacuum filtration. In fact, in the case of digested sludge con-
taining waste activated sludge, it may be a necessary process if
reasonable dewatering costs are desired. Since chemical condition-
ing is a major cost and nuisance factor in the vacuum filtration of
sludge, elutriation can be used to minimize these factors in sludge
conditioning.
Elutriation can also thicken digested sludge by diluting and degassing
solids. Thickening, of course, reduces the cost of subsequent sludge
disposal processes, such as barging and lagooning. Using inter-
stage •lutriation between primary and secondary digesters,
elutriation results in increased digester capacity and improves the
supernatant liquor. Elutriation of dirty digester supernatant
liquors can break the cycle of suspended solids recirculation in a
treatment plant(172).
Treatment plants reporting operational problems with elutriation
usually complain about poor recovery of fine solids. At Los Angeles,
i*0 percent of the elutriated solids escaped to the ocean^180). This
could have caused a serious problem if the ocean had not been avail-
able for elutriate disposal. A gradual build-up of recycling
elutriate solids can jeopardize the efficiency of activated sludge
processes by exerting excessive requirements for oxygen. At New
York, the elutriate has contained from 5 to 30 percent of the solids
applied*171>.
The problem of losing solids in the elutriate has been solved in
three ways. First, additional elutriation basins have been con-
structed to decrease the solids and hydraulic loading rates to a
level conducive to good solids capture. Second, flocculents have
been used to agglomerate the fine solids causing them to settle more
effectively. Third, the elutriation basins have been operated on a
batch basis rather than a continuous basis. Batch operation produces
a much better elutriate but it sometimes is not feasible due to
limited facilities or labor. The importance of the solids demand in
relation to the liquid demand in a sludge chemical conditioning
process is shown when one of the above techniques is used to increase
the capture of fine solids. Chemical conditioning costs increase
greatly due to the flocculent demand exerted by the fine solids
washed out of the elutriation basin.
Another disadvantage of elutriation results from the washing out of
nitrogen compounds in the sludge. This decreases the value of the
-------
-101-
sludge as a fertilizer and ultimately results in a higher concentra-
tion of nitrogen compounds in the treatment plant effluent. Increased
fertilization of the receiving water is a major water pollution
control problem.
Many consulting engineers do not consider elutriation when designing
new waste treatment facilities. They believe the loss of solids in
the elutriate is unavoidable and, therefore, the process is unsatis-
factory even if high chemical costs for mechanical dewatering are
required as a substitute. Better process design and operation or
the use of chemicals can solve the major operational disadvantage of
elutriation—poor solids capture, but this would eliminate what may
be the major function of the process — washing out fine solids.
Center(172) over-emphasized the importance of the liquid flocculent
demand. Capturing these solids in the elutriation tank would,
therefore, destroy the objective of the process.
Perhaps the best solution to the above dilemma is to treat the
elutriated solids separately or in combination with solids contained
in digester supernatant, thickening tank overflow, filtrates, and
centrates. For example, these solids could be dried on sand dry-
ing beds. Another possibility is to add lime to the combined liquid
wastewater and sell the mixture as a liquid fertilizer. This and
other new approaches should be evaluated soon.
-------
-102-
9. LAGQONING - LANDFILLING
General - Lagooning is the most popular sludge disposal technique
at industrial wastewater treatment plants. It is also a very
popular method for disposal of sewage sludges and water treatment
plant sludges. Lagoons may be natural or artificial depressions
in the ground. They may be used for either digested or undigested
sludge. However, using lagoons to store raw organic sludges such
as sewage sludge is rare due to problems of odor and insect breed-
ing. Lagooning may be considered as a stage process in the
handling of sludge or as a final sludge disposal process. Some
lagoons are used only in emergency situations when other sludge
handling processes are temporarily overloaded or out-of-service.
Because lagooning is cheap and simple, it will continue to be a
popular sludge handling and disposal technique until land becomes
so valuable that space limitations force the use of another
process.
Landfills are used as final disposal sites for dewatered sludges
of all types. If the distance from the treatment plant to the
disposal area is not too great, landfilling can be a relatively
inexpensive disposal technique.
Class ifications - Lagoons may be divided into three classes^70):
(1) thickening, storage, and digesting lagoons; (2) drying
lagoons; and (3) permanent lagoons.
The first type of lagoon is used for thickening, storage, and
digestion of sludge when mechanical thickeners, storage tanks,
and conventional digestion units are overloaded or sometimes
as substitutes for conventional processes. When used as a
substitute, there should be multiple units and equipment for
decanting the overhead liquid and returning it to the head of
the treatment plant. Digestion in a lagoon can be a lengthy
process and one that creates multiple nuisance problems. After
digestion is complete, the sludge can be discharged to sand
drying beds for dewatering.
Drying lagoons are used as substitutes for sand drying beds. The
sludge is periodically removed and the lagoon refilled. Many
months are required to dry the sludge sufficiently for removal,
so multiple units must be provided. Two lagoons could be operated
in series, the first receiving the heavier solids while the
second receives the lighter solids decanted from the first. The
-------
-103-
sludge may not dry to less than 70 percent moisture, but at that
point, it can be removed by mechanical excavation(70). After
removal, the dried sludge can be used as a fertilizer or soil
conditioner.
A permanent lagoon where the sludge is never removed, or at
least not removed for many years, is one of the cheapest methods
of sludge disposal. Many plants, both large and small, use this
technique. It is obvious that large and inexpensive land areas
are required to justify this method rather than other sludge
disposal methods. A supernatant liquor decanting system is
recommended to optimize the usefulness of permanent sludge lagoons.
Parameters and Design - Some of the parameters related to lagoon-
ing are:(1) land area, (2) climate, (3) subsoil permeability,
(H) lagoon depth, (5) lagoon sludge loading rates, and (6) sludge
characteristics.
Lagoons are often constructed in areas where the soils are porous
unless contamination of the ground water is a threat. Many states
require that the lagoon bottom is at least 18 inches above the
maximum water table level in order to prevent contamination'58).
If permeable sandy or gravel-type soils are unavailable, under-
drains may be constructed to facilitate removal of the drainage
water. Some observers, however, believe that under-drains do not
benefit the drying process.
The design of lagoons should provide for uniform distribution of
the sludge when applied and for an easy method of removing the dry
solids. MacLaren recommended a discharge system that limits
sludge travel to 200 feet. He also recommended diked embankments
with a 1:2 slope on the exterior side and 1:3 on the interior to
prevent erosion^53). The width of the embankment at the top should
be sufficient to allow vehicle transport during the cleaning opera-
tion. Again, provisions for supernatant decanting should be
provided to promote drying. Decanted supernatant can be returned
to the treatment plant influent.
Jeffrey made some interesting observations about lagooning based
on laboratory studies relating sludge drying to drainage,
evaporation, and transpiration(205, 216). He observed that dry-
ing caused by drainage is independent of lagoon depth, at least
during the first 20 days, and does not produce a sludge
sufficiently dry for easy removal. Therefore, evaporation or
-------
-104-
transpiration is necessary for satisfactory lagooning. Areas where
climatic conditions encourage evaporation are particularly conducive
to lagoon operations.
Jeffrey also conducted a laboratory study which related lagoon depth,
soil permeability, and dewatering rates(216). He loaded a "lab
lagoon" to 18.7 feet with 8 percent sludge; it dewatered to 3.7 feet.
The study concluded initial dewatering rates were affected by
supporting media permeability. However, after a short time, the
rate decreased and the effect of the support media was insignificant.
Solids loading criteria for lagoons have been recommended by
VanKleeck<8>:
(1) Raw sludge 6 Ibs/yr./cu. ft.
(Using lagoon as a digester) of lagoon capacity
(2) Digested sludge 2.2 to 2.4 Ibs./yr./cu. ft.
(Using lagoon for dewatering) of lagoon capacity
Others have prescribed a loading rate of 500 tons per acre^68).
Jeffrey quoted loading rates for lagoons in Canada as being 1.8U
pounds of dry solids per square foot of lagoon per 30 days of
bed use. In this case, the sludge was removed after 18 months'^16).
He also reviewed the Iowa City experience where the loading rate
was 1.7 pounds of dry solids per square foot per 30 days of bed use.
Lagoon performance is affected by rainfall and temperature so these
factors should be considered in the design and operation.
Operation - The standard operating procedure for lagoons is to
discharge digested sludge to the lagoon at regular intervals as
determined by the solids accumulations in digesters^20.5). Jeffrey
proposed a 3-year cycle for lagooning digested sludge^216'. The
lagoon is loaded through one year; it dries for 18 months and is
cleaned; and finally the supporting media is "rested" for six months.
In northern climates this operation would start in November. Three
lagoons are necessary for operational flexibility.
When the lagoon is used for dewatering and not permanent storage,
MacLaren recommended filling to a depth of from 2.5 to 4 feet,
(the greater depths in warmer weather)(53), He believed the best
filling practice was to add one foot of sludge, switch to another
lagoon to permit some drying in the first unit, and then add the
-------
-105-
remaining amount to the first lagoon. A 4-foot depth provides 2 to
3 years' capacity, assuming one wet year in that period. This schedule
is equivalent to a loading of 40 pounds dry solids per square foot per
year. Bubbis suggested a similar schedule: filling lagoons with 10
inches of well digested sludge, allowing it to settle for 14 days,
decanting the supernatant liquid, and repeating the process until the
lagoon is full of sludge<204).
Eventually the "dried" sludge must be removed from the lagoons or
new lagoons constructed. Sometimes if natural low-lying land is
filled by sludge lagooning, removal is not necessary because the
objective may be land reclamation. MacLaren reported that minimum
cleaning costs for Winnipeg lagoons are achieved by using rippers
during winter months and removing the dislodged sludge by earth-
moving equipment(53). Dredging the top layer of sludge by dragline
after a 3-month drying period speeds up the dewatering of lower
sludge layers^70). Berger reported that paper-mill sludge lagoons
require cleaning or expansion every 3 to 5 years(51).
Dried sludge removed from lagoons is landfilled or used as a soil
conditioner on parks or other areas.
Performance - Sewage sludge stored in lagoons may be dewatered from
about 95 percent moisture to 55 or 60 percent moisture in a 2 to 3
year period^53). In England sludge dewatering to less than 70 percent
is rare(?0). Many large cities have used sludge lagoons satisfac-
torily for many years. These cities include Chicago, Philadelphia,
Dallas, Fort Worth, Akron, and Toledo. The literature contains very
little performance data from either large or small treatment plants,
describing the efficiency of lagoons.
This lack of information may reflect the fact that lagoon operations
can be simple, economical, and satisfactory to all concerned. How-
ever, lagoons are not a panacea. The following sections discuss
economics and the pros-and-cons of sludge lagoon operations.
Economics - Lagooning of sludge is popular because it is often
cheaper than any other dewatering or disposal technique. Most
industrial sludges are lagooned for this reason. For example,
industrial sludge is frequently inorganic and, therefore, creates
a minimum of odor; thus, a sludge digestion step preceding lagoon-
ing is not required. Also, many industrial sludges, such as oil
and metal finishing sludges, are lagoon-dried because they are
difficult to dewater by vacuum filtration.
-------
-106-
MacLaren stated that lagoons can be constructed at an approximate
cost of $12,000 per acre or $1.10 per ton per year, based on 30-
year amortization at 5% interest (53). This cost included land
and all piping. He also stated that cleaning costs range from
$1,000 to $2,000 per acre per year depending on ultimate sludge-
disposal points, or $1.20 to $2. 40 'per ton of dry solids applied
to the lagoon. Berger reported that lagoon cleaning costs for
paper mills were $2.00 to $3.00 per ton of dry solids applied^51).
Because these lagoons are undeveloped land depressions , this
figure is an estimate of the total sludge disposal cost. The
cost of excavation and diking has been about $0.48 per cubic
yard for southern paper mills. Lagoon drying at Winnipeg cost
$0.80 to $1.00 per ton, excluding lagoon maintenance (2Q1O.
In Green Bay, Wisconsin, 20 acres of lagoons were constructed at
a cost of $50,638 (or $2,530 per acre) in 1960. It was estimated
that 25 years will be required to fill the lagoons , so operating
costs (cleaning) are
Howe 11s and Dubois studied the cost of sewage stabilization ponds in
the midwest' 206). Representative costs of sludge lagoons are shown
in Figure 9.1. Multiplying the cost from the graph by a factor of
1.2 accounts for the increase in the ENR Cost Index. Caron reported
that sludge disposal in shallow lagoons costs between $1.00 and
$3.00 per ton depending on hauling costs and final sludge
moisture* ^).
An average lagoon capital and operating cost ranges from $1.00 to
$3.50 per dry ton of solids handled. These costs include an
assumption that lagoons are near the wastewater treatment plant.
If the sludge has to be piped long distances , the costs would
naturally be greater.
The operating cost for sanitary landfills is generally considered
to be $0.50 to $2.00 per ton(3, 69, 362). Average capital costs
are about $1.00 per ton. These figures appear very low but haul-
ing costs of the dewatered sludge determine whether landfilling
can be economical. If they exceed $5.00 per ton, incineration of
the sludge may be a cheaper means of disposal.
Summary - Lagoon disposal of well digested sewage sludge and many
industrial sludges is a very common practice basically because it
is economical. In addition, lagoons have the advantages of
simplicity of operation, flexibility, and suitability for plants
-------
-107-
Figure 9.1
4000r
3000
I
I
§2000
8.
3
WOO
E>
• 12
Juf'ac* An* (Acm)
20
24
-Stabilisation pond costs in Missouri based on a statewide
average cost of excavation and adjusted excavation quantities.
(Reprinted by permission JWPCP, Vol. 31* No. 7*
p. 815, July 1959)
-------
-108-
of any size. Sludge disposal to lagoons can be a regular operational
procedure or used temporarily during peak-load periods or when other
disposal processes are out of service.
Lagoons are usually located at the treatment plant site when land is
available. If inexpensive land is not available close by, it is
often economical to pump sludge to remote areas within 5 to 10 miles
of the treatment plant.
Lagoons, however, are not applicable to all sludge disposal situations.
For example, they can be a source of odor and insect nuisance if used
for the disposal of raw sludge or incompletely digested sludge. Their
satisfactory use depends, in the latter case, on a well-operated treat-
ment plant. Odor control chemicals could be applied when nuisance
problems develop, but their use has rarely been satisfactory.
Two other disadvantages of lagoons are the large land area required
and the possibility of ground water pollution. Lagooning of all
sludge at medium to large-size urban cities is uncommon because land
is too expensive or unavailable. Ground water pollution by micro-
organisms or toxic industrial wastes is a threat that must be con-
sidered in the design of sludge disposal facilities. These same
two disadvantages, large land area requirement and potential for
ground water pollution, are also applicable to sanitary landfills.
In addition, landfills have the disadvantages of needing cover dirt,
and their operation is affected by the weather.
Lagooning of sludge will continue to be popular so long as inexpen-
sive land is available relatively close to the treatment plant site.
However, such land becomes less available as population and manu-
facturing grows. The trend to mechanical dewatering already is
obvious; space that might have been used for lagoons is now being
used for landfilling mechanically dewatered sludge cake. More ex-
pensive but more compact sludge disposal processes will be sub-
stituted for lagoons in the future. As landfill areas disappear,
especially in large urban areas, incineration processes will be
installed.
-------
-109-
10. LAND DISPOSAL OF LIQUID SLUDGE
General - A survey of consulting engineers and State Pollution
Control Agencies' revealed that the disposal of liquid digested
sewage sludge to open land surfaces is very common among smaller
waste treatment plants(25). Large cities such as New York, San
Diego, and Miami Beach have also used this technique of sewage sludge
disposal in conjunction with land reclamation projects. In
England, the disposal of liquid sludge to farmland is very popular.
The process is often economically attractive because it eliminates
a costly solid-liquid separation step. But, land disposal of
liquid sludge can be very expensive if truck hauling distances
are very long.
Liquid digested sludge and supernatant liquor are being applied
to land for final disposal, to fertilize grass or agriculture crops,
and to condition soils on sandy park land. These operations have
been very satisfactory with few exceptions. The success of this
sludge disposal method usually depends on the availability of
suitable land close to the waste treatment plant. Pumping the
sludge to disposal areas within 10 miles of the treatment plant is
usually justifiable but less control is possible than with disposal
near the plant site. Liquid sludge disposal is regulated by health
authorities because nuisance conditions from odors and insects are
possible, and potential public health hazards from microorganisms
need to be considered. The disposal of liquid sludge in association
with composting is discussed in the Compost chapter of this report.
Parameters and Operations - Digestion, aerobic or anaerobic, is
almost always required before disposing of liquid sewage sludge.
Nuisance-free disposal requires the sludge to be well digested;
therefore, the plant operator has a responsibility for good treat-
ment operation. Climate influences the disposal of liquid sludge;
for example: sludge cannot be hauled onto farmland during wet
weather. Digesters and lagoons are usually designed with excess
capacity so they can be used for storage until the weather permits
hauling.
Sludge is distributed on the land and processed in a variety of ways.
Treatment at small plants may include only digging of shallow
trenches, filling them with liquid raw or digested sludge, and
covering the sludge with soil to prevent nuisance conditions.
Sludge may be pumped or gravity fed through pipelines to agricul-
tural fields or land to be reclaimed. At some orchards, the liquid
-------
-110-
sludge is injected into the subsoil under pressure. A very common
technique is disposal of liquid-digested sludge directly to land by
spraying from tank wagons having a capacity of 1,000 gallons. Large
unloading lines hasten disposal and thereby reduce labor costs.
Agricultural land is best used by clearing the previous crops, plow-
ing deeply, and forming furrows to contain the liquid sludge. An
ideal sludge-application system uses a two-step procedure; a shallow
layer is spread, dried, harrowed, followed by a deeper application of
sludge with drying and harrowing. Multiple applications at low
dosages form a thin sludge layer that is easily worked into the soil.
A sludge loading on cropland of 100 dry tons per acre is successful
under average conditions. In areas of low rainfall, 300 tons per
acre is practical^ 3*>8).
Land reclamation by spreading liquid digested sludge is practiced
in some coastal areas. In New York City, barges and tank trucks
are used to transfer liquid digested sludge from sewage treatment
plants to a disposal site where the sludge is pumped into a pipe
distribution system for application to the land. Liquid sludge
is dried 2 to 3 days and then disced into the soil. This process
is repeated about 16 times in an area until t inches of topsoil
are formed from a mixture of sludge and sand^369* 372). At San
Diego, land reclamation incorporates furrow plowing in the sandy
soil, filling with liquid sludge and immediate covering of the
furrow. After drying for 1 to 2 weeks, cross furrows are plowed
and the process repeated^3"). One thousand tons of sludge have
been applied per acre.
The canning and paper industry investigated the disposal of wet
solids by spray irrigation. Solids are disintegrated and sprayed
with liquid wastes through irrigation piping. For this process,
a good, inexpensive, solids grinder is required for successful
disposal of wet solids<216» 5°2>.
Performance - Studies at San Diego, California, have generated
significant performance data concerning liquid sludge-disposal.
From studies of a digested-sewage sludge described in Table
10.1^369), Nuflbaum and Cook made the following conclusions^3"):
1. Liquid sludge can be used to reclaim waste land for
agricultural purposes at a lower cost than heat-dried
sewage sludge.
2. Sludge can be applied at a rate of 100 tons per acre
without impairing the growth of crops.
-------
-111-
3. Applying sludge at a rate of 25 tons per acre
achieves a crop growth rate equal to that of
commercial fertilizers applied at conventional
rates.
4. Superior crops can be attained over a 2-year
period at a sludge dosing rate of 50 tons per
acre without applying sludge the second year.
5. Liquid land disposal can be achieved without
serious handling or nuisance problems.
Table 10.1
ANALYSIS OF DIGESTED SEWAGE SLUDGE - SAN DIEGO
(Results in Percent, Dry Basis)
Prior Average During
Average Spreading
Total dry solids 3.44
Grease 11.15
Fatty acids 24.23
Chlorides 0.75 0.87
Total nitrogen 2.73 2.78
Phosphorus as ?2°H 4.78 4.70
Sodium 1.07 0.56
Potash as K 0 0.83
The San Diego studies demonstrated the usefulness of liquid
sludge as a fertilizer for agriculture crops, grasses, and
shrubs, and as a soil conditioner for relatively sterile dredged
sand<10\ 368> 369). Many acres of sandy soil in the Mission
Bay Park have been reclaimed by the use of sludge to build top-
soil. A 7-mile pipeline from the new San Diego treatment plant
conveniently delivers the digested primary sludge to the park
area. It has been estimated that the use of liquid sludge
eliminated the need to import one million cubic yards of topsoil
to Mission Bay Park<10>.
In New York City, land, proposed for future parks, has been re-
claimed by spreading liquid sludge in place of natural topsoil.
-------
-112-
In 1956 five million cubic feet of liquid digested sludge was
sprayed on landfill areas prior to a dressing of topsoil^372).
The sludge was also used on sandy tidal areas devoted to a bird
santuary. These sludge land-disposal operations were satis-
factory on the basis of cost and performance. They served as
an inexpensive method of sludge disposal and at the same time
provided a cheap topsoil material.
Scott reported on the use of a 5 to 6 percent total solids whey
waste as a liquid fertilizer(365). Crop yields of land receiv-
ing the waste increased if the loading did not exceed 50 tons
per acre per year. No odors developed at this loading, but
higher rates produced both odor and insect nuisance problems.
Canham described the application of wet cannery solids to
land^385). After comminution to particle sizes of about one-
eighth inch, the solids were sprayed with liquid wastes through
irrigation piping. The solids did not form a mat in the fields
and did not produce odors if the application rate was not ex-
cessive. It was suggested that a variety of crops could benefit
from the land disposal of comminuted cannery solids because they
contribute beneficial humus to the soil.
Caron and Blosser studied the efficacy of land disposal for
dilute paper-mill sludgesC*02). The system proved to be feasible
because cellulose is decomposed by aerobic and anaerobic
bacteria in the soil. The rate of cellulose decomposition was
influenced by the nitrogen content, as indicated by the following
lab data:
90% cellulose decomposition achieved in: H5 days 50 days 80 days
with carbon:nitrogen ratios of- 5:1 10:1 550:1
Loading rates of 20 to 25 tons of sludge per acre per year were
feasible.
The use of liquid organic sludges on land improves the soil structure,
its moisture retention ability, and it contributes valuable nutrients
to stimulate vegetative growth.
Economics - Liquid sludge disposal at San Diego offers a stark
example of favorable economics in comparison with mechanical
dewatering and heat drying. Between 1951 and 1956, the average
solids handling costs were about $40 per ton. This figure
included sludge digestion, elutriation, vacuum filtration, and heat
-------
-113-
drying(369» 389). Because the dried sludge was sold as fertilizer
at about $5 per ton, the net cost for solids handling was approxi-
mately $35 per ton of dry solids. Average liquid sludge disposal
costs during 1959 and 1960 were $9.91 per ton (sludge solids =6%).
Of this total, $2 per ton was spent on preparing and finishing the
site disposal area. The truck haul was 21 miles round-trip at a
cost of $0.0019 per gallon^389). Hauling liquid sludge eliminated
the need for sludge elutriation which was a source of water
pollution and increased use of chlorine because of the poor,
solids-capture efficiency in the elutriation basin. San Diego
included a pipeline for sludge transport when they constructed
their new treatment plant, so the solids handling costs were
further reduced to about $4.00 per ton, $1.50 per ton for pipe-
line operation and maintenance and $2.50 per ton for site
preparation and finishing.
In New York liquid sludge disposal to land areas cost about the
same as barging costs or $7.50 per ton of dry solids. They estimated
that many thousands of dollars have been saved by substituting
digested sludge for natural topsoil. The in-place cost for sludge
was reported to be $1,600 per acre while the cost for natural top-
soil is $4,500 per acre(372).
Some cities sell their liquid digested sludge to private groups;
others less fortunate must give it away or even pay someone to
haul it off the treatment plant property. A few examples of each
situation have been reported:
1. Orlando, Florida, sells liquid digested sludge to
fruit growers for $1.00 per 1,000 gallons(24).
2. Two northwestern cities sell their liquid sludge
for $2.00 to $10.00 per 1,000 gallons f.o.b. the
treatment plant(359).
3. In California, liquid sewage sludge is often blended
with other materials and sold as fertilizer. The sludge
is usually given free-of-charge to commercial fertilizer
companies or sold at a cost less than $2.00 per dry ton.
4. Olympia, Washington, is paid $4.00 per ton and Chehalis,
Washington, $10.00 per ton for liquid sludge. At
Chehalis, the sludge is hauled to hayfields<359» 365).
-------
5. Some cities and industries in the midwest pay $7.00
to $10.00 per 1,000 gallons to have their waste sludge
hauled to disposal sites'28)^
Canham believed that the capital investment and operating costs for
disposal of pea wastes favors land disposal by comminution and
spray irrigation instead of solids separation and hauling to land-
fills^385). He estimated the annual cost for liquid waste disposal
to be 64 percent of the solids separation and landfill technique.
HacLaren estimated the cost of liquid sludge disposal to land in
Canada to be $5.00 to $10.00 per 1,000 gallons of sludge hauled.
Assuming a sludge concentration of 5 percent, this cost is relative-
ly expensive ($20 to $UO per ton of dry solids)*53).
Not including digestion, liquid sludge disposal generally costs $<4
to $30 per ton of dry solids. The average cost is about $10 per
ton. Including the capital and operating cost for anaerobic
digestion increases the range to $8 to $50 per ton and the average
to $15 per ton.
Summary - Disposal of liquid digested sludge on land areas is
quite popular in the United States and foreign countries.
Basically, the reasons for this popularity are simplicity and
economy. A close look at the advantages reveals:
1. The process represents final disposal because the
sludge is normally hauled off the treatment plant
grounds by someone assuming responsibility for the
material.
2. The sludge is useful as a soil conditioner and
fertilizer; therefore, it often can be sold for
$1 to $10 per 1,000 gallons.
3. Small capital investment is required, particularly
if a contract for hauling is negotiated. Many
consulting engineers routinely design digester
piping with enough flexibility for loading tank
trucks. The required additional piping costs very
little.
4. Complex mechanical operation and the use of
chemicals is avoided.
-------
-115-
5. Related to item (**), solid-liquid separation processes
can be eliminated, thereby improving treatment plant
economics and efficiency. Overall treatment plant
efficiency is improved because there is no need for
digested sludge elutriation and dewatering, steps that
usually produce a recycle of fine solids in the treat-
ment processes.
The major negative aspect of land disposal for liquid sludge is
that it is not applicable to all waste treatment plants, mainly
because acceptable disposal sites are not always conveniently
available. Hauling costs to acceptable areas can be very
expensive because large quantities of water are included with
the sludge solids. Land disposal areas must be within a short
hauling distance of the treatment plant if a pipeline is not
available for sludge transportation. If the disposal area is not
owned by the sludge discharger, the success of this technique de-
pends on continued acceptance by the land owner. One application
of odoriferous sludge could result in a law suit and the denial
of the disposal area to the discharger.
/<
Digestion of sewage sludge, and perhaps some types of industrial
sludge, is a prerequisite to acceptable land disposal. This
means of stabilizing sludge is costly and must be considered in
an evaluation of alternative disposal methods. Anaerobic digesters
serve as storage tanks which is necessary provision for liquid
sludge disposal systems because weather delays tank-truck hauling
to farmland. The use of liquid sludge as a fertilizer or soil
conditioner also involves public health considerations. California
has adopted regulations preventing the use of sludge for fertilizing
vegetables, berries, and low growing fruit unless the sludge has
been digested for 30 days, is practically odorless, drains readily,
and has a volatile solids concentration less than 50 percent^369).
The Ontario Water Resources Commission restricts the use of liquid
sludge fertilizer to crops that are cooked before consumption^3).
Liquid sludge disposal will continue to be popular at small plants
because it offers many advantages. The hauling costs may be high
on a per ton basis, but the total volume of sludge, and the total
operating budget for its disposal are not very great. Also, the
number of sludge handling processes and their related mechanical
equipment is minimized. MacLaren considered land disposal of
liquid sludge to be applicable to all plants serving less than
50,000 persons^53). San Diego and New York have proven its
-------
-116-
application to large cities where adequate land disposal areas are
available. It is a technique that should be considered even if the
sludge has to be transported 10 to 15 miles by pipeline. Economi-
cally, land disposal can be promising especially if the sludge
producer does not have to pay to have the material hauled away. Ifi
this case, solids handling ends at the digester. Additional
information is in the chapters on Pipeline Transportation and Sludge
As a Fertilizer or Soil Conditioner.
-------
-117-
11. PIPELINE TRANSPORTATION
General - Many large cities pump sludge relatively long distances
through pipelines. The list includes Chicago, Philadelphia,
Cleveland, Camden, San Diego, and San Francisco. Pumping is done
to optimize sludge handling and disposal costs. When a city has
multiple wastewater treatment plants, costs are often decreased
by centralizing certain sludge handling steps such as digestion
and dewatering.
Data - Wirts surveyed various sludge, force-main pumping operations
partially summarized in the following table^32).
Table 11.1
Location
Length Diameter Total Head
(miles) (inches) (feet) Sludge Type
Solids
Cone .
Digested
Digested
Digested
4.0 to 5.0
8.5 to 10.0
4.0 to 5.0
Digested
Raw
Raw
Raw
Raw
-
1
2
3
3
4
3.73
.0
.0
.0
.0
.0
to
to
to
to
to
2
4
4
4
5
.0
.0
.0
.0
.0
Mogden, England 7 12 142
Birmingham, England 4 9 and 12
The Hague, Nether- 78
lands
Los Angeles, Calif. 7.5 24
Chicago, Illinois 17 14 210
5 12 170
Cleveland, Ohio 13 12 391
Philadelphia, Pa. 5 8 225
Columbus, Ohio 5 -
At Nogden, England, the friction loss of the digested sludge is 1.4
times that of water under the same conditions. Birmingham has a
friction loss of 2.6 times that of water for "thick" sludge. In
general, digested sludge containing 7.5 percent solids has a friction
loss about 2.5 times that of watert^1*9). At Chicago, the pressure in
the 17-mile line, carrying a 2 percent mixture of raw primary and
activated sludge, varied from 100 psi at the beginning to 4 psi at
the end.
Operating and maintenance costs for pumping sludge are $1.00 to $2.00
per ton for Cleveland and other cities. Capital costs, of course,
vary with the length of the pipeline, costs of easements, and other
factors. Total annual costs of existing sludge pipeline systems are
probably $3.00 to $7.00 per ton of dry solids.
-------
-118-
Wirts proposed the interesting idea of pumping sewage sludge 80 miles
to abandoned, coal strip-mines^1*32K In addition to being a means
of sludge disposal, he suggested that alkaline digested sludge could
partially solve the acid mine drainage problem. A pipeline service
charge of 10 to 15 cents per ton mile is suggested for cities along
the pipeline route. Hirts believed this sludge disposal technique
would be much more economical than the $40 or $50 per ton spent on
other disposal methods.
Summary - Transporting wastewater sludges by pipeline deserves
more attention by researchers and design engineers. It can be
simple, inexpensive, and relatively trouble-free. Being an
enclosed system the operation is practically odor free. Most
important, pipeline transportation can reduce sludge handling and
disposal costs.
As is practiced in a number of cities, combined dewatering and
ultimate disposal of sludges from multiple sources can be more
economical than separate systems. Perhaps in urban-industrial
areas, all wastewater sludges from all sources could be pumped to
a central handling and disposal treatment plant. Combining a
variety of industrial and sewage sludges could have the secondary
advantages of neutralizing pH, odor control, sludge lubrication,
and disinfection.
Pipeline transportation allows flexibility in ultimate disposal
techniques. It permits the relocation of large quantities of
sludge away from land-short urban areas, where air pollution
regulations are in effect, to less populated areas or the ocean
where disposal methods are feasible. Land disposal methods could
include lagooning, liquid land disposal as a fertilizer, under-
ground disposal, and disposal in quarries. The cost of trans-
portation might be easily absorbed by savings from relatively
cheap methods of disposal not possible in urban areas.
Digestion before pipeline transportation is sometimes specified
for non-activated sewage sludges due to potential grease and
abrasion problems. Any sludge pipeline system, however, should
have the capability of being cleaned at regular intervals with-
out interrupting pumping operations. A design that permits the
addition of chlorine to aid pipeline cleaning is also recommended.
-------
-119-
The use of pipelines for sludge transportation successfully reduces
tbtal sludge handling and disposal costs in many cities. It should
be routinely considered in economic evaluations of all sludge
disposal systems. One area of needed research is the development of
a chemical to fluidize sludge to ease pumping over long distances.
Another is a method of preventing sludge septicity during transporta-
tion. Also, advanced waste treatment of sludge should be explored to
facilitate ultimate disposal at the end of the pipeline. The solids
might be used as a fertilizer and the liquid for irrigation. Volumes
involved would be much less than the original wastewater, so the
costs could be reasonable.
-------
-120-
12. OCEAN DISPOSAL - DILUTION
General - Sludge disposal by dilution in fresh water is generally
unacceptable except for those sludges produced from water treat-
ment plants. However, in large seacoast communities, the disposal
of sewage sludge is commonly disposed to the ocean via pipeline or
barge. One example of the popularity of ocean disposal is that
one-third of the sludge produced in England and Wales is discharged
to the ocean^70). Some of the largest cities in the United States,
including Boston, Mew York, and Los Angeles, dispose of their sludge
in this fashion. The canning industry in the San Francisco Bay area
barges thousands of tons of wet solids each year to the ocean. This
technique has the tremendous advantages of being more economical than
most other disposal methods. It represents ultimate solids disposal
to the treatment plant operator because the sludge is permanently
removed from his property.
Obviously, the pumping or barging of sludge to the ocean can be
economically justified only for communities or industries on or
relatively near the seacoast, but "relatively near" could be a
round-trip barging distance of 400 miles. Recent studies by a
few, large, eastern cities indicated barging that distance was still
cheaper than alternative sludge disposal methods. Disposal of
sludge to the ocean has some potential dangers, however, so a
thorough scientific study is necessary before adoption of this
technique.
Design Requirements - Ocean disposal of sludge normally requires
that the following conditions be met:
1. Safe bacteria levels at ocean beaches.
2. No floating solids or scum at the shore line.
3. No objectionable odors.
4. No accumulation of bottom deposits.
5. No concentrations of toxic material detrimental
to plant or animal life.
Because raw sewage solids can create a variety of nuisance conditions,
it is the usual custom to digest sewage sludge prior to ocean dis-
charge. Digestion changes the nature of the sludge so that it is
conducive to rapid mixing and dispersion in the saline waterC*39).
In addition to reducing the volume of solids, it homogenizes the sludge
to produce a uniform, end product more predictable upon dilution. A
considerable reduction of the pathogenic bacteria population in the
sludge and the biochemical oxygen demand is also achieved. Bacteria
-------
-121-
reductions of 99.8 percent after 30 days digestion at 95 to 100°F have
been reported and B.O.D. reductions of 90 percent are possible(^39).
Digestion also decreases the chance of odor development and damage
to aquatic life. Because digestion is so important, over-design of
the system is a good idea.
Sludge floatables, if not removed, have a tendency to travel long
distances on the surface of the ocean. In that environment they
undergo very little physical or chemical change and, therefore, may
cause odors, be unsightly, and carry pathogenic bacteria to recreational
areas. Floating material can be reduced by burying or incinerating
screenings and skimmings rather than letting them pass into the digester.
Digestion reduces and homogenizes some of these materials but the
process is incomplete. Some West Coast cities practicing ocean dis-
posal screen the digested sludge prior to pumping through the outfall
line.
The success of ocean disposal of sludge via pipeline depends in part
on the design of the sludge outfall line. Outfall lines have been very
thoroughly and scientifically studied in California. They consider
the following factors of major importance to the outfall
A. Beneficial Use of Receiving Water
1. Shellfish
2. Bathing
3. General
B. Public Health
1. Shellfish
2. Bathing
3. General
C. Marine Resources
1. Fishery
2. Shell fishery
3. Other (kelp-game, etc.)
D. Oceanographic
1. Current structure
2. Eddy diffusion
3. Density structure
4. Wind structure
5. Submarine
a. Topography
b. Geology
-------
-122-
E. Nuisance
F. Aesthetic
G. Economic
Sludge must be discharged at a depth that prevents solids from
rising to the surface and the discharge must be far enough
from shore so that dispersion by currents does not carry the
sludge to the shore. Careful planning and investigation of the
sludge and dilution water to include specific gravity, temperature,
winds, and ocean currents is necessary.
At the end of the submerged outfall line, effective mixing of
sludge and seawater is required to promote rapid dilution. The
California studies have stressed the need for a sludge diffuser
that distributes the flow uniformly at all rates of flow and with
maximum dilution near the outfall^1*6). Mixing the digested
sludge with treatment plant effluent or seawater prior to discharge
facilitates sludge handling and dispersion*43**). Some plants dis-
charge the sludge through effluent lines, others discharge through
separate sludge outfall lines. Separate sludge lines, being
relatively small diameter pipes, can be installed more economically
and, therefore, carried to greater distances and depths than the
larger effluent lines.
Operation and Performance - Under some conditions, it may be desirable
to discharge the digested sludge to the ocean intermittently. This
is done to take advantage of strong currents, ebbing tides, and non-
bathing periods when public health problems are not so critical.
Digestion offers the advantage of storage flexibility so intermittent
discharge is possible*1*39).
Henry reported that the Vancouver Lions Gate sewage treatment plant
stores digested primary sludge until the winter months when favorable
ebb tides carry the material rapidly to sea. The sludge is discharged
through the effluent outfall line when the tidal flow in the narrows
is about 500,000 cfs* ****!'. At the Nut Island sewage plant in Boston,
digested sludge disposal is also timed to mix with strong currents.
Disposal is through a t.5 mile, 12 inch line into deep water where
the sludge is distributed along the bottom and does not rise to the
surface*1*38). The sludge at Boston is diluted prior to discharge,
UO.OOO gpd sludge with 1 mgd plant effluent. Once each year the
sludge outfall line is cleaned with mechanical cleaning equipment* ^S).
-------
-123-
At the Los Angeles Hyperion plant, digested sewage sludge is diluted
with plant effluent to a concentration less than one percent solids
before being pumped through a 7 mile, 22 inch submerged pipeline.
The outfall line terminates at a depth of 320 feet but the submarine
canyon in which the sludge settles is at a depth of 590 feet'^5)^
Dilution to make the sludge-solids characteristics in the pipeline
similar to water is thought to be important. The flow is kept con-
tinuous and the volume relatively constant^6).
As part of a water quality monitoring program, scientists in
California are thoroughly studying the effects of sludge disposal
on the marine environment'435), They observed no accumulation of
solids around the outfall even though deposition is known to occur.
In fact, they estimated that 60 percent of the solids are deposited
within 500 feet of the outfall. But, the sludge deposits were
periodically removed by ocean currents. Marine investigators
reported an increased population of plankton and aquatic animals but
no toxic effects on fish have been observed. The long term effects
are unknown. Ocean disposal of sludge undoubtedly affects the
marine environment. However, with a properly designed system the
effect is probably small because the amount of dilution water is so
large.
Economics - Of the two methods of ocean disposal (pipeline or barge)
pipeline disposal is much cheaper. Operating costs for the sludge
outfall line at the Los Angeles Hyperion plant were expected to be
$1.15 per ton of dry solids. Before ocean disposal, the cost of
producing dried sludge-fertilizer by digestion, elutriation, vacuum
filtration, and heat drying was $38.00 per ton. The dried sludge
sold for $4.50 per ton, so the net cost was $33.50 per ton^66^.
Construction of the pipeline cost 2.72 million dollars or $73.50 per
foot. In 1951, engineers estimated a 30-inch outfall line on the
Atlantic Coast would cost $77.00 per foot^438*. Using the ENR Con-
struction Cost Index, the 1966 cost would be $140.00 per foot.
Los Angeles is also saving money on chlorine. Before the sludge
outfall was constructed, 68 tons per day of solids were lost to the
plant effluent line, primarily from the elutriation basins. This
material resulted in an extra chlorine demand amounting to $150.00
per daydBO).
New York City has been barging digested sludge to the ocean for
many years. Their total cost of sludge handling (digestion and
barging) used to be about $9.31 per ton of dry solids. Operating
and maintenance costs accounted for greater than 75 percent of
-------
-124-
this total/372). In recent years, New York has reduced the total
sludge handling cost to about $7.50 per ton by the use of larger,
more mechanized barges plus more rapid unloading of the sludge
into the ocean. The round trip distance depends on the treatment
plant location, but the average is 25 miles.
Middlesex County Sewerage Authority pays $11.30 per dry ton for
disposal of stored chemically precipitated sludge. Their round
trip barging distance is 50 milesC^8). Barging costs at
Elizabeth, New Jersey, are reported to be $4.34 per dry
Digested sludge is barged from Yonkers, New York, at a cost of
$16.05 per dry ton. This figure includes $2.98 per ton capital
cost, $5.99 per ton for operation and maintenance, and $7.08 per
ton for barge towing. The total cost ($16.05 per ton) compares
very favorably with another Hestchester County sewage treatment
plant New Rochelle, New York. There, sludge handling and
disposal by vacuum filtration and incineration costs $62.02 per
ton.
Four large eastern cities recently evaluated the economics of
various methods of sludge disposal. Generally, the economics of
barging sludge to the ocean appeared favorable in relation to
most of the processes studied.
At Baltimore, Maryland, the sludge handling oosts were estimated
for the year 1980. Barging costs were based on a digested primary
and secondary sewage-sludge mixture having a solids concentration
of 7.5 percent. The estimated unit costs were:
Barging $22.81/Ton
Heat drying and granulating $36.UO/Ton
Incineration $38.07/Ton
Dewatering, mixing with soil $28.00/Ton
Sludge-refuse combustion $30.00/Ton
These costs are based on dry solids and include thickening,
digestion, elutriation, and vacuum filtration. Equipment related
to these processes has already been installed at Baltimore.
Capital costs for new facilities were computed using an interest
rate of 5.63%. Barging costs were based on disposal of 93 tons
of dry sludge per day with a round trip of over 300 miles v1*1*9'.
Philadelphia evaluated 14 different methods, or combinations of
methods, for disposal of digested, secondary sludge from their
-------
-125-
Northeast sewage treatment plant. A cost comparison of the six
major sludge disposal techniques is as follows^1*1*2* ****7):
$/Ton of Dry
Proposals Solids
Pumping wet sludge to
Southwest Lagoons 7.23
Barging wet sludge to sea -
10% solids 8.78
Barging wet sludge to sea -
5% solids 11.81
Trucking wet sludge to South-
west Lagoons - 10% solids 12.05
Excavating lagoons at intervals -
trucking 18% solids to Southwest 12.67
Trucking wet sludge to South-
west Lagoons - 5% solids 17.64
Trucking sludge cake to Southwest 18.90
Dewatering and Incinerating Sludge 28.01
While lagooning had the lowest cost, it was not selected because
of the high price of land and the uncertainty of future operations.
Cost estimates were calculated on the basis of 30 year bonds at 3.5%
interest. The thickened sludge would be dumped in an area requiring
round-trip barge travel of 227 miles.
The Washington, D. C. Department of Sanitary Engineering had studied
possible future sludge disposal costs at the Blue Plains treatment
plant. Estimated unit costs for 1970 are:
Sludge
Concen-
tration
% Suspended
Solids
Barging
$/Ton
Inciner-
ation
$/Ton
Drying Raw
Activated Sludge
Barging Digested
Primary Sludge
Wet Oxidation
$/Ton
7.5% $17.95/T - -- $38.95(Raw Sludge)
$25.25/T $27.95/T
»».0% $20.75/T - - $31. 90( Digested
Sludge)
-------
-126-
Above costs are based on dry solids and include thickening, digestion,
elutriation, vacuum filtration, and, as may be applicable, final dis-
posal. Capital costs for new facilities were computed at an interest
and amortization rate of 5.8% (capital costs for existing facilities
are not included). Barging costs were based on a round trip of UOO
miles, a requirement for channel dredging at the plant, and the
construction of a dock.
A solids handling and disposal study completed for Camden, New Jersey,
concluded that barging of sludge rather than vacuum filtration and
dewatering would save $200,000 over a 5-year periodv^SOa)^ Current
filtration and incineration costs were $20 to $28 per ton. It was
assumed that present filtration and incineration equipment would be
overhauled or replaced during the next 5 years. Also, ash lagoons
would have to be constructed. Barging costs were based on the disposal
of a 7 percent stored, primary sludge at a round trip distance greater
than 200 miles.
In general, ocean dilution is the cheapest method of sludge disposal
for cities near seacoasts. Disposal through a pipeline is less
expensive than barging unless very long outfalls are required. Capital
and operating costs for pipeline disposal would generally be $5 per
ton or less, while barging costs have been reported to be $5 to $25 per
dry ton. As the data show, barging costs are a function of the distance
the barge must travel to a safe disposal area.
Summary - Ocean disposal of sludge has a number of advantages for sea-
coast cities when compared with other disposal methods:
1. The removal of sludge from the treatment plant is
complete, not even an ash residue remains.
2. Disposal of sludge at sea is relatively inexpensive.
3. Ocean disposal permits nuisance-free sludge handling
assuming the sludge is digested.
U. Ocean disposal permits flexibility in plant operation,
assuming once again the sludge is digested. There are
few problems with sludge-volume fluctuations and the
dumping schedule can be varied.
Ocean disposal of sludge lessens the effectiveness of sewage treatment,
but it is being accomplished without any significant, known, detri-
mental effects to the receiving water that might impair the beneficial
uses of the water. Long term effects are not known, so continuous
surveillance is needed. While much research has been conducted by
-------
-127-
California engineers and scientists, they recommended the following
additional invest igat ions C^e),
1. Detailed monitoring of the area near ocean outfalls
for physical, chemical, and biological factors,
2. Studies of bacterial viability in sea water.
3. Effects of wind and waves on the dispersion of a
seawater-sludge mixture .
4. Evaluation of water standards for bathing.
5. Grease and coliform bacteria factors in sludge disposal.
Before a decision is made to use the sea for dilution, beneficial uses
of the water should be evaluated with the biologic, geologic, and
oceanographic characteristics of the disposal area. Then, decisions
can be made concerning the degree of sludge treatment required and
the best location for outfalls or barging dumps. Once adopted, a
control method is required to insure that the sludge is being dumped
in the prescribed location.
The disposal of sludge to the ocean will become more common in the
next few years at cities located near seacoasts because it is simple,
final, and less expensive than other methods. Ocean disposal, however,
may not always be acceptable to the public and regulatory agencies .
In the future, a decision could be made to eliminate this practice
based on esthetic, scientific, or other reasons.
-------
-128-
13. UNDERGROUND DISPOSAL
General - Industrial wastewaters have been disposed of underground
through deep injection wells for many years—why not sludge?
Economic analyses indicate that underground disposal could cost at
least one-half that of conventional sludge disposal techniques, if
subsurface strata would accept the high concentration of suspended
solids in sludge.
Koenig reported on ultimate disposal of well treated wastewater
into subsurface strata that contain natural and man-made cavities'^'
such as caves, depleted mines and wells. His report indicated that
subsurface disposal is very competitive with other forms of disposal.
There are several problems in addition to suspended solids, that are
associated with subsurface disposal. First, the filling of natural
caverns, depleted mines, or wells is specifically legislated against
in a number of States^?1*). Second, finding sites that are geologi-
cally and legally acceptable can be a problem. Excavating underground
sites for sludge disposal is very costly, and the material removed
presents a disposal problem in itself. Koenig suggested nuclear
blasting^55). A consideration is whether devices of sufficient size
could be detonated close enough to population centers to produce
caverns, economically usable for sludge disposal.
Summary - Assuming that man-made cavities, specifically created for
disposal, are eliminated due to the high cost involved, subsurface
disposal is limited then to natural cavities and deep-well injection
into a suitable subsurface strata. Mineral production in the United
States creates 152 million gallons per day of underground cavity
capacity. About 116 million gpd of this total exist as bituminous
coal cavities, this is significant because their distribution is
relatively near the urban areas creating waste solids^55).
As Wirts proposed, sludge could be transported to underground
cavities by pipe lines (W2). Koenig estimated the cost of pipelin-
ing waste from advanced waste treatment processes
1961
10 miles $0.56/1,000 gals.
100 miles $3.90/1,000 gals.
Assuming a 5 percent sewage sludge, the 1966 costs could be $2.50
to $3.50 per ton of dry solids for 10 miles and $15 to $22 per ton
for 100 miles. Koenig*s figures were not based on sewage sludge
so the accuracy of these estimates is questionable.
-------
-129-
A subsurface disposal cavity should be below the level from which
ground water is pumped. If this is not possible the cavity must
be sealed permanently against seepage. This would be difficult
and costly. The problem of acid mine drainage from bituminous
coal mines is a good example of the seriousness of surface and
ground water pollution. Any cavity considered for sludge disposal
must be thoroughly studied and monitored to determine such factors
as elevations relative to ground water, permeability of the forma-
tion, and its ability to be sealed.
Sludge disposal by deep-well injection could be much cheaper than
conventional sludge handling methods. But, this technique depends
on suitable subsurface geology and the approval of governmental
regulatory agencies. It can be accomplished successfully as
demonstrated by the fact that The Dow Chemical Company disposes of
waste activated sludge in this fashion^71*) at one of their large
production facilities. Lowes reported that the economics of sludge
handling and disposal at Dow were considerably improved by separa-
ting the difficult-to-dewater activated sludge from the primary
sludge and pumping the activated sludge underground.
Deep-well injection costs are influenced by several parameters
such as: (1) volume of material to be injected, (2) well depth,
(3) well head pressure (affected by physical and chemical charac-
teristics of the formation), (H) sludge concentration, and
(5) required surface treatment. The surface treatment required
depends on the nature of the receiving formation and the nature
of the sludge to be injected. Accurate costs for subsurface
disposal were not available when this report was written. Koenig
estimated that the average cost of drilling oil and gas wells
(5,991 ft. to 7,500 ft. deep) was $100 per foot<55>.
Underground disposal of all types of sludge should be considered
where the geology and legal environment are favorable. It can be
less expensive than conventional disposal schemes even if the
sludge must be transported long distances by pipeline to a suitable
disposal area. However, because permanent contamination of water
supplies can occur, extensive preliminary studies of subsurface
geology are necessary as is surveillance after the system is
completed.
-------
-130-
m. SLUDGE DEW ATE RING
The primary objective of any dewatering operation is to reduce
the sludge moisture content to a degree which allows ultimate
disposal by incineration, landfilling, heat drying, or other
means. Dewatering differs from sludge thickening in that the
sludge is processed into a non-fluid form.
This chapter discusses the following dewatering processes:
(1) vacuum filtration, (2) pressure filtration, 13) centrifuga-
tion, CO sand bed drying, and (5) screening.
A. Vacuum Filtration
General - Vacuum filtration of process-industry slurries has been
in common use in Europe and the U.S.A. for many years. The process
has also been used in the sewage treatment field; Milwaukee and
Chicago initiated vacuum filtration more than 35 years ago(53).
After World War II the popularity of vacuum filters decreased.
Surveys revealed that many installations were abandoned between
19U5 and 1960 due to maintenance and operational problems as well
as increasing labor, equipment, and chemical costsU-51).
Since 1960, however, the popularity of vacuum filtration has
increased for a number of reasons that include: (1) development
of self-cleaning filter media, (2) increased cost of alternative
sludge dewatering methods, and (3) the increased acceptance of
sludge incineration as a final disposal technique. At one time
vacuum filters were not considered for sewage treatment plants
serving less than 25,000 persons. Today, they are considered
in economic studies and adopted for cities serving 10,000 persons
or less. It was recently estimated that 1,300 vacuum filters
have been installed in sewage treatment plants(251).
Mechanical dewatering by vacuum filters is applicable to all
types of sewage sludge and to many industrial wastewater sludges.
Centrifuges are becoming increasingly competitive with filters.
But, they will not in the foreseeable future completely replace
filters because they capture fewer solids and, in general, are
less efficient for dewatering certain difficult biological and
industrial waste sludges.
Theory and Parameters - Filtration has been defined by Hubbell
as a "Process of separating solids from a liquid by passing the
liquid through a porous medium on which the solids remain to
form a cake"^65). In rotary-drum vacuum filtration the drum is
continuously passed through the sludge where it picks up solids
-------
-131-
to form a cake which is partially dewatered and discharged. Figure
m.I shows a typical installation. The filter is a cylindrical
drum with some filter media covering the outside surface. Internally,
the drum is divided into drainage compartments which connect to the
filtrate system. About 20 to 10 percent of the drum is submerged
in the filter "pan" containing the sludge as the drum is rotated.
A sludge mat is formed on the filter media as a result of a vacuum
(10 to 26 inches Hg) applied to the drainage compartments servicing
this submerged portionCB). As the mat, or cake, rotates out of
submergence, vacuum and dewatering are continued. The cake is
removed from the drum just before it would be submerged in the pan
once again.
Disc type vacuum filters are sometimes used for dewatering industrial
slurries. They are particularly conducive to filtering large
volumes of easily dewatered solids.
Rich described vacuum filtration as a special case of flow through
a bed of solids<6)* He stated that permeability through the solids
depended on cake characteristics such as porosity, average specific
surface area of. the cake particles, and sludge compressibility.
Resistance to filtration is effected by the filter media and by the
filter cake. A number of sludge and operating variables affect the
filtration performance.
Sludge Variables - Factors that affect the ability to dewater sludge
are: ID solids concentration; (2) sludge age and temperature;
(3) sludge and filtrate (liquid) viscosity; U) sludge compressi-
bility; (5) chemical composition; and (6) the nature of the sludge
solids including volatile content, size, shape, electrical charge
of the particles, density, ratio of slimes to coarse particles,
content of "bound" water and whether or not the sludge has been
exposed to biological treatment.
Operating Variables - (D Vacuum, (2) amount of drum submergence,
(3) drum speed, CO degree of agitation, (5) filter media, and
(6) conditioning of the sludge prior to filtration, all affect the
operation of vacuum filtration equipment.
Increased solids concentration in the filter feed increases filter
production (yield). In general, the sludge filtration rates
increase directly in proportion to the increase in feed sludge-
solids concentration; a smaller filtrate volume has to be removed
per pound of filter-cake formed. There is, however, a practical
upper limit (probably 8 to 10 percent solids for sewage sludge)
which, when exceeded, makes chemical conditioning and sludge
distribution difficult. Figure 11.11 illustrates the improved
-------
-132-
Figure 14.1
Filter
Vacuum control
regulators
Sludge feed
from —
mix tank
Dewatered
Sludge
Filter overflow
to sludge wel 1
Filtrate
receivers
Filtrate
to
plant
Filtrate
pumps
Muffler
Vacuum
pumps
Piping layout for filter vacuum system with vacuum filtration.
(Reprinted by permission from Vol. 28, No. 12, p. 1451,
December 1956, Sewage and Industrial Wastes)
-------
-133-
Figure l4.II
4.0
55
c
X
S 3.0
•a
h
- 2.5
V
1
>»
o 2.0
•
I
1
..1.6
o
I*
to.
0.5
/
f
/
/
1
i
i
/
/
3
/
7
-
234
Solid* in FMtf. X
Filtration rat* vtriui lolidi concentration
(Reprinted by permission from Vol. 28, No. 12, p. 1449,
December 1956, Sewage and Industrial Wastes)
-------
-134-
filter rates possible with increased feed solids concentration
for a primary-activated sewage sludge mixture(232).
Other advantages of increased solids concentration are that less
conditioning chemicals are usually required per pound of dry
solids and filter—cake moisture is reduced. These facts make a
tremendous difference in operating costs at many filter installa-
tions. McCarty presented the data in Figure 14.Ill, which show
the relation between decreasing filter-cake moisture and increasing
sludge solids concentrations(61). He also described vacuum filtra-
tion rates in terms of moisture removal rates. Figure I1*,IV shows
data averaged from many treatment plants that relate solids
concentration, filter yields, and moisture removal rates. The
median moisture removal rate is 4.9 gallons per square foot per
hour.
Most people agree that as sludge ages, it becomes more difficult
to dewater on vacuum filters. Ettelt and Kennedy observed the
aging effect on filter-cake moisture using thickened activated
sludge at the Chicago Sanitary District56). Figure m.V shows
the relation between increasing cake moisture and increasing age
of the thickened sludge. It also shows that freshening of sludge
by re-aeration dramatically reduced the cake moisture even though
the sludge had aged only 3.5 hours. Wishart and co-workers found
that sludge aeration for 1 to 2 hours significantly reduced the
required ferric chloride conditioning dosage(239). They theorized
that this was due to decreased alkalinity and the oxidation of
reducing compounds that exert a ferric chloride demand.
Up to a point, the higher the vacuum, the greater the filter yield.
This relationship is shown in Figure 14.VI for a variety of sludge
dewatered on pilot plant filters(232). Trubnick and Mueller
questioned, however, whether vacuums in excess of 15 inches Hg
significantly affect cake yields and moisture(227^. It seems that
the importance of increasing the vacuum beyond this point depends
on the sludge cake compressibility and the filter media. An ideal
vacuum filter design would incorporate two independent vacuum
systems; one operating while the cake is being formed, and the
other after it comes out of submergence and is being dried. With
very compressible cakes, a moderate forming vacuum may be desirable
to prevent media plugging. However, maximum drying vacuum is
almost always desirable to produce a cake having a minimum moisture
content.
Drum submergence and drum speed influence the filter yield and
filter—cake moisture. Increasing the drum submergence increases
the detention period of the form cycle, thereby generally resulting
-------
c 90
8
80
JO
§ 70
iii
-------
Figure 14. IV
CIO
£ "(MOISTURE REMOVAL RATE = 6.2 gal/sqft/hr
ui
£ *
3
lL
2 2
4 6 8 10 12
SLUDGE SOLIDS CONCENTRATION (percent)
14
CO
O)
I
Relational? Between Sludge Solids Concentration and Dry Solids
Vacuum Filtration Rate for Various Moisture Removal Rates
(Courtesy P. L. McCarty, Stanford University)
-------
Figure 14.v EXAMPLE OF EFFECT ON CAKE MOISTURE OF ELAPSED TIME
AFTER THICKENING SLUDGE. (|.I2% ACTIVATED SOLIDS,)
(Reprinted by permission from Vol. 38, No. 2, p. 256, Feb. 1966, J. V.'ater Pollution Control Federation)
90
88
86
iii
a:
84
82
80
REAERATED
0 40 80 120 160 200 240 280
ELAPSED TIME AFTER THICKENING (MINUTES)
-------
-138-
Plgure 14.VI
6 7 B 9 10 IS
Form Vacuum, lnch«l Mtrcwy
Filtration rat* versus form vacuum; pilot-
plant data.
(Reprinted by permission from Vol. 28, No. 12, p.
December 1956, Sewage and IndustrlaT Wastes)
-------
-139-
in a thicker cake or increased filter yield, but often a higher
cake moisture. Increasing the filter cycle time (or slowing
the speed) would, as expected, decrease the yield or production
rate. This parameter is described in Figure m.VII for a number
of different sewage sludges. While yield may be sacrificed,
increasing the cycle time can be expected to decrease the filter-
cake moisture because the drying cycle is extended.
Agitation of the sludge during and after chemical conditioning
is an important operating parameter. The proper evaluation of
this parameter requires variable-speed mixing equipment for
chemical conditioning tanks and the vacuum filter pan. When
being mixed with chemicals, some sludges require more violent
agitation than others. After chemical conditioning, the general
rule is to handle the sludge as gently as is practical. This
usually means only enough filter pan agitation to prevent solids
classification and to keep the solids in suspension and well
distributed along the length of the filter pan. Because sludge
viscosities vary, optimum control requires variable speed pan
agitation equipment.
Proper selection of the vacuum filter media is very important
to efficient filter performance. Early filter installations
were limited in media selection, but now a wide variety is
available including natural and synthetic fabrics, metal coils,
and flat metal belts. Sludge characteristics and chemical
conditioning play an important part in media selection. So,
the selection should be based as nearly as possible on actual
plant conditions, using a laboratory filter leaf test or by
the reaction of different media panels on an operating filter(66),
In general, a media is selected on the basis of the cleanest
filtrate consistent with high filtration rates and reasonable
replacement costs.
Most raw primary sewage sludges and certain industrial waste
sludges have fibrous and non-uniform solids. Vacuum filtration
of these solids is most efficient with media having comparatively
large openings that do not clog or "blind" with sludge particles.
Media blinding leads to lower filter yields, increased chemical
consumption, and the need for frequent media washing. Increased
filtrate solids is one disadvantage associated with "open" media,
but often these solids easily resettle when returned to primary
clarifiers. In some evaluations of raw sludge dewatering after
polymeric flocculent conditioning, practical vacuum filtration
was not possible until open media was substituted for "tight"
media.
-------
-140-
Flgure 14.VII
C 2»
*"
I 18
&
•
5
o IO
t 9
ci 8
i ?
2 6
a.
« 5
1*
«•
o
r 3
\
\
A
O i
X - R
A - S
0 - G
• - 0
\
"\
jng B«(
ockford
ch«nec
r«enwic
alia*,
\
X
X
ich. N
. III.
lady.
h. Co
T«x.
\
s
^
N.Y.
nn.
\
O
\,
^L
X,
\
S
V
1.9 2 Z.5 3 4 9 6 7 8 910
Cyclt Tlm«, Mlnutct
Filtration rate venua cycl* time; pilot-
plant data; primary-digested sludges; form vacuum, 20 to
25 in. Hg; submergence, 25 per cent.
(Reprinted by permission from Vol. 28, No. 12, p. 1454,
December 1956, Sewage and Industrial Wastes)
-------
The nature of the sludge to be filtered is an important filtra-
tion parameter, but sometimes its modification is limited as so
many characteristics vary with the sludge source and methods of
treatment's). in general, raw sludge is easier to filter than
digested sludges, and primary sludge is easier to filter than
secondary sludges. Sludge particle size, shape, electrical
charge and density affect filter-ability because they affect
compaction and flocculating chemical demand. For example,
small particles (fines) tend to form a compact mat under vacuum
which leaves a small void ratio for passage of the liquid(227).
Small particles also exert a greater chemical flocculent demand
per unit weight than large particles. Center reported that
small particles have a greater surface attraction for water than
large particles. If solids have a low specific gravity, the
wetting effect is increased^21). Usually the larger the particle
size is, the higher the filter rate (in dry pounds per square
foot), and the lower the cake moisture. An increase in slimes
or extremely fine particles decreases the filtration rate and
increases the cake moisture.
Compressible sludge solids tend to deform as pressure increases
and the result is a tight filter-cake that resists liquid separa-
tion. According to Genter, compressibility of sludge solids is
a direct function of the volatile content^!). Many operators
have noticed increased chemical consumption with increased sludge
volatile content. The structure of the solids and their tendency
to deform can be controlled in part by chemical conditioning.
The chemical composition of a sludge has a large influence on
the requirement for chemical flocculents. Genter believed that
the total flocculent demand is the sum of the liquid and solids
demand(21). The liquid demand is thought to be from the alka-
linity of the sludge, and it is particularly important in digested
sludge because of the bicarbonate alkalinity formed during the
digestion process. The solids demand is proportional to the
ratio of volatiles to inerts. Digestion reduces this ratio but
increases the alkalinity in the process. (For additional
details, see the Elutriation chapter).
Chemical conditioning is usually a necessary step before sludge
vacuum filtration. It enhances maximum efficiency of sludge
dewatering. Only a few sewage sludges do not require it. Some
industrial sludges can be filtered without chemical treatment
but they often have, due to process requirements, a high compo-
sition of the same chemicals normally used to condition sludges.
Chemical flocculents agglomerate solids and cause a release of
-------
-142-
water. By doing so, they create large uniform voids in the sludge
so water can pass through. Genter believed chemicals to be the
simplest and most effective way of keeping void channels relatively
open before and during vacuum filtration^^O).
A wide variety of chemicals have been evaluated for conditioning
sludges prior to vacuum fi It ration (19 5) m T}je iist includes the
following: ferric chloride, ferrous chloride, ferric sulfate,
ferrous sulfate, sulfuric acid, nitric acid, hydrochloric acid,
lime, sodium dichromate, chromic chloride, aluminum chloride,
aluminum chlorohydrate, zinc chloride, titanium tetrachloride,
chlorine, sodium chloride, potassium permanganate, cupric chloride,
soap, aluminum sulfate, sulfur dioxide, phosphoric acid, dicalcium
phosphate, and organic polyelectrolytes.
Physical filter aids used along or in conjunction with chemicals
to increase porosity and filtration rates include: coke, bone
ash, peat, paper pulp, ground blast furnace slag, diatomaceous
earth, ground garbage, fly ash, clay, sawdust, crushed coal,
animal blood and activated carbon.
In this country the most popular chemical conditioning materials
are ferric chloride, lime and cationic polyelectrolytes. Over-
seas, aluminum chlorohydrate is a common flocculating agent along
with lime and ferric salts. Often combinations of ferric salts
and lime are used to optimize chemical conditioning costs. The
dual use of anionic and cationic polymers has been very successful
because of the two basic phenomena thought to be involved with
flocculation—charge neutralization and particle bridging (or
agglomeration). The first function is performed by the cationic
materials; the second by the anionic materials.
The success of synthetic polymeric flocculents represents one of
the few recent major advances in the sludge handling field. These
materials have captured the major portion of the municipal sludge
conditioning market, exclusive of raw waste activated sludge.
To a point, increased chemical dosages increase the vacuum filter
yield and decrease the cake moisture. A typical curve relating
dosage and filter yield is presented in Figure It.VIII1232).
As indicated, a dosage is reached where additional yield increases
are not achieved. The curves in Figure 1**.VIII show a plateauing
of the yield, but yields may decrease drastically from overdosing.
Ferric salts and lime added to raw sewage sludge will change the
pH and decrease the microorganism population. This is important
for odor control, but the reduction is not sufficient to eliminate
-------
-143-
public health hazards from improper use of filter-cake. Figure
14.IX shows a substantial reduction in coliform MPN, but the
sludge could not be called sterilized. Organic polymeric floccu-
lents do not affect the coliforms in the filter-cake.
Beck and co-workers developed a Buchner funnel laboratory test
to evaluate chemical conditioning and other parameters(236),
It is a simple test that indicates: the best flocculentls) for
a particular sludge, the optimum sludge and flocculent dilution,
the best sludge-flocculent mixing procedures, the effect of system
vacuum, sludge age, and drying time, and allows an estimate of
the filter-cake moisture. The Buchner funnel test has certain
limitations, so a filter leaf test and/or pilot filter runs should
be considered. Testing on a filter leaf permits fairly accurate
predictions of filter yields, the best filter media, cake moisture,
cake discharge characteristics, and necessary media maintenance.
Coackley and Jones modified the Buchner funnel test so they could
compare filtration resistance for different types of sludge. They
compared sludge dewatering characteristics by developing a specific
resistance value: it was defined as the pressure difference required
to produce a unit rate of flow of filtrate having a unit viscosity
through a unit weight of cake. For any conditioned sludge and
filter vacuum, tests can determine the specific resistance of the
sludge^233). The test is useful for comparing different prefiltra-
tion procedures and flocculents. Laboratory tests can be applied
to full-scale design and operation along with filter leaf and/or
pilot filter tests.
Design and Operation - Vacuum filter systems are designed from
data showing quantities of sludge to be filtered, sludge charac-
teristics, filtration rates, cake moisture, and filter operation
cycles(66). As discussed previously, this data could be generated
from laboratory or pilot filter tests on the sludge. If the
actual sludge to be filtered is not available, vacuum filter
systems are designed using averaged data from plants treating
sludge with similar characteristics.
Yield or production rate is the basic factor in sizing vacuum
filter installations. A conservative design rate of 3.5 pounds
per square foot per hour has been widely used^5c)f but a more
accurate rule-of-thumb is to assume that the yield is equal to
the solids concentration of the sludge to be filtered. In
general, this means the yield may vary from 2 to 10 pounds per
square foot per hour. The low values represent filtration of
fresh and digested activated sludge; the high values are typical
for raw primary, or primary plus trickling filter humus, sludge
filtration.
-------
Flgure 14.VIII
14
13
i*
as
a
I
«/>
2
cr
O
Lu
O
u
a
2
s
?
MPN
X
a.
234
* FtCI, ADDED
Effect of ferric chloride dosage on coliform count in dewatered
fresh sludge.
(Reprinted by permission from Vol. 30, No. 11, p. 1373,
November 1958, Sewage and Industrial Wastes)
-------
-1U5-
Vacuum filtration facilities are generally sold as a package by
filter manufacturers. In addition to the filter itself the
package normally includes vacuum pumps, sludge feed pumps,
filtrate pumps, sludge conditioning tanks, chemical feed pumps,
and belt conveyors to transport dewatered filter-cake. Except
for cake conveyors, an optimum design includes complete individual
accessories for each vacuum filter^ 251). This enhances operational
flexibility and reduces the opporutnity for hydraulic inbalance.
Because it is difficult to predict exactly the performance of
vacuum filters before installation, a maximum amount of flexibility
consistent with reasonable economy should be designed into the
system. An optimum design would incorporate the following features:
1. Separate chemical conditioning tanks having two mixing
stages: (a) a flash mix variable from 100 to 1000 rpm,
and (b) a slow mix variable from 10 to 100 rpm. Variable
detention periods for each stage is desirable. The
tanks should be open so the filter operator can observe
the effect of the chemical additives, and it should be
possible to change the chemical addition point^6*) .
Sludge and additives should be mixed with agitation
that produces the best floe and at minimum chemical
dosage. Conditioning tanks should be adjacent to the
filter and have flume discharges over the lip of the
filter pan to broadly distribute the treated sludge.
Filtering sludge as soon as possible after chemical
conditioning is desirable.
2. Sludge and chemical dilution facilities. Each sludge
has an optimum solids concentration for filtration.
If the feed sludge has a high solids concentration
it may, therefore, be desirable to add dilution
water. Chemical flocculents are often more efficient
if diluted so adequate water lines should be provided
for this operation.
3. Variable speed filter pan agitator drives. The
stability of sludges and their need for agitation in
the pan varies, so flexibility in the agitation speed
is very desirable.
H. Delivery of a uniform sludge feed.
5. Effective filter media cleaning facilities.
-------
-146-
6. Convenient means of positioning the internal bridges
that control the point of cake pick-up, drying and
discharge.
One interesting design modification discussed by Emmett and
Dahlstrom is the top feed filterl21*5). in this unit, cake
formation occurs with gravity rather than against it as is the
case with conventional rotary drum filters. The use of top feed
filters is dictated by the feed solids size distribution and the
rate of cake formation. There is no limitation on larger particle
sizes, but the solids should contain no more than 30 percent of
material passing a 200 mesh and only very small quantities of
5-micron-diameter particles. A one-inch cake should be formed in
10 seconds or less.
Top feed filters are not used for dewatering sewage sludges and they
are used infrequently to dewater industrial sludges because: (1) they
are more expensive per ton of solids processed than other filters,
(2) there is a lack of understanding of the basic phenomena, and
(3) there is a need for improved engineering design. The concept of
top feed filtration is good; hopefully, the design will be improved.
The primary goals in the operation of vacuum filters usually are to
attain high filter yields and minimum cake moistures^6). These
goals generally require that the following important design and
operating parameters be observed:
1. Provide a thick, feed sludge.
2. Effectively condition the feed sludge.
3. Operate the vacuum filter at a minimum cycle time
consistent with adequate cake discharge and desired
cake moisture.
U. Operate the filter with a maximum drying vacuum.
5. Operate the filter with low submergence.
6. Prevent media blinding*
In some cases, the highest yields and lowest cake moistures may
not be the most economical or reasonable way to operate. Depend-
ing on the method of ultimate cake disposal, other goals may be
more reasonable. Each vacuum filter operator must decide on the
particular goals appropriate to his plant.
-------
-147-
Because filtration is more of an art than a science, it is
desirable to have well-trained filter operators. It is possible
to substitute excessive chemical dosages for close control of
the filtration operation, but this is uneconomical; chemical
costs may account for 50 percent of the total vacuum filter
operating expense. Filter operations should never be considered
as having reached the ultimate in performance because new refine-
ments in techniques such as dilution, agitation, and chemical
treatment allow process improvements.
Filtered digested sludge is used as a soil conditioner on farmland
and public parks. It also may be incinerated, heat-dried, com-
posted, or deposited in landfills. The disposal of filtered raw
sewage sludge is not quite as simple. It often is heat-dried,
incinerated, or buried in sanitary landfills. Some filtered
sewage sludge is dumped on public land, but this procedure is
risky due to possible odor and health problems from the high
bacteria content. Many inorganic industrial sludges are filtered
and satisfactorily dumped in landfill areas.
Performance - The performance of vacuum filters is usually
measured by filter yield, the filter-cake characteristics, and
the quality of the filtrate. Cake production and moisture are
particularly important, especially if the sludge is ultimately
incinerated or heat-dried. For example, the importance of filter-
cake moisture has been emphasized by Ettelt and Kennedy for the
Chicago Sanitary District operation^56). As shown in Figure 1H.X,
there is a direct relationship between filter-cake solids and the
capacity of their flash drying plant. Note that 15 percent cake
solids allows the disposal of t80 tons per day but an 18 percent
cake solids allows the disposal of 610 tons per day. Vacuum
filtration is subject to many variables; generalized data should
be viewed with caution.
The following general and specific data are offered with only
minor interpretation.
Typical yield data for a variety of sludges were listed in the
design portion of this section. Cake moistures vary from 55 to
85 percent by weight depending on the type of sludge handled and
the filter operating conditions. The cake moisture should be
adjusted to the method of final disposal because it is inefficient
to dry a cake more than is required. For example, at some
treatment plants using incineration for final disposal, the
filter cake has had to be wetted in order to reduce incinerator
temperatures.
-------
-148-
FIGURE i4.y-EFFECT OF DRIER FILTER CAKE ON
CAPACITY OF FLASH DRYING PLANT
AT CHICAGO
800
H 700
-J
CQ
0.
O
600
CO
o
CL
CO
o
g 500
O
CO
400
13
14
15
16
17
18
19
20
21
VACUUM FILTRATION CAKE SOLIDS (%)
(Reprinted by permission from Vol. 38, No. 2, p. 255, February 1966,
J. Water Pollution Control Federation)
-------
-1U9-
Incinerati.cn of the filter-cake without the continuous use of
auxiliary fuel is desirable; if possible, the filter operation
should be adjusted to produce a cake sufficiently dry that fuel
is not needed except for start-ups. Burial of filter-cake on the
treatment plant property, however, eliminates the need for a cake
with equal dryness. In many instances, the cake moisture content
is controlled by discharge characteristics of filter-cake from
the drum. "Wet" cakes often do not allow a clean discharge and
therefore blind the media.
The concentration of total solids in the vacuum filter filtrate
is the primary measure of filtrate quality. It may vary between
100 and 20,000 mg/1 depending on the sludge type, the filter
media, and the vacuum*°5'. Activated sludges, particularly
when digested, contain higher proportions of fine particles than
primary sludge and, therefore, produce poorer filtrates. For
this reason, plants filtering activated sludge usually employ
tight filter media.
Filtrate is often returned to the head of the treatment plant.
While the solids in the filtrate normally resettle readily,
fine solids may build up and reduce overall plant treatment
efficiencies, in part due to high oxygen demand and demand for
additional chemical conditioning. Filtrate solids, therefore,
should be kept as low as possible, consistent with efficient
filter operation. Disposal of filtrate to elutriation or
thickening tanks has been investigated in the hope that any
residual flocculent activity would improve basin solids capture
efficiencies; varying degrees of success have been reported.
One vacuum filter manufacturer reported the following cake
moisture and inorganic chemical conditioning requirement for
a variety of sewage sludges^ ':
Thickened Chemical
Sludge Requirements Cake
Cone. FteCl3 CaO Moisture
Treatment Process (t Solids) (%) (%) (%)
Primary Settling
Undigested 10 1.0 6.0 66
Digested 10 2.5 7.5 70
Standard Rate Trickling
Filter
Mixed primary 6 secondary 8 1.5 7.0 68
Digested mixture 8 3.0 8.0 71
-------
-150-
Thickened
Sludge
Cone.
Treatment Process (% Solids)
High-Rate Trickling Filter
Mixed primary 6 secondary 7
Digested mixture 7
Activated Sludge
Mixed primary 6 secondary 6
Digested mixture 6
Chemical
Requirements
FeCl3 CaO
2.0
3.0
3.5
3.5
8.0
8.0
5.0
9.0
Cake
Moisture
70
72
75
76
A review of the operating records of about 60 sewage treatment
plants having used ferric salts and/or lime yielded the average
data listed below(242, 244, 253).
Chemical Dose Rate
(%) Yield
Cake
Moisture
Type of Sludge Ferric Chloride Lime (Lbs. /sq.ft. /hr. ) (%)
1.
2.
3.
4.
5.
6.
7.
8.
9.
Raw Primary
Digested
primary
Elutriated di-
gested primary
Raw primary -t-
filter humus
Raw primary +
activated sludge
Raw activated
sludge
Digested primary
+ filter humus
Digested primary
+ activated
sludge
Elutriated di-
gested primary +
activated sludge:
(a) Average
w/o lime
(b) Average
w/lime
2.1
3.8
3.4
2.6
2.6
7.5
5.3
5.6
8.4
2.5
8.8 6.9
12.1 7.2
-0- 7.5
11.0 7.1
10.1 4.5
-0-
15.0 4.6
18.6 4.0
-0- 3.8
6.2 3.8
69.0
73.0
69.0
75.0
77.5
84.0
77.5
78.5
79.0
76.2
Most of the 60 locations providing the above data have changed to
the use of polymeric flocculents in place of ferric salts and lime.
-------
-151-
Interestingly, in each sludge category, digestion increased the
cost of chemical treatment and increased cake moistures, while
decreasing filter yields. This fact should be considered in any
decision of whether to digest raw sludges.
Elutriation lowers chemical costs for digested primary sludge but
not for digested secondary sludges. At secondary waste-treatment
plants, elutriation often increases overall treatment costs because
fine elutriated solids are recycled to other treatment processes.
Average data from the many sewage treatment plants using polymeric
flocculents to condition sludges show the following^2^^':
Dose Rate Yield Cake Moisture
Type of Sludge (%) (Lbs./sq.ft./hr.) (%)
Raw primary or
raw primary and 0.2 - 1.2 6 - 20 63 - 72
filter humus
Digested primary 0.2-1.5 4-15 66-74
Digested primary _ && - 7&
and activated
Garber and co-workers studied vacuum filtration in great detail at
the Los Angeles Hyperion Treatment PlantdSO). They determined that
particle size, shape, and compressibility were very important param-
eters in filter operation. After prolonging elutriation until most
of the fines were washed out of the sludge, they obtained 30 percent
greater filter yields at 80 percent of the flocculent demand.
This investigation proved the importance of particle size, but it
does not offer a practical solution to vacuum filtration problems
because the elutriate solids must still be disposed of in an
acceptable manner. It was decided that chemical conditioning offered
a better chance to improve vacuum filtration than did equipment
redesign. After investigating dozens of chemical flocculating
materials, the Hyperion personnel chose to condition their sludge
with a combination of ferric chloride and organic polyelectrolyte.
The use of the polymer increased the filter yield by 45 percent.
Sherbeck reported the following advantages upon substituting organic
polymeric flocculents for ferric chloride and lime at Bay City,
Michigan*182*:
-------
-152-
1. Less chemical handling equipment and space was required
because the average inorganic flocculent dosage of *f«*0
pounds per ton was replaced by an organic flocculent
dose of 17 pounds per ton.
2. Less incinerator ash was produced, easing ash disposal
problems (cake volatiles increased from 63 to 75 percent).
3. The vacuum filter and incinerator operating time was
reduced due to the increase in filter yield from 3.1 to
6.3 pounds per square foot per hour.
4. Employee morale increased due to improved safety and
cleanliness.
Thirty percent of the vacuum filter surface area was sealed off in
order to prevent overloading of the incinerator as a result of
increased filter yields.
Morris reported that plants in Atlanta, Georgia also converted to
the use of a cationic polymer in place of ferric salts and lime^1^3),
He summarized chemical evaluations and plant data from Atlanta as
follows:
1. The conversion to polymers was simple because existing
equipment could be used.
2. Laboratory Buchner funnel tests were good indicators
of plant efficiency.
3. Polymer operating efficiencies were significantly
affected by the feed-sludge solids concentration
and alkalinity.
•*. Polymers allowed exceptional production rates at
only a small loss in drying efficiency.
5. Polymeric flocculents showed no corrosiveness, toxic
effects, or inorganic residues and left an easily
cleaned filter media and filter drum.
6. Polymer treatment resulted in a filtrate clarity
equal to, or better than, that obtained under
previous conditions.
The advantage of decreased handling of chemicals, provided by
polymers, was emphasized by Goodman^03). At Battle Creek,
-------
-153-
Michigan, he reported that 420 pounds per ton of ferric chloride
and lime were replaced by 101 pounds per ton of combined ferric
chloride and cationic polymer. The substitution resulted in
higher cake moistures, but less incinerator ash.
At Baltimore, Md., Keefer evaluated the use of aluminum chloro-
hydrate as a sludge conditioning agent and found that it produced
higher yields than ferric chloride, but also higher cake moistures
This material has been commonly used in England and Western Europe.
Keefer's interest was academic because aluminum chlorohydrate was
not manufactured in the U.S.A. At Buffalo, New York, the use of
aluminum chloride, when substituted for ferric chloride, decreased
the chemical costs by 53.1 percent^190). Buffalo also used a lime
slurry generated from the manufacture of acetylene; the waste
carbide lime allowed significant cost reductions at many wastewater
treatment plants.
Carpenter and Caron conducted numerous laboratory tests on the
dewatering of board-mill and deinking paper sludges by vacuum filtra
tion^68). Their evaluation of fiber addition and polymer treatment
of the sludges produced the following findings:
1. Drainage time was significantly reduced by the addition
of fiber (35 to 70% with a 25% fiber addition).
2. Increasing the sludge fiber concentration improved
filtrate clarity and cake discharge.
3. Filter loading rates of a 2 percent slurry could be
doubled by a 20 percent fiber addition, but 4 to 6
percent slurries were unaffected by fiber additions
up to HO percent. It was concluded that a 15 percent
fiber content allowed successful vacuum filtration of
most board-mill and deinking sludges.
4. Polyelectrolytes increased the dewatering rate of
board-mill sludge by 184 percent at a dosage of 10
pounds per ton. The increase for deinking sludge
was the same (184%) at a dosage of 5 pounds per
ton.
5. Polyelectrolytes were concluded to be competitive
in performance with inorganic flocculents and
involved less handling and maintenance.
-------
-15U-
Carpenter and co-workers also evaluated fly ash as a vacuum
filter precoat for dewatering board-mill and deinking
These sludges represented the two extremes in paper mill sludges;
because the board-mill sample was high in organic solids and the
deinking sample was high in inorganic material. Conclusions from
the study were:
1. Sludges not "dewaterable" by conventional vacuum
filtration could be handled by fly ash precoat
techniques.
2. By using a fly ash precoat, drainage rates increased
when dewatering low inorganic-content sludges, but
the rates decreased when dewatering highly inorganic
sludges.
3. The portion of ash passing a 20 mesh screen, but
retained on an 80 mesh, appeared to be the most
effective ash particle size.
4. Fly ash precoating allowed a good separation of
sticky cakes from filter media surfaces. Minor
media blinding was noticed.
5. The ash requirement for precoating was usually
less than the quantity available at board-mills
burning pulverized fuel.
Sludge ash can be used instead of chemicals to improve filtration
of raw sludged*79). The ash is added to the raw sludge in amounts
equal to or greater than the dry solids content of the sludge.
Reported advantages were: less solids in the filtrate and lower
cake moisture(as little as 30 percent). Of course, adding that
much dry ash decreases the sludge moisture.
Loading a rotary, vacuum filter with chemically conditioned
digested primary sludge at the top rather than at the bottom
was investigated at the Midland, Michigan, sewage treatment
plant^251*). This procedure takes advantage of gravity and the
rapid drainage characteristics of polymer treated sludge. After
a 15-second drying time, a filter yield of Ut pounds per square
foot per hour and a cake moisture of 70 to 75 percent was obtained.
Filtrate solids were 100 to 500 ppm. Loading involved the spreading
of a 2.5 inch sludge-layer across the top of a rotary filter covered
with an "open" saran cloth. The filter bridges were readjusted to
allow maximum dewatering during the brief cycle.
-------
-155-
Economics - The capital and operating costs of vacuum filtration
vary widely, making it difficult to generalize about the process
economy. Filters themselves, including necessary auxiliaries,
cost $95 to $275 per square foot, depending on the size of the
installation and the filter media (stainless steel media represents
a very high cost). The cost of a building to house the equipment
doubles the capital outlay^53).
Operating costs of vacuum filtration are usually greater than the
capital costs. These costs normally include labor, power, chemicals
and maintenance, but they should also include a portion of the
administrative overhead and the cost of hauling filter cake to
landfill sites, etc. HacLaren estimated that operating costs were
$10 to $16 per ton of cake, depending on the size of the facility
and the amount of chemicals used^53). His figures are low for many
U.S. installations.
Simpson and Sutton surveyed costs from numerous sewage treatment
plants and concluded that total operating costs varied from $5.3H
per ton to $30.17 per ton*650'. They gave the following breakdown
of the direct operating cost:
Labor and direct supervision
Chemicals and supplies
Electric power
Maintenance
The direct costs represent 74 percent of the total operating cost
figures ($5.3U to $30.17/ton). As expected, digested and activated
sludges were more costly to dewater than raw and primary sludges.
Hauling the filter cake from the plant site (costing at least
10 cents per ton-mile) could significantly increase the operating
cost figure.
In 1958, Dietz reported on a survey of vacuum filtration costs at
sewage treatment plantsC*6). The following capital costs, based
on 25 years' depreciation and a 5% interest rate, were listed
for different sizes of installations, determined by population
served:
-------
-156-
No. of People Served
6,500 10,000 20,000 30,000 HO,000
1958 costs $38,811 $38,811 $52,209 $78,239 $106,233
1966 costs (Est.) $48,500 $48,500 $65,250 $97,800 $133,000
The above costs for vacuum filtration were in all cases lower than
costs for digesters with sand beds or lagoons.
Annual capital costs were also reported by Dietz:
No. of People Served
6,500 10,000 20,000 30,000 40,000
1958 costs $7,670 $ 8,344 $11,527 $14,908 $19,750
1966 costs (Est.) $9,600 $10,400 $14,400 $18,650 $24,700
For cities of the size listed, the annual cost of vacuum filtration
was always more expensive than digestion plus lagooning, but it was
cheaper than digestion plus sand beds for populations over 25,000
people.
Operating costs per ton were $8.20 to $32.40 per ton with a median
of about $20 per ton. The costs included chemicals, power, and
labor.
Actual operating records from nearly 60 sewage treatment plants
showed the following chemical costs, classified by type of sludge
and plant size(253). Small plants usually had higher chemical
costs because they often purchased anhydrous ferric chloride and,
of course, the volume of chemicals used is relatively small (hence,
higher prices). A total operating cost figure is given in paren-
theses, assuming chemical costs equal 40 percent of the total
operating figure.
-------
-157-
Small Plants Large Plants
Sludge Type ($/ton) ($/ton)
Raw primary $ 7.00 ($17.50) $ 3.00 ($ 7.50)
Digested primary $11.50 ($38.70) $ 5.50 ($13.75)
Elutriated digested $ ^.00 ($10.00) $ 3.50 ($ 8.75)
primary
Raw primary + $10.20 ($25.50) $ 6.50 ($16.30)
filter humus
Raw primary + - $10.50 ($26.20)
activated
Digested primary + $21.50 ($53.80) 5 9.50 ($23.80)
filter humus
Digested primary + $13.00 ($32.50) $12.50 ($31.25)
activated
Raw activated - - $ 6.50 ($16.30)
Elutriated digested
primary + activated - - $ 8.50 ($21.28)
In each category, the chemical and total operating costs vary
widely depending on many variables such as the fraction and
characteristics of industrial wastes in the sewage, the efficiency
of the filtration system equipment, the cost accounting technique,
and the skill of the filter operator. As mentioned before, the
argument that digestion reduces overall chemical treatment costs
is open to question after a review of the data. In all categories
digestion increased chemical conditioning costs.
A review of the District of Columbia annual- reports revealed that
the cost of vacuum filtration increased significantly when treat-
ment process at the large District plant was converted from a
primary process to a high-rate aeration activated sludge process.
The data listed in Table 14.1 include 3-year averages for both
treatment classifications:
-------
-158-
Sludge Type
Elutriated,
digested primary
Elutriated,
digested primary
+ activated
Table 1'
FeCl3 dose (%)
4.72
7.48
Filter Yield *Annual Sludge
(Lbs./sq.ft./hr.) Disposal Cost
5.96
4.82
$105,333
($16/ton)
$340,000
($31/ton)
*Includes operating costs for elutriation, vacuum filtration, and
preparation for use as a soil conditioner.
In conclusion, the total annual cost of vacuum filtration is generally
$8 to $50 per ton of dry solids dewatered.
Summary - Since 1960, vacuum filtration as a method of mechanically
dewatering sludge has become very popular at cities serving popula-
tions of 10,000 and greater. It has also been a very popular
technique for dewatering numerous industrial sludges. This popularity
recognizes the following vacuum filter advantages: (1) a wide variety
of sludges can be dewatered, (2) filters occupy a smaller space than
sand beds or lagoons and are unaffected by climate, (3) a relatively
dry filter cake that can be incinerated is produced, which eliminates
the need for digesters, (4) the solids capture can be very good,
and (5) plant operations are improved because filters offer some
flexibility in scheduling so dewatering can be coordinated with other
treatment processes.
However, vacuum filtration has significant disadvantages as indicated
by many abandoned units*151). A survey of West Coast States revealed
that 12 of 19 cities had abandoned or were considering abandoning
vacuum filter facilities^180). Most of the cities reported high
operating costs as the primary reason for their displeasure with
filtration. Excessive chemical requirements are usually the basis
for the high operating cost. Other factors involved in the unsatis-
factory operations include: (1) frequent media blinding requiring
shutdowns, washing and a resultant high labor cost; (2) odors
from filtering raw sludge; (3) the need for duplicate units when
filtering raw sludge; (4) the need for more highly trained, filter
operators than are required with other dewatering techniques;
(5) the lack of scientific control to accommodate fluctuations
in sludge quantity and quality; and (6) the necessity for additional
handling steps because filtration does not represent ultimate sludge
disposal*
-------
-159-
The most common problems in sludge filtration involve chemical
conditioning and filter media blinding. Often these problems
are associated with erratic sludge flows and sludge quality. To
compensate for this situation, overdosing with chemicals is
practiced, which leads to high chemical and maintenance costs.
The latter occurs because of chemical deposits in the filter
media and related equipment.
A survey of dissatisfied users of vacuum filters revealed one
interesting fact—vacuum filters have been installed at locations
where failure was predictable due to sludge characteristics not
conducive to this dewatering technique. This situation usually
occurs where biological sludges are digested, producing dilute
sludges with a high concentration of small particles.
Because the filter operator is faced with a sludge whose basic
character he cannot control plus equipment of fixed design, he
usually considers chemical conditioning as a solution to dewater-
ing problems. Chemicals can agglomerate small particles and
produce a sludge of greater porosity and a filter cake that is
less compressible.
Small treatment plants are increasingly adopting mechanical
dewatering techniques. The improvements previously mentioned
have encouraged this trend. However, vacuum filtration cannot
usually compete economically with digestion plus sand beds,
liquid land disposal or lagoons at plants with less than 10,000
population or 3.5 mgd total flow. At larger plants vacuum
filters are encountering some competition from centrifuges
because they offer simplicity and lower costs. But, vacuum
filters will continue to be used because they can dewater
many difficult sludges and capture a high percentage of solids.
Certain significant improvements have been made recently in
the filtration process, but more are required. Such improve-
ments are non-clog media, a wider selection of synthetic media,
designs that include individual sludge contitioning tanks for
each filter with short and gentle distribution of conditioned
sludge, and the development of polymeric flocculents which
have lower capital and operating costs.
Additional improvements are in order to replace some of the art
of filtration with new technology. A basic step would be the
design of equipment to deliver sludge to the filter at a uniform
rate. Another would be to incorporate the equipment flexibility
discussed in the design parameters. Sludge quality is difficult
to predict* particularly over the life of equipment that might
last as long as 25 years, so flexibility to accommodate unexpected
sludge characteristics is desirable.
-------
-160-
Instrumentation to accurately proportion conditioning chemicals
to the sludge could greatly reduce operating costs. This
instrumentation could measure sludge flows as well as the demand
for flocculating chemicals.
Further research in sludge conditioning is warranted. This may
involve heat treatment, freezing, or the use of electricity,
inert additives or any number of other techniques (see Sludge
Conditioning and Dewatering - Unusual Processes chapter).
Garber's work on thermophilic digestion^1*0) with an objective
of producing an easily dewatered sludge should be renewed.
Economics of systems that produce a better ratio of primary to
secondary sludge should be explored because increases in the
proportion of primary sludge decrease dewatering costs. This
can be achieved by chemical flocculation in primary sedimentation
basins and perhaps by adjusting biological treatment so that it
creates a minimum of solids.
Handling primary and secondary sludge separately should be
investigated. Digesting primary sludge and later mixing it
with raw secondary sludges may produce lower dewatering costs.
New filtration equipment such as the top load filter described
by Eramett and Dahlstrom^21*5^ should be explored further.
B. Pressure Filtration and Miscellaneous Processes
General - Mechanical pressure filtration of sewage sludge is a
common practice outside the United States. In the United States
pressure filtration has been used by industry in a variety of
process dewatering systems but its use for dewatering waste
sludges is uncommon. In recent years the use of "plug" presses
for dewatering waste sludges has been promoted in this country.
The design of these systems takes advantage of free water drain-
age followed by the application of low pressures.
Pressure Filtration - It has long been known that pressure
Filtration could effectively remove both the free and interstitial
water from a wide variety of sludges. Pressure filtration has
never been enthusiastically accepted in this country because
it is a batch operation involving high labor and maintenance
costs. Where pressure filtration is in use, leaf filters are
the most common type of unit.
Like vacuum filtration, a porous media is used in leaf filters
to separate solids from the liquid. The solids are captured
in the media pores; they build up on the media surface; and
they reinforce the media in its solid-liquid separation action.
Sludge pumps provide the energy to force the water through the
media.
-------
-161-
Thompson and Proctor described the use of a mechanical filter
press in England to produce a filter-cake with an unusually
low moisture content of HO percent, the feed solids averaged
10 percent(280). The operational procedure described is
typical: (1) lime is added for sludge conditioning (20 Ibs.
per dry ton), (2) the lime treated sludge is stored 48 hours,
(3) the filter is filled and the sludge pressed in 3 hours,
and (U) the filter is unloaded in one hour and the equipment
cleaned. Two pressings per day shift per unit are possible with
the above schedule. During warmer weather the sludge is also
treated with 6 ppm chlorine to prevent odors and filter cloth
blinding.
In Hamilton, Scotland, presses were successfully used for
dewatering sewage sludges^67^. Elutriated digested sludge was
conditioned with 120 pounds per ton aluminum chloride and pumped
to presses at pressures up to 90 psi. The 4 percent feed sludge
required a filtration time of 5-1/2 to 18 hours to produce a
sludge cake 1-lA inches thick.
Lime, aluminum chloride, aluminum chlorohydrate, and ferric
salts have been commonly used overseas to condition sludge prior
to pressing. The successful use of ash precoating has also been
reported. Minimum chemical costs are supposed to be the major
advantage of press filters over vacuum filters.
Leaf filters represent an attempt to dewater sludge in a small
space quickly. But when compared to other dewatering methods,
they have major disadvantages: (1) batch operation, (2) high
operation and maintenance costs, and (3) high filter-cake moisture,
The above disadvantages together with the high labor costs in the
United States preclude the use of leaf filters in most situations.
These disadvantages were apparent in a recent evaluation of
pressure and vacuum filters for dewatering board-mill and
deinking sludges. The conclusion reached after comparative tests
was that vacuum filters are superior^2**9'.
Caron and Carpenter reported on the use of other mechanical
presses, screw and hydraulic, to dewater papermill sludges^8).
The hydraulic press dewatered board-mill sludge to 40 percent
solids at a pressure of 300 psi and a pressing time of five
minutes. It appears that little additional dewatering was
gained by increasing the pressures above 300 psi.
Hydraulic and screw presses, while effectively dewatering sludges,
have the major disadvantage of requiring a thickened sludge feed.
-------
-162-
Their use may therefore be limited to situations similar to
those reported by Coogan and Stovall*325). They described the
use of a circular V-press to dewater papermill sludge already
partially dewatered by a centrifuge. Further dewatering was
recommended because ultimate disposal was by incineration,
and papermill sludges are difficult to incinerate without the
use of auxiliary fuel. Feed solids to the press averaged 20
to 25 percent solids and the press delivered sludge to the
incinerator averaging 33 to 38 percent solids. In the future,
more industrial sludges will be incinerated as space for lagoons
disappears. Then, pressing as described above, may become more
popular.
Free Drainage and "Plug" Presses - In recent years a few small
communities and some industries have installed dewatering
equipment that takes advantage of free water drainage followed
by the application of low pressures. The two proprietary
systems using this technique are the "Roto-Plug"^263) and
"DCG Solids Concentrator. M(2^ 8) One objective of both systems
is to avoid a critical pressure whereby the sludge solids
structure breaks down blinding the filter media. Another
objective is to avoid large dosages of flocculents necessary
to build a firm solids structure. Most sewage sludges consist
of gelatinous particles that collapse under low mechanical
pressures. The use of chemicals or fibrous materials to
condition sludges increases the allowable critical pressure,
but the "plug" presses were developed to minimize the need
for these materials.
Dewatering in the two proprietary systems is accomplished in
successive stages with increasing pressure in each stage.
This delays the period of heavier pressures until the sludge
concentration has increased to a point where pressure can be
applied without collapsing the solids structure^278). The
manufacturers of the proprietary equipment have claimed that
sludge conditioning materials are unnecessary with fresh sludges
because a natural floe already exists, but polymers or waste
paper pulp are useful for conditioning septic or digested sludges
to prevent structural collapse of the
The "plug" processes begin with a thickening step using free
drainage of easily separated water through a nylon cloth under
a low pressure of 1 to 1.5 inches of water. As solids accumulate,
a rolling mass of sludge or plug is formed which further squeezes
water from the sludge due to it own weight. Figure 14. XI depicts
this and following steps. The plug forces the thickened sludge
into the cake formation or compression unit. Here the sludge is
-------
-163-
Plgure 14.XI
Roto-Plug Flow Diagram.
THICKENING CELL
;a&3JI 2 ipr.i)
COMPRESSION UNIT
UbMt 2 ipm)
(Reprinted by permission Nichols Engineering & Research Corp.)
-------
-164-
pressed at about 10 to 15 psi between a wedge-wire drum and
rubber covered rollers. Additional compression filters can
be installed in series if further dewatering is required.
Pressed sludge is incinerated or hauled away to land disposal.
Advantages claimed by both manufacturers of the "plug" process
are: (1) little if any sludge conditioning materials are necessary,
(2) power consumption is low, (3) only a small area is required
for equipment, CO uniform cake production is possible, and (5)
the equipment is simple and economical*278* 283),
Basically the "plug" manufacturers have acknowledged the difficult
characteristics of sludge such as its weak solids structure. They
attempt to work with the sludge as it is rather than to alter its
basic character. By taking advantage of free drainage, they
remove large quantities of water very economically, and by opera-
ting below a critical pressure of about 20 psi, they avoid
operational problems common with vacuum filters.
How well has the equipment performed? The published data show
cake moistures from 65 to 85 percent*66* 27°» 282>. At Caldwell,
New Jersey, cationic polymers were used to condition the digested
sludge at a cost of S8 to $10 per dry ton of solids*282).
Polyelectrolytes or waste paper pulp were also used at a number
of other "plug" press locations* Because of the excellent idea
of using free drainage of unbound sludge water, it seems logical
to encourage the use of flocculents for solids agglomeration and
even more rapid drainage of free water.
A variation of the "plug" press was described by
He reported on the use of a Rice Barton Water Extractor for
dewatering primary and activated sludge from a papermill. This
equipment consisted of an inclined constant-pitch screw rotating
at low speeds in a perforated basket. The dilute sludge was fed
at the bottom and conveyed by the screw to the top where a plug
was formed and later broken up prior to discharge.
Data collected from test runs showed that a 2.7 percent solids
feed sludge can be dewatered to a 15 percent sludge cake. This
degree of dewatering would not be adequate for many installations.
Another major disadvantage was the reported low solids capture of
70 percent. If the solids capture could be improved by the use
of conditioning agents, this equipment might be useful as a
first step in a two-step dewatering system.
Heymann Sludge Thickening Process - More than 20 sewage treatment
plants in Germany and Switzerland have used the "Heymann" process
consisting of vibrating screens and rotary filter presses* I32).
-------
-165-
Digested sludge was dewatered to 35 to HO percent solids using
a combination of a coarse screen, a sonic screen, a sonic filter,
and a roll press (66). xhe screening of sludge by vibrating screens
is discussed in the Screening section of this chapter.
Data from Germany, where the Heymann process was developed in 195H,
disclosed that the average raw sludge moisture was reduced from
a feed of 93.4 percent to a roller press cake of 7H.5 percent.
Swiss data showed that the moisture was reduced from roller pressing
alone of 80.6 percent in the roller feed to 75.8 percent in the
pressed
The advantages claimed for the Heymann process are that no chemical
conditioning of sludge is required and the dewatered sludge has
good drying characteristics. However, two major disadvantages pre-
clude its use in most installations. First, the filtrate solids
were reported as an excessive 2.16 to 9.2 percent in U.S.A. tests
and, second, the roller pressed cake had a high moisture content
(65 to 7«*.3%)<132>.
Two other sewage sludge press-dewatering processes have been
described in the literature. One involved sludge thickening
followed by mixing with previously dried sludge; moisture con-
centrations of 50 to 55 percent were obtained. This mixture
was then extruded into hollow shapes similar to building tile.
The extruded shapes were air or heat-dried, used for fertilizer,
or incinerated'2®1*). The other process started with a 5 percent
sludge which passes through sieves and roller mills. A 20
percent reduction in the sludge water content was
Summary - Dewatering waste sludges by pressing has not been
widely adopted for two major reasons: (1) the resultant cake
is not sufficiently dried, and (2) the separated water contains
excessive solids. Improvements in equipment and/or the use of
chemicals or other sludge conditioning materials could at least
partially solve these limitations. The limitations are important
because recycling effluent solids decreases overall treatment
plant efficiencies, and the high cake moisture limits the
ultimate sludge disposal choices or makes them unnecessarily
expensive. Pressing techniques may be limited to a two-stage
dewatering system installed prior to incineration or heat
drying.
C. Centrifugation
General - Numerous consulting engineers and equipment manufacturers
have believed that the centrifuge will replace the vacuum filter
-------
-166-
as the most popular mechanical device for dewatering sludge.
The centrifuge is not new to waste treatment; a perforated
basket type was used in Germany to dewater raw primary sludge
as long ago as 1902. In Milwaukee, a centrifuge was evaluated
in 1920, but the operating results on sewage sludge dewater-
ing were disappointing^12**'. However, centrifuges are becoming
increasingly popular today due to design improvement, greater
sales promotion by manufacturers, and increased dissemination
of successful performance data.
Centrifugation has some inherent advantages over vacuum filtra-
tion and other processes used to dewater sludge. It is simple,
compact, totally enclosed, flexible, normally used without
chemical aids, and the costs are moderate. These advantages
have resulted in the installation of 50 units around the country
for dewatering sludges^262^. Industry particularly has accepted
centrifuges in part due to their low capital cost, simplicity
of operation, and effectiveness with difficult-tc-dewater sludges,
Theory and Operation - The most effective centrifuges to dewater
waste sludges are horizontal, cylindrical-conical, solid bowl
machines. Basket centrifuges dewater sludges effectively but
liquid clarification is poor. Disc-type machines do a good job
of clarification but their dewatering capabilities leave much
to be desired(126J.
Basically, centrifuges separate solids from the liquid through
sedimentation and centrifugal force. In a typical unit, (Figure
It.XII) sludge is fed through a stationary feed tube along the
centerline of the bowl through the hub of the screw conveyor.
The screw conveyor is mounted inside the rotating conical bowl.
It rotates at a slightly lower speed than the bowl. Sludge
leaves the end of the feed tube, is accelerated, passes through
the ports in the conveyor shaft, and is distributed to the
periphery of the bowl. Solids settle through the liquid pool,
are compacted by centrifugal force against the walls of the
bowl, and are conveyed by the screw conveyor to the drying or
beach area of the bowl. The beach area is an inclined section
of the bowl where further dewatering occurs before the solids
are discharged. Separated liquid is discharged continuously
over adjustable weirs at the opposite end of the bowl.
Parameters - In centrifugation, process variables are: (1) feed
rate, (2) sludge solids characteristics, (3) feed consistency,
(t) temperature, and (5) chemical additives. Machine variables
are: (1) bowl design. (2) bowl speed, (3) pool volume, and
conveyor
-------
-167-
i-'iguru m. XII
LIQUIDS
DISCHARGE
Operation of horizontal Supor-D-Cantei centrifuges
SOLIDS
DISCHARGE
(Reprinted by permission Sharpies-Equipment Division Pennsalt
(.'hemieal Corporation)
-------
-168-
Two factors usually determine the success or failure of centrifuga-
tion — cake dryness and solids recovery. Guidi summarized the effect
of the various parameters on these two f actors d2**':
To Increase Cake Dryness To Increase Solids Recovery
1. Increase bowl speed 1. Increase bowl speed
2, Decrease pool volume 2. Increase pool volume
3. Decrease conveyor speed 3. Decrease conveyor speed
4. Increase feed rate 1. Decrease feed rate
5. Decrease feed consistency 5. Increase temperature
6. Increase temperature 6. Use flocculents
7. Do not use flocculents 7. Increase feed consistency
The effect of bowl speed on solids recovery or capture is shown
in Figure 14. XIII for a mixture of raw primary and activated
sewage sludge^126). Blosser described very similar data for
papermill sludges^260). While the ideal speed for each appli-
cation is determined by many factors, the data showed improved
captures at higher speeds.
Blosser and Guidi agreed that an increase in the pool volume, or
detention time, increased the solids recovery, as shown in
Figure It.XIV^26'. This increased recovery, however, is achieved
at the sacrifice of cake dryness, as described in Figure 1<*.XV(126>.
The incompatibility of cake dryness and solids recovery is also
shown—in Figure 1U.XVI-- for digested primary and activated sewage
sludge.
Papermill sludge and digested primary sewage sludge also responded
similarly to changes in the feed rate(l26» 260). Figure Pt.XVII
by Guidi shows a decreasing solids recovery with increasing feed
The effect of bowl speed on cake dryness is described
by Figure m.XVIH*126'. Increases in bowl speed increased the
centrifuge cake-solids content.
Figure 1M. XIX shows the effect of chemical additives on the
centrifuge dewatering process t^93). The graph was developed from
data generated from the dewatering of a chemical plant sludge
consisting of both raw primary and activated sludge. Low
dosages of organic polyelectrolytes greatly increased solids
-------
Figure 14.XIII
EFFECT of BOWL SPEED on RECOVERY
% recovery
94 •
9O -
86 -
82 •
78
74
52OORPM
445O RPM
J 4 5
feed rate-GPM
STP G9 Connecticut
Raw Q Activated
345Ox'Vt
255O x "G
T~
6
0>
10
i
(Courtesy Dorr-Oliver, Inc.)
cylrcon.
-------
Figure 14.XIV
RECOVERY Vs POOL VOLUME
90-
86-
82-
% recovery
78-
74-
70
I 2
shallow
2 GPM
4 GPM
•j
3
I
STP E, Massachusettes
Raw Primary Q Bio filter
~T
J
5 6
>deep
pool volume
(Courtesy Dorr-Oliver, Inc.)
cyl-con.
-------
Figure 14.XV
CAKE DRYNESS VsPOOL VOLUME
35-
%cake solids
29-
27-
25
I 2
shallow
STP F, Connecticut
Raw Primary
pool volume
(Courtesy Dorr-Oliver, Inc.)
con.
-------
Figure 14.XVI
CAKE DRYNESS V$ RECOVERY
32\
3O-
28-
%cake solids 26-
24-
20
76 80
STP B, Indiana
Digested Primary 8 Activated
Polymer added
~84 00 92 $£~
% recovery
(Courtesy Dorr-Oliver, Inc.)
IOO
cy/.-con.
-------
Figure 14,XVII
RECOVERY Vs FEED RATE
94- -
92 -
% recovery 91 -
9O -
89 -
88
STP D, Pennsylvania
Digested Primary
WO Polymer
(25.1 % TS)
(29.6)
2345
feed rate - GPM
(Courtesy Dorr-Oliver, Inc.)
cyl.-con.
-------
Figure 14.XVIII
EFFECT of BOWL SPEED on CAKE DRYNESS
34^
32-
3O-
% cake solids 28 -
26-
24-
22
OORPM
-400Ox G
-2310x6
"4250 RPM
STP H, Washington
Digested Primary
feed rate GPM
(Courtesy Dorr-Oliver, Inc.)
con.
-------
-175-
Flgure 14.XIX
100
SOLIDS RECOVERY, %
02468
Chemical DOSE, LBS./TON
Crnlrifugation of primary-activated sludge mixture,
(Reprinted by permission Schools of Engineering Purdue University
-------
-176-
recovery. Chemical treatment, however, usually lowers the cake
dryness (probably due to the capture of the fine solids) so a
compromise on objectives is necessary (dryness vs. recovery).
Amero stated that polymers permit higher unit loadings as well
as higher solids recovery. He believed 90 percent recovery is
possible with chemical treatment<262'.
Obviously from the graphs, many parameters affect the ultimate
centrifuge performance* Fortunately, the machines made today
are designed with some flexibility, so adjustments can be
made for varying conditions. The many parameters and how they
are inter-related must be evaluated in each case to attain
operating procedures that will deliver the "best" dewatered
cake.
Performance - For many years the Los Angeles County Sanitary
Districts have been using centrifuges to dewater digested
primary sludge^18* 258). Sludge with an average solids content
of 5 percent is dewatered to 30 or 35 percent. Solids capture
up to 66 percent is possible when centrifuges are operated at
speeds of 1,020 to 1,580 rpm. Centrate is screened to remove
floatables and then discharged to the ocean. Centrifuge cake
is sold as a fertilizer ingredient.
A number of other California cities have used centrifuges to
dewater sewage sludges. A sludge cake of 30 percent is obtained
from a digested sludge feed of 4.5 to 5 percent at the North
San Mateo County Sanitary District plant*255» 256). The centri-
fuge is operated 9 hours per week; the cake produced is used to
fertilize city parks. Solids capture is very poor, as indicated
by a centrate containing from 2 to 2.5 percent solids. Existing
centrate drying beds are not used however for additional solids
removal because the centrate solids resettle upon recycle to
the head of the treatment plant. At San Leandro, California,
a cake of 26 percent is generated from a 3.5 percent feed of
raw sewage and industrial sludges. The centrate contains 1.9
percent solids; it causes some deterioration in overall treat-
ment plant efficiency upon recycle25 .
Caron, Blosser and Jenkins reported data from centrifuge
dewatering of many paper sludges*263* 2fi1*). Typical results
are:
-------
-177-
Feed Cake Solids
Solids Solids Capture
Mill and Sludge Type (%) (%) (%)
1. Save-all sludge from 6 35 to "40 90
felt manufacture
2. Fine paper 3 to 5 22 to 30 88 to 95
3. Save-all sludge from 1 to 5 22 to "*0 75 to 98
speciality mill
4. Boardmill 2 to 5 22 to 30 85 to 95
5. Deinking 5 to 7 25 to 30 85 to 90
6. Tissue 2 to 3 20 to 35 85 to 92
7. Kraft mill 1 to 5 22 to 3* 82 to 95
According to Jenkins dewatering allows ultimate solids disposal
by landfilling, incineration, or by-product use in rough paper or
low price fiber boards(263). He stated that the use of flocculents
increase centrifuge capacity and dewatering efficiency.
The use of polymeric flocculents in a centrifuge to dewater primary
and waste-activated sludge, generated from chemical process, improved
efficiencies as follows^25).
Control (No Flocculents) (With Flocculents)
Cake production 25,000 Ibs/day 50,000 Ibs/day
Solids capture 30 to 45 percent 95 percent
Centrate solids 27,000 ppm 300 ppm
Efficiency improvements of a similar magnitude were reported for
the dewatering of digested primary and secondary sewage sludge(200):
-------
-178-
Control Treatment With
(Without Cationic Polymer) 10 Lbs./Ton Flocculent
Feed solids 7.2% 7.5%
Bowl speed 1500 rpm 1500 rpm
Centrate solids 40,500 ppm 1700 ppm
Cake solids 18.2% 15.7%
Removal efficiency 43.8% 97.8%
Centrifuges have demonstrated their usefulness in by-product recovery
at meat packing plants. Dewatering floated and screened fats to 35
percent solids has been possible in a centrifuge^ . Because the
feed solids vary from 0 to 15 percent, the flexibility offered by
centrifuges is important.
Guidi summarized the performance of conical and cylindrical-conical
centrifuges in dewatering many different types of pulp and paper mill
sludges (see Figure 14.XX) as well as sewage sludges (see Figure 14.XXI).
He found that: (1) primary sewage sludge can be dewatered to 28 to 35
percent. (2) biofilter 20 to 26 percent, and (3) activated 18 to 24
percent'126^.
Economics - Centrifuge dewatering costs vary with the sludge to be
treated, the daily volume and consistency of the sludge, and whether
fine centrate solids must be captured in the machine. Caron and
Blosser estimated that centrifuge operation and maintenance costs for
the paper industry were $4 to $20 per ton of dry solids excluding
cake hauling(264). They listed f.o.b. capital costs as follows
(assume 1 gpm per rated HP):
25 HP 40 HP 100 HP 250 HP
$20,000 $28,000 $40,000 $50,000
Installation and auxiliary equipment, not including housing, adds
$12,000 to $25,000 to the cost. The machine-cost figures compare
favorably with those from San Leandro, Calif., ($30,000 for 75 HP)
and N. San Mateo ($11,000 for 15 HP)(256).
The centrifuge dewatering costs at the Los Angeles County Sanitary
District have been reported to be about $4.25 per ton of dry solids
recovered(258). if 60 percent solids recovery is assumed, the cost
per ton of solids feed is $7.10 per ton. This figure includes
capital, power, labor and maintenance. Whey polymeric flocculents
are required, the operating costs of centrifuges could double as
indicated by the data shown in Figure 14.XXII*126}. Chemical costs
for various sludges are $6 to $20 per ton.
-------
Figure 14.XX
PULP a PAPER WASTE
WASTE SLUDGE
UNIT
% CAKE SOLIDS
% RECOVERY
BOXBOARD
conical
28-36
86-94
BOXBOARD
conical
22-28
88-93
HARDBOARD
conical
26-28
85-95
WHITE WA TER
conical
21 -3O
78-94
-j
ID
BARKER WASTE
conical
32-40
90-93
KRAFT MILL
cy/.- con.
36-43
78-89
LIME a PAPER
cy/. -con.
45-5O
9O
PULP WASTE
cy/. -con
15
(Courtesy Dorr-Oliver, Inc.)
9O
-------
Figure 14.XXI
36-
CAKE DRYNESS
Vs
RAW SLUDGES
% cake
solids
32-
28-
24-
20-
28-35 %
16
primary
2O~26%
biofilter
18-24
activated
sludge classification
o
I
(Courtesy Dorr-Oliver, Inc. )
-------
Figure 14.XXII
POLYMER REQUIREMENTS
polymer
cost
$/ ton
20-
16-
12-
8-
0
O-8 $/T
primary
6"I6$/T
biofilter
6-2O&/T
activated
sludge classification
(Courtesy Dorr-Oliver, Inc.)
i
\->
CD
M
I
-------
-182-
In general,for any specific dewatering application, centrifuge
capital costs are about 30 percent less than the capital cost of
vacuum filters. The operating costs of the two pieces of
mechanical equipment are nearly equal. Vacuum filtration almost
always requires chemical conditioning of the sludge, which can
cost a substantial sum. Chemicals have not been used in most
centrifuge operations. Maintenance is more costly with
centrifuges than with vacuum filters because certain parts
regularly wear out. Overall, the dewatering costs for centri-
fugation appear to be less than vacuum filtration except perhaps
when biological sewage sludges or difficult industrial sludges
are dewatered. The range of total annual costs is $5 to $35 per
ton. A typical average is $12 per ton.
Summary - Centrifuges are being installed in more and more waste-
water treatment plants for the following reasons: (1) the capital
cost is low in comparison with other mechanical equipment, (2) the
operating and maintenance costs are moderate, (3) the unit is
totally enclosed so odors are minimized, (*O the unit is simple and
will fit in a small space, (5) chemical conditioning of the sludge
is often not required, (6) the unit is flexible in that it can
handle a wide variety of solids and function as a thickening as
well as a dewatering device, (7) little supervision is required,
and (8) the centrifuge can dewater some industrial sludges that
cannot be handled by vacuum filters.
The disadvantages associated with centrifugation are: (1) without
the use of chemicals the solids capture is often very poor, and
chemical costs can be substantial; (2) trash must often be removed
from the centrifuge feed by screening; (3) cake solids are often
lower than those resulting from vacuum filtration; and (4) mainte-
nance costs are high.
The poor quality of the centrate is a major problem with centri-
fuges. The fine solids in centrate recycled to the head of the
treatment plant sometimes resist settling and as a result, their
concentrations in the treatment system gradually build up. The
centrate from raw sludge dewatering can also cause odor problems
when recycled. Flocculents can be used to increase solids
captures, often to any degree desired, as well as to materially
increase the capacity (solids loading) of the centrifuges. However,
the use of chemicals nullifies the major advandate claimed for
centrifuges—moderate operating costs. New techniques should be
explored to handle centrate separately without returning it to
other conventional treatment plant units. These may include
-------
-183-
aeration for stabilization, mixing with incinerator ash prior to
filtration, or combining with digester supernatant liquor and
lime to produce a liquid fertilizer.
Centrifuges will continue to become more popular because of their
many advantages. However, while they dewater fibrous and lime
sludge easily, they do not completely replace vacuum filters because
they do not easily dewater biological sludge and clay slurries.
Research in two areas has made centrifuges more acceptable to the
waste treatment field in recent years — improved machine design and
the application of chemicals. Continued research in these areas is
recommended.
-------
-184-
D. Sand Bed Drying
General - The most common method of dewatering sewage sludge is
by drying on open or covered sand beds. U. S. Public Health
Service Publication Number 609 (1958) stated that 71 percent of
all municipal plants in the United States operated sludge drying
beds. A survey of 2M leading consulting engineers and 30 State
water pollution control agencies in many different geographic
locations indicated sand bed dewatering is the most common
dewatering method(59). Over 6,000 sewage treatment plants in
the United States use this system of sludge drying. Sand bed
drying is also the most common technique in England and Europe.
While sand beds are particularly suitable for small installations,
they are used at treatment plants of all sizes and in geographical
areas of widely varying climates. U.S.P.H.S. Publication 609
pointed out that cities with less than 10,000 people especially
favor air drying of sludge on sand beds. Kelman and Priesing
reported that 60 percent of the cities having treatment plants and
a population between 25,000 and 100,000 use drying beds. Also,
38 percent of the cities with treatment plants and populations
greater than 100,000 have drying beds^^°K
Many industrial sludges are also dewatered on drying beds. Water
plant sludge too can be dewatered in this fashion. Air drying of
sewage sludge is more or less restricted to well digested sludge
because raw sludge is odorous; it attracts insects and it does not
dry satisfactorily when applied to sand beds at reasonable depths.
Oil and grease discharged with the slimy raw sludge clog the sand
bed pores and thereby seriously retard drainage.
Parameters - The design and use of drying beds are affected by
many parameters such as: (1) weather conditions, (2) sludge
characteristics, (3) land values and proximity of residences,
(•*) use of sludge conditioning aids, and (5) subsoil permeability.
Climatic conditions are most important. Factors such as amount
and rate of precipitation, percentage of sunshine, air temperature,
relative humidity, and wind velocity determine the effectiveness
of air drying. Weather, being uncontrollable, prevents the
establishment of a reproducible scientific dewatering procedure.
Drying on sand beds occurs by drainage and evaporation. Studies
in England showed that the proportion of water removed by drainage
-------
-185-
varied between 22 and 85 percent of the total sludge moisture(221).
Factors considered to be significant in drainage are sludge
dewatering characteristics and the initial solids concentration
of the sludge. The higher the initial water content, the larger
the percentage of water removed by drainage(213).
Swanwick stated that 85 percent of the water lost from secondary
sludge is lost by drainage(13). He believed that drainage was
influenced by sludge characteristics and bed loading rates. Vogler
and Rudolfs estimated that 60 percent of the sludge water is free
or drainable, 35 percent is capillary or occluded water, and 5
percent is combined or bound water that must be removed by heat'224).
No matter which drainage figure is used, it is apparent that
evaporation is also important and, therefore, so is climate.
Sludge exposed to air dries to a moisture content that depends on
the temperature, wind velocity, and relative humidity of the air in
contact with the sludge^). Evaporation is particularly important
one to two days after sludge is applied to beds because most of the
drainage is completed by that time. After a few days the sludge cake
shrinks horizontally producing cracks at the surface which accelerate
evaporation by exposing additional sludge surface areas^2). Cracking
also enhances drainage. While rain lengthens the drying time, its
effect is less important if the sludge has dried to the point of
cracking. In addition to lengthening the drying time, excessive
rainfall on a drying sludge has a secondary disadvantage of reducing
the sludge fertilizer value because soluble nutrient compounds are
removed^8).
The effect of temperature on the rate of drying has been well
established. Quon and Ward reported that the evaporation rate
doubled when converting from a low temperature-low humidity
environment for sludge drying to one of high temperature — high
humidity(217). Fleming stated that sludge dries in 6 weeks during
the summer but requires 12 weeks to dry in the winter^*8). In
England, average sludges dry in 1 to 3 months during the summer, but
in the winter, 6 months or more may be required(?0). Records from
Birmingham, England, show that the summertime sludge drying rate
was three times greater than the winter rate<289). Because
temperature is so important, many operators of wastewater treat-
ment plants store sludge in digesters during the winter and apply
it to drying beds only from April to September.
More favorable conditions for drying may be created by covering
drying beds and providing artificial heat. These modifications
-------
-186-
will be discussed in the following design section. Alternate
freezing and thawing of sludge encourages dewatering, therefore,
disposal plants in cool climates could operate drying beds through-
out the year.
The nature and moisture content of the sludge discharged to drying
beds affects the drying process. Sludges containing grit dry fairly
rapidly, those containing grease more slowly; aged sludge dries
slower than new sludge; primary sludge dries faster than secondary
sludge; and digested sludge cracks earlier and dries faster than
fresh sludge^70). It is important that sewage sludge be well
digested for optimum drying. In well digested material, entrained
gases tend to float the sludge solids while leaving a layer of
relatively clear liquid that readily drains through the sand*2'.
However, Haseltine reported that sludge can also be "over-digested"
causing a reduction in the drying rate^209). The more water
removed by drainage, the less is required to be removed by
evaporation; the overall effect is reduced drying time.
Vogler and Rudolfs determined the effect of initial solids con-
centration on sand bed dewatering by studying paper mill white
water sludge in the laboratory< 224 ) . Figures 14. - XXIII, XXIV and
XXV depict the results of their experimentations. Note that the
percentage of drainable water in the total sludge water decreased
on a straight line basis as the sludge solids concentration increased.
Also, drainage time increased as the solids concentration increased,
therefore, the lower the initial solids, the more rapid is the
drainage. The final cake moisture in the dried solids increased
again as the solids concentration increased. Therefore, the lower
the initial solids concentration, the more complete is the drainage.
De s ign- Standard - Most drying beds are open and completely exposed.
Others are glass covered to reduce the effects of weather on the
drying process.
The "10 States' Standards" and Seelye design criteria for drying bed
size recommend the following'^ » 2':
Size of Digested Sewage Sludge Beds for an Area
and 15° N Latitude
_ _
Bed Area in Sq. Ft. /Capita
Type of Sludge Open Beds Covered Beds
Primary 1.00 0.75
High-rate trickling filter 1.50 1.25
Activated sludge 1.75 1.35
-------
-187-
too
Figure
14. XXIII
o
*
i
»
.70
_c
EFFECT OF SOLIDS CONCENTRATION
ON DRAINABLE WATER
0
o
^
o
0 0
k
o
o
,0
^\
Solid* Coke. - X
FlCUKR 1.
DG
,,
C
id
B
-
Figure
14.XXIV
EFFECT OF SOLIDS CONCENTRATION
ON DRAINA9C TIME
o of
0 0
0 ^^
o
o
°o ./
&S o
o
S S 4
Solid* Ceie, X
FlGUU 2.
Figure
XXV
C«k» Mol.lur* Con!**! X
3« « S
o » o
EFFECT. OF SOLIDS CONCENTRATION
ON CAKE MOISTURE CONTENT
o
jS^e
o
2}f
^
0
»° o^X^
o *^S^
*r
^
0° 0
o
U
8
„_
w
DO
r-
C
c
-
.•
J
g
--;
.
a
C
'
--
•
-,
•-
O
.
K
Solid* Cen«. X
Ficou S.
-------
-188-
Areas south of the standard latitude can reduce the recommended
bed area by 25 percent while areas north of this latitude should
increase the above recommended areas by 25 percent.
Design standards, formulated by the Texas State Department of Health,
for the area required for drying were based on rainfall and relative
humidity'-*'. They use the following formula:
area in sq. ft./capita = 0.01 R + F
R = average annual rainfall
F is related to humidity as specified
in the following chart:
Average annual relative humidity
(based on average readings taken
at 6:30 AM and 12:30 PH) F
<60% 0.3
60-70% 0.5
>70% 1.0
The drying bed area selected by the "Texas formula" would be very
similar to that recommended by Seelye and the "10 States"
Standards."
Other specific design standards have been discussed in the
literature. Furman said only 0.3 square feet per capita for
drying bed area was required in semi-tropical climates such as
Florida. His conclusion was based on the drying of digested
primary sludge and trickling filter humus^26). Ryan recommended
the following criteria for northern (cool) areas(212):
Open beds operated 6 months/year 0.6 to 0.8 sq. ft./capita
Covered beds 0.2 to 0.3 sq. ft./capita
In England, the following capacities have proven to be adequate
for normal sludges under normal climatic conditions(70):
Sq. Ft./Capita
Sewage Sludge Type (Open Beds)
Primary sludge 1.30
Trickling filter sludge 1.50
Digested mixed sludges 1.00
Undigested mixed sludges 2.25
Sludges inclined to be greasy 3.00
-------
-189-
English standards for covered beds are one-third less than the
area recommended for open beds. Zack stated that glass covered
beds required 30 to 50 percent less area than open beds(28).
Another criteria for the design of sand beds could be pounds of
dry solids per square foot per year. Fischer recommended the
following digested sewage sludge bed loadings based on actual
plant operation data^57>:
Lbs. Dry Solids/Sq. Ft/Year
Primary Sludge
35
70
Primary +
Filter Humus
30
60
Primary +
Activated
30
60
Activated
25
50
Open beds
Covered beds
Somewhat lower values were presented by Eckenfelder and O'Connor
for open beds to dry sewage sludge(5):
Sludge Loading
Area
Type of Digested
Sludge
Primary
Primary and standard
trickling filter
Primary and activated
sludge
Chemically precipitated
sludge
Dry Solids
Sq. Ft./Capita Lbs./Sq. Ft./Year
1.0
1.6
3.0
2.0
27.5
22.0
15.0
22.0
Drying beds usually consist of 4 to 9 inches of sand over 8 to 18
inches of graded gravel or stone'•*-» 2, 5)0 The Sand has an effective
size of 0.3 to 1.2 mm and a uniformity coefficient less than 5.0.
Gravel is normally graded from 1/8 to 1.0 inches. Drying beds are
drained by underdrains spaced from 8 to 20 feet apart. Underdrain
piping is often vitrified clay laid with open joints, having a
minimum diameter of U inches and a minimum slope of about 1 percent.
Collected filtrate is usually returned to the treatment plant.
Ideally, the applied sludge should be well distributed in the drying
bed or uneven loading rates will occur. A 5 percent sludge, for
example, may not travel more than 50 feet due to its relatively high
viscosity, which increases as it dewaters. At the Maple Lodge Works
in England, rapid distribution of sludge is achieved because each
-------
-190-
drying bed has 32 sludge application points(223). The end of the
application pipe should not be submerged and splash plates should
be used to distribute the sludge more evenly.
As indicated in the design criteria, the use of covers reduces
the required area for drying beds. Bed enclosures, usually glass,
protect the drying sludge from rain; they help control odors and
insects; they reduce the drying periods during cold weather; and
they can improve the appearance of a waste treatment plant. Fair
and Geyer stated that if the enclosures are properly ventilated,
the number of sludge applications per bed can be increased from 33
to 100 percent over beds without coversO. Good ventilation to
reduce humidity is important because enclosures restrict air
circulation, resulting in reduced evaporation. As expected, evapo-
ration is more rapid from open beds in warm fair weather, but,
during rainy or cold periods , evaporation from covered beds is
faster .
In England, many engineers have believed that covered beds are
over-rated. Swanwick and Baskerville believed that covering of
beds is useful only when there is a requirement to dewater sludge
beyond the minimum required for lifting. Also, they believed that
covering would be useful in areas of very high rainfall' 221 ).
MacLaren predicted that covered beds will be used less and less due
to difficulties involving the maintenance of mechanical equipment (53).
Adapting mechanical equipment to a relatively small enclosure is more
difficult than with open drying beds. The suggestion has been made
to combine open and closed bcds^). This system would offer flexi-
bility for varying weather conditions and have the advantages and
disadvantages of both types of drying beds.
Des ign-Modif icat ions - Some drying beds have been constructed with
asphalt or concrete bottoms to facilitate removal of the dried
sludge. These beds seem to function well in areas where evapor-
ation rates are
Lynd described an asphalt bed where 3 inches of asphalt, rather
than sand, were placed over a base-layer of gravel*208'. He
reported that the advantages were: reduced drying time and less
saint enance. Twelve beds at Salt Lake City, Utah, were paved with
asphalt; result was that reduced equipment, operation, and
maintenance costs saved $84.50 each time a bed was filled^66). At
the Maple Lodge Works in England, a 2-1/2 inch water-tight asphalt
-------
-191-
layer was placed beneath layers of gravel and fine sand to
prevent pollution of underlying water-bearing gravel(223).
Grove concluded that concrete-bottom drying beds take twice as
long to dry sludge as sand beds, but the labor cost is much
less(215). Particularly, rain caused problems due to the limited
drainage area installed at the center of the bed. Because power
equipment could be used to lift dried sludge off the beds, it took
one-seventh the time to clean the concrete bed as it did a
conventional sand bed.
In England, bed media have consisted of clinkers with a top layer
of fine ashes(7°). Comparative performance data from the use of
unsolidified media other than sand and gravel have not been
reported.
Heating sand drying-beds, particularly the glass-enclosed type,
have shortened the drying time. One theory suggested that the
heat accelerates sludge biological decomposition, producing gas
which floats the sludge solids and allows the water to drain away(221).
Heat also decreases the sludge viscosity causing it to drain more
rapidly. At Durham, North Carolina, heating coils, placed on 5-inch
centers in covered drying beds, greatly increased the drying
capacity(211). The heat was supplied by circulating cooling water
from gas engines through the coils.
Wedge-wire drying beds have been used in England with great success.
They consist of a perforated sheet laid on top of conventional
drying bed media^6*). "Support water" is first added to the drying
bed to prevent blinding of the media when sludge is added. As the
sludge is applied and forms its own filtering layer, the, support
water is slowly removed. This procedure prevents solids from
breaking through the wedge wire and plugging the drying bed.
Advantages claimed for wedge wire beds include: (1) no clogging of
the media, (2) constant and rapid drainage, (3) increased bed
capacity because more loadings are possible, (4) easy bed maintenance,
(5) easier dried-sludge removal, (6) less susceptibility to adverse .
weather, and (7) difficult-to-dewater sludges can be dried*°^> -LOJ-» *•*•
Levin quoting Stokes and Harwood reported that the following difficult
sludges can be dewatered: hydroxide sludge, bacterial slimes,
vegetable wastes, slag fines, and tannery sludges^62'.
Frequently, sludges, applied to drying beds, are conditioned with
chemicals or other materials. An increased rate of drying is the
major advantage sought in the use of conditioning aids. This
-------
-192-
advantage is very important when an inadequate drying area is avail-
able, when the sludge has poor drying characteristics, or when un-
favorable weather threatens to delay the drying process. In addition
to increasing dewatering rates, conditioning aids reduce sand-bed
maintenance because uniform sludge drying throughout its depth permits
more complete removal of cake from the sand.
Materials used to condition sludges have included inorganic
flocculents, polymeric flocculents, sawdust, sulfuric acid,
anthracite, and activated carbon. Alum and aluminum chlorohydrate
have been particularly popular as conditioning aids in England.
These aids make a sludge more porous which allows more rapid and
uniform dewatering. They also improve the structure of the sludge
solids, thereby decreasing solids compression and subsequent media
blinding.
Mechanical facilities to assist dried sludge removal can be a
solution to one of the biggest disadvantages of sand drying beds,
the availability and cost of labor. Mechanical lifting of sludge
has been practiced for many years at some large treatment plants,
such as Chicago, but now it is receiving more attention as the need
to minimize labor costs grows.
Kershaw reported that machine lifting and conveying of sludge at
Maple Lodge, England, has been successful for two reasons: (1) it
decreases labor costs and (2) it effectively increases the drying
capacity of the system because dried sludge can be removed faster,
allowing the beds to be filled more frequently'223). -j^e equip-
ment consisted of adjustable tines that lifted the sludge from the
sand, a flight-elevator fed by rotating crocodile-toothed paddles,
and conveyor belts that transferred the lifted sludge to a storage
area. The main sludge-lifting equipment rides on rail tracks
placed on top of drying bed dividing walls. Other interesting
features of the lifting unit include equipment for scraping off
excess sand from the bottom of the lifted sludge cake, disc harrows
to agitate the drying bed surface, and equipment to level the sand
surface after lifting so that the beds can be immediately refilled.
In lieu of mechanical lifting, the dried sludge is usually forked
or shoveled into wheelbarrows or trucks. The ideal situation,
however, is probably one where the treatment plant operator con-
vinces the public to fork their own sludge and carry it away after
shredding.
-------
-193-
Operations - Sand bed operations are more of an art than a science
because of the large number of uncontrolled variables. When
sludge is first applied to a drying bed rapid drainage occurs, but
soon the hydrostatic pressure of the sludge draws sludge solids
into the sand and compresses the layer of sludge in immediate con-
tact with the sand. Eventually the drying bed becomes spongy and
resists liquid flow, so evaporation has to complete the drying
process. The operation is necessarily intermittent because dried
sludge is almost always removed before more sludge is applied.
Sludge is normally applied to a drying bed at a depth of 8 to 12
inches. This varies at different locations depending on the
sludge characteristics, weather, rate of drainage and method of
sludge removal. Bowers believed that the lower end of the range is
advisable because the water drains better, there is less chance
of sealing the sand with sludge particles, and the sludge cracks
faster than it does if 10 to 12 inches are appliedC207). Digested
secondary sludges are applied at depths less than that of primary
sludge.
VanKleeck said that the vret_sludge drying time is increased by each
inch of additional sludge depth and also by increases in the dry
solids concentration. As an example he presented this data:
5% solids sludge dries in one week of good drying weather
8% solids sludge dries in two weeks of the same weather^8. 12).
Figures 14.XXVI and 14.XXVII also show the importance of sludge
depth*221*). Drainage time and cake moistures measured after 24
hours increase as the depth of applied board-mill sludge is increased.
Data from England showed the following relationships between sludge
depth and drying time*289):
18 inch depth, one bed loading dried in 203 days
12 inch depth, two sludge applications of 12 inches each,
dried within 203 days
6-9 inch depths, three sludge applications of 6-9 inches
each, dried within 203 days.
Observing drying bed performance and applying a reasonable sludge
depth to promote the most efficient use of the beds appears to be
good advice.
MacLaren recommended that the optimum solids loading of drying beds
was 15 pounds (dry) per square foot for uncovered beds and 25 pounds
-------
-194-
Figure 14.XXVI
25
10
EFFECT OF INITIAL DEPTH
ON DRAINAGE TIME
10
I t 3
initial D»plk - ft
Figure 14.XXVII
»s
EFFECT OF INITIAL DCPTN ON
CAKE MOISTURE CONTENT
IB
80
78
t 3
Ullltl O.plk- fl.
(Reprinted by permission Schools of Engineering Purdue University)
-------
-195-
per square foot for glass covered beds. The first loading applies
to seven fillings per year, the second to twelve times per year*53).
Uniform solids loading over the entire bed is encouraged by
multiple application points. This technique, while not commonly
designed into a system, could significantly increase drying bed
efficiencies.
Pilot plant investigations in England proved the usefulness of
decanting and sludge elutriation for most efficient use of drying
beds'221). Decanting the easily separated sludge water puts less
of a demand on the drainage system. Elutriation improves the
drying rate of sludge probably because it removes the fine solids
that have a tendency to seal the sand bed. The technique used
in England and described for wedge-wire beds, which included
"support water" applied by flooding the bed above the sand surface,
before sludge is added, is also recommended to prevent sand-bed
sealing*2217.
Filling the beds when rain or cold weather is not expected is
recommended for rapid drying. A heavy rain beating on the sludge
can seal voids at the sand surface and thereby retard drainage.
Once cracks are formed the effects of rain are minimized. Crack-
ing often occurs within 2 days in the summertime*223).
The solids content at which various sludges reach a "liftable"
state differs considerably (state at which sludges are removed
from the drying beds). Many references cite 50 percent solids as
the normal liftable state. In England, 55 percent solids is
considered liftable for badly drained sludge, but 16 percent solids
is considered liftable for a well drained sludge because it dewaters
more uniformly*221). Sludges that require extensive drying before
removal reduce the efficiency of drying beds.
After the dried sludge is removed by manual labor or machine, the
drying beds require maintenance. Small sludge particles and
weeds should be removed from the sand surface. Periodically the
bed should be disced and the top layer of sand replaced. Usually
resanding is advisable when 50 percent of the original sand depth
is lost*8).
Sludge dried to at least 70 percent moisture is often hauled to
landfill sites or used as a soil conditioner. Weathering of the
dried sludge makes it friable and, therefore, more suitable for
spreading. In Dayton, Ohio, drying-bed sludge was heat-dried to
less than 10 percent moisture, bagged, and sold as fertilizer*374).
-------
-196-
Sludge dried on open beds probably never attains this level of
dryness. Shredders have often been installed at drying bed sites
to make the sludge more appealing for soil conditioning purposes.
Some beds have been kept open to the public in the hope that the
sludge will be shredded and hauled away "free-of-charge" by the
taxpayers.
An analysis of plant records has shown that few drying beds are
utilized at full capacity. Their capacity depends upon the bed
area, maintenance, weather conditions, sludge characteristics and
the moisture content of the "lifted" sludge. The following good
operational techniques enhance efficient utilization of bed
capacity:
1. A properly digested sludge is essential for good
dewatering. The digesters should be operated to
produce a sludge that dries rapidly.
2. A clean bed with porous, level, and loose media
should be available for each sludge filling.
3. Beds should not be filled during periods of rainfall
and low temperatures.
Performance - A typical curve describing sludge drying on sand
beds during warm weather shows rapid dewatering for 1 or 2 days
(drainage) followed by a 2 to 5 week period of slow dewatering
(evaporation).
MacLaren said that a well digested sewage sludge dewaters from
95 percent to about 55 percent in 6 weeks of good weather if
applied at a depth of 6 to 9 inches*53'. English observers
believed that many industrial sludges dewater better on sand beds
than do sewage sludges*225^. Vogler and Rudolfs after testing
paper mill sludges, confirmed this belief because board-mill
sludge drained a little faster than sewage sludge'221*'. They
believed that the effectiveness of dewatering paper-mill white-
water sludge on a drying bed depends on the raw materials used,
the mill operations, and the particular "saveall" process.
Variations are expected because paper mill sludges will have
different concentrations of fiber, filler, sizing, and cellulose
debris.
Studies of the performance of open versus covered drying beds
showed inconsistent results. However, covered beds can be expected
to handle a higher solids-loading rate because they can be filled
more frequently than open beds.
-------
-197-
The literature contains many reports showing that drying-bed
performance can be greatly improved by conditioning the sludges
with chemicals and other materials before their application to
the beds. Sperry made the following conclusions concerning the
use of chemicals to treat sewage sludge prior to sand bed dewater-
ing at Aurora, Illinois^188): (1) alum is the most effective and
economical sludge conditioning agent, (2) all trivalent iron salts
are effective flocculating agents but they should not be used
because iron oxides plug the pores, and (3) for sludges containing
less than 3 percent solids, alum dosages of 1 pound to 200 or 300
gallons of sludge are recommended.
After a scientific investigation at Butler, Pennsylvania, it was
concluded that the greatest benefit from chemical sludge con-
ditioning is realized during the months of October to December.
Little, if any, benefit is realized during the summer and spring
months'209). VanKleeck stated that 6 to 10 pounds of alum per
1,000 gallons of sludge improve the drying character of normally
slow-to-dewater sludge^12). The WPCF Manual of Practice No. 11
recommended that alum at a dosage of 1 pound per 100 gallons of
sewage sludge should be mixed with the sludge as it flows to the
beds^H). Alum treatment may reduce the sludge drying time by
50 percent.
At Tenafly, New Jersey, ferric chloride was determined to be the
best flocculent for conditioning sewage sludge prior to sand bed
dewatering^32). A dosage of about 90 pounds per ton (dry)
permitted the removal of sludge from the drying bed after 10 to
20 hours. The thin feed sludge (0.5 to 0.75% solids) was applied
at a depth of 8 to 9 inches. A liftable cake was produced at a
moisture content of 87 to 89 percent solids.
Kelman and Priesing have presented interesting pilot plant data
showing improved drying-bed performance after conditioning the
sewage sludge with organic polyelectrolytes*198'. They claimed
that polymeric flocculents could increase the rate of bed de-
watering up to 30 times the rate of conventional bed performance.
Unprecedented drying bed yields were obtained by loading the beds
to depths exceeding 3 feet.
Figure 14.XXVIII from the Kelman-Priesing report**98> shows a
greatly increased drying rate with sludges conditioned with a
cationic polymer at a dosage of 60 pounds per ton (dry). It was
concluded that the drying bed yield increases linearly with
-------
TYPICAL DRYING DATA-CONSTANT SOLIDS LOADING
r
CAKE % SOLIDS CONC.
10 -
X 8% 60 LBS./TON PLOCCULANT
SOLIDS LOADING
10 L8S./SQ.FT.
TO 30% SOLIDS
AT I MONTH
4 6
TIME, DAYS
8
(O
I—I oo
-fcr I
><
H
10
(Courtesy of The Dow Chemical Company
-------
-199-
increasing polymer dose. This is described in Figure 14. XXIX*19 '
for a number of different drying periods. Depths of applied sludge
up to 6 feet were studied in contrast to the normal 8 to 12 inch
depths. It was observed that dewatering was retarded up to a depth
of 3 feet, but beyond that depth, no further retardation occurred
and, of course, the yield increased. Beyond 3 feet, the drying rate
was identical for all depths. Figure 1U.XXX*198) describes the
relationship of extraordinary depths with time and cake moisture
when the sludge is conditioned with 60 pounds per ton of polymeric
flocculent. It was noted that unconditioned sludge stored in
lagoons 2 to 6 feet deep may require several months to a year to
dry satisfactorily, as compared to the few days required for the
conditioned sludge.
Kelman and Priesingd^B) believed that dewatering was essentially
a filtration process with evaporation being of secondary importance,
except for the top 2 inches of the sludge layer. Rain, even in the
early drying stages, did not retard the rate of dewatering.
Figure I1*. XXXI presents data from an actual sewage plant operation
where organic polyelectrolytes were used to condition sludge prior
to sand-bed dewateringd"X Upon loading at conventional depths,
the chemically treated sludge exhibited greater increases in
dewatering than the control. A good solution to the supernatant
disposal problem was indicated by the data*1") in Figure 14. XXXII.
Supernatant liquor can be rapidly dewatered on drying beds, after
treatment with 2.8 to 4.5 pounds of a cationic polymer per 1,000
gallons of supernatant liquor (66 to 106 pounds per ton). The
supernatant liquor was applied to a depth of 18 inches; after 12
days the material treated with a polymer dosage of 106 pounds per
ton had a solids concentration of 51 percent. Polyelectrolytes
have also been evaluated in England where researchers concluded
that they promote bed cracking and, therefore, speed up evapora-
Ullrich and Smith mixed 3.5 cubic yards of sawdust with 10,000
gallons of digested sludge before applying the sludge to a sand
drying bed*37'). They concluded that, by keeping the sludge
porous, the sawdust allowed the sludge solids to dry faster and
to be removed from the beds 1 to 3 weeks earlier than expected.
The sawdust also eliminated offensive odors.
Economics - MacLaren estimated the cost of drying beds in Canada
for the year 1961, as follows(53):
-------
EFFECT OF DOSE ON CAKE SOLIDS CONC.
CAKE % SOLIDS CONC.
DOSE LBS./TON FLOCCULANT
(Courtesy of The Dow Chemical Company* 9 ')
60
3
M
X
-------
EFFECT OF SLUDGE DEPTH ON RESULTING SOLIDS CONC.
30
25
CAKE % SOLIDS CONC
20
15
PLOCCULANT
DOSE 60 LBS./TON
8% SLUDGE
5 DAYS
1234
APPLIED DEPTH.FEET LIQUID SLUDGE
(Courtesy of The Dow Chemical Company
ro
M
.Cr
I
to
o
M
I
X
-------
-202-
Figure 14.XXXI
DIGESTED SLUDGE
DEWATERED ON SAND DRYING BEDS
AT VARIOUS CHEMICAL DOSAGES
301.BS./TON FLOCCULANT
20 LBS./TON
FLOCCULANT
10 LBS./TON
FLOCCULANT
CONTROL (NO TREATMENT)
16 24 32
HOURS
0 8
(Courtesy of The Dow Chemical Company'
-------
TREATMENT OF SUPERNATANT ON DRYING BEDS
SOLIDS LOADING: 18 INCHES OF 1% SUPERNATANT
Figure JL^vKXXII
(Courtesy of The Dow Chemical Company^ *'}
(199)
1.3% SOLIDS
CONTROL
Flocculant dose
2.8 LBS./1000 GAL
locculant
/1000
SOL DS
8 10
TIME DAYS
-
•
-------
-20U-
Capital cost installed for 10,000 Population Equivalent =
$0.25/sq. ft. or
$2.65/ton
Operating cost = $1 to $10/ton of
dry solids removed
(30 years amortization at 5% interest)
The specific operating cost depends on the local labor situation and
the hauling costs to the ultimate sludge-disposal point.
In England, the 1963 annual cost for dewatering sludge on drying
beds was $11.90 to $19.60 per ton of dry solids<218>.
In 1966, an estimated cost for constructing concrete-bottom drying
beds was $0.80 per square foot. The cost of asphalt beds would
probably be similar<215'.
Seelye's design manual listed the cost of conventional drying beds
as follows^7!':
Open sand beds $0.9H/sq. ft. or $1.15/sq. ft.
at January 1, 1966 costs
Glass covered beds $2.05/sq. ft. or $2.50/sq. ft.
at January 1, 1966 costs
Kershaw reported very high construction costs for the drying-bed
at Maple Lodge Works in England*223'. The bed itself cost $2.62
per square foot or $1.70 per square foot when the cost of the
mechanical lifting and conveying equipment were included. The
high bed costs were probably due to adverse weather conditions and
the need to protect ground water by incorporating an impervious
asphalt layer beneath the sand. Annual operating costs of $76.«*5
per ton (dry) were reported for the year 1963-1964<223>.
The sale of dried sludge as a soil conditioner offsets the drying
bed operating costs. Shredded sludge has been sold for as much
as $6 to $10 per cubic yard. Many waste treatment plants, however,
give the sludge away and thereby eliminate the cost of hauling it
to land-fill sites. In general, the capital and operating cost for
dewatering sludge on sand beds varies from $3 to $20 per ton of dry
-------
-205-
solids applied. Digestion before dewatering significantly increases
the total sludge-disposal process cost. A typical value for
digestion plus sand bed dewatering would be $25 per ton.
Summary - Sand drying beds will continue to be a useful and common
tool for sludge dewatering. As reported by Chambers,digesters plus
sand beds are the most economical dewatering technique for sewage
treatment plants serving cities with flows less than 10 mgd, when
compared with vacuum filtration or raw sludge filtration with subse-
quent incineration^58'. MacLaren believed that rising labor costs
restrict the use of sand-drying beds to cities having less than
5,000 people^53'. The "10 States' Standards" stated that sand beds
are in common use at small to medium-sized plants, particularly
where land is inexpensive and where a demand exists for the sludge
as a soil conditioner^2^.
The popularity of sand drying beds cannot be disputed. They are
popular fipst because of economics. Only sludge lagoons, ocean
disposal, or liquid sludge disposal to nearby land are generally
considered to be less expensive than sand bed dewatering. A second
reason for their popularity is simplicity. Sludge beds can be
filled and essentially forgotten until the sludge has dried. This
is a major factor in the acceptance of drying beds by smaller
communities and some industrial plants.
Sand drying beds have, however, certain disadvantages. These
include: (1) the area required, (2) potential nuisance problems,
(3) cost of removing the sludge, CO susceptibility to adverse
weather conditions, and (5) the general need for a digestion step
preceding sand bed dewatering.
The area requirement often eliminates drying beds as a practical
technique for large municipalities. However, there are still a
few large cities using beds for at least a portion of their sludge
dewatering needs. Obviously the local situation in each case will
determine the suitability of using drying beds.
Digesting organic sludges before sand bed dewatering reduces the
potential threat of nuisance conditions. Sewage sludges are
almost always digested before drying on sand beds, but digestion
is not often specified for industrial sludges. Lime may be added
to organic sludges to control odors, but it has a tendency to plug
sand pores and thereby retard drainage.
-------
-206-
The labor cost of lifting dried sludge from beds can be substantial
because it takes a few days to remove sludge from a medium-sized
bed. Improvements in mechanical lifting and sludge conveying equip-
ment could make the economics of drying beds much more attractive
at medium to large-sized treatment plants.
The climate (rainfall, temperature, and humidity) is the most
important factor in sand bed dewatering. It cannot be very well
controlled, even though rainfall can be eliminated by covering the
beds with glass structures. Rain interferes with drying and bed
cleaning; however, the specific effect has not been determined
because it depends on the state of sludge dryness at the time of
precipitation. But, the normal, covered bed is expensive, it does
not prevent freezing, and it is very hot to work under when cleaning
beds during the summer months(23), jn northern climates, drying
beds are generally not used from November to March unless the beds
are covered. Sludge storage for 3 to 5 months, usually in digesters,
is an additional expense.
Future sand bed designs should consider decanting facilities;
decantation can remove a significant part of the total water con-
tained in sludge and can therefore increase bed capacities. Other
process improvements may result from the use of chemicals or heat
and from the installation of mechanical lifting equipment. Con-
sideration should be given to transporting liquid sludge through
pipelines to rural areas where the sand bed drying process would
be more suitable for many reasons.
-------
-207-
E. Screening
General - Dewatering of sludge on vibrating screens has been
investigated in a number of sewage treatment plants in the
United States. Overseas, there are more than 20 sewage plants
using the Heymann process which consists of vibrating screens
followed by rotary filter presses(132). The use of screens is
advantageous because of their simplicity and small size. Up to
now, however, the disadvantages of screens have generally pre-
cluded their use in sewage treatment plants except for floatables
and trash removal.
Design - The Rhewum (sonic) vibrating screen filter, described by
Kiess and Schreckegastd28), consists basically of three screens
in series. The first is a coarse screen (8 x 2U mm mesh) that
removes 2 percent of the total sludge solids. Next in line is a
sonic screen (1.2 - 2 mm mesh) that is vibrated by electro-
magnets attached to one edge of the filter cloth. Longitudinal
and transverse waves are formed which agglomerate and concentrate
the solids while moving them to the,next screen in series. Twenty-
three percent of the total solids are removed in the second step.
The last screen is called a sonic filter (0.1 - 0.5 mm mesh);
it also is vibrated by electromagnets. This screen removes 55
percent of the total solids. The screens are vibrated at 1,200 rpm
with a double amplitude of •*.** - 7.8 mm. Synthetic fabrics are
used for coarse screening, but the finer screens may be stainless
steel.
The Southwestern Engineering Company (SWECO) built a vibrating
screen unit that has been evaluated in municipal and industrial
wastewater treatment plants. Their "Vibro-Energy Separator"
develops a variable, low amplitude, three-dimensional motion
that moves sludge across a metallic screen(133). A combination
of radial and tangential motions move the sludge in a long spiral
path from a vertical and well distributed center feed point to an
outer rim discharge. Vertical amplitude creates a vertical motion
that accelerates the drainage of the liquid through the media.
Sludge particles as small as U«» microns may be removed.
Operation - The efficiency of the Rhewum screen depends on the
solids loading, sludge particle-size distribution, the type of
sludge, and the angle of sludge discharge to the filter cloth^28).
The best solid-liquid separation occurs when the sludge discharge
nozzle approaches a tangential direction to the filter cloth.
-------
-208-
Sludge fed to a SWECO screen must be at a constant rate and
perpendicular to the screen surface. The three dimensional
motion of the screen is regulated to form the pattern on the
screen which produces the most efficient dewatering. The required
pattern varies with the sludge solids concentration, the particle
size distribution, and the sludge feed rate.
Performance - Sludge, having a solids concentration from 20 to 25
percent, has been produced by the Rhewum vibrating screen'128).
The screen filtrate contained considerable solids but the solids
seemed to have excellent settling characteristics, so it is assumed
they could be handled in the primary sedimentation tanks.
Budd reported on the use of a vibrating screen and filter press
at Lake Hiawatha, New Jersey(23) to dewater raw organic sludges.
The combined use of a screen plus press preduced a 25 percent
total solids cake from a sewage sludge having a solids concentra-
tion of 3.76 percent. Screen filtrate solids were 2 to 3 percent
total solids. It was suggested that this very low solids capture
would lower overall treatment plant efficiencies and, therefore,
would be unacceptable.
An English reference stated that the vibrating screens, which are
a component of the Heymann process, produced a filtrate containing
a high concentration of solids(131*). A study of the
dewatering characteristics of the filtrate from the screening
operation using digested primary sewage sludge as the feed, indicated
that filtrate could be dried on covered drying beds in 2 to 14 days.
Treating raw sludge filtrate in aeratipn tanks was suggested.
Studies during 1966 in California indicated that the use of
cationic polyelectrolytes to condition sewage sludge prior to
screening increased the efficiency of the screening process.
Economics - In Germany, the estimated cost of a complete
installation including screens and a filter press for a city of
100,000 was $4.50 per ton. Operating costs were $1.35 per ton and
capital costs, $3.15 per ton^S). These costs assumed that the
feed sludge concentration was 10 percent. The operating costs of
vibrating screens should be low if chemical conditioning of the
sludge is not required. Power consumption is low and labor costs
are relatively low due to the ease of operation and maintenance.
Summary - The use of vibrating screens for thickening or dewater-
ing waste sludges is promising because the equipment is simple;
-------
-209-
it uses very little power; the space requirement is minimal; and
the screens are applicable to many different types of materials.
However, screens have not been adopted for thickening and dewater-
ing processes because of two major disadvantages; first, the solids-
capture efficiency is very low; second, the degree of dewatering is
unsatisfactory for subsequent disposal steps. An additional, but
less basic, disadvantage is the requirement for regular screen
maintenance. The screening media can be readily plugged by grease
and other materials.
One of the major problems at wastewater treatment plants has been
recycling of fine solids not captured by various process steps.
This recycling of solids from the screening operation, or other
processes, can significantly lower the overall treatment plant
efficiency. Ultimate solids disposal by incineration, landfilling,
or other methods often require, for practical reasons, a well
dewatered material. Vibrating screens usually do not produce as
high a degree of dewatering as other processes such as vacuum
filtration. The use of chemicals for sludge conditioning prior to
screening may eliminate the two major disadvantages; but this, of
course, impairs, the economic attractiveness of screens.
Vibrating screens are useful, however, for removing trash and
floatables from sludge prior to ocean disposal or dewatering in
other mechanical equipment. Based on available data this appears
to be one of the best uses for a fine screening sludge treatment
process. Industrial waste sludge dewatering is another promising
use for screens. Wet solids such as paunch manure and waste
tomatoes could be effectively dewatered on vibrating screens.
-------
-210-
15. SLUDGE CONDITIONING AND DEWATERING
UNUSUAL PROCESSES
General - To improve conventional sludge conditioning techniques,
numerous unconventional approaches have been investigated. The
usual research incentive is to eliminate the need for chemicals
and to increase production rates.
A. Freezing
Through the years a number of people have observed that sludge
frozen by nature and later thawed in sand drying beds or lagoons
had good dewatering and fertilizer or soil-conditioning char-
acteristics. Thawed sludge was stable and dewatered rapidly if
provisions were made for water drainage. These observations
encouraged researchers, particularly in Great Britain, to
evaluate artificial freezing of sludge as a means to promote
rapid dewatering. They speculated that freezing disrupted the
cell walls retaining the internal moisture in sludge, thereby
allowing water release and drainage^).
Bruce and others reported the following negative results with an
artificial freezing and thawing process(265): (i) fine solids, that
were produced, blinded vacuum filter media; (2) centrifugal
dewatering was impractical because the fine solids were discharged
with the centrate; and (3) digestion of activated sludge before
freezing did not improve its response to freezing and subsequent
dewatering.
Clements and co-workers investigated freezing as a sludge
conditioning technique prior to vacuum filtration, and obtained a
number of positive results from laboratory, pilot plant, and small-
scale plant tests, as follows*267^: (1) Freezing was an effective
sludge conditioning process for all types of sludges. (2) The use
of flocculents with freezing was helpful but not necessary. (3) Slow
and complete freezing of the total sludge was required for good
results (cooling without freezing produced no dewatering improvement).
CO The length of time that sludge was kept frozen before dewatering
was not important. (5) The method of thawing was not critical, but
it must not be accompanied by vigorous agitation. (6) Freezing
accelerated the rate of sludge settling. (7) The supernatant
liquid from sludge frozen, thawed and settled approximates the
strength of raw sewage.
-------
-211-
The use of chemicals, in conjunction with freezing, to condition
sludges prior to dewatering was reported to allow unprecedented
filtration rates(267). Table 15.1 describes these rates along with
cake solids, filtration yields, chemicals used and their dosages.
As indicated, the most successful chemicals were aluminum sulphate,
chlorine and chlorinated iron sulphate (copperas); they were most
effective if added after cooling but before freezing. With
vacuum filtration, dewatering occurred in a few seconds and produced
a cake with a solids content of 22 to 29 percent.
Table 15.1
Method of Treatment
Chemical
Cone.
(ppm)
Time
(a) Digested Sludge
No chemicals, no freezing
No chemicals, freezing
Chlorine, freezing
Chlorinated FeSOH,
freezing
Aluminum sulfate,
freezing
Aluminum sulfate,
freezing
Aluminum sulfate,
freezing
(b)
No chemicals, no freezing
Aluminum sulfate,
freezing
250 C12
250 Fe
100 Al
200 Al
500 Al
120 min.
6 min.
3 min.
1.2 min.
3 min.
1 min.
5 sec.
23.6
22.0
26.9
26.9
25.6
28.6
Filtration
Dry Solids
Cake Rate
(%) (Lbs./Sq.Ft./Hr.)
7
14
31
9
27
321
Activated Sludge
10 min.
100 Al
30 sec. 20.0
24
The above plus other experiments showed that the filter cakes were
porous, friable, and easily removed from the filter media. Filter
yields as high as 350 pounds (dry) per square foot per hour were
attained with digested sludge when large dosages of alum were used
with freezing*26'). Activated sludge yields as high as 70 pounds per
square foot per hour were possible. A yield of 40 to 50 pounds per
square foot per hour was achieved with more realistic chemical
dosages of 100 ppm chlorinated copperas or 20 ppm alun>(267). Filter
yields from freezing without chemicals were good but not spectacular.
-------
-212-
Dewatering frozen and thawed sludge on sand drying beds rather than
by vacuum filters also greatly increased the rate over that obtained
with'unfrozen sludge.
Doe and others applied the freezing technique to water plant sludge
concentration(**59). At the Lancashire, England, water filtration
plant, 33,000 gallons per day of a 0.5 percent sludge were concen-
trated to a gel that occupied an area one-eighth the size of that
previously necessary for untreated sludge. The process steps were:
(1) gravity thickening with slow picket stirring, concentrating the
sludge to 1.9 percent, (2) storage and decanting for at least 16
hours in other tanks which further concentrate the sludge to 2.4
percent; and (3) freezing and thawing to concentrate the sludge to
a gel.
The freezing process was independent of freezing temperature and
the sludge detention time in the frozen state but, as Clements
observed, the process required slow and complete freezing of the
sludgers?). Construction cost for the small-scale plant in
Lancashire was $178.500 or about $51 per ton (20 year amortization
at 4% interest)C*59). Operating costs were $57 to $79 per ton,
assuming that the feed sludge contained 2.4 percent solids. Doe
and co-workers believe that the freezing process offered a secondary
advantage: it produced a material that was ideally suited for the
recovery of aluminum sulfate.
Freezing operating costs included power, flocculents and refri-
gerants. It takes 28 B.t.u. to lower the temperature of one pound
of sludge from 60°F to 32°F and 142 B.t.u. to freeze a pound of
sludge(267). Possibly, the freezing expense may be reduced by
using sludge cake thawing for pre-cooling. Clements quoted a total
operating cost for freezing of $0.28 (2 shillings) per ton of wet
sludge or $5.60 per ton of dry sludge(267). Operating costs of
$6.77 to $9.49 per 1,000 gallons have been reported for freeze and
thaw cycles, lasting 50 to 120 minutes, t'sing a 5 percent sludge
as a basis, this translates into $32.25 to $45.25 per dry ton<286).
This very high operating cost has been the major reason why freezing
has not been adopted as a conditioning technique for wastewater
sludges. Another problem is the need for dewatering equipment
capable of handling the very high production rates possible with the
freezing process.
Undoubtedly, artificial freezing can aid sludge dewatering. However,
it probably will never be practical, except in isolated cases, unless
the economics are improved greatly.
-------
-213-
Ek Heat Treatment
In 1951, ten years of full scale operating experience with a
heat treatment—sludge conditioning process was completed success-
fully'^^'. The unique process, called the "Porteous Process,"
reduced the water affinity of sludge solids by heating the sludge
for short periods^266).
Exposing the sludge to heat and pressure coagulates the solids,
breaks down the gel structure, and reduces the hydration and
hydrophilic nature of the solids. The liquid portion of the
sludge can then be easily separated from the solid by decanting
and pressing.
The following data show the relative dewatering rates of sludge
conditioned by different agents.
Table 15.2(275)
Relative Dewatering Rates
Conditioning Agent Primary Secondary
Sludge Sludge-1
None 30 1
Sulfuric acid2 100 2
Aluminum sulfate3 200 10
Ferric sulfate3 300 15
Ferric chlorideS 400 20
LimeS 1,000 80
Heat treatment1* 6,000 1,000
iMixed humus and activated sludge
2At optimum pH value
3At optimum dosage
H0ne-half hour at 360°F
As Table 15.2 shows, the dewatering rates of both primary and secondary
sludges after heat treatment, exceeded markedly the rates when using
chemical conditioners.
As the process was operated at the Halifax, England, sewage treatment
plant*2'5), raw sludge was heated by live steam to temperatures of
290°F to 370°F in pressure vessels. After "cooking" for 1/2 to 3/U
hour at a pressure of 150 psi, the sludge was passed through a heat
-------
-214-
exchanger, countercurrent to the raw sludge inflow. The sludge was
stored and settled overnight and the next day, dewatered in a filter
press. Raw sludge solids were 4.9 percent; heated and stored sludge
averaged 10.1 percent solids; and the filter press cake averaged
52 percent solids. No chemicals were used. Maximum economy required
the use of heat exchange equipment. At Halifax, 75 percent of the
heat was recovered with intermittent operations.
In 1951, Lamb gave the total operating cost of the Halifax "Porteous"
process as $6.58 per ton of dry solids(275). This cost would be
competitive with sludge conditioning and dewatering costs in the
U.S.A.
Jepson and Klein conducted numerous laboratory experiments involving
the heat treatment of sludge'276'. They reported that under high
pressures (150 psi), sludges could be thickened from 0.8 to 2.49
percent and from 4.84 to 7.8 percent. The final sludge concentration
depended on the steam pressure applied and the treatment detention
time. Solid-liquid separation was rapid using the heat treated sludge,
but the supernatant contained many solids (up to 36 percent of the
original sludge solids) and most of the nitrogen (59 percent of the
original nitrogen). The thickened sludge filtered well, but heat
treatment inhibited digestion.
Teletzke advocated the use of the Zimpro wet oxidation unit for heat
treatment of sludges prior to conventional dewatering(328)_
Operation of the unit at low pressures (150-300 psig) and temperatures
(300-350°F), he stated,can reduce organic sludges to a sterile, non-
putrescible end product that can be dewatered without chemical condi-
tioning by vacuum filters, centrifuges, or sand beds. Reliable
cost information was not available because heat conditioning prior
to mechanical dewatering had not been in full-scale operation at any
U.S. sewage treatment plant. The closest approximation was the batch
wet-oxidation operation at South Milwaukee, Wisconsin, where digested
primary sludge was partially oxidized at relatively low temperatures.
Their fuel costs were $11.50 per ton and power costs, $5.25 per
ton<328).
Advantages claimed for the Porteous (heat-treatment) process included:
1. No odors.
2. No chemicals required for sludge conditioning.
3. The treated sludge was completely sterilized.
4. Capital and operating costs were similar to those
of digesters and sand beds.
-------
-215-
A major disadvantage was the newness of the process. Reliable
performance and cost data were not available for U.S. operations.
Another disadvantage was the high concentration of soluble B.O.D.
in the storage-tank decantant and in the filtrate.
Research and development of heat conditioning processes should
be encouraged because they offer some very important technical
advantages. It appears that costs could be competitive with other
methods of sludge treatment. Perhaps heat treatment should be
considered wherever solids are incinerated because of the opportunity
for optimum heat utilization. It may be particularly useful for
conditioning hydrous biologic sludges.
-------
-216-
C. Solvent Extraction
Sludge conditioning by solvent extraction has been tested at the
Rockford. Illinois, treatment plant. Described as the "McDonald
Process"'269)^ it involved a series of steps including: (1) de-
watering by centrifugation, (2) solvent extraction with carbon
tetrachloroethylene, and (3) distillation. The end products were
dried oils, fats, and greases. Solvent extraction was an interest-
ing approach to sludge dewatering. but the "McDonald Process" has
been described as impractical^269/.
D. Electrical Treatment
Sludge conditioning with electricity has fascinated a number of
researchers. Slagle and Roberts treated both sewage and sludge
by "electrodialysis"^271*'. In laboratory experiments, they put
sludge in a vessel which was placed in a copper dish containing
an equal amount of sewage. The copper dish served as the cathode
while a carbon anode was placed in the sludge. Following passage
of a direct current, the filterability of the sludge increased,
as indicated by the following data:
Hater removed by
vacuum filtration
Untreated sludge 12%
Sludge electrodialyzed for 15 min. 13%
Sludge electrodialyzed for 30 min. 65%
In their second series of laboratory tests, Slagle and Roberts(274)
used three cells: the center cell contained sludge and the outside
two contained sewage. A graphite anode was placed in the sludge
while iron cathodes were placed in the sewage. Upon the appli-
cation of a direct current, the cations migrated to the sewage
and collected at the cathodes forming metal hydrates which in turn
flocculated the sewage. Anions migrating to the sludge caused acid
hydrolysis and loss of solids. The sludge again was filterable
without chemical conditioning.
Next, Slagle and Roberts*271*) tested the process in a pilot plant,
where they also used graphite anodes and iron cathodes. Asbestos
cloth was used to separate the sewage-sludge compartments. Electro-
dialysis in this larger system reduced the sludge pH to 3.4, and
the sludge could be filtered without the use of chemical conditioners.
-------
-217-
The solids in the filter cake (laboratory filtration) varied from
32 to 45 percent. After electrodialysis, the sludge settle
rapidly and seemed to be stabilized because there was very little
gas after extended detention.
A comparison of electrodialysis conditioning versus chemical
conditioning produced the following data, using fresh sludge having
a 6.56 percent solids concentration^274^:
Chemical Treatment(per ton of Electrodialysis
dry solids) Treatment(per ton
of dry solids)
89 Ibs. of ferric chloride used 181 KWH expended
70% filter cake moisture 59.5%
2,065 Ibs. filter cake solids 1,440 Ibs.
4,665 Ibs. filter cake water 2,130 Ibs.
6.2 pH 3.4
Conclusions drawn from the results of the above studies were:
(1) electrodialysis can condition sludge for filtration and
flocculate sewage at the same time; (2) optimum filtration occurred
at a pH of 3.4 and required no chemicals; (3) sludge treated by
electrodialysis produced a filter cake much drier than chemically
treated sludge; (4) as the pH increased the filterability decreased,
and conditioning chemicals become necessary; (5) electrodialysis
caused a solids loss of 20 percent and an ash loss of 30 to 40
percent; and (6) the most economical current density was about
0.3 amp per square foot of anode surface with a potential drop
of 4 volts between the electrodes.
Costs of electrodialysis and chemical sludge conditioning depend,
of course, on the price of flocculents and electricity. Typical
data indicated that 181 KHH were equivalent to 408 pounds of ferric
chloride and 416 pounds of lime. Using a price of $0.01 per KWH,
results in much lower conditioning costs for electrodialysis than
for chemical treatment(274)^
Cooling and co-workers also reported on their experiments with
electricity(268). "Electro-osmosis" was the term Cooling used
to describe the passing of a current through digested sludge.
He concluded that the quantity of water removed from sludge was
proportional to the electricity transported. An electro-osmosis
permeability of 0.006 gallons per square foot per hour per inch
-------
-218-
per volt and a constant equal to 0.02 gallons per ampere-hour
was used. It was observed that thinner sludges required less
current. While electro-osmosis was promising technically,
Cooling and his co-workers concluded that the consumption of
electricity was too high to be considered practical and that the
maintenance problems would result in a very high labor cost.
Dried crusts formed on the anodes, reducing the electrical
efficiency, and corroded anodes had to be replaced frequently.
Laboratory studies described by Coackley also confirmed the
need for greater amounts of energy to remove a given quantity
of water as the solids concentration in sludge increased(277).
Hicks was able to filter sludge by substituting electrolytic
conditioning for chemical treatment(30). He used carbon and
iron electrodes. With the application of a direct current, the
pH decreased very rapidly to a point where the sludge became
filterable.
Beaudoin further dewatered activated sludge filter cake at
Chicago by electrical means'273). He also called the process
electro-osmosis because water moves through porous sludge after
the application of an electric current. High, sludge-drying
costs provided the incentive to seek a way to decrease the
vacuum filter-cake moisture. Beaudoin concluded that the use
of electricity was effective, but it was not economically
feasible. He obtained the best results by conditioning the filter
cake with 25 volts for 2 minutes.
In summary, it appears that electrical treatment can be
substituted for chemical conditioning of a wide variety of liquid
sludges, if power economics were the only major consideration.
However, the sludge handling and equipment maintenance problems
appear to preclude this process until improved techniques are
developed.
-------
-219-
E. Ultrasonic Treatment
Conditioning of sewage sludges by ultra or supersonic vibration
has been explored in British and European laboratories(270, 277)j
The available data indicate that this process is not a very
successful method of sludge treatment. Ultrasonic vibrations
degasify sludge, which is beneficial, but the vibrations also
tend to destroy sludge floes resulting in fine solids that are
more difficult to dewater.
F. Bacteria Treatment
The addition of autotrophic sulfur bacilli to digested sewage
sludge has been investigated as a means of sludge conditioning,
prior to dewatering. Under aerobic conditions sulfur-oxidizing
bacteria stimulate the production of acids. The pH of the sludge
is lowered by the sulfuric acid produced, thereby enhancing
dewatering processes. Unfortunately no data are available to
describe the performance or economics of the process.
-------
-220-
16. COMPOSTING
General - Composting of wastewater solids, to convert the organic
wastes into a humus valuable as a soil conditioner and nutrient
source, is a controversial process. It has many supporters attracted
by the potential of producing a useful end product from a material con-
sidered by most people to be something you should completely eliminate
as quickly and cheaply as possible. The conservation principle of
returning nutrients to the soil is appealing and, as a result,
composting techniques will continue to be discussed if not installed.
It is frequently stated that composting has not caught on in the
U.S.A. because we have a tradition of waste that obstructs any
interest in reclaiming organic solids. But, attempts to produce
and market compost have been made and have failed. Failure is
usually due to the inability of the producer to market the compost
in competition with other materials such as chemical fertilizers
which are relatively cheap, plentiful, easy to apply, and very
familiar to the farmers. In comparison, composting has many disad-
vantages including the fact that the technology is limited, the
economics of the process are uncertain, and inorganic fertilizers
are often required to be used along with compost.
Definition - Composting has been defined as the "aerobic thermophilic
decomposition of organic wastes to a relatively stable humus.
Decomposition results from the biological activity of microorganisms
which exist in the waste" '"*18). A good compost could contain to
2 percent nitrogen, about 1 percent phosphoric acid, and many trace
elements. Its most valuable features, however, are not its nutrient
content, but its moisture retaining and humus forming properties (70).
Many types of microorganisms are involved in converting the complex
organic compounds such as carbohydrates and proteins into simpler
materials, but the bacteria, actinomycetes, and fungi, predominate.
These organisms function in a composting environment that is optimized
by copying the natural decomposition process of nature where, with an
adequate air supply, the organic solids are biochemically degraded to
stable humus and minerals.
History - Composting was widely used in Europe and Asia many centuries
ago because there was ah agricultural demand for its soil conditioning
and nutrient properties. At the beginning of the 20th century, however,
compost was replaced by chemical fertilizers that gave better results
and were easier to apply (69). Starting about 1930 and accelerating
rapidly after World War II, the European interest in composting was
revived. Two factors were important in this revival; first, there
was a shortage of chemical fertilizers and second, there was a need
to do something with the wastewater solids resulting from programs
to control water pollution. Composting has become most widely
accepted in Holland where at least 25 percent of the organic waste
solids have been converted to compost
-------
-221-
Use - Compost is generally considered as a material to be used in
conjunction with fertilizer, rather than as a replacement for fertilizer
unless it is fortified with additional chemical nutrients.
Compost benefits the soil by replenishing the humus, improving the soil
structure, and providing useful nutrients and minerals. It is particu-
larly useful on old, depleted soils and soils that are drought-sensitive
t69). In horticulture applications, compost has been useful on heavy
soils as well as sandy and peat soil. It has been commonly applied to
parks and gardens because it increases the soil water absorbing capacity
and improves the soil structure.
Parameters - All composting processes attempt to create a suitable
environment for thermophilic facultative aerobic microorganisms.
If the environmental conditions for biological decomposition are
appropriate, a wide variety of organic wastes can be composted. The
most important criteria for successful composting are: (1) complete
mixing of organic solids, (2) nearly uniform particle size, (3) adequate
aeration, (4) proper moisture content, (5) proper temperature and pH,
and (6) proper carbon-nitrogen ratio in the raw solids *356» 118).
The smaller the particles, the more rapidly they will decompose;
size is controlled by grinding. Air is necessary for aerobic
organisms to function in a fast, odor-free manner. Aeration is
enhanced by blending wastes to form a porous solids structure in
the composting materials. Some composting systems use blowers while
others aerate by frequent turning of compost placed in windrows and
bins. The solids to be composted must not, of course, contain high
concentrations of materials toxic to the decomposing microorganisms.
A proper moisture content is the most important composting criteria
(118). Microorganisms need moisture to function but too much moisture
can cause the process to become anaerobic and develop the characteristic
odor and slow decomposition rate associated with anaerobic processes.
Kneiss recommended a maximum moisture content of 50 to 60 percent in
the composting mixture '357)^
Composting mixtures should have a pH near 7 (neutral) for optimum
efficiency. The temperatures vary a great deal but those in the
thermophilic range (greater than 110°F) produce a more rapid rate
of decomposition than those in the lower mesophilic range. Higher
temperatures also cause a more efficient destruction of pathogenic
organisms and weed seeds.
-------
-222-
An essential requirement of the composting process is control of
the ratio of carbon to nitrogen (C/N) in the raw materials. Micro-
organisms need both carbon and nitrogen, but they must be available
in the proper amounts or decomposition will be prolonged. Braun
and Kneiss (396, 357) recommended C/N ratios of 25 to 30.
The time required to complete composting varies, depending on the
climate, materials composted, the degree of mechanization, whether
the process is enclosed, and the desired moisture content of the final
product. Composting detention times from a couple of weeks to several
months have been reported.
Composting Materials - Many types of wet solids have been success-
fully used in composting operations. These include sewage sludge,
cannery solids, pharmaceutical sludge, and meat packing wastes.
Sewage sludge has been frequently used as an additive when composting
dry refuse and garbage. It enhances the composting operation because:
(1) it serves as a seeding material to encourage biological action,
(2) it helps to control the moisture content in the composting mixture,
(3) it enhances the value of the compost by contributing nitrogen and
other nutrients, and (4) it can be used to control the important C/N
ratio. In recent years, the carbon content of refuse has steadily
increased in part due to changes in the packaging of consumer products.
The use of sewage sludge to increase the nitrogen content of compost
is, therefore, useful in promoting an ideal C/N ratio.
Normally, blending sewage sludge with other compost raw materials
required prior dewatering of the sludge. If the dewatering step
is omitted, the moisture content of the mixture is too high and
odors develop. Reducing sludge moisture from 90 to 70 percent by
vacuum filtration or centrifugation allows good aerobic composting
with garbage at a blended moisture content of 53 percent (3*1, 357)^
Gothard recommended a sludge-dry refuse ratio of 0.7 to 1.1 to produce
an optimum moisture content of 65 percent (359). Davies recommended
a 2:1 refuse-sludge ratio and the use of raw sludge rather than
digested because of its higher nitrogen value (360). Black (355)
stated that any amount of sludge could be mixed with refuse for
composting, provided the sludge was adequately dewatered.
In favorable climates, the composting of digested sludge with
sawdust, straw, and wood shavings has been successful 1355).
Pharmaceutical laboratories have composted their wastewater solids
with sawdust and animal manures (378X Mercer described the com-
posting of a mixture of cannery wastes, refuse, and rice hulls (356).
It appears that many combinations of waste solids are compatible so
long as the critical composting parameters are not ignored. But,
wastewater solids from municipalities, food industries, pulp and paper
mills, and the pharmaceutical industry are particularly conducive to
composting because they have a high organic content which promotes a
good C/N ratio.
-------
-223-
Processes - Basically, composting consists of three stages:
(1) mixing, (2) composting, and (3) maturing. If pieces of metal,
glass, and cinders are in the refuse to be composted, sorting and
grinding is necessary to remove them. Figures 16.1 and 16.11
describe two common composting techniques.
The simpler process (Fig. I)(353) is called the rasping or hammermill
system. Here the refuse is first sorted by screening and magnetic
separation. The remaining particles are then pulverized in a grinder
called a rasping machine or a hammermill. Following this, sewage
sludge is uniformly blended with the pulverized refuse solids and
the mixture is placed in windrows, pits or silos for decomposition
and stabilization. The stored material is kept moist and aerated
while decomposing.
A more complicated process called the Dano system is described in
Figure 16.11 (360). After sorting and particle crushing, the material
to be composted is conveyed to a large rotating drum called a bio-
stabilizer (69). Composting material is held for 5 days in a slowly
rotating drum while air and water are added to optimize the environment.
After partial decomposition, the material is ground again and placed
in windrows for further decomposition and stabilization.
Both systems produce a material that is non-odorous and easily handled.
As a final step, some European plants have used inertial separation
processes to improve the value of the finished compost by separating
the lighter organic compost from heavier inorganic material.
Wiley and Spillane stated that refuse-sludge mixtures placed in
windrows can be effectively decomposed in about 6 weeks, provided
that the windrows are thoroughly turned five times during the first
3 weeks of curing (395). ne recommended small cross-sectional piles
plus frequent turning to prevent oxygen depletion and odor production.
Regrinding and moisture adjustment are desirable when turning the
compost.
A unique composting system called the "Brikollare Process" has been
constructed at Schweinfurt, Germany (351). After pulverization and
magnetic separation, sewage sludge and refuse are blended and
briquetted under a pressure of ^00 psi. The resultant bricks are
stored in a curing room and fermented at 160°F. After stabilization,
the composted bricks are used for agricultural purposes.
-------
-224-
Flgure 16.1
(353)
RECEPTION. SEGREGATION AND PREPARATION
DECOMPOSITION AND STABILIZATION
MARKET PREPARATION
Figure 16.II
(360)
RECEIVING BIN
MAGNETIC CAN SEPARATION
TO MET*
£ia HAND-SORTED LARGE NON-COMPOSTABLES I* [\ SALVAGE
TO CLEAN FILL.
TO METAL
AIR & WATER
REVOLVING BIOSTABILIZER
COMPOST
Q_
CONVEYOR
u u u
HEAVY MATERIALS
TO CLEAN FILL
7
WIMDROWS
\
ScKemotic flow thcet for o Dono composting plant.
(Reprinted by permission Compost Science, Emmaus, Pa.)
-------
-225-
Performance - Black reported on a composting study at Chandler,
Arizona (aaa). Sewage sludge and refuse were mixed and composted
outdoors in windrows and bins. Periodically the material was reground,
wetted, and replaced. Within 30 days, 35 to HO percent of the
volatiles were lost. After composting U3 days, the volatile solids
reduction was 50 percent. It was estimated that the same volatile
reduction could be accomplished in 9 days with the use of mechanical
units having continuous aeration and mixing. Little nuisance was
observed from the Chandler tests even when raw trickling filter
humus was blended with the refuse. Composting temperatures up to
160°F were reported which should kill pathogenic microorganisms.
Composting in Jersey, the British Isles, was described by Gothard
(359). Garbage and refuse were screened, pulverized, and mixed with
sewage sludge. The mixture was held for 1 week in fermentation
bins and then transferred to outside, but covered, maturing sheds
for 6 weeks. Composted mixtures exceeding 10,000 tons per year were
used for general agriculture purposes, on golf courses, and for
greenhouse soil conditioning.
The Tillo Products Company near San Francisco has been successfully
marketing a composted mixture of Bay Area wastes for 15 years. Basic
ingredients have included digested sewage sludge, coffee grounds, and
rice hulls. On occasion, wood shavings, sawdust, and slaughter-house
wastes have also been used. The liquid digested sludge was blended
with the food industry wastes and composted for several weeks during
which time a temperature of 140°F is generated. At this temperature,
it is assumed that weed seeds and pathogenic bacteria are killed.
Typical nutrient and trace element concentrations of the Tillo compost
were: 1.5% nitrogen, 2% phosphoric acid, 0.25% potash, 3% calcium, and 0.5%
magnesium. The Tillo material has a total porosity of 90 percent and a
moisture retention factor of 280 percent.
Hercer and others described the composting of wet cannery solids mixed
with other materials such as municipal refuse or rice hulls (356).
Mixing a maximum of 250 pounds of fruit solids (85% moisture) per 100
pounds of municipal waste achieved an optimum moisture concentration
of 70 to 65 percent. The mixture was stored in bins, turned daily
for the first week and every other day until composting was essentially
complete. At first, a low pH of H.5 to 5.0 lengthened the required
composting period, but the addition of lime corrected this problem.
Finished compost had a slight fermentation odor and attracted a few
flies but no serious nuisance problems developed. Mercer concluded
that air dried compost can be used as the dry component in succeeding
blends of fruit waste-compost mixtures.
-------
-226-
Subsequent tests by Mercer and co-workers determined that a critical
moisture content was 70 percent (**06). if the moisture content was
kept below this figure by blending wet canning solids with absorbent
materials, aerobic composting proceeded satisfactorily. Above
70 percent, the composting proceeded slowly in an odorous anaerobic
state. Grinding of the fruit wastes to expose more surface area,
and the addition of nitrogen in the form of urea improved the
composting process.
Other studies of windrow composting of wet cannery fruit solids with
rice hulls as an absorbent material resulted in the following conclu-
sions by Mercer and others representing the National Canner's
Association research staff: (1) cannery wastes can be disposed of by
composting, (2) mechanized turning of the compost pile is possible,
(3) absorbent materials can be used through an entire season because
they resist biological degradation, and (H) thermophilic temperatures
are developed in the composting process (**06).
Composting of 6 to 10 tons per day of pharmaceutical wastes was
described by Gabaccia (353, 378). Organic sludges from various
fermentation and animal operations are dewatered, mixed, and shredded
with sawdust and animal manures. The optimum blend was determined
to be:
Waste treatment plant sludge 65% by weight
Stable manures 25%
Sawdust 10%
After blending, one part of ground rock phosphate was added to each
200 parts of fresh mixture. The material was composted for 5 months
in windrows, and then turned over, and restored in cone-shaped
piles. After stabilization, the compost was reshredded and applied
to lawns as a soil conditioner.
Kneiss reported on the successful use of garbage-sludge compost in
Europe as a fertilizer and soil conditioner for orchards and vine-
yards (357). He also suggested that compost could be used to reclaim
barren strip-mined areas.
Economics - Accurate production costs of compost were not available;
but they are estimated to be high because of the numerous steps in
the process. The steps include: (1) raw material transportation to
the compost site, (2) dewatering of wet solids, (3) trash separation,
CO grinding and blending of solids, (5) turning of compost, and
(6) regrinding and further processing for commercial sale. It has
been estimated the real production costs approximate those for
incineration ($35 to $45/ton). Operating costs can, of course, be
defrayed by selling the compost, but a market must exist for the
material.
-------
-227-
Avaliable technical literature contains widely varying compost
cost estimates having questionable value because most of them
describe overseas operations. The few cost estimates for United
States based facilities were as follows:
1. Davies reported that U.S. processing costs in 1956 were
less than $3.25 per ton for full time operation and
$8.00 per ton for part time operation (exclusive of
salvage )^3t>o;.
2. In Compost Science, a reported 1952 composting cost
was $3.0~per ton, if the raw material was delivered
free to a 200-300 ton per day facility and if the value
of salvaged material was deducted (354).
3. Snell was quoted as stating that the cost of composting
in the United States should be $2.00 to $4.00 per ton
for a reasonably sized plant (354).
<•
4. Gotaas reported that composting costs were $2.00 to $6.00
per ton (364).
The bases for the above costs are unknown, but they undoubtedly
reflected an optimistic attitude and probably included only operating
costs. Gothard and Shuval presented more detailed cost for composting
analyses in the British Isles and Israel, respectively, but, again,
the value is questionable because they describe overseas installations:
Compost Costs - British Isles (359) (1961)
Design capacity = 60 Ton/day
Capital cost = $600,000
Capital cost per ton = $5 per ton at 1961 production rates
Operating costs = $25_ per ton (allows for sale of compost)
Total = $30 per ton
The output in 1961 was at 40 percent of design capacity. Operating
costs were much lower than the previous incineration operating cost
of $75.00 per ton.
Compost Costs - Israeli (363$ (1959) - Dano System
Design capacity = 50 Ton/day 500 T/day
Capital cost = $278,000 $1,220,000
Estimated 1962
production costs = $86,500 per year $642,000 per year
or, $4.75 per ton $3.52 per ton
The actual 1962 production cost was $7.65 per ton of compost produced.
-------
-228-
Dependable markets for compost have not been established, at
least in the United States. Rodale <39D and Scott C»0»O said
that compost was being sold for $2.00 to $90.00 per ton. The
smaller figure was the price for large quantities of raw compost;
the larger figure was the price to small specialty markets. Sales
to specialty markets (use on gardens and golf courses) require
further processing such as the addition of inorganic fertilizer,
bagging and other commercial presale preparations. The specialty
market is very small when considering the tremendous volumes of
wastewater sludges available for composting. Therefore, the farm
market with its associated requirement for low compost prices should
be developed.
Unfortunately, the true market value of compost has never been
determined. Most everyone agrees, however, the economic situation
in the United States favors the use of concentrated commercial ferti-
lizers rather than organic soil conditioners '362).
In general, transporting compost from the urban areas providing
the raw material to distant farmland would cost more than the value
of the fertilizer components.
Summary - In addition to the conservation appeal of "returning
nutrients from the city to the country," (351) composting is claimed
to offer the following advantages: (1) many types of organic sludges
can be composted, (2) it can solve the critical problem of elimina-
ting troublesome sludge, (3) it has an appeal because the sludge can
be utilized as opposed to sludge disposal, CO sludge volumes are
reduced and stabilized to a sanitary nuisance-free product, and
(5) the end product has real value as a soil conditioner and source
of nutrients.
The major disadvantages to composting are high production costs,
limited technology, and the uncertainty of a market for the compost.
Composting is generally considered to be an untried innovation which
discourages support from public officials responsible for municipal
waste disposal t*»0t)t This fact, together with the lack of a
municipal organization for marketing the end product, usually prevents
composting from being seriously considered as a practical sludge disposal
method.
Less-often-mentioned disadvantages of composting are the possible
health hazards due to the inclusion of sewage sludge in composting
mixtures and the potential for public nuisance problems involving
odors, insects, and rodents. Reeves isolated many strains of
Shigella and Salmonella from a composted mixture of air dried sewage
sludge and sawdust that had been wetted and turned (366). He cautioned
against the use of such compost on vegetable crops. Others have believed,
however, that the high temperatures in composting mixtures kill all
microorganisms as well as weed seeds in a period of 4 to 6 weeks
(356, 401). Cleanliness and prompt handling of the wastes reduces the
chances of public nuisance problems.
-------
-229-
In 1962 there were more than 100 engineered compost facilities in
operation or under construction in various parts of the world but
very few were in this country. One survey reported that only three
cities in the United States have composted a blend of refuse and
sewage sludge (**05). The inability to dispose of large quantities
of compost at a favorable price was suggested as being the major
factor in closing 6 United States composting plants during the
period 1962-196H. One factor that has encouraged composting,
overseas, is governmental subsidies. The subsidy concept was
necessary to attract private investors to assume composting con-
cessions on a profit sharing basis (363).
Composting would be more acceptable if research and development efforts
produced improved technology and information on the true value of the
end product. Some specific needs are as follows:
1. New information concerning the effects of compost,
methods of application, and time of application on soil
structure, nutrient release, soil biochemistry, and
crop yields <»*0»O.
2. New data proving whether or not composting processes
disinfected sewage sludge.
3. Improvements in equipment design to sort, grind, and
dewater solids and to accelerate the composting process.
4. Realistic capital and operating costs for composting
processes.
5. Development of package compost units for use by
farmers, small communities and industries generating
organic wastes near rural areas.
Some additional cost information describing sewage sludge and refuse
composting will probably be generated by the cities of Houston, Texas;
Mobile, Alabama; and Johnson City, Tennessee. Houston has been
negotiating with various compost companies to process their wastes
at a cost to the city of $2.75 to $3.50 per ton. Mobile has a
composting plant under construction. A ioint TVA-Public Health Service
study has been planned for Johnson City f405). The question of whether
large amounts of compost can be marketed at a profit should be answered
by the Houston and Mobile operations.
The National Canner's Association is working on the fourth phase of a
study that will determine compost production costs for the canning
industry
-------
-230-
When research and development efforts result in lower production
costs and determine the real market value of compost, the com-
posting process may become attractive to small rural communities
or industries located in areas where a market for the end product
is nearby. But, even with system improvements, composting will
always be a minor sludge disposal process due to excessive trans-
portation costs of the raw material and finished end product. Its
popularity is declining in Europe in favor of mechanical dewatering
and incineration. Perhaps there is a message in this trend for the
United States.
-------
-231-
17. DISPOSAL OF DRIED SLUDGE AS A FERTILIZER
OR SOIL CONDITIONER
General - Sewage sludge has been used as a fertilizer and soil
conditioner for many years. Some industrial sludges such as filter
cake from sugar cane mills also have valuable soil conditioning
properties. Preservation of organic matter in this fashion has a
great appeal to conservationists but the trend is to alternate methods
of disposal because of economics. This chapter is concerned with
waste sludges that have been air dried on drying beds; mechanically
dewatered; or dried by artificial heat. Some systems combine the
three drying methods. Detailed information on these dewatering
techniques is in other chapters. The use of liquid sludge as a
fertilizer or soil conditioner, after preparation by composting or
digestion, is also discussed in other chapters.
One outstanding characteristic of sewage and industrial waste sludges
is their large difference in fertilizer value. To a great extent
these differences are associated with the method of waste treatment,
but certainly, the composition of the wastes collected in the sewerage
system plays an important role in the ultimate fertilizer value.
Usually, the value of sludge as a fertilizer is limited because the
nitrogen, phosphoric acid, and potash content is too low. The organic
material in sewage sludge does, however, make it a desirable soil
conditioner.
Sludge Processing and Composition - The beneficial use of raw sewage
sludge on land involves a potential health hazard from pathogenic
organisms unless it is artificially heat treated or composted. In
addition to the public health aspect, untreated raw primary sewage
sludge is not recommended for agricultural use because its physical
structure and high grease content have a detrimental effect on the
soil structure and the growing plants (383).
Raw waste activated sludge that has been vacuum filtered and heat
dried has become a valuable fertilizer and soil conditioner. The
largest producers of dried activated sludge are the Milwaukee
Sewerage Commission and the Chicago Sanitary District. Because
their treatment processes minimize the removal of solids before the
activated sludge process, the sludges retain most of the organic
solids. The sludges have a higher than normal nitrogen content in
part due to the type of industrial waste discharged to the sewerage
system.
Digested sludges from all sewage treatment processes are commonly
applied to the land after dewatering in mechanical equipment or
on sand drying beds. The digestion process, however, decreases the
value of the sludge as a fertilizer because nitrogen is lost as is
the readily digestible organic matter. Digested sludge primarily
consists then of inorganics plus residual organics that soil micro-
organisms are slow to degrade v
-------
-232-
Many chemical analyses have been made of dried sewage sludge.
Anderson reported an average nitrogen content for activated
sewage sludges of 5.6 percent and an average for digested sludge
of 2.6 percent (407). A survey of 10 sewage plants in Ohio showed
an average nitrogen content for digested sludge of 1.77 percent
and a total phosphoric oxide average of 2.4 percent (407).
Fair and Geyer gave the following approximate fertilizer values for
sewage sludge (7):
Sludge from plain sedimentation 0.8 to 5% nitrogen
Activated sludge 3.0 to 10% nitrogen
Trickling filter humus 1.5 to 5% nitrogen
Digestion will reduce the nitrogen content by 40 to 50 percent.
Raw sludge may contain from 1 to 3 percent P2<>5 and 0.1 to 0.3
percent potash.
Table I presents chemical analyses describing treatment plant
influent solids as well as raw activated and digested sludge solids
(373). it is interesting to note that aeration (activated sludge)
causes an increase in the sewage solids nitrogen content. Carbon-
nitrogen ratios are of particular interest to agronomists, but they
also serve as an indication of organic solids decomposition during
different treatment stages (373).
-------
-233-
Table I
CHEMICAL COMPOSITION OF SEWAGE SLUDGES
(Dry Weight Basis)
Sewage Treatment
Plant
Washington, D.C.
(Primary Treatment)
Influent solids:
Spring
Summer
Digested Sludge
Baltimore, Md.
Influent solids
Activated sludge
Humus tank sludge
Heat-dried
digested sludge
Jasper, Indiana
Influent solids
Activated sludge
Digested sludge
Richmond , Indiana
Influent solids
Activated sludge
Digested sludge
Chicago, Illinois
(Southwest plant)
Raw sludge
Activated sludge
Heat-dried sludge
Nitrogen
(%)
2.42
2.39
2.06
2.23
2.36
5.34
3.05
2.90
3.51
5.89
3.80
3.02
2.24
2.70
4.98
5.56
Carbon
(%)
43.46
43.69
28.59
,47.09
30.37
37.90
36.53
42.31
23.01
22.95
28.21
44.04
26.36
46.62
28.62
29.41
Carbon-
Nitrogen
Ratio
18.0
18.3
13.9
21.1
12.9
7.1
12.0
14.6
6.6
3.9
7.4
14.6
11.8
17.3
5.7
5.3
Phos-
phoric
Oxide
(%)
1.14
1.09
1.44
1.29
11.01
3.96
2.97
1.62
2.81
3.49
5.19
3.64
4.34
2.71
5.58
6.56
Ash
Milwaukee, Wisconsin
Heat-dried sludge
5.96
20.88
3.5
3.96
32.35
37.59
52.83
24.16
29.70
32.30
39.73
32.29
52.43
36.96
40.94
31.37
50.09
28.24
34.82
37.42
27.73
-------
-234-
Because the composition of raw sewage has changed in recent years,
Anderson compared certain sludge analyses made over a 20-year period.
The change in raw sewage has resulted from industrial wastes, wide-
spread use of phosphate-containing detergents, and the increased use
of home garbage grinders. Table 2 lists the nitrogen and phosphorus
percentages over 20 years (373), AS indicated there has been little
change in the nitrogen content of sludge, but the phosphorus content
has increased significantly.
Table 2
COMPARISON OF THE NITROGEN AND PHOSPHORUS CONTENT
OF ACTIVATED SLUDGE AND DIGESTED SLUDGE
_ 1931-35 and 1951-55 _
Activated Sludge Digested Sludge
Determination (%) 1931-35 1951-55 1931-35 1951-55
Nitrogen:
Min. 4.4 4.8 1.3 1.8
Av. 6.0 5.6 2.2 2.4
Max. 6.4 6.0 3.0 3.1
Phosphorus
Min. 2.0 4.0 0.8 0.9
Av. 3.2 5.7 2.1 2.7
Max. 3.8 7.4 3.8 5.0
Minor elements in fertilizer are often considered to be important
in crop nutrition. Table 3 shows typical values of some minor
chemical elements in activated and digested sludge (373)^ -phe
concentrations vary widely due to differences in industrial waste
discharges
-------
-235-
Table 3
MINOR CHEMICAL ELEMENTS IN
ACTIVATED SLUDGE AND DIGESTED SLUDGE
Elements (p.p.m. )
Sludge Copper Zinc Boron Manganese Molybdenum
Activated sludge 385 950 6 65 6
Min. 916 2,500 33 134 16
Av. 1,500 3,650 74 190 45
Max.
Digested sludge
Min. 315 1,350 4 30 2
Av. 643 2,459 9 262 6
Max. 1,980 3,700 15 790 12
Comparing dried sewage sludge on the basis of its humus content, has
been suggested by Husmann (^08)^ After a detailed sludge analysis,
he totalized those constituents readily available for humus formation.
The amount of humus in various sludges was as follows:
Type of Sludge Humus (%)
Fresh 33
Digested 35
Activated 41
Trickling filter 47
Obviously, primary treatment does not remove all of the humus-
like material.
Performance - In general, sewage sludge has proven its value as a
fertilizer and soil conditioner. The specific value is obviously
influenced by its nitrogen content. In addition, the sludge nitri-
fication rate, or conversion of nitrogen to nitrates, is very important
because it indicates the usefulness of sludge to growing plants. Data
by Anderson indicated that the nitrification rate for undigested activated
sludge is two and one-half times that of digested sludge (373)> ms
data also showed that most of the nitrification accomplished during a
growing season takes place within a month after the sludge is added
to the soil.
-------
-236-
The exact rate of nitrification has been studied by Anderson and
other staff members of the U.S. Department of Agriculture (**07).
They determined that the degree of nitrification of sewage sludge
is much less than that of ammonium sulfate. Less than 20 to 25
percent of the sludge nitrogen was converted to the nitrate form
in 16 weeks under optimum conditions for nitrification; the con-
version was 90 percent for ammonium sulfate. It was concluded that
the rate for activated sludge was similar to other natural organic
materials. Industrial wastewater sludges had an almost nonexistent
nitrification rate.
In addition to their fertilizer value, sludges are useful because
they contain necessary trace elements and they improve the soil
structure. The total effect increases the soil moisture holding
capacity, the organic content, the total nitrogen content, and it
improves the soil aggregation (29i).
Numerous field and greenhouse evaluations of sewage sludges have
been completed. In general, application rates of 10 to 40 tons
per acre are recommended ^"7» 383). Higher rates reduce crop yields
due to toxicity from trace elements in the sludge. Sewage sludge has
been usually handled in the same manner as farm manure. It is spread
and turned under or harrowed, before the crop is planted.
VanKleeck recommended liming of soils receiving dried sewage sludge,
that has not been conditioned with lime in a prior vacuum filtration
step (383). He listed the following reasons for liming:
1. Lime neutralizes excess acidity and precipitates some
metals that may be present in excessive concentrations.
2. It encourages bacterial decomposition of organic
solids thereby increasing the effectiveness of sewage
sludges.
3. It improves the physical structure of heavy soils
and supplies the necessary element calcium.
1. It makes phosphorus more available.
An application of .5 to 1 ton of lime per acre in the fall before
the use of fertilizers is suggested.
-------
-237-
The use of dried sludge as a fertilizer benefited the following
crops: citrus, tobacco, cotton, corn, potatoes, cabbage and various
grasses. Yields increased as much as 18 percent above unfertilized
crops *W7'm The increased yields often extended into the third
year after a single sludge application (&). Lunt surveyed the use
of digested sewage sludge on farmland in Connecticut and arrived at
the following conclusions (382;).
1. Sandy soils benefited more than loams, as expected.
2. Soils increased 3 to 23 percent in field moisture capacity,
non-capillary porosity, and cation exchange capacity.
3. Soil organic matter content increased by 35 to to percent.
4. Total nitrogen content increased up to 70 percent.
5. Soil aggregation increased by 25 to 600 percent.
While good results have been achieved from its use alone, the maximum
effectiveness of dried sewage sludge usually has been obtained by
combining it with inorganic chemical fertilizers.
Marketing Economics - Most treatment plants with heat drying
equipment have changed from fertilizer production to sludge incinera-
tion or landfilling. The basic reason for abandoning heat drvine was the
sludge market has not developed to the potential that many predicted
for it. Getting a reasonable price for dried sludge has not been
possible due to a limited demand. While it seemed reasonable that
more value should be attached to soil conditioning with natural
organics, the facts are that inroganic chemical fertilizers have
effectively and economically increased crop production.
There are some exceptions to the low agricultural market for sewage
sludges. For example, Milwaukee, Chicago, and Houston have successfully
marketed large quantities of heat dried activated sludge for many years.
The price depended on the nitrogen content (5 to 6%); generally it was
$12 to $18 per ton. Over 200,000 tons each year were wold by these
cities for fertilizing agricultural crops, golf courses, and park land.
Using the common values placed on nitrogen, phosphorus and potash as
fertilizersi 20*, 10*, and 5* per pound, respectively, the average
activated sludge should be worth $20 per ton and digested primary sludge
should be worth $11 per ton C*9>.
-------
-238-
The Milwaukee Sewerage Commission marketed their heat-dried activated
sludge under the trade name Milorganite. It was sold to large distri-
butors who in turn marketed the material through jobbers in all 50
States, plus some foreign countries. Home-owners could buy the
material in 50 pound bags. An average analysis of Milorganite showed:
(1) 6% nitrogen, (2) H% phosphate, (3) 0.4% notash, (4) 5% moisture,
and (5) numerous beneficial trace elements (367'. A marketing
organization consisting of a sales manager, agronomist, and three
office workers were all the personnel required in the sales effort.
The marketing successes at Chicago and Milwaukee were unique in part
due to the high nitrogen content of the dried sludge, an objective
pursued in the treatment plant design and operation. Other factors
were long term contracts, bulk sales, and the effectiveness of the
sludge as a fertilizer. The usual situation revealed by surveys of
sewage treatment plants across the country was that heat drying of
sludge was not economical and,therefore, was replaced by other handling
techniques ((66, 323).
A survey in 1955 of 23 cities, 35,000 to 4,500,000 population, indicated
that only Schenectady, New York, was satisfied with its sale of dried
sludge (66). Schenectady sold 65 pound bags of Orgo for $1.00 (30.80/
Ton). At this price much of the operating costs for heat drying were
recovered.
Sludge was sold in small quantities (outside of Chicago, Milwaukee,
and Houston) at $1.00 to $80.00 per ton. The price that a buyer was
willing to pay for sludge depended on several factors such as the
method of preparation, type of raw materials in the system, location
of the sludge with respect to the point of use, and kinds of crops or
vegetation to be fertilized CK>7).
Many plants found that granulating the dried sludge make it more
appealing to potential users. Most cities that dried sludge on sand
beds, gave the material to anyone willing to haul it away. Even at
no charge, this had the advantage of ultimate disposal because the
sludge was moved off the plant property. Not even the ash from
incineration processes was left for disposal.
In addition to competing with chemical fertilizers, sewage sludges
have to compete with other organic wastes. Table 4 compares the
chemical composition of some wastes with sewage sludges (369)t
-------
-239-
Table U
CHEMICAL COMPOSITION OF VARIOUS FERTILIZERS (_%)
Potassium
Fertilizer
Cottonseed Meal
Chicken Manure
Average Farm Manure
Average Activated Sludge
Average Digested Sludge
San Diego Digested Sludge
Nitrogen
7.0
4.1
1.2
5.6
2.0
2.7
Phosphorus ]
as P205
2.5
3.7
1.2
.4
.2
.8
Sewage sludge competed most often with farmyard manure and certain
seed meals because their effectiveness as fertilizers was similar.
Table 5 shows the 1958 prices of some nitrogen in the form of organic
sludges and inorganic fertilizers (367). Farmyard manure is relatively
inexpensive and often close to the point of use, but it is in short
supply.
Table 5
WHOLESALE PRICES OF NITROGEN
AT POINTS OF ORIGIN
Approximate
Nitrogen Price per Pound
Content (cents)
Material (%)
Milwaukee, Wisconsin, sludge 6 35
Chicago, Illinois, sludge 5-6 25
Baltimore, Maryland, Sludge 3 10
Washington, D.C., sludge 2 Free
Lancaster, Pennsylvania,
stockyard manure 1.5 10
Cottonseed meal 6.5 U8
Urea-form 38 36
Anhydrous ammonia 82 5
Ammonium sulfate, synthetic 21 9
Summary - Many tons of dried sewage sludge have been sold each year
as a fertilizer or soil conditioner, but the sales have been generally
by three large activated sludge plants or small towns in rural areas.
Large industrial towns would generally have a difficult time marketing
sludge because of the quantities handled and the distant location of
the markets. Other problems associated with the agricultural use of
sludge are: (1) occasional odors when the sludge is wetted, (2) excessive
labor to remove it from drying beds, (3) necessity for grinding in most
cases, (4) difficulty in proper spreading, and (5) possible disease
transmission
-------
-240-
Odors can be reduced by storing the sludge for a few months before
spreading and by proper treatment plant operation. There have been
no reported cases of disease transmitted by the use of heat-dried
raw sludge or dewatered digested sludge, but the concern about disease
transmission is legitimate, therefore, the use of dried sludge is regula-
ted by various health departments. Regulations promulgated by the
Connecticut Department of Health are typical of those throughout the
country <383);
1. Heat-dried sludge is safe for use under all conditions
because heat destroys microorganisms.
2. Air-dried or mechanically dewatered digested sludge may
be used on lawns, shrubs, flowers, and beneath trees.
Storage for several months before spreading is recommended.
The same sludge may be used on vegetable plots provided
that the edible portion of the crops grow above ground or
the crops are cooked before eating.
3. When the above sludge is used on soil where vegetables
are grown in contact with the soil and eaten raw, the
sludge should be applied the previous fall and plowed
under before the crop is planted.
t. The public should be warned to avoid handling sludge
with bare hands.
The two most important advantages of using dried waste organic sludges
for agriculture are: (1) it returns a natural resource to the land,
and (2) it represents ultimate disposal of solids. Sewage sludge is
generally recognized as a material that can increase soil fertility.
In the future, as intensive farming continues, more attention will
have to be given to returning essential minerals and organics to the
soil.
After reviewing the advantages and disadvantages of drying sludge
agricultural use, few consulting engineers today seriously consider
the installation of heat drying equipment. Heat drying is the most
expensive of all conventional sludge processing techniques. This
fact, plus a low market price and a limited demand, makes investment
in heat drying equipment a risky business.
-------
-241-
Sludge is a liability at all waste treatment plants, even Chicago
and Milwaukee lose many dollars in the sale of their heat-dried
activated sludge. But sludge from small treatment plants naturally
dried on sand beds will continue to be used as a soil conditioner
because the volume is small and the material is usually given away.
At small plants this method of sludge disposal is simpler and less
expensive than most other accepted techniques. It will continue to
be successful as long as sludge disposal has top priority over
sludge sales.
Improving the economics of dewatering and heat drying would increase
the use of sludge, as would fortifying the sludge with nitrates,
potash and phosphates. Obviously, fortifying sludge with inorganic
fertilizers involves higher costs, a definite disadvantage.
New research is needed to determine the soil conditioning value of
dried organic sludges. Many studies have been made but their scope
was too limited. A broad scientific study could probably prove that
sludges are of inestimable values to agricultural businesses.
Effective public relations campaigns to promote dried sludge are
also needed. For example, most cities with dried sludge have made
no major effort to sell nearby farmers on the value of the sludge
as a fertilizer and solid conditioner.
-------
-242-
18. NON-FERTILIZER BY-PRODUCT RECOVERY
General - Using water and wastewater solids rather than disposing of
them, is certainly an applaudable concept. But, waste utilization
is not necessarily profitable, more likely it is a means of reducing
sludge disposal costs.
Factors that should be considered when evaluating by-product recovery
include: (1) relative costs of sludge disposal and by-product recovery,
(2) market value of by-product and marketing problems that may be
encountered, and (3) research and development problems that may be
involved with by-product systems.
The marketplace determines the by-product specifications. These could
be very rigid limitations involving product purity and concentration.
The waste sludge processing cost to meet rigid specifications could,
of course, be very high. Most industries who have investigated waste
by-product recovery, concluded that it is much easier to sell or give
the material to a refiner who assumes responsibility for its ultimate
disposition.
Reported By-Product Recovery Methods and Materials;
Sewage Sludge-Vitamin 812, Grease and Protein - The recovery of
vitamin B12, particularly from sewage sludge, has been frequently
discussed in the literature. Vitamin 812 is a valuable supplement
to animal feeds.
Two factors influence the Bj.2 concentration in sewage sludges:
(1) nature of the incoming solids (whether food industries contribute
a significant portion of the total waste solids), and (2) manner in
which the sewage is treated. Does it receive secondary treatment?
Are the solids digested anaerobically? Vitamin Bj.2 has been success-
fully recovered from dried, undigested activated sludge in pilot plant
facilities. Extensive studies at Milwaukee determined that B^ is
derived in part from raw sewage and partly from biological synthesis
in the activated sludge aeration tanks (4l2). The 812 production
process at Milwaukee started with the dried waste activated sludge
(Milorganite). Commercial heat drying of the sludge caused a 60 to
75 percent loss of vitamin B12 from the starting concentration of 3.5
to 4 mg/kg of sludge (**12). The Milorganite was subjected to multiple-
effect washing, followed by a continuous draw off of concentrated
extracted liquor C*24'. The liquor was concentrated to a syrup in
vacuum evaporators and then spray dried. Milorganite, with 812
extracted, can be redried and sold with no decrease in value. The
pilot plant operation indicated that 308 pounds of pure vitamin Bi2
could be produced each year from Milwaukee's 70,000 ton per year
sludge supply.
-------
-243-
In a cooperative study by the University of Wisconsin and City of
Milwaukee, dried activated sludge was fed to hogs as a protein
source ^24^. Leary summarized that while the sludge contained
40 percent protein, it could not be used as a hog feed. Milorganite
was also fed to rats and chicks as a vitamin supplement; its benefits
were comparable to that of yeast.
The Metropolitan Sanitary District of Greater Chicago supported
research studies at the University of Illinois to determine the
value of heat dried activated sludge as an additive to animal
feeds (66, *+23). They considered that activated sludge was a good
source of vitamin 812 and essential amino acids. After feeding
studies with chicks, pigs, lambs and steers, Hurwitz concluded that
sludge was a successful beneficial additive if limited to a small
percentage of the total animal feed. For example, activated sludge
levels of 0.5 to 2.0 percent were a satisfactory source of vitamin
812 for pigs and chickens '66). The studies detected no pathological
symptoms in the pig or chicken tissues.
Rudolfs and Cleary described the "Miles Acid Process" for extracting
grease from raw sewage solids (35'. Sewage was mixed with sulfurous
acid which precipitated the solids and killed the microorganisms.
After investigations in Boston and New Haven, it was concluded that
the process could be worthwhile if the sewage had a very high grease
content and low alkalinity.
Food Industry Sludge - The food industry has been very interested
in using waste products to increase profits and reduce air and
water pollution. Food waste sludges have the disadvantages, however,
of high moisture and fiber content plus a lack of stability which
complicates storage of the seasonal production.
1. Citrus. The use of waste activated sludge from the
citrus industry has been evaluated as a source of
vitamin B and protein. Vitamins of the B group
contained in this sludge include: thiamin, 8^2>
riboflavin and niacin (409). After the sludge is
filtered, dried and pulverized, it may be used
as an animal feed supplement.
Citrus pulp has been extensively used by itself as a
cattle feed. After dehydration the pulp has been sold
for $30 to $15 per ton (416, 430). Without dehydration,
citrus wastes have been sold as cattle feed for $4 per
ton or converted to molasses and sold for $40 per ton.
Pectin is also made from citrus wastes and used in jams
and jellies.
-------
-244-
2. Winery. During World War II tartaric acid and tartrate
salts were manufactured from winery wastes. But today,
the process is not considered economical (428). The
pomace portion of the waste has been used as a vineyard
soil conditioner or sometimes dehydrated and sold as a
cattle feed supplement and plant mulch.
3. Canning. The disposal of wet solids from the canning
industry has been a major problem. In California, some
canning wastes have been barged to sea at a relatively
high cost. Composting has been investigated and seemed
promising. A small portion of the total canning waste
has been dehydrated and used for animal feed (^l?).
About 15 tons of bulk tomatoes are required to make one
ton of dehydrated feed. The cost of producing this feed
was reported to be $47 to $53 per ton of feed (capital
and operating costs) (130). As in most waste disposal
operations there was a net cost to the industry genera-
ting the waste, but by-product utilization can be cheaper
than other methods of disposal.
4. Meat. Grease and fats recovered in meat processing are
valuable raw materials for soaps, gelatin and glue.
By-product recovery techniques have been easy to justify
in this industry because of the large U.S. and overseas
market for high quality grease and fat. A price of $.045
per pound for the recovered material was quoted in 1963
Semi-solid meat processing wastes were sold at $.04 per
pound for use as animal feed. The Carver-Greenfield
dehydration system installed at Hershey Estates, Pennsylvania,
can recover greases from combined sewage and industrial
wastes. The process includes sludge grinding, fluidizing
with oil, three-stage evaporation, centrifuging of solids,
and solid pressing.
5. Brewery and Distilling. Wastes from brewery and distillery
fermentation have been converted to valuable by-products for
many years. The two major by-products are high priced
animal feed and Brewer's yeast.
In the 5-year period from 1945 to 1950, the nation's distillers
sold nearly $116 million worth of by-product feed (**27).
Spent grain mash used to be a serious water pollution problem
but now the distilling industry has recovered more than
90 percent of its fermentation residues for sale as animal
feed C*15). The residue has a high concentration of vitamins
and protein that, when mixed with other materials, makes a
premium cattle feed*
-------
-245-
The spent grain drying process usually involves:
(1) screening to an 83 percent moisture content,
(2) pressing to 65 percent moisture, and (3) drying
in a rotary dryer to 8 percent moisture (**27).
Residual yeast from brewery and distillery fermentation
is also valuable as an animal feed because it contains
vitamin B complex, protein, and essential minerals. *
Lipsett recorded the breakdourn as: (1) 46 percent
protein, (2) 37 percent carbohydrate, (3) 8 percent
minerals, and (4) 1 percent fat C*27). jn 1946 the
dried yeast sold for $.08 to $.20 per pound. Eighty-
five to 90 percent of the drum-dried yeast market has
been for poultry, livestock, and specialty feeds.
Some has been used in dog food (426), Drying stabilizes
the residual yeast by deactivating the enzymes. It also
improves the yeast's digestability.
Singruen believes that a brewery must have one million
pounds of yeast solids per year to justify drying equip-
ment. This quantity requires a brewery capacity equal to
two million barrels per year. He suggested that residual
yeast could be collected from several breweries and dried
at a central point.
6. Potato. Douglass observed that potato processing pulp
is a good cattle feed v420)< Because the solids are
hydrophilic, dewatering and drying the pulp is difficult.
The 5 to 13 percent solids pulp can be dewatered to
30 to 35 percent solids by pressure filtration after
conditioning the pulp with lime.
Paper Industry - The paper industry has investigated the possibility
of using deinking wastewater sludges in the following applications:
(1) lightweight aggregate for building blocks, (2) filler for asphalt
tile and. liquid emulsions, (3) filler for fiberboard and other
lined boards, and (4) filler for paper, rubber, and other manufacturing
materials. Deinking sludge solids contain 50 to 75 percent clay,
making them particularly useful in the above applications
Bottenfield and Burbank described the recovery of lime from
causticizing operations in the kraft wood pulping process
Lime muds from a kraft process mill and the mill water softening
plant were first dewatered on vacuum filters to a 55 percent solids
concentration. This concentrated slurry was then calcined in a
natural gas fired kiln at a temperature of 1600°F. The kiln output
-------
-2U6-
of 160 tons of lime per day reflected a lime recovery rate of 97
percent. This recovery of a valuable raw material, in addition to
solving a solids disposal problem, saved the pulp mill $1,600 per
day. Other lime reclamation data are included in the chapter on
Water Plant Sludge Disposal.
Other Industrial Sludge By-Product Techniques:
1. Chemical and Petroleum. Waste tars, "spent" catalysts, and
other materials from petrochemical processing have often been
used by reclamation companies to recover metal from the tars
and catalyst complexes v!6). Giving away the waste tars
eliminates the necessity of on-site disposal by incineration
or some other relatively expensive procedure.
Carbide lime slurries, an acetylene process by-product,
and aluminum chloride are two other "waste" materials from
the chemical industry that can be salvaged. They are
useful as sludge conditioning agents prior to dewatering.
2. Mineral Industry - The refining of alumina from bauxite ore
produces a mud in the filtering operations that is a source
of alumina oxide. Alumina oxide is recovered by remixing the
refuse mud with chemicals and sintering it at 1800°F in rotary
kilns C*W).
Other by-product possibilities mentioned in the literature concern
fish processing, beet sugar processing and metal finishing.
Summary - By-product recovery of useful materials from municipal
and industrial sludges should be given serious consideration because
it may: (1) eliminate or reduce a water pollution problem, (2) save
raw materials cost by reuse, (3) reduce the cost of operation, and
(•O cause greater stability in process operations. But, recovery
processes involve risks and, therefore, careful planning is required
for their success. Some criteria are: (1) the process must be in-
expensive in order for the by-product to compete with other materials,
(2) if producing animal feed the material must be free of substances
harmful to animals and it must be easily digested, and (3) the
materials should have a narrow concentration range of valuable
constituents C*^5).
Many industrial sludges often contain too great a variety of materials
which make by-product recovery uneconomical. However, the success
of the brewery and distilling industry certainly indicates by-product
recovery is possible and economical in some situations. Because
incineration as a sludge disposal technique is becoming so popular,
perhaps particular attention should be directed at finding a bene-
ficial use for the incinerator ash.
-------
-247-
19. SLUDGE COMBUSTION
Introduction - Incineration of municipal and industrial refuse has
been a standard practice for many years. The combustion of semi-
solid sewage and industrial waste sludge is a relatively recent
innovation. Dearborn, Michigan, installed the first full-scale
sewage sludge incinerator in 1935 (287). Sludge combustion is
becoming increasingly popular as land areas for sand beds, lagoons,
and landfills become more difficult to find and more expensive.
The land situation is magnified by ever-increasing volumes of sludge
to dispose of and by the encroachment of urban neighborhoods on land
disposal areas. There is no doubt that sludge combustion will in
the future be adopted by more and more industries and cities because
it represents a sanitary method for ultimate sludge disposal (at
least for the organic portion of the sludge) in a relatively small
land area. For many industries and cities, incineration will provide
the only practical answer to sludge disposal.
Incineration is practiced for two basic purposes, volume reduction
and solids sterilization. Fulfilling these purposes is expensive
and it may cause an air pollution problem. But the economics of
combustion appear more favorable each year as the alternative
sludge disposal methods become more expensive. Air pollution problems
can be solved by proper equipment design and operation.
Sludge Characteristics - Russell says the following sludge parameters
are most important in the incineration process: (1) moisture,
(2) volatiles, (3) inerts, and (H) calorific value (65d). Of the
above, moisture is the principal characteristic, over which the
treatment plant operator has some control. Moisture is generally
reduced by mechanical sludge-dewatering techniques before incinera-
tion. It is important because of the thermal load it imposes on the
incineration process with consequent effects on self-sustained sludge
combustion.
Volatile and inert materials affect the calorie or B.t.u. value of the
sludge. They are, to some extent, controlled by other treatment
processes such as degritting, mechanical dewatering, and sludge
digestion. Almost all of the combustibles are present in the sludge
as volatiles, much of it in the form of grease. The volatile per-
centage and, therefore, the heat value can vary widely, so incineration
equipment must be designed to handle a broad range of values.
Table 19.1 by Owen describes the most important incineration
parameters for many of the solids generated at a sewage treatment
plant <321).
-------
-248-
Table 19.1
Combustibles Ash
Material (%) (%) B.t.u./ Pound
Grease and scum 88.5 11.5 16,750
Raw sewage solids 74.0 26.0 10,285
Fine screenings 86.4 13.6 8,990
Ground garbage 84.8 15.2 8,245
Digested sewage solids
and ground garbage 49.6 50.4 8,020
Digested sludge 59.6 40.4 5,290
Grit 33.2 69.8 4,000
As a general rule, the thermal value of sewage sludge is considered
to be 10,000 B.t.u. per pound of volatile solids. If the ultimate
analysis is known, Owen claimed that the heat value can be precisely
computed by using the DuLong formula '321):
Q = 14,600 C + 62,000 (H- - )
8
where Q = H. t.u./lb.
C = % carbon
H = % hydrogen
0 = % oxygen
Theory - Incineration processes involve two steps: (1) drying and
(2) combustion. In addition to fuel and air; time, temperature, and
turbulence are necessary for a complete reaction. The drying step
should not be confused with preliminary dewatering; this dewatering,
usually by mechanical means, precedes the incineration process in
most systems. A sludge, having a moisture content of about 75 percent
is delivered to the most common types of incinerators. Since the
typical sludge contains about 3 pounds of water for each pound of dry
solids, the heat required to evaporate the water nearly balances the
heat available from combustion of the dry solids (65d)s
Drying and combustion may be done in separate pieces of equipment
or successively in the same unit. Manufacturers have developed
widely varying types of sludge drying and combustion equipment.
These include traveling-grate furnaces, rotary-kiln-type furnaces,
fluidized-bed units, wet-oxidation units, atomized spray units,
multiple hearth furnaces, and flash-drying units.
The drying and combustion process consists of the following phases:
(1) raising the temperature of the feed sludge to 212°F, <2) evapo-
rating water from the sludge, (3) increasing the water vapor and air
temperature of the gas, and (4) increasing the temperature of the
dried sludge volatiles to the ignition point (65d)
-------
-2U9-
Practical operation of an incinerator requires that air in excess
of theoretical requirements must be supplied for complete combustion
of the fuel. The introduction of excess air has the effect of
reducing the burning temperature and increasing the heat losses from
the furnace. For this reason, a closely controlled minimum excess
air flow is desirable for maximum thermal economy. The amount of
excess air required varies with the type of burning equipment, the
nature of the sludge to be burned, and the disposition of the stack
gases.
Fuel (sludge) burned in a furnace emits heat; some is absorbed by the
furnace and is lost by radiation, a larger portion is lost with the
stack gases, and a smaller portion is lost with the ash. The difference
between the heat generated and the heat lost is available for heating
the incoming sludge and air. Self-sustained combustion is often
possible with dewatered raw sludges once the burning of auxiliary
fuel raises incinerator temperatures to the ignition point.
The primary end products of combustion are considered to be water,
sulfur dioxide, carbon dioxide, and inert ash.
-------
-250-
A. Multiple Hearth
General - The multiple hearth furnace is the most popular sewage
sludge incinerator in use today. There are about 115 of these
units installed for sewage sludge combustion (333). Incineration
in multiple hearth units is particularly popular in large cities
where alternative final sludge disposal techniques are inconvenient
or too expensive.
Dewatering the sewage sludge feed, usually by vacuum filtration, is
almost always a standard operating procedure. Either raw or digested
sludge can be incinerated but digestion causes a loss of combustible
organic matter, which reduces the heat value of the sludge. As a
result, auto-combustion (or burning without auxiliary fuel except for
start-ups) is usually not possible as it is when burning raw sludges
having a higher volatile content.
Multiple hearth units are popular because they are simple, durable,
and have the flexibility of burning a wide variety of materials
even with fluctuations in the feed rate.
Design and Operation - The multiple hearth furnace consists of a
circular steel shell surrounding a number of solid refractory hearths
and a central rotating shaft to which rabble arms are attached. Since
the operating capacity of these furnaces is related to the total area
of the enclosed hearths, they are designed with various diameters and
a varying number of hearths, usually between four and eleven (321).
Each hearth will have openings that allow the sludge to be dropped
to the next lower hearth.
The central shaft and rabble arms are cooled by air supplied in
regulated quantity and pressure from a blower discharging air into
a housing at the bottom of the shaft. An annular space between
the inner "cold air tube" and the outer wall of the shaft exposed
to furnace heat serves as a conduit for hot air. Rabbling is very
important to combustion because it breaks up the large sludge
particles, thereby exposing more surface area to the hot furnace
gases that induce rapid and complete combustion. A typical multiple
hearth furnace is described in Figure 19.1.
Owen recognized- four distinct zones in the multiple hearth furnace
when combustion of wet sludge is carried out to practical comple-
tion (321). Partially dewatered sludge is continuously fed to the
upper hearths which form a drying and cooling zone. Here, vapori-
zation of some free moisture occurs as well as cooling of exhaust
gases, all by transfer of heat from the hot gases to the sludge.
-------
-251-
Figure 19.1
-COOLING AIR DISCHARGE
FLOATING DAMPER
SLUDGE INLET
FLUE GASES OUT
RABBLE ARM
AT EACH HEARTH
COMBUSTION
ZONE
COMBUSTION
AIR RETURN
COOLING ZONE
ASH
DISCHARGE—1141^
RABBLE ARM
DRIVE
COOLING AIR FAN
TYPICAL SECTION
MULTIPLE HEARTH
INCINERATOR
(Reprinted by permission Black and Veatch Consulting Engineers)
-------
-252-
Intermediate hearths form a high temperature burning zone, or
combustion chamber zone, where all volatile gases and solids are
burned. Combustion of most of the total fixed carbon takes place
on the lowest hearth of the combustion zone.
The bottom hearth of the furnace functions as a cooling and air-
preheating zone where ash is cooled by giving up heat to the shaft
cooling air which is returned to the furnace in this zone. Incinerator
temperatures range from 1,000°F on the top hearths to 1,600-1,800°F
on the middle hearths to 600°F at the bottom. In many incinerators
the waste gases from combustion are heated to 1,250°F or higher to
guard against odor nuisance. Exhaust gases leaving the incinerator
at the top are scrubbed in a wet scrubber to remove fly ash.
Some incinerator specifications have been written to require
deodorization of gases leaving the furnace by increasing the
temperature to about 1,500°F. The simplest method of doing this
involves conducting the gases to a chamber where the temperature
is raised by burning auxiliary fuel in direct contact with the
gases before venting to the atmosphere. This method is expensive
due to the fuel cost. A heat recovery device can improve the
economy of high temperature deodorization but this requires
expensive deodorization and combustion air preheating equipment.
The ash discharged from multiple hearth furnaces is essentially free
of all organic solids; therefore, nuisance problems are not expected.
Ash is generally transported from a furnace by one of three methods:
(1) hydraulic, (2) mechanical, and (3) pneumatic. The hydraulic
system is preferable if land for lagooning is available nearby
because this method is simple and adaptable.
Hydraulic systems can handle most types of ash regardless of
temperature and particle size.
When land availability prevents the use of lagoons, ash is stored dry
and periodically hauled away to convenient fill areas. Mechanical
conveyors are preferable to pneumatic systems for transporting ash
from the furnace to storage bins t302)>
-------
-253-
Performance and Economics - Incineration is generally considered to
be more expensive than other sludge disposal processes. Bartlett-
Snow-Pacific, Inc., showed the following approximate costs for
sewage sludge incineration at cities of varying populations '
Population System Cost Annual Operating Cost
6,000 $ 90,000 $ 2,300
10,000 100,000 3,500
25,000 135,000 5,700
50,000 150,000 8,800
100,000 185,000 17,500
250,000 375,000 40,000
The figures include vacuum filtration equipment, chemicals, power,
fuel, and maintenance.
Assuming that the incinerator handles 0,15 pound of dry solids
per person, the annual operating cost on a per ton basis would be:
/<
Population Annual Operating Cost ($ per Ton)
6,000 $13.95
10,000 $12.80
25,000 $ 8.33
50,000 $ 6.42
100,000 $ 6.38
250,000 $ 5.84
The above costs appear to be a little low, particularly for digested
sludge filtration and incineration.
MacLaren estimated that the capital cost of incinerators, assuming
the capacity is divided between two or more units, was $5 to $10
per ton of dry solids based on a 30-year amortization at 5%
interest (53). He estimated that operating costs were $4 to $7
per, ton. The total annual cost would, therefore, be $9 to $17
per ton of dry solids.
Quirk made a precise study of sewage sludge incineration costs
using, as a model, digested sludge from a city of about 100,000
contributing 2,530 tons of solids per year. In summary, his
incineration cost data show (65e):
Capital Cost (includes vacuum filtration but not external
ash disposal facilities)
1. Incineration w/o deodorization $11.75/Ton
2. Incineration w/ deodorization $12.07/Ton
-------
-254-
Operating Cost
1. Vacuum filtration $7.91/Ton
2. Incineration w/o deodorization $6.36/Ton
3. Incineration w/ deodorization $9.50/Ton
Total Annual Cost of Solids Disposal
1. Without deodorization $26.02/Ton
2. With deodorization $29.«48/Ton
Variables in the cost of sludge incineration have been due generally
to the following parameters^3-*-0);
1. Size and design of treatment plant.
2. Nature of waste sludge.
3. Amount and type of chemicals used for sludge conditioning
prior to mechanical dewatering.
**. Efficiency of mechanical dewatering.
5. Site conditions and degree of nuisance control.
6. Skill of operating labor, their productivity,
and operating schedule.
7. Management competence.
8. Record keeping procedures.
9. Extent of standby facilities built in.
10. Cost of utilities (fuel, water, power).
The literature reported wide variations in operating costs. Part of
this variation was due to the variation in supplemental fuel require-
ments. A survey of numerous incineration facilities showed a
variation from less than 1 percent to 35 percent of the heat value
supplied by the sludge cake itself '296)^ j^ should be remembered that
all installations use some fuel during start-up. Schroepfer reported,
as expected, a wide difference between the fuel required for raw sludge
incineration and for digested sludge incineration^96). Raw sludge units
required an average of 1.75 percent additional heat in the form of fuel
while digested sludge required 17.9 percent, or ten times as much. This
variation could mean a difference in operating costs of $1.06 per ton.
The difference in heat value between polymer conditioned sludges and
those conditioned with inorganic flocculents is significant. A typical
comparison is as reported below:
Ferric Chloride and Lime Conditioned Sludge
Total Solids Volatile Solids
Assume 100 Ibs. 70 Ibs.
5% FeCl3 adds 3.3 Ibs. 0 Ibs.
15% lime adds 20.0 Ibs. _£ Ibs.
123.3 Ibs. 70 Ibs.
Therefore, the true percentage of volatiles = 7.0 or 57%
-------
-255-
Assume average filter cake solids of 25% and a heat value per
pound of volatile solids of 10,000 B.t.u.
The heat value of 1 pound of wet cake = .25 x .57 x 10,000
But, the calcining effect on ferric chloride and lime requires heat:
(1) lime = 388 B.t.u./lb., and
(2) ferric chloride = 6J3 B.t.u./lb.
for a total of 451 B.t.u./lb.
So, the actual heat value is 1,425 - 451 or 974 B.t.u./lb.
Organic Polymer Conditioned Sludge
The percentage of volatiles remains at approximately 70%.
The heat value of 1 pound of wet cake = .25 x .70 x 10,000
= 1,750 B.t.u./lb.
There is no heat consumed by calcining.
Therefore, the difference in heat value between these two sludges
is 1,750 - 974 or 776 B.t.u./lb.
which equals 1,552,000 B.t.u. /ton of wet solids
or 6,000,000 B.t.u. /ton of dry solids
A general review of incineration costs showed the following:
Average ($) per Ton Range
1. Total cost (dewatering + $30 $10-$50
incineration, etc.) $20 $ 8-$40
Some interesting modifications of conventional multiple hearth
incineration have been reported in the literature. For example,
Piqua and Ashland, Ohio, have incinerated raw unfiltered sewage
sludge for over 15 years '28)0 The furnaces incorporated no special
features for handling the liquid sludge other than the sludge feeding
arrangement and the addition of extra fuel burners. At Piqua, the
average concentration of sludge burned was 14.2 percent (the range
was 8 to 17 percent). As would be expected, the incineration of
liquid sludge resulted in excessive supplemental fuel costs; the
average requirement was 88 gallons per ton of dry solids. At 10 cents
per gallon this equaled a fuel cost of $8.80 per ton.
-------
-256-
Multiple hearth furnaces may also be operated as sludge dryers.
When operated for drying, the flow of sludge and the rabbling action
in multiple hearth furnaces is identical to incineration procedures.
The differences are that fuel is burned at the top hearth and the
gases are down-drafted to exit from the bottom hearth (also see the
chapter on Heat Drying). A similar procedure, parallel flow of solids
and gases, is used when incinerating skimmings in a multiple hearth
furnace.
Summary - The multiple hearth unit is the most popular furnace for
sewage sludge incineration because of its many advantages that include:
(1) simplicity; (2) durability and low requirement for maintenance;
(3) moderate operating costs; CO ability to burn grit, screenings,
skimmings, and sludge in the same unit; and (5) its flexibility to
accept fluctuating loads. Also, combustion in a multiple hearth
unit is "complete" (nearly 100 percent destruction of organic solids).
The disadvantages associated with the multiple hearth unit generally
involve the capital cost, ash, nuisances, and explosions. Capital
costs are not considered high in relation to other incinerator
designs, but they could be considered high in relation to other sludge
disposal techniques that use land or ocean disposal.
Theoretically, incineration should not be considered an ultimate
sludge disposal technique because ash remains for subsequent handling.
Lagoon space is not always conveniently located near the treatment
plant site and hauling dry ash to a distant landfill area can be
expensive. Unfortunately, many operating cost figures in the litera-
ture have not included a value for ash disposal. The weight of the
ash may average 30 percent of the weight of the dry solids incinerated.
Nuisance conditions involving air pollution can occur, usually of
a fly ash nature rather than odors. The multiple hearth incinerator
effectively eliminates odors due to the high combustion temperature.
Nickerson, Sawyer and Kahn, and others have generally agreed that
temperatures above 1,200°F deodorize exhaust gases (291, 337). To
be safe, most designers have recommended temperatures of 1,HOO°F.
Fly ash can be effectively controlled by centrifugal dust collectors
or water scrubbers 1291>. Centrifugal collections remove 75 to 80
percent of the particles and are suitable for exhaust gas temperatures
of 650 to 700°F. Water scrubbers are at least equally effective,
they are less sensitive to loadings and gas temperatures, plus, they
collect the condensable portion of the exit gases.
-------
-257-
Explosions that damage equipment may occur in the multiple hearth
furnace from the combustion of grease. For this reason, separate
feed openings in the furnace are desirable for grease and screenings.
If the unit is used for only grease and skimmings incineration, a
parallel flow of feed solids and hot gases is desirable.
Incineration is being adopted by many municipalities and industries;
therefore, the multiple hearth units will be installed at progressively
more locations. Some improvements in the design and operation would
be desirable even though the present unit operates quite satisfactorily.
Recovery of heat offers one potential way of improving the economy of
incineration. Research on new designs to accomplish this is advisable.
Perhaps the heat could be used to condition the sludge to be incinerated.
The development of additional instrumentation to control the com-
bustion process and the development of some beneficial uses for the
ash, perhaps as an aid to sludge conditioning, would be desirable.
-------
-258-
B. Flash Drying - Incineration
General - Flash drying - incineration processes are rarely adopted today
for sewage sludge disposal. While not as popular as multiple hearth
furnaces, they were installed at quite a few locations because of the
flexibility of drying or incinerating the feed sludge.
Because drying of sludge for sale as a fertilizer is not often seriously
considered today, this flexibility is of little interest. Flash drying-
incinerators are more complex than multiple hearth units, so they do
not compete very well on a straight incinerator basis.
Operation - Basically, this process involves three steps: (1) sludge
dewatering in mechanical equipment, (2) heat drying, and (3) incineration.
First, the dewatered sludge feed is mixed with dry sludge to reduce
its moisture content and particle size. Then, the mixture is fed into
the drying system where it moves at a velocity of several thousand
feet per minute in a stream of gas having a temperature of about 1,100°F.
The sludge passes through this high temperature-turbulent zone in a
few seconds during which time the moisture is reduced to about 10
percent. Heat dried sludge is separated from the gases and vapors in
a cyclone separator.
The fluffy, dried sludge produced by the flash dryer is blown into
a furnace in a manner similar to that of powdered fuel. Temperatures
near 1,HOO°F are usually maintained for deodorization. The heat of
combustion is used in the dryer operation.
The Laboon Process at Pittsburgh, Pennsylvania, incorporates flash
drying-incineration of their biologically floated and concentrated
primary sludge ^131). Thickened liquid sludge (about 18% solids)
rather than dewatered filter cake is mixed with previously dried
sludge. Drying and incineration proceed in the normal fashion,
except that significant quantities of auxiliary fuel are burned in
the furnace along with the sludge: O.U pound of coal and 0.9U cubic
foot of natural gas for each pound of dry sewage sludge.
Summary - Flash drying-incineration is rarely installed in treatment
plants today. The lack of a fertilizer market for dried sewage sludge
has eliminated one advantage for this unit, the flexibility of drying
or burning. As an incineration unit, the flash drying system has the
major disadvantages of complexity, potential for explosions, and
potential for air pollution by fine particles. In comparative
situations, it is not equal to other furnace designs.
-------
-259-
C. Fluidized Bed
General - A fairly recent development in sewage sludge incineration
is a fluid-bed technique similar to that used in industrial processing
for many years. In addition to sewage sludge destruction, this tech-
nique has been used for: "Drying, sizing, roasting, calcining and
other heat treatment operations of solids-with-gases in the chemical,
metallurgical, nonmetallic, food, and pharmaceutical process industries,
in the pulp and paper field, and in municipal water treatment" (350)^
The fluidized bed process represents "complete destruction" of organic
solids (at least 99%).
Theory and Operation - The term "fluidized" is used because the
sludge particles are fed into a bed of fluidized sand supported by
upward moving air. Sufficient air is used to keep the sand in
suspension but not to carry it out of the reactor. The reactor serves
as a large heat reservoir where rapid mixing of the sludge throughout
the bed provides efficient contact between the sludge particles and
oxygen and allows rapid heat transfer. Sludge particles are kept
suspended in the moving stream of gases causing the mixture of gases
and particles to behave as a liquid. The agitated sand bed retains
the organic particles until they are oxidized; in addition, this
reduces the size of the sludge and ash particles.
Mixing is very important in the fluidized bed process because combustion
must be completed quickly and in a small combustion space. Albertson
reported that: "Intense and violent mixing of the solids and gases
results in uniform conditions of temperature, composition, and
particle size distribution throughout the bed. Heat transfer between
the gases and the solids is extremely rapid because of the large
surface area available" (330). Heat required for combustion basically
comes from the combustion zone where sludges are burned. Auxiliary
fuel (oil or gas) is required when burning secondary sludges but, after
start-up, dewatered raw primary sludge can be burned without this
supplementing fuel.
The fluidized bed reactor is the main unit in the disposal system.
A typical fluidized bed disposal system incorporates the following
process steps: (1) solids preparation, (2) solids dewatering,
(3) solids combustion, and («O stack gas treatment (330,331).
Solids preparation starts with degritting. The two basic reasons
are to prevent wear on equipment in subsequent steps and to produce
a sludge with as high a volatile content as is practical. Sludge
solids grinding or comminution usually follows the degritting.
This procedure reduces the particle size to about 10 mm (3/8 inch),
a size that can be conveniently handled by pumps, centrifuges, and^
the reactor feed system. Next is sludge thickening. This thickening
-------
-260-
is beneficial because it equalizes the sludge flow and increases the
solids concentration. Degritting and thickening are particularly
important in the second step ~ dewatering. Sludge is dewatered
before combustion by either a centrifuge or vacuum filter. (Both
of these unit processes are discussed in other chapters). Dewatering
reduces the amount of water fed to the reactor and thereby signifi-
cantly improves the economics of solids combustion.
Thermal oxidation of the dewatered solids (solids combustion) is the
third step. The solids are extruded into the reactor operating at
a pressure of about 2 psi and a temperature of 1,<*00-1,5000F. They
are retained in the bed until the rapid combustion reduces them to
an inert ash. Sludge is not fed to the reactor until the fluidized
sand bed is heated to a temperature of 1,250°F or higher. At this
temperature, the sludge quickly dries and burns, thereby maintaining
the bed temperature. The ash is removed from the fluidized bed by
the upward flowing combustion gases.
Gases released from thermal oxidation of the solids are scrubbed and
cooled in wet gas scrubbing equipment using general treatment plant
effluent as the scrubbing medium. The method of disposal of inert
solids contained in the scrubber water depends on local conditions.
Ash solids can be separated from the liquid in a hydrocyclone, if
necessary, and the liquid can be returned to the raw waste stream
or recycled to the scrubber.
An instrument control system is used to control the combustion process
in the reactor. In addition to standard combustion controls, the
fluixized bed system uses an oxygen analyzer on the exit combustion
gases. The oxygen content is continuously measured and used to
automatically adjust the air rate and sludge pumping rate to the
reactor. A slight excess oxygen concentration is maintained. The
instrumentation system also includes an automatic shutdown safety
feature in case any component fails.
Performance - The first developmental work on sewage sludge combustion
was described by Albertson t330). A pilot plant at New Rochelle,
New York, successfully oxidized raw primary sludge in a system similar
to the standard design described above. Pilot plant tests showed that
10 to 15 percent excess air was adequate for complete combustion of
the carbon and hydrogen, and smoke and odor nuisances could be
eliminated at temperatures above 1,110-1,150°F.
-------
-261-
The first commercial fluidized bed system was installed at Lynnwood,
Washington, for the combustion of raw primary sewage sludge *331)^
After gravity thickening and centrifuge dewatering, the sludge was
fed to the reactor at a solids concentration of about 35 percent.
The fluidized bed reactor has been operated with 20 percent excess
air or about 360 scfm at a sludge feed rate of about 210 pounds per
hour. Number 2 fuel oil was used for daily reheating and as
auxiliary fuel. A reactor bed temperature of about 1.3000F and a
stack gas oxygen level of about 4 percent were typical operating
parameters. The auxiliary fuel system was controlled by the bed
temperature; fuel could be added automatically if the temperature
fell below a predetermined level.
Because the reactor at Lynnwood has not been operated continuously,
reheating with auxiliary fuel has been required. The fuel require-
ment for reheating and reheat time is a function of the shutdown
period.
Operating power for the complete disposal system at Lynwood has been
estimated to be 237 KWH per ton' of dry solids. A mass spectrographic
analysis of the reactor ash showed 66 percent Si02i 15 percent Al20a;
7.5 percent CaO; and 9.25 percent ferric, magnesium, and sodium oxides
(330; an
-------
14 OO
Uj
/ 3OO. -
12OO-
/ /OO -
g
IOOO-
9OO
O
12
Figure 19.11
REHEAT TIME-MINS.
18 24 30
SHUTDOWN-HRS.
Cooling and heating times of e. small reactor for various reactor temperatures.
(Reprinted by permission Dorr-Olive, Inc.)
10
ro
42 48
-------
-263-
Economics - Capital and operating costs for the fluidized-bed
combustion system have been reported for most of the installations
in service. At Lynnwood, Washington, capital and operating costs
were compared with single stage digestion followed by land disposal
of the liquid sludge. Albertson listed the following cost
data (3307;
Combustion System($/Ton) Digestion System($/Ton)
8,000 Pop. 22,000 Pop. 8,000 Pop. 22,000 Pop.
Capital Costs $15.00$15.00$7.50$7.50
(25 year amorti-
zation at 1%
interest)
Operating Costs $20.41 $11.38 $38.18 $26.80
Total $35.11 $26.38 $15.98 $31.30
Labor costs for both combustion and digestion are similar and represent
a major portion of the total operating cost figure. Hauling the
digested liquid sludge to land disposal areas accounts for the signi-
ficant difference between the two cost figures. Albertson used a unit
cost figure of 0.9 cent per gallon of liquid sludge. In the combustion
system, power and fuel accounted for 21.7 percent of the operating cost
at the 8,000-population level and 38.6 percent at the 22,000 level.
Power was computed at 1 cent per KWH and fuel at 12 cents per gallon.
At the East Cliff Sanitary District plant, California, an operating
cost of approximately $25.32 per ton has been reported (317). This
figure includes: (1) $2.50 per ton for fuel, (2) $1.17 per ton for
power, and (3) $18.35 per ton for labor.
The economy of the fluidized bed system is a function of the percentage
of excess air; therefore, automatic controls are used to keep the excess
air at about 20 percent which minimizes the loss of input heat and
reduces the fuel demand. Figure 19.Ill shows the impact of excess air
on the cost of fuel in sludge incineration ^62'. An air preheater
is an optional piece of equipment which, according to Walter and
Millward can reduce the auxiliary fuel cost C*62). He used an example
where air preheated from 70°F to 1,000°F allowed a reduction in fuel
costs from $9 per ton to $3.50 per ton. The air preheater could
represent 15 percent of a fluidized-bed plant's investment, so its
cost must be compared with the economics of using additional auxiliary
fuel.
-------
u_
o
o
o
a 4
fc x 3
Q CO
<&
to I
x 5
31
-26*-
Figure 19.111
% EXCESS AiR Vs AUXILIARY FUEL
SLUDGE@30%75, 70% VOL & 10,000 BTU/LB
VS
EXIT TEMPERATURE @ 1500° F
$3.70 /TON
$0.92/TON
20
40
60
80
100
% EXCESS AIR FOR SLUDGE
EXCESS AIR FOR NATURAL GAS @ 20% (CONSTANT)
The impact of excess air on the cost of fuel in sludge incineration.
(Reprinted by permission Dorr-Oliver, Inc.)
-------
-265-
Reactor-feed solids concentration is the major factor in combustion
economy. The drier the cake from vacuum filtration or centrifugation,
the lower the cost of operation. Figure 19.IV shows the effect of
cake moisture content on the cost of auxiliary fuel (462). While
the fluidized bed reactor can handle a 10 percent solids feed, it is
uneconomical to eliminate the sludge dewatering step due to the required
increase in auxiliary fuel. Through the use of thickening and
degritting processes, the fluidized bed system produces sludges for
combustion having high volatile solids and a minimum of excess water.
This combined with air preheating and close control of excess air
optimizes combustion efficiency. Figure 19.V shows the effect of
cake volatile solids concentration on the cost of auxiliary fuel for
sludge incineration.
There are, of course, increased operating costs if chemicals must be
used in the dewatering step. The cost for fuel, power, and chemicals
will be about $5 per ton for handling raw primary sewage sludge in a
fluidized bed system and $15 to $18 per ton for raw primary and
secondary sludge. Most centrifuges have been operated without chemicals,
but vacuum filtration almost always requires chemical conditioning of
the sludge.
The annual cost of the fluidized bed system appears to be from $25 to
$50 per ton of dry solids.
Summary - The advantages claimed for the fluidized bed combustion
system include: (1) small area requirement when compared with digesters,
lagoons and sand beds; (2) high degree of organic sludge oxidation with
low excess air requirements; (3) low operating expense; ("O no air
pollution nuisance, and (5) automatic control of combustion to give
optimum results (331).
Temperatures from 1,250°F to 1,400°F have eliminated stack gas odor.
The use of automatic devices to control combustion after start-up
maintains optimum combustion conditions with a minimum of attention.
An oxygen analyzer in the stack controls the air rate into the reactor
and the auxiliary fuel feed rate is controlled by a temperature
recorder. Shutdown controls for emergency situations further
decrease the need for operator attention.
-------
-266-
Flgure_ Jig. IV
u.
Z>
o
o
o
co 9.
9 "
co
ss
x i
3 <
** DC
H
% TOTAL SOLIDS Vs AUXILIARY FUEL
EXIT TEMPERATURE @ 1500°F EXCESS AIR 20%
^2.56/TON
I.76/ TON
0.92/TON
25 27.5 30
% TOTAL SOLIDS IN SLUDGE
SLUDGE 75% VOL. 8 10,000 BTU/LB V.S.
The effect of moisture content on the cost of sludge combustion.
(Reprinted by permission Dorr-Oliver, Inc.)
-------
-267-
u.
o
II
o --
> o
D
1.5
IX)
Figure 19.V
% VOLATILE Vs AUXILIARY FUEL
SLUDGE 30% TS
EXIT TEMP <2>I500° F
EXCESS AIR §20%
i.68/ TON
TON
65
70
75
80
85
,% VOLATILE
@ 10,000 BTU/LBV.S.
The effect of Che percent of volatile solids on the cost of
auxiliary fuel for sludge incineration
(Reprinted by permission Dorr-Oliver, Inc.)
-------
-268-
Burning sludge in a fluidized bed or any other system may not
always be the most economical solution to ultimate sludge disposal.
However, incineration is becoming increasingly popular. More accu-
rate fluidized bed cost and performance data are needed and will
become available. Current information indicates that the fluidized
bed system is applicable to cities as small as 10,000 population.
Several years ago, mechanical dewatering and incineration were
considered feasible only for cities with a population of at least
25,000. The major operational problem with the fluidized-bed system
seems to be the centrifugal dewatering step. (Centrifuge operation
and economics are discussed in the section on dewatering.)
In general, the fluidized-bed systems are operating satisfactorily
and they appear to be competitive with other incineration techniques
if deodorization to 1,UOO°F is required for effluent gases. If
deodorization of effluent gases is not required, fluidized bed systems
could be somewhat more expensive than multiple hearth furnaces.
-------
-269-
D. Atomized Spraying
General - A recently developed sludge destruction technique referred
to as "spray evaporation" has received much publicity (53). it has
also been designated as the Atomized Suspension Technique (AST) and
the Thermosonic Reactor System. The process has some resemblance
to spray drying techniques discussed since 1872. In general, the
atomized suspension technique has been designed for high temperature-
low pressure thermal processing of wastewater sludges. Sludges are
reduced to an innocuous ash, and bacteria and odors are destroyed.
Theory and Design - Basically, the process includes the following
steps: (1) thickening the feed sludge to a range of 4.5 to 11 percent
solids, preferably greater than 8 percent; (2) grinding the sludge to
reduce the particle size, generally to less than 25 microns;
(3) spraying the sludge into the top of a reactor to form an "atomized
suspension"; (4) drying and burning the sludge within the reactor;
and (5) collecting and separating the ash from the hot gases. Details
of the process have been described in numerous literature references
(53, 309, 3m, 316). Figure 19.VI shows the basic components of the
system (316).
The unique features of the atomized sludge incineration process start
with a sonic atomizer that produces a mist and fine particle spray at
the top of the reactor. The reactor feed pump supplies the required
atomizing pressure of 20 to 40 psig. Within the top zone of the
reactor the atomized suspension is formed quickly because the walls
of the reactor are very hot. The atomized sludge particles pass down
through the inner shell of the reactor. Heat is supplied by hot gases
and dust passing at high velocities up the annulus of the reactor.
This heat is created by an oil or gas fired burner at the bottom of
the reactor plus the exothermic oxidation of sludge particles also
at the bottom.
As the sludge particles pass downward through the reactor, the
following actions occur: atomization heating to 212°F, evaporation
at 212°F, heating to combustion temperatures, and high tempera-
ture combustion. At a temperature of about 600°F, the dried sludge
solids ignite contributing to gas temperatures that ultimately reach
2,000°F. The destruction of odor components is essentially instan-
taneous and complete at this temperature. Evaporation, combustion,
and gasification is not inhibited in this system because it operates
at low pressure. Heat transfer in the annulus results from convection
to the metallic walls and then by radiation to the atomized sludge
particles. The high velocity of the upward stream in the annulus
serves two functions: first, it increases the heat transfer, and,
second, it suspends the dry fine dust so that it is carried out of
the reactor.
-------
-270-
Flgure 19.VI
^^— ^
/ \ SONIC NOZZLE
I ^-^
\
I
RAW SLUDGE
i SLUDGE
THICKENER
FILTRATE
1 "»>.
THICKENED— |
SLUDGE 1
\7
r
i — i ^**^
A-S-T
REACTOR
\/
1
If
k|:
/T
/V
'-AIR
PREHE/
«•
i^»
— >
XTER
y
O
" ODOR FREE
O GASES
O
\
• -STACK
DUST
SEPARATOR
T
F ~~l f*l T ~~ INERT ASH
IJ ' AUXILIARY
1 / r-. .4. FUEL a AIR FEED
GRINDER7 REACTOR
FEED PUMP
Thennosonlc RMCtor System for treatment and disposal of raw sludge.
(Reprinted by permission of Water and Wastes Engineering)
-------
-271-
Following the reactor combustion step, there is an air preheater-
heat recovery system, a cyclone scrubber to remove dust particles
from the gas, and a simple stack for venting the solids-free
combustion gases.
The system could be thermally self-sufficient after start-up.
Whether supplemental fuel would be needed along with the fuel
provided by the dried organic compounds in the waste sludge depends
on the type and concentration of the sludge and on the amount of
excess air used.
Parameters - Various parameters that are important in the design and
performance of atomized suspension incineration have included:
sludge type, sludge solids concentration, amount of excess air used,
pressure in the reactor, and sludge particle size.
The sludge solids concentration necessary for self-sustaining com-
bustion is a function of the type of sludge and the excess air used.
Sewage sludges would probably not be thermally self-sufficient unless
first dewatered in mechanical equipment. It has been estimated that
a raw sludge having a heating value of 8,780 B.t.u. per pound of dry
solids would have to be thickened to m percent to be thermally self-
sufficient (309). Achieving this concentration may be more expensive
than burning oil or gas as a supplemental fuel. Figure 19.VII
relates the necessary fuel consumption as a function of raw sewage
sludge solids concentration (309).
Particle size distribution has been an important factor in sludge
stoppages in lines and in the atomizing nozzle; it also affects
combustion. The rates of evaporation and heat transfer in the
reactor are directly proportional to the particle volume i316>.
The system has been operated at a pressure equal to less than 30
inches of water to prevent leakage from the equipment and to insure
no inhibition of evaporation and gasification.
Application and Performance - According to the manufacturer, atomized
suspension techniques have been versatile to the point of being
applicable for "Combinations of evaporation, drying, pyrolysis,
oxidation, and chemical reactions" 1314). In addition to the demon-
strated use for processing sewage sludge, it was claimed to be useful
for chemical recovery in the pulp and paper industry and for oxidizing
organic wastes from the food industry.
-------
-272-
Figure 19.VII
FUEL OIL.
IMP. GAL.
PER DAY
ISO
100 .
46 8 10 12
SLUDGE CONCENTRATION TO AST UNIT
Fuel consumption at a function of sludge concentration.
(Reprinted by permission Schools of Engineering, Purdue University)
-------
-273-
Martin and Bryden discussed pilot plant performance of the
atomized suspension technique using raw primary sludge as the
reactor feed (288). A 4.5 to 6 percent sludge was concentrated
in a cyclone evaporator to 11 percent prior to injection into
the reactor at a feed rate of 750 pounds per hour. The pressure
to the atomizing nozzles was 30 psi and the reactor wall temperature
was 1,400-1,450°F. A heat supply of 970,000 B.t.u. per hour, or
1,300B.t«u-per pound of sludge, was required to produce 36 percent
useful work. The flue gases accounted for 46 percent of the heat
supply.
Beaconsfield,Quebec, installed an atomized suspension facility for
the combustion of raw primary sewage sludge. An 8 to 10 percent
thickened sludge was fed to a reactor having a maximum temperature
of 1,400°F. After a I-minute detention time, the system produced
an effluent consisting of odorless gases, condensed water, and an
inorganic ash. About 400 pounds of ash per day have been produced
from 12,000 pounds of sludge (53» 66> 298).
A U.S. chemical company evaluated the spraying of thickened activated
sludge into a conventional furnace at a power generation plant.
Nearly 8 tons per day of a 2.2 to 2.6 percent sludge was burned along
with coal, the usual fuel. Plugging of the sludge feed device was
a major operational problem but this was solved by using a strainer
to eliminate oversized solids (332).
Economics - Being a new process for sludge handling, very little
capital and operating cost data have been available. MacLaren
reported that the Beaconsfield capital cost as $70,000 for the
sludge thickening tank, AST equipment, and associated buildings
The design feed rate was 0.65 ton per day; therefore, the equivalent
capital cost was $100,000 per ton. A capital cost of $45,000 per
ton of capacity for a 50-ton-per-day unit was also reported by
MacLaren. He quoted general capital costs of $15 to $30 per ton
(30 year amortization at 5%) and operating costs based on Beaconsfield
experience of $15 to $25 per ton. Total capital and operating costs
have been, therefore, estimated to be from $30 to $55 per dry ton of
solids. The recovery of additional heat for beneficial uses such as
self-sustained combustion could lower costs, if this recovery were
determined to be feasible.
-------
-274-
Summary - In general, the atomized suspension combustion technique
has been claimed to offer the following advantages: (1) versatility
in sludge handled; (2) continuous and rapid conversion of raw sludge
to innocuous ash, steam, and C02; (3) small space requirement;
(4) closed system operation; (5) little or no nuisance conditions;
and (6) flexibility in accomplishing drying or complete oxidation
of the sludge solids <3°9>.
A discussion of disadvantages may be premature because of the newness
of this system. However, it has been estimated that the cost will
be somewhat higher than conventional incineration processes due to
maintenance and the need for supplemental fuel oil or gas. Thermal
self-sufficiency is possible with primary sludge at solids concen-
trations of 20 to 25 percent, but this material would be difficult
to handle; so, fuel must be used along with the more dilute sludges
(316)^ Capital costs do not appear to be less than for other
incineration techniques. The AST process does, however, have an
advantage over the Zimpro wet oxidation process in that the operating
pressures are much lower. A more valid evaluation will be possible
after additional plant experience has been gained. The possibility
of incinerating a dilute sludge, thereby eliminating costly dewatering
steps, is very attractive. Continued research and development of
atomized spray systems should be encouraged.
-------
-275-
E. Wet Oxidation
General - The terms wet oxidation, wet incineration, and wet
combustion have all been applied to a commercialized process
commonly called the Zimmerman or Zimpro Process. This process
refers to the oxidation of sludge solids in an aqueous medium by
applying heat and pressure. Wet oxidation can accomplish different
degrees of organic matter destruction depending on the temperature
and pressure applied.
This section will include only "complete destruction" of putrescible
solids which is often defined as at least 90 percent reduction. At
this lever, wet oxidation competes with other incineration techniques
such as multiple hearth furnaces, fluidized beds, atomized suspension
processes, flash drying and incineration.
Less than "complete destruction" of organic solids by the wet oxidation
process is discussed in the sludge conditioning section because it
essentially competes with digestion and chemical sludge conditioning
prior to dewatering.
Theory - An A.S.C.E. Committee report gave a good thorough discussion
of wet oxidation theory (311). Basically, it stated that waste sludge
organic solids are "Chemically oxidized in an aqueous phase by
dissolved oxygen in a specially designed reactor at elevated tempera-
ture and pressure." Figure 19.VIII describes a typical flow sheet for
the wet oxidation process.
Basic equipment includes a reactor, air compressor, heat exchanger,
and high pressure sludge pump (66). Other related equipment.
could include sludge concentration-storage tanks, sludge grinders,
solid-liquid separation tanks, power generating equipment, and vacuum
filters for wet ash dewatering.
In general, the process steps are as follows:
1. Thickened sludge is passed through a grinder to reduce
the particle size.
2. The sludge is pressurized to the necessary operating level.
3. The sludge is preheated in heat exchangers by reactor
gases, steam, and water.
t. Air is combined with the preheated sludge and the resultant
mixture is injected at the bottom of the reactor.
5. Oxidation occurs as the sludge-air mixture follows a
baffled path through the reactor.
6. The reactor effluent is cooled while passing through the
heat exchangers.
7. A gas-liquid separation is made followed by an ash-liquid
separation.
8. The separate ash is dewatered in lagoons, on vacuum filters,
or by some other means.
-------
-276-
Figure 19.VIII
FLOW DIAGRAM OF ZIMPRO WET AIR OXIDATION UNIT REACTOR
SLUDGE
FROM GRINDER
EFFLUENT
TO ASH
SETTLING
SLUDGE
STORAGE
TANK
FEED HIGH
PUMP PRESSURE
PUMP
AIR EXPANSION
COMPRESSOR ENGINE
(Reprinted by permission Zimpro Division of Sterling Drug, Inc.)
-------
-277-
In the above process insoluble organic matter is converted to
soluble organic compounds which are then oxidized to form mainly
C02 and water. Organic nitrogen compounds are converted to
ammonia, and sulfur to sulfates (30*0.
Parameters - Four important parameters control the performance of
wet oxidation units: temperature, air supply, pressure, and feed
solids concentration.
The degree and rate of sludge solids oxidation are significantly
influenced by the reactor temperature. A much higher degree of
oxidation and shorter reaction times are possible with increased
temperatures. Hurwitz and co-workers showed this in their data
presented in Figure 19.IX relating C.O.D. reduction with tempera-
ture (100°C to 300°C) for a number of different sludges (30O.
As is the case in conventional incinerators, an external supply of
oxygen (air) is required to attain nearly complete oxidation. The
air requirement for the wet oxidation process is determined by the
heat value of the sludge being oxidized (311). an(j jjy ^he degree
of oxidation accomplished.
Thermal efficiency and process economy are a function of air input,
so it is important that the optimum amount be determined. Because
the input air becomes saturated with steam from contact with
reactor water, it is important to control the air also to prevent
excessive evaporation of the water. Table 19.2 describes the heat
value of different materials and the air utilization in the
oxidation process '^93)t
-------
-278-
Figure 19.IX
100
ONE HOUR OXIDATION AT TEMPERATURE
o RAW PRIMARY
RAW ACTIVATED
o DIGESTED PRIMARY ACTIVATED
100 140 180 220 260 300
COD reduction vs. temperature.
(Reprinted by permission Water and Sewage Works )
-------
-279-
Table 19.2
Material
Acetic Acid
Carbon
Casein
Ethylene
Fuel Oil
Hydrogen
Lactose
Oxalic acid
Pyridine
Semi-Chemical solids
Sewage sludge,primary
Sewage sludge,
activated
Waste sewage sludge
solids
Heat Value
(B.t.u./lb)
6,270
1H,093
10,550
21,460
19,376
61,000
7,100
1,203
11,950
5,812
7,820
6,540
^
7,900
Oxygen Air
Utilization Utilization
(Ib/lb
1.07
2.66
1.75
3.42
3.26
7.937
1.13
0.178
2.53
0.955
1.334
Material )
4.6
11.53
7.55
14.8
14.0
34.34
4.87
0.77
10.9
4.13
5.75
1.191
1.32
5.14
5.70
Heat in Terms
of Air Use
(B.t.u./lb air)
1,365
1,220
1,395
1,450
1,380
1,780
1,455
1,565
1,370
1,410
1,365
1,270
1,385
Oxidation in an aqueous system requires sufficient pressure in the
reactor to condense the water vapor, because temperatures are above
212°F. Operating pressures have varied from 150 to 3,000 psi,
depending on the size of the plant and the degree of oxidation
desired*326>.
Ettelt and Kennedy reported on the significance of the feed solids
concentration, as it applies to wet oxidation at the Chicago Sanitary
District*56'. Figure 19.X shows that the cost can be reduced by
$15.50 per ton if the feed sludge solids are thickened from 3 to 6
percent^56). Future modifications of the heat exchangers and pump
capacity may reduce this cost further.
Thickening the sludge feed is important to keep the oxidation self-
sustaining. After an original estimate of 2,350 to 3,000 B.t.u. per
gallon of feed as being satisfactory, it was determined that 4000 B.t.u.
per gallon was necessary for self-sustained combustion at the Chicago
facility.
A typical design problem for a wet oxidation facility involves the
determination of the required pumping capacity, the proper size of
. reactors and heat exchangers, and the size of compressors to deliver
the optimum quantity of
-------
-280-
Figure 19.X
0, FEED SOLIDS CONCENTRATION
ON CAPACITY AND COSTS OF WET AIR
OXIDATION PROCESS AT CHICAGO
o
o
(T
UJ
0.
w
8
42
38
34
30
26
22
18
FUTURE
\
CAPAC
ITY
PRESEN" CAPACITY
240 ~
o.
H
220 H
_j
CD
200
180
160
140
C
-
o
V.
C
C
G
•:
O
I
O
CO
FEED SOLIDS CONCENTRATION (%)
(Reprinted by permission from Vol. 36, No. 2, p. 250
Feb. 1966, J. Water Pollution Control Federation)
-------
-281-
Application and Performance - Hurwitz, et al, reported on the successful
application of the wet oxidation technique to many types of sewage
sludges vSOU)^ Tney stated tnat all sludges, whether oxidized to a
low or high degree, were suitable for ultimate disposal because they were
sterile, biologically stable, and had good settling and dewatering
characteristics. Teletzke said that wet air oxidation was also
applicable to industrial organic sludges (305), He thought that
this process was particularly applicable to paper and textile-mill
sludges because the oxidation converts nitrogen to ammonia which can
be returned in the effluent liquid to a biologic treatment process
where it serves as an added nutrient.
One of the first technical articles describing wet oxidation of
sludge was written by Moran and co-workers (2=IO. They oxidized
raw primary sludge in laboratory experiments, using oxygen gas at
elevated temperatures and pressures. The feed sludge, containing
4.8 to 6.1+ percent solids, was macerated in a blender and oxidized
in an autoclave. Temperatures from 150°C to 300°C and oxygen
pressures from 300 to 1,000,-psia were investigated. These tempera-
tures and pressures were maintained for 2 hours.
At 150°C and 300 psi, less than 10 percent of the initial carbon
was converted to C02. However, the sludge was suitable for further
processing because the odor and slimy nature of the solids were
eliminated.
At 250°C and 1,000 psi, 75 percent of the initial carbon was
converted to C02« An essentially odorless and colorless liquid
remained, having an insoluble residue containing less than
1 percent of the initial carbon. Moran and his co-workers
speculated that complete conversion to C02 might be possible: at
higher temperatures or pressures, with better contact between
the oxygen and waste liquid, or with the use of a catalyst (294),
Data from pilot-plant operations at the Chicago Sanitary District
indicated that 90 percent of the organic matter in sewage sludge
can be oxidized at 500°F and 1,200 psig. Also, it was concluded
that if the feed solids concentration is high enough, sufficient
thermal energy can be recovered to operate the entire wet
oxidation process (311).
-------
-282-
Full-scale plant data showed that the C.O.D. can be reduced by
75 to 80 percent at temperatures near 525QF and pressures about
1,750 psig (312, 326), xne fee
-------
-283-
Table 19.4
Typical Analysis of Ash from Wet Oxidation Process
Percent ft
Iron (Fe) 4.92
Silicon (Si) 3.78
Potassium (K) 0.76
Manganese (Mn) 0.025
Calcium (Ca) 0.87
Aluminum (Al ) 3.90
Zinc (Zn) 0.04
Copper (Cu) 0.24
Magnesium (Mg) 0.03
Phosphorus (P) 2.62
Boron (B) 0.03
Nickel (Ni) 0.01
Sodium (Na) 0.12
Specific gravity 2.23
*Metals determined spectrometrically.
Walters and Ettelt made a very thorough study of dewatering the wet
oxidation ash by vacuum filtration and centrifugation (327>- Their
search for a technique to dewater the ash was prompted by the
inadequacy of the current ash lagoon disposal system. First, they
determined that sedimentation followed by sand-bed drying was impractical
due to weather restrictions. The next step was to characterize the ash.
For the Chicago facility, it was determined that ash dewatering
properties depend on the degree of sludge oxidation and on the nature
of the material fed to the wet oxidation process. The average ash
particle has a density of 2.1 grams per cubic centimeter and a size range
between 7 and 44 microns. Figure 19.XI describes the ash particle
size distribution.
Because solids concentration before dewatering was usually advisable,
Walters and Ettelt (327)^ conducted batch gravity settling tests and
concluded that a 10 percent wet oxidation slurry could be attained
in 6 hours. The average wet oxidation effluent carried about 1.5
percent solids in suspension.
Laboratory bench and pilot-plant vacuum filtration tests indicated
that the wet oxidation ash could be satisfactorily dewatered on con-
ventional filtration equipment. A belt vacuum filter with continuous
washing of the filter media produced a constant filtration rate of
5.6 pounds per square foot per hour of cake when the feed solids
concentration was 9.4 percent. The dewatered cake contained 40.5
percent solids and the filtrate solids varied from 0.16 to 0,6 percent.
No chemical conditioning of the slurry was required.
-------
-284-
Figure 19.xi ZSMPRG ASH PARTICLE
IE DISTRIBUTION
LJ
M
O
LJ
CO
U
O
CD
<
$e
2 5 10 20
MICRONS DIAMETER
(Reprinted by permission Schools of Engineering, Purdue University)
-------
-285-
The pilot-plant centrifuge tests run by Walters and Ettelt^327^
on wet oxidation ash determined that solids captures of 60 to 70
percent could be obtained, starting with a 10.8 percent solids feed.
The dewatered ash contained 52 to 56 percent solids. Solids captures
greater than 99 percent were possible with the use of reasonable
doses of an anionic polyelectrolyte; however, the cake solids concen-
tration dropped to 30 to 40 percent.
The long term goal at Chicago has been expressed as the desire to use
the effluent in such a way that it reduces the cost of operation. In
this regard, Koenig has suggested the use of wet oxidation ash to
make various ceramic products^4'.
Wet oxidation of raw primary sludge at Rye, New York, has been
described by Harding and Griffin^295'. The Blind Brook treatment
plant at Rye achieved a 90 percent reduction of insoluble organic
matter by operating at a temperature of 237.8°C and a pressure of
750 psi. This is a small plant having a sewage sludge solids
concentration from 5 to 7 percent and a volatile solids concentration
of about 65 percent. The wet oxidation facility has been operated
intermittently on a 7 days on, 7 days off, schedule^72* 295 J^
Auxiliary fuel has been used only when starting the unit.
The oxidized sludge (ash) had an organic content of 18.6 percent
during the first year's operation (1964). After cooling and solid-
liquid separation, the B.O.D. of the supernatant effluent averaged
8,400 ppm. This represented only a small quantity in comparison
with the entire treatment plant effluent B.O.D. Separated ash
disposal has been a problem at times due to odors from the ash drying
beds. Because the ash dewaters readily without chemical conditioning,
a small vacuum filter may be used in the future.
Economics - The cost of wet oxidation as a means of "complete sludge
destruction" depends largely on the degree of oxidation desired, the
plant size, and the nature of the feed solids. Ettelt and Kennedy
reported that the current wet oxidation costs at the Chicago Sanitary
District for 70 to 80 percent C.O.D. reduction were $34 to $38 per
ton of sludge^56*. This cost included capital (interest at %5)
and operating figures, but not lagooning or any alternative ash de-
watering and disposal processes. It seems reasonable then that the
total sludge handling cost at Chicago would be near $40 per ton.
-------
-286-
Teletzke recently related wet oxidation costs to C.O.D. reduction.
He stated that the capital and operating costs for 70 percent C.O.D.
reduction were double those for 25 percent C.O.D. reduction (328).
Teletzke described a typical cost for wet oxidation plus ash dewatering
as follows:
5% sludge feed, 67% volatile, 1,000 Ibs./hour, unknown
degree of oxidation
Capital cost = $290,000 with a vacuum filter
$225,000 with sand beds
Operating cost = $H.07/Ton (This included $1.92/Ton for power
at K7KHW; fuel cost = $0.60/million B.t-u-
dewatered ash hauling and labor costs were
not included. The fuel cost for a 3%
sludge feed was $2/Ton and for a 6% sludge
feed $0.50/Ton.)
The operating costs for the Blind Brook treatment plant at Rye,
New York, have been reported as $26.80 per ton (295). Nearly complete
destruction of raw primary sludge was accomplished. A breakdown
of the total operating cost shows the following unit charges:
Power = $13.60/Ton ($0.023/KWH)
Chemicals = $ 3.60/Ton
Water = $ 3.60/Ton
Labor = $ 6.00/Ton
HcKinley described the Wheeling, West Virginia, wet oxidation capital
and operating costs as '318).
1) Installed capital cost = $284,000 for a 5.6Ton/day facility.
2) Operating cost = $19.97/Ton which was broken down as follows:
Power = $6.11/Ton
Chemicals = $4.13/Ton
Fuel = $1.65/Ton
Maintenance = $1.17/Ton
Labor = $6.91/Ton
An insoluble organic destruction of 90 percent was achieved, starting
with a raw primary sludge feed of 7.35 percent solids.
-------
-287-
Weller and Condon compared the economics of wet oxidation with
other sludge disposal processes for the Kansas City, Missouri,
sewage treatment facility. They concluded that wet oxidation
capital and operating costs would be much higher than for other
systems ^329). in fact, the capital cost was 97 percent greater
than the selected alternative and the operating cost, 54 percent
higher.
For "complete oxidation" systems, the average total annual cost
for wet combustion would be probably about $42 per ton of dry solids.
Summary - The advantages often claimed for the wet air oxidation
process included: (1) flexibility in achieving any degree of
oxidation; (2) flexibility in type of sludge handled; (3) production
of a small volume of oxidized material that settles rapidly, compacts
well, dewaters easily, is susceptible to biologic treatment, and
offers few nuisance problems; and (4) operation in a small closed
system.
Certain disadvantages are associated with the wet oxidation process.
First, odor problems can develop from the off-gases and from
lagooning of the ash containing effluent. Air pollution caused by
the stack gases can be controlled by catalytic burning at high
temperatures, but this is an unknown added expense. Odors from
lagooning or sand drying bed operations might best be solved by
dewatering the ash in a system that includes gravity separation-
thickening followed by dewatering on vacuum filters or in centri-
fuges. Walters and Ettelt estimated the total cost (capital and
operation) of handling the wet oxidation effluent at Chicago in this
manner (327).
Operation Estimated Cost/Ton
Sedimentation-Thickening $0.27
Vacuum filtration $0.30
Centrifugation $1.60
Another suggested disadvantage of wet combustion systems is the
need for high quality supervision and frequent maintenance.
Operating at the high temperatures and pressures required for a
high degree of oxidation necessarily involves relatively sophis-
ticated equipment and controls. One operational disadvantage could
be the need to recycle wet oxidation liquors back through the
wastewater treatment processes. This may represent a considerable
organic load and the fine ash could plug air diffusion plates and
sludge vacuum filter media.
-------
-288-
The major disadvantage of wet oxidation is the cost of construc-
tion and operation. Recent studies have shown that this system
of solids handling and disposal is the most expensive of those
processes often considered in the design of sewage treatment
plants. As mentioned previously, the specific cost depends on
the required degree of oxidation which in turn depends on factors
unique to a local situation such as the size of the plant, the
land available for ultimate disposal, and the cost of power
Many engineers believe that wet combustion has the potential of being
the best method for ultimate sludge disposal. Further research and
development of this technique is certainly warranted.
-------
-289-
F. Burning with Refuse and Miscellaneous Techniques
General - Solids disposal whether refuse or semi-solid wastewater
sludges has been an urban problem that becomes more critical as
population and manufacturing increases. Perhaps combining all
the different solids discarded in an entire urban area, and
incinerating them together might be the best eventual procedure.
Some data are already available demonstrating successful incin-
eration of refuse and sludge. Los Angeles has been investigating
the use of sanitary sewers to transport all waste solids to a
central collection point - the sewage treatment plant. They have
envisioned the grinding of solid refuse before discharge to the
sewers.
For many years, European refuse combustion practice has been to
use the refuse as fuel to generate steam or hot water for power
generation and heating. Incineration of wastewater sludges in
boiler furnaces has been accomplished in the United States. It
represented an appealing approach to combustion because the
liberated heat is put to useful work.
Combined^ Refuse-Sewage Sludge Incineration - Many cities incinerate
dewatered sewage sludge and refuse in separate units. On the surface,
combining the two incinerator operations into one would appear to be
economical. Three factors, however, must be contended with in any
combined burning process: (1) hauling costs, (2) sewage sludge moisture,
and (3) waste production rates which affect uniform blending of the two
diverse materials.
Hauling costs often account for most of the operating budget in
refuse incineration. Because cities frequently install refuse
burners in a central location and sewage treatment plants at one
end of town, hauling costs for a combined operation may be prohibi-
tive. This is one reason why the Los Angeles investigation is so
interesting.
It was generally thought that sewage sludge must be dewatered to at
least 80 percent moisture before combining it with refuse for
incineration. Of utmost importance is the maintenance of a uniform
low moisture in the feed to a combined incinerator. Various ratios
of refuse to sludge for moisture control have been reported (2:1 to
20:1), but the specific level at any one location must have been
determined on the basis of sludge type and moisture, heat value of
the various solids, type of furnace, operating procedures, and other
factors.
-------
-290-
Obviously, producing a uniform material for the furnace requires
a coordination of the volume of refuse and sewage sludge fed to
the blending device. Raw sewage solids are produced continually;
refuse availability depends on municipal collection schedules and
on the operating schedule of industries producing refuse for
disposal. Storage facilities for the solids are necessary, there-
fore, to assure a uniform mixture.
The literature contained a number of successful examples of sludge-
refuse incineration. Whitemarsh Township, Pennsylvania, burned
vacuum filtered raw primary and trickling filter sludge with
refuse (322, 355). The 75 percent moisture filter cake was combined
with refuse at a ratio of 24:1 (refuse to sludge) to produce a
mixture with a moisture content only 3 percent greater than the
refuse alone. Sewage plant effluent was used to cool the incinerator
furnace walls and to scrub stack gases. Excess heat from the com-
bustion process could be used to pre-dry the filter cake. Operating
costs were reported to be $3.00 to $6.00 per ton.
Frederick, Maryland, has also successfully incinerated a vacuum
filtered raw sewage (70 to 75% moisture) with refuse (28» 355).
Effective and economical incineration was possible with a mixture
of 2.9 parts refuse to 1 part wet sludge. This ratio must be
strictly maintained for adequate operation. Little auxiliary fuel
was required due to the large amount of heat released by refuse
combustion.
Haterbury, Connecticut, has burned a dewatered sludge with a high
grease and fiber content in a refuse incinerator (291). Hot gases
from the incinerator were used to dry the vacuum filtered sludge;
cool gases were returned for deodorization in the furnace combustion
chamber. Operating costs for filtration and incineration were
reported as $14.34 per ton (chemicals not included) and capital costs
as $8.50 per ton. The total annual cost was, therefore, $22.84
per ton.
Two cities in Wisconsin have incinerated wastewater sludges with
refuse, Neenah-Menasha and Kewaskum (306)^ Neenah-Menasha sludge,
mostly papermill waste, was vacuum filtered to 70 percent moisture
and heat dried to about 17 percent moisture before incineration
in a traveling—grate-type furnace. The furnace feed consisted of
20% garbage (80% moisture), 57% rubbish (10% moisture), and 23%
sewage sludge (17% moisture). Operating temperatures were 1,900°F
to 2,000°F. Reported advantages were a reduction in hauling costs
and the cost of auxiliary fuel that would have been required, if
sewage sludge were incinerated alone.
-------
-291-
Kewaskum activated and primary sludge also had a high industrial
component (milk and malt wastes). It was vacuum filtered and
burned with municipal refuse at a temperature of 1,400°F to 1,600°F.
No nuisance problems or need for auxiliary fuel have developed.
The costs of vacuum filtration and incineration were estimated to
be equal to the cost of digestion.
Figure 19.XII illustrates a typical combined sewage sludge-refuse
combustion system.
The successful operations described in the preceding paragraphs
indicate more consideration should be given to combined refuse and
raw sludge incineration systems. It may be particularly useful in
small cities where hauling costs could be reasonable. Centrally
located refuse collection and sewage treatment could make this system
also conducive to larger cities. Or perhaps, some potentially inex-
pensive refuse collection technique, as is being investigated in
Los Angeles, could make combined incineration desirable.
Improved mechanical design would undoubtedly encourage combined
incineration. This could include systems to lower the moisture
content of sludge feed as well as to break the sludge into small
particles and distribute it uniformly throughout the refuse,
Certainly using the waste heat from refuse combustion has much appeal;
using it to burn or dry sludge deserves more attention.
Burning Sludge as a Useful Fuel - Incinerating sludge in conjunction
with a waste heat boiler to use the excess heat in generating steam
for heat and power has seemed to be a reasonable approach. At the
Chicago Sanitary District, facilities were originally installed that
would allow the burning of dried sewage sludge for steam production
but this sludge was too valuable as a fertilizer to justify its
burning. At Hershey, Pennsylvania, somewhat the opposite was true,
because dried sludge appeared to be more valuable as a fuel than as
a raw material for grease recovery. The Carver-Greenfield dehydra-
tion process was installed at Hershey. It comprised the following
steps: (1) sludge disintegration, (2) fluidizing and water separation
by the addition of oil, (3) triple stage evaporation, CO centri-
fugation, (5) screw pressing, and (6) incineration. The dried sludge
(ll,OOOB.t.u./lb)was fed to the boiler furnace at a moisture content
of 2.5 percent. A portion of the steam produced was used in the
evaporation stages. Supplemental fuel oil was also burned in the
furnace.
-------
Figure 19.XII
(Reprinted by permission
Bartlett-Snow-Pacific, Inc.)
N)
ID
VACUUM
FlL TEft
SLtJ&GS
HOLYOKE.MASS.
CAtfE: tOOO tss./Hff. oft? SOL/DS
re CD rtotsrcsfte: £5 ro 7S %
f»fte>ZHJCr HOtSr»Jf?£ .'/O TO 2O%
GAS Tf/*fF>£/?ATVf?£! //OO
AUXILIARY risen.: //? 4 ruei o/L
SLUDGE
BAft Ti eT T -
S9G4- ro
BOfSZ) - COfJSUt. T/fJG
-------
-293-
A large chemical company has successfully incinerated a thickened
waste activated sludge (2.2 to 2.6%) in a boiler furnace along
with conventional fuels (332). However, inorganic deposits,
accumulated on the boiler tubes, and eventually forced suspension
of this disposal technique. It has been suggested that sludge
cake could be used as fuel for a gas turbine (66). Sewage sludge
may be considered a low grade fuel, but new combustion units tailored
to burn this material and recover the waste heat should be investigated.
-------
-294-
G. Summary
As the volume of sludge inexorably increases and land areas
become less available and more expensive, incineration becomes
a logical and economical process that is being considered and
installed by more and more municipalities and industry. While
this chapter and others has emphasized sewage sludge, a wide
variety of industrial sludges are being incinerated: e.g. chemical
sludges, petroleum tars, paunch manure, papermill sludges, etc.
The advantages of incineration include: (1) nearly complete
combustion of organics, (2) large reduction of sludge volumes,
(3) production of an inert ash that is usually easy to dispose of,
and («O the destruction of microorganisms and potential nuisance-
causing materials.
These advantages are achieved in a wide variety of incineration
equipment including: (1) multiple hearth furnaces, (2) flash-
drying incineration units, (3) rotary kiln incinerators, (4) flui-
dized sand bed incinerators, (5) atomized spray units, (6) conven-
tional boiler furnaces, and (7) wet combustion units. Multiple
hearth furnaces have been the standard units for sewage sludge
combustion but fluidized bed units are becoming increasingly
popular particularly at small treatment plants. Rotary kiln
incinerators are successfully used by industry for burning a wide
variety of sludges. Atomized spray units have the appeal of not
requiring prior mechanical dewatering of the sludge, but performance
and economic data are not very plentiful due to the newness of the
system. The wet oxidation process has a number of disadvantages
that include cost, complexity, less than complete destruction of
organics, and air pollution problems.
One factor in the selection of combustion systems is the solids
content of the thickened sludge. Rotary kilns can operate over
a wide range of 7 to 70 percent solids. Lower solids levels,
such as 2 to 10 percent require fluidized bed, atomized spray,
or wet combustion techniques. Sludges with high solids concen-
trations, 25 to 70 percent, can be burned in simple stationary
incinerators '^0),
The broad question of whether to adopt incineration or some other
sludge disposal technique involves a number of factors including
the size of the city or industry, climate as it affects land
disposal, land area available and its proximity to residences,
proximity to the ocean where sludge dilution may be feasible, and
the potential fertilizer market. Incineration is considered to
be applicable to cities with population equivalents of at least
10,000. Many engineers believe raw sludge incineration is more
economical than digestion followed by land disposal techniques for
cities exceeding a population of 20,000.
-------
-295-
One fascinating new idea is submerged combustion where sludges are
incinerated as they are collected in sludge hoppers of sedimenta-
tion basins. Solids handling and disposal would certainly be
simplified by eliminating the costly handling steps between clari-
fication and ultimate disposal.
Elimination of at least the mechanical dewatering step similar to the
Ashland and Piqua operation would be a forward step for multiple
hearth and fluidized bed incineration. To accomplish this, the
liquid sludge would have to be consistently thickened to a level
greater than the 15 percent accomplished at Piqua. Otherwise,
direct incineration of the liquid sludge would be uneconomical.
But, with or without design improvements, incineration is the one
sludge disposal process that, without a doubt, has a bright future
because it meets future sludge disposal criteria.
-------
-296-
20. PYROLYSIS
Heat treatment of waste solids by pyrolysis is being investigated
as a substitute for incineration. Pyrolysis differs from incin-
eration in that it involves heating without oxygen. The basic
purpose is to decompose complex organics to simpler materials.
In the case of refuse solids, this may be accomplished at 1,200°F
to produce compounds of commercial value such as combustible gas
for boiler furnace fuel, elemental carbon, tars, resins, and various
acids.
Pyrolysis, like incineration, reduces the sludge volumes and steri-
lizes the end product. Unlike incineration, it offers the potential
advantages of: (1) eliminating air pollution, and (2) the production
of useful by-products. Air pollution can be controlled because
heating takes place in a closed system that allows the collection of
gases for beneficial uses or flaring.
Research on pyrolysis techniques for refuse solids has been underway.
Its application to wastewater sludge could be evaluated when the
refuse data are available. A major reservation about the pyrolysis
approach would appear to be the assumption that by-products could be
sold to reduce operating costs.
-------
-297-
21. HEAT DRYING
General - Any decision to heat-dry sludge includes the assumption
that a market exists for the sale of the dried product as a ferti-
lizer or soil conditioner. As discussed in the chapter on Fertilizer,
a market often does not exist and, as a result, heat drying has not
been given serious consideration by many consulting engineers.
If a thermal disposal process were adopted, due to site limitations
or the cost of alternate sludge handling techniques, Quirk recommended
resolution of the following questions '*>5e):
1. What is the difference in cost between drying and
incineration systems or combinations of the two?
2. What is a realistic market price for the dried sludge?
3. What market conditions are required to justify heat
drying over cheaper alternatives? What is the local
interest in heat drying and what public relations
value may come from selling dried sludge?
*». What portion of the annual sludge production could
be marketed? What is the minimum price that could
be expected in a declining market?
5. What additional costs are necessary for deodorization?
A detailed analysis of these questions and their answers generally
preclude heat drying processes. Many, if not most, of the heat
drying processes installed at sewage treatment plants have been
abandoned in favor of incineration or other sludge disposal methods.
MacLaren stated that the installation of a complete heat-drying plant
without facilities for burning could no longer be recommended 153).
He knew of no example in Ontario where a fertilizer manufacturer
had offered to purchase the entire sludge production of a sewage
treatment plant. Price quotations may be made, but unless specified
in a contract, there is no guarantee that all heat dried sludge
will be removed from the plant site.
Equipment - Units to dry wastewater sludges to less than 10 percent
moisture include the following types: (1) flash dryers, (2) multiple
hearth dryers, (3) rotary dryers, and CO atomizing spray dryers.
Flash dryers have been the most common type in use at sewage treat-
ment plants; industry has used various designs but frequently the
choice was a rotary type. Most systems can be made flexible enough
to dry or incinerate. Heat drying processes usually have been
preceded by a mechanical sludge dewatering step.
-------
-298-
Flash drying instantaneously removes moisture from solids by contact
with a hot gas stream. The process has included blending wet filter
cake with previously dried sludge to lower the moisture level, violent
mixing of the semi-dry sludge with hot gases (1,300°F) in a cage mill,
and separation of dried sludge from the gases in a cyclone. A portion
of the dried material is returned automatically to the mixer for use
in conditioning incoming wet sludge. Combustion of fuel in the form
of gas, oil, coal, refuse or sewage sludge itself provides the heat
for the process. Vapors from the flash drying cycle are returned
through preheaters to the furnace where they are heated to 1,200°F
to destroy odors. Effluent gases pass through a centrifugal-type
dust collector or a wet scrubber, depending on local air pollution
control regulations.
The same multiple hearth furnace, conventionally used for sludge
incineration, has been adapted for heat drying. Modifications to
the basic furnace design included fuel burners at the top and
bottom hearths plus down-drafting of the gases. The wet feed sludge
was mixed in a pug mill with previously dried sludge. As the solids
moved downward through the furnace, the gases became cooler and the
solids became drier. At the point of exit from the furnace the gas
temperature was about 325°F and the solids temperature about 100°F.
Rotary dryers have been basically long steel cylinders into which
hot gases are introduced for drying. As the cylinder revolves at
H to 8 rpm the hot gas enters at one end along with the dewatered
sludge cake. Baffles or flights break-up the sludge as it passes
from the inlet to the outlet. Wet sludge is also mixed with previously
dried material in a pug mill, similar to other drying units. The
complete system includes cyclone dust collection and a gas deodorizing
incinerator.
Atomized drying by spraying liquid sludge into a vertical tower
through which hot gases are passed downward was a process first
patented in 1872. The water is evaporated from the atomized
particles and passes off with the hot gases as the dried sludge
drops to the bottom of the tower. Dust carried with the hot gases
is removed by a dust collector or separator. Atomized drying is a
procedure used by many process industries for drying many commercial
products.
Performance and Economics - Schenectady, New York, represents one
of the very few locations where heat drying of sewage sludge is
considered satisfactory. They have sold enough dried sludge as
fertilizer to offset a significant portion of their operating
costs (315, 379). Digested primary sludge is vacuum filtered to
26.5 percent solids and flash-dried; operating costs are:
-------
-299-
Once the decision to incinerate is made, a number of other questions
must be considered for each particular situation. Among these are:
(1) to digest or not, (2) to mechanically dewater or not, (3) to
combine with refuse incineration, and (4) to attempt to recover heat.
It seems reasonable not to digest sludge because the raw material has
a higher heat value, it dewaters easier than digested sludge, and
auxiliary fuel is usually not required. The argument that digestion
reduces sludge volumes and produces valuable digester gas is of
minor importance compared to other factors.
With multiple hearth and fluidized bed equipment, mechanical dewatering
in vacuum filters or centrifuges become an accepted pretreatment step.
Mechanical dewatering is expensive, but the use of the large quanti-
ties of auxiliary fuel necessary to burn liquid sludge is also expen-
sive and, therefore, not generally given serious consideration.
Incinerating combined refuse and wastewater sludge has been accomplished
successfully and should be considered in economic studies of incinerator
systems. Using the excess heat available from refuse incineration to
dry and burn sludge solids is a desirable approach to economical solids
disposal. Recent advances in equipment design and technology make the
combination process more suitable than it was previously. In the
future, designs will be simpler and the system more easily started
and stopped. These improvements will encourage small cities to adopt
incineration processes.
An example of possible design improvements was reported by Sercu (308).
He described a modified design of a rotary kiln incinerator that was
able to burn both solid and liquid wastes having high heat values.
The incinerator could handle 81 million B.t.u. per hour of liquid tars,
plus 60 million B.t.u. per hour of solid wastes.
Recovering excess heat generated from incineration processes is
usually considered uneconomical. However, some units have been
installed and operated with the flexibility of generating steam for
heating buildings, drying sludge, and producing electric power.
Improved heat recovery designs are apparently required to justify
the added expense of this equipment.
Substantial improvements in incineration processes have been made,
but this area still needs further study and development to reduce
costs and to increase thermal efficiency, dependability, and freedom
from nuisance (31°). Very often the requirement to lower costs
eliminates the possibility of accomplishing the other three objectives.
-------
-300-
Heat Drying (315)($/Ton
Labor
Fuel oil O.VSC/gal.)
Power (1.88t/kwh)
Bags supplies and services
Vacuum Filtration
Not reported, assume to be:
$15.00
Total operating costs for filtration and
drying is, therefore -
$32.04
$15.00
$47.04
The City of Schenectady received $27.60 per ton for the sludge
fertilizer, so, the net cost of operation would appear to be $19.44
per ton. However, the operating cost of sludge digestion was not
included, and this could increase the net operating cost to about
$23.00 per ton. Adding on the capital cost for expensive flash-
drying equipment with facilities for deodorization increases the
total net annual cost figure to a level that is less attractive
than the literature indicated.
Quirk made a detailed comparison of the cost of heat drying
versus incineration (65e). por a model, he used a medium sized
installation (2,530 tons/year) and drying equipment that had the
flexibility of functioning as an incinerator. A summary of his cost
data showed:
Operating cost
Capital cost
Cost ($/Ton
dry solids)
Vacuum
filtration
annual cost
Total sludge
handling cost
($/Ton)
Incineration or Drying Equipment
Incineration
Equipment
Incinerated Sludge
w/
Deodor.
9.82
12.12
$21.94
W/0
Deodor .
6.60
11.59
$18.19
Dry
w/
Deodor .
24.90
12.52
$37.42
Sludge
W/0
Deodor .
17.50
11.59
$29.09
Incinerated Sludge
W/
Deodor.
9.50
9.47
$18.97
W/0
Deodor .
6.36
9.15
$15.51
10.51
10.51
10.51
10.51 10.51
10.51
$32.45 $28.70 $47.93 $39.60 $29.48 $26.02
-------
-301-
A comparison of these costs revealed some interesting facts:
1. High temperature deodorization increased costs of
either drying or incineration by at least 20 to 30
percent. Deodorization is commonly required at new
installations.
2. Incinerating sludge in a system designed for either
drying or incineration would require a 17 percent
increase in basic unit costs over a system designed
for incineration alone.
3. The cost of heat drying sludge is much higher than
the cost of incinerating sludge.
Obviously, a market for the dried sludge must exist at a certain volume
and price to justify heat drying.
MacLaren estimated that the cost of heat drying over incineration
was at least $8 per ton. due mainly to the fuel required to
evaporate the moisture (53). He reported that the cost of equip-
ment varied widely depending on the size of the installation.
Generally, though, assuming the capacity is provided in two units
or more, he believed that the range was approximately $5 to $10
per ton of dry solids based on a 30-year amortization and 5 percent
interest. Operating costs were estimated to be $9 to $15 per ton
of solids.
At Baltimore, Maryland, the distribution of operating costs was
estimated to be as follows: (1) labor - 2U%, (2) sludge combustion
fuel - 7.9%, (3) fuel for deodorization - 33.4%, CO materials -
15.3%, (5) power - 10.3%, and (6) administrative and lab - 9.1%
If a price of $8 per ton on a guaranteed basis for the total
production of dried product can be obtained, MacLaren believed that
heat drying could be cheaper than incineration. He pointed out,
however, that prices obtained in actual operation were lower than
those expected during the plant design phase. An example was used
where prices of$16 to $18 per ton were quoted for heat dried^sludge;
yet, a price of only $U per ton was obtained when the production
became available. Zack believed that heat drying should only be
considered if a price of $12 or more per ton can be obtained for
the dry sludge (28).
-------
-302-
Milwaukee, Chicago, and Houston have been selling heat dried
activated sludge generally for $12 to $18 per ton. Their opera-
ting costs are unknown but at Coral Gables, Florida, the total
cost of heat drying was $50 per ton of dry solids (**0). Using
this figure for the three large activated sludge plants, results
in a net capital and operating cost of $32 to $38 per ton.
Fifty dollars per ton was estimated to be the average total cost
for heat drying in the United States; the range was from $UO to
$55 per ton.
The economy of heat drying waste sludges might be improved by using
waste heat from refuse incineration. Stamford, Connecticut, used
hot stack gases from a refuse incinerator to pre-dry sewage sludge
filter cake to a moisture content of 6 to 10 percent. The dried
cake was sold as a fertilizer or burned in the refuse incinerators
(28). At Louisville, Kentucky, vacuum filtered digested sewage
sludge was hauled 6 miles to a city incinerator where it was dried
or burned. Waste heat from the burning refuse dried the sludge in
flash dryers for fertilizer or further burning in the refuse incin-
erators ^66). other locations have burned digester gas to produce
heat cheaply and lower operating costs. Burning dried sludge in
boiler furnaces to produce power and/or heat for sludge dehydration
has been another possible way to economize on the drying process.
Summary - Waste sludges have been heat dried in an attempt to improve
the economy of sludge incineration. This assumes that the sludge
can be sold as a fertilizer or soil conditioner at a good price,
an assumption that has been proven false in most cases. The sale
of sludge is necessary because heat drying represents one of the
most costly sludge handling techniques.
Selling dried sludge as a fertilizer is basically appealing because
it represents conservation of a natural resource by returning
organics to the land. In addition, heat drying offers the advan-
tages of volume reduction, odor reduction, and the destruction
of pathogenic microorganisms. These advantages make land disposal
of raw sludge feasible (^'. Heat drying is also attractive
because it represents a way for ultimate sludge disposal; the
sludge is permanently removed when sold as fertilizer.
Many sludge drying operations have been suspended, mainly because
of cost. The costs are high basically because it takes about
8,OOOB.t-uper pound of product to produce a material with 10 percent
moisture from a sludge having 80 percent moisture even at 50 percent
thermal efficiency (223). This high production cost coupled with a
generally unreliable market results in an uneconomical sludge
disposal technique.
-------
-303-
Other disadvantages involve control, maintenance and air pollution
problems. Air pollution is a serious by-product of heat drying
unless expensive deodorizing equipment has been installed.
Raising stack gas temperatures to 1,200°F to 1,400°F is costly,
a heat exchanger almost becomes an economic necessity. At
Baltimore, three operational problems plagued the heat drying
process: (1) fires and explosions occurred in the drying system
from grease accumulations, (2) storage of dried sludge was compli-
cated because the material compacted and absorbed moisture, and
(3) a product of predominantly very fine particles was produced (202)
The fine, dusty solids were unacceptable to fertilizer companies.
A pelletizing or granulating step seemed necessary. For the year
1980, this step was estimated to add $2.90 per ton to the cost of
heat drying (*M»9).
Heat drying of sewage sludge has been rarely seriously considered
by consulting engineers. Unlike incineration where auto-combustion
is possible, heat drying has usually required the burning of
significant quantities of oil, gas, or coal. If drying costs could
be reduced and the market value of the dried product increased, heat
drying could be economically feasible.
A broad scientific research effort to prove the value of dried
sludge as a soil conditioner or fertilizer is needed.
This could, of course, increase the market value of the product.
Also, lower cost methods of heat drying should be developed. In
this regard, combining waste sludge drying with refuse combustion
offers the potential of improved economics. The use of food
industry sludges as an animal feed or soil conditioner would be
enhanced by lower drying costs. Drying without the need for prior
mechanical dewatering would be desirable if the overall economics
were acceptable. (See the Fertilizer and By-Product chapters for
further information.)
-------
-30U-
22. SLUDGE ODOR CONTROL AND DISINFECTION
A. Odor Control
General - Air pollution control has become a problem of increasing
importance at wastewater treatment plants as urban areas have
expanded and virtually surrounded plants that were formerly in
isolated areas. Public awareness of air pollution has led to
increased concern about treatment plant odors. Paradoxically, the
solving of a water pollution problem may result in the creation
of an air pollution problem. This can, for example, happen when
waste sludges are incinerated ^339).
At a typical sewage treatment plant odors may emanate from many
sources including the following: (1) accumulation of grit, screenings,
and skimmings; (2) operation of "sick" digesters; (3) sand bed drying
or lagooning of incompletely digested sludge; (1) raw sludge thickening
or storage in tanks; (5) vacuum filtration of raw sludge; and
(6) incineration or heat drying of sludge. Industrial organic sludges
may develop odors from many of the same sources.
Corrective Measures - Eliassen and Vath commented that there are two
approaches to eliminating or reducing odor problems (339). Odors
can be eliminated at the source, or the odors can be prevented from
reaching the atmosphere. The approach taken will depend on the nature
of the problem, the economics, and the treatment plant design.
The basic requirements for preventing odor are intelligent plant
design and good plant operation. Intelligent design can include:
totally enclosed units; prevention of sludge septicity by providing
adequate sludge hopper designs and flexibility in pumping schedules;
digesters that cannot be easily upset; and piping flexibility to
allow the addition of well-aerated plant effluent to sedimentation
and thickening basins. Location of plants in isolated areas or
disguising them by plantings are aids due to the "out-of-sight, out-
of-mind" psychology.
Good housekeeping and treatment procedures prevent many odors.
Examples of such procedures are 1338).
1. Prompt burying or burning of screenings, etc.
2. Regular removal of sludge and skimmings from
sedimentation tanks.
3. Discharge of only well digested sludge to sand beds,
lagoons* etc.
1. Scientific control of digestion and other unit
process operations.
-------
-305-
Once odors are emitted, control is generally accomplished by
one of five methods: (1) combustion, (2) chemical oxidation,
(3) adsorption, (H) dilution, and (5) masking. Masking of
odorous gases including hydrogen sulfide, methyl mercaptans, and
methyl amines is not a very satisfactory method of control.
First, it is not acceptable to many people because they object to
the masking agent fragrance. Also, odor masking is often not
complete, and the effect is an intolerable combination of sludge
odors plus masking agent odors. Effective spraying of masking
agents outside on windy days is very difficult at small sludge
thickening tanks, and even worse at large lagoon areas. Masking
has some uses but at waste treatment plants, it should be limited
to temporary emergency situations that improve public relations on
hot humid days.
All of the other methods of odor control require the basic first
step of odor confinement and collection. Effective confinement
requires the covering of grit chambers and sludge blending, storage
and thickening tanks. Keeping the interior atmospheric pressure
slightly below that of the exterior helps to prevent gas from
escaping to the exterior atmosphere. Confinement also implies
ventilation in rooms, such as those used for vacuum filtration,
where people are working. Once confined the odorous gases must
then be collected at a central treatment station.
Combustion as a method of removing odors is promising because it
can be complete. But, to produce the principal end products of
carbon dioxide, water, and sulfur dioxide is costly. Incomplete
combustion can be disastrous because intermediate products may be
formed that are more unpleasant than the original odors.
Several combustion techniques are used to destroy odors. It is
sometimes practical to discharge the gases into existing furnaces
so that no extra costs for fuel are necessary. If, however, there
are large volumes of odorous gases requiring complete oxidation, a
separate combustion system is desirable. Two systems have been in
use, representing high and low temperature combustion.
High temperature odor combustion has been practiced at the San Diego
Point Loma sewage treatment plant (349). Gases are collected
through a suction blower and supplied to a fume incinerator at
zero pressure. The incinerator burns digester gas to attain a
combustion temperature of 1,100°F to 1,500°F. A heat exchanger,
incorporated into the system, provides about two-thirds of the
necessary heat requirement. The stack discharge temperature is
about 600°F. Incinerator operation, once started and pre-set at
incineration temperature, is automatic.
-------
-306-
The use of digester gas at San Diego obviously has had its advantages.
High temperature combustion is undoubtedly effective in eliminating
odors, but it can be expensive. Fuel requirements and the power
requirement for induced-draft fans can result in high operating costs.
Low temperature combustion has been possible by using a catalyst,
such as some form of a platinum - metallic screen. Such a catalyst
enables combustion to be carried out at a temperature much lower
than would be possible without it because it accelerates the
combustion reaction. A temperature of 600°F has been commonly used.
Natural gas has been often burned as a fuel if digester gas were not
available. The operating costs can be very high due to maintenance
of the screens and the cost of fuel. However, this process can
successfully control odor.
Jaffe stated that two approaches to chemical oxidation of gases were
possible: (1) oxidizing the gases in a dry environment or (2) scrubbing
the gases with a liquid containing oxidants (317). The first approach
commonly used ozone to oxidize the odor causing agents. Ozonation
has been a fairly common odor control technique because it is rela-
tively inexpensive and usually effective. It has had the disadvan-
tages of requiring close control for safety reasons, and it may
generate odors that are objectionable. Ozone in small concentrations
is toxic. Monitoring equipment for ozone is available to maintain
the slight residual in the exhaust stack necessary for complete odor
destruction.
Chemical oxidants such as chlorine, hydrogen peroxide, and hypo-
chlorite have been used in absorption processes to control odors.
The process usually involves a scrubbing tower where odorous gases
and trickling liquid pass in a countercurrent fashion, allowing a
gradual oxidation of the gases.
At the Midland, Michigan, sewage treatment plant, chlorine was added
directly to raw sewage sludge to reduce odors during vacuum filtra-
tion C*°3). The average dosage was 5 to 10 pounds of chlorine
per ton of dry solids, added just prior to the chemical sludge
conditioning tank. The cost of 30 to 75 cents per ton was more than
offset by the reduced flocculent dosage after sludge chlorination.
Ferric salts and lime partially control raw sludge odors during
filtration; but, because the sludge is exposed to the filter room
atmosphere before conditioning, control is not complete. Lime has
often been used to control odors from dewatered sludge applied to
land.
-------
-307-
At New York City, an experimental scrubber that used a No. 3 diesel
fuel oil along with naphthalene (0.55 Ib. in 10 gals, of oil)
successfully removed odorous gases from sewage sludge storage
rooms (345). One gallon of oil could scrub 200,000 cubic feet
of air. The spent oil could be used in the diesel engines of sludge
barges.
Adsorption of odorous gases on activated carbon and other inorganic
materials has been successfully demonstrated. However, carbon
adsorption by channeling odorous gases through towers packed with
carbon has been expensive. Regeneration of the carbon by heat treat-
ment is required at frequent intervals. Therefore, for this tech-
nique to be accepted for sludge odor control, the cost must be
reduced.
The use of high stacks and fans to dilute odorous gases with the
outside atmosphere has not been a satisfactory odor control tech-
nique. It lessens odors in the immediate plant vicinity, but down-
wind odor problems are bound to develop. High stacks and fans are
recommended equipment when used with combustion or chemical oxida-
tion processes.
More and more, design engineers have been asked to provide facilities
for air pollution control at wastewater treatment plants. These
facilities include the covering and ventilation of many structures.
As a result, the cost of waste treatment increases, but odor control
is necessary. Well-engineered plants allow the plant operator to do
a better job of odor control. Certainly, consulting engineers should
always consider odor control in their designs.
Many techniques and materials are available for odor control. All
in all, high temperature combustion and ozonation have been the
most practical means for waste treatment plants. There has been
a trend towards raw sludge handling and away from anaerobic digestion.
As a result, odor control will become more critical because raw
sludge is not stabilized. Raw sludge has been frequently incinera-
ted; so, the opportunity exists to collect gases from unit processes
such as sludge thickening and combust them in sludge incinerators.
New research to find better and cheaper oxidants is desirable.
Finding a material that could be added directly to sludges in
economical low doses and that could keep the sludge odor-free
would be a substantial development.
-------
-308-
B. Disinfection
General - Sewage sludge can be disinfected by heat treatment, chemical
treatment, and radiation. Digestion and conventional chemical condi-
tioning prior to vacuum filtration will partially disinfect sludge.
Because sewage sludge has been disposed of in a manner that allows
contact with people, disinfection to eliminate the public health
hazard associated with pathogenic organisms has often been required.
The disposal of sludge to the ocean and its use as a fertilizer
or soil conditioner involves potential contact with people.
Data - Heat drying of raw sewage sludge is an effective disinfection
technique. Data from four large cities selling dried sludge as a
fertilizer have shown low bacteria content (33lO. Based on the
relative heat sensitivity of coliform bacteria and enteric pathogens
that may live in sewage sludge, it is reasonable to assume that if
heat kills the coliform bacteria, the enteric pathogens will also
be killed. Because the bacterial, parasitic, and viral enteric
pathogens found in sewage sludge have the same order of heat sensi-
tivity as coliform bacteria, the use of heat dried sludge even on
vegetables could be considered as safe.
A study of the survival of E. coli in digested primary sludge showed
that they survived for 7 weeks at 37°C and for 2 weeks at 22°C.
The coliform organisms apparently disappeared because of competi-
tion from other microorganisms better adapted to the digestion
environment I34**). Disease organisms such as typhoid-dysentery
bacilli, polio virus, anthrax, ova of parasitic worms, and brucella
have been thought to have a rapid mortality rate due to their sensi-
tivity to the unacceptable digestion environment. One study where
raw and digested sludge was exposed to 55°C for 2 hours resulted in
100 percent destruction or inactivation of Ascaris Lumbricoides
ova *31*3'. Keller reported that thermophilic digestion destroyed
all ova of parasitic worms and cysts of amoebae parasitic to man in
24 hours <35l>.
Digested sewage sludge can be disinfected by chlorine if the sludge
is thoroughly digested <342>. An actively digesting sludge requires
a much higher chlorine dose than a well digested sludge. If sludge
and chlorine are thoroughly mixed, the following relationship between
chlorine dose and contact time exists:
-------
-309-
tt
Contact Time Lbs. Active Chlorine/Gal, of Sludge $/Ton
0.5 hr. .0334 11.90
1.0 hr. .0416 14.90
2.0 hrs. .0500 17.85
3.0 hrs. .1333 47.70
4.0 hrs. .1758 62.75
*Assume a 5% sludge and 7-1/2 cents per pound for chlorine.
Connell discussed laboratory studies of raw primary sewage sludge
disinfection using the following chemicals: chlorine, bromine,
iodine, calcium and sodium hypochlorite (348^ ^e founcj that
microorganisms were protected by clumps of solids which limited
complete disinfection. Homogenizing the sludge in a blender reduced
the chemical requirement and increased the sterilizing power. Raw
sludge exerts a high chlorine demand; using a 2 hours' contact time
the following dosages effectively disinfect a raw sludge having 5 per-
cent solids:
Chemical Dose (%)
Chlorine 6 to 7
Sodium hypochlorite 4 to 5
Calcium hypochlorite 6 to 7
The use of ferric salts and lime to condition sewage sludges before
vacuum filtration has often reduced the bacteria population in
sludge by at least 50 percent (227)^ jju1- ^he remaining bacteria
constitute a potential health hazard.
Radiation has been studied as a means of sterilizing sewage sludge.
A threshold absorbed dose of 10** rads is necessary before large
numbers of the bacteria are inactivated. Apparent sterilization
(100 percent kill) was achieved at a radiation dose of 5 megarads
per pound (350). The fact that sludge can be sterilized by radiation
may be interesting, but the very high costs involved make it
impractical.
Summary - Because large quantities of sewage sludge are ultimately
used as a fertilizer or soil conditioner, the question of sludge
disinfection becomes of vital interest. The opinion has been
expressed by numerous people that there is no record of disease
transmission to humans as a result of using sludge as a fertilizer.
This good record may be a reflection of various health department
regulations. It seems logical that pathogenic organisms would not
-------
-310-
survive through the various sludge handling processes in common
use because the environment is so foreign. Heat drying and digestion
followed by dewatering and storage seem to produce a material that
can be safely used on land for beneficial purposes.
There does not appear to be an urgent need to improve disinfection
processes except when such improvement may affect odor control.
New techniques for odor control may involve sludge stabilization and,
therefore, sludge disinfection.
-------
-311-
23. WATER PLANT SLUDGE DISPOSAL
General - The disposal of sludges resulting from the clarification
and softening of raw water at water treatment plants has not been
as much of a problem as wastewater sludge disposal, but it is
becoming more critical every day. Two primary reasons have been
responsible for the increased concern about water plant sludge
disposal: (1) urban areas are growing and less and less inexpensive
land is therefore available for land disposal and (2) increased
interest in water pollution is restricting the dumping of water
plant sludges into surface waters. At many plants, sludge disposal
has become a major operational problem. As is the case in waste
treatment, the least costly disposal method is the one normally
used.
Methods of Disposal and Sludge Characteristics - Black conducted
an extensive survey of water plant sludge disposal practices, with
the following results C*60).
Method of Disposal Percentage of Water Plants Using
To flowing streams 58.4%
To drying beds or lagoons 29.6%
To storm or sanitary sewers 8.6%
Other methods 3.4%
In contrast to waste sludges, water sludges frequently can be
disposed of because of their basic nature, by dilution in surface
water. Water sludges contain mostly inorganic matter and cause few
odor problems even when the raw water has a high concentration of
organic material. Softening-plant sludges consist primarily of
calcium carbonate with small amounts of ferric, magnesium, and
aluminum hydroxide. The sludge particle sizes are very small,
most between 5 and 15 microns ^4°°'.
It is difficult to predict what the softening plant sludge volumes
and the settled sludge solids concentration will be. It is known,
however, that 2.5 pounds of sludge (dry) is produced per pound of
commercial quicklime added for softening. The concentration of
solids in the settled sludge may vary from 5 to 33 percent.
Volumes range from 0.4 to 6.0 percent of the water softened
Disposal by dilution into surface waters is simple, inexpensive, and
acceptable to many State regulatory agencies without prior treatment.
It does not have much effect on the dissolved oxygen content of the
receiving waters, but it can be a nuisance because it produces turbidity
and forms sludge banks. More and more States are classifying this
disposal technique as water pollution.
-------
-3.12-
Lagoon disposal is simple and inexpensive. There is a large land
requirement, but low operational costs. Few legal problems have
resulted from this disposal method. The lagoon capacity is affected
by: (1) whether sludge solids concentration is high or low,
(2) whether continuous or intermittent lagoon loading is practiced,
(3) whether the supernatant liquid is decanted, and CO whether the
climate is tempered or cold (**52). Sludges can be dried to 50 percent
moisture in lagoons and then removed.
The lagoon capacity requirement in five midwestern cities (assuming
that the sludge was dewatered to 50 percent moisture) was O.U5 to
0.66 acre-feet/year/mgd/100 ppm of hardness removed. According to
Howson, multiple basins were desirable so sludge could be air-dried
before additional sludge was added C*52). He recommended filling
to depths of 3 to 5 feet and supernatant decanting so that the sludge
was exposed to the air to facilitate drying. Where sludge can be
lagooned to 10 foot depths, it is common to provide lagoon land
areas of 3 to 5 acres per mdg of water treated.
According to Howson, pipeline transportation of water sludges to
lagoons would be simple and inexpensive. Asbestos-cement pipe-
lines (4 to 6 inches in diameter) are adequate and can be constructed
at a relatively low cost. It has been estimated that the total annual
cost for lagooning water plant sludge from a 10 mgd facility removing
100 ppm hardness would be only $1,250 to $2,500 ^52'.
Water plant sludge has been occasionally dried on sand drying beds or
vacuum filters prior to land disposal. Ultimate disposal may be to
landfills or to agricultural land for use as a soil conditioner.
At Boca Raton, Florida, the water plant sludge was vacuum filtered
and used as a roadway base stabilizer (**56). The use of the filter was
a tremendous improvement over trying to dewater by dumping the wet
sludge into unsightly piles. Dewatering sludge was definitely more
expensive than dilution or lagoon disposal, but the operation was
fairly simple and the costs were moderate. Some revenue may be
secured by selling the sludge to farmers. Sometimes water plant
sludge has been sold as a soil conditioner without prior dewatering;
the liquid sludge was simply spread from a tank truck used to haul the waste.
At many cities the water plant sludge has been added to the municipal
sewerage system. Some observers believe that the sludge functioned
as a raw sewage flocculent and thereby improved the overall sewage
treatment efficiency (**51). If the dumping of water plant sludge
is not proportioned over a period of time, it may inhibit biological
treatment processes at waste treatment plants.
-------
-313-
One interesting and discernible trend in water plant sludge handling
has been the increased use of recovery techniques. At locations where
dilution and lagooning were not feasible and hauling to distant farm-
land was not economical, waterworks sludge recovery was an alternative
solution to disposal that was sometimes adopted. Sludges from both
clarification and softening processes can be recovered.
Alum has been commonly used to clarify raw water at water purification
plants. The sludges removed from sedimentation basins at these plants
consisted of a dilute suspension of aluminum hydroxide floe containing
the substances removed from the water, such as algae and silt.
Reclaiming alum (aluminum sulfate) from the hydrous sludge and reusing
it for raw water flocculation was possible.
Investigations in England determined that sulfuric acid could be used
to convert insoluble aluminum hydroxide to aluminum sulfate, which in
turn could be reused as a flocculent C^58'. The amount of aluminum
hydroxide dissolved by the addition of sulfuric acid was a function
of the pH of the acid treated sludge. Solubility was very rapid
when the sludge pH was less than 4, and about 60 to 65 percent of
the alum was recovered when the pH was decreased to about 3. The
addition of acid increased the sludge settling rate, its compaction,
and its dewatering rate. This phenomenon was apparently due to the
release of bound water.
Roberts and Roddy reported that 3.8 pounds of commercial alum
produced 1.0 pound of aluminum hydroxide floe which theoretically
required 1.9 pounds of sulfuric acid for recovery (^53). jn a wet
process design for Tampa, Florida, they estimated that 1, 183 pounds
of 93.0 percent sulfuric acid were needed to produce 2,000 pounds
of reclaimed alum (17% Al203> at a 90 percent yield. At Tampa,
the cost of commercial alum was $38.08 per ton and sulfuric acid,
$19.48 per ton C*53). They assumed that $26.59 per ton could be
saved by using reclaimed alum.
The above figures must be modified and interpreted in relation to the
overall water treatment costs; the recovered flocculent will have a
lower efficiency than the origianl commercial alum so higher dosages
will be required. Also, recovery is not complete, so some commercial
alum must be added.
-------
-31U-
A typical alum recovery process has consisted of the following unit
operations:
1. Collection of the 1.5 to 2.5 percent waste sludge in
storage tanks.
2. Acid treatment.
3. Separation of aluminum sulfate by centrifuging or
filtration.
*». Pumping the solution to storage tanks.
The recovered alum could be mechanically dewatered and heat-dried,
but the added expense seemed unnecessary.
Accurate plant-scale performance and cost data have not been reported
for alum recovery processes. Recovery offers potential reductions in
chemical costs and in the volume of water plant sludge to be disposed
of; however, these advantages must be evaluated in relation to certain
disadvantages. These include the high cost of sulfuric acid in some
locations, the increased dosage required for recovered alum, the cost
of operating a centrifuge, and the need for sludge disposal facilities
to be used in conjunction with a recovery process.
Additional research is warranted, but a recovery process that is
economically competitive with the other common disposal techniques
should not be expected.
Lime recovery processes have been installed at numerous water
softening plants. Where lime costs are high, calcining to recover
quicklime for reuse in the softening process has, unlike alum
recovery, proven to be more economical than purchasing commercial
lime and wasting the softening plant sludge.
According to Black, one pound of lime (CaO) produced 3.57 pounds
of sludge that can be dried and calcined to recover 2 pounds of
lime C*°0). in practice, 100 percent recovery was not obtained,
but, because of the calcium and bicarbonate alkalinity present in
the hard water, more lime can often be produced than is required
for the softening process.
The basic recovery process included storage-thickening of the
clarifier underflow, mechanical dewatering, and calcining. Another
step often included before dewatering was recarbonation of the
waste sludge. This treatment improved the settling rate and,
later, the separation of unwanted magnesium hydroxide from calcium
carbonate. The most popular calcining units have been rotary kilns
and fluidized beds. Mechanical dewatering was accomplished in
vacuum filters or centrifuges. Centrifuges have the important advan-
tage of separating a portion of the magnesium, ferric and silica
compounds from the calcium carbonate slurry. Magnesium build-up
was also reduced by adopting split treatment procedures where
magnesium was precipitated separately from calcium.
-------
-315-
In Miami, Florida, lime was recovered by the following steps:
(1) gravity thickening sludge to about 25 percent solids,
(2) centrifuge dewatering to 66.8 percent solids, and (3) oil-
fired rotary kiln calcining at 2,100°F to 2,200°F (451). The capital
cost for the recovery plant was $0.8 million and operating costs
were $135,910 per year. No lime was purchased; in fact, lime was
sold -- the annual gross was $29,000.
San Diego has recovered lime, but, because softening was no longer
practiced, the process has been abandoned. Dewatering was accom-
plished in a centrifuge and calcining in a rotary kiln heated to
2,000°F by burning natural gas. Nine million B.T.U. 's were required
per ton of kilned product (45H). While the centrifuge rejected some
iron and magnesium from the waste sludge, silica remained. A wet
cyclone was used to remove the silica before centrifugation.
Calcining was done at only one water treatment plant in San Diego
but enough lime was recovered to make the city self-sufficient at
three water treatment plants, once a production level of 25 tons
per day was achieved.
Dayton, Ohio also became a lime supplier rather than a lime purchaser.
Sludge was recarbonated with carbon dioxide-containing gases from
the calciner, and subsequently dewatered by centrifuges and burned
at 2,000°F (**55). The recovery plant cost $1.5 million and produced
150 tons per day of pebble lime.
Lansing, Michigan, has been recovering lime from water-plant sludge
for 10 years. The process includes recarbonation, centrifuge
dewatering and fluidized bed "roasting" C*61>. in 1957 operating
costs for fuel, labor, power, and water were estimated to be $15.50
per ton at a production level of 25 tons per day. Assuming that
commercial lime cost $30 per ton, this left a generous margin for
capital costs.
Reclaiming lime sludge has been a common procedure at paper mills
using equipment similar to that discussed for water treatment
plants. Other industrial sludges such as those from the food
industry may also be conducive to lime recovery.
Summary - The disposal of sludges from water treatment plants is
becoming more of a problem for many of the same reasons as the
disposal of wastewater sludges. Simple procedures such as lagooning
and dilution are becoming impracticable due to limited land avail-
ability and restrictive water pollution control regulations. A
pattern similar to the history of wastewater sludge handling and
disposal has been established. With the elimination of "easy" techniques,
a logical next step has become mechanical dewatering and land disposal.
As this method, in turn, becomes impractical, heat treatment will
be adopted.
-------
-316-
Recovery of softening plant sludges can reduce the overall plant
operating costs, but it involves numerous operational problems.
First, magnesium should be excluded from the sludge. To some
extent, centrifuges accomplish this but often at the expense of
solids capture efficiency. Second, split treatment procedures can
be adopted but neither calcium carbonate nor magnesium hydroxide
floes settle as well separately as when they are precipitated
together. Third, dewatering and calcining equipment requires
substantial maintenance and moderate operating budgets. Fourth,
sludge reclamation often does not eliminate the need for alternate
sludge disposal facilities because not all of the sludge is processed.
A final disadvantage results from changing water quality with the
seasons. This, of course, means a variable sludge quality which
complicates calcining operations.
Inevitably, more and more water treatment plants will adopt lime
reclamation processes and alum reclamation. While reclamation
has not been enthusiastically accepted, the cost of water treatment
has been reduced by varying degrees at those locations where it
has been in operation. Additional research should be undertaken
to improve chemical and mechanical procedures so that costs are
reduced, efficiency improved, and controls simplified. The complete
water treatment plant should be designed with reclamation in mind if
the process might be adopted.
-------
-317-
24. SUMMARY OF SLUDGE HANDLING AND
DISPOSAL ECONOMICS
General - Fleming quoted an appropriate statement concerning sludge,
"The man who owns the most of it is the worst off" (21O. While
sludge does have some fuel and fertilizer value, it is a definite
liability at any sewage treatment plant.
A specific system to treat and dispose of the sludge should be
selected only after all the factors have been scientifically studied.
A good system for one location may not be applicable somewhere else
because local conditions vary greatly. Weller and Condon stated
that the major factors in selecting a sludge processing system were:
(1) plant location, (2) plant operation, and (3) economic evaluation
(329).
Plant location and operation factors are affected by the climate,
physical environment, esthetics, system complexity and efficiency,
and the characteristics of the waste. The economic evaluation
includes a consideration of relative capital and operating costs
and the plant operating agencies' capabilities and desire to meet
these costs (329).
Data - Numerous comparative economic studies of sludge treatment
processes have been reported in the literature. These studies
are useful as general information but the data are applicable only
to the specific location and conditions under consideration.
Again, a separate and complete economic analysis is required for
each particular treatment plant system. The usefulness of economic
data are often nullified by the lack of uniformity in accounting and
reporting procedures, but if the data are generated by reliable
consulting engineers they can be accepted as being fairly accurate.
Each chapter in this report has included a discussion of the
economics of specific sludge handling and disposal processes.
This chapter reviews and summarizes the economic data.
A few literature references presented a general review of sludge
treatment costs. These include the following:
1. Weller and Condon compared relative costs for
sewage sludge treatment at Kansas City (329).
-------
-318-
System Original Cost Annual Cost
Dewatering and incineration
of raw sludge 1.0 1.0
Digestion, mechanical dewatering
and landfilling 1.05 1.41
Digestion, mechanical dewatering
and incineration 1.43 1.38
Wet combustion 1.97 1.54
2. In England,the Ministry summarized sewage sludge treatment
costs ^'0). (Average figures reported were arbitrarily
increased by 50% to account for increased costs for labor,
equipment, etc. in England. Mixed sludge refers to a
combination of primary and biologic sludge.)
System Disposal Cost ($/Ton)
Lagooning raw mixed sludge 3.15
Free distribution of liquid
digested mixed sludge 8.00
Barging raw primary sludge to sea 8.40
Air drying digested mixed sludge
to 65% moisture 8.84
Landfilling raw liquid sludge 10.10
Air drying raw mixed sludge to
65% moisture 10.10
Filter pressing raw primary sludge
to 60% moisture 11.15
Filter pressing and heat drying
secondary sludge 19.75
Air drying mixed digested sludge to
15% moisture, granulating and bagging 22.30
By-product recovery of grease from
raw primary sludge 31.15
Filter pressing and heat drying digested
elutriated primary sludge to 35%
moisture, granulated and sold in bulk 32.60
-------
-319-
3. Dietz presented data showing the economy of using
digesters at small sewage treatment plants (^6).
(Annual costs were increased by 25% in accordance
with the Engineering News Record (ENR) Cost Index
Increase.)
Population 6,500 10,000 20,000 30,000 HO, OOP
System Annual Cost in Dollars
Digesters with
sand beds 5,700 7,700 13,580 19,300 25,250
Digesters with
lagoons 4,765 5,700 9,100 13,700 20,500
Vacuum filtration
of raw sludge 9,600 10,400 14,400 18,650 24,650
4. Logan surveyed construction costs for sludge treatment
at small sewage treatment plants (6°). (His figures were
increased by 18% to account for the change in the ENR
Index.)
System Construction Cost ($)
Plant Size 1^ jngd 5 mgd 10_mgd_
A. Vacuum filtration of
raw primary sludge 52,800 76,000 152,000
B. Vacuum filtration of
raw pri. + sec.
sludge 56,600 150,500 227,000
C. Digestion and sand
bed drying 98,300 397,500 812,500
D. Digestion and vacuum
filtration 131,500 389,000 693,000
The above figures were based on a digester design capacity of 3 cubic
feet per capita, a sand bed capacity of 1.25 square feet per capita,
and a vacuum filter rate of 6 pounds per square foot per hour.
A general review of all available economic data including the recent
studies completed for Baltimore, Maryland,- Washington, D.C.,- and
Philadelphia, Pennsylvania (see the Ocean Disposal chapter) yielded
the average sludge handling and disposal costs listed below. Most
of the data used to compile the average figures were based on sewage
sludge processing costs. Industrial sludge treatment costs could
vary considerably from the average figures because of their different
characteristics.
-------
-320-
1. Ultimate disposal (includes cost of preparation, such
as dewatering, digestion, etc., as described specifically
in item 2).
Capital and Operating Costs
($/Dry Ton>
System Average Range
A. Composting Not accurately known
B. Heat drying* 50 40-65
C. Incineration
(1) wet combustion 42
(2) multiple hearth and
fluidized bed 30 10-50
D. Landfilling dewatered sludge 25 10-50
E. Disposal as a soil conditioner
w/o heat drying* (dewatered) 25 10-50
F. Disposal on land as a liquid
soil conditioner* 15 8-50
G. Lagooning 12 6-25
H. Barging to sea 12 5-25
I. Underground disposal Unknown, potentially inexpensive
J. Pipeline to sea 11
*Gross cost, does not account for money received from sale
of sludge.
2. Sludge handling (specific process costs)
Capital and Operating Costs
($/Dry Ton)
System Average Range
A. Thickening
(1) gravity _ 1.50-5
(2) air flotation* _ 5 .15
(3) centrifugation* _ 3 .20
B. Dewatering
(1) vacuum filtration ^5 9 .50
(2) centrifugation 12 5 -39
(3) sand bed drying _ 3-20
C. Anaerobic digestion _ 4 _±Q
D. Elutriation _ 2-5
E. Lagooning 2 1-5
F. Landfilling _ j_ 5**
G. Pipeline transportation 5
H. Liquid sludge disposal on land ^g 4. .30
as a soil conditioner
I. Heat drying 35 25 -HO
J. Incineration 20 8 -40
K. Barging to sea 10 4-25
2nding on the need for chemicals
ler
***Moderate distances, cost varies with length.
x*Varies tremendously deper
**Long hauls would be higher
-------
-321-
A review of the above costs and the general literature on sludge
handling and disposal economics led to the following conclusions:
1. Anaerobic digestion of sewage sludges for all small
cities and cities located near the coasts is justified.
It allows relatively inexpensive final disposal methods
such as ocean dilution, lagooning, and spreading liquid
sludge on land as a soil conditioner.
2. Lagooning industrial sludges is an inexpensive disposal
technique.
3. Pipeline transportation of sludge to desirable disposal
areas should be considered because it is relatively
inexpensive.
4. If heat dried sludge can be sold as a fertilizer for
about $15 per ton, only then should the process be
considered.
5. Digesting sludge before incineration cannot be
justified on the basis of economics.
6. Water plant sludges are normally disposed of very
inexpensively in sewerage systems, lagoons, or surface
waters.
Obviously, sludge treatment costs vary greatly. This statement is
valid even though two similar treatment plants use identical sludge
treatment processes. For this reason, design engineers should make
comparative studies and cost estimates for each particular sludge
disposal situation. The fact that sludge handling and disposal
represents 25 to 50 percent of the total treatment plant capital
and operating cost justifies a thorough economic evaluation.
-------
-322-
25. FUTURE APPROACHES
General - One obvious and general statement that can be made
concerning a new approach to sludge handling and disposal is that
it deserves more attention from researchers, design engineers, and
others involved with water and waste treatment.
To start with, attention could be gained by scheduling national
conferences, similar to those convened for solid waste disposal,
to discuss thoroughly the problems and their possible solutions
among a variety of interested parties. In fact, joint conferences
concerning all types of waste solids, whether they be car bodies,
garbage, or wastewater sludges, should be called because joint
treatment and disposal of all of the waste solids in a particular
watershed may be the best solution to individual disposal problems.
Another useful initial step would be to standardize accounting
and reporting procedures in the sanitary engineering field. The
lack of uniformity today makes it very difficult for researchers,
design engineers, and plant operators to compare cost and per-
formance data. .Uniformity will enhance the scientific and orderly
development of technical procedures in the water and wastewater
treatment field.
New approaches to sludge handling and disposal should concentrate
on minimizing the number of process steps. Submerged combustion
of the sludge in the initial collection basin would represent the
ultimate development in this regard. Numerous sewage treatment
plants have incorporated multiple process steps between sludge
collection and ultimate disposal; including thickening, digestion,
elutriation, and dewatering. Incomplete solids capture has been
the usual situation in all of these treatment steps. Water pollu-
tion has been increased along with waste treatment costs because
the "uncaptured" solids were recycled but not totally removed from
the waste stream.
Better analytical tools are needed to control sludge handling
processes. To start with, a sludge characterization study would
be useful; it would provide more data about the fundamental nature
of sludge particles and how this nature may be altered to permit
more effective dewatering. New instrumentation to control precisely
various sludge treatment operations could reduce costs of wastewater
treatment and operator frustration.
-------
-323-
Because labor often accounts for two-thirds of the operating cost
of sludge handling processes, systems and equipment should be
designed to maximize the efficient use of labor in order to reduce
costs. Improved design of all types of equipment is continually
desirable. The improved designs have already resulted in lower
costs and the use of mechanical equipment at treatment plants with
volumes formerly considered too small to justify mechanization.
Research into methods of reducing sludge volumes should be
supported. This is particularly important where biological sludges
are involved because their hydrous nature resists dewatering.
Perhaps a biological wastewater treatment system could be designed
that minimizes the sludge volume while still maintaining efficient
B.O.D. removal.
To encourage research into new approaches to sludge handling and
disposal, government financial support should continue to be made
available. Research and demonstration contracts should continue
to be awarded to private industry having specialized knowledge in
certain areas of sludge treatment, to operators of treatment
facilities having well-qualified staffs, and to university research
groups. Water Pollution Control Administration Laboratories should,
of course, also continue to take an active role in investigating
new techniques.
Specifics - Individual sections of this report include specific
recommendations for new research and development. The following
are particularly important:
1. Better separation of grit from organic solids; in
order to optimize digestion and incineration operations.
The hydrocyclone is a step in the right direction.
2. Improved solids concentration and freshness from
sedimentation units. Pre-aeration and the use of
polymeric flocculents have accomplished this to
some degree. New sedimentation basin designs might
be a partial answer.
3. Improved techniques for thickening sludge,
particularly to a level that would allow economical
incineration without mechanical dewatering.
U. Thorough evaluation of aerobic digestion parameters,
economics, and sludge dewatering characteristics.
-------
-324-
5. Improvements in the anaerobic digestion process. A
process is needed that is more stable, produces a clean
supernatant liquor or none at all, produces an easy-to-
dewater sludge without elutriation, and removes nitrogen
and phosphates. "Densludge" thickening, ahead of digestion
as practiced at New York, is a good example of how the
process may be improved. An evaluation of the effects of
detergents on digestion is in order because numerous
operators believe this material has a deleterious effect
on the process. Continuation of the high temperature
digestion studies started at Los Angeles is in order.
6. Means of separately treating digester supernatant liquor
alone or in combination with thickening tank effluents,
elutriates, centrates, and filtrates. The addition of
lime or some other chemical to produce a useful ferti-
lizer would take advantage of the nutrients concentrated
in digester supernatant liquor. Studies are needed to
supply information on the nitrogen and phosphate content
of filtrate, centrate, elutriate and digester, and
thickening tank supernatant liquor.
7. Better means of mechanically removing dried sludge from
sand drying beds and improved techniques for liquid
decanting.
8. Thorough evaluation of the economics of pipeline trans-
portation of various sludges. Research into methods of
fluidizing sludge to be pumped through pipelines would
be desirable as would the control of septicity.
9. Evaluation of underground disposal techniques for liquid
sludge to include deep wells and discharge into abandoned
mines, etc.
10. Support of studies showing the effects on aquatic plants
and animals of dumping sludge into the ocean.
11. Continued efforts to develop more active chemicals for
conditioning sludges prior to mechanical dewatering
and simpler, more effective dewatering units. Chemicals
that would eliminate the elutriation process would be
welcome in the field.
-------
-325-
12. Greater research emphasis on sludge conditioning by
heat treatment. Perhaps waste heat from incineration
processes can be used for this purpose.
13. A broad, thorough and scientific study of the value of
liquid, composted or dried sludge as a fertilizer or
soil conditioner. This study is long overdue. Some
studies are underway, but they should be expanded.
14. Investigation into by-product use of some industrial
waste sludges. This might include an inexpensive
method to dry food-industry sludges so the material
could be used as an animal feed or soil conditioner.
15. Expanded research investigations into sludge combustion
techniques. Such studies are justified because
combustion is the one disposal method that will meet
future criteria. Improvements in the following areas
should be considered:
(a) Incineration of liquid sludge so mechanical
dewatering can be eliminated.
(b) Combustion of wastewater sludges with refuse.
(c) Combustion of sludge in boiler furnaces to
develop steam.
(d) Heat recovery to distill sewage effluents,
condition sludges, etc.
(e) Combustion of skimmings and screenings.
16. Continued support for the Los Angeles investigations of
combining the collection of refuse and sewage sludge
by grinding and adding the refuse to the sewerage system.
17. New analytical tools to enable digester troubles to
be predicted and to proportion sludge conditioning
chemicals to the precise sludge demand. These would
be well received by budget supervisors and over-
burdened plant operators.
-------
-326-
18. Investigation into improved methods for odor control.
19. Evaluation of the use of compost or liquid digested
sludge to reclaim barren strip mine areas.
20. Continued efforts to develop better methods for
reclaiming alum sludges from water treatment plants.
21. Increased evaluation of the feasibility of separate
treatment and disposal of primary and secondary sludges.
22. Improved laboratory bench techniques and larger
scale tests for developing sludge handling design criteria.
The cost and troublesome nature of existing sludge handling and disposal
processes warrant a large research effort. In the future, the situa-
tion could be more critical if new techniques are not developed
because sludge volumes are rising and increased wastewater treatment
efficiencies are producing more difficult-to-handle sludge. Once
new and reliable technology is generated, it is important to disseminate
it throughout those groups involved in water and waste treatment so
that an early effort can be made to solve sludge treatment problems.
THE MENTION OF PRODUCTS OR MANUFACTURERS IN THIS REPORT DOES NOT IMPLY
ENDORSEMENT BY THE FEDERAL WATER POLLUTION CONTROL ADMINISTRATION,
U.S. DEPARTMENT OF THE INTERIOR.
-------
REFERENCES
(Grouped by Unit Processes)
General
1. Seelye, E. E., "Design, Data Book for Civil Engineers." 3rd
Edition, John Wiley and Sons, Inc., New York. (I960).
2. "Recommended Standards for Sewage Works (10 States Standards),"
by Great Lakes-Upper Mississippi River Board of State Sanitary
Engineers, 1960.
3. "Design Criteria for Sewerage Systems," by Texas State Dept.
of Health, 1961.
4. "Advances in Sewage Treatment Design," Proceedings Manhattan
College Symposium, New York, 1961.
5. Eckenfelder, W. W., and O'Connor, D. J., "Biological Waste
Treatment." Macmillan Co,, New York, 1961.
6. Rich, L. G., "Unit Operations of Sanitary Engineering." John
Wiley and Sons, New York.(1961)
7- Fair, G. M. and Geyer, J0 C., "Water Supply and Wastewater Disposal."
John Wiley and Sons, New York, 1954.
8« VanKleeck, L. W., "How to get top Performance From Your Sewage
Treatment Plant." Reprinty Wastes Engr., New York.
9. Bloodgood, D. E., "Sewage Treatment Practices." Reprint, Water
and Sewage Works.
10. Dodson, R. E. and Stone, Ralph, "Advances in Sludge Disposal."
Journal of the Sanitary Engineering Division--ASCE, Vol. 88, No.SA4,
pp. 71-72 (July 1962).
11. "Operation of Wastewater Treatment Plants," Manual of Practice,
No. 11, WPCF, Washington, D. C,, 1961.
12. VanKleeck, L.W., "Sewage Works Guide." Reprint, Wastes Engr., New
York City, 1961.
13. Swanwick, J. D., "Recent Work on the Treatment and Dewatering of
Sewage Sludge." Journal Water Pollution, (Control Federation, Vol. 34,
No. 3, pp. 239-240 (Mar., 1962).
-------
14. Fleming, J. R., "Sludge Utilization and Disposal." Sewage
and Industrial Wastes, Vol. 31, No. 11, pp. 1342-1346 (Nov. 1959).
15. "1958 Operator's Forum," Sewage and Industrial Wastes. Vol. 31,
No. 6, pp. 739-742. (1959)
16. Garrett, J. T., "Tars, Spent Catalysts, and Complexes as
Petrochemical Waste Sources." Sewage and Industrial Wastes,
Vol. 31, No. 7, pp. 841-845 (July 1959).
17. Reefer, C. E., "Sewage Treatment Works." McGraw-Hill Book Co.,
Inc., New York, 1940.
18. Compton, C. R., "Taming Sludge by Density Control, Non-Clog
Pumping and Dewatering." Wastes Engineering (Oct. 1959).
19. Haseltine, T. R., "Tannery Wastes Treatment with Sewage at
Williamsport, Pa.." Sewage and Industrial Wastes, Vol. 30,
No. 1, pp. 65-85. (1958).
20. Leaver, R. E., "Sludge Disposal Practices in the Pacific Northwest."
Sewage and Industrial Wastes. Vol. 28, No. 3, pp. 323-328 (Mar. 1956).
21. Center, A. L., "Conditioning and Vacuum Filtration of Sludge."
Sewage and Industrial Wastes, Vol. 28, No. 7, pp. 829-840. (1956).
22. "Seminar Papers on Waste Water Treatment and Disposal," Boston
Society of Civil Engineers Sanitary Section, R. S. Rankin, pp. 96-
121, 1959.
23. Budd, W. E., "Thickening of Raw Organic Sludges." Proc. of the 8th
So. Municipal and Industrial Waste Conf.. pp. 163-177, 1959.
24. Fleming, J. R., "Sludge Utilization and Disposal." Proc. of the
8th So. Municipal and Industrial Waste Conf.. pp. 198-218, 1959.
25. "Survey of Design Trends and Developments for Small Sewage Treatment
Plants in Past Decade," Editors, Wastes Engineering, pp. 520-523
(Oct. 1962).
26. Furman, T., "Sewage Plant Design Criteria for the Semitropics."
Sewage and Industrial Wastes. Vol. 26, No. 6, pp. 745-758 (June
1954).
27. Wirts, J. J. and Ausflug, F. J., "Concentration of Activated
Sludge." Sewage and Industrial Wastes. Vol. 23, No. 10, pp. 1219-
1226 (Oct. 1951).
-------
-3-
28. Zack, S. I., "Sludge Dewatering and Disposal." Sewage and
Industrial Wastes. Vol. 22, No. 8 pp. 975-996 (Aug. 195o).
29. Turney, J. G., "Sludge Disposal Study at Houston, Texas."
Sewage and Industrial Wastes. Vol. 21, No. 5, pp. 807-810
(May 1949).
30. Hicks, R., "Disposal of Sewage Sludge." The Surveyor, pp. 105,
303-306 (Apr. 19, 1946).
31. Komline, T. R., "Experiences in Thickening and Drying of
Sludge." Sewage Works Journal, Vol. 19, No. 5, pp. 806-810.
(1947).
32. Adams, J. K., "Sludge Dewatering." Sewage Works Journal,
Vol. 15, No. 4, pp. 704-712. (1943^
33. Rudolfs, W., "Concentration of Activated Sludge by Compacting
and Flotation." Sewage Works Journal, Vol. 15, No. 4, pp. 642-657.
(1943)
34. Rudolfs, W. and Logan, R. P., "Effect of Temperature on Sludge
Concentration." Sewage Works Journal, Vol. 15, No. 5, pp. 894-907.
(1943).
35. Rudolfs, W. and Cleary, E. J., "Sludge Disposal and Future
Trends." Sewage Works Journal, Vol. 5, No. 3, pp. 409-428.
(1933).
36. Schroepfer, G. J. and Ziemke, N. R., "Development of the
Anaerobic Contact Process." Sewage and Industrial Wastes, Vol.
31, No. 6, pp. 697-711. (1959)"!
37. "Sludge Handling Sidelights," Editors, Wastes Engr., p. 95
(Feb. 1960).
38. Estrada, A. A., "New Design Criteria Needed for Sludge Handling
Units." Wastes Engr.. pp. 599-600 (Oct 1960).
39. Gehm, H. W., "New Wrinkles for Dewatering Difficult Paper Mill
Sludges." Wastes Engr., pp. 256-258 (May 1959).
40. Fleming, J. R., "Sludge Utilization and Disposal." Water and
Sewage Works. Vol. 107, Ref. No. pp. R-282 to R-284. (1960).
41. Brink, R. J., "Operating Costs of Waste Treatment in General
Motors." Proc. of the 19th Purdue Industrial Waste Conference,
pp. 12-16, 1964.
42. Washington, D. R. and Symons, J. M., "Volatile Sludge Accumula-
tion in Activated Sludge Systems." Journal Water Pollution Control
Federation. Vol. 34, No. 8, pp. 767-790 (Aug. 1962).
-------
43. Symons, G. E., "The Present and Future of Industrial Waste
Treatment." Proc. of the 17th Purdue Industrial Waste
Conference, pp. 717-730, 1962.
44. Caron, A. L., "Economic Aspect of Industrial Effluent Treatment."
TAPPI, Vol. 47, No. 9, pp. 62A-67A, 72A. (Sept. 1964).
45. Carpenter, W. L., "Factors Affecting Selection of Equipment for
Treatment of Deinking Waste." TAPPI, Vol. 47, No, 5, pp. 144A-146A.
(May 1964).
46. Dietz, J. C., "Some Factors Influencing the Selection of
Sludge Handling and Disposal Methods for Municipal Sewage
Treatment Plants." Proc. of the 13th Purdue Industrial Waste
Conference, pp. 555-565, 1958.
47. Nelson, F. G. and Budd, W. E., "New Developments in Sewage
Sludge Treatment." Proc. of the ASCE, Journal of the San. Engr.
Div., Vol. 85, No. SA6, pp. 1-16 (Nov. 1959).
48. Fleming, J. R., "Sludge Utilization and Disposal." Public Works,
Vol. 90, No. 8, pp. 120-122, 202-204 (Aug. 1959).
49. Schepman, B. A., "New Horizons in Sludge Disposal." Public Works,
Vol. 86, No. 12, pp. 74-76 (Dec. 1955).
50. Rabb, A., "Sludge Disposal: A Growing Problem." Hydrocarbon
Processing, Vol. 44, No. 4, pp. 149-150 (Apr. 1965).
51. "Solids Removal Practices in Southern Kraft and Newsprint Mills,"
Paper Trade Journal, pp. 42-45 (Sept. 21, 1959).
52. Seidel, Harris, "Solids Handling and Disposal." Water Works and
Wastes Engineering, Vol. 2, No. 9, p. 63 (Sept. 1965).
53. MacLaren, J. W., "Evaluation of Sludge Treatment and Disposal."
Canadian Municipal Utilities, pp. 23-33, 51-59 (May 1961).
54. Koenig, L., "Ultimate Disposal of Advanced-Treatment Waste."
U.S.P.H.S. AWTR-3. Publication No. 999-WP-3 (Oct. 1963).
55. Koenig, L., "Ultimate Disposal of Advanced-Treatment Waste."
U.S.P.H.S. AWTR-8. Publication No. 999-WP-10, 1964.
-------
-5-
56. Ettelt, G. A. and Kennedy, T. J., "Research and Operation
Experience in Sludge Dewatering." Journal Water Pollution
Control Federation, Vol. 38, No. 2, p. 250, (Feb. 1966).
57. Fischer,A. J. , "The Economics of Various Methods of Sludge
Disposal." Water Works and Sewage, Vol. 83, No. 3, pp. 74-77,
1936.
58. Chambers, Jr., J. T., "Digested Sludge Disposal Practices."
Proc. of the 25th Short Course^ Louisiana State University,
Baton Rouge, La., 1962.
59. "Survey of Design Trends and Developments for Small Sewage
Treatment Plants in Past Decade," Editors, Wastes Engr., pp. 520-
523 (Oct. 1962).
60. Logan, J. A. and others, "An Analysis of the Economics of
Wastewater Treatment." Journal Water Pollution Control
Federation, Vol. 34, No. 9, pp. 860-882. (Sept. 1962).
61. McCarty, P. L., "Sludge Concentration - Needs, Accomplishments
and Future Goals." Paper presented at the 38th WPCF Conf.
(Oct. 1965).
62. Lewin, V. H., "Survey of Some Methods of Sludge Dewatering."
The Surveyor. Vol. 121, No. 3680, pp. 1521-1523, 1962.
63. Babbitt, H. E., "Sewerage and Sewage Treatment." John Wiley
and Sons (London) 1932.
64. "The Sewerage and Refuse Digest," Editors, Public Works, pp. 128,
130 (Jan. 1956).
65a. Katz, W. J. and Geinopolos, A., "Dissolved-air Flotation as a
Method of Thickening Aerobic Biological Solids," in "Sludge
Concentration -- Filtration and Incineration," Continued
Education Series No. 113, U. of Michigan, Ann Arbor, pp. 17-36,
1964.
65b. Kennedy, Richard R., "Thickening by Elutriation and Chemical
Coagulation" In "Sludge Concentration -- Filtration and Incinera-
tion" Continued Education Series No. 113, U. of Michigan, Ann
Arbor, pp. 37-49, 1964.
65c. Simpson, George D. and Sutton, Stanley H., "Performance of Vacuum
Filters" f.n "Sludge Concentration -- Filtration and Incineration"
Continued Education Series No. 113, U. of Michigan, Ann Arbor,
pp. 126-138, 1964.
-------
-6-
65d. Russell, Robert A., "Theory of Combustion of Sludge" in
"Sludge Concentration -- Filtration and Incineration"
Continued Education Series No. 113, U. of Michigan, Ann
Arbor, pp. 380-382, 1964.
65e. Quirk, Thomas P., "Economic Aspects of Incineration Versus
Incineration-Drying" in "Sludge Concentration -- Filtration
and Incineration" Continued Education Series No. 113, U. of
Michigan, Ann Arbor, pp. 389, 1964.
66. Committee on Sewerage and Sewage Treatment, "Advances in
Sludge Disposal in the Period from October 1, 1954 to
February 1, I960." A Progress Report of the ASCE - San.
Engr. Div., Vol. 88, No. SA2, pp. 13-51 (Mar. 1962).
67. "Sludge Drying Still the Weakest Link at Sewage Works,"
Editors, The Surveyor, Vol. 123, No. 3740, p. 35, London,
1964.
68. Carpenter, W. L. and Caron, A. L., "Research on Solids
Dewatering." Pulp and Paper Magazineo^Canada, pp. 3, 19-23
(June 1964).
69. "Proceedings, National Conference on Solid Waste Research,"
published by the APWA, Chicago, 111., 1963.
70. "Treatment and Disposal of Sewage Sludge." Ministry of
Housing and Local Government, Her Majesty's Stationery Office,
(London) 1954.
71. Seelye, E. E., "Specifications and Costs, Data Book for Civil
Engineers." 3rd ed., John Wiley and Sons, New York. (1957).
72. 1964 Annual Report, Westchester County Department of Public
Works, Division of Sewers, New York, (1964).
73. Fleming, R. R., "Solid-Waste Disposal." The American City,
pp. 94-96 (Feb. 1966).
74. Lowes, F. J., Personal Communication, The Dow Chemical Company,
1963.
-------
-7-
Screening and Grit Removal
75. Ketcham, L., "Operator Ingenuity at the Tacoma, Washington
Sewage Treatment Plant." Sewage and Industrial Wastes, Vol.
30, No. 12, pp. 1506-1518 (Dec. 1958).
76. Smith, E. J., "Incineration of Fine Screening at Niagara
Falls, New York." Sewage Works Journal. Vol. 18, No. 2,
pp. 221-227. (1946).
77. Flood, F. L., "Handling and Disposal of Screenings." Sewage
Works Journal. Vol. 3, No. 2, pp. 223-231. (1931).
78. Sager, J. C., "Minneapolis-St. Paul S. D. Incinerates Scum."
Water and Sewage Works, Vol. Ill, No. 9, pp. 393-396. (1964).
79. Dreier, D. E. and Walker, J. D., "Grease Incineration." Proc.
of the 19th Purdue Industrial Waste Conference, pp. 161-166,
1964.
80. "Seminar Papers on Waste Water Treatment and Disposal,"
Boston Society of Civil Engineers Sanitary Section, F. L.
Flood, pp. 35-62, (April 1961).
-------
-8-
Sedimentation
81. Seidel, H. F. and Baumann, E. R., "Effect of Preaeration
on the Primary Treatment of Sewage." Journal Water Pollution
Control Federation. Vol. 23, No. 3, pp. 339-355. (1951).
82. Garrison, W. E. and Nagel, C. A., "A Density Meter to
Control Sludge Pumping." Sewage and Industrial Wastes, Vol.
31, No. 11, pp. 1327-1335.
83. Ross, E. E., "Raw Sludge Pumping Systems." Sewage and
Industrial Wastes. Vol. 31, No. 1, pp. 105-110. (1959).
84. Sparr, A. E., "Operation of Sedimentation Tanks." Journal
Water Pollution Control Federation, Vol. 36, No. 6, pp. 760-
766. (1964).
85. Eckenfelder, Jr., W. W. and Melbinger, N., "Settling and
Compaction Characteristics of Biological Sludges." Sewage
and Industrial Wastes, Vol. 29, No. 10, pp. 1114-1122. (1957).
86. Fitch, E.- B., "The Significance of Detention in Sedimentation."
Sewage and Industrial Wastes, Vol. 29, No. 10, pp. 1123-1133.
(1957).
87. Heukelekian, H. and Weisberg,E., "Bound Water and Activated
Sludge Bulking." Sewage and Industrial Wastes. Vol. 28, No. 4,
pp. 558-574. (1956).
i
88. Laboon, J. F., "Construction and Operation of the Pittsburgh
Project." Journal Water Pollution Control Federation, Vol. 33,
No. 7, pp. 758-782. (1961).
89. Hazen, R., "Performance and Economics in Settling Tank Design."
Proc. of the 8th Southern Mun. and Industrial Waste Conference,
pp. 60-75, 1959.
90. Katz, W. J. and others, "Concepts of Sedimentation Applied to
Design." Water and Sewage Works, pp. 257-258 (July 1962).
91. Camp, T. R., "Studies of Sedimentation Basin Design." Sewage
and Industrial Wastes, Vol. 25, No. 1, pp. 1-14 (Jan. 1953).
92. Mancini, J. L., "Gravity Clarifier and Thickener Design."
Proc. of the 17th Purdue Industrial Waste Conference, pp. 267-277,
1962.
-------
-9-
93. Sawyer, C. N. and Bradney, L., "Rising of Activated Sludge
in Final Settling Tanks." Sewage Works Journal, Vol. 17,
No. 6, pp. 1191-1209. (1945).
94. Tapleshay, J. A., "Control of Sludge Index by Chlorination
of Return Sludge." Sewage Works Journal, Vol. 17, No. 6,
pp. 1210-1226. (1945).
95. Keefer, C. E., "Checking Density of On-Stream Sludge with
Nucleonic Analyzer." Wastes Engr., pp. 24-26 (Jan. 1960).
96. Katz, W. A. and others, "Concepts of Sedimentation Applied
to Design." Wastes and Sewage Works, pp. 162-165 (April,
1962) pp. 169-171 (May, 1962) pp. 257-259 (June 1962).
97. McKee, J. E. and others, "Fundamental Concepts of Rectangular
Settling Tanks." ASCE Transactions. Vol. 121, pp. 1179-1218,
1956.
98. Foster, J. H., "Fundamentals of Primary Treatment." TAPPI,
Vol. 46, No. 5, pp. 155A-160A, (May 1963).
99. Comings, E. W., "Continuous Settling and Thickening."
Industrial and Engr. Chemistry, Vol. 46, No. 6, pp. 1164-1172,
(June 1954).
100. Kynch, G. J., "A Theory of Sedimentation." Trans, the Faraday
Society, Vol. 48, pp. 166-176, 1952.
101. Fitch, E. B., "Sedimentation Process Fundamentals." Trans.
Society of Mining Engrs., Vol. 223, pp. 129-137, (1962).
102. Crowe, R. E. and Johnson, E. E., "Organic Polyelectrolyte -
A New Approach to Raw Sewage Flocculation." Paper presented
at the 38th WPCF Conference, (Oct. 1965).
103. McKee, J. E. and others, "Fundamental Concepts of Rectangular
Settling Tanks." Proc. of the ASCEf Sep. No. 590. Vol. 81
(Jan. 1955).
104. Camp, T. R., "Sedimentation and the Design of Settling Tanks."
Trans, of the ASCE, Vol. Ill, No. 2285, 1946.
105. "Industry's Idea Clinic, Journal Water Pollution Control
Federation. Vol. 36, No. 8, pp. 948-970, (1964).
-------
-10-
106. Brisbin, S. G., "Sewage Sludge Thickening Tests." Sewage
and Industrial Wastes. Vol. 28, No. 2, pp. 158-165 (Feb.
1956).
107. Torpey, W. N., "Concentration of Combined Primary and
Activated Sludges in Separate Thickening Tanks." Proc.
ASCE, 80, Sep. No. 443 (May 1954).
108. Dust, J., "Sludge Thickening Proves Economical in Beaumont,
Texas Sewage Treatment Plant." Civil Engineering, pp. 68-
72 (April 1956).
109. Reynolds, E. C., "Sludge Washing and Thickening Eliminate
Need for Grit Removal." Wastes Engr.. pp. 640-641 (Dec.
1956).
110. Reefer, C. E., "Thickening of Raw Sludge." Water and Sewage
Works. Vol. 107, Ho. 7, pp. 218-221. (1960).
111. Behn, V. C. and Liebman, J. C., "Analysis of Thickener
Operation." Proc. of the ASCE. Sanitary Engr. Div.. Vol.
89, No. SA3, pp. 1-12, 1963.
112. Fitch, E. B. and Talmage, W. P., "Determining Thickner
Unit Areas." Industrial and Engr. Chemistry. Vol. 47, No. 1,
pp. 38-41, (Jan. 1955).
113. Rudolfs, W., "Sludge Concentration-- Some Results in the State
of New Jersey." Proc. of the ASCE - San. Engr. Div.. Vol. 70,
pp. 1283-1289 (Oct. 1944).
114. Torpey, W. N., "Concentration of Combined Primary and
Activated Sludges in Separate Thickening Tanks." Proc. of the
ASCE. Vol. 80, Sep. No. 443 (May 1954).
115. Lauer, W. N., "Sludge Concentration." Proc. of the 6th
Southern Municipal and Industrial Waste Conference, pp. 130-
140, 1957.
116. Glace, Jr., I. M., "Discussion of Thickening of Raw Organic
Sludges." Proc. of the 8th Southern Municipal and Industrial
Waste Conference, pp. 179-186, 1959.
117. Bek, C. M., Personal Communication, The Dow Chemical Company,
1963.
118. Priesing, C. P., and Milbrandt, L., Personal Communication,
The Dow Chemical Company, 1962.
-------
-11-
Sludge Thickening - Air Flotation
119. "Fundamental Principles of Dissolved Air Flotation of
Industrial Wastes," Proc. of the 14th Purdue Industrial
Waste Conference. 1959.
120. Braithwaite, R. L., "Polymers as Aids to the Pressure
Flotation of Waste Activated Sludge." Water and Sewage
Works. Vol. Ill, No. 12, pp. 545-547, (1964).
121. Ettelt, G. A., "Activated Sludge Thickening by Dissolved
Air Flotation." Proc. of the 19th Purdue Industrial Waste
Conference, pp. 210-244, 1964.
122. Katz, W. J., "Sewage Sludge Thickening by Flotation."
Public Works. Vol. 89, No. 12, pp. 114-115 (Dec 1958).
123. Jones, W. H., "Developments with Pressurized Flotation."
Paper presented at the 38th WPCF Conference (Oct. 1965).
124. Mayo, E., "Industrial Applications of Air Flotation."
Paper presented at the 38th WPCF Conference (Oct. 1965).
-------
-12-
Sludge Thickening - Centrifugation
125. Bradney, L. and Bragstad, R. E., "Concentration of
Activated Sludge by Centrifuge." Sewage and Industrial
Wastes, Vol. 27, No. 4, pp. 402-412, (1955).
126. Cuidi, Jr., E. J., "Centrifugation of Waste Sludges."
Paper presented at the 38th WPCF Conference (Oct. 1965)
127. Griffin, G. E. and Brown, J. M., "Studies on Concentrating
Digested Sludge Prior to Barging." Paper presented at the
38th WPCF Conference (Oct. 1965).
-------
-13-
Sludge Thickening - Screening and Biologic Flotation
128. Kiess, F., "Sludge Dewatering by Vibrating Screens."
Water and Sewage Works, p. 479 (Nov. 1959).
129. Laboon, J. F., "Experimental Studies on the Concentration
of Raw Sludge." Sewage and Industrial Wastes, Vol. 24,
No. 4, pp. 423-444, (1952).
130. Laboon, J. F., "Further Investigations of Concentration
of Raw Sludge." Froc. of the ASCE, Sep. No. 314. Vol. 79,
(Oct. 1953).
131. Laboon, J. F., "Pittsburgh Plans Unique Project to Abate
Stream Pollution." Civil Engr., Vol. 24, No. 1, pp. 44,45,
100, (Jan. 1954).
132. Spohr, G. and Eckenfelder, Jr., W. W., "Sewage Sludge Thicken-
ing by Mechanical Vibration." Public Works, p. Ill (Mar.
1958).
133. "SWECO Vibro-Energy Separator," Bulletin No. IM 4-6 Yr.,
Southwestern Engineering Co., Los Angeles, Calif, (1964).
134. Hulzebos, W., "Artificial Dewatering of Sewage Sludge on
a Dynamic Basis." Water, Vol. 47, pp. 42-45, 1963.
-------
-14-
Sludge Digestion - Anaerobic
135. Schreiber, H. A., "Digester Mixing." Journal Water
Pollution Control Federation. Vol. 34, No. 5, pp. 513-516,
(1962).
136. Hindin, E. and Dunstan, G., "Effect of Detention Time on
Anaerobic Digestion." Journal Water Pollution Control
Federation, Vol. 32, No. 9, pp. 930-938, (1960).
137. Lohmeyer, G. T., "A Review of Sludge Digestion." Sewage
and Industrial Wastes. Vol. 31, No. 2, pp. 221-234, (1959).
138. Gould, R. H., "Economical Practices in the Activated Sludge
and Sludge Digestion Processes." Sewage and Industrial
Wastes, Vol. 31, No. 4, pp. 399-405, (1959).
139 Blidgett, J. H., "Accelerated Sludge Digestion Experience
at Columbus, Ohio." Sewage and Industrial Wastes. Vol. 28,
No. 7, pp. 926-931 (July 1952).
140. Garber, W. F., "Plant-Scale Studies of Thermophilic Digestion
at Los Angeles." Sewage and Industrial Wastes, Vol. 26,
No. 6, pp. 1202-1216, (1954).
141. Morgan, P. F., "Studies of Accelerated Digestion of Sewage
Sludge." Sewage and Industrial Wastes. Vol. 26, No. 4,
pp. 462-478, (1954).
142. Torpey, W. N., "High Rate Digestion of Concentrated Primary
and Activated Sludge." Sewage and Industrial Wastes, Vol. 26,
No. 4, pp. 479-496, (1954JT
143. "High-Rate Digester Design Developed by Plant Studies,"
Editors, Wastes Engr., pp. 152-153 (Mar. 1960).
144. Steffen, A. J. and Lemen, R. M., "Degasifying Digester
Liquor." Wastes Engr., pp. 400-402 (Aug. 1961).
145. Voshel, D., "Gas Recirculation and CRP Operation." Wastes
Engr., p. 452 (Sept. 1963).
146. Spohr, G., "Electrical Stimulation of Bacteria in Wastewater
and Anaerobic Digestion." Water and Sewage Works, Vol. 112,
No. 5, pp. 168-170, (1965)
147. Suhr, C. J. and Brown, J. M., "High-Rate Digestion Tamed."
Water Works and Wastes Engineering. Vol. 1, No. 8, pp. 44-46,
70, (1964).
-------
-15-
148. Rasmussen, A. E., "Digesters Beat Incineration." The
American City, pp. 100-101 (Dec. 1964).
149. Malina, J. F., "The Effect of Temperature on High Rate
Digestion of Activated Sludge." Proc. of the 16th Purdue
Industrial Waste Conference, pp. 232-250, (1961).
150. Garrison, W. E. and others, "Gas Recirculation-Natural,
Artificial." Water Works and Wastes Engineering. Vol. 1,
No. 2,, pp. 58-63, (1964).
151. Spitzer, E. F., "Digesters vs. Vacuum Filtration." The
American City, Vol. 77, No. 3, pp. 96-98, (March 1962).
152. Davis, E. R., "Sludge Digestion or Incinceration." Proc.
of the ASCE - Sanitary Engr. Div., Proc. Paper No. 1173,
Vol. 83, No. SA1 (Feb. 1957).
153. Sawyer, C. N. and Grumbling, J. S., "Fundamental Considera-
tions in High Rate Digestion." Proc. of the ASCE - San.
Engr. Div.. Vol. 86, pp. 49-63 (Mar. 1960).
154. Estrada, A. A., "Design and Cost Considerations in High
Rate Sludge Digestion." Proc. of the ASCE - San. Engr.
Div.. Vol. 86, pp. 111-127 (May. 1960)
155. Rawn, A. M. and Candel, E. J., "Some Effects of Anaerobic
Digestion on Sewage Sludge." Proc. of the ASCE - San. Engr.
Div., Vol. 74, pp. 1467-1479 (Nov. 1948).
156. "Built-in Convertibility Cuts Cost of Expanding a Sewage
Plant." Editors, Engineering News-Record (Nov. 15, 1962).
157. Preliminary Report, WPCF Technical Practices Comm. -
Sludge Digestion (Oct. 1964)
158. Lynam, B. and others, "Start-Up and Operation of Two New High
Rate Digestion Systems." A Paper Presented at the 38th WPCF
Conference, 1965.
159. Agardy, F. J. and Shepherd, W. C., "DNA - A Rational Basis
for Digester Loadings." Journal Water Pollution Control
Federation. Vol. 37, pp. 1236-1242, (1965).
-------
-16-
Sludge Digestion - Aerobic
160. McPhee, W. T., "Duquesne, Pa., Breaks Precedent." The
American City, pp. 93-96 (Oct. 1965).
161. Dreier, D. E., "Aerobic Digestion of Solids." Proc. of
the 18th Purdue Industrial Wastes Conference, pp. 123-140, 1963.
162. Morris, G. L. and others, "Study of an Extended Aeration
Plant and Effluent Effect on the Receiving Water Course."
Proc. of the 18th Purdue Industrial Waste Conference,
pp. 331-347, 1963.
163. Carpenter, W. L. and Blosser, R. 0., "Aerobic Decomposition
of Secondary Papermill Sludges." Proc. of the 17th Purdue
Industrial Waste Conference, pp. 126-135, 1962.
164. Barnhart, E. L., "Application of Aerobic Digestion to
Industrial Waste Treatment." Proc. of the 16th Purdue Industrial
Waste Conference, pp. 612-618, 1961.
165. Lawton, G. W. and Norman, J. D., "Aerobic Sludge Digestion
Studies." Journal Water Pollution Control Federation, Vol. 36,
No. 4, pp. 495-504, (1964).
166. Viraraghavan, V., "Digesting Sludge by Aeration." Water Works
and Wastes Engr.. Vol. 2, No. 9, pp. 86-89, (1965).
167. Dreier, D. E. and Obma, C. A., "Aerobic Digestion of Solids."
Walker Proc. Equip. Co. Bulletin No. 26-S-18194, (Jan. 1963).
168. Loehr, R. C., "Aerobic Digestion - Factors Affecting Design."
Paper Presented at the 9th Great Plains Sew. Works Design
Conference (Mar. 1965).
-------
-17-
Sludge Conditioning - Elutriation
169. Chasick, A. H. and Dewling, R. T., "Interstage Elutriation
of Digested Sludge." Journal Water Pollution Control
Federation. Vol. 34, No. 4, pp. 390-400, (1962).
170. Sparr, A. E., "Elutriation Experience at the Bay Park
Sewage Treatment Plant." Sewage and Industrial Wastes,
Vol. 26, No. 6, pp. 1443-1449 (Dec. 1954).
171. Torpey, W. N. and Lang, M., "Elutriation as a Substitute
for Secondary Digestion." Sewage and Industrial Wastes,
Vol. 24, No. 7, pp. 813-825, (1952).
172. Center, A. L., "Elutriation - How it Aids in Dewatering
Sludge." Reprint from Public Works Magazine. 1934.
173. Fraschina, K., "Sludge Elutriation at the Richmond-Sunset
Plant, San Francisco, California." Sewage and Industrial
Wastes. Vol. 22, No. 11, pp. 1413-1416, (1950).
174. Center, A. L., "Small Sludge Elutriation Plants." Sewage
Works Journal, Vol. 18, No. 1, pp. 26-45, (1946).
175. McNamee, P.D., "Filtration Tests with Elutriated and
Unelutriated Digested Solids." Sewage Works Journal, Vol. 11,
No. 5, pp. 764-773, (1939).
176. "Interstage Elutriation Thickens Digested Sludge to 770,"
Editors, Wastes Engr.. p. 193 (Apr. 1961).
177. Center, A. L., "Elutriation - How it Aids in Dewatering
Sludge." Public Works, 1934.
178. Bushee, R. J., "Evaluation of a Sewage Plant." Wastes
Engineering, p. 24 (Jan. 1961).
-------
-18-
Sludge Conditioning - Chemicals and Other Materials
179. Literature Review, Journal Water Pollution Control
Federation, Vol. 35, No. 5, p. 575, (1963).
180. Bargman, R. D., Garber, W. F., and Nagano, J., "Sludge
Filtration and the Use of Synthetic Organic Coagulants
at Hyperion." Sewage and Industrial Wastes, Vol. 30,
No. 9, pp. 1079-1099 (Sept. 1958).
181. Stokes, F. E. and Harwood, J. H., "Aluminum Chlorohydrate
in Sludge Treatment." Effluent and Water Treatment
Journal. Vol. 4, No. 7, pp. 329-332, (1964)
182. Sherbeck, J. M., "Synthetic Organic Flocculants Used for
Sludge Conditioning." Journal Water Pollution Control
Federation, Vol. 37, No. 8, pp. 1180-1183, (1965).
183. Morris, R. H., "Polymer Conditioned Sludge Filtration."
Water Works and Wastes Engineering (Mar. 1965).
184. Center, A. L., "Computing Coagulant Requirements in
Sludge Conditioning." Proc. ASCE. 71, 307-30, 1945.
185. Jones, F. W. and Faber, H. A., "Discussion of Progress
in the Conditioning of Sewage Sludges for Dewatering."
Sewage Works Journal. Vol. 13, No. 1, pp. 101-104, (1941)
186. Reefer, C. E., "The Manufacture of Chlorinated Copperas at
the Baltimore Sewage Works." Sewage Works Journal, Vol. 12,
No. 6, pp. 1113-1115, (1940).
187. Agar, C. C., "Chemical Treatment of Sludge to Facilitate
Disposal." Sewage Works Journal, Vol. 5, No. 2, pp. 270-
277, (1933).
188. Sperry, W. A., "Alum Treatment of Digested Sludge to Hasten
Dewatering." Sewage Works Journal, Vol. 13, No. 5, pp. 855-
867, (1941).
189. Reefer, C. E., "Conditioning Sludge with Aluminum Chlorohydrate
for Vacuum Dewatering." Wastes Engr., pp. 380-382 (July 1960).
190. Rilbride, A. J., "Use of Waste Chemical Cuts Cost of Sludge
Conditioning." Wastes Engr., p. 568 (Oct. 1961).
191. "Lime Aids Sludge Thickening," Editors, Wastes Engr., p. 101,
(Feb. 1958).
-------
-19-
192. Thompson, W. E., "Alum Speeds Up Drying of Sludge."
Wastes Engr.. pp. 297-300 (June 1955).
193. Schaffer, R. B., "Polyelectrolytes in Industrial
Waste Treatment." Prog, oj the 18th Purdue Industrial
Waste Conference, pp. 447-459, 1963.
194. Caron, A. L. and Carpenter, W. L., "Effects of Polyelec-
trolytes on Primary Deinking and Boardmill Sludge and on
Effluent Clarification of Deinking Effluent." Proc. of
the 19th Purdue Industrial Waste Conference, pp. 139-145,
1964.
195. Mohlman, F. W. and Edwards, G. P., "Prefiltration Treatment
of Sewage Sludge." Industrial and Engr. Chem., Vol. 26,
No. 2, pp. 226-230, (Feb. 1934).
196. Carpenter, W. L. and others, "Evaluation of Fly Ash as a
Precoat on Vacuum Sludge Filters." Paper Trade Journal,
Vol. 147, No. 21, pp. 26-31, (May 1963).
197. Burd, R. S., "Use of New Polyelectrolytes in Sewage Sludge
Conditioning." Proc. of the 2nd Vanderbilt San. Engr. Conf.
(May 1963).
198. Kelman, S. and Priesing, C. P., "Polyelectrolyte Flocculation
- Sand Bed Dewatering." Paper presented at the Michigan WPCA
Conf. (June 1964).
199. The Dow Chemical Company Bulletin No. 164-133-464, "Purifloc
C 31 for Slurry Sand Bed Dewatering," 1965.
200. The Dow Chemical Company Bulletin No. 125-440-64, "Sludge
Conditioning with Purifloc C 31," 1965.
201. Croft, D. K., "Waste Activated Sludge Thickening at Hagerstown,
Md." Sewage and Industrial Wastes, Vol. 21, No. 5, pp. 892-896,
(1949).
202. Keefer, C. E., "Improvements and Operation at Baltimore's Back
River Sewage Works." Journal Water Pollution Control Federation,
Vol. 33, No. 1, pp. 22-47, (1961).
203. Goodman, B. L., "Chemical Conditioning of Sludges: Six Case
Histories." Water and Wastes Engr., Vol. 3, No. 2, pp. 62-65,
(Feb. 1966).
-------
-20-
Sludge Dewatering - Sand Beds and Lagoons
204. Bubbis, N. S., "Sludge Drying Lagoons at Winnipeg."
Journal Water Pollution Control Federation. Vol. 34, No. 8,
pp. 830-832 (Aug. 1962).
205. Jeffrey, E. A., "Dewatering Rates for Digested Sludge in
Lagoons." Journal Water Pollution Control Federation. Vol. 32,
No. 11, pp. 1153-1160, (1960).
206. Howells, D. H. and Dubois, D. P., "The Design and Cost of
Stabilization Ponds in the Midwest." Sewage and Industrial
Wastes, Vol. 31, No. 7, pp. 811-818, (July 1959).
207. Bowers, M., "Tips on Sludge Drying Bed Care." Sewage and
Industrial Wastes. Vol. 29, No. 7, pp. 835-836 (July 1957).
208. Lynd, E. R. , "Asphalt-Paved Sludge Drying Beds." Sewage
and Industrial Wastes. Vol. 28, No. 5, pp. 697-699 (May 1956).
209. Haseltine, T. R., "Measurement of Sludge Drying Bed
Performance." Sewage and Industrial Wastes. Vol. 23, No. 9,
pp. 1065-1083 (Sept. 1951).
210. "Sludge Lagoons," Report of the Comtn. on Sewage Disposal,
Engr. Sect., APHA. Sewage Works Journal. Vol. 20, No. 5, pp. 817-
831, (1948).
211. Lubow, L. A., "Drying of Sludge on Heated Sludge Beds." Journal
N. Carolina Section AWWA. Vol. 16, 118, 1941.
212. Ryan, W. A., "Operation of Open Sludge Beds." Sewage Works
Journal. Vol. 10, No. 1, pp. 153-158, (1938).
213. Carpenter, L. V., "Sludge Drying on Open and Covered Drying
Beds." Sewage Works Journal. Vol. 10, No. 3, pp. 503-512, (1938),
214. Martin, G., "Second Stage Digestion Eliminated by Lagooning."
Wastes Engr., pp. 538-539 (Oct. 1961).
215. Grove, M., "Do Concrete Bottom Sludge Drying Beds Work?" 47th
Annual Short School, Water and Sewage Works Assoc., Texas, 1965.
216. Jeffrey, E. A., "Laboratory Study of Dewatering Rates for
Digested Sludge in Lagoons." Proc. of the 14th Purdue Industrial
Waste Conference, pp. 359-384, 1959.
-------
-21-
217. Quon, J. E. and Ward, G. B., "Convective Drying of Sewage
Sludge." International Journal of Air and Water Pollution.
Vol. 9, pp. 311-322 (May 1955).
218. Swanwick, J. D. and Baskerville, R. C., "Dewatering of
Industrial Sludges on Drying Beds." Chemistry and Industry,
p. 338 (Feb. 20, 1965).
219. "Large Scale Sludge Drying in Thin Beds," Editors, Engineer-
ing, pp. 206-207, (Feb. 9, 1962).
220. Imhoff, K., "Natural Processes for Sludge Dehydration are
Still Economic." Gash-Wasserfach. Vol. 105, No. 26, pp. 710-
715, 1964.
221. Swanwick, J. D. and Baskerville, R. C., "Sludge Dewatering
on Drying Beds." Paper presented at London Int. Engr. Exhib.
(Apr. 1965).
222. ASCE Manual on Sewage Treatment Plant Design, New York City,
1959.
223. Kershaw, M. A., "Developments in Sludge Treatment and Disposal
at the Maple Lodge Works, England." Journal Water Pollution
Control Federation, Vol. 37, No. 5, pp. 674-691 (May 1965).
224. Vogler, J. F. and Rudolfs, W., "Factors Involved in the
Drainage of White-Water Sludge." Proc. of the 5th Purdue
Industrial Waste Conference, pp. 305-315, 1949.
225. Water Pollution Research - 1964, Ministry of Technology, Her
Majesty's Stationery Office (London), 108-110, (1964).
-------
-22-
Sludge Dewatering - Vacuum Filters
226. Simpson, G. D., "Operation of Vacuum Filters." Journal
Water Pollution Control Federation, Vol. 36, No. 12, pp. 1460-
1467 (Dec. 1964).
227. Trubnick, E. H. and Mueller, P. K., "Sludge Dewatering
Practice." Sewage and Industrial Wastes, Vol. 30, No. 11,
pp. 1364-1378 (Nov. 1958).
228. Dahlstrom, D. A. and Cornell, C. F., "Improved Equipment for
Vacuum Filtration of Sludge." Sewage and Industrial Wastes,
Vol. 30, No. 7, pp. 891-900 (July 1958).
229. Martin, D. W. and Mayo, E., "Sludge Dewatering at Union City,
Tennessee." Sewage and Industrial Wastes, Vol. 29, No. 5,
pp. 594-601 (May 1957).
230. Zablatzky, Herman R., "Disposal of Industrial Waste Treatment
Plant Sludge." Part II. Journal Water Pollution Control
Federation. Vol. 37, No. 4, pp. 535-563 (Apr. 1965).
231. Jones, B. R. S., "Vacuum Sludge Filtration. II. Prediction
of Filter Performance." Sewage and Industrial Wastes, Vol. 28,
No. 9, pp. 1103-1115 (Sept. 1956).
232. Schempman, B. A. and Cornell, C. F., "Fundamental Operating
Variables in Sewage Sludge Filtration." Sewage and Industrial
Wastes, Vol. 28, No. 12, pp. 1443-1460 (Dec. 1956).
233. Coakley, P. and Jones, B.R.S., "Vacuum Sludge Filtration. I.
Interpretation of Results by the Concept of Specific Resistance."
Sewage and Industrial Wastes, Vol. 28, No. 8, pp. 963-976
(Aug. 1956).
234. Spaeder, H. J., "Fresh Solids Dewatering as a Unit Operation."
Proc. of the 8th Southern Municipal and Industrial Wastes
Conference, pp. 192-197, 1959.
235. "Sewage Sludge Filter Blankets," Editors, Water and Sewage
Works, pp. 217-221 (May 1955).
236. Beck, A. J., Sakellarian, E. N., and Krup, M., "A Method for
Evaluating the Variable in Vacuum Filtration of Sludge." Sewage
and Industrial Wastes, Vol. 27, No. 6, pp. 689-705 (June 1955).
237. Halff, A. H., "An Investigation of the Rotary Vacuum Filter
Cycle as Applied to Sewage Sludge." Sewage and Industrial
Wastes, Vol. 24, No. 8, pp. 962-984 (Aug. 1952).
-------
-23-
238. Joseph, A. B., "Comparative Study of Filter Cloths." Sewage
and Industrial Wastes, Vol. 23, No. 8, pp. 977-981 (Aug. 1951).
239. Wishart, J. M. and others, "Dewatering of Sewage Sludge by
Coagulation and Vacuum Filtration, Part I Laboratory Experi-
ments." Report of the Rivers Cotmn. of the Manchester Corp.
Brit. (1959).
240. Center, A. L., "Principles and Factors Influencing Vacuum
Filtration of Sludge." Sewage Works Journal, Vol. 13, No. 6,
pp. 1164-1208, (1941).
241. VanKleeck, L. W., "Vacuum Filtration of Sludge." Sewage
Works Journal, Vol. 10, No. 6, pp. 949-974, (1938)1
242. Schepman, B. A., "Designing Vacuum Filter Systems to Fit the
Type of Sludge." Wastes Engr., pp. 162-165 (Apr. 1956).
243. Brown, J. M., "Vacuum Filtration of Digested Sludge'J Water
and Sewage Works, Vol. 107, No. 5, pp. 193-195, (1960)1
244. Trubnick, E. H., "Vacuum Filtration of Raw Sludge." Water and
Sewage Works, Vol. 107, Ref. No., pp. R-287-R-292(196U71
245. Emmett, R. C. and Dahlstrom, D. A., "Top Feed Filtration and
Drying." Chemical Engr., Vol. 57, No. 7, pp. 63-67, (July 1950).
246. "Five Factors in Evaluating Vacuum Filter Performance,"
Editors, Wastes Engr., pp. 296-297 (June 1963).
247. Cornell, C. F. and Dahlstrom, D. A., "The Belt Filter Concept."
Chemical Engr. Progress, Vol. 55, No. 12, pp. 68-73, (Dec. 1959).
248. Trubnick, E. N., "Simplified Sludge Handling at Lower Cost."
Public Works, pp. 116-118 (May 1956).
249. Carpenter, W. L. and Lardieri, N. J., "Review of Current
Experimentation on Dewatering of Paper Mill Sludges." Proc. of
the 18th Purdue Industrial Wastes Conference, pp. 9-19, 1963.
250. Merman, R. G. and others, "Sludge Disposal at a Philadelphia
Refinery." Journal Water Pollution Control Federation, Vol. 33,
No. 11, pp. 1153-1165.
251. Jones, W. H., "Sewage Sludge Dewatering - Selection/Sizing
and Application of Equipment." A Paper Presented at the 9th
Great Plains Sewage Works Design Conf.(March 1965).
-------
-24-
252. Eimco Corp. Bulletin No. F-2067A-10M-1-62-E.
253. Burd, R. S., Personal Communication, The Dow Chemical
Company, 1964.
254. Priesing, C. P. and Mogelnicki, S. J., Personal Communica-
tion, 1965.
-------
-25-
Sludge Dewatering - Centrifuges
255. White, W. F. and Burns, T. E., "Continuous Centrifugal
Treatment of Sewage Sludge." Water and Sewage Works
(Oct. 1960).
256. Jenks, J. H., "Continuous Centrifuging Used to Dewater
Variety of Sludges." Wastes Engr.. pp. 360-361 (July 1958).
257. Bischofsberger, W. and Stalmann, V., "Investigation of the
Dewatering of Biological Sludge With a New Centrifuge
Apparatus." Water and Sewage Works, pp. 411-414 (Nov. 1963).
258. White, W. F. and Burns, T. E., "Continuous Centrifugal
Treatment of Sewage Sludge." Water and Sewage Works, pp.
384-386 (Oct. 1962).
259. White, W. F., "Centrifugal Treatment of Industrial Waste
Waters." Industrial Water and Wastes. Vol. 8, No. 6, pp. 40-
42, (Nov-Dec. 1963).
260. Blosser, R. 0., "Centrifugal Dewatering of Paper Mill
Sludges." Proc. of the 15th Purdue Industrial Waste
Conference, pp. 515-528, 1960.
261. Bamford, R. A., "Centrifugal Dewatering of Paper Mill Waste."
TAPPI, First Water Conf. Cincinnati, Ohio (June 1963).
262. Amero, C. L., "Application of Field Data for Selection of
a Centrifuge to Dewater Municipal Sewage Sludges." Paper
presented at the 38th WPCF Conf. (Oct. 1965).
263. Jenkins, A., "Dewatering and Disposal of Paper Mill Wastes."
Paper Trade Journal, Vol. 147, No. 34, pp. 30-32.
264. Caron, A. L. and Blosser, R. 0., "Centrifugal Dewatering
of Primary Paper Industry Sludges." Proc. of the 20th Purdue
Ind. Wastes Conf. (May.1965).
-------
-26-
Sludge Conditioning and Dewatering - Other Processes
265. Bruce, A. and others, "Further Work of the Sludge Freezing
Process." Surveyor, Vol. 112, p. 849 (Dec. 5, 1953).
266. "Sludge Dewatering by the Porteous Process," Editors, Water
and Sewage (Toronto), Vol. 82, p. 71, (Feb. 1944).
267. Clements, G. S. and others, "Sludge Dewatering by Freezing
With Added Chemicals." Journal Inst. of Sew. Purif., Part 4,
pp. 318-337, 1950.
268. Cooling, L. F. and others, "Dewatering of Sewage Sludge by
Electro-Osmosis." Water and San. Engr., Vol. 3, No. 7, p. 246,
1952.
269. Stallery, R. H. and Eauth, E. H., "Treatment of Sewage Sludge
by the McDonald Process." Public Works, p. Ill (Mar. 1957).
270. Husmann, W., "Effect of Supersonics in Sewage and Sludge."
Gesundh. Ing., Vol. 73, 7/8, 127, 1952.
271. Clements, G. S. and others, "Sludge Dewatering by Freezing
With Added Chemicals." Water and San. Engr. (Brit.), 1, 7
and 8, p. 280 (Nov. 1950).
272. "Sludge Dewatering by the Porteous Process," Editors, Water
and Sewage (Toronto, Vol. 82, p. 17 (Feb. 1944).
273. Beaudoin, R. E., "Reduction of Moisture in Activated Sludge
Filter Cake by Electro-Osmosis." Sewage Works Journal, Vol. 15,
No. 6, pp. 1153-1163, (1943).
274. Slagle, E. A. and Roberts, L. M., "Treatment of Sewage and
Sewage Sludge by Electrodialysis." Sewage Works Journal. Vol. 14,
No. 5, pp. 1021-1029, (1942).
275. Lamb, C., "Heat Treatment as an Aid to Sludge Dewatering - Ten
Year's Full-Scale Operation." Journal Inst. of Sew. Purif.,
Part I, pp. 5-15, 1951.
276. Jepson, C. and Klein, L., "Heat Treatment of Sewage Sludge."
Journal Inst. of Sew. Purif.. Part I, pp. 36-45, 1951.
-------
-27-
277. Coackley, P., "Research on Sewage Sludge Carried Out
In the Civil Engr. Dept. of University College." Journal
Inst. of Sew. Purif.. Part I, pp. 59-72, 1955.
278. Smith, E. G., "Features of a Mechanical Sludge Concentrator
for Dewatering Sludge." Sewage and Industrial Wastes,
Vol. 29, No. 5, pp. 601-606 (May 1957).
279. MacNeal, J. A., "Solids Dewatering at Downington Paper
Company." TAPPI, Vol. 47, No. 1, pp. 189A-191A, (Jan. 1964).
280. Thompson, J. T. and Proctor, J. W., "Filter Pressing of
Sludge." The Surveyor, Vol. 93, pp. 235-236, 1938.
281. Molyneux, F., "Continuous Pressure Filtration and Adiabatic
Drying of Sludge." Fluid Handling, n!64, pp. 305-307 (Sept.
1963).
282. Valente, G. A., "A New Idea in Sludge Dewatering." The
American City, pp. 95-98 (July 1965).
283. "Nichols-Roto-Plug Sludge,Concentrator," Bulletin No. 254,
Nichols Engr. and Research Corp., New York, (1963).
284. Triebel, W., "Press Drying of Sewage Sludge." Gas-Wasserfach,
Vol. 105, No. 2, pp. 35-38, 1964 (Germany).
285. Wegmann, E., "Dehydration of Sewage Sludge." Industrieabwaesser,
May 1962 (Germany).
286. "Sludge Concentration by Freezing," Editors, Water and Sewage
Works (Nov. 1965).
-------
-28-
Heat Drying of Sludge and Incineration
287. Groen, M. A., "Sludge Drying and Incineration at Dearborn,
Michigan." Sewage and Industrial Wastes, Vol. 31, No. 12,
pp. 1432-1434 (Dec. 1959).
288. Martin, R. and Bryden, W., "An Assessment: Drying versus
Pyrolysis of Sewage Sludge." Journal and Proc. Inst. Sewage
Purif., Part 3, p. 307 (1960).
289. "Sludge Drying," Editors, Sewage and Industrial Wastes,
Vol. 31, No. 2, p. 239, (1959)
290. Frazee, J., "Sludge Filtration and Incineration at Camden,
New Jersey." Sewage and Industrial Wastes, Vol. 31, No. 10,
pp. 1224-1230, (1959).
291. Nickerson, R. D., "Sludge Drying and Incineration." Journal
Water Pollution Control Federation, Vol. 32, No. 1, pp. 90-98,
(1960).
292. Pettit, C.,"20 Years of Sewage Sludge Burning at Barberton,
Ohio." Journal San. Engr. Div., ASCE, Vol. 85, SA6, Part
I, pp. 17-24 (Nov. 1959).
293. Hurwitz, E. and Dundas, W., "Wet Oxidation of Sewage Sludge."
Journal Water Pollution Control Federation, Vol. 32, No. 9,
pp. 918-929, (1960).
294. Moren, R. J. and others, "Oxidation and Stabilization of
Sewage Sludges with Oxygen at Elevated Temperatures and
Pressures." Sewage and Industrial Wastes, Vol. 26, No. 6,
pp. 1450-1452, (1954).
295. Harding, J. C. and Griffin, G. E., "Sludge Disposal by Wet
Air Oxidation at a Five MGD Plant." Journal Water Pollution
Control Federation. Vol. 37, No. 8, pp. 1134-1141 (Aug. 1965).
296. Schroepfer, G. J., "Incinerator Problems." Sewage Works
Journal, Vol. 19, No. 4, pp. 559-579, (1946).
297. Smith, A. R., "Incineration of Sludge - Some Governing
Principles." Sewage Works Journal, Vol. 11, No. 1, pp. 35-42,
(1939).
-------
-29-
298. "Atomized Suspension Method Destroys Sewage Sludges,"
Editors, Wastes Engineering, p. 156 (Mar. 1960).
299. Hurwitz, E. M. and Dundas, W. A., "What are the Residues of
Wet Combustion of Sludge?" Wastes Engineering, p. 168 (Mar.
1960).
300. Owen, M. B., "Sewage Solids Combustion." Water and Sewage
Works, Vol. 106, No.,10, pp. 442-444 (Oct. 1959).
301. Dewatering, Drying and Incineration," Editors, Water and
Sewage Works. Vol. 106, Ref. No. p. R-394, (1959).
302. Hampton, R. K., "Methods of Handling Dried or Incinerated
Sewage Solids." Water and Sewage Works, Vol. 105, Ref. No.
pp. R-321- R-325, (1958)
303. Mick, K. L. and Linsley, S. E., "Sewage Solids Incineration
Costs." Water and Sewage Works, Vol. 105, Ref. No.,pp.R-327 -
R-337, (1958).
304. Hurwitz, E. and others, "Wet Air Oxidation of Sewage Sludge."
Water and Sewage Works, Vol. 112, No. 8, pp. 298-305, (Aug. 1965).
305. Teletzke, G. H., "Wet Air Oxidation." Chemical Engr. Prog.,
Vol. 60, No. 1, pp. 33-38, Jan. 1964).
306. Clinton, M. 0., "Experience With Incineration of Industrial
Waste and Sewage Sludge Cake With Municipal Refuse." Proc. of
the 14th Purdue Industrial Waste Conference, p. 155, 1959.
307. Hurwitz, E. and Dundas, W. A., "Ultimate Disposal of Sewage
Sludge by Wet Oxidation." Proc. of the 14th Purdue Industrial
Waste Conference, pp. 211-226, 1959.
308. Sercu, C. L., "New Incineration Facilities at Dow, Midland."
Proc. of the 14th Purdue Industrial Waste Conference, pp. 612-
621 (1959).
309. Finder, K. L. and Gauvin, W. H., "Applications of the Atomized
Suspension Technique to the Treatment of Waste Effluents." Proc.
of the 12th Purdue Industrial Waste Conference, pp. 217-249 (1957).
310. "Municipal Incineration of Refuse," Progress Report of the
Committee on Municipal Refuse Practices, Proc. of the ASCE,
Sanitary Engr. Div., Vol. 90, SA3, pp. 13-26 (1964).
311. "Sludge Treatment and Disposal by the Zimmerman Process,"
Sanitary Engr. Comm. Report, Proc. of the ASCE, Sanitary Engr.,
Div., Vol. 85, SA4, pp. 13-23 (1959).
-------
-30-
312. "Wet Combustion of Sewage Sludge Solves Disposal
Problems," Editors, Chemical Engineering, Vol. 71, pp. 118-
120 (May 25, 1964).
313. "Omaha Plans Paunch Manure Incinerator," Editors, Engr.
News-Record, pp. 59-62 (Nov. 12, 1964).
314. "Atomized Suspension Adopted for Sludge," Editors, Chem.
and Engr. News, p. 79 (Sept. 14, 1964).
315. Irving, C. E., "Flash Drying and Incineration." Water
Works and Wastes Engr.. Vol. 2, No. 9, pp. 70-72, (1965).
316. Helfgott, T. and Webber, P., "Atomized Suspension
Technique Incinerates Sewage Sludge." Water Works and
Wastes Engr.. Vol. 2, No. 9, pp. 76-79, (1965).
317. Sohr, W. H. and others, "Fluidized Sewage Sludge Combustion."
Water Works and Wastes Engr., Vol. 2, No. 9, pp. 90-93,
(1965).
318. McKinley, J. B., "Wet Air Oxidation Process." Water Works
and Wastes Engr., Vol. 2, No. 9, pp. 97-99, (1965).
319. Bartlett-Snow-Pacific, Inc. Bulletin No. HI-2M-1-65, (1964).
320. Bartlett-Snow-Pacific, Inc. Bulletin No. SllO, 1965.
321. Owen, M. B., "Sludge Incineration." Proc. of the ASCE, San.
Engr. Div., Proc. Paper No. 1172 (Feb. 1957).
322. Reilly, B. B., "Incinerator and Sewage Treatment Plant Work
Together." Public Works, p. 109 (July 1961).
323. Quirk, T. P., "Economic Aspects of Incineration vs. Incinera-
tion-Drying." Journal Water Pollution Control Federation,
Vol. 36, No. 11, pp. 1355-1368, (1964).
324. "New Sludge Disposer," Editors, Engr. News-Record, p. 20
(Aug. 9, 1962).
325. Coogan, F. J. and Stovall, J. H., "Incineration of Sludge
from Kraft Pulp Mill Effluents." TAPPI. Vol. 48, No. pp. 94A-
96A (June 1965).
-------
-31-
326. Zimpro, Wet Air Oxidation Unit, Sterling Drug Inc., 1964.
327. Walters, W. R. and Ettelt, G., "Dewatering of the Ash By-
Product From the Wet Oxidation Process." Paper presented
at the 1965 Purdue Industrial Waste Conference, (1965).
328. Teletzke, G. H., "Wet Air Oxidation of Sewage Sludge."
A Paper presented at the 16th University of Kansas Sanitary
Engr. Conf., 1966.
329. Weller, L. and Condon, W. , "Problems in Designing Systems
for Sludge Incineration." A paper presented at the 16th
University of Kansas Sanitary jSngr^ Conf., 1966.
330. Albertson, 0. E., "Low-Cost Combustion of Sewage Sludges."
Proc . of the 9th Great Plains Sew. Works Design Conf. ,
1965.
331. "The Dorr-Oliver FS Disposal System," Bulletin No. 6051,
Dorr-Oliver Inc., Stamford, Connecticut, (1965).
332. Smith, R. M. , Personal Communication, The Dow Chemical
Company, 1960.
333. Davy, T. E., "Operation and Maintenance of Incineration
Systems." A Paper presented at the 16th University of Kansas
San. Engr. Conf. , 1966.
-------
-32-
Sludge Disinfection and Odor Control
334. Cornell, C. H. and Garrett, M. T., "Disinfection Effec-
tiveness of Heat Drying ofSludge." Journal Water Pollution
Control Federation. Vol. 35, No. 10, pp. 1262-1268 (Oct. 1963).
335. Wiley, j. 3., "Pathogen Survival in Composting Municipal
Wastes." Journal Water Pollution Control Federation, Vol. 34,
No. 1, pp. 80-90 (Jan. 1962).
336. McKinney, R. E. and others, "Survival of Salmonella Typhosa
During Anaerobic Digestion." Sewage and Industrial Wastes,
Vol. 31, No. 1, pp. 23-32, (1959).
337. Sawyer, C. N. and Kahn, P. A., "Temperature Requirements for
Odor Destruction in Sludge Incineration." Journal Water
Pollution Control Federation, Vol. 32, No. 12, pp. 1274-1278,
(1960).
338. "Odors and Their Control," Editors, Journal Water Pollution
Control Federation, Vol. 36, No. 11, pp. 1424-1426, (1964).
339. Eliassen, R. and Vath, C., "Air Pollution Control in Sewage
Treatment Plants." Journal Water Pollution Control Federation,
Vol. 32, No. 4, pp. 424-426, (1960).
340. Post, N., "Counteraction of Sewage Odors." Sewage and
Industrial Wastes. Vol. 28, No. 2, pp. 221-225 (Feb. 1956).
341. Keller, P., "The Influence of Heat Treatment on the Ova of
Ascaris Lumbricoides in Sewage." Journal and Proc. Inst. Sew.
Purif.. Part 1, p. 100 (1951).
342. Ammen, F. V. and Wieselsberger, F., "Chlorination of Digested
Municipal Sewage Sludge." Gesundh, Ing., Vol. 73, No. 1 and 2,
p. 15 (1952).
343. Keller, P., "Sterilization of Sewage Sludges." Public Health,
(So. Africa) Vol. 15, No. 1, p. 11 (1951).
344. Fuller, J. E. and Litsky, W., "E. Coli in Digested Sludge."
Sewage and Industrial Wastes, Vol. 22, No. 7, pp. 853-859,
(1950).
345. Edwards, G. P., "Sludge Odor Control by Diesel Oil Scrubbing."
Sewage and Industrial Wastes, Vol. 21, No. 5, pp. 795-799,
(1949).
-------
-33-
346. Shook, W. M. and others, "The Bacteriology of Vacuum
Filtered Raw Sludge." Water and Sewage Works, pp. 152-
154 (April 1962).
347. Jaffe, T., "Odor Control in Sewage Treatment." Water and
Sewage Works, Vol. 104, No. 4, pp. 175-178, (1957).
348. Connell, C. H., "Disinfection of Sewage Sludge by Halogens."
Public Works, Vol. 89, No. 12, pp. 106-107, (1958).
349. "Special Incinerator at Sewage Plant Allows Advantageous
Site Selection," Editors, Air Engineering, Vol. 8, No. 4,
pp. 112, 12, 14 (April 1966).
350. Kelman, S. and Priesing, C. P., "Personal Communication,
The Dow Chemical Co., 1964.
-------
-34-
Sludge Disposal - Fertilizer and Soil Condition
351. Compost Science, Editors, Spring 1964.
352. Acevedo-Ramos, G. and others, "Effect of Filter-Press Cake
on Crop Yields and Soil Properties." Compost Science.
Winter 1963, p. 34.
353. Gabaccia, A. J., "Composting Waste Sludge from Pharmaceutical
Mnfr." Compost Science, pp. 8-11, Summer 1960.
354. Compost Science. Editors, Spring 1960.
355. Black, R. J., "Combined Disposal of Sewage Sludge and
Refuse." Compost Science, pp. 16-17, Winter 1962.
356. Mercer, W. A. and others,"Aerobic Composting of Vegetable
and Fruit Wastes." Compost Science. Autumn 1962.
357. Kneiss, I. F., "Combined Sludge-Garbage Composting."
Compost Science, pp. 13-14, Summer 1962.
358. Compton, C. R. and Bowerman, F. R., "Composting Operation
in L. A. County." Compost Science, Winter 1961.
359. Gothard, S. A., "Garbage Processing in Jersey, British
Isles." Compost Science, Spring 1961.
360. Davies, A. G., "Composting Sewage Sludge with Municipal
Refuse." Compost Science, pp. 9-11, Autumn 1960.
361. Olds, J., "How Cities Distribute Sludge as a Soil Conditioner."
Compost Science, pp. 26-30, Autumn 1960.
362. Stone, R., "Economics of Composting Municipal Refuse." ASCE -
Journal San. Engr. Div., Vol. 88, No. SA6, pp. 123-124 (Nov.
1962).
363. Shuval, H. I., "Economics of Composting Municipal Refuse."
ASCE - Journal San. Engr. Div., Part I, Vol. 88, No. SA4,
pp. 47-56 (July 1962).
364. Gotaas, H. B., "Planning Municipal Composting Facilities."
3rd Pittsburgh San. Engr. Conf. Mun. Solid Wastes Disp., 33
(Nov. 1961).
-------
-35-
365. Scott, R. H., "Disposal of High Organic Content Wastes on Land."
Journal Water Pollution Control Federation, Vol. 34, No. 9,
pp. 932-950 (1962)
366. Reeves, J. B., "Sanitary Aspects of Composted Sewage Sludge and Sawdust."
Sewage and Industrial Wastes, Vol. 31, No. 5, pp. 557-563 (May 1959)
367. Anderson, M. S., "Fertilizing Characteristics of Sewage Sludge."
Sewage and Industrial Wastes, Vol. 31, No. 6, pp. 678-682(June 1959)
368. "Third Report on the Sludge of Waste Water Reclamation and Utilization,"
Pub. No. 18 (1957), California State Water Pollution Control Board.
369. "Continued Study of Waste Water Reclamation and Utilization," Pub. No. 15
(1956), California State Water Pollution Control Board.
370. Thompson, R. N. and others, "Spectrographic Analysis of Air-Dried Sewage
Sludge," Journal Water Pollution Control Federation, Vol. 36, No. 6,
pp. 752-759 (June 1964)
371. Rapp, Jr., W. F., "Nitrogen Content of Nebraska Sewage Sludge," Sewage
and Industrial Wastes, Vol. 30, No. 8, pp. 1072-1074 (Aug. 1958)
372. Scanlon, A. J., "Utilization of Sewage Sludge From the Product of Topsoil."
Sewage and Industrial Wastes, Vol. 29, No. 8, pp. 944-950 (Aug. 1957)
373. Anderson, M. S., "Comparative Analyses of Sewage Sludges." Sewage and
Industrial Wastes, Vol. 28, No. 2, pp. 132-135 (Feb. 1956)
374. Olds, J., "The Use and Marketing of Sludge as a Soil Conditioner," Proc.
of the 8th Southern^Municipal and Industrial Wastes Conference, pp. 219-
225 (1959)
375. Fuller, J. E. and Jourdian, G. W., "Effect of Dried Sewage Sludge on
Nitrification in Soil," Sewage and Industrial Wastes, Vol. 27, No. 2,
pp. 161-165 (Feb. 1955)
376. "The Agricultural Use of Sewage Sludge and Sludge Composts", Tech. Comm.
No. 7, Ministry of Agriculture and Fisheries, Great Britain (Oct. 1948)
377. Ullrich, A. H. and Smith, M. W., "Experiments in Composting Digested
Sludge at Austin, Texas," Sewage and Industrial Wastes, Vol. 22, No. 4,
pp. 567-570 (April 1950)
378. Gabaccia, A. J., "Composting Waste Sludge from Pharmaceutical Manufacturing,"
Sewage and Industrial Wastes, Vol. 31, No. 10, pp. 1175-1180 (1959)
-------
-36-
379. "Sewage Sludge as Soil Conditioner," Editors, Water and Sewage Works,
Vol. 106, Ref. No., pp. R-403 - R-424 (1959)
380. Keefer, C. E., "Sludge Granulating Plant Expected to Increase Revenue,"
Public Works, pp. im-117 (Feb. 1965)
381. Olds, J., "Fertilizer from Solid Wastes," Industrial Wastes, pp. D-44-
D-46 (Sept.-Oct. 1958)
382. Lunt, H. A., "The Case for Sludge as a Soil Improver," Water and Sewage
Works, Vol. 100, No. 8, pp. 295-301 (1953)
383. VanKleeck, L. W., "Utilization of Sewage Sludge," Water and Sewage Works,
Ref. Edition, pp. R-203 - R-206 (1953)
384. Anderson, M. S., "Economic Potential in Utilization of Organic Wastes,"
Agriculture Chemicals, pp. 30-33 (Feb. 1959) pp. 41-42 (Mar. 1959)
385. Canham, R. A., "Comminuted Solids Inclusion With Spray Irrigated Canning
Waste," Sewage and Industrial Wastes, Vol. 30, No. 8, pp. 1028-1049
(Aug. 1958)
386. Dundas, W. A. and McLaughlin, C. P., "Experience of Chicago, Illinois in
the Preparation of Fertilizer," Proc. of the ASCE - San. Engr. Div.,
Vol. 69, pp. 80-102 (Jan. 1943)
387. Wylie, J. C., "The Use of Sewage Sludge in Waste Treatment," Pergamon
Press, London (1961)
388. "Sanitary Landfill," ASCE - Manual of Engineering Practice, No. 39, New
York (1959)
389. Nusbaum, I. and Cook, Jr., L., "Making Topsoil with Wet Sludge," Waste
Engineering, pp. 438-440 (Aug. 1960)
390. Toth, S. J., "Using Organic Wastes in Agriculture," Compost Science,
pp. 10-14, Spring 1960.
391. Rodale, R., "Marketing of Large Amounts of Compost," Compost Science,
pp. 15-17, Spring 1960
392. Irving, C. E., "How One City Sells its Sludge," Compost Science,
pp. 18-20, Spring 1960
393. Snell, J. R., "Composting Around the World," Compost Science,
pp. 30-35, Spring 1960
-------
-37-
394. Monson, H., "Cannery Waste Disposal by Spray Irrigation,"
Compost Science, pp. 41-44, Spring 1960.
395. Wiley, J. S. and Spillane, J. T., "Refuse-Sludge Composting in Windrows
and Bins," Compost Science, pp. 18-25, Winter 1962
396. Braun, R., "Utilization of Organic Industrial Wastes by Composting,"
Compost Science, pp. 34-36, Autumn 1962
397. Compost Science, Editors, p. 47, Winter 1961
398. "Financing a City Compost Plant - Four Views," Editors,
Compost Science, Spring 1961
399. Vlamis, J. and Williams, D. E., "Test of Sewage Sludges for Fertility
and Toxicity in Soils," Compost Science, Spring 1961
400. Leaver, R. E., "Marketing Sewage Sludge in the Northwest,"
Compost Science, pp. 44-47, Spring 1961.
401. Golueke, C. G., "Composting Refuse at Sacramento, California,"
Compost Science, pp. 11-15, Autumn 1960
402. Caron, A. L. and Blosser, R. 0., "Recent Progress in Land Disposal of
Pulp and Paper Mill Effluents," TAPPI, Vol. 48, No. 5, pp. 43A-46A,
(May 1965)
403. Trubnick, E. H., "Disposal of Dewatered Fresh Sewage Sludge,"
Public Works, pp. 112-114 (Dec. 1958)
404. Scott, G. T. G., "Solids Wastes Disposal," Paper Presented at the
Canadian Inst. on Pollution Control Meeting (Oct. 1964)
405. Wiley, J. S. and Kochtitzky, 0. W., "Composting Developments in the
United States," Compost Science, Summer 1965
406. Mercer, W. A. and others, "Treatment of Liquid and Solid Food
Processing Wastes," Paper Presented at the 38th WPCF Conf. (Oct. 1965)
407. Anderson, M. S., "Sewage Sludge for Soil Improvement," Circular No. 972,
U.S.D.A., 1955
408. Skibniewski, L., "Chemical Problems in the Utilization of Sewage in
Agriculture," Gaz, Wodai Tech. Sanitarna (Polish), Vol. 23, No. 52
(Feb. 1949)
-------
-38-
Sludge Disposal - By-Product Recovery
409. Doughery, M. H. and McNary, R. R., "Activated Citrus Sludge - Vitamin
Content and Animal Feed Potential," Sewage and Industrial Wastes,
Vol. 30, No. 9, pp. 1151-1155 (Sept. 1958)
110. Stevens, W. F., and others, "Extractive Production of Vitamin 812
form Milorganite," Chemical Engr . Prog . , Vol. 51, No. 4, p. 163
(Apr. 1955)
411. "Vitamin Bj^ Test Project Completed," Editors, Sewage and Industrial
Wastes , Vol. 26, No. 3, p. 260 (Mar. 1954)
412. Hoover, S. R. and others, "Activated Sludge as a Source of Vitamin 6^2
for Animal Feeds," Sewage and Industrial Wastes, Vol. 24, No. 1,
pp. 38-44 (Jan. 19521
413. Bottenfield, W. and Burbank, N. C., "Putting Industrial Waste to Work;
Mead's New Lime Kiln Recovers Waste Lime Mud," Industrial Water and Wastes,
Vol. 9, No. 1, pp. 18-20 (Jan. - Feb. 1964)
414. Dick, C. J., "Alumina Reclaimed from Refuse Mud," Industrial Water and
Wastes, Vol. 6, No. 1, pp. 1-3 (Jan. -Feb. 1961)
415. Boruff, C. S., "By-Product Recovery - Pollution Control Measure,"
Chemical Engr. Prog., Vol. 55, No. 11, pp. 82-86 (Nov. 1959)
416. Burch, J. E. and others, "The Utilization of Waste Products,"
Food Technology, pp. 54-60 (Oct. 1963)
417. Hart, S. A. and McGauhey, P. H., "The Management of Wastes in the
Food Industry," Food Technology, pp. 30-34 (Apr. 1964)
418. Wiley, J. S., "Utilization and Disposal of Poultry Manure," Proc. of the
18th Purdue Industrial Waste Conference, pp. 515-522 (1963)
419. Burch, J. E. and Lipinsky, E. S., "Planning for the Chemical Utilization
of Waste Products," Proc. of the 17th Purdue Industrial Waste Conference,
pp. 29-36 (1962)
420. Douglass, I. B. "By-Products and Waste in Potato Processing," Proc. of
the 15th Purdue Industrial Waste Conference, pp. 99-106 (1960)
421. Nadelman, A. H. and Newton, L. P., "A Practical Approach to Utilization
of Solids from Deinking Mill Effluent," TAPPI , Vol. 43, No. 2,
pp. 120-128 (Feb. I960)
-------
-39-
422. Neujahr, H. Y., "On Vitamins in Sewage Sludge," Acta Chem. Scand.,
Vol. 9, pp. 803-806 (1955)
423. Hurwitz, E., "The Use of Activated Sludge as an Adjuvant to Animal Feeds,"
Proc. of the 12th Purdue Industrial Waste Conference, pp. 395-414 (1957)
424. Leary, R. D., "Production of Vitamin B]_2 from Milorganite," Proc. of
the 9th Purdue Industrial Waste Conference, pp. 173-183 (1954)
425. Carrick, C. W., "Use of Industrial Wastes in Poultry Feeding," Proc.
of the 6th Purdue Industrial Waste Conference, pp. 130-134 (1951)
426. Singruen, E., "Uses for Yeasts," Proc. of the 5th Purdue Industrial
Waste Conference, pp. 68-78 (19491
427. Lipsett, C. H., "Industrial Wastes - Their Conservation and Utilization,"
Atlas Publishing Co., New York, N.Y. (1951)
428. Vaughn, R. H. and Marsh, G. L., ."Problems in Disposal of California
Winery Wastes," American Journal of Enology, Vol. 6, No.l, pp. 26-34
(1955)
429. "Studies in Waste Disposal," Editors, Food Engineering, pp. 85-86
(Apr. 1963)
430. "An Economic Analysis of Alternative Methods of Cull Tomato Disposal
in Dade County, Florida," Florida Agricultural Experiment Station,
Report 59-2 (Sept. 1958)
431. Sathyanarayana, S. and others, "Vitamin Bi2 in Sewage Sludge
Science, Vol. 129, No. 3344, p. 276 (1942)
-------
-Uo-
Sludge Transportation - Pipeline
H32. Wirts, J. J., "Pipe Line Transportation and Disposal of Digested Sludge,"
Sewage and Industrial Wastes, Vol. 28, No. 2, pp. 121-131 (Feb. 1956)
1*33. Weller, L. W., "Pipeline Transport and Incineration," Water Works
and Wastes Engr., Vol. 2, No. 9, pp. 66-69 (1965)
-------
-41-
Water Plant Sludge Handling and Disposal
451. Russell, G. D. and G. S., "The Disposal of Sludge from a Lime-Soda
Softening Plant as Industrial Waste," Proc._of the 9th Purdue
Industrial Waste Conference, pp. 201-216 (1954)
452. Howson, L. R., "Lagoon Disposal of Lime Sludge," JAWWA, pp. 1169-1173
(Sept. 1961)
153. Roberts, J. M. and Roddy, C. P., "Recovery and Reuse of Alum Sludge at
Tampa," JAWWA, pp. 857-866 (July 1960)
454. Kohanowski, F. I., "San Diego has its Own Lime Recalcining Plant,"
Water and Sewage Works, Vol. 108, No. 3, pp. 81-84 (1961)
455. "Dayton, Ohio Recalcines Water Softening Sludge," Editors, Water and
Sewage Works, Vol. 107, No. 4, pp. 137-139 (1960)
456. Hager, G., "We Vacuum Filter Lime Softening Sludge," The American City,
pp. 105-106 (June 1965)
457. Chojnacki, A., "Recovery of Coagulants from the Sludge After Waste
Treatment," Inst. Hydrotech. Res. Sci. Sess., Bucharest, Sect. 4,
25-26, 1964 (Water Pollution Abs., Sept. 1965)
458. Vahidi, I. and Isaac, P. C. G., "Recovery of Waterworks Sludge,"
Journal Inst. of Water Engineers, Vol. 14, pp.454-458 (1960)
459. Doe, P. W. and others, "Sludge Concentration by Freezing," Water and
Sewage Works, Vol. 112, No. 11, pp. 401-406 (1965)
460. Black, A. B., "Disposal of Softening Plant Wastes," Journal AWWA,
Vol. 41, No. 9, pp. 819-829 (Sept. 1949)
461. Krause, F., "Softening Plant Reclaims Lime Sludge by Fluid Bed Roasting,"
Water Works Engr. (Apr. 1957)
462. Walter, L. H. and Millward, R. S., "Sludge Disposal by the F/S System,"
A Paper presented at the 16th U. of Kansas San.Engr. Conf. (1966)
463. Maass, A., Personal Communication, Midland, Michigan (Apr. 1966)
-------
-42-
Sludge Disposal - Ocean
434. Sylvester, R. 0., "Sludge Disposal by Dilution in Puget Sound,"
Journal Water Pollution Control Federation, Vol. 34, No. 9,
pp. 891-900 (1962)
435. Hume, N. B. and others, "Characteristics and Effects of Hyperion
Effluent," Journal Water Pollution Control Federation, Vol. 34,
No. 1, pp. 15-35 (Jan. 1962)
436. Hume, N. B., and others, "Operation of a 7-Mile Digested Sludge
Outfall," Journal San. Engr. Div. ASCE, Vol. 85, No. SA4, Part I,
pp. 71-87 (July 1959)
437. Griffin, G., "Sewage Sludge Disposal in Westchester County," Journal
San. Engr. Div. ASCE, Vol. 85, No. SA5, Part I, pp. 1-7 (Sept. 1959)
438. Heaney, F. L., "Design, Construction and Operation of Sewer Outfalls in
Estuarine and Tidal Waters," Journal Water Pollution Control Federation,
Vol. 32, No. 6, pp. 610-621 (1960)
439. Rawn, A. M. and Bowerman, F. R., "Disposal of Digested Sludge by
Dilution," Sewage and Industrial Wastes, Vol. 26, No. 11, pp. 1309-1314
(Nov. 1954)
440. Rudolfs, W,, "Sludge Concentration and Barging at the Elizabeth (N. J.)
Joint Meeting Sewage Treatment Works," Sewage Works Journal, Vol. 16,
No. 3, pp. 617-622, (1944)
441. Henry, W., "Sludge and Effluent Disposal by Dispersal," Wastes Engr.,
pp. 677-698 (Dec. 1961)
442. "Down to the Sea ... in Sludge Barges," Editors, Wastes Engr.,
pp. 78-79, 92 (Feb. 1959)
443. Theroux, R. J. and others, "Removal of Flotables from Digested Sludge,"
ASCE Transactions, Vol. 127, Part III, Paper No. 3324 (1962)
444. Hume, N. B. and others, "Operation of a 7-Mile Digested Sludge Outfall,"
ASCE Transactions, Vol. 126, Part III, Paper No. 3176 (1961)
445. Theroux, R. J. and others, "Removal of Flotables from Digested Sludge,"
Proc. of the ASCE - San. Engr. Div., Vol. 87, pp. 21-48 (Mar. 1961)
-------
-43-
446. "An Investigation of the Efficacy of Submarine Outfall Disposal of
Sweage and Sludge," California Water Pollution Control Board,
Publication No. 14 (1956)
447. Baxter, S. S., "Sludge Disposal in Philadelphia," Proc. of the ASCE,
Sanitary Engr. Div., Vol. 85, No. SAG, pp. 127-141 (1959)
448. Seid, S., Annual Report, Middlesex County Sewerage Authority, with
additional note (1964)
449. Whitman, Requardt and Associates, Report to the City of Baltimore
(1965)
450. Orndorff, R. L., Personal Communication, Dept. of Sanitary Engr.,
District of Columbia,(1966)
450a. Frazee, John, Personal Communication, Supt. of Sewers and Sewage
Treatment, Camden, N. J., (1966)
-------
|