Environmental Protection Technology Series
CHARACTERIZATION AND UTILIZATION OF
MUNICIPAL AND UTILITY SLUDGES AND ASHES
Volume II. Municipal Sludges
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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EPA-670/2-75-033b
May 1975
CHARACTERIZATION AND UTILIZATION OF
MUNICIPAL AND UTILITY SLUDGES AND ASHES
Volume II
Municipal Sludges
by
N. L. Hecht, D. S. Duvall, and A. S. Rashidi
University of Dayton Research Institute
Dayton, Ohio 45469
Program Element No. 1DB064
Research Grant No. R800432
Project Officers
R. A. Carnes and D. F. Bender
Solid and Hazardous Waste Research Laboratory
National Environmental Research Center
Cincinnati, Ohio 45268
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESERACH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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REVIEW NOTICE
The National Environmental Research Center--Cincinnati has
reviewed this report and approved its publication. Approval
does not signify that the contents necessarily reflect the views
and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
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FOREWORD
Man and his environment must be protected from the adverse effects
of pesticides, radiation, noise and other forms of pollution, and the unwise
management of solid waste. Efforts to protect the environment require a focus
that recognizes the interplay between the components of our physical environ-
ment—air, water, and land. The National Environmental Research Centers
provide this multidisciplinary focus through programs engaged in
• studies on the effects of environmental contaminants on man
and the biosphere, and
• a search for ways to prevent contamination and to recycle
valuable resources.
Wastewater and water treatment sludges pose a serious disposal
problem in the United States. The quantities of these sludges, their com-
position, and handling and treatment techniques now employed have been com-
piled in this report. Environmental contamination in disposing of these sludges
is of major concern. On the other hand, utilization of the sludges as a val-
uable resource continues to be an important research objective.
A. W. Breidenbach, Ph.D.
Director
National Environmental
Research Center, Cincinnati
iii
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ABSTRACT
A comprehensive characterization and evaluation was performed
of disposal and utilization practices for sludges from municipal
wastewater and water treatment plants. The nature and quantities
of the sludges were discussed. Various sludge handling and
treatment techniques were detailed. Problems encountered in
sludge disposal were reviewed, and the economics of wastewater
sludge disposal were discussed.
IV
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TABLE OF CONTENTS
PAGE
SUMMARY 1
INTRODUCTION 15
SLUDGE ORIGIN AND TYPE 17
SLUDGE QUANTITIES 19
Waste-water Treatment Plant 19
Water Treatment Plant 19
WASTEWATER TREATMENT PLANT SLUDGE
CHARACTERISTICS 22
Sludge Settling Characteristics 24
Specific Resistance of Sludge 32
Sludge Flow Characteristics 37
Calorific Value of Sludges 43
Chemical Composition of Sludge 45
Fertilizer Value of Sludge 47
CHARACTERISTICS OF WATER TREATMENT PLANT
SLUDGE 52
Purification or .Coagulation Sludge 57
Softening Sludges 60
Diatomite Earth Waste Characteristics 65
SLUDGE HANDLING AND TREATMENT 68
SLUDGE CONCENTRATION 68
Gravity Thickening 70
Flotation Thickening 75
Centrifuge Thickening 81
SLUDGE STABILIZATION 81
Anaerobic Sludge Digestion 84
Aerobic Sludge Digestion 90
Composting to Sludge 94
Lagooning of Sludge 99
Heat Treatment 99
Chemical Stabilization of Sludge 110
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PAGE
Solvent Extraction 113
Electrical Treatment 113
SLUDGE CONDITIONING 113
Elutriation of Sludge 114
Chemical Conditioning 116
Polyelectrolytes 117
Freezing 122
Fly Ash Conditioning of Sludges 124
Filter Aids 128
SLUDGE DEWATERING 128
Sand Bed Drying 132
Centrifugation 140
Vacuum Filtration 148
Filter Presses and Plug Presses 155
Vibration 156
Heat Drying 1 57
SLUDGE TRANSPORTATION 159
SLUDGE REDUCTION PROCESS 164
Sludge Incineration 164
Pyrolysis 174
ULTIMATE DISPOSAL AND/OR UTILIZATION OF
MUNICIPAL WASTEWATER SLUDGES 178
Ocean Disposal of Sludge 178
Land Spreading of Liquid Sludge 181
Land Reclamation 187
Lagooning and Landfilling of Sewage Sludge 190
Disposal of Dried Sludge as Fertilizer or
Soil Conditioner 190
Underground Disposal 192
By-Products Recovery 192
WATER TREATMENT PLANT SLUDGE DISPOSAL
AND/OR UTILIZATION 194
Direct Discharge to Streams, Lakes, and Ocean 194
Lagooning of Sludge 195
vi
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PAGE
Agricultural Utilization 195
Discharge to Sewers 197
Disposal in Sanitary Landfill 197
Application of Sludge to Strip Mine Spoils 197
By-Product Recovery 199
ECONOMICS OF WASTEWATER SLUDGE DISPOSAL 200
vii
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ACKNOWLEDGEMENT
The authors wish to acknowledge the assistance and support rendered
during this study by Professor S. J. Ryckman, G. S. Sk1v1ngton, and
R. A. Ralston.
The authors also wish to acknowledge the assistance provided by the
two project officers: R. A. Games and D. F. Bender..
viii
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SUMMARY
The primary objective of a waste water treatment plant is to prevent
pollution of streams, lakes, and ground water supplies, by removing
the solid pollutants from the wastewater. The solids removed are
in the form of a liquid slurry, called sludge. Sludges are concen-
trated pollutants. They must be disposed of in a manner that assures
public health and environmental safety. Furthermore, disposal
methods applied must account for recycling of the organic and use-
ful material of the sludge back to nature; also, the disposal method
should be economically feasible.
Sludge handling, treatment and disposal requires many steps, in-
cluding: concentration, stabilization, conditioning, dewatering and dry-
ing, transporting, and solids reduction. A flow diagram of the most
common methods used for sludge processing and disposal is shown
in Figure 1.
Concentration processes are applied primarily to increase the solids
content of the sludge, thus reducing its volume significantly. Sludge
concentration might be desirable when: (1) liquid sludge is applied to
farmland; (2) sludge is disposed of in the sea (barging); and (3) when
savings of chemicals, heat energy, and auxiliary fuel in the sub-
sequent steps are questions of concern. Methods used for sludge
concentration or thickening include: (1) gravity thickening; (2)
dissolved air-flotation; and (3) centrifugation. Gravity thickening
is the most widely used, simplest and least expensive sludge con-
centration method. This process is basically the same as sedi-
mentation settling, but relitively slow in action. The degree of sludge
concentration obtained and the efficiency of thickener operation de-
pends on such factors as: (1) initial solids concentration and temper-
ature of the sludge in the thickener; (2) type of sludge and volitile
content; and (3) addition of chemicals and inert weighing agents.
Typical concentration that can be obtained for various types of sludges
are: 8% for primary, or trickling filter sludges, 5% for modified
aeration; and 3% for activated sludge. Flotation thickening of sludge
is becoming Increasingly popular. The basic principle in flotation
thickening is to attach minute air bubbles to the suspended solids of
the sludge. This reduces the specific gravity of the solid particles
below that of water and causes the solids to separate from the liquid
phase while moving in an upward direction.
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Four methods of flotation thickening that are used in wastewater
treatment plants include: (1) dispersed air flotation; (2) dissolved
air pressure flotation; (3) dissolved air vacuum flotation; and (4)
biological flotation. Among the above four methods, dissolved-air
flotation, and biological flotation have received wide use because
of the higher solids concentration obtained. The major variables
influencing flotation thickening include: (1) pressure; (2) recycle
ratio; (3) feed solids concentration; (4) detention period; (5) air
to solid ratio; (6) type and quality of sludge; (7) solids and hy-
draulic loading rates; and (8) use of chemical aids. Flotation
thickeners are used primarily in conjunction with waste activated
sludge. Typical concentration obtained from combined primary
and activated sludge is 6%. For activated sludge alone, the con-
centration is 4%; and when chemical aids are employed, 6%.
Centrifugation of sludge for thickening and dewatering purposes
has long been practiced in wastewater treatment plants. Typical
concentration obtained is 7%. Higher concentration is possible
using chemical aids.
Raw sludge contains a substantial amount of organic material. Sludges
provide food for microorganisms which grow in and upon them. These
microorganisms are mostly of fecal origin and many of them are
pathogenic, which may be definite health hazards, Therefore, decompo-
sition of the organic material, or sludge stabilization seems an ob-
vious necessity. Methods applied for stabilization of wastewater
sludges include: (1) anaerobic digestion; (2) aerobic digestion; (3)
composting; (4) lagooning; (5) heat treatment; and (6) chemical
stabilization.
Anaerobic digestion of sludge involves decomposition of organic
material in the absence of free oxygen. Digestion occurs in two
separate stages, called liquefaction and gasification stages. The
end product of the first stage is utilized in the second stage as
fast as it is produced. Responsibilities of the group of micro-
organisims (acid forming bacteria) in the first stage are the
hydrolysis and fermentation of the complex organic compounds to
simpler organic acids. A responsibility of the group of micro-
organisims (methane forming bacteria) in the second stage is to
convert the organic acids to methane gas and CO-. Anaerobic
sludge digestion is the oldest method of stabilization, and it will
continue to be used particularly at small sewage treatment plants
and in large coastal cities. Factors effecting digestion of waste-
water in sludges include: sludge type and volatile content; digester
temperature; digestion detention period; feed sludge concentration;
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degree of digester mixing; digester loading rates; and presence of
toxic materials. Three types of anaerobic digesters being used in
wastewater treatment plants are: (1) conventional digester, (2)
high-rate digesters, and (3) anaerobic contact process.
Aerobic digestion is a biological process during which gross oxi-
dation is completed in two stages: (1) direct oxidation of any biode-
gradable matter by biologically active masses of organisms, and
(2) oxidation of microbial cellular material by endogenous respi-
ration. Aerobic digestion has been used extensively in treating
waste activated sludge. Factors effecting design and operation of
this process include: rate of sludge oxidation; sludge temperature;
oxygen requirements of the system; sludge loading rates; sludge
age; sludge solids characteristics; and characteristics of the
residue and supernatant liquid. In aerobic digestion volatile solids
reduction of up to 50% can be obtained. Rate of reduction is very
high in the initial 10-15 days, whereas after 15 days the rate of
reduction slows down drastically.
Composting of sludge can be defined as the decomposition of organic
waste by aerobic, thermophilic organisms to produce a stable
humus-like material* The compost produced can be used as a
soil conditioner or fertilizer depending on its nutrient content.
Composting methods used include: (1) indoor (mechanical), and (2)
outdoor (windrows and bins) processes. In the mechanical com-
posting process there are three phases. These are: (1) de water ing
of the sludge, (2) composting, and (3) final curing. The factors
which affect the composting process include: mixing moisture
content; percent of recycled compost; aeration; and temperature
and pH. Volume reduction of about 70% with solids reduction of
30% can be obtained. The heat generated (140 + °F) is sufficient
to destroy most of the pathogenic bacteria* Composting of waste-
water sludge, mixed with refuse, has been found to be more
advantageous than composting of either sludge or refuse alone.
Sludge lagoons have been used for sludge stabilization purposes
quite frequently, particularly in small plants. They are relatively
inexpensive and very simple to operate. Where a lagoon is used
for digestion of raw sludge, odor, and insect breeding may be
problems. Design parameters commonly used are: land area,
climate, subsoil permeability, lagoon depth, sludge loading rate,
sludge characteristics and types.
Heat treatment of the wastewater sludge has increasingly gained
popularity in recent years. When sludge is heated at high tempera-
tures under pressure, the gel-like structure of the sludge is
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destroyed and the bound water is liberated. The resulting product
is sterilized, deodorized and readily dewaterable. Major processes
used include: (1) Zimpro process, (2) Porteous process, and (3)
Farrer system.
Application of the heat treatment processes, particularly the Zimpro
process ( also called wet oxidation), in the treatment of wastewater
sludge, is growing rapidly. Operation of these processes includes
the following steps: (1) sludge is ground up and pressurized to the
selecting operating pressure; (2) sludge is pre-heated by passing
it through a heat exchanger; (3) the pre-heated sludge (in the case
of wet oxidation, mixture of the sludge and air) is fed into the
reactor, where sludge is further heated to about 300 to 400 F for a
period of 30 to 60 minutes; (4) the heat conditioned sludge is cooled
by heat exchange with the incoming sludge; (5) gases are separated
and released through a catalytic after burner or similar devices;
and (6) stabilized sludge is concentrated and dewatered to about
40 to 50% solids content without use of chemicals. It has been
observed that heat treatment destroys most of the pathogens. The
major difference between the Zimpro process and the other two
is that the Zimpro process air is introduced into the reactor, and
it is not in the Porteous or Farrer systems.
Chemicals have also been used to stabilize the sludge produced from
wastewater treatment processes. The two types of chemicals that
are very common are chlorine and lime. Both chemicals have been
found to be very effective in microbial destruction.
Other methods used for sludge stabilization include solvent extrac-
tion, and electrical treatment. Though these methods are very
effective and technically promising, they are not economically
feasible and many operational problems are associated with them.
Sludge is conditioned principally to improve its dewatering charac-
teristics. Various methods of sludge conditioning that are currently
being practiced include: (1) elutriation, (2) chemical conditioning,
(3) freezing, and (4) addition of flocculating agents and filter aids,
such as fly ash, diatomaceous earth, etc. The sludge moisture
content can be reduced to about 70% by means of proper conditioning,
followed by a dewatering process.
Elutriation is a washing operation which removes sludge consti-
tuents that interfere with sludge thickening and dewatering processes,
As the elutriation comes in contact with the sludge particles,
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dispersants such as carbonates and phosphates are extracted from
the sludge as well as non- settle able fine particles, the products of
decomposition, and the toxic materials. The elutriation operation
can be performed as a batch or continuous process. It could be
single stage with single contact between the solids and the liquid,
or it could be multistage contact in one or more stages or multistage
counter current contact.
Use of chemicals to condition sludge results in coagulation of the
solids and the release of the absorbed water from the solid particles.
The most common types of chemicals used in conditioning of waste-
water sludge include: ferric chloride (either alone or combined with
lime); a combination of ferric chloride and alum; or lime alone.
Ferric chloride is used in plants dewatering activated sludge, where
ferric chloride and lime are used in plants dewatering raw and/or
digested sludge. The chemical dosage required for a given sludge
is determined in the laboratory. Polyelectrolytes have gained much
popularity in recent years as a sludge conditioner. Dewaterability
of the sludge and solids content of the filter cake solids captured
during the elutriation have significantly been improved in sludge
which was conditioned with polymers. Dosage rates of polymers
as compared with other chemicals are significantly lower, on the
order of 1/50 to 1/100 times of other chemicals.
Freezing has also been applied to sludge conditioning. The condi-
tioning effect produced by freezing is believed to result from dehy-
dration, and the pressure exerted on the sludge particles by the
ice structure. A significant increase in solids content and dewater-
ability of the sludge is attained by slow freezing and thawing,
provided that the freezing cycle is complete. Freezing is generally
used in cold climates, however, it can be applied in mild climates
if the sludge is applied in thin layers 0. 5 to 1.0 inch thick.
Fly ash and other filter aids such as diatomaceous earth, sawdust,
newsprints,etc., have also been used for sludge conditioning.
These materials together with the solid phases of the sludge
form a porous, permeable and rigid lattice structure which filters
particulates but allows passage of liquid. The amount of fly ash
required to improve filterability of activated sludge ranges from
500 to 700% of the initial solids content of the sludge; whereas,
for digested sludge it ranges from 100 to 150%.
Sludge dewatering is generally applied to serve such purposes as:
reducing the volume of the sludge; reducing the moisture content of
the sludge, thus, reducing the fuel coats when sludge is dried or
incinerated; and increasing the solids content so that the sludge is
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easily handled and disposed of. The methods commonly used for
sludge dewatering include: (1) sand bed drying; (2) centrifugation;
(3) vacuum filtration; (4) filter press; (5) heat drying; and (6)
vibration.
Drying of the sludge on open or covered sand beds is the most com-
mon de water ing method presently in use. Open sand bed de water ing
is probably the least expensive method of all. It has been used exten-
sively in small (^10,000 pop.) communities. Variables effecting
de water ing rates include: (1) climate and atmospheric variations;
(Z) sludge depth applications; (3) sludge moisture content; (4) origin
and type of the sludge; (5) sludge age; (6) drying beds construction;
and (7) presence or absence of coagulants. In general, drainage is
the predominant de water ing mechanism during the first 2 to 3 days
after the application of the sludge, during which, 60 to 85% of the
sludge moisture loss occurs. After 2 to 3 days, evaporation
becomes the major factor and its rate depends solely on the
climatic conditions. The optimum sludge application depth is about
9 inches.
Centrifuges have also been used for municipal sludge de watering
purposes. The three major types of centrifuge used are: (1) basket-
design centrifuges; (2) disk-design centrifuge; and (3) solid bowl
conveyor centrifuge. The solid bowl conveyor centrifuge has been
the most effective of all. Parameters that effect the efficiency of the
unit include machine variables, such as bowl design, bowl speed,
pool volume, and conveyor speed, and pitch; and process variables,
such as feed rate, solid characteristics, feed consistency, tem-
perature, and chemical aids. Total solids of the cake ranges from
18 to 35%, and the solids recovery varies from 50 to 90% without
use of chemicals, and 90 to 98% when chemicals are used.
Vacuum filters are the most common type of mechanical de watering
facilities used today, particularly when incineration is used for
final disposal. Various types of vacuum filters used include: (1)
drum-type filters; (2) string-discharge filters; (3) belt-type filters;
and (4) coil-type filters. Factors effecting de water ability of the
sludge include: (1) sludge solids content; (2) sludge age and tempera-
ture; (3) sludge and filtrate viscosity; (4) sludge compressibility;
and (5) the nature of the sludge solids. Chemicals are frequently
used for conditioning the sludge. Cake solids content for various
types of sludges, ranges from 15 to 35%.
Filter presses and plug presses have also been used for sludge
de water ing purposes. However, their use ia very limited in the
U.S. because of the high cost of manual labor and maintenance of
the system.
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Wastewater sludge has been heat dried particularly in conjunction
with production of fertilizer or soil conditioner, in which sludge
is generally dewatered to a 10% moisture content, utilizing heat
dry facilities. Heat drying permits the end products to grind well,
reduces sludge weight and odor, destroys pathogens and most
important of all provides an ultimate means of disposal by
recycling the organic material back into the land. The various types
of equipment used for sludge drying include: (1) flash drying
system; (2) multiple hearth dryer; (3) rotary kiln dryer; and (4)
atomized spray dryer. The most common types are the flash drying
and multiple hearth systems. Because of the high costs involved
in sludge drying, sludge is generally de watered by mechanical
means prior to drying. The sludge drying temperature commonly
used is about 700 F.
Common methods for conveying the resulting liquid or dried sludge
from the waste water treatment plant to a final disposal site include:
(1) barging transport; (2) pipeline transport; (3) truck transport;
and (4) railroad transport. Barging is generally utilized in coastal
cities. Approximately 4. 5 million tons (on a wet basis) of waste -
water sludge was barged to the sea in 1968. Liquid treated sludge
is generally thickened to about 4 to 10% solids content prior to
barging. Pipeline transporting of sludge is very common. Its uses
include: transport to farmland, to land for land reclamation pur-
poses, to disposal site at sea, and others. Truck transporting of
liquid and dried sludge from the plant to the final disposal site is
a very common method particularly in smaller treatment plants.
Sludge is generally incinerated for two main reasons: (1) volume
reduction, and (2) solids reduction and sterilization. Methods used
for sludge incineration are the same as those mentioned for sludge
drying purposes with some process modifications. Incineration
temperature ranges from 1350 to 1600°F. Sludge is generally
dewatered to about 70% moisture content prior to incineration.
Sludge combustion is effected by a number of factors, including
sludge calorific value, sludge volatile content, and sludge inert
content. A typical calorific content of the sludge is about 10,000
Btu/lb of volatile solids. A complete incineration process involves
two steps: (1) drying, and (2) combustion. The first step requires
auxilliary fuel, whereas the second step could be an endogenous
process, depending on the volatile content of the sludge and
deodorizing requirements. Sludge incineration has become very
common, particularly in large cities, despite the high costs involved.
The major types used are multiple hearth and flash drying units.
The principal end products of incineration are gases (CO,,
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and inert ashes and residues. It appears that incineration of the
sludge involves some degree of hazardous environmental and
health effects; however, sufficient data are not available to establish
these potential hazardous effects.
Pyrolysis of wastewater sludge has also been practiced to produce a
marketable by-product (absorbent materials). The pyrolysis unit
generally consists of a closed stainless steel cylinder, in which the
sludge is retained for 60 to 90 minutes in the absence of air at a
temperature of "1200 F.
The main objectives for the application of physical, biological and
chemical processes in a wastewater treatment plant are to remove
and concentrate that portion of the wastewater that is responsible
for its offensive nature. The sludge which constitutes a significant
portion of the total pollutants removed is by far the most important
by-product of the wastewater treatment processes. Handling and
disposal of such concentrated pollutants present some of the most
complex problems that engineers and plant operators face. In addition,
the cost of the sludge treatment and disposal is very high-as much as
50% of the total capital and operating costs of the entire waste
treatment processes.
Sludge consists of a mixture of organic, and inorganic solid phases,
suspended in an aqueous solution. Sludges are the solids which
settle from the water and wastewater, and the colloidal particles that
are precipitated by biological flocculation and chemical coagulation. The
sludge has a very high moisture content ranging from 90 to 99%.
The characteristics of the sludge are dictated by such factors as:
source of the wastewater, i. e., municipal, or municipal combined
with industrial; the degree of treatment the wastewater receives,
i.e., primary, secondary, advanced,or any combination of the
above; the type of treatment processes to which the sludge is subjected,
i. e., raw (untreated), digested, digested and elutriated; and the
type of collection system, i. e., separate or combined systems.
In a municipal water treatment plant, sludge is obtained from two
major sources: sedimentation basins, and filter backwashes. The
characteristics of this sludge depend upon the source of the raw
water and the type of the processes utilized.
The quantity of sludge generated by municipal wastewater treat-
ment plants throughout the U.S. is about 13 million tons /year on
a dry solids basis. Intensified water quality enhancement programs,
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stringent water pollution standards, and the construction of addi-
tional waste water treatment facilities, and/or the upgrading and
expansion of the existing facilities, and the continual growth of
population will contribute significantly to the increase of the sludge
quantities being produced. The quantities of sludge produced by
water treatment plants are far less than those produced by waste-
water treatment plants, amounting to 4 million tons/year on a
dry solids basis in the U.S.
In general, primary sludge is gray in color, slimy in nature, and
gives off an offensive odor. It has solids content ranging from 1 to
5 %. Activated sludge is generally brown in color. When the color
darkens, it indicates septic conditions. The fresh sludge has no
characteristic odor, but in septic conditions it has the disagreeable
odor of putrefaction, and the solids content ranges from 3 to 8%.
Trickling filter humus has a brownish color; the fresh sludge has
relatively inoffensive odor, but when it undergoes decomposition,
it gives off a very offensive odor. The solids content of the humus
ranges from 3 to 7%. Digested sludge has a dark brown to black
color and contains large quantities of gases. A completely digested
sludge has an odor similar to that of hot tar or burnt rubber. It has
solids contant ranging from 0.2 to 4.0%.
The solids fraction of the wastewater sludge is primarily composed
of biodegradeable material (30%), stable organic matter (35%), and
inert material (35%) . Further, about 60% of the total solids are
dissolved solids, 20% are settleable solids, and 20% are colloidal
solids.
The characteristics effecting ultimate disposal and effective utiliza-
tion of the wastewater sludges include: settling characteristics,
specific resistance, flow characterisitcs, calorific values, chemical
composition, and fertilizing ingredients of the sludge.
Settling characteristics of the sludge are vital parameters, since
one of the primary objectives in sludge treatment is to concentrate
the residues and reduce the overall volume of the sludge. In the
design of treatment processes, zone settling and compression
settling are of major importance; however, discrete settling and
flocculant settling may also occur at a given period during sedi-
mentation. The Talmadge and Fitch method is generally applied
to determine the settling tank area requirements for both sludge
thickening and clarification purposes, for zone settling. The
concentration of the settled sludge is effected by such factors as:
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waste water characteristics; type of biological treatment the waste-
water receives; design and operation of the settling tank; promotion
of settling, using mechanical means or chemicals; settleable solids
characteristics; solids concentration in the original suspension;
and sludge detention time, etc. The solids concentration of the
sludge varies from 0. 5 to 10% for raw sludges and from 2 to 10%
after digestion.
Another physical characteristic of the sludge that is important in
the design of treatment processes such as vacuum filters, centri-
fuges, and drying beds is the specific resistance. A specific
resistance concept is generally used to evaluate the filtration
characteristics of the sludge; by this is meant the difficulty en-
countered in removing water or conversely, the ability of the sludge
to retain water. Specific resistance values for waste water sludge
range from 10 x 10? sec^/gr, when chemical coagulants are em-
ployed, to as high as 2800 x 10' sec /gr for pure activated sludge.
Low values are indicative of a sludge with rapid draining or filter-
ing characteristics, and for high values the reverse is true.
The heterogeneous nature of the wastewater sludge causes complex
flow phenomena; consequently, the direct application of the existing
hydraulic equations for measuring pipe friction loss is not adequate
inmost cases. Flow characteristics of the sludge become a signifi-
cant parameter when liquid sludge is transported via pipelines to the
disposal site. The most significant factor effecting sludge flow is
the moisture content of the sludge. Based on the moisture content,
the flow is classified as: (1) flow in suspension, and (2) plastic
flow. Numerous methods for calculating critical velocities, head
loss, and other flow characteristics are available in the literature.
Calorific value of the sludge is a vital parameter in the design and
operation of sludge incinerators. Calorific value of the sludge
combined with its ultimate analysis determines the quality of
the sludge for incineration. The combustible fraction of the sludge
ranges from 50% for digested sludge to 75% for raw sludge. The
heat content of the sludge ranges from 5000 Btu/lb to 14,000 Btu/lb
of combustible solids, for digested and undigested sludges respec-
tively.
A proper assessment of the usefulness of the sludge can be made if
its chemical composition is known. Analysis of the specific organic
constituents of a sludge is important in predicting what conditions
will prevail during dewatering and also helps in determining what
specific effects the sludge might have on solids characteristics. The
organic fraction ranges from 60 to 80% on a dry weight basis, for
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primary sludge; 62 to 75%, for activated sludge; and 45 to 60%, for
digested sludge. The organic content of the sludge is usually
measured from the BOD and volatile solids determination. However,
more useful and implicit tools are the COD and TOG tests. Sludge
also contains a variety of metallic ions, including toxic metals.
Their relative concentrations depend mainly upon the origin of
the waste water.
Waste water sludge contains many fertilizing elements. In compari-
son with commercial fertilizers, they are rated principally on
their content of three substances: nitrogen, ranging from 0. 8 to
10% as N; phosphorous, ranging from 1 to 4% as phosphoric acid;
and potassium, ranging from 0.1 to 0. 5% as potash.
Characteristics of the sludges from water treatment plants are
highly variable depending on such factors as: sources of raw water;
type of treatment processes employed; type of chemicals added;
efficiency of unit operation; degree of solids removed; and the
time between basin and filter backwash clean-out. The significant
parameters for characterizing these sludges include: solids;
settleability; filterability; viscosity; and to a lesser extent, coliform
content, BOD, COD and chemical composition.
The two major types of sludge resulting from a municipal water
treatment plant are: (1) basin sludges, and (2) filter backwash
wastes. Basin sludge is comprised of the organic, inorganic, and
biological organisms present in the raw water and may be in a
state of solution, colloidal suspension or readily settleable form;
or, it may be mixed with coagulant aids employed in the treatment
process. Basin sludge appears as a fluffy agglomeration of
chemically precipitated and organic debris, with a high moisture
content (96 - 99%). The color of the sludge approaches yellow-
orange, pale-green, dark-brown and sometimes black, depending
upon the type of chemicals used, nature of the impurities extracted,
and stage of decomposition of the organic material. Filter backwash
waste essentially contains a finer fraction of the basin sludge in a
much lower concentration, plus a small portion of the filter media
itself.
The ultimate disposal of the vast quantities of sludge being generated
throughout the U.S. is one of the most complex problems facing
sanitary engineers today. It appears that the disposal problem will
continue to grow due to tightening of water, air and land pollution
control standards, lack of land availability resulting from rapid
growth of urbanized communities, growth of population, etc.
11
-------
Numerous methods for disposal or utilization of municipal sludges
have been practiced. These methods are summarized in Table 1.
Ultimate disposal of the sludge is the last step in the treatment
processes. It must comply with the state, interstate and federal
standards and requirements; it should not adversely effect the
surface or ground water, air, or land surfaces; it should be economi-
cally feasible; and it should assure public health safety.
Ocean disposal of wastewater sludge by barging or through submarine
outfalls is very common practice for major coastal cities. It is the
most economical disposal method for coastal cities when compared
with the alternative methods. However, because of the observed
adverse environmental effects of this method, indications are that
this method may be banned in the future.
Land spreading of treated liquid sludge on crop lands appears to
have gained much popularity in the last decade. It offers many
advantages, including: (1) economy; (2) recycling of water, nutrients,
and organic materials; (3) a final treatment process and ultimate
disposal; and (4) freedom from nuisance if done properly. Possibil-
ities of toxic metal accumulation and surface and ground water
contamination appear to be negligible; however, extensive research
is required to confirm this.
Wastewater sludge has been utilized to reclaim sandy soils and strip
mine spoils by converting them into valuable crop land or recreation
parks. Numerous projects throughout the U.S. are underway. It
appears that this is a satisfactory method of sludge disposal to the
extent that it does not cause pollution; it is relatively economical;
and it utilizes the organic content of the sludge for beneficial pur-
poses.
Application of dried treated sludge on land as fertilizer, or soil
conditioner is a viable ultimate disposal method. From the stand-
point of fertilizing ingredients content, dried sludge may not compete
with commercial fertilizer, however several municipalities have
reported satisfactory marketing of the dried sludge. A growing
demand for the use of dried sludge on parks, golf courses, and
lawns is apparent.
Recovery of certain by-products such as Vitamin B-12, grease,
metals (Ag, Cu, Ti, etc.), absorbent material, etc. from waste-
water sludges has been reported in the literature. However, market-
ing and the value of the product and associated marketing problems
have not been worked out. In general, the market place determines
the by-product specifications; and, inmost cases, the specifications
12
-------
TABLE I
PRESENT METHODS OF MUNICIPAL SLUDGE DISPOSAL AND/OR UTILIZATION
Waste water Treatment Water Treatment
1. Ocean dumping or discharging 1. Direct discharge to streams, ocean
or lake.
2. Land spreading of liquid sludge.
2. Lagooning of the sludge.
3. Land reclamation.
3. Agriculture utilization.
4. Lagooning and landfilling.
4. Discharge to sewers.
5. Disposal of dried sludge as fertilizer
or soil conditioner. 5. Disposal in sanitary landfill.
6. Underground disposal. 6. Application of sludge to strip mines.
7. Incineration and landfill of ash. 7. By-product recovery.
8. By-product recovery.
-------
are rigid concerning product purity and concentration. A great
amount of development and additional research needs to be done
before by-product recovery appears to be practical.
Disposal of the sludge generated from municipal water treatment
plants has been predominantly dumped into the nearest available
water course. It appears that an intensified effort is in progress
by the water industry and other organizations, to put the task of
handling and disposal of these wastes in the proper perspective.
A majority of the states has banned direct discharge of these
wastes into state or federally owned waters. Methods for disposal
and utilization of these wastes presently in use are listed in Table 1.
Direct disposal of these wastes into awater course may not cause
serious health hazards as compared with wastewater and waste-
water sludges, because of low organic content. However, the high
solids content of these sludges may color the receiving water,
increase its turbidity, and may settle and form sludge beds, inter-
ferring with the natural aquatic life cycle. Disposal of these sludges
into sanitary sewers has resulted in enhancement of the wastewater
treatment process operation. However, it has some disadvantages,
including precipitation and solids formation on the walls of the
sewers if flow velocities are below 25 ft/sec;-and in excessive amounts
it may hinder the biological activities of the secondary process.
Disposal of these wastes into sanitary landfills appears to have many
advantages. However, leaching has been reported, when liquid
sludge was applied in excessive amounts. Application of water
softening plant sludge as an agricultural lime, or for reclaiming
strip mine spoils has shown very promising results. When used as
agricultural lime, care must be taken so that possible accumulation
of toxic metals is prevented-
By-product recovery of the wastes from water treatment plants
includes lime recovery, magnesium recovery, and alum re covery.
Recalcination (lime recovery) from large softening plants has been
very successful and has almost eliminated the disposal problem.
Recovery of magnesium carbonate and utilization of it as a replace-
ment for alum coagulant seems very promising. Recovery of
aluminum sulfate from water purification wastes has been practiced
very extensively in Japan and France. Although this process sub-
stantially reduces the volume of sludge for final disposal, complete
separation of the impurities, such as iron, manganese, etc. is a
difficult job.
14
-------
INTRODUCTION
The fraction of wastewater which is primarily responsible for the offen-
sive nature of the wastewater is removed by physical, biological, and/or
chemical processes in a municipal water or wastewater treatment plant.
These unwanted materials, which include grit, screenings, skimmings,
and sludge, present a potential pollution hazard. These sludges which
constitute a significant portion of the total wastes removed are by far
the most important by-products of the waste treatment processes.
Sludge handling and disposal represent some of the most complex prob-
lems for the engineers and operators.
Sludge consists of a mixture of organic and inorganic phases, suspended
in an aqueous solution. Sludges are predominantly the solids which
settle from the water and wastewater, and/or the colloidal particles that
are precipitated by biological flocculation and chemical coagulation*.
The sludge has a very high moisture content, ranging from 90 to 99%.
Variations of moisture content and other characteristics, will depend
on such factors as: wastewater origin, type of treatment processes to
which both the wastewater and sludge had been subjected, and the type
of seweragfe system.
Proper handling and disposal of sludge is a serious problem today. Dis-
posal practices must be consistent with an awareness of the aesthetics,
pollution potentials, and economics. The magnitude of the sludge dis-
posal problem is ever-increasing due to implementation of more strin-
gent pollution control standards for water, land, and air along with the
rapid growth of population (projected to reach over 350 million people
by the year 2000, with over 85% living in the urbanized communities).
Because technological advancement in the treatment and disposal of
sludge has not kept pace with the advancements of water and waste-
water treatment, the cost of sludge disposal may increase to the point
where it will be as much as 50% of the total treatment costs (1). Blood-*
good (2) states the problem of sludge disposal is as great, or greater
than that of purifying the sewage. Thomson and Morgan (3) say that
"the enigma of sewage treatment is the disposal of ever-increasing
sludge". Dean (4) says disposal of the sludge has always been a prob-
lem and one that requires the expenditure of money without the prod-
uction of anything that could be sold for a profit.
* Sludges also include a fraction of the dissolved solid phases.
15
-------
Various sludge disposal methods existing today include landfill, ocean
disposal, deepwell injection, incineration, land spreading and land
reclamation. Another possible method of disposal is outer space
dumping. However, not all of the above methods have solved the problem
of ultimate disposal. The most promising ultimate disposal methods
are: reuse and utilization of the sludge as tsoil fertilizer or soil con-
ditioner; recovery and recycling of certain by-products; and land
reclamation.
The purpose of this report is to summarize the information pertaining
to wastewater and water treatment sludges that has been gathered
through literature searches and personal contacts with individual
investigators, research institutes, governmental agencies and other
available sources*
16
-------
SLUDGE ORIGIN AND TYPE
The characteristics of the sludge generated from water and waste-
water treatment facilities are directly related to the source of the
water and wastewater; the type of treatment processes to which the
water, wastewater, and the sludge are subjected; the type of sewer-
age system; and the degree of treatment required. Considering these
factors, sludges may be classified as follows:
I. Sludge Origin
A. Municipal
1. Domestic and industrial wastewater
2. Potable water treatment plant
B. Industrial Wastewater Treatment
H. Sludge Treatment Processes
A. Raw (untreated)
B. Digested
1. Aerobic
2. Anaerobic
C. Digested and Elutriated
D. Air Floating Sludge
E. Chemically Treated
F. Heat Treated
III. Degree of Wastewater Treatment
A. Primary
B. Secondary
1. Activated
2. Waste Activated
3. Trickling filter humus
C. Advanced
D. Others
1. Primary combined with waste activated sludge
2. Primary combined with trickling filter humus
3, Waste activated sludge combined with chemically
precipitated sludge, etc.
17
-------
The nature and characteristics of the sludges from industrial wastes are
extremely diverse, when compared with municipal water and wastewater
sludges. This study deals with the municipal sludges only,although the
municipal wastewater treatment plant may receive both domestic and
industrial wastewater.
The sludge produced from municipal water treatment plants are much
different in character from those produced from wastewater treatment
plants. In a municipal water treatment plant, sludge is obtained from
two major sources: (1) sedimentation basins, and (2) filter backwashes.
The specific character of this sludge depends upon the source of the raw
water and the types of processes utilized.
18
-------
SLUDGE QUANTITIES
The literature review did not reveal an exact value for the volume of
the sludge being produced from municipal treatment plants throughout
the U.S. However, several investigators have made attempts to esti-
mate the sludge quantities. These estimates are rather fragmentary
and, in some cases, conflicting. Nevertheless, these estimated values
provide an order of magnitude value of the sludge quantities being gen-
erated throughout the U.S.
Waste water Treatment Plant
The quantities of sludge generated from municipal wastewater treatment
plants throughout the U.S. approach 120 Ibs./capita/year or over 13
million tons/year, on a dry solids basis (5). The total volume of liquid
wastes before treatment has been estimated to exceed 23 billion gallons/
day (6) with the volume of the sludge produced from secondary treat-
ment of the domestic wastewater alone being estimated to exceed 120
million gallons/day (7). Reported average values of the total digested
sludge was 25 Ibs. /capita/year (8). Table 1 shows typical quantities
of sludge produced by different treatment processes. Quantities of the
sludge on a dry solids per capita basis are highly variable, since they
depend on the type of unit process and unit operation applied in the
treatment of wastewater. This variation in the quantities of sludge pro-
duced by different treatment processes is evident from Table 1.
Because of the growing concern and greater emphasis on the enhance-
ment of water quality and the environment, more sophisticated processes
and equipment will be used in wastewater treatments. This will prob-
ably result in a great increase in the quantity of the sludge generated.
Also, more communities will be served by the wastewater utilities. It
is reported (5), that over 170 million people in the U.S. are being
served by municipal utilities. It is further estimated that this figure
will increase at a rate of 2.25%/year for the next 10 years.
Water Treatment Plant
The quantities of the sludge produced in municipal water treatment
plants are naturally far less than those produced from wastewater
treatment plants. The controlling factor in the production of sludge at
a municipal water treatment plant is the source of the raw water.
Ground water supplies are normally high in hardness and frequently
high in manganese, magnesium, and iron. Commonly, lime, lime-soda,
or lime-caustic are used to soften such waters. The resulting sludge
19
-------
TABLE 1: Normal Quantities of Sludge Produced by Different Treatment Processes* (14)
Treatment Process
Primary sedimentation:
Undigested
Digested in separate
Normal
Gal/
million
gal of
s ewage
2.950
tanks 1,450
Digested and dewatered
on sand beds
Digested and dewatered
on vacuum filters
Trickling Filter
Chemical Precipitation
Dewatered on vacuum
filters
745
5, 120
Primary sedimentation and
activated sludge:
Undigested
6,900
Undigested and dewatered
on vacuum filters
Digested in separate
1.480
tanks 2, 700
Digested and dewatered
on sand beds
Digested and dewatere*
on vacuum filters
Activated sludge:
Wet Sludge
Dewatered on vacuum
filters
Dried by heat dryers
Septic tanks, digested
Imhoff tanks, digested
1
19, 400
900
500
Quantity
Tons/
million
gal of
sewage
12.5
6. 25
0. 94
1.36
3. 17
22.0
6.0
29.25
5.85
11.67
1*7 C
. 1-5
3.5
75,0
5,62
1. 17
of Sludge
Cu ft/
1,000
persons
daily
39.0
19. 0
5.7
4.3
9.9
63.5
19.3
92.0
20.0
36.0
10 ft
ID* \)
11.7
Moisture
%
95
94
60
72.5
SZ. 5
92,5
72.5
96
80
94
f.0
v\J
80
i
258.0
19. 0
3. 0
12.0
6.7
98.5
60
4
90
85
Specific
gravity of Specific
sludge gravity of
solids sludge
1.40 1.02
1.03
1 no
1.33 1.025
1.93 1.03
..... If] 7
n 95
----- 103
0.95
1.25 1,005
OQ**
1.40 1.04
1.27 1.04
Dry
Lb/
million
gal of
sewage
1,250
750
750
750
476
3, 300
3. 300
2, 340
2, 340
1,400
1, 400
1,400
2,250
2,250
2,500
810
690
Solids
Lb/
1, 000
persons
daily
125
75
75
75
48
330
330
234
234
140
140
140
225
225
225
81
69
Based on a sewage flow of 100 gpcd and 300 ppm, or 0. 25 Ib per capita daily, of suspended solids in sewage
-------
production is quite high, with an average of 2 tons /million gallons of
water treated on a dry solids basis. Surface waters are generally low
in hardness, but high in turbidity and microorganisms. Alum, chlor-
ides of iron, and organic polymers, etc. are generally applied for
coagulation, floeculation, and purification purposes. The quantities of
the sludge produced are less, being approximately 0.16 ton/million
gallons of water treated on a dry solids basis.
21
-------
WASTEWATER TREATMENT PLANT SLUDGE CHARACTERISTICS
Wastewater treatment plant sludges have been defined as a mixture of
solid phases suspended in an aqueous solution of dissolved substances.
Characteristics of the sludges from wastewater treatment plant are
highly variable, and depend on the sources of the wastewater, type of
sewerage system, unit of operation and unit process to which both the
wastewater and the sludge are subjected, and the degree of treatment
required.
In general, primary sludge is gray in color, slimy in nature, and gives
an offensive odor. It has a solids content ranging from 1 to 5% and is
readily digested under suitable conditions of operation. The fresh
solids include chemicals that have been used for flocculation and sedi-
mentation, in more or less stoichiometric amounts, and up to 90% of
the suspended solids present in the raw wastewater (10).
Activated sludge is generally brown in color. When the color darkens,
it indicates septic conditions. The fresh sludge has an inoffensive
characteristic odor. In septic conditions it has the disagreeable odor
of putrefaction. Some general properties are presented in Table 2.
Trickling filter humus has a brownish color and is in the flocculated
condition. When sludge is fresh, it has a relatively inoffensive odor;
but when it undergoes decomposition, which is relatively slower than
for other undigested sludges, it gives off a very offensive odor. The
solids content of humus approaches 3% for high-rate filter and 7% for
low-rate filters (11). The low-rate trickling filter captures from 50
to 60% of the dissolved solids reaching the filter; while for high-rate
filter the percentage ranges from 80 to 90% (10).
Digested sludge has a dark-brown to black color. If the sludge is
thoroughly digested it will not have an offensive odor; it has an odor
similar to that of hot tar or burnt rubber. During drying, both
moisture and entrapped gases are released, leaving a well-cracked
surface, with an odor similar to that of garden loams. Some general
properties of digested sludges are presented in Table 2.
The solids fraction of the wastewater sludge are primarily composed
of biodegradable material, stable organic matter and inert material.
The average reported proportions of these constituents are as follows (1):
22
-------
TABE.E 2: Some General Properties of Activated and Digested Sludge
Investigated by Howe (15)
K>
Color
•Digested Sludge
Brown
Physical State
ph
Gelatinous mass or
jelly-like
7. 0 - 9. 0
Percent Solids, Total
0. 2 - 4. 0
Carbon Nitrogen
Ratio
Density (gm/ml)
4. 0 - 7. 0
0. 95 - 1. 10
Ion Exchange
Capacity (Milliequivalent
per 100 gm solids) 350 +_ of both cation and inion
O-R Potential (mv) + 100 to +450
Chemical Elements Found
C, N, H, O, P, K, S,
Al, Ca, Mg. Cl, Na, Fe
Activated Sludge
Color Blackish
Physical State
Amorphous, non-plastic
and heterogenic mixture
ph 6. 5 - 8. 0
Percent Solids, Total 3. 0 - 8. 0
Carbon-Nitrogen Ratio 14. 0 - 22. 0
Density (gm/ml) 1.050 - 1.200
Cation Exchange Capacity
(Milliequivalent per 100
gm solids) 300 - 450
O-R Potential (mv) -400 to - 420
Chemical Elements
Found
Major Contents
Other Contents
Major Contents
Gelatinous matter of microbial matrix
... ., . Other Contents
Ammo acids, vitamins, sugar, nitrate
sulfate, phosphate, etc.
K, Cr, Zn, Sn, Mn
Fe, Cu, Ca, Mg, Si, etc.
Varied according to the
origin of raw sludge
Humus-like and lignin-
like material
Phosphates, sulfides,
ammonium, biocarbonates,
carbonates, organic acids,
polyuronides, alcohols, etc.
-------
Biodegradable organics 30%
Stable organics 35%
Inert material 35%
The physical condition and composition of the solid fraction for an aver-
age domestic sewage are presented in Table 3 (1).
General classification and significant characteristics for these sludges
are presented in Table 4 (12). A schematic of those analyses which is
most helpful in characterization of wastewater sludges is shown in
Figure 1 (13).
The effective utilization of wastewater sludges will be dictated by six
major physical and chemical properties. In order to properly describe
the sludge characteristics these six properties, outlined below will be
discussed in much greater detail: Settling Characteristics, Specific
Resistance, Flow Characteristics, Calorific Values, Chemical Compo-
sition, and Fertilizer Values. Other characteristics not pertaining to
this study include solids, pH, odor, oxygen demand, alkalinity, tem-
perature, hexane solubles, etc.
Sludge Settling Characteristics
The settling characteristics of the sludge are vital parameters, since
the main objective of the sludge treatment is to decrease the volume of
the solid residue of the sludge. Based on the particle size concentra-
tion and, tendency for interaction, settling is divided into four types.
In type I settling, particles settle as an individual entity, and strong
interaction force does not exist between the particles. This type of
settling is called "discrete settling".
In type II settling, particles coalesce or form floes during sedimenta-
tion. This coalescence increases the particle's mass which causes a
faster settling rate. This is referred to as "flocculant settling".
In type III settling, intraparticular forces are strong enough to hinder
the settling nature of the neighboring particles. Thus, particles remain
in a fixed position with respect to each other and settle as a unit. A
distinct interface between the solids and the liquid develops. This type
of settling is referred to as "Zone Settling".
In type IV settling, the particles at the bottom of the tank are further
compacted by the weight of the particles which are constantly being
added. Thus, settling occurs only by compression of the structure or
mass of the settled particles. This type of settling is known as "Com-
pression Settling". It is very common that more than one type of settling
will occur at a given period during sedimentation operation.
24
-------
TABLE 3: Physical Condition and Composition of Solids in
an Average Domestic Sewage (1)
(Numbers are in mg/1)
Suspended
Solids
200
Total
Solids S
600
Dissolved
Solids
400
Settleable
Solids
1ZO
Colloidal
Solids
80
Colloidal
Solids
40
Dissolved
Solids
360
Organic 90
Inorganic 30
Organic 55
Inorganic 25
Organic 30
Inorganic 10
Organic 125
Inorganic 235
Settleable
Solids 120
Colloidal
Solids
120
Dissolved
Solids 360
Total
.Solids
600
-------
TABLE 4: General Categorization of Sludges (12)
Sludge Source
Influent ss (primary sludge)
Chemical Treatment Sludge
to
Waste Activated Sludge
Effluent As
Unit Process Function
Collection and concentration of
suspended material
Coagulation, precipitation, and
concentration of participates
Concentration, digestion, de-
watering, heat drying, and
combustion
Filtration
Significant Characteristics
Concentration of SB, inorganic con-
stituents, organic constituents,
specific gravity (settleability),
and dewaterability.
All of those mentioned above,
• effects of pH changes, and effect
of treatment chemicals on subse-
quent processes or water use.
Fraction of inert materials,
amenability of organic fraction
to digestion, heat value per unit
weight of sludge, settleability,
and dewaterability.
Concentration of ss, biological
activity in terms of oxygen demand,
composition of effluent sludge in
terms of nutrients, both organic
and inorganic, and microbial
analyses.
-------
SLUDGES
1*1 INDUSTRIAL
(B) IMMORAL
(C) BIOLOGICAL SYNTHE9S
I0t OCMtCAL
GENERAL CHARACTERISTICS
SPEOFtt GRAVITY
MUSS
MLVSS
SVI
SETTLW6 VELOCITY
SPECIFIC RESISTANCE
COEFFKXNT OF OOMFRESSiaLITY
10
1 1
| INORGANIC]
| CHEMICAL
HM«y MdoH AMtn* Eortti
Mtloit
ANALYSIS
Hongm
| ORGANIC]
ItALOmFIC CONTENT
»X~" |
PhOHMMM 1
cpOjjrocTTOpi
ing VS5 I
NON-aOLOGlCAL 8
BIOLOGICAL-
NOW - ACTIVE
I
INON-BCLOGCAL
Oil a
(Eitar EilrwMIn)
TTC
<-*i5|
«f VSS
PWt GOMII
'^v^>
titoerotic
Figure 1. Analysis of Wastewater Solids (13)
-------
In the treatment of wastewater sludge, zone settling and compression
settling are of major significance. Settling tests are usually required
to determine the settling characteristics of the suspension. In the
design of settling tanks, both clarification of the liquid over-flow and
thickening of the sludge under-flow is involved. The over-flow rate for
clarification requires that the average rise velocity of the liquid over-
flowing the tank be less than the zone settling velocity of the suspension.
Over-flow rate determination is based on such factors as are needed for
free-settling and discrete-settling in the discrete-settling region (see
Figure 2). These factors include the area needed on the basis of the
rate of settling of the interface between the discrete-settling region and
zone- settling region, and the rate of sludge withdrawal from the com-
pression region. However, rate of zone settling seems to be the con-
trolling factor because: (1) it is less than the rate of free particle
settling, and (2) the upper portion of the sludge, just below the interface,
acts as a filter to entrap and strain-out slower settling particles that
otherwise might be carried out by the rising water through the sludge
mass to the over-flow weirs (14).
The Talmadge and Fitch (16) method is generally applied to determine
settling tank area requirement for zone settling. This method utilizes
a settling column (1 liter graduated cylinder) filled with a homogeneous
mixture of the sludge. The sludge solids are then allowed to settle
under quiescent conditions. As the suspension settles, the position of
the interface with respect to time is recorded. The settling curve is
then plotted as shown in Figure 3. Tangents are projected from both
hindered and compression settling zones. The point where the bisector
of the angle formed by the two tangents intersects the settling curve,
is called the point of critical compression or critical concentration
"C2". Furthermore, the height of the sludge in the settling column at
the desired underflow concentration "C " is noted as H . This height
u u
can be determined using a mass-balance relationship:
H C = H C (1)
o o u u
where,
H = initial height of interface in column,
C = initial concentration of the sludge.
To determine the settling time required to reach the desired underflow
concentration "C ", a horizontal line is drawn from the computed H
u u
value to a line tangent to the settling curve at the point of critical com-
pression. The intersecting point represents the required settling time
28
-------
t>o
'«>
B
Region of clear water
Region of discrete settling
Region of hindered settling
Region of zone settling
Region of compression settling
Settling time Settling column
Figure 2. Schematic of Sludge Settling
H
Critical Compression
Compression
'2 "u
Settling time
Figure 3. Graphical Presentation of Zone Settling Curve
29
-------
The area required for sludge thickening is then found using the following
relationship:
i -w -M _ *j ^ **-»*.»,*» ' f\ i.
A • TT <2>
O
where,
A = area required for sludge thickening, sq. ft.,
Q = flow rate into tank, cfs,
H = initial height of interface in column, ft,
o
t = time to reach desired underflow concentration, sec.
u
The tank area required for clarification purposes is generally based on
the underflow rate, i.e.,
A - -S-
A OR
and
C C -C
OR = 77-r —- ( —^—-)
O U
where,
Q = flow rate into tank, MGD,
OR = overflow rate, gal/day/ft ,
C, = 120, conversion factor from mg/1 to Ibs/gal,
U.A. = unit area, ft /lb solid/day,
C = influent solids concentration,
o
C = underflow suspended solids concentration.
The concentration of the settled sludge is effected by a number of fac-
tors (17) which include: wastewater characteristics; type of biological
treatment the wastewater receives and whether the sludge from bio-
logical treatment is handled separately, or is returned to the settling
basin; design and operation of the settling tank; whether settling was
30
-------
promoted, using mechanical means or chemicals; settleable solids
characteristics, density, shape percentage of volatile solids, viscosity,
electrostatic charges; and solids concentration in the original suspen-
sion, sludge detention time, etc.
Settling rates of the biiogical sludge depend on the type of micro-
organism which is responsible for the nature of the substrate and
organic loading in case of an activated sludge aeration basin. Further
limitation of the settling rate parameters as noted by Ford (12) are as
follows: (1) the effect of initial sludge depth on settling velocity; (2)
stirring speed and configuration of racking mechanism; (3) lack of
reproducibility when measuring sludge settleability in test cylinders,
and (4) the effect of test-cylinder diameter on settling rates.
Larson (59) has carried out a series of tests, using various concentra-
tions of similar sludge in different sizes of cylinders and has measured
the settling rates. Results of his tests are graphically represented in
Figure (4). It is apparent that for normal mixed liquorsolids concentra-
tions applied in activated sludge systems, the smaller the test cylinder
size (large circumference/area ratio) the higher would be the settling
velocity of the solids.
0)
•o
VD
CT
L.
•3 1.0
g 0.9
CJ
0.8
8000 mg/l
4000 mg/1
2000 mg/I
Concentrations Normally
Employed in Activated
Sludge Aeration Basins
0 4 8 12 16 20 24 28 32 36
Cylinder Diameter, inches
Figure 4. Effect of test cylinder diameter on sludge settling rate (59)
31
-------
Heat treatment of sludge under pressure for one hour at temperatures
below 250°F has been shown to improve settling characteristics of the
sludge significantly. Substantial solids volume reduction, and sludge
solids concentration have been observed. Results of the study made by
Hurwitz et al., (19) on three types of sludges listed in Table 5, are
given in Figure 5 and Figure 6.
Table 5. Sewage Sludge Characteristics (19)
Sludge Designation
Type of Sludge
Raw Digested Raw
Primary Primary Activated
Activated
Total Solids
Concentration - g/1 49.7
Ash-g/1 10.4
Volatile-g/1 39.3
% Volatile Solids 79.0
COD-g/1 61.8
COD/Volatile Ratio 1.57
61.3
26.0
35.3
57.6
57.6
1.64
23.0
8.6
14.4
62.5
31.3
2.17
The concentration of solids in sludges as a result of treatment has been
reported in the literature and is summarized in Table 6. These values
indicate that the solids, content of sludge varies from 0.5 to 10% fox jaw
sludges, and from 2 to 15% after digestion. Digested sludge shows a
definite thickening when the solids content is higher than 5%. It develops
the consistency of syrup and is less free flowing compared to water. At
a solids content of 15 to 20% the sludge will no longer flow, and it can
be handled with a spading fork at solids content of 25%. At 50% solids
content the sludge appears moisture free and at 90% concentration it is
dust dry.
Specific Resistance of Sludge
Another important physical characteristic of the sludge in the design
of treatment processes is the specific resistance; the difficulty en-
countered in removing water or conversely the ability of the sludge to
retain water. Some of the processes where specific resistance is im-
portant are vacuum filtration; centrifugation; and <3rying on sand beds (1),
32
-------
TWO HOUR SETTLING PERIOD
100
1
CO
o
Ul
N
e
X
o
§
V)
SLUDGE
o RAW PRIMARY
RAW ACTIVATED
a DIGESTED PRIMARY ACTIVATED
40 60
REDUCTION-%
Figure 5
COD reduction vs. settled oxidized solids volume
TWO HOUR SETTLING PERIOD
SETTLED OXIDIZED SOLIDS CONCENTRATK
- ro w
o o o
D O O O
^
*A
i i i ' L
fW'1^J***J>^^™'
SLUDGE
o RAW PRIMARY
A RAW ACTIVATED
D DIGESTED PRIMARY ACTIVAT
^
a
fi
*
ED
80
10O
"0 20 40 60
C.O.D. REDUCTION -%
Figure 6
Settled oxidized solids concentration vs. COD reduction
33
-------
Table 6. Solids Concentration of Sludges
Type Solids Cone. (14,18,33
Raw Sludge
Plain Sedimentation 2.5 - 5.5%
Trickling Filter 4 - 10%
Trickling Filter, Mixed 3 - 6%
Activated 0.5 - 1.2%
Activated, Thickened 1 - 2%
Activated, Mixed 2.6 - 5%
Modified Aeration 2 - 4%
Modified Aeration, Mixed 3 - 4%
Digested Sludge
Plain Sedimentation 10 - 15%
Trickling Filter 10% -
Activated 2 - 3%
Activated, Mixed 6 - 8%
The specific resistance "r" concept is used to evaluate filtration
characteristics of the sludge. Specific resistance is numerically
equal to the pressure difference required to produce a unit rate of
filtrate flow of unit viscosity through a unit weight of cake (20,21).
Studies on the theoretical approach to filtration go back to 1908, to
Hatschek (60); and, reached a satisfactory stage by Ruth et al. ,
(61,62) and Carman (63,64) in 1933-1935. However, the application
of these theories and selection of the best method for filtration of
sewage sludge was made by Coakley and Jones (22) in 1956. They
have found that among all of the filtration theories available, the
simplest and most easily applicable theory is that of Carman's,
Carman's basic equation for a rigid cake is:
dt 0(rivV+RA)
whe re ,
34
-------
-~ = rate of flow of liquid across the bed thickness,
dt
P = pressure difference across the bed,
A = area of the bed,
[i = viscosity of the filtrate,
v = constant of proportionality between volume of filtrate
and volume of solids deposited,
V = volume of filtrate,
R = initial resistance per unit area for a liquid of unit
viscosity.
r. = the resistance of the unit weight of cake per unit area,
Integrating,e. g., at constant pressure we will have:
or
(7)
This equation gives a straight line when t/v is plotted against V
with a slope b equal to:
(8)
and the specific resistance is obtained by simple rearrangements, i. e.,
(9)
Coakley and Jones determined that Carman1 s filtration theory for
rigid cake is also applicable for compressible cakes like those
obtained from sewage sludge, if "r. is regarded as a function of
"P". They have also noted that it is easier to express the sludge
concentration by weight of dry cake solids per unit volume and
liquid, C, than by unit volume of cake per unit volume of filtrate,
v. Substituting C for v in equation 9, the relationship for "r"
applicable to sewage sludges, was develaped, i. e.,
35
-------
,.. 2bA2P
and
- ?£
100 - C. 1 Cf
where,
P = pressure difference across the cake, gr/cm ,
2
A = area of filtering surface, cm ,
H = viscosity of filtrate, poise,
r = specific resistance, as resistance per cm of cake,
sec /gr,
b = slope of the t/v vs. V plot,
V = volume of filtration,
t = time interval during test,
C. = the percent liquid of the sludge,
C = the percent liquid of the filter cake.
It was further suggested that in any experimental determination
of specific resistance, t and V should not be measured from the
beginning of the filtration. It is necessary to record the filtrated
volume after a thickness of cake having a resistance equal to the
resistance of the filter cloth has been deposited. This problem is
normally over come if the first two minutes of the test are
neglected.
The specific resistance "r" is also a function of the pressure
difference in accordance with the relationship:
= r'
PS, (12)
where,
r1 = specific resistance at unit pressure,
P = pressure difference of filtration apparatus,
s = coefficient of compressibility.
-------
Typical values of coefficient of compressibility reported (22) for
various types of sludges are;
Type of Sludge Coefficient of Compress.
Digested 0. 70 to 0. 86
Activated 0. 60 to 0. 79
Raw 0. 87
Humus 0.80
The parameters "r" and "s" indicate the dewatering abilities
of the sludge. Low "r" values are indicative of a sludge with rapid
draining or filtering characteristics and the reverse is true for
high values of "r"'l°'. Typical values of "r" for various types of
sludges are listed in Table 7.
Specific resistivity of sludge can be improved dramatically
by using chemical coagulants. Among the most common chemicals
used are tri valent metal ions such as Al+++ and Fe+++; and
organic polyelectrolytes (cationic, nonionic, anionic). A com-
prehensive study of chemical flocculants has been made by Tenney
et al. (23)
The draining or filtering characteristics of the sludge can be
significantly improved by employing a wet-air oxidation process
(heat treatment at below 300 psi pressure and up to 300°F
temperature). In this process, insoluble organic matter is converted
to simpler, soluble organic compounds which are then oxidized and
converted to CC«2 and water. It has been reported**'' that small
amounts of oxidation, 15-20%twill convert the solid structure of the
sludge from a hydrophilic nature to an easily drained or hydro-
phobic structure. Results of the effect of heat treatment for one
hour on filtering characteristics of various sludges are illustrated
in Table 8.
Sludge Flow Characteristics
In a wastewater treatment plant the solids which have been
removed through various processes are hydraulically transported
by pipeline. The densities of these solid particles are generally
higher than the transporting fluid and in order to prevent the
settling of these particles, a continuous mixing of the flow is
essential (high turbulence).
37
-------
TABLE 7
*(12)
Reported Sludge Specific Resistance Values
Specific
Resistance
Description (sec2/gram) x 107
Domestic Activated Sludge 2, 800. 0
Activated (digested) 800. 0
Primary (raw) 1,310-2, 110
Primary (digested) 380-2, 170
Detention Time Stage
7. 5 days 1 1,590.0
10. 0 days 1 1,540.0
15. 0 days 1 1,230.0
20. 0 days 1 530. 0
30.0 days 1 760.0
15. 0 days 2 400.0
20. 0 days 2 400. 0
30.0 days 2 480.0
Activated sludge + 13. 5% FeCl3 45. 0
Activated sludge + 10. 0% FeCls 75. 0
Activated sludge + 125% by wt.
newsprint 15. 0
Activated digested sludge + 6%
FeCl3 + 10% CaO 5.0
Activated digested sludge + 1 25%
by wt. newsprint + 5% CaO 4. 5
2
* All values at 500 gms/cm pressure
38
-------
u>
vo
TABLE 8
Specific Resistance Values of Oxidized Sludges
Raw Primary Sludge
(19)
Oxidation Temperature,
%COD Reduction
Specific Filtration
Resistance
sec2/gx 10?
°F 100 150 180 190
Untreated 0.0 19.3 28.2 34.2
heated
500-800 800 26 3.1 4.7
200
45.1
5.2
210
53. 1
4.2
220 250
65.7 83.4
9. 4 4. 3
Digested Primary-Activated Sludge
Oxidation Temperature,
%COD Reduction
Specific Filtration
Resistance
sec2/g x 107
Oxidation Temperature,
°F 120 160 170 180 190
Untreated 6.9 19.5 34.8 46.3 56.4
2,170 91 3.5 5.2 6.7 6.6
Raw Activated Sludge
°F 150 170 180 190
200
62.5
6.7
220
210
69.9
10.1
260
220 230 24C
76. 0 80. 7 84.
8. 6 8. 8 4.
%COD Reduction
%ecific Filtration
2 7
sec /g x 10
Untreated 9.5 15.4 23.3 39.3
18,700 58 14.1 14.6 11.5
63.7 78.3
10.9 6.6
-------
The heterogeneous nature of wastewater sludge causes
flow phenomena. Thus, direct application of existing hydraulic
formulas for head loss computation is not adequate in most cases.
Although sludge flow is affected by a number of factors, the most
significant one is the moisture content (M) of the sludge. Based on
the moisture content, the sludge flow is classified by: (1) flow in
suspension, and (2) plastic flow.
The moisture content which delineates between the two types
of flow is called the limiting moisture content (ML) and is defined
as the critical moisture content in percent, where a measurable
yield value (S ) first occurs. Thus, below MT the flow is plastic
and above ML it is in suspension. The major parameters which
have to be considered in the study of sludge flow are as follows:
(1) Reynolds Number (R^) and critical velocity; (2) specific gravity;
(3) coefficient of rigidity; and (4) head loss.
In the case of flow in suspension, the presence of suspended
particles has a negligible effect on the viscosity of the liquid. Thus,
the characteristics of the sludge flow are very close to that of
viscous liquids, and the head loss is independent of M. The Reynolds
number, RN, can be expressed by the relationship (24):
RN = J°VD ^ PVD (13)
r? [i
where,
2 4
p = density of the fluid, Ibs. - sec. /ft ,
V = flow velocity, ft/sec. ,
D = pipe diameter, ft,
?7 = coefficient of rigidity, Ibs. - sec. /ft
fj, = coefficient of viscosity, Ibs. - sec. /ft .
The suggested lower and upper values for RN are 2000 and 3000,
respectively(25). Thus, the critical velocities are:
V = 2000n (14)
and
V = 3000u (16)
cu "
40
-------
and the Reynolds number becomes,
R = PVD = 3p DV2 (17)
3r? V + 16 S D
The critical velocities are
f 103^947? 2 + D'
Ic
and
_ 1500TJ +127Vl40T?2+D2S p
uc - _ - y
pD
From Table I it can be seen that the specific gravity for the
various typed sludges does not vary significantly. The margin of
variation is only 7% (24). The specific gravity value for a sludge can
be used to determine its specific weight from the relationship,
p=62. 4 G.
The coefficient of rigidity of sludge is a direct function of the
sludge moisture content. The correlation between moisture content
and coefficient of rigidity is shown in Figure 7.
"High" and "Mean" values are offered for design purposes.
However, as Chou(24) states, "The experimental determination of
T] by either viscometer or from, pressure drop measurement is very
delicate". Thus, any over estimate will increase the critical
velocity and to some extent the head loss.
Head loss of the sludge flow in pipelines depends on such factors
as type and stage of flow. When sludge flow is typified by
flow in suspension and the velocity of the flow is in the laminar
stage, head loss is practically the same as that for water, i. e. ,
(20)
pD pD
where,
h = head loss between any two points in a pipeline, ft. ,
L = distance between two points in a pipeline, ft.
41
-------
ACTIVATED » OOOI x
PRIMARY . 0019 *
DIGESTED I
DIGESTED (GERMANY) 6
IMHOFF °
UNCLASSIFIED v
IN Ib/ftAec
Figure 7. Coefficient of Rigidity rj of Sludges(24)
-------
When the velocity of the flow is turbulent, the head loss can be
computed by the relation:
h =G2hw, (21)
s
where,
h = head loss of flow in suspension with moisture content
M, ft. S
hw = corresponding head loss of water, ft. ,
G = specific gravity of sludge.
Laminar plastic flow is the most common type of sludge flow
and the suggested formulas for determining the head loss are (25):
Sludge Head h, _ 16Sw A v (22)
Water Head h, 16S
3WD WD
I = ~""v + JiX, (23)
wn^
The literature offers no specific formula? for computing the
head loss for plastic flow where turbulent conditions prevail.
Available information is rather inconsistent and in some cases
contradictory. Some studies have shown that head loss is a function of
M and varies from 1. 5 to 8 times that of water. Other studies
claim that the head loss is independent of M and is almost the
same as that of water (24). The most reliable information appears
to be from the work of Brisbin(26) ancj Chou(27) .
Calorific Values of Sludges
Calorific values of the sludge is a vital parameter in the design
and operation of incinerators for the disposal of sludges. Calorific
values of the sludge, combined with its ultimate analysis, determine
the quality of the sludge for incineration. Reported (28) /aiues
indicate that the combustible fraction of the sludge is variable
although a large portion is grease. Raw sludge has been found to
contain 75% combustible matter, and digested sludge may contain
only 50% combustible matter on an average basis. Available in-
formation on calorific values of various sludge has been collected
and is listed in Table 9. As seen from Table 9 the heat content
varies from 5000 to 14000 Bt u's per pound on a dry basis. The
43
-------
TABLE 9
Calorific Values of Various Sludges
Description
Undigested Sludge
Digested Sludge
Grease and Skimmings
Screenings
Grit
Activated Sludge
Sludge Digester Gases
% Combustible
71.
72.
79.
71.
80.
74.
48.
52.
59.
49.
48.
59.
88.
80.
74.
92.
84.
86.
33.
33.
5
6
9
7
0
0
3
5
6
6
0
6
5
2
8
0
4
4
2
2
•
-
% Ash
28.
27.
20.
28.
20.
26.
51.
47.
40.
50.
52.
40.
11.
19.
25.
8.
15.
13.
69.
69.
5
4
3
3
6
0
7
5
4
4
0
4
5
8
2
0
6
6
8
8
-
-
Btu/lb
13,
12,
11.
11,
11,
10,
9.
9,
8,
8,
10,
5,
16,
10,
10,
10,
9,
6,
10,
4,
6,
900
320
775
335
420
285
825
500
980
020
750
290
750
350
000
000
950
980
146
000
540
650
Ref.
40
40
40
40
12,
41
40
40
40
40
40
12,
40
40
40
40
40
40
40
12,
41
42
29
29
29
44
-------
calorific value of the sludge will increase dramatically if organic
polymers are employed in conditioning the sludge. It is believed
that an increase from 1500 to 4000 Btu's/lb of dry solids, and ash
content reductions of 5 to 20% can be obtained using organic
polymers (29).
The heat value of the sludge can be determined if the chemical
analysis of the sludge is known, using DuLong's equation,
Q = 14,600 C + 62, 000 (H-2) (24)
8
where,
Q = Btu/lb
C = % Carbon
H = % Hydrogen
O = % Oxygen
Typical values for the ultimate analysis and calorific values
of the sludge and other waste materials are presented in Table 10.
Chemical Composition of Sludge
A proper assessment of the usefulness of the sludge can be
made if the chemical composition is known. A compilation of the
organic fractions for the various sludges reported in the literature
is presented in Table 11. Analysis of the specific organic consti-
tuents of a sludge is important in predicting what conditions will
prevail during dewatering and also helps in determining what
specific effects the sludge might have on solid characteristics.
TABLE 10: Ultimate Analysis of Waste Materials (28)
Material C% H% O% N% Btu/lb
Wood 53.5 6.0 40.4 0.1 9,900
Garbage 54.5 7.5 35.0 1.0 10,000
Rubbish 53.0 6.0 37.0 3.5 9,000
Sludge 55.0 7.0 35.0 3.0 10,000
Hunter and Heukelekian (30) have made a comprehensive
investigation of the composition of various sewage fractions.
They found that the particulate-fraction solids were about 80%
organic matter, while the soluble-fraction solids contained
about 30% organic matter. The particulate-fraction solids
45
-------
TABLE 11: Organic Fractions for Various Sludges
(Percent of Dry Weight) (12)
Constituents
Organic matter
Total Ash
Pentosams
Grease and Fat
f* olllll f\Q&
v^cllUlOHC
Liignin
Protein
Plain
Settled
60 - 80
20 - 40
1. 00
7-35
3?n
• bU
3RO
• ou
5 an
t OU
22 - 28
Digested
45 - 60
40 - 55
1.6
3-17
l 6
0 6
8 A.
i t
16 - 41
Activated
62 - 75
25 - 38
2. 1
5 - 12
7 n
i • U
32 - 41
Raw
Activated
67. 0
33. 0
7.2
21. 0
12.4
15% t
Oxidized
64.0
36.0
2.4
19.3
8.4
80% t
Oxidized
23.4
0, 0
1.8
0.5
*«
Ether extract
Solids fraction only
-------
contained from 4 to 7% organic nitrogen, while the soluble-
fraction solids contained less than 1%. The particulate fraction
was composed mainly of carbohydrates, 24%, amino acids, 19%,
and grease, 17%, and free fatty acids were present only in small
amounts. The soluble organic matter was found to be composed
largely of ethyl-ether extr actable matter, of which the organic
acids were the primary constituents, 56%.
A study made by Hunter and Rickert (31) showed that activ-
ated sludge treatment resulted in the following solids reduction:
(a) total solids were reduced 42% and volatile solids 66%; (b)
particulate total solids were reduced 86% and particulate vola-
tile solids 86%; and (c) soluble total solids were reduced 20%
and soluble volatile solids 46%. This study also showed that
activated sludge treatment is very effective in reducing par-
ticulate organic matter but ineffective in reducing soluble
organic matter. However, the author and others believe that
they may have had an atypical substrate and system, partic-
ularly in reference to their results showing the ineffectiveness
of an activated sludge process in reducing soluble organic
matter.
The organic content of sewage sludge is usually measured
from the BOD and the volatile solids determination. However,
in recent years the trend is to use other means so that the chance
that inorganics might also volatilize during the course of the
experiment is eliminated. More useful and implicit tools for
determining the organic content are the COD and the TOG tests.
Rudolfs (32) has made a thorough analysis of the inorganic
constituents of digested and activated sludges. The results of his
investigation are listed in Table 12. A comprehensive investiga-
tion of the metallic content of the various air-dried sludges were
made by Thompson et al.(8). Results of their studies are presented
in Table 13. A more or less complete list of minor and major
elements found in the various types of sewage sludge is presented
in Table 14.
Fertilizer Value of Sludge
Disposal of waste water treatment stabilized sludges in either
the liquid or dried solid form, as soil fertilizer has received sig-
nificant attention in the U. S. because of the high cost and environ-
mental effects of alternate means of disposal.
47
-------
TABLE 12
Inorganic Constituents in Sludges
(Percent of Dry Weight) (32)
Chemical
Constituents
Silicon
Iron
Aluminum
Calcium
Magnesium
Potassium
Sodium
Titanium
Manganese
Copper
Barium
Zinc ZnO
Lead
Nickel
Cobalt
Sulfur
Chlorine
Chromium
Arsenic
Boron
Iodine
Phosphorus
Ignition Loss
TOTAL
Chemical
Symbol
Si02
Fe2°3
A12°3
CaO
MgO
K20
a2
MnO
CuO
BaO
ZnO
PbO
NiO
CoO
so3
C12
Cr2°3
AS2°3
B2°3
I
P2°5
Digested Sludge
(Toledo)
15.60
5.43
6.80
6.64
1.83
0.42-
0. 78
1.00
0.06
1.24
0. 81
8. 12
0. 37
0. 13
--..
2. 02
44.40
Activated S]
(Milwauke<
8. 45200
7. 14800
3.21100
1. 67500
1.81000
0. 86200
0. 32700
0. 46500
0.06110
0. 01627
0. 22500
0.00561
0. 00020
2. 90000
0. 50100
0.21900
0.01347
0. 00426
0.01130
3. 08900
68. 55000
95. 55
99. 90360
48
-------
TABLE 13
Spectrqgraphic Analysis for Metals Content of Air-Dried
Sewage Sludge (As Percent of Air-Dried Sludge) (8)
Kk'tiicnts
MtJOR
vMuiuiimm
Calciimi
] roil
Magnesium
Silifuii
T:it;iiumi
1\TKUXIO>1.\TK
( 'upper
.\icki-l
Tin
y.m>:
Sil UT
Unroll
JliTylliiim
f iutlilllll
Nichols
lUlls
(I.)
Major
2.00
1.00
Major
1.50 to 2.00
0 ^10
0.50
0.10-1-
0.05
O.U8
0.10
1.50
o.:io
O.UI
0.01
0.06
0.1)1
U.OI
Molylidi-iruiiil OIKI7
Vttriini!
Yiti-rbinin
A»ll C'OXTKNT
C/i)
T
T
,,70
Oklu. City
No.'"!'
(2.)
Major
Major
2.00
1.50
Major
1.50 to 2.00
0.30
0.30
0.10 +
0.07
U.0.1
0.10
U 10
0.50
0.01
0.05
0.01
U.OI
0.05
0.01
0.0(17
T
T
.,,.30
Okla. City
aNo'a2 "
(2.)
Major
Major
2.00
1.00
Mttjor
1.50 to 2.00
0.30
0.30
O.lO-f-
0.05
0.05
O.Oli
0.10
0.50
0.01
0.05
0.01
o.oi
0.05
0.01
0.005
T
T
5-1.50
Okla, City
Soutlisidc
(3.)
Major
Major
1.30
1.50
Minor
1.00 to 1.50
0.30
O.liO
e.io
1.00
0.30
0.20
0.10
0.70
0.10
0.05
0.007
0.10
T
-±0.10
—
—
•17.00
llrokcn
Bow
(4.)
Major
1.30
2.00
1.00
Major
1.50 to 2.00
0.30
0.10
O.07
0.10
0.01
0.05
U.07
0.50
0.01
i.oo
0.01
0.01
T
0.005
0.01
T
—
7U.OO
Idubcl
(S.)
Major
Mujur
2.00
1.00
Major
1.50 to 2.00
0.50
0.08
0.00
0.10
0.01
0.05
0.20
0.70
0.01
1.00
0.01
0.01
T
0.005
U.OI
T
—
CC.OO
Norman
No. 1
(U.)
Major
Minor
1.50
1.30
Major
1.50 to 2.00
0.70
0.20
0.10
1.00
0.01
0.07
1.00
0.70
0.01
0.50
U.OI
0.01
—
0.005
0.01
T
—
50.00
Nonnuil
No. 2
Major
3.00
1.50
1 30
Major
1.50 to 2.00
0.10
0.20
0,10
1.00
0.01
0.00
1.00
0.70
0.01
0.10
0.01
0.01
—
0.005
0.01
•r
—
04.00
Holdcnville
(7.)
Minor
1.00
2.00
a 10
Minor
0.10
0.01
0.10
0.01
0.20
0.01
0.05
0.05
0.50
0.05
0.01
0.01
T
0,005
T
— —
—
05.20
Hugo
a.)
Major
Minor
2.00
O.SO
Major
1.00 to 2.00
0.10
0.10
0.10
0.20
0.01
0.05
0.05
0.50
0.05
0.01
0.01
T
0.005
O.OOf.
T
T
—
U3.20
Lawton
(1.)
Major
M inor
2.00
1.00
Major
0.50
0.10
0.1U to 0.20
0.05
1.00
0.01
0.10
O.ttt
0.30
0.01
0.03
0.01
T
T
0.005
0.003
0.03
T
—
43.01
Enid
C2.)
Major
Major
2.00
1.00
Major
0.70
050
O.:>0
0.10
0.70
0.01
0.10
0.07
0.30
0.03
0.<*5
0.01
T
0.05
O.OI
0.001
0.0.3
T
T
54. DO
(!.) I'rii ary settling, stuiidurd-rHta liicklint: filler, linul srUliim, M-panitu siudcc diction.
li.J I'rii urv settling, nctivatrd sludge, liiuii scUliilK, SL'paruU' aliMilsc diicnliun.
i:i.) 1'rii urv SL'ttlititf. iwo-sliifB filtration, iinul guttliui;, separate Kludge digestion.
i l.J flu ;ruMi»T. utiindiirii-rntr lii^li-niti? irirklini.' Jiiti;r, final ucltiixtfr.
(5.J 1'iii ury .scrltlin^, rjluiidard-ralc trickling liluir, sctJtirutc ^liKUii di^t^itiuii.
(<>.} Hiu^orptiuli, finul aftllintt. Srimilltt1 ttkid^i1 dij£i:atioti.
(7.)'iiulmll tnnU, standurd-ritU! lixt-d-uoz/.li' liltiir, linal sc'ttlinij.
-------
TABLE 14
Summary of Major and Minor Elements in Sludge
(Milligram per Gram Dried Sludge) (34)
01
o
Elemental
Analysis
Al
Sb
As
Ba
Be
B
Cd
Ca
Cr
Co
Cu
Fe
Pb
Mg
Mn
Hg
Mo
Ni
P
K
Si
Ag
Na
Sr
S
Primary Sludge
Vverage
5. 10
1.24
2.25
0. 0025
0. 104
0. IBB
2. 05
0. 217
2.00
16. 1
1.01
10.6
0.781
0. 0046
0.362
0.522
3.78
0.243
3.96
0. 13
Range
10. 78rl. 83
1.49 0.83
5.0-0. 11
0. 0030-0. 0017
0. 15-0. 07
0. 30-0.0034
9.0-0.08
0. 5-0. 05
6.0-0.0083
20. 0-2. 86
2. 14-0. 33
15.0-5.0
1.0-0. 16
0.006-0.0030
1. 0-0. 05
2.0-0.0014
6.83-1.49
1. 0-0. 08
10. -0. 5
0. 14-0. 12
No. in
Sample
3
3
11
3
11
4
15
6
17
12
3
8
11
2
11
17
3
11
8
3
Activated Sludge
Digested Sludge
Average
10.0
Range
17.0-4. 35
No. in
Sample
1.20
1. 15
0. 0035
0.070
0. 35
13.0
4.31
0. 0016
1. 10
40.5
1. 52
7.04
0.310
0.016
0. 197
0. 378
19.9
4.21
39.5
0. 150
4.44
0. 155
10. 1
22.22-0. 101
3. 0-0.22
0. 0044-0.0026
0.22-0.006
0.44-0.26
18.0-9- 0
17.0-0. 1
0. 0016
2. 6-0. 37Z
96.6-4. 83
2.09-0. 51
10.9-3.01
0. 93-0.055
0. 020-0.012
0. 89-0. 006
2. 0-0.04
32.2-11.07
7. 16-2. 49
39.5
0. 22 -0. 1
7. 88-1. 0
0.21-10.0
11.6-7.6
3
4
2
9
2
7
8
1
13
9
3
7
9
2
8
8
8
6
1
3
2
2
6
iverage
17.86
0.897
1. 36
0. 0025
0.046
0. 264
33. 5
2.28
1.65
30. 65
1. 89
7.49
0. 976
0. 254
0. 372
12.75
2. 76
162.0
0. 195
6. 15
0.26
12.3
Range
36-7.75
0.984-0.81
4.01-0. 10
0. 0065-0.0012
0. 149-0.003
0.50-0.001
112. 0-4.9
11.0-0. 10
16.0-0. 10
60.0-10.09
7.52-0. 18
13.0-1.0
6.04-0.06
1.29-0. 002
3.0-0.0.3
25. 13-1. 18
6. 15-0.83
334. 0-73. 0
0.50-0.08
10.0-2.0
0.26
32.5-1.64
No. in
Sample
4
2
15
11
17
10
15
28
39
19
18
17
17
24
27
15
13
3
4
5
1
14
-------
TABLE 14 Cont'd.
en
Elemental
Summary of Major and Minor Elements in Sludge
(Milligram per Gram Dried Sludge)(34)
Activated Sludge
Primary Sludge
No. in
Range Sample Average
Digested Sludge
Average Range
No. in
Sample
Sn
Ti
V
Zn
Zr
Ca
0. 95
14.8
2. 09
6.87
1.72
0.063
2.0-0.5
20.0-5.0
15.0-0.3
25.0-0.34
10. 9-0. 3
0. 1-0.01
8
8
11
18
8
7
0.5
11.8
0.70
3.29
10.0
0.05
0. 5
20.0-0.50
0.89-0.51
6. 3-0. 13
10.0
0.05
1
3
3
13
1
1
0.60
14.2
5.20
4.04
2.03
0.05
0.70-0. 50
20.0-1. 0
10. 0-0. 32
11.0-0.5
5. 0-0. 10
0.05-0.05
3
3
4
39
3
3
-------
Wastewater sludges contain many fertilizing elements. In com-
parison with commercial fertilizers, they are rated principally
on their content of three substances: nitrogen as N, phosphorus
as P or as phosphoric acid (P €)_), and potassium as K or potash
(K O). The concentration of these three elements is expressed as
a percentage of the dry weight of the solids. Compositional data
obtained from various literature sources are compiled in Table 15.
It appears that the fertilizing ingredients of the sludge depend on
the type of processes used in the wastewater treatment plant.
In general, N ranges from 0. 8 to 5% for plain sedimentation
solids, and 3. 0 to 10% for activated sludges. Nitrogen content
in trickling filter humus depends on the length of its storage in
the filter and it will vary from 1. 5 to 5%. Anaerobic digestion
drastically reduces the nitrogen content of the sludge (40 to 50%).
The phosphate content of the sludge is rather small ranging from
1 to 4%, and the potash content is even smaller ranging from 0. 1
to 0.5% (10).
For a better understanding of the sludge fertilizing potential, the
fertilizing ingredients of wastewater sludge compared with various
manures and organic nitrogen material are presented in Table 16.
Minor elements present in sludge are also often important in crop
nutrition. A list of these elements for activated and digested sludge
is presented in Table 17.
A comparison of various types of sludge based on their humus con-
tent was made by Hasmann (39). Results of his study are tabulated
as follows:
Sludge Type Humus Content %
Fresh 33
Digested 35
Activated 41
Trickling Filter 47
CHARACTERISTICS OF WATER TREATMENT PLANT SLUDGE
Characteristics of the sludge from water treatment plants are
highly variable depending on such factors as : sources of raw
water; type of treatment process employed; types of chemicals
added; efficiency of unit operation: degree of solids removed;
and the time between basin and filter backwash clean-out.
The sources and hence, the quality of the raw water will have a
significant effect on the physical, chemical, biological, and quanti-
tative characteristics of the sludge produced. Sources of raw
52
-------
TABLE 15
Chemical Composition of Sewage Sludges
(Dry Weight Basis)
Sewage Treatment Plant
Washington, D.C.
Influent solids (spring)
Influent solids (summer)
Digested sludge
Baltimore, Md.
Influent Solids
Activated sludge
Humus tank sludge
Heat-dried dig. si.
Jasper, Ind.
Influent solids
Activated sludge
Digested sludge
Richmond, Ind.3
Influent solids
Activated sludge
Digested sludge
Chicago, HI. (Southwest plant)
Raw sludge
Activated sludge
Heat* dried sludge
Milwaukee, Win.*
Heat- dried sludge
Rochester, N. Y.
Digested sludge
Des Moines, la.
Digested Prim. & Act. Sludge
Phoenix, Ariz.
Chandler, Aziz.
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
5.96
2.54
1.81
1.07
2.48
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
20.88
--
--
12.90
43.30
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
3.5
--
--
12.1
17.5
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
3.96
1.16
3.31
0.78
—
Ash
(%)
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
42.80
61.40
74.80
31.70
pH
5.3
5.6
5.8
5.7
5.5
5.9
5.0
5.7
6.8
5.8
7.6
6.2
6.9
4.5
6.2
6.0
4.8
—
--
__
—
Ref.
No.
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
12
12
36
37
Primary treatment.
Prevalence of home garbage grinders affects sludge composition.
Ground garbage discharged directly to digesters.
Sludge not digested.
53
-------
171
*».
TABLE 16
Fertilizing Ingredients in Sewage Sludges,
Various Manures and Organic Nitrogenous Materials
Material
Digested settled sludge
Digested settled sludge with
filter sludge
Digested activated sludge
Heat dired -activated sludge
Commercial pulverized
Sheep manure
Cattle manure
Poultry manure
Animal tankage
Blood
Fish scrap
(** f*ttf\1\ (2 C*F*A TYltfkZll
vs UllQIlo CCU IZlCdl.
("".stst-cir1 nrimar*"
Fertilizing
Nitrogen (N)
0.8-3.5
trickling
1.0-4.5
2. 0-4. 5
4. 0-7. 0
1.2-2.5
1.6-2. 1
1. 9-4. 0
5-10
9-13
6.5-10
5-R
— o
c,-A
Ingredients (%)
Phosphoric Acid
2
2
tr. -4. 0
1. 0-3. 6
1.7-2.5
1. 0-2. 0
1.0
2.5-3.7
7-16
0.5-14
5-10
2.7
J
7
Potash Organic Matti
3 34
0.8-1.6 30-60 '
01 ^ -___--
2. 0-4. 0 48
1.0-2.2 66
0.8-1.3 64
1 -? ......
l
_See Reference 1
-Dry basis
.Estimated from other tablets
Volatile matter
-------
Ul
U1
TABLE 17
Minor Chemical Elements in Activated Sludge and Digested Sludge
(38)
Sludge
Activated sludge:
Min
Max
Avg.
Digested sludge:
Min
Max
Avg.
Copper
385
1,500
916
315
1,980
643
Zinc
950
3,650
2, 500
1, 350
3,700
2,459
Element
Boron
6
74
33
4
15
9
(ppm)
Manganese
65
190
134
30
790
262
Molybdenum
6
45
16
2
12
6
-------
water can be influenced by many factors, such as seasonal varia-
tions; changes in the utilization of a watershed area; the nature of
the discharge from municipalities and industries at the upstream
portion of the source; and the degree of ground water contamina-
tion.
Some of the more significant reasons for knowing the character-
istics of the water treatment plant sludge include: (1) to provide
a means for assessing the pollution hazards to the immediate
environment of the receiving water; (2) to establish a basis for
advancing the technology for facility design in conditioning, thick-
ening, dewatering, and disposing of the sludge; and (3) to allow
for recovery of chemicals and effective utilization of the sludge
by-products.
The literature reveals that not much effort has been made to
characterize these sludges in the past as compared with waste -
water and industrial sludges. However, more recently the AWWA
Research Foundation, EPA, and State agencies for New York and
Ohio have increased their efforts in this area. Standard methods
were not found for sampling and analyzing the physical, chemical,
and biological composition of these sludges. The information that
appears in the literature is rather fragmentary and inconsistent.
Therefore, it is necessary to first, outline the parameters that
define the characteristics of these sludges and then collect and
organize the reported information pertaining to the characteristics
of sludges produced from the various types of treatment processes.
The significant parameters for characterizing sludges from water
treatment plants are: (1) solids, (2) settleability, (3) filterability,
(4) viscosity, and (5) to a lesser extent, coliform content, BOD,
COD, TOG, and pH, and chemical composition.
Solids content of the sludge is important to the extent that it
adversely affects the immediate environment of the receiving
waters by increasing water turbidity, dissolved solids, dis-
coloring of water, formation of sludge blankets, and possible
toxicity. These solids can be acidic, alkaline, or neutral. Solids
containing biodegradable organic material wiU exert an oxygen
demand (43).
Settleability of the sludge is another important factor which is in-
fluenced by many variables, such as: viscosity of the liquid; solids
content; size and configuration of the settling basins; and the physical,
chemical, and electrostatic nature of the suspended material.
56
-------
Viscosity of the sludge is needed to predict the friction head loss
of the sludge transported by pipelines. It has been reported (48)
that sludge with solids content above 1% exhibits non-Newtonian
flow characteristics and, therefore, the viscosity of the sludge is
referred to as "apparent viscosity".
Coliform content of the sludge depends on the source and quality
of the raw water used. Sludge with a high concentration of path-
ogenic bacteria could be extremely detrimental to the quality of
the receiving water. For determining the pathogenic bacteria of
the sludge, E. coli concentration is normally used as an indicator.
Inorganic constituents of the sludge from water treatment plants
are important parameters which reflect the chemical nature of
the sludge. Knowledge of these constituents is useful, when re-
covery of chemicals or utilization of the sludge is to be con-
sidered. The level and concentration of the chemical content
depend upon the source of raw water and the type of chemicals
employed in the treatment processes.
Water treatment plant sludge characteristics vary significantly
depending upon the parameters mentioned above. Sludge charac-
terization can be studied thoroughly if sludge from various
treatment processes are considered separately.
Purification or Coagulation Sludge
The two major types of sludge resulting from a municipal water
purification plant are: (1) basin sludge; and (2) filter backwash
sludge. Basin sludge is highly complex in nature. It is comprised
of the organic, inorganic, and biological organisms present in
raw water in a state of solution, colloidal suspension or readily
settleable form; or it may be mixed with the coagulant or coagu-
lant aids employed in the treatment process. The sludge is described
as a fluffy agglomeration of chemical precipitates and organic
debris, and is high in moisture content (98-99%) (43). The agglom-
eration is difficult to dewater, but is readily settleable. The color
of the sludge approaches yellow-orange, pale-green, dark-brown,
and sometimes black; depending on the type of chemical process-
ing used, the nature of impurities extracted, and the stage of the
organic material.
A series of tables and figures have been assembled to describe
the general characteristics reported for basin sludge. Table 18
is an outline of the principal constituents of basin sludge; Table 19
presents data on solids, BOD, COD and pH of basin sludge as re-
ported by Russelman(43). Table 20 is a compilation of the same
57
-------
Ln
oo
TABLE 19 ,43)
Coagulation Sludge Character!sties
Plant
A
B
C(3)
D
Treatment
Alum
Coagulation Ic
Sedimentation
Alum
Coagulation,
Clarifier
Alum
Coagulation ,
Clarifier
Alum
Coagulation,
Clarifier
Plant
Capac.
mgd
25
2
1
10
BOD
(5- day)
mg/1
41
72(1)
144 (2)
90
108
44
Total
COD Solids
mg/1 pH mg/1
540 7.1 1,159
2,100 7.1 10,016
15,500 6.0 16,830
6.0
Volatile
Solids
mg/1
571
3,656
10,166
«_ _
Total
Suspended
Solids
mg/1
1,110
5,105
19,044
15,790
Volatile
Suspended
Solids
mg/1
620
2,285
.10,722
4,130
(1) BOD after 7 days
(2) BOD after 27 days
(3) Activate carbon in •ample
-------
cn
TABLE 20
Characteristics of Basin Sludge
T. S. S.S. V.S.
mg/1 mg/1 mg/1 BOD COD pH
Plant Location x 103 x 103- x 103 mg/1 mg/1
Shermount Plant
Rochester, N. Y. 4.35 3.61 1.50 33-77 500-1000 7
Wolcott Plant
Wolcott N. Y. 4. 26 3. 58 1. 02 45 1048 7
Ithaca, N. Y. 6.32-19.48 6.19-18.84 24.04-34.51*
<;_* 7
Reference
#
44,47
44,47
4ft 47
Blacksburg -Chris tamburg,
V. P. I. Water Authority
17. 35
4. 12 380
1320
6.6
42
Howard Ave. Plant
Milwaukee, Wise. 8. 050
Piqua Municipal Plant
Piqua, Ohio 23-31
7.75
3.32
95-97* 26-28* 14-21 320-350
7.0
10.4
50
of T.S.
-------
sludge characteristics as reported for several different plants.
Table 21 shows the changes of viscosity as the solids content changes,
and Table 22 is a tabulation of the chemical composition of basin
sludge. Reported (48) values of specific resistance range from 0. 1 x
iTr — 10 2
10 to 0. 44 x 10 sec /gr. Lime is often used for coagulation
purposes and this greatly improves the sludge handling character-
istics. However, sludge filters more like a tertiary sludge and
generally much poorer than a raw primary sludge. Studies on the
settleability of the sludge showed that basin sludge exhibits zone
settling characteristics (48,49). The bacterial content of the
basin sludge ranges from 2 to 190% of comparable concentrations
in the river water supply, as reported by Sutherland (44). Dean
(45) has reported that the coliform content of the basin sludge
varies from 0 to 42% as E. coli concentration of the river water
supply.
TABLE 18: Principal Constituents of Purification Wastes
£ # Lime and Iron Salts
Alum Coagulation Iron Salt Coagulation Coagulation
Al (OH)3 Fe (OH)3 CaCO3
Inorganic Solids Inorganic Solids Inorganic Solids
Organic Solids Organic Solids Organic Solids
Activated Carbon Activated Carbon Activated Carbon
Coagulant Aids Coagulant Aids Coagulant Aids
*
Reference 43
Characteristics of the filter backwash waste are essentially the
same as the basin sludge, but in far less concentration. This
sludge contains fine clay particles, hydroxides of aluminum and
iron, activated carbon, oxides of iron, debris, chemicals preci-
pitated from the filter media, and small portions of the media itself.
Typical values for the various characteristics of the sludge are shown
in Tables 23 to 25.
Softening Sludges
In municipal water softening plants, the hardness producing
minerals are removed either by a conventional softening process
which utilizes chemical precipitation using lime, lime-soda,
or lime caustic; or, by an ion-exchange softener.
Conventional or chemical precipitation sludge is more predic-
table than coagulation sludges. The principal constituents are
60
-------
TABL.E 22
Chemical Analysis of Basin Sludge
Location It
Ref. No.
CentralU<53)
Ol»«»
T.S.
0.5
0.9
CaC03
41.9
41.3
Mg(OH)2
13.3
20.1
A1(OH)3 Fe203
8.7 6.4
4.4 4.6
Si02
6.4
3.4
Volatile
Matter
-•
Remainder
23.3
26.2
-------
TABLE 23
Filter Backwash Characteristics
(43)
P
L,
A
N
T
Treatment
BOD
(5-day)
mg/1
COD
mg/1
pH
Total
Solids
mg/1
Volatile
Solids
mg/1
Total
Suspended
Solids
mg/1
Volatile
Suspended
Solids
mg/1
ro
Alum
Coagulation &
Sedimentation
Alum
Coagulation &
Clarifier
4.2
3.7
C Alum
Coagulation & Clarifier 2. 8
D Alum
Coagulation & Clarifier 1. 8
E Lime -ferric.
Coagulation &
Sedimentation
28
75
160
7.8
7. 2
7.8
121
378
166
3.2-4.1 18-27 9.9 731-864
44 47
115 104
45 75
100
216-285 695-825
31
53
40
60
-------
OJ
TABLE 24
Suspended Solids in Filter Backwash
Municipality
Centralia, 111.
Evan s ton, 111.
Olney, 111.
Streator, 111.
Dayton, Ohio
. , n-, .
Sidney, Ohio
**
Pounds / sq.
Range
0. 11-0. 16
0. 0800.20
0.23-1. 10
0.04-0. 15
**
ft.
Average
0. 14
0. 14
0.58
0. 10
C one entra tion,
Maximum
1, 167
958
5,212
1,053
22C f\
, J3U
i 17ft
1* l&v
mg/1
Avg. Max.
920
766
2,932
687
IAcn
I O3w
074
O I**
Filter
-------
TABLE 25
Chemical Analysis of Filter Washwater* '
Purification
Volatile Matter
SiO.
it
FC2°3
ft J
*2°3
CaCO,
^
Mg(OH)
ft
Remainder
Range
Samples
Range
Samplers
Range
Samples
Range
Samples
Range
Samples
Range
Samples
Range
Samples
0. 5-40
15
0.65-69a
15
0.08-18.4
11
0 -45b
13
1-55.8°
12
0. 3-22. 1
10
d
2-70. 3
10
Softening
4.5-99.06'
3
0. 1-18.8
7
0. 06-20. 1
7
0--34. 1
6
0. 35 -98f
7
0. 1-6
6
0.53-54.4
6
a - Rochester, N. Y.
b - Detroit, Michigan
c - San Diego, California
d - Pittsburgh, Pennsylvania
e - Bismarck, North Dakota
£ - Bismarck, North Dakota
-------
Mg(OH)2» CaCOo, turbid matter, suspended solids, and coag-
ulant aids. The volume of the sludge produced is greater than
that of coagulation sludges. Reportedly, up to 3. 5 Ibs. of dry
solids are produced per pound of hardness removed when lime-
soda are the principal coagulants (54, 55), and up to 1. 7 Ibs.
when lime-caustic are the principal coagulants (55). Sludge
color ranges from pure white to pale-yellow. Solids content of
the sludge reportedly ranges from 2 to 33% (54). The physical
characteristics of the sludge are such that it remains a fluid
up to about 15% solids content. The sludge is thixotropic between
a solids content of 25 to 70%. For solids content above 70%, the
sludge appears compactible and loses its thickness (55). Chemical
compositions of these sludges are presented in Table 26. The
specific resistivity of the sludge ranged from 0. 13 x 10 to 4. 2
x 10 sec2/gr for sludges containing 1. 5 and 14. 5% solids, res-
pectively.
TABLE 21
Apparent Viscosity of Settling Basin Sludge
(Measured at a Shear Rate of 2000) (48, 51)
Apparent Viscosity at
Percent Solids by Weight 20°C- g/cm-sec.
Distilled Water 0. 010
1.2 0.0347
1.88 0.0458
2. 58 0. 0583
3.27 0.117
Characteristics of the ion-exhange brine wastes produced as
the results of regeneration are given in Table 27. Volume of the
waste has been reported to range from 3 to 10% of the volume of
water treated. The chloride content is very high, ranging from
9000 to 25, 000 mg/1. Additional information regarding the
characteristics of the sludge was not available in the literature.
Diatomite Earth Waste Characteristics
Characteristics of the sludge are almost the same as that of
diatomite earth itself. The reason is that the amount of filter
aid used is usually twice that of the impurities extracted from
the raw water. It has a dry density of about 10#/cu ft. , and specific
gravity of about 2. 0 (57), Russelmann (43) has reported BOD
values of 105 mg/1, COD of 340 mg/l,pH of 7. 6 and total solids
content reaching over 7000 mg/1. Other information found in
the literature pertaining to the characteristics of these sludges
is presented in Table 28.
65
-------
TABLE 26
Dry Solids Content of Basin Sludge Mean Values
1.
2.
3.
4.
5.
6.
7.
Municipality T
0
i
(surf. - soft)
Bloomington 2
Canton 1
Decatur 2
Eureka 2
(well - soft)
Champaign (E) 20
Elgin (Slade) 32
Normal 22
.S. CaCO3 Mg(OH)2 A1(OH)3 Fe O SiO *
.3 85. 2 9. 5 3. 7
.9 67.3 9.0 1.3
.8 80.0 11.7 2.0
.3 66.1 18.7 2.2
. 0 79.6 6.3 0.4
.2 86.0 4.3 0.3
.2 82.4 11.8 0.3
TABLE 27
I"
0. 3
0.7
2.7
0.5
2.2
0.9
1. 5
i" /«
0. 6 0.
0.9 21.
1.7 1.
0.7 13.
2.0 9.
0.2 8.
0.7 3.
1
7
8
9
8
5
3
3
Analysis of Waste Brines from Ion -Exchange Softener
Constituent
Calcium
Magnesium
Sodium
Sulfate
Chloride
Total Dissolved
Tntal T-Tafrfr»A««
Amount
Reference #56
1, 720
600
3, 323*
328
9,600
Solids 15, 656
7. 7^7.
mg/1
Reference
3,000 - 6,
1, 000 - 2,
2, 000 - 5,
9, 000 - 22
#54
000
000
000
,000
•
includes potassium also
66
-------
TABLE Z8
Characteristics of Diatomite Earth Filter Waste
Process Wastes Item
Origin of Was tea
Typical Magnitude of Wastes
Volume, percent of throughput,
range;
Suspended solids, concentration,
range, rng/1;
Suspended solids, weight, Ibs/mil-
lion gals throughput.
Typical Characteristic*
Settleability
Putrescibility
Abrasiveness
Porosity
Orainability
Form of Solid*
Diatomite Filtration
Filter Backwash
Conventional Filtration
Sedimentation
Basin Sludge
Filter
Backwash
0.1 - 0.5
10,000 - 15,000
100 - 400
0.05 - 0.1 0.5 - 3
5,000 - 50,000 150 - 400
100 - 250 10 - 50
Filter aid- good
Entrapped turbidity-
varies from poor to good
Negligible or minor
Good
Fair
Varies with Negligible
nature of solids
removed and whether
or not sludge removal
is continuous or
intermittent
Pronounced
Pronounced
Good
Rigid
Usually None
Usually None
None
Amorphous
None
None
Poor to fair
Amorphous
-------
SLUDGE HANDLING AND TREATMENT
The aim and ultimate goal for application of physical, biological
and chemical processes in a wastewater treatment plant is to
remove and concentrate that portion of the wastewater that is
responsible for the offensive nature of the waste flow. Thereby,
the major part of the flow is allowed to return to the environ-
ment. The concentrated pollutants are called sludge. They are
generally in suspended solids form with little tendency for con-
solidation. Most sludges are biologically unstable substances.
They provide food for microorganisms which grow in and upon
them, resulting in a stable material, and obnoxious and odor-
ous byproducts. The enormous quantities of microorganisms
associated with sludge are a definite hazard to human health.
These microorganisms are mostly of fecal origin and many
of them are pathogenic. Therefore, sludge stabilization and
treatment seem an obvious necessity. Sludge treatment often
includes stabilization (making the sludge biologically inert
and reducing its content of pathogens by many orders of
magnitude), volume reduction, and solid-liquid separation.
A flow diagram, of the most common methods used for sludge
processing and disposal is shown in Figure 8.
SLUDGE CONCENTRATION
Concentration is applied to increase the solids content of the
sludge, thus reducing its volume significantly. Sludge concen-
tration might be desirable when: (1) pipeline transportation of the
sludge is involved, (2) liquid sludge is applied on farm land,
and (3) savings of chemicals, heat energy, and auxiliary fuel
in the subsequent steps are questions of concern.
Where sludge is disposed of by dumping into the sea, as is
being practiced in many coastal cities, sludge concentration
could result in significant cost reduction by reducing pipe size
and pumping equipment and barge sizes and trips. Sludge con-
centration also results in reduction of the amount of chemicals
used for conditioning and dewatering of the sludge; reducing the
digester capacity requirements and digester heat requirements;
and the amount of auxiliary fuel required for heat drying and/or
incineration etc. (17)
Methods used for sludge concentration or thickening include:
(1) gravity thickening, (2) dissolved air flotation, and (3) cen-
trifugation.
68
-------
Figure 8
Unit Processes for Sludge Handling, Treatment,
and Ultimate Disposal
[Ultimate Disj
I RT
lUtiliT-.ation
Lagooning
Land
Spreading
Ocean
Under
Ground
Land
Fill
Fertilizer or
Soil Cond.
Land Reel.
By Product
Recovery
-------
Gravity Thickening
Gravity thickening is the most simple, inexpensive, and widely
used sludge concentration method in wastewater treatment plants.
This process is basically the same as sedimentation settling, but
relatively slow in action.
According to Burd (17) gravity thickening usually exhibits the
"hindered" settling phenomenon. Hindered settling is influenced
by such factors as particle size distribution, density, concen-
tration and agglomeration of the particles, as well as hydraulic
conditions in the settling basin (65). In a gravity thickening process,
different zones have been distinguished, as shown in Figure 9.
These zones include: (A) clarification zone or zone of clear super-
natant liquid, (B) settling zone characterized by a constant rate of
solids settling, (C) compression zone characterized by a reduction
in the rate of solids settling, and (D) compaction zone or zone of
minimal settling rate. Several investigators (16, 67) have reported
that data obtained from a batch settling test can be used safely as a
basis for design of a continuous thickening unit. Settling curves
can be utilized to determine the surface area required for clari-
fication and the surface area required to thicken the sludge to a
particular solids concentration. Plotting and utilization of settling
curves for design purposes have been discussed previously. A
comprehensive investigation of the relationship between settling
properties of continuous thickening has been reported by Dick (68).
Solids concentration obtained using mechanical thickeners varies
considerably, depending on factors such as sludge types and charac-
teristics; initial concentration of the sludge; particle size distri-
bution, shape and density; sludge age and temperature; and the ratio
of organic to inorganic content of the sludge. Sludge thickening
using separate handling units has been found to produce a greater
concentration of solids than by thickening in the initial wastewater
clarification unit (17).
Degree of sludge concentration obtained and the efficiency of thick-
ener operation depend on the following factors. Initial solids concen-
tration and temperature of the sludge in the thickener is important.
As shown in Figure 10, an increase in initial solids content reduces
the efficiency of thickener drastically, where an increase in tem-
perature to a certain degree enhances degree of concentration obtain-
ed. Also, the type of sludge and its volatile content effect sludge
concentration in a manner shown in Figure 11 (70). The reason for
lower degree of concentration of the combined primary and biofilter
70
-------
Figure 9
Thickening Zones
(66)
II
III
-:.. D =
A- represents clarification zone
B- represents settling zone
C- represents compression zone
D- represents compaction zone
71
-------
Figure 10
Effect of Temperature and Initial Concentration
on the % Increase in Fresh Solids Concentration (after 96 hours)
(69)
n)
Vi
4)
o
c
o
U
m
n)
0)
M
U
c
1000
500
100
50
10
\
12345
% Initial Concentration
\
72
-------
Figure 11
(70)
Relationship of Total Solids to Percent Volatile Solids
0
13
C
M
fl)
a
0)
to
•a
•H
O
H
20
18
16
14
12
10
8
6
4
2
Total solids
vs.
% volatile
90
80
70
60
50
40
% Volatile Solids
73
-------
sludge is due to the presence of biological sludge. It is believed that
biological sludges, particularly activated sludge, are generally
bulky in nature and have shown a relatively lower degree of concen-
tration as compared with non-biological sludges. Concentration is
influenced by the addition of chemicals and inert weighing agents.
Rudolfs (71) has observed that alum and ferric salts did not change the
concentration significantly and sulfuric acid, iron oxides, diatoma-
ceous earth, and fly ash were insignificant at reasonable dosage
rates. Lime dosages of 250 to 500 ppm, on the other hand, increased
sludge concentration significantly. Successful use of polyelectrolytes
as an aid to sludge compaction,incr ease in settling rates and overhead
clarity have also been reported in the literature.
Dust (72) has reported that under normal operations thickening pro-
duced about 8. 7% solids concentration for combinations of primary
and trickling filter sludge. Keefer (73) has reported that raw primary,
waste activated and trickling filter sludge was concentrated to as
much as 8. 5% solids content. Voshel (74) has reported that sludge
concentration of up to 7% solids content was obtained using picket-
type thickener for activated sludge. Typical concentration that can be
obtained for various types of sludges by applying mechanical thick-
eners are presented in Table 29 (14).
TABLE 29
Concentrations of Unthickened and Thickened Sludges
(14)
and Solids Loadings for Mechanical Thickeners
Type of sludge
Separate sludges:
Primary
Trickling filter
Modified aeration
Activated
Combined sludges:
Primary and trickling filter
Primary and modified aeration
Primary and activated
Sludge ,
percent solids
Unthickened Thickened
«
2.5-5.5
4-7
2-4
0.5-1.2
3-6
3-4
2.6-4.8
8-10
7-9
4.3-7.9
2.5-3.3
7-9
8.3-11.6
4.6-9.0
Solids
loading
for
mechanical
j i * i
thickeners ,
Ib/sq ft/dav
20-30
8-10
7-18
4-8
12-20
12-20
8-16
74
-------
Flotation Thickening
The literature shows that flotation thickening of wastewater sludges
is becoming increasingly popular over gravity thickeners. The basic
principle in flotation thickening is to attach the minute air bubbles to
the suspended solids of the sludge. This reduces the specific gravity
of the solid particles below that of water and causes the solids to
separate from the liquid phase while moving in an upward direction.
According to Rich (66), entrapment of rising air bubbles in the floe
particles is the predominant action with flocculant materials such
as activated sludge, and in the case of non-flocculant particles,
contact of air bubbles and particles by adhesion has more signifi-
cance, which depends highly on the surface properties of the solid
particles.
Flotation thickeners have been widely used in wastewater treatment
plants, primarily in conjunction with waste activated sludge. Thick-
ened sludge having solids content of 4% with 85% solids recovery
without use of chemicals has been reported (14). Where a mixture
of primary and activated sludge was used, solids concentration
averaging 6% and reaching as high as 8% was obtained (14). Table 30
shows average sludge concentration obtained without use of chemical
aids.
TABLE 30
Sludge Solids Produced by Flotation Thickener
Combined primary and activated sludge 6. 1% Solids
Combined primary and activated sludge 7. 4% "
Activated sludge only 4. 9% "
Activated sludge only* 3. 7% "
Primary 4- activated sludge 1:1
Cannery waste in season 5. 3% "
Cannery waste our of season 7. 1% "
$
Same as above, but higher volatile content
Use of chemical aids has resulted in an increase in solids concen-
tration, as well as percent solids recovery. Dick (68) has reported
from the results of other studies that for plants not using polymers,
float solids content averaged about 4% with solids recovery of about
90%. For plants using polymers, float solids contents were up to
5.8% with solids recovery of 98.6%. Table 31 shows results of a
survey of various treatment plants applying air-flotation thickening
for activated sludge by Bernard and Eckenfelder (76).
75
-------
TABLE 31
Flotation Thickening Units
Installation
Bernardsville, N.J.
Hatboro, Penna.
Omaha, Neb.
Belleville, 111.
Indianapolis, Ind.
Warren, Mich.
Frankenmuth, Mich.
Oakmont, Penna.
Columbus , Ohio
Levittown, Penna.
Nassau Co. , N.Y.
Nashville, Tenn.
Fort Worth, Texas
Toledo, Ohio
Cleveland, Ohio
Type
Sludge
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
Feed
Solids
Cone.
(%)
1.70
0.73
2.00
1.83
0.30
0.60
0.90
0.63
0.68
0.57
0.81
1.54
0.90
0.82
0.45
Thickened
Sludge
(%)
4.3
4.0
5.9
5.7
5.0
7.0
7.0
8.0
5.0
5.5
4.4
12.4
4.5
7.3
4.5
Solids
Recovery
{%)
98.8
96.0
99.8
98.7
95.0
95.0
99,1
98.7
99.5
99.4
99.6
99.6
99.8
99.6
99.3
Loading
(r»sfd)
103.0
69.5
183.0
91.0
50.5
125.0
156.0
72.0
79.0
70.0
118.0
123.0
108.0
89.0
77.0
Chemical
Aids
+
+
+
+
f
+
+
+
+
+
+
+
+
+
f
A = waste-activated sludge
-------
Burd (17) has discussed four methods of flotation thickening used in
wastewater sludges. The basic principle which is common in all of
the four methods is that of rising of gas bubbles to increase the
buoyancy of the solid particles. These methods included: (1) disper-
sion-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
pressure and later released under atmospheric pressure; (3) dissolved-
air vacuum flotation, which applies a vacuum to wastewater aerated
at atmospheric pressure; and (4) biological flotation, where the
gases formed by natural biological activities are utilized to float
solids. Among the above four methods dissolved-air flotation and
biological flotation have received wide use in sludge handling because
of higher solids concentration obtained. Dispersed-air and dissolved-
air vacuum flotation are generally applied for wastewater clarifi-
cation purposes.
Mechanism of solids thickening by flotation thickening is analogous
to that of gravity sedimentation. Therefore, it is referred to as
"hindered" separation or flotation. A typical flotation curve is shown
in Figure 12. The degree of sludge thickening depends on factors
such as compressional force and the surface active properties of the
solids resisting compression (78). The major variables influencing
flotation thickening given by Burd (17) are as follows: (1) pressure;
(2) recycle ratios; (3) feed solids concentration; (4) detention period;
(5) air to solid ratio; (6) type and quality of sludge; (7) solids and
hydraulic loading rates; and (8) use of chemical aids.
Generally speaking, greater float concentration is obtained as
pressure is increased. However, depending on the type of the sludge
and other parameters, a critical pressure exists beyond which air
pressure breaks up fragile floes. Optimum value of air recommended
by Mayo (79) is 0. 03 ft /ft2/min. Katz (80) has observed that increase
in the recycle ratio resulted in a significant increase in the rate of
rise of particles, as shown in Figure 13. Recycle ratio and feed
solids are interrelated. It hae been observed that dilution of the
feed sludge to a lower concentration increased the concentration
of the floated solids (71). Detention period has also been found to
influence the concentration of floated solids. Data presented by Katz
(80) as shown in Figure 14 supports this fact.
The effect of air-to-solids ratio on the concentration of floats was
investigated by Ettlet (78). As shown in Figure 15, increase in air-
to-solids ratio caused increase in floated solids production. Because
of rapid separation of solids from the sewage, higher loading can be
77
-------
Figure 12
Rate of Rise of Floated Solids Interface
(77)
a
u
W
w
u
30
25
20
15
10
= 0.27 cm/sec
Laboratory cell
Cs = 0.3135
A/S = 0.006
SI = 84
0
50
100
150
200
250
300
350
TIME (sec. )
78
-------
400
Figure 13
Ee LABORATORY
THE EFFECT OF THICKENING
TIME ON SOLIDS CONCENTRATION
OF ACTIVATED SLUDGE _
THICKENING TIME (hours)
Figure 14
79
-------
Figure 15
oated Solid
Terminal Velocity at Constant Overflow Ratex
Effect of Air Content on Floated Solids Production and Effective
(78)
UJ
> .020
o
L-J
U_
L-.
LJ
I
8
V)
.015
.010
I
V,.
to
I?Z
p
O
r-
O
Q
O
cr
o.
CO
Q
o
L'J
I
u.
15
II
10
1
1
FLOTATION UNIT
INLET DESIGN NO.G
AVERSE LOADING OF 22 _
OVERFLGY/ RATE OF 0.214 GPM/fta
si = as
FLOATED
SOLIDO
I
MAXIMUM
AIR IMPUT—
lECUAL TO
65% OF
I SATO.IATICN
• C3*F Q
'OOPSIO _
I
I
J_L
O .005 .OIO .015 J02O
LBS OF AIR PER LB OF SOLIDS (A/S)
80
-------
used. However, higher loadings may impair the performance of
the thickening unit. Typical solids loading values are shown in
Table 32.
TABLE 32
(14)
Loading of Dissolved-Air Flotation Units
Type of Sludge Loading, Ib/sq ft/day
Activated (mixed liquor) 5-15
Activated (settled) 10-20
50% primary + 50% activated (settled) 20-40
Primary only to 55
Centrifuge Thickening
Centrif.ugation of sludge for thickening and dewatering purposes has
long been practiced in wastewater treatment plants. When centrifu-
gation is employed for sludge thickening the purpose is to reduce the
sludge volume to a point where the sludge is still fluid and can be
pumped easily. Bradney and Bragstad (81) are among the first inves-
tigators reporting on the subject. Activated sludge containing a solids
content of 0. 5% to 0. 8% was thickened to a concentration of 7%. An
extensive investigation in the application of centrifugation was made
by Ettelt and Kennedy (82) in the Chicago Sanitary District, where
the objective was to thicken the waste activated sludge. Results showed
that application of centrifugation for sludge thickening up to 7% was
feasible although operation problems were noted. Centrifuge perfor-
mance was noticed to improve when a mixture of activated sludge and
primary sludge was used. The solids concentration obtained reached
9.8%. Further, the tests were carried out for various liquid
levels in the centrifuge. It was observed that activated sludge was
thickened to 16%, but the solid recovery was unsatisfactory, as
shown in Figure 16. They have also used cationic poly electrolytes
for conditioning and have reported very satisfactory results, as
shown in Figure 17.
SLUDGE STABILIZATION
The main objectives for application of treatment processes on waste-
water sludge is to stabilize or sterilize the sludge and reduce its
volume for the subsequent unit processes and final disposal. Methods
81
-------
Figure 16
Decrease in Recovery with Corresponding Increase
(821
in Solids Concentration from Lowering Liqxiid Level
O
y
m
•8
(X
n
100
80
60
40
20
(H
Activated
SVI = 91
% Solids cone.
J I L
8
10
12
14
16
82
-------
c
o
•
Q>
n
Figure 17
Addition or
by Concurrent Centrifuge at 90% Solids Recovery
Effect of Chemical Addition on Solids Concentration
(82)
13
12
11
10
8
50% Preliminary
50% Activated
Activated
SVI = 69
Activated
SVI = 93
Polymer Dosage (Ibs/ton)
i I I
10
20
30
40
50
83
-------
applied for stabilization of wastewater sludges include: (1) anaerobic
digestion; (2) aerobic digestion; (3) composting; (4) lagooning; (5)
heat treatment; (6) chemical stabilization; arid (7) other processes.
Anaerobic Digestion
Anaerobic digestion of sludge involves decomposition of organic
and/or inorganic material in the absence of free oxygen. The
anaerobic digestion of organic matter occurs in two separate
stagesr called-liquefaction and gasification stages. The end
products of the first stage are utilized in the second stage as
fast as they are produced. The general biochemical reactions
for the entire process are summarized (83) as follows:
First stage reactions:
Complex Organic
Matter
Facultative and
Anaerobic Bacteria^
Simple Sugar,
peptides, glycerol,
fatty acids, amino acids
Facultative and
Anaerobic Bacteria
: >
(Acid forming bacteria)
Second stage reactions:
Organic Acids
Aldehydes
Alcohols
Organic Acids
Aldehydes
Alcohols
and
Amines
Ammonia
Acid Carbonates
N2
Soluble
Nitrogenous
Compounds
Mer captains
Indole
Skatol
Methane forming
Bacteria
r
Amines
Ammonia
Acid Carbonates
Methane forming | |
Bacteria
CHA+ CO +
Very small
amount of
other gases
84
-------
The flow-sheet diagram of the two stages involved in the anaerobic
digestion process is given in Figure 18.
Figure 18
Flow Chart Diagram of Anaerobic Digestion Process
« 1
Carbohydrates {
, *• • . ~1
Propionic
Acid
*
Butyric
Acid
Fats ;
• .. k '
9 T
Acetic
Acid
A
j
_ |
CHL+ CO,,
4 2
Decomposition of organic material is completed by two groups of
microorganisms. Responsibilities of the first group are the hydrolysis
and fermentation of the complex organic compounds to simple
organic acids such as propionic, butyric, and acetic acids. The
microorganisms in this group are both facultative and anaerobic
bacteria, collectively called acid forming bacteria. Responsibilities
of the second group of microorganisms is to convert the organic
acids formed in the first stage to methane gas and CC^. These
bacteria are strictly anaerobic and are called methane forming
bacteria. These bacteria have a very slow growth rate. Since
complete decomposition occurs in this stage, anaerobic digestion
generally takes a long time.
The main purpose of sludge digesting is to stabilize the sludge and
make it fit for final disposal. Volume reduction and byproduct (gases)
recovery are considered to be insignificant when compared with
the conversion of raw materials such as fats, carbohydrates, and
proteins into a stable and acceptable form.
Other reported objectives include: reduction of pathogenic organisms;
production of more easily dewatered solids; volume reduction by
concentrating the remaining solids to a dense sludge; and homogen-
izing sludge solids to facilitate subsequent handling steps (17).
Sludge digestion is the oldest method used for sludge stabilization.
Burd (17) stated that "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. "
85
-------
Factors effecting digestion of wastewater sludge include: sludge type
and the volatile content of the solids; digester temperature; digestion
detention period; feed sludge concentration; degree of digester mix-
ing; digester loading rates; and presence of toxic materials. Volatile
content of the sludge is an important factor. Generally the higher
the volatile content of the sludge the better the digester efficiency.
Temperature is an important factor influencing the rate of growth
of bacteria. As the temperature increases, the minimum cell resi-
dence time of the bacteria in the digester is equivalent to the hydraulic
detention period of the sludge in the digester, an increase in tem-
perature could cut down on the digester volume (14). The general
temperature range in which optimum biological activities take place
is the mesophilic range (85 to 95°F) and the thermophilic range (125
to 135 F), Figure 19, shows that biological activity is not a direct
mathematical function of the temperature. However, the most
common range of temperature used in the digestion of wastewater
sludge is in the range of 80 to 95° F (83).
The pH and volatile acid production influence the digester perfor-
mance. The most desirable pH range for biological life in a digester
is between 6. 6 and 7. 4 with a maximum tolerable range of about
6. 4 to 7. 8 (83). A pH below 6. 4 is indicative of a rise in the volatile
acid concentration. Increase in volatile acid concentration in turn
suggests that the methane forming bacteria are not utilizing the
volatile acids as fast as they are produced as the result of the
first stage reaction. A low pH and a correspondingly high volatile
acid concentration is referred to as an unbalanced digester. However,
investigators (84) have taken issue by debating this theory. In
general, most investigators feel that the volatile acid concentrations
are a critical indicator of the state a digester is in, and the pH
control is of the utmost importance. Low pH can be very toxic
to the digesting process.
Presence of toxic material can hinder digester operation. At low
pH, ammonia (NH^ is usually found in the digester) is converted
to NH4 (cation) and toxicity will take place. Heavy metals such
as copper, nickel, zinc, iron and chromium VI also cause toxicity
at specified concentrations. Toxicity of heavy metals is eliminated
by addition of sulfides (below 200 mg/1). The heavy metal ions
are precipitated out as sulfides. Other cations causing digester
toxicity include Na , K , Ca , and Mg . Synthetic detergents
(ABS type) and soluble fatty acids have also shown toxicity at a
concentration range of 40-1000 mg/1 (85). Four methods are
86
-------
60
C
•rl
(0
rt
I
fl
O
•T-»
-IJ
0)
V
O
<*-(
T3
0)
t3
O
9)
G
•H
0}
(4
0)
m
u
V
Q
U
a)
bo
O
V
6 -3
.H .H
H CQ
Figure 19
(83)
Temperature vs. Biological Activity
Mesophilic range
Thermophilic range
60
80
100
120
140
Temperature F
87
-------
TABLE 33
Anaerobic Digesters- Design Criteria
L . Solids Loading
(Ib vs/cu-ft/day)
Conventional
Digester
0.03
0.04-0.1
High- Rate
Digester
0.1-0.2
0.15-0.40
Anaer. Cont.
Process
0.1-0.2
Ref.
86
17
#
GO
00
2. Design Criteria
(cu-ft/cap.)
a) Primary Sludge
b) Trickling Filters
c) Activated Sludge
2-3
4-5
4-6
1-1/3 - 2
2-2/3 - 3-1/3
2-2/3 - 4
87
3. Digestion Retention
Period
30-60 days
39 days
10- 15 days
14.5 days
3-4 hrs
86
-------
available for eliminating toxicity in an anaerobic digestion system
(86). These methods include: (1) dilution of the waste itself, (2)
precipitation of toxic material, (3) use of antagonistic ions, and
(4) modification of the industrial process.
In high rate digesters, efficiency of operation depends on how well
the digester content is mixed. Mixing not only brings the micro-
organisms in contact with the available food supply which increases
the efficiency of microorganisms, it also keeps the concentration
of biological end products uniform and prevents scum accumulation.
Performance and efficiency of a digester are greatly affected by
the solids loading rate. In conventional digesters, loading rate is
normally low and is added at a uniform rate. Loading in an unsteady
rate or shock loading upsets the microorganisms drastically. In
high-rate digesters and anaerobic contact process, loading rates
are higher and continuous. Solids loading of different types of
digesters, design criteria and digestion retention period required,
as reported in the literature (17, 86, 87),are shown in Table 33.
Suhr and Brown (88) have done extensive work in evaluation of
conventional digester versus high-rate digester. It was found
that a high-rate digester can produce the same volatile solids
reduction as the conventional digester with much lower digester
capacity requirements. It was also observed that solid-liquid
separation in the high-rate digester was much poorer than the
conventional. Summary of their results is given in Table 34.
TABLE 34
Anaerobic Digesters- Design Criteria
Design cap.
Digestion Period
Volatile solids reduction
Digested sludge solids
Conventional
3. 0 cu-ft/cap.
30 days
65. 9%
6-9%
High -Rate
0.4
16
64. 5
3.5-4
Numerous papers discuss the sludge digestion process, and compare
the advantages and disadvantages of the conventional method with the
high-rate method. In conclusion, one could say that proper operation
of the digesters is an art and that no single rule of thumb exists that
could be applied equally in any two digester units. Although it is a
very effective process for sludge stabilization and perhaps sludge
volume reduction, it is however an expensive process, contributes a
vast quantity of nitrogen into the receiving water, and causes a lot of
operational problems. Also, the resulting sludges have been shown to
impose difficulty in the dewatering step.
89
-------
Aerobic Sludge Digestion
Aerobic digestion is another method used for stabilizing the organic
fraction of the sludge produced in wastewater treatment plants. The
aerobic digestion process has not been used very extensively, as
its application has been limited to small wastewater treatment plants,
Aerobic digesters could be used to treat primary sludge, trickling
filter sludge, and waste-activated sludge. To date, it has been
extensively used in treating waste-activated sludge.
According to Loehr (89) aerobic digestion is a biological process
having a long solids retention period, during which gross oxidation
is completed in two stages:tl) direct oxidation of any biodegradable
matter by a biologically active mass of organisms, and (2) oxi-
dation of microbial cellular material by endogenous respiration.
Whereas, endogenous metabolism depends on the living or active
portion of the bacterial mass and becomes significant when food-
limitation increases. Thus, bacterial growth and cell synthesis
becomes minimal; furthermore,the bacteria is forced to
utilize the food stored within their cells and endogenous meta-
bolism is responsible for further decrease of the remaining
sludge mass.
Reported (17, 89) factors affecting design of aerobic digesters
include: (1) rate of sludge oxidation, (2) sludge temperature,
(3) oxygen requirements of the system, (4) sludge loading
rates, (5) sludge age, (6) sludge solids characteristics, and
(7) characteristics of the residue and supernatant liquid.
Oxidation rate of the sludge in an aerobic system is believed to
be a function of many factors, such as sludge age, temperature,
microbial content of the sludge, and the waste characteristics
used to grow the sludge (89). The investigation of Lawton and
Norman was reviewed by Loehr (89) and indicated that a high
degree of correlation existed between percent volatile solids
reduction and sludge age. Furthermore, Lawton and Norman
used sludge age as a major parameter in obtaining volatile solids
reduction in an aerobic digestion system.
In general, fresh sludge has a higher rate of volatile solids reduc-
tion as compared with old sludge which has already been oxidized
partially prior to aerobic digestion. Loehr also reported that in
an aerobic system, oxidation and sludge reduction are minimal
during the winter, when temperatures are the lowest. Sludge age
of about 15-20 days at 20 C is sufficient, whereas sludge age of
30-40 days is required if the temperature is 10 C.
90
-------
The effect of retention time on percent volatile solids reduction in
an aerobic system was investigated by Viraraghovan (90) and
reveals that reduction of volatile solids is a function of retention
time of up to 15 days as shown in Figure 20.
Burd has reported on the research work on aerobic digestion
using a mixture of raw primary and waste activated sludge. As
shown in Figure Zl, at a temperature of 20°C, volatile solids
reduction rate was very rapid during the first 12 days of detention
period and was moderate thereafter. Figure 21 also reveals that
as the temperature increased, the percent volatile reduction
increased too.
Change of digestion temperature affects the rate of metabolic
activity of the microorganisms. Eckenfelder (91) has reported
that a 10°C change in temperature will change the rate of meta-
bolism of microorganisms by a factor of 2. Loehr (89) has repor-
ted that the effect of temperature is more significant at short
detention periods (less than 15 days). This effect will decrease as
the time lengthens and the loading rate decreases. Indeed, for
long detention times temperature effect is insignificant because
digestion is essentially completed at all temperatures. Loehr
(89) has also reported the work of Jaworski and Lawton in
connection with the effect of temperature on aerobic digestion
of activated sludge. Volatile solids reduction of 3. 2%/day at
15°C, 4. 0%/day at 20°C and 4. 5%/day at 35°C for the first ten
days, and then 0.15%/day at 15°C, 0.1%/day at 20°C for the
remainder of the study was reported.
It has been observed that aerobic digestion of activated sludge
results in lowering the pH drastically. Values of pH as low as 4. 5
have been reported (92) after 15 days of sludge digestion. Randal
(93) et al conducted a comprehensive investigation of the effects of
low pH environment on the aerobic digestion process with respect
to volatile solids destruction and sludge dewaterability, both on
laboratory and pilot-plant scales. The conclusions on the sludge
made are as follows:
1. The destruction of activated sludge solids during aerobic
digestion is a strong direct function of the initial volatile
solids fraction of the sludge.
2. The influence of mixed liquor pH on solids destruction is
small when compared to variation with initial volatile
solids fraction.
3. The final mixed liquor pH value achieved during aerobic
digestion is primarily a function of the nature of the specific
sludge being digested.
4. Substantial activated sludge solids destruction can occur during
91
-------
Figure 20
(90)
Volatile Solids Reduction- Effect of Detention Time
50
fc
540
S20
10
10 15. 20
DETENTION TIME IN DAYS
25
30
92
-------
50
025
co
LO\OlNG-lbVCuft J0y-V0lolil«
24 32 40
DETENTION TtME-Ooy»
Figure 21
(17)
-------
aerobic digestion at mixed liquor pH levels as widely
separated as 3. 5 and 9. 5.
5. The optimum pH range for aerobic digestion from a solids
destruction viewpoint is from 6 to 7.
6. The pH of the mixed liquor during aerobic digestion has a
strong effect on the levels of nitrates and phosphates in the
supernatant. Extreme pH values completely inhibit nitrate
formation and cause extremely large releases of soluble
phosphorus.
7. Large protozoa populations can produce very significant in-
creases in waste activated sludge settleability, filterability
and drainability during aerobic digestion even at very low pH
levels.
Advantages claimed (17,89,90) for an aerobic digestion system are
as follows: (1) volatile solids reduction obtained from aerobic diges-
tion is comparable to that obtained from anaerobic systems; (2) aero-
bically digested wastewater sludge is humus-like and has no disagree-
able or objectional odor, thus, lagoons can be used for final disposal;
(3) supernatant liquor has lower BOD concentration; (4) product sludge
has excellent de water ing characteristics; (5) capital costs are
generally low; (6) it is a relatively problem-free operation; and (7)
product sludge contains more basic fertilizer value. The major
disadvantage of the aerobic digestion process is the high power cost
associated with the required oxygen. Lack of methane gas production
and variation of the performance as the temperature change.s are
believed to be the minor disadvantages.
Composting of Sludge
Aerobic composting of sewage sludge is a biological process. It
can be defined as decomposition of organic waste by aerobic
thermophilic organisms to produce a stable humus-like material.
The byproducts of this treatment process are carbon dioxide,
water and heat. The compost produced can be used as soil
conditioner or fertilizer depending on its nutrient content.
Composting of wastewater sludge and other solid wastes has
been extensively practiced in Asia and Europe for centuries
with the products formed being used as soil conditioner and
fertilizer. Composting of sludge and refuse has received special
attention in Europe for the past thirty years because of the
enactment of a water pollution program which has resulted in
the formation of tremendous quantities of sludge and refuse.
Also, there has been a shortage of chemical fertilizer. Com-
posting of community solid wastes in the U.S. was first studied
by the University of Californic during the years 1950-1952. It
was concluded that composting should be considered as a
94
-------
means of disposal and reclamation for municipal refuse (94).
Beginning in 1953, Wiley and others of the Public Health Ser-
vice, Communicable Disease Center, (95,96,97), conducted
extensive research in composting of ground refuse and refuse-
sludge mixtures on a laboratory scale project at Savannah,
Georgia, and in pilot plant projects at Chandle, Arizona. After
ten years of studies, they concluded that composting of muni-
cipal refuse with or without sludge is a feasible and a sanitary
method of treatment. In the early 1960's, compost plants were
built primarily for sale of compost as a substitute for chemical
fertilizer. However the sale of compost did not appear to be
profitable and most plants closed down after two or three years
of operation.
In recent years, composting of sludge mixed with refuse has
been reconsidered in the U.S. due to the higher cost and other
problems associated with conventional methods of sludge dis-
posal.
Since composting of sewage sludge is an aerobic biological pro-
cess, it is necessary to provide an adequate method for supply-
ing oxygen and thorough mixing of the sludge during the com-
posting operation. This is required in order to obtain an effi-
cient odor-free sludge treatment. Composting methods used
include indoor (mechanical), and outdoor (windrows and bins)
processes. In the mechanical composting process of sewage
sludge there are are three phases. These are: (1 )dewatering
of the sludge, (2) composting, and (3) final curing. A flow
diagram of a mechanical composting process is shown in
Figure 20.
Dewatering of the sludge accomplishes the following: (1) reduces
the volume and (2) obtains an optimum moisture content required
for proper composting. Mechanical dewatering processes such
as vacuum filtration and centrifugation are frequently employed
along with chemical conditioning agents which enhance the de-
waterability of the sludge. The cake moisture content produced
often ranges from 70 to 80 %.
In the second phase, dewatered sludge is mixed with recycled
compost from the com poster to accomplish the following pur-
poses: (1) to adjust the moisture content of the feed sludge, and
(2) to seed the incoming sludge with microorganisms. The mix-
ture of filter cake and recycled compost is then transferred to
the mechanical composter, where it is continuously mixed and
aerated. The temperature of the mix during composting averages
95
-------
ID
CT1
LIME
STORAGE
SLUDGE
SOURCE
SLUDGE
STORAGE
METERING I I
PUMP L_|
Figure 20
The process for composting of dewatered sewage sludge '
F.Cla
STORAGE
,. J"| METERING
J I PUMP
iii 11 • j
D FLOCCULATOR
CMIX TANK)
c :>
CO
SLUDGE
PUMP
EIMCOBELT
VACUUM FILTER
CAKE
COMPOST
STORAGE
OFF GASES
MECHANICAL
COMPOSTER
^—AIR
RECYCLE AND CAKE
FILTRATE
CONVEYORS
-------
about 140 F. The heat is produced through biological action.
In the third phase, the end products obtained from the composter
are stored for curing.
The factors which affect the sludge composting process are: (1)
mixing, (2) moisture content, (3) percentage of recycled compost,
(4) aeration, and (5) temperature and pH. Continuous mixing of
the sludge in the composter is essential since it keeps the bed
loose and prevents air channeling, and supplies the microorgan-
isms with adequate air (98). The moisture content of compost
depends upon the feed cake moisture content, retention time
within the composter, and the rate of aeration and mixing.
Moisture content of the feed to the composter depends upon the
moisture content of the filter cake and the percentage of the
recycled compost and its moisture content.
Observations have shown that efficient composting is obtained
when the moisture content of the mix during composting ranges
from 25 to 40%. Biological action of the microorganisms was
greatest at moisture contents of 20 to 30%. Below 15% moisture
or above 40% moisture biological activity slowed down signifi-
cantly (98). With a moisture .content above 40%, mixing and other
operational problems occur. When the moisture content is below
15%,dusting problems arise (98).
Optimum conditions for composting are obtained by recycling the
compost and thoroughly mixing. The affect of recycling is indica-
ted in results obtained which reported that a 45% solids reduction
was possible with a 1. 5 recycle ratio, while only 18% solids reduc-
tion was obtained using 1.0 recycle ratio. Both were for a seven
day retention time (98). Recycling of the compost results in an
increase in the temperature of 10 to 20 F and changes the pH of
the feed from 10-11 to 7.
A temperature in the thermophilic range of greater than 110 F
has been shown to produce a more rapid rate of decomposition
than those in the mesophilic range (<110 F). It has been reported
(14) that 140 F is an optimum temperature, and is adequate for
destruction of most pathogens.
Aeration of the compost has two functions: (1) it provides a means
for rapid evaporation of excess moisture fed to the compost, and
(2) it maintains aerobic conditions. According to Shell and Boyd
(98) the first step in composting of dewatered sludge is moisture
97
-------
reduction through evaporation (reduction of 80 to 90%) followed by
biological destruction of volatile solids. An air flow rate of 0. 25
scfm-ft is believed to be adequate.
Physical and chemical characteristics of a mechanically composted
sewage sludge (combined primary and trickling filter) are present-
ed in Table 35.
TABLE 35
Physical and Chemical Characteristics
of Wastewater Sludge Compost
Total Kjeldahl nitrogen
(% N by weight) 2. 21
Total nitrate (% NO3 by weight) 0.01
Total phosphorus (% P^OS by weight).. 2.16
Total potassium {% K2O by weight). ... 0. 27
pH 6-6. 5
Bulk Density (Ib/cu ft) 48. 2
Moisture (% by weight) 20-35
Color Dark brown
Odor Musty
Wiley (99) in discussing composting of wastewater sludge with refuse
solids has stated that "treatment of sewage sludge with refuse or
other solid wastes offers several advantages over their separate
treatment by conventional processes. These advantages include
enhancement of the composting process, reductions in problems
of sludge processing and disposal, and reduction in processing
costs. "
Composting of sewage sludge mixed with refuse has been investi-
gated very thoroughly, both by mechanical composters and outdoor
composting. It is believed that addition of wastewater sludge indeed
enhances the composting operation for such reasons as: (1) con-
trolling the moisture content in the composting mixture; (2) pro-
viding a seed material necessary for initiation of biological action;
(3) enhancing the nutritious values of the final compost; and (4)
providing a means to control the C/N ratio. Preparation of refuse-
sludge moisture for composting requires sorting of the refuse and
mixing of sludge with refuse. The sorting has the purpose of remov-
ing the metals, rags, and other material which would adversely
affect the composting operation and alter the quality of the end
product.
98
-------
It appears that the parameters affecting sludge-refuse composting
operation are essentially the same as those used for mechanical
sludge composting. The moisture content of the compost mixture
has been recommended (100) not to exceed 50 to 60%. Gothard (101)
has noted that a sludge with a dry refuse ratio of 0.7 to 1.1 is
required to produce an optimum moisture content of 65%. The
recommended C/N ratio that is essential for optimum activity
of microorganisms ranges from 25 to 30 (100).
Successful composting of digested sludge with straw, sawdust,
and wood shavings under favorable climatic conditions has also
been reported (102).
Lagooning of Sludge
Sludge lagooning is the most economical method of treatment and
disposal, particularly for smaller plants and where land is rela-
tively inexpensive. A sludge lagoon is essentially a large shallow
unheated digester. It may be a natural or artificial depression in
the ground. Sludge lagoons have been used for a variety of purposes
such as for thickening, storage, digestion, drying and final dis-
posal of sludge. When a lagoon is used for digestion of raw sludge,
odor and insect breeding may be problems. When a lagoon be-
comes filled with digested sludge, it is either abandoned or drain-
ed and the digested sludge is excavated. Some of the design
parameters commonly used are ; (1) land area, (2) climate,
(3) subsoil permeability, (4) lagoon depth, (5) lagoon-sludge load-
ing rate, and (6) sludge characteristics and types. Several investi-
gators (6,45,103) have made studies on lagoon design parameters,
dewatering rate, etc. Typical loading rates reported (17) are
presented in Table 36.
TABLE 36(1?)
Sludge Types Loading Rate
Raw 6 Ibs/yr/cu-ft of lagoon
(using lagoon as digester) cap.
Undigested 2. 2 to 2. 4 Ib/yr/cu-ft of
(using lagoon for dewatering) lagoon cap.
Digested 1. 7 to 1. 84 lb/30 days/
(using lagoon for dewatering) sq-ft of lagoon surface area
Heat Treatment
Heat treatment of the wastewater sludges has gained popularity in
recent years. This process takes advantage of the combined prop-
99
-------
erties of heat and pressure. When sludge is heated at high temper-
atures under pressure, the gel-like structure of the sludge is
destroyed and the bound water is liberated. The resulting product
is sterilized, deodorized and readily dewaterable (40,41,108).
Major processes used for heat treatment of wastewater sludges
include: (1) Zimpro process, (2) Porteous process, and (3) Farrer
system.
Application of the Zimpro process in treatment of wastewater sludge
is growing rapidly. Table 37 indicates municipal wastewater treat-
ment plants equipped with this process, or additions utilizing it^ire under
construction. A schematic of this process is shown in Figure 23.
Operation of the process includes these following steps: (1) sludge
is ground up and pumped to a sludge holding tank, (2)
compressed air is introduced into the sludge and the mixture is
brought to an operating temperature above 350 F by heat exchange
and direct steam injection, (3) the heated conditioned sludge is
cooled by heat exchange with the incoming sludge, (4) gases are
separated and released through a catalytic afterburner or equiva-
lent odor control device, (5) conditioned sludge is concentrated
by settling and dewatered by filters, centrifuge or on drying beds,
and (6) liquor,readily biodegradable,flows to secondary plants or to
separate treatment units. Short reaction time assures minimum
color and BOD.
Figure 23
Schematic flow diagram of the Zimpro
wet-air oxidation system
(111)
Sludge
Grinde
Sludge
Holding
Tank
Reactor
Steam
Pump
Heat Exchanger
Exhaust
Gas
Separator
^•»-~ -
i
i
K
ji
ta -i
Air
Vacuum Filter
or
Filter Press
100
-------
TABLE 37
ZIMPRO THERMAL SLUDGE CONDITIONING INSTALLATIONS
(UNITED STATES ONLY)
PROJECT
lower Bucks County Joint
Municipal Authority
Levittown, Pennsylvania
Cannon Mills
Xennapolis, N. C.
Valley Sanitary District
Xndio, California
Rothschild, Wisconsin
Wgusau, Wisconsin
Millville. N. J.
Defiance, Ohio
Den ton, Texas
Fort Lauderdale, Florida
i
West Memphis, Arkansas
Jef f ersonvllle « Indiana
Si loan Springs, Ark*
Glowtrsvill*-JbhMtOMA(
MMf York
ENGINEER
Paul X. Blattler,
Managing Engineer
LBC7MA
Engineering Department
Cannon Mills
Kocbig & Koebiq. Inc.
Becher-Hoppe
&»cher-4toppe
Allx?rt C. Jones. Assoc.
Jones ft Henry Engineers,
Limited
Freese, Nichols ft Endress
Philpott, Ross ft Saarinen
Albert Switzer ft Assoc.,
Inc.
Moore ft Heqer
McGoodwin, Williams &
Yates, Inc.
Korrcll Vrooaap Encrlnacr*
IYPE or
PLANT"
LO
LO
LO
LO
LO
LO
LO
LO
LO
LO
LO
LO
LO
TYPE OF WASTE
Primary & Activated Sludge
and Digested Sludqe
Activated ft Primary Sludge
(Textile Plant Waste and
Domestic Sewage)
Primary & Activated Sludqe.
Primary ft Activated Sludge
Primary ft Activated Sludge
(Raw or Digested)
Primary & Activated Sludge
Primary ft Wqested Sludge
Primary ft Digested Sludqe
Primary & Activated Sludge
Primary ft Activated Sludge
Primary & Activated Sludqe
Primary Sludge ft Trickling
Filter Humus
Prinary ft Trickling Filtar
ft Activated Sluda*
TONS OP
SOLIDS
PER DAY
7.36
15.0
6.4
1.44
€.75
8.7
4.0
6.0
15.0
2.7
4.25
5.0
30.6
GALLONS
PER
MINITTE
20.0
50.0
30,0
6.7
37.0
36.0
12.0
33.0
61.0
16.3
23.0
14.0
100.0
INITIAL
OPERATION
1967
&
1970
1967
1969
1969
1970
1970
1970
1970
1971
1970
UC»
1970
uc«
-------
TABLE 37 cont'd.
.
PROJECT
Kalamazoo, Michigan
Bedford Heights, Ohio
Cambridge, Maryland
Amsterdam, N'ew York
Kay fie Id. Kentucky
Terre Haute, Indiana
Kewport, Tennessee
Midland. Michiaan
Columbus, Ohio
Worth Olmsted. Ohio
Maumee River-
Lucas County. Ohio
Speedway. Indiana
Canton. Ohio
ZIHPRQ THERMAT. STIITVM- n
(UNITED S
ENGINEER
Jones ft Henry Engineers.
Limited
Kedrick-Cox-Dancull
Benjamin E. Beavin Co.
O'Brien & Gere
Allen & Hoahall
Henry B. Steeq & Assoc.
Allen and Hoshall
McNamse, Porter ft Seeley^
Bonham, Grant & Brundaqe
Floyd G. Browne ft Assoc..
Ltd.
Finkbeiner, Pettis ft
Strout, Ltd.
Henry B. Steeq & Assoc.
Floyd G. Browne & Assoc. ,
Ltd.
TYPE OP
PLANT*
•'!!•• || ..
10
••VM^MM^^WW
10
10
LO
in
10
10
10
ID
10
LO
LO
* Ifrv4*v» fVM*«*>«>*t**+4***»
OfJDITIONING INSTALLATIONS
FATES ONLY) ""
TYPE OF WASTE
Primary ft Activate siudae
Primary ft Agtivftfc»d Sludqe
Primary ft Activated Siudae
Primary ft Actlvaf«»H <:inr)r.&
Primary ft Activated Siudae
Diqested Siudae
Primary Activated Sludqe
Raw Primary & Trickling
Filter
Primary ft Activated Sludge
(Raw or Digested)
Primary ft Activated Sludge
(Raw or Digested)
Waste Activated Sludqe
Digested Primary & Activated
Sludge
Primary ft Activated Sludge 1
(Raw or Diaested) 1
TONS OF
SOLIDS
PER DAY
97.5
8.0
16.0
• • i •••^•— .
12.3
2O
• J
12 0
4.9
16.32
49.0
10.4
10.0
,5 1
1
2-28.75 1
GALLONS
PER
MTNIITP
3-125
26.0
65.0
50.0
— •• .•-•-a^,^
10.0
-
10 n
on n
33 3
200.0
1A Ft
•ty e
40.8
-75.0
i
INITIAL
Ur & KA 11UN
1971
UC*
UC»
UC«
1971 1
uc*
1971
iir**
uc*
-_as_J
uc» 1
tie* j
••
IX) - tow Oxidation
-------
TABLE 37 cont'd.
ZIMPRO THERMAL SLUDGE CONDITIONING INSTALLATIONS
(FOREIGN)
PROJECT
Ape Idoorn , Holland
Staz, Switzerland
(St. Horitx)
ENGINEER
City Engineering Dept.
Toscano-Bernardi-Frey
Zurich. Switzerland
TYPE OP
PLANT*
ID
W
TYPE OF WASTE
Primary and Activated
Sludqe
Raw Primary and Activated
Sludge
TONS OF
SOLIDS
PER DAY
11.6
4.0
GALLONS
PER
MINUTE
32.6
16.2
INITIAL
OPERATION
1969
1970
o
U)
-------
From a plant operator's viewpoint, the Zimpro process offers many ad-
vantages, according to Mann (109). The advantages include: (1)
reduction of detention time from weeks and days to minutes, (2)
solids produced are more concentrated and much easier to dewater,
(3) process is virtually free of upsets due to types of sludge and
loading, (4) eliminates chemicals and increases production, and
(6) the operator is not exposed to the sludge as a daily routine.
Advantages claimed for this process are that the resulting end
products are: (1) dewaterable withour chemical conditioning by
filters, centrifuges, or sand drying beds, (2) sterile residue of
humus-like appearance, and (3) cake with a low moisture content
(50 to 70 %) suitable for ultimate disposal by dumping or incin-
eration without supplementary fuel. Furthermore, the total cost
for the Zimpro process (including land, amortization, fuel, power,
water, labor and maintenance) has been claimed to be less than
the costs of chemicals alone when chemical conditioning is used
(108). However, the cost for treating the supernatant is not in-
cluded in the above total cost.
Figure 24 represents a schematic of the Porteous process used
for wastewater slud.ee treatmeit. In this process sludge is pre-
heated to about 320 F by passing it through a heat exchanger.
Sludge is then discharged into the reactor, where the sludge is
being heated at approximately 380 F and maintained at this tem-
perature for about 30 minutes to an hour before it is discharged
back into the heat exchanger and out from the system. Operating
pressure may vary from 160 to 250 psi depending on the type of
sludge being treated (37). The resulting end product is easily
dewaterable; it can be dewatered to solids content of 40 to 60%
without use of chemicals (14).
The advantages claimed for the Porteous process are basically the
same as those outlined for the Zimpro process. The major difference
between the two processes is that in the Zimpro process, air is intro-
duced into the reactor, and it is not in the Porteous system. Table
38 indicates typical characteristics of liquor after heat treatment
of sludge by the Porteous process (111).
The Farrer system is based on the same principles as the Porteous
process. The Farrer Company, after purchasing the Porteous
patents, modified the process to make these improvements: (1)
overcome the odorous steam released, and (2) prevent short cir-
cuiting in the vessel-type reactor when heat treating continuously
(156).
104
-------
Figure 24
Schematic flow diagram of the Porteous Heat Treatment Process
(111)
SLUDGE
_SLUOG€
I
I
HEAT
EXCHANGE*
RAW
DECANTER
.
i REQUIRES ADDITIONAL
i TR£ATM£NT
J
SLUDGE
—O~—
PUMP i
I
SUJOCE TO DEWATERIN6
UNIT (RUER PP£SS)
(VACUUM r
REACTION
VESSEL
TREATED SLUDGE
STEAM
105
-------
TABLE 38
Characteristics of Liquor After Heat Treatment: Porteous Process
(HI)
Heat- Treatment Liquor
Total solids
(mg/1)
Suspended solids
(mg/1)
BOD5 (mg/1)
Organic nitrogen
(mg/1)
Ammonia nitrogen
(mg/1)
Activated All
Sludge Sludges
10,820 7,500
__ - -
4,620 4,500
1,110 410
418 830
Primary and
Activated
16,626
197
10,440
1,440
531
Press Filtrate
Primary Primary and
Sludge Activated
10,440 10,430
__ -_
3,780 4,170
150 504
220 190
-------
The Farrer system is a continuous sludge conditioning process in
comparison to batch treatment of the Porteous process. Heat treat-
ment of the sludge is a time-temperature relationship, where tem-
perature ranges from 350-400 F and time varies from 20 to 30
minutes. Figure 25 shows a schematic of this system. The system
consists of a thickener, disintegrator, heat exchanger, boilers,
decanting and storage tank, and a dewatering device. Heating of the
sludge is accomplished in the tube-type heat exchanger by indirect
heat exchange using hot water through a closed loop. This is the
major improvement over the Porteous process, in which steam is
injected directly into the sludge.
Major advantages of this system over the Porteous process are
claimed to be: (1) the need for sophisticated deodorizing devices is
eliminated as there is no odorous steam released; (2) the feed vol-
ume to the reactor is not increased by condensed steam; and (3)
since water is used in a closed loop, the need for continuous water
supply and treatment is eliminated (156).
In reviewing other investigators papers, Balakrishnan et al (156)
have reported that the filtration rate of domestic wastewater sludge
has been improved drastically through application of heat treat-
ment, with temperatures in excess of 130 C. When the temperature is
raised between 160 and 190 C and held for 10 to 15 minutes, and then
believed that a complete breakdown of the colloidal structure occurs.
This resulted in a sludge that was 200 to 1000 times more filter-
able than untreated sludge and is 15 to 50 times more filterable
than chemically conditioned sludge. Table 39 represents relative
dewatering rates of sludge conditioned by different agents.
The major disadvantages of heat treatment include: (1) BOD of the
filtrate is very high, ranging from 4000 nag/1 to amost 20,000 mg/1,
depending on the type of sludge being heated, and the amount of
time spent in the reactor, (2) fuel requirement to produce neces-
sary steam and temperature is high, and (3) need for separate
treatment of the overflow liquor from the solid-liquid separation
process and the filtrate of the dewatering system (111).
The atomized suspension technique has also been proposed for
wastewater sludge treatment. This method is described as being a
method for physical, thermal separation of dissolved or suspen-
sion solids from liquid, and subsequent chemical treatment of these
solids by means of gas-solid reaction in a continuous operation
(112).
107
-------
Figure 25
Flow Sheet for the Dorr-Oliver Farrer System
(156)
o
00
REACTOR
SECOND H6AT
EXCHANGER
PRE-HEATER
THICKENER
CONTROL
PANEL
BOILER
CIRCULATING
PUMP
COMPRESSOR
AUTOMA'TI
VALVES
(ONE BACK-UP)
LEVELING
VESSEL
DECANTING
AND STORAGE
TANK
CENTRIFUGE
* +
TO PS SOIL LAND PILL
CONDITIONING
GRINDER PUVP
-------
TABLE 39
Relative Dewatering Rates of Sludge
Conditioned by Different Conditioning Agents
Relative Dewatering Rates
Conditioning Agent Primary Sludge Secondary Sludge
None 30 1
Sulfuric acid2 100 2
Aluminum sulfate3 200 10
Ferric sulfate 300 15
Ferric chloride 400 20
Lime3 1000 80
Heat treatment 6000 1000
Note: 1. Mixed humus and activated sludge
2. At optimum pH value
3. At optimum dosage
4. One-half hour at 360° F
109
-------
Chemica^Stabilization of Sludge
Chemicals have also been applied to stabilize the sludge produced
from wastewater treatment processes. The two chemicals most
often used for this purpose are chlorine and lime. Both chemicals
have been found to be very effective in microbial destruction. How-
ever, it has been reported that lime is less effective than chlorine
in converting sludge to an inert material, but it is intrinsically
softer and easier to handle (113).
Sludge stabilization by chlorine is also referred to as the chemical
oxidation process. This process is generally performed using
chlorine with low pressure.
According to Games (13) this process not only improves the de-
waterability of the organic wastewater sludge, it has also been
noted that attachment of chlorine gas bubbles to the solid particles
causes flotation when applied to sand drying beds, resulting in
improving the drainability significantly. The sludge solids dis-
charged from this process are generally bleached to a light buff
color. This method is not very effective in reduction of volatile
content of the sludge; but, it is an effective method for the treat-
ment of combined waste-activated sludge and oily sludge in which
the heat causes the oil to float, making it easy to skim.
The literature reveals that numerous investigators (114,115,116)
have studied the effectiveness of lime in reducing the microbial
hazards in water, wastewater, and sludge. Lime was found to be
very effective in sludge stabilization; however, most information
available is qualitative and a minimum pH for satisfactory perfor-
mance has not been established. Lime stabilization of the waste-
water treatment plant sludge at Lebanon, Ohio was investigated
by the Ultimate Disposal Research Group of NERC, EPA, Cin-
cinnati, Ohio (113), recently. Results for the lime dose
requirement are presented in Table 40. It was concluded that
the lime demand is associated with the sludge solids and the
wastewater itself. For raw sludge, the lime demand was low due
to its high solids content. For chemical primary sludges ( alum
sludge and iron sludge) lime demand was not a function of solids
content of the sludge. It is believed that higher lime demand for
alum sludge was due to the fact that aluminum is amphoteric and
reacts with CafOH)? to form calcium aluminate, where iron does
not react in this way. Plant-scale test results shown in Table 41
indicated that typical enteric pathogens, Salmonella species and P.
aeruginosa, are destroyed by lime treatment, and the organisms
remaining after lime treatment appear to be non-pathogenic spore-
forming bacilli.
110
-------
TABLE 40
Preliminary Comparison of Lime Costs. .
for Raw, Alum, and Iron Sludges
Sludge Type
Chemical Dose to Primary (mg/1)
Sludge Suspended Solid (g/1)
Sludge Volatile Solids (% of SS)
pH before lime
pH after lime
Ca(OH)2 added, (g/1 sludge)
(g/g dried solids)
Cost of lime ($/ton dry solids)*
Raw
0
89.1
64.0
5.2
11.0
8.7
0.098
1.96
Alum
27.3 Al
3Z.1
55.9
6.2
11.0
12.9
0.40
8.00
Fed-
20.7 Fe
20.1
45.7
6.2
11.0
5.5
0.27
5.40
tf
Based on Ca(OH)2 cost of $20/ton. Multiply by 1.10 for $/metric ton.
TABLE 41
Bacteriological Studies of Sludge ,..,.
Produced in Plant-Scale Tests
Bacterial Count (organisms /liter of sludge
Salmonella Pseudomonas Total Aerobic
Sludge Species aeruginosa Count x 10
Alum-primary 110 1300 41
Limed alum-primary None detected None detected 5.0
Ferric-primary > 24, 000 610 190
Limed ferric-primary None detected None detected 0. 29
The addition of lime has effectively reduced the percent volatile
solids of the sludge, as is shown in Table 42. Furthermore, filter-
ability of alum and iron sludges was improved by the addition of
lime. Data obtained are presented in Table 43.
TABLE 42
Effect of Lime Addition on
Dissolved Solids and Percent Volatile Solids
Percent Volatile
Dissolved Solids (g/1) Suspended Solids
_ , „
Lame Treatment Al Fe Al Fe
Before treatment 1.7 1.7 66 64
After treatment 2.1 1.7 55 60
Increase 0.4 0 -11 -4
111
-------
TABLE 43
Effect of Lime on Filterability of Al and Fe Sludges
f-«
{-•
ro
SLUDGE PEDEER2Y
Buclmer Funnel Tests
Specific Resistance at
20" HG (ca/G x 10"11)
Compressibility
Filter Leaf Tests
Yield
(Ib/hr/ft^)*
Cake Moisture
(Ib w/lb d.s.)
LIME
ADDITION
Before
After
Before
After
Before
After
Before
After
AlHH'Dose(ng/l)
jl.8
19.0
3.8
0.50
0.70
0.98
1.97
4.31
3.87
22.7
28.0
4.5
0.46
0.91
0.9^
2.10
4.35
3.92
13.6
29.0
3.8
0.59
0.80
0.95
2.58
4.37
3.83
i^i^i
Fe Dose (os/l)
31.0
14.9
7.0
0.53
0.43
1.C6
1.57
4.10
4.23
15.5
21.6
5.5
0.60
0.76
1.57
2.40
3.75
3.75
Multiply by U.86 to get 3cg/hr/m2
-------
Solvent Extraction
Application of solvent extraction methods known as the "McDonald
Process" for treatment of wastewater sludge is discussed by Stal-
lery (117). This process involves a number of steps, including
dewatering of the sludge by centrifugation, solvent extraction with
carbon tetrachloroethylene, and distillation. The resulting end
products are generally fats, dried oils and grease. Although this
process proved to be very efficient, it is believed to be impractical
in wastewater treatment because of high expense.
Electrical Treatment
Slagle and Roberts(118) have made an extensive investigation at both
laboratory and pilot plant scales of sludge treatment by electro-
dialysis. They found this method of sludge treatment to have advan-
tages over chemical conditioning and other methods. To compare
this method with chemical conditioning, a fresh sludge containing
6.5% solids was used. The resulting comparative data are shown in
Table 44.
TABLE 44
Comparison of Chemical Treatment .
and Electrodialysis Treatment
Chemical Treatment Electrodialysis Treatment
(per ton of dry solids) (per ton of dry solids)
89 Ibs of ferric chloride used 181 KWH expended
70% filter cake moisture 59.5%
2065 Ibs filter cake solids 1440 Ibs
4665 Ibs filter cake water 2130 Ibs
6.2pH 3-4 pH
The costs of sludge treatment using electrodialysis were found to
be much more attractive than chemical conditioning of the sludge.
The electro-osmosis process has also been investigated and reported
by Cooling et al (119), and Beaudoin (120). It was concluded that
although the process of sludge treatment is technically very pro-
mising, it is not economically feasible.
SLUDGE CONDITIONING
Sludge is conditioned principally to improve its dewatering charac-
teristics. Various methods of sludge conditioning are being prac-
ticed in wastewater treatment plants with the most common methods
113
-------
being elutriation and chemical conditioning. Other methods include:
freezing of the sludge and addition of fly ash, diatomaceous earth,
or other flocculating agents. The sludge moisture content can be
reduced from 95 to 99% to about 70 to 75% (9) by means of proper
conditioning followed by a dewatering process. One of the mechanisms of
sludge conditioning using chemicals reduces the charge of the sus-
pended particle thus decreasing the water affinity of the
sludge solids and promoting particle coalescence. According to
Carnes (13) conditioning can be applied at any point in the train of
unit processes included in solids handling. The ideal point of
application is dependent on the performance required, subsequent
handling, and the characteristics of the waste solids.
Elutriation of Sludge
Elutriation has been defined as a washing operation which removes
sludge constituents that interfere with sludge thickening and dewater-
ing processes (17).
The elutriant used is usually settled or biologically treated waste-
water. As the elutriant comes in contact with the sludge particles,
dispersants such as carbonates and phosphates are extracted from the
sludge as well as non-settleable fine particles and the products
of decomposition. This results in a sludge of high porosity, with
lower requirements for coagulant chemicals because of less
surfaces to be treated and lower concentration of chemical inter-
ference (9). It has been observed that the bicarbonate alkalinity of
the sludge liquor increases dramatically during the digestion process,
which in turn causes a drastic increase in the chemicals required
for conditioning. Thus, elutriation is applied, reducing the
alkalinity and consequently lowering the chemical requirements.
Other advantages of elutriation as noted by Center (121) include the
washing out of the toxic materials that inhibit sludge digestion or
other biological processes, and the treating of dirty digester
supernatant liquor.
Elutriation operations can be performed as a batch or continuous
process. Figure 26 shows flow diagrams of the three methods of
operation by Rich (66). Operation can be single stage with single
contact between the solids and the liquid. It could be multistage
contact in one or more basins, or it can be multistage counter-
current contact.
For determination of the alkalinity of the elutriated sludge, the
following equation has commonly been used.
114
-------
Figure 26
Flow Diagrams Illustrating Various
Arrangements Employed in Dispersed- Contact
Leaching Operations (66)
(a) Single Contact
(b) Multistage Co-current Contact
(c) Multistage Countercurrent Contact
Single.
1
V*
Stye /
x/ .
**,.*
t
t
I
,0
Y*
115
-------
1. Single stage elutriation E =
D+RW
R+l
2. Multistage elutriation E =
3. Multistage counter cur rent E = p ZXP.LI
elutriation
2-t-R
where;
D = sludge liquor alkalinity, mg/1
E = elutriated sludge alkalinity, mg/1
W= elutriated water alkalinity, mg/1
R = ratio of elutriating water to sludge alkalinity.
Elutriation of digested sludge removes the constituents that inter-
fere with chemical flocculation. The work of Center (123) (Figure
27) supports this fact for (a) primary sludge, (b) primary and
filter humus, and (c) primary and waste-activated sludge.
Figure 27
Quantity of ferric chloride required for vacuum filtration of various
domestic sludges (45)
.(a)
Chemical Conditioning
Use of chemicals for the purpose of sludge conditioning results in
coagulation of the solids and the release of the absorbed water from
the solid particles. The literature reveals that many investigators
have made extensive studies on the use of various types of chemicals
116
-------
for sludge conditioning. At present, the most common types of
chemicals used in conditioning of waste water sludge include: ferric
chloride; ferric chloride combined with lime; ferric chloride com-
bined with alum; or lime alone (17,124). Reviewing the sludge con-
ditioning method of 18 plants, Zack (125) found that ferric chloride,
alone* was used in plants dewatering waste activated sludge, while
ferric chloride and lime were commonly used for those plants
dewatering raw and/or digested sludge. Table 45 represents typical
dosages used for various types of sludges. However, actual dosage
for a given plant may vary considerably from these figures (14).
Dosage variations depend on many factors. For example, digested
sludge dewaters in a mannner varying with the duration of the
digestion. Activated sludge dewatering depends on its state of
oxidation (9). The chemical dosage required for a given sludge
is determined in the laboratory, using a filter leaf test kit, or a
modified Buchner funnel procedure (9,14).
TABLE 45
Dosage of Chemicals for Various Types of Sludges
(14)
(Conditioners in Percentage of Dry Sludge)
Description
Primary
Primary and
trickling filter
Primary and
activated
Activated (alone)
Fresh
FeCl2
1-2
2-3
1.5-2.5
4-6
solids
CaO
6-8
6-8
7-8
Digested
FeCl2 CaO
1.5-3.5 6-10
1.5-3.5 6-10
1.5-4 6-12
Elutriated
digested
Fed 2 CaO
2-4
2-4
2-4
Table 46 gives a partial listing of waste water treatment plants
using chemical conditioning, together with their dosage rates, and
resulting cake yield and solids recovery. The solids recovery varied
from 88 to 95%.
Polyele ctrolyte s
Polyelectrolytes or organic polymers have gained much popularity
in recent years as a sludge conditioner. Dahl et al (126) has made
extensive studies using synthetic organic polymers as sludge con-
ditioners. Results showed that: (1) conditioning of sludge signifi-
cantly improved its de water ability, and {2) solids captured during
117
-------
TABLE 46
(9)
Illustrative Data, Vacuum Filters with Lime and Ferric Chloride Conditioning
00
Installation
Cleveland.
southerly
Cleveland,
westerly
Cincinnati,
Mill Creek
Dayton
Detroit
Indianapolis
Minneapolis-
St. Paul
Feed Sludge
Dry Soiids
Solids
(%)
4.0
8.6
11.8
6.4
10.4
5.6
7.2
Dry
Weight
(tons/day)
37.0
4.4
41.0
4.8
157.0
51.0
134.0
Chemical Dosage
As % Dry Feed
Sludge Solids
FeCl3
4.5
2.6
1.9
5.6
1.6
4.0
0.8
CaO
10.7
12.3
6.0
16.1
10.2
14.9
2.2
Sludge Cake
Solids
(%)
25.2
34.6
37.4
24.4
36.9
26.8
1 27.2-
1
Volatile
(%)
41.6
45.3
30.8
..
53.0
61.1
72.9
Yield
(Ib/hr/sq ft)
1.84
3.98
4.17
3.80
3.95
5.48
3.11
Dry Solids
Recovery
(% of total)
93
94
95
88
99
95
95
Type of Sludge
*V f w««^.^»
elutriated, digested
primary, and activated
digested primary
elutriated, digested
primary
digested
primary and humus
primary with 75 to
80 percent digested
and 20 to 25 per-
cent raw
primary
raw
primary
raw
Note: Tons X 907 - kg; Ib/hr/sq ft x 4.88 * kg/hr/xj m.
-------
the elutriation were increased from 57 to 92%. Bugg et al (127)
reported on conditioning of alum sludge from water treatment
plants with polymers. The results of their investigation indicated
the following: (1) anionic polymers were somewhat superior to
nonionic agents, and performed exceptionally well when compared
with cationic chemicals, (2) cationic polymers were effective only
at low pH ranges, whereas nonionic and anionic polymers were
most effective at pH ranges of 6 to 10. Sharman (128) reported
that the results of substituting polymers for ferric chloride in-
cluded; (1) the solids content of the filter cake was increased dra-
stically; (2) the non- combustible content of the filter cake was
lowered substantially; and (3) the chemical dosage rate was lowered
significantly, e.g. from 220-359 Ib/ton of dry solids filtered when
ferric chloride was used, to 2.5-3.6 Ib/ton of dry solids filtered
when organic polymer was used.
Sherbeck (134) reported on the use of synthetic organic polymers for
sludge conditioning in Bay City Sewage Treatment Plant. The results
of 36 months of operation, costs of inorganic conditioners and or-
ganic polymers were reported. These data are presented in Tables
47 and 481 Table 49 represents typical dosages of polymers used
for sludge conditioning, the resulting cake yield, and the final
solids content.
TABLE 48
Cost of Chemicals for Sludge Conditioning
Cost/Ton
Dry Solids
Substance
Kiln-dried lime and ferric chloride 9.93
Carbide lime and ferric chloride 6. 85
Polyelectrolyte C-32 14.50
Polyelectrolyte C-31 11.20
Polyelectrolyte C-149 7.00
Polyelectrolyte A -21 _ 2.75
The application of polymers has also been discussed by other
investigators, such as Dickert (129), Goodman (130,131), Burd
(17), Morris (132), and Bargman (133). Reviewing these dis-
cussions and others, the following observations are made: (1) every
individual polymer tested behaved differently, however, some
general similarities do exist; (2) a wide variety of polymers are pre-
sently available which imposes difficulties in selecting the one
that gives optimum results for a given sludge; (3) some polymers
used for elutriation of digested sludge performed excellently
while others required addition of ferric chloride; (4) some polymers,
119
-------
TABLE 47
A Comparison between Lime, Ferric Chloride and Polyelectrolytes
for Conditioning Sludge
Year
1959
1960
1961-62
1962-63
1963-64
Dry
Solids
(tons)
461
580
424
415
437
Solids
in
Sludge
(%)
11.2
11.2
10.9
10.7
10.2
Yield
(psf/
hr)
3.1
3.1
5.3
5.5
6.3
Amount of Substance Added
db)
Lime
162,000
225,000
--
--
*» •
FeCl_
£t
31,000
44,000
--
--
--
C-31
--
--
--
10,300
--
C-32
—
--
5,562
—
C-149
--
--
--
A-21
—
--
--
--
Total of all three
polymers
7,671
Solids
in
Sludge
(%)
40.1
39.0
35.9
34.5
34.6
Vol. in
Cake
(%)
64.1
62.1
75.6
73.7
75.9
Opera-
tion
Time
(hr)
2,125
2,420
1,119
1,114
1,301
t\J
o
-------
TABLE 49
Illustrative Data, Vacuum Filters with Polymer Conditioning
(9)
Installation
Atlanta,
S. River(53)
Erie, Pa. (91)
Erie, Pa. (91)
Hamilton,
Ontario (92)
Ann Arbor,
Mich. (42)
Battle Creek,
Mich. (42)
Battle Creek,
Mich. (42)
Feed Sludge
Dry Solids
Solids
(%)
8.0
6.2
5.3
_
2.0
5.5
4,5
)ryWeight
(tons/flay)
11.34
10.65
16.55
— .
7.02
21.64
19.76
Chemical Dosage
As % Dry Feed
Sludge Solids
Polymer
(cation ic)
0.95
1.4
1.56*
0.14
Polymer
FcCI3
(cationic)
0.72
0.89
0.72
Polymer
(anionic)
12.0
0.93
4.47
Sludge Cake
Solids
(%)
23.0
—
-
29.5
20.0
23.5
20.0
Volatile
(%)
—
-
-
—
..
_
—
Yield
(Ib/hr/sqft)
6.7
2.5
2.5
8.9
_
—
—
Dry Solids
Recovery
(% of total)
~
—
-
—
—
—
••
Type of Sludge
elutriated, digested pri-
mary, and activated
digested primary
and activated
elutriated, digested pri-
mary, and activated
digested primary
elutriated, digested pri-
mary, and activated
digested primary and
humus (3/4) + raw (1/4)
digested primary and
humus
*0.075- to 0.125-percent anionic polymer used in elutriation prior to conditioning and filtration.
Note: TOM x 907 • kg; Ib/hr/aqft x 4.88 • kg/hr/tq m.
-------
while performing as an excellent coagulant agent as in the elutri-
ation process, did not improve dewaterability of sludge; (5) poly-
mers used to condition sludge for subsequent dewatering, in gen-
eral, showed little improvement in the settling rate; and (6) pre-
dictions as to the action of a particular polymer on a particular
sludge seem to have little validity with the most practical approach
to the problem being a trial and error procedure.
Freezing
Katz and Mason (135) have reported on the application of a freezing
method for conditioning activated sludge prior to dewatering. Results
of their comprehensive investigation were as follows: (1) solids
production rate was found to be significantly high, on the order of
50 Ib/sq-ft/hr; (2) quality of filtrate and filter cake produced from
freeze-conditioning was equivalent or better than from conventional
vacuum filter operation; (3) gravity draining using wire screen
cloth (40-80 mesh) satisfactorily dewatered freeze-conditioned
sludge; and (4) the conditioning effect produced by freezing was
believed to be a result of dehydration and the pressure exerted on
the sludge particles by the ice structure.
Farrell and co-workers (136) have made a comprehensive study of
the application of natural freezing for dewatering of aluminum hy-
droxide sludge. According to Farrell "aluminum hydroxide sludges,
when frozen slowly and completely, change structure character
completely upon thawing. A rapidly, settleable, high density
precipitate is obtained. " Conclusions of the study made were: (1)
distinct similarity between freezing rate of sludge to that of water
was observed with the sludge freezing rate being about 10% slower
because the natural convection is reduced during the stage
when the liquid is being cooled to the freezing point; (2) snow cover
was believed to be detrimental to freezing of sludge, even in
extremely cold climates;(3) significant increase in solids content
and dewaterability of the sludge is attainable by slow freezing and
thawing, provided that the freezing is complete as shown in Figure
28; (4) freezing can be used in mild climates only if sludge is
applied in thin layers of 0.5 to 1.0 inch thickness; and (5) presence
of phosphate from 0 to 1 mole phosphorus per mole of aluminum did
not change filterability and solids content of the frozen sludge
significantly, as shown in Table 50. Conditioning of sludge by
freezing has been investigated. Results of these investigations are
summarized as follows: (1) freezing is applicable to all types of
sludge; (2) addition of flocculant agent enhanced conditioning of
aludge but is not required; (3) good dewatering results can be
122
-------
Figure 28
Effect of Repeated Partial Freezing on Sludge Properties*136*
00
o
S-
Ol
CO
o
0>
0>
u
c
-M
tn
in
C£.
u
u
SL
00
CD
I
O)
u
XJ
"o
/•completely frozen
0
freeze-thaw cycles
123
-------
obtained if freezing is slow and complete; (4) increasing the freezing
period did not affect dewaterability; (5) thawing of sludge can be done
in any manner, provided that it is not accompanied by vigorous agi-
tation; and (6) freezing increased the rate of settling.
TABLE 50
Effect of Freeze-Thaw Cycle on Properties of
Aluminum Hydroxide Sludges with Varying ,.
Amounts of Phosphate
Nominal
P/A1
Molar
Ratio
0
0. 34
0.69
1.04
Before
Solids
Content
e/l
3.20
2.50
3.20
2.84
Freezing
Specific
Resist-
ance
sec^/g
x 108
10
19
16
16
After Freezing
Specific
Resist-
Solids ance
Content sec^/g
g/1 xK)8
176 0.04
168 0.06
180 0.06
178 0.07
Theoretically, if water containing impurities is frozen at a slow
rate, the impurities are rejected from the ice structure a.id become
concentrated in the liquid fraction (137). The freezing rate for the
rejection of impurities is about 0.0005 cc/sec, and when the rate
exceeds 0.001 cc/sec, impurities will be captured in the ice crys-
tals. The freezing rate used in sludge is about 0.0067 cc/sec, (137)
resulting in the capture of some solids in the ice structure. The
effect of freezing on the concentration of suspended solids in the
unfrozen fraction of the sludge was demonstrated, (135) as shown
in Figure 29. The effect of extent of freezing on the dissolved solids
in the unfrozen fraction of sludge is shown in Figure 30.
Fly Ash Conditioning of Sludge s
Use of fly ash to condition the sludge from wastewater treatment
plants, for subsequent dewatering, has recently been investigated
by a number of investigators. Eye and Basu (138) made an extensive
study on the effect of fly ash in improving filterability of digested
sludge. A summary of their results is as .follows: (1) it was found
that a 1:1 mixture of fly ash to sludge filtered better than digested
sludge alone; (2) addition of chemical conditioning to this mixture
increased filterability significantly, as shown in Table 51; (3) and
optimum filtering condition (minimum filtering time) was obtained
when sludge-fly ash ratios ranged from 3:1 to 4:1, as shown in
Figure 31.
124
-------
Figure
-------
TABLE 51
Typical Sludge Filterability by Buchner Funnel Test
(Secondary Effluent at Vacuum of 15 in. Hg. Sample Volume = 50 ml)
(138)
Fly ash Sludge
22% Solids by
Weight
Time
(sec)
10
15
20
25
30
35
40
45
50
60
80
100
120
Filtrate
Vol (ml)
12
16
23
27
31.5
34.0
35.0
35.5
36.0
36.2
36.5
36.8
37.0
Digested Sludge
(No Chemicals)
13% Solids
by Weight
Time
(sec)
25
35
60
100
190
230
335
555
630
730
945
1190
1410
2200
Filtrate
Vol (ml)
3
4.5
7
10
15
17
21
28
30
32
35
37
38
39.2
Flyash Sludge
& Digested
Sludge 1:1
Time
(sec)
10
15
20
40
60
80
100
145
200
250
300
360
400
Filtrate
Vol (ml)
3
4
5.0
8.5
12.0
14.5
16.5
22.0
28.0
32.0
35.0
37.0
37.5
Digested Sludge
with Chemicals
Time
(sec)
10
15
20
25
30
35
40
50
70
100
Filtrate
Vol (ml)
12
22
28
31
32.5
34.'0
35.5
36.5
37.0
37.0
Flyash
Sludge & Digested
Sludge 1:1 with
Chemicals
Time
(sec)
10
15
20
25
30
35
40
50
85
110
Filtrate
Vol (ml)
16
24
27
30
31.5
32.5
33.5
34.5
35.5
35.8
Solids Content in
Cake (% by Weight)
• / O *
Only flyash sludge
= 65%
Only digested
sludge with
chemicals = 52%
1: 1 mixture with-
out chemicals
= 56%
1: 1 mixture with
chemicals
= 59%
-------
Figure 31
Flyash dosage vs. time to collect 50 ml of filtrate.
(138)
2000
1600
1200
o
UJ
v>
a eoo
400
DIGESTED SEWAGE SLUDGE - 12* SOLOS
WSAGE Of fLYASH IN CMS/100 ML Of SIU06£
0 S 10 li 20 25 30 35 40
Tanney and Cole (139) investigated the use of fly ash for condition-
ing of activated and digested sludge. Results showed that the amounts
of fly ash required for improving filterability of activated sludge
ranged from 500 to 700 percent of the initial solids content of the
sludge, whereas for digested sludge they ranged from 100 to 150
percent.
Malina (111) in discussing the Beloit-Pas savant "SLUDGE ALL"
process has indicated that fly ash has been used as the condition-
er agent with the ratio of fly ash-solid ranging from 1:1 to 4:1 on
dry-weight basis, depending on the type of sludge used. He further
noted that one advantage of adding fly ash as a conditioning agent
is that the fine fly ash particles have the capability of trapping many
of the particles present in the sludge. Table 52 presents typical
pilot-plant operation data obtained using fly ash as the sole con-
ditioning agent.
TABLE 52
Operating Data: Beloit-Passavant "SLUDGE ALL" System
(111)
Sludge
Type of
Sludge
Raw + activated
Raw + activated
Activated
Activated
Digested
Digested
Waste activated
Waste activated
Suspended
Solids
(Percent)
4.90
4.10
0.90
0.98
6.20
6.36
1.20
1.00
Volatile
Solids
(Percent)
66.5
69.0
77.0
69.0
50.0
47.5
71.0
64.0
Ash-
Sludge
(Ratio)
1:1
3.1
3:1
2:1*
2:1
3:1
2:1
3:1
Filter Cake
Solids
(Percent)
40.0
55.0
29.0
40.0
52.2
59.4
58.2
62.1
Volatile
(Percent)
29.0
23.0
29.0
27.0
17.4
18.4
29.0
26.9
127
-------
Filter Aids
It has been observed that the specific resistance of certain sludges
can be reduced by adding fibrous or particulate material. These
added materials together with the solid particles of the sludge form
a porous, permeable, and rigid lattice structure which filters
particulates but allows passage of liquid. These materials are
particularly effective for sludge structures that are highly corn-
re ssible and tend to blind or slime the filtration medium (9).
Filter aids include diatomaceous earth, sawdust, news prints,
pulped magazine paper, wood chips, etc.
Malina (111) reported the results of laboratory-scale investigation
of the use of filter aids as well as organic and inorganic polymers.
Results shown in Figures 32, 33, and 34 indicated that newsprints
as well as pulped magazine paper could be used very effectively
as conditioning agents.
SLUDGE DEWATERING
Sludge is the most important by-product of the waste water treat-
ment plant. Disposal of these materials is a problem comparable
in magnitude and importance to that of treating the waste water.
Any reduction in volume of these by-products would be a signifi-
cant step in improving and solving the disposal problem. Reduc-
tion of the volume of the sludge, by removal of water,has been a
difficult problem since the very first days of waste water treat-
ment. For many years sludge de water ing was done by land
methods, i. ef, lagooning, and sand beds. However, recently
attention has been focused on use of other techniques for sludge
dewatering. Increased land costs, along with stringent land, air,
and water pollution standards have made sludge dewatering an
economically attractive process since it significantly reduces
the volume to be disposed of and may retard biological decomposition.
The methods commonly used for sludge dewatering to be discussed
briefly in this paper include:
1. Sand Bed Drying
2. Centrifugation
3. Vacuum Filtration
4. Filter Press
5. Vibration
6. Heat Drying
128
-------
Figure 32
Effect of Conditioners on Specific Resistance of Activated Sludge
(111)
14
12
10
o
~ 8
W
*•
o
g
o
5 6
to
a:
o
IE >t
o 4
a.
CO
Return Activated Sludge
Suspended Solids = 22.7 gm/l
~0 2 4 6 8 10 12
Dosqgs of FeCI3 or Polymer ( % Dry Wt. Solids )
, i i i i i i
O 50 100 ISO
Dosage of Newsprint or Magazine (% Dry Wt. Solids )
129
-------
Figure 33
Effect of Conditioners on Specific Resistance of Digested Primary
Sludge and Humus.
Digested Sludge
Total Solids = 52.6 gm/l
Magazine
•HO%CaO
--C-3I Polymer
FeCI3+5%CaO
FeCI
Newsprint
+10% CaO
FeCI +15% CoO i
"0 2 4 6 8 10 12
Dosage of FeCI3 or Polymer (% Dry Wt Solids )
0 50 100 150
Dosage of Newsprint or Magazine (% Dry Wt. Solids)
130
-------
Figure 34
Effect of Conditioners on Specific Resistance of Primary Sludge
(111)
14
12
10
E
<5*
u
0>
CA
CO
o
C. 8
0>
o
6
tfi
rr
_o
»H
'o
0>
CL
(O
Raw Primary Sludge
Suspended Solids = 23.9 gm/l
Newsprint
FeCI3
*5%CaO
Magazine
Newsprint + 5%CaO
0 2 4 6 8 10 12
Dosage of FeCI3 or Polymer (% Dry Wt Solids)
i i 1 1.
0 GO (00 ISO
Dosage of Newsprint or Magazine (% Dry Wt. Solids)
131
-------
Sand Bed Drying
Drying of sludge on open or covered sand beds is perhaps the oldest
and yet the most common method used to reduce the water content
of the wastewater sludge to a forkable condition prior to final dis-
posal. It is the most economical method of sludge disposal, partic-
ularly for municipal wastewater treatment plants serving small and
medium size communities. Thoman and Jenkins (140) in their report
on the "Statistical Summary of Sewage Works in the U.S. in 1957"
stated that "over 71% of the nation's 7518 municipal wastewater
treatment plants used drying beds for sludge dewatering purposes. "
A 1969 survey report from 27 State Health Departments (9) showed
that the percentage given previously was down, as shown in Table
53. Lack of land availability, high costs of land and labor were
believed to be the main reasons.
TABLE 53
1967 Report from 27 State Health Departments,
(9)
Number of Sludge Drying Beds
Population of Cities
Less than 5,000
5,000 to 25,000
More than 25,000
Total
Number
1,886
750
168
2,804
With Sludge Beds
%* %+
67
27
6
100
73
22
5
100
* Percent of reported totals for 1967 survey.
+ Percent of reported totals for 1957 survey.
Note: Only 6 states reported having covered beds; 14 states reported
they will continue with sludge beds in new plants; 13 states reported
the use of beds with paved surfaces in lieu of sand.
Both open and closed drying beds have been used with the former
being more common.
According to Jennett and Harris (141) the variables affecting the
drying rate of sludge on open beds include: (1) climate and atmos-
pheric variations, (2) sludge depth application, (3) sludge mois-
ture content, (4) types and origin of sludge, (5) sludge age, (6)
drying beds construction, and (7) presence or absence of coagulants.
The two major dewatering mechanisms influencing sludge dewater-
ing rates on drying beds, are evaporation and drainage (9,17,141,
142). Quon and Tamblyn (143) have reported that an inverse rela-
tionship exists between the two, and that maximization of either
132
-------
mechanism will retard the other one. Other investigators (141, 144)
have supported this theory to some extent and have pointed out
that drainage is indeed the major and more significant factor.
Swanwick (145) reported that 85% of the water lost from secondary
sludge was by drainage. Templeton (146) found that drainage account-
ed for 21% by weight of the sludge applied without use of chemicals,
and Volger and Rudolfe (147) have reported that 60% of the sludge
water is free or drainable. Drainage is influenced by many factors,
such as : initial solids content of the sludge (142,148); depth of
supporting media (149); sand grain size distribution (149); and
presence or absence of coagulants (144,149).
Nebicker (150) reported that sludge moisture loss due to evaporation
could be approximated to the removal rate from a free water surface,
provided that there is free sludge surface moisture. He also noted
that sludge evaporation rates would be modified by the darker sludge
surface which would absorb more radiant energy and thus increase
moisture loss. The evaporative rate would be decreased because
of dissolved solids, and oils and fats on the supernatant surface.
He also indicated that the drainage of water from the applied sludge
is almost completed within three days after application of the sludge
and further dewatering is caused by evaporation.
Evaporation dewatering of sludge occurs in two distinct phases:
constant rate drying, and decreased rate drying. In the first phase
evaporation continues until the free surface moisture is exhausted
and no longer can be replenished by the internal transport of water
to the sludge surface. Evaporation in the second phase proceeds
at a decreasing rate, depending on the nature and character-
istics of the dewatering material. It may or may not have a linear
relationship with time (122).
Rate of evaporation is affected generally by factors such as air
temperature, amount of sunshine, relative humidity and wind
velocity. Laboratory and field investigation of these factors have
been reported by numerous investigators (17,141,144).
In general, drainage is the predominant dewatering mechanism
during the first 2 to 3 days after the application of the sludge on
drying beds. After 2 to 3 days, evaporation becomes the major
factor and in the course of a few days the sludge cake shrinks
horizontally, and produces cracks at the surface which accelerate
evaporation by exposing additional sludge surface area (151).
133
-------
Effect of the dosing depth and solids content of the applied sludge on
the drying time was studied by Downes. A summary of results re-
ported by Jennett (141) is presented in Table 54.
TABLE 54
Days Required to Dry Different Depths of .
Sludge of Varying Solid Contents
Percent
Solids
4
5
6
7
8
Dosing Depth, inch.
8
Drying
5.5
6.5
9.0
12.0
14.0
9
Time,
6.0
8.5
12.0
14.0
18.0
10
Days
6.0
10.0
13.0
16.0
23.0
Haseltine (148) found that the optimum depths for the open and
covered beds were 8 and 14 inches respectively. Quon and Johnson
(142) reported that 8" depth for open bed gave the greatest drainage
volume. The effect of polymers on dewatering rate was investiga-
ted by Jennett and Santry (144). It was found that in general, poly-
mers were effective in increasing dewatering rate. It was also
noted that greater amounts (200 mg/1 dosages) of polymer did not
increase dewatering rate significantly more than smaller amounts
of 50 mg/1 or less.
A comprehensive investigation of the effect of air temperature and
relative humidity on the dewatering of anaerobically digested
activated sludge was made by Jennett and Harris (141). They have
reported that sludge dewatering on drying beds is not a function
of relative humidity, as had been originally assumed. Also*it is
shown in Figure 35 that a definite relationship exists between
sludge moisture content, evaporation rate from a free water surface,
and the parameters AT (the difference between wet and dry bulb
temperature), and AH (the difference between saturation and absol-
ute humidity). Data showing these relationships are presented in
Table 55 and shown graphically in Figure 36. The moisture gra-
dient (moisture content of the top layer minus the moisture content
of the bottom layer) developed within the dewatering sludge gen-
erally increased with time and AT, as shown graphically in Figure
37. An inverse relationship was noted between drainage and eva-
poration; and also wind significantly affected the removal of sludge
moisture with time. Total drainage and evaporation losses are
summarized in Table 56.
134
-------
Figure 35
Moisture Content Versus Time for Runs 1, 2, and 3
(HI)
IOOC
80
U
o
Of
Ul
60
ui
ui
oc
40
20
0
100
z 80
UI
U
U
O.
X
III
60
O
0 401-
tal
CC
2 20
OPEN.ORAINED MODELS
RUN DRY BULB RELATIVE
NO. TEMP.,°F HUMIDITY, PERCENT
«—• 95 42
2o—a 92 66
3o—o 95 25
CLOSED, DRAINED MODEL
100
HOURS
200
DAYS
8
135
-------
TABLE 55 ^
Runs 1, 4, 5, 6, 7, and 8 Ranked on the Basis
of Environmental Conditions and Applicable Results
(HI)
Temperature
Dry
Bulb
t
(1)
1
7
6, 8
8, 6
k
5
Wet
Bulb
t
(2)
1
6
8
7
5
^
AT
T
(3)
1
7
8
k
6
5
Humidity
Pel.
;
<<0
7
^
1
8
5. 6
6, 5
Abs.
;
(5)
fc
5
7
8
6
1
Sat.
I
(6)
k
5
7
8
6
1
AH
r
(7)
1
7
8
U
6
5
Moisture, #
In it.
1
(8)
6
7
5
1
U
6
Final
4
(9)
1
7
6
U
6
5
Evaporation
Sludge
t
(10}
1
7
8
6
k
5
Water
t
(il)
1
7
8
4
6
5
Ratio
t
(12)
1
7
5
6
8
4
Drainage
Total
1
H3)
k
8
7
1
6
S
56
Applied
1
(VO
u
8
7
1
6
5
*l- Indicates increasing order of magnitude*
f- Indicates decreasing order of magnitude.
-------
Figure 36
Moisture Content Versus Time for Runs 1, 4, 5, 6, 7, and 8.
(141)
100
OJ
60
w
u
0.
01
z
o
o
60
40
o
20
iOO
HOURS
200
900
DAYS
8
-------
Figure 37
Moisture Gradient Versus Time for Runs 1, 4, 5, 6, 7, and 8.
HOURS
DAYS
8
-------
TABLE 56
Total Moisture Losses from the Open, Drained Models
(141)
Run
No.
1
2
3
4
5
6
7
8
Average
Applied
Weight of
Sludge
Dsf*
42.5
41.3
42.6
42.3
44.0
43.5
43.1
42.6
Initial
Moisture
Content
%
96.6
97.6
97.1
96.8
96.5
97.2
96.3
95.6
Initial
Water
Content
psf*
41,0
40.4
M.I*
M.O
42.5
42.3
M.5
41.0
Drainage
Total
psf*
31. o
31.9
33.4
24.4
39.5
34.5
30.6
28.3
% of Initial
Water
Content
75.6
79.0
81.0
59.5
93.0
81.5
73.7
69.0
Svaporation
Total
psf*
10.2
2.9
5.5
9.5
0.3
4.8
8.2
7.8
% of Initial
Water
Content
24.9
7.2
13.3
23.2
0.7
11.4
19.8
19.0
Dewatering
Drainage
+ Evap.
psf*
41.2
34.8
38.9
33.9
39.8
39.3
38.8
36.1
% of Initial
water
Content
100.4
66.2
94.0
82.7
93.7
93.0
93.5
88.0
to
»Multiply by 4.88 to convert psf to kg/sq m.
-------
In summary, dewatering characteristics of the sludge are dictated
by many factors, such as the wastewater characteristics, type
of sludge, and the efficiency and characteristics of the wastewater
treatment process. Raw sludge generally contains a high percent-
age of fibrous and greasy materials, resulting in a bed of dense
dewatered sludge cake containing only a few large surface cracks.
On the other hand, beds of digested sludge contain many small
cracks which allow more surface exposure to the drying air and
thus greater drainage of water (9). Sludges containing grits dry
faster, where sludge containing grease dries more slowly. Fresh
sludge has a tendency to dry faster than aged sludge; primary sludge
drains faster than secondary sludge; and digested sludge dries
faster than fresh sludge (17).
For the sake of brevity, design criteria of drying beds will not be
discussed here, however, some data are provided in Table 57 and
Table 58.
C
entrifugation
Centrifugation of wastewater sludge is a mechanical means of de-
watering. The application of centrifugal force for dewatering of
wastewater solids has been recognized by engineers, however, it
was not until a decade ago that centrifugation gained popularity in
wastewater treatment systems. Reports.indicate that during the
years 1967 and 1968, a total of 233 installations of centrifugal
dewatering units, for processing municipal and industrial sludge
were made in the U.S. (9).
Centrifuges that have been used for sludge dewatering include (1)
basket-design centrifuge, (2) disk design centrifuge, and (3) solid
bowl conveyor centrifuge. The basket-design centrifuge did not gain
popularity because liquid clarification was generally poor, although
it provided a well dewatered cake. The disc-type centrifuge was effec-
tive in clarification, but operational problems and their dewatering
capabilities caused their abandonment. The solid bowl conveyor
centrifuge solves the problems by combining the good dewatering
of one unit with the effective clarification of the other unit. A
schematic of this type of centrifuge is shown in Figure 38.
Alberton and Guidi (152) have pointed out that the greatest advantage
of centrifugation is its flexibility. It may be modified to meet
specific process needs; or, in some cases, the process conditions
themselves may be modified to improve the performance of the
dewatering unit. Parameters that are affecting the efficiency of the
unit include machine variables and process variables listed in
Table 59.
140
-------
TABLE 57
Sludge Drying Bed Area Needed for
(9)
Dewatering Digested Sludge
Type of Wastewater Treatment
Primary
Trickling Filters
Activated Sludge
Chemical Coagulation
Area (sq ft/cap)*
Open Beds
1.00
1.50
1.75
2.00
Covered Beds
0.75
1.25
1.35
1.50
#"South of 40 degrees north latitudes, these figures can be reduced
25 percent; and north of 45 degrees north latitude, they should be
increased 25 percent." (2)
Note: Sq ft x 0.0929 = sq m.
TABLE 58
Sludge Drying Bed Area Needed for
(9)
Dewatering Digested Sludge
Type of Sludge
Primary digested
Primary and humus digested
Primary and activated digested
Primary and chemically
precipitated digested
Area (sq ft/ cap)*
Open Beds
1.0 to 1.5
1.25 to 1.75
1.75 to 2.5
2.0 to 2.25
Covered Beds
0.75 to 1.0
1.0 to 1.25
1.25 to 1.5
1.25 to 1.5
#"With glass covered beds, a greater number of sludge drawings
per year is obtained because of protection against rain and snow --
actually a combination of open and enclosed beds provides maximum
usage. . . .open beds may evaporate cake moisture faster than covered
beds under favorable weather conditions." (1)
Note: Sq ft x 0.0929 = sq m.
141
-------
TABLE 59
Machine and Process Variables
(152)
Machine Variables
1. Bowl Design
a. Length-to-diameter ratio
b. Bowl Angle
?.. Bowl Speed
3. Pool Volume (depth)
4. Conveyor Speed and Pitch
Process Variables
1. Feed Rate
2. Solids Characteristics
a. Particle Size
b. Density
3. Feed Consistency
4. Temperature
5. Chemical Aids
Figure 38
The solid bowl-conveyor sludge dewatering
centrifuge assembly consists of a rotating unit com-
prising a bowl and conveyor.
(Courtesy Bird Machine Co.)
COVER
DIFFERENTIAL SPEEO
GEAR Box -i
MAIN
DRIVE
SHEAVE
JI3— FEED PIPES
(SLUDGE a
CHEMICAL)
BEARING
BASE NOT SHOWN
CENTRATE
DISCHARGE
\
SLUDGE CAKE
DISCHARGE
142
-------
Burd (17) reported that according to Guidi, the two major factors
determining the success or failure of a centrifuge are solids recov-
ery and cake dryness. These two factors are affected by the
parameters given previously in the following manner;
To Increase Cake Dryness
1. Increase bowl speed
2* Decrease pool volume
3. Decrease conveyor speed
4. Increase feed rate
5. Decrease feed consistency
6. Increase temperature
7. Do not use floccuiants
To Increase Solids Recovery
1.. Increase bowl speed
2. Increase pool volume
3. Decrease conveyor speed
4. Decrease feed rate
5. Increase feed consistency
6. Increase temperature
7. Use floccuiants
Results of the investigations of Alberton and Guidi (152) concerning
the above parameters are as follows. The effect of bowl speed on
the feed rate and solids recovery is shown in Figure 39. Data
indicates that a higher percentage recovery is possible for the
same feed when bowl speed is increased. Furthermore, if bowl
speed is increased about 40%, feed rate would be double while
the same solids recovery is achieved.
The effect of pool depth is shown in Figure 40. It is obvious that
the deeper pool produces higher solids recovery, whereas the
medium pool gives lower solids recovery, but higher cake solids.
Figure 41 shows the effect of feed rate on solids recovery for two
types of sludges; whereas^Figure 42 indicates a typical relation-
ship exists between the two factors.
Chemical conditioning of the sludge prior to centrifugation has been
found to improve solids recovery dramatically. Alberton and Sher-
wood (153) investigated the use of polymer for enhancing and de-
watering of various types of sludges. A summary of their results
is: (1) in case of digested primary sludge, there is no need for poly-
mer conditioning if the unit is hydraulically loaded within its clar-
ification capacity, however, if polymers are used to enhance clar-
ification, machine capacity will be greatly increased; (2) in the
case of a combination of digested primary and biofilter sludge,
polymer conditioning increased solids recovery to only 85% at a
feed rate of 1 to 2 gpm, using a short-bowl unit; and (3) in the case
of digested primary mixed with activated sludge, polymer condi-
tioning increased solids recovery from 75% to 96% of the feed
solids, but the cake solids decreased from 23% to 20. 5% as shown
in Figure 43.
143
-------
Figure 39
Effect of Bowl Speed on Recovery
(152)
o
u
0)
94
90
86
82
78
74
(16.0$. T.S.)
3450 x "6"
Raw & Act. Sludge
(21.0%)
(18.0%)
Figure 40
Effect of Pool Depth on Percent Recovery
(152)
86
82
£ 78
e
Ji 74
M
70
66
Raw and Activated Sludge-A
M9.ll T.S.)
(21.3X)
(26.31)
3456
Feed Rate gpn
144
-------
(152)
Figure 41
Effect of Feed Rate on Percent Recovery1
Primary Sludge - C
(28.0t TS)
881
80
75
Raw
Digested
(28.W)
2 3 4 5 5 7
Feed Rate gpa
Figure 42
Relationship Between Cake Dryness and Percent Recovery
(152)
CAKE ORYNESS vs RECOVERY
32
30
28
I cake solids X
24
22-1
20
Digested PriMry t Activated - 0
Poly.er.dded*
•W12/T)
76 80 8» 88 92 96 100
I recovery
145
-------
M
•o
$
-------
TABLE 61
Summary of Results-- Sanitary Sludges
(152)
Type Sludge
RP
DP
R (P + TF)
D (P + TF)
R (P + AS)
D (P + AS)
Cake Cone.
%TS
28-35
25-35
20-26
18-25
18-24
18-24
Recovery - %
w/o Chem
85-90
80-90
65-85
60-85
50-80
50-70
w/Chem
95+
95++
95+
95+
95+
95+
Chem Cost
$/Ton
3-8
3-8
6-16
8-16
6-20
10-20
R = Raw
D = Digested
P = Primary
RP = Raw Primary
DP = Digested Primary
TF = Trickling Filter
AS = ActivatedS ludge
-------
A summary of the advantages claimed for sludge centrifugation
includes the following: (1) lower capital cost than other mechanical
dewatering equipment; (2) lower space requi-rement; (3) no need
for chemical conditioning of the sludge; (4) chemical dosage rate
not critical (operators can proceed with only intermittent dosage
of chemicals); (5) minimal odor; (6) supervision requirement mini-
mal; (7) simple and easy unit operation; and (8) high degree of
flexibility of the unit (e. g. it can handle dilute or thick feed
equally well; and can function as a thickening or dewatering device).
The disadvantages of sludge centrifugation as stated by Burd (17)
are: (1) without use of chemicals the solids capture is often poor,
and chemical cost could be substantial, (2) screening must often
be used to remove the trash from the centrifuge feed, and (3) cake
solids are often lower than those resulting from vacuum filtration.
Vacuum Filtration
Vacuum filtration of sludge is the most common mechanical method
of dewatering used in both municipal and industrial waste treatment
plants. It has been used very successfully in the West Southwest
Wastewater Treatment Plant, Chicago, Illinois with 96 operating
units. Reports of the four leading manufacturers of vacuum filters
in Dec., 1967, showed that over 1523 units were installed for all
types of sludges originating from municipal wastewater treatment
plants, alone (9). According to Burd (17) the rapid increase in the
use of vacuum filters in wastewater treatment in the past decade is
believed to be due to: (1) increased cost of the alternative methods
of sludge dewatering; (2) development of self-cleaning filter media;
and (3) the increased us-e of incineration as the final disposal
technique.
The sole purpose for the use of a vacuum filter is to reduce the water
content of a sludge, whether the sludge is raw, digested, or
elutriated. The solids content is increased to a level of 25 to
30% by filtration (14). Various types of vacuum filters used for this
purpose are:(l) drum-type filters, which were the first continuous
filters using scraper-discharge mechanism, with functioning zones,
such as cake forming zone, wash zone, dewatering zone, and
discharge zone (See Figure 44); (2) string-discharge filters, utilizing
closely spaced (about 0. 5 to 1. 3 cm apart) strings, which pass around
the filter drum and medium, and a set of discharge and return rolls
to carry the filter cake free of the medium and discharge it; and
(3) belt-type filters, utilizing a traveling woven cloth or metal
belt serving as a filter medium. The discharge mechanism is the
148
-------
Figure 44
Diagrammatic Cross Section of Rotary Drum Vacuum
Filter'14*
Cake saturated
with wash liquoi
-Cake forming
149
-------
same as for the string-discharge filters. The filter medium is
washed from both sides to minimize any blinding effect of the sludge
solids. A schematic of a belt-type filter is shown in Figure 45.
Coil-type filters use two layers of stainless steel coil springs
arranged in corduroy fashion around the drum. These springs act
as the filter medium. After the dewatering cycle is completed
the two layers of springs are separated from each other in such a
manner that the filtered sludge cake is lifted off the lower layer of
the coil springs and is discharged from the upper layer with the aid
of a positioned time bar.
Factors affecting dewaterability, of the sludge as stated by Burd (17)
are: (1) sludge solids content, (2) sludge age and temperature, (3)
sludge and filtrate viscosity, (4) sludge compressibility, «tnd (5) the
nature of the sludge solids including the volatile content, size, shape,
electrical charge, density, content of bound water and whether or
not the sludge has been exposed to biological treatment.
Factors influencing the operating conditions of a vacuum filter have
been reported (17) to be: (1) vacuum pressure applied, (2) filter sub-
mergence, (3) type of filter media, (4) degree of agitation, (5) drum
speed, and (6) conditioning of sludge prior to filtration.
Figure 45
Belt-type filters have facilities for continuous
(9)
intermittent washing of belt media.
FILTER MEDIUM (BELT)
I—RETURN ROLL
-WASH ROLL
DISCHARGE
ROLL
V— CAKE
* DEFLECTOR
WASH
HEADER
WASH
TROUGH
SEFftRATE DISCHARGE
TO DRAIN OR RECYCLE
FEED TANK
150
-------
Figure 46
Filtration rate versus solids concentration;
leaf test data for primary-activated sludge,
East Lansing, Mich. ; medium, polyethylene
802 HF; vacuum, 20 inc. Hg; cycle, 3.6 min;
submergence, 25 percent; chemicals, 2 1/2
percent FeCl3 and 10 percent lime. (154)
4.0
3.5
ik
X
I 3.0
z
o Z.O
i
0.5
Solids in F««d, %
151
-------
Figure 47
Example of effect on cake moisture
£ iguj. t; if
on cake moisture of elapsed time after thickening
(82)
(1.12-percent activated solids.)
sludge.
O 40 80 120 ISO tOO 24O Z9O
ELAPSED TIME AFTER THICKENING (MIN)
Figure 48
Filtration rate versus form vacuum; pilot-plant data.
(154)
Form Vacuum. Inclwt IMrcur*
152
-------
Feed sludge concentration is the most important variable in
vacuum filtration. In general an increase in feed sludge solids
content increases the vacuum filtration rate (shown in Figure 46).
Although filter production increases as feed solids content increases,
feed solids concentration must be limited to 8 to 10%, beyond
which chemical conditioning and sludge distribution becomes diffi-
cult (17).
The effect of sludge age on filtration rate and cake moisture con-
tent was investigated by Ettelt and Kennedy (82). As shown in Figure
47 an increase in sludge age resulted in a high cake moisture con-
tent. It was also observed that freshening of the aged sludge
reduces the cake moisture content dramatically.
Schempman (154) has presented data showing the relationship
between the amount of pressure applied and the filter cake yield,
as shown in Figure 48.
Pretreatment of most sludges is practiced commonly to reduce
the moisture content of the sludge prior to vacuum filtration. This
is accomplished by thickening, which produces higher solids
content; or by elutriation as in the case of digested sludge, to reduce
chemical demands used for conditioning. Chemical conditioning of
sludge, as frequently practiced, is used to enhance the dewater-
ability of the sludge cake. Although a long list of the chemicals used
for conditioning of the sludge is available in the literature, the
most frequently used chemicals reported are ferric chloride;
ferric chloride and lime; and cationic polyelectrolytes. The
chemical dosage used for various types of sludges and the result-
ing cake moisture contents are presented in Table 62 and Table 63.
TABLE 62
Dose Rate Yield Cake Moisture
Type of Sludge % (Lbs/sq-ft/hr) %
Raw primary or
raw primary and
filter humus 0.2-1.2 6-20 63-72
Digested primary 0.2-1.5 4-15 66-74
Digested primary
and activated 0.5-2.0 4-8 68-76
153
-------
TABLE 63
Chemical Dose Rate
(17)
Yield
Cake
Moisture
1.
2.
3.
4.
5.
6.
7.
8.
9.
Type of Sludge
Raw primary
Digested primary
Elutriated di-
gested primary
Raw primary +
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
Ferric Chloride
2.1
3.8
3.4
2.6
2.6
7.5
5.3
5.6
8.4
2.5
Lime
8.8
12.1
-0-
11.0
10.1
-0-
15.0
18.6
-0-
6.2
(Lbs/sq ft/hr)
6.9
7.2
7.5
7.1
4.5
-
4.6
4.0
3.8
3.8
%
69.0
73.0
69.0
75.0
77.5
84.0
77.5
78.5
79.0
76.2
w/lime
154
-------
Advantages claimed for use of vacuum filters in sludge dewatering
include: (1) a filter cake of relatively low moisture content which is
important when sludge is to be incinerated; (2) high solids recovery;
(3) small space requirement; and (4) applicability to all types of
sludges.
The disadvantages of vacuum filters reportedly are: (1) high cost
of chemicals required for conditioning; (Z) frequent shutdown of
operation for cleaning purposes; (3) odor from filtration of raw
sludge; and (4) requirement of highly trained operators.
Filter Presses and Plug Presses
A review of the literature reveals that use of the filter press for
municipal sludge dewatering is not very common in the U.S.
because of the high cost of manual labor and maintenance of this
system. However, its application in Europe is very common.
Vater (155) has stated that "of all available mechanical sludge
dewatering equipment, the filter press yields the greatest possible
cake solids content. This cake can be ground without previous
drying and can be incinerated with little or no supplemental
heat...... Recently, improved construction, using automatic
cake removal has made this process popular in many European
communities. "
The filter press uses the principle of free water drainage fol-
lowed by the application of low pressure. Filter presses are
operated in batches and chemicals have generally been used for
sludge conditioning. Games and Eller (13) reported that by
employing the filter press system, cake with as high as 50% solids
may be obtained, if filter aids such as diatomaceous earth or
ferric chlorides are used. The capital investment for filter
presses is believed to be higher than for centrifuges or vacuum filters.
However, it is a much more attractive method of dewatering if
sludge is incinerated; because fly ash may be recycled and used
for precoating or as a conditioning agent.
The filter press is still used by industry in the U.S. for the main
reason that it can handle all types of sludges. Results of filter
press dewatering of several industrial sludges are presented in
Table 64 (13).
Plug presses are similar in principle to filter presses. Pressing
techniques are limited to a two-stage dewatering system, generally
installed prior to incineration. Plug presses have the advantages
of free water drainage followed by the application of low pressure,
155
-------
thus minimizing the need for chemicals. The two proprietary
systems using this technique are the "Roto-Plug" and "DEC Solids
Concentrator. " These units have been used in smaller commun-
ities and in industries. The main objectives in these systems are:
(1) to avoid the critical pressures that would break the structure
of the sludge solids and blind the filter media, and (2) to avoid
large dosages of flocculants necessary to build a firm solids
structure (17,156).
The dewatering of the sludge by the filter press is generally
accomplished in successive stages with increasing pressure in each
stage. Sludge conditioning depends on the presence of natural
floes. It has been found that conditioning is unnecessary for fresh
sludge where an abundance of fresh floe exists, but is desirable
for digested or other aged sludge (157).
TABLE 64
Result of Pilot-Scale Testing
of Filter Press*
Sludge
Properties
Waste Digested Primary/
Activated Activated Primary Activated
Feed
PH
Solids (%)
Chemicals (%)
Water (%)
Sludge (%)
Total cake (%)
9.5
1.2
20.7
62.5
16.7
37.4
11.3
1.5
24.7
60.5
17.6
42.3
10.9
1.3
19.1
64.0
15.3
34.4
12.1
1.4
23.7
65.5
25.7
49.4
Advantages claimed for the system include: (1) minimal sludge con-
ditioning requirements; (2) low power requirements; (3) low capital
cost of the equipment; (4) low area requirement; and (5) simple
system operation (156). However these systems have not received
wide application because the resultant cake is not sufficiently dry
and the solids content of the separated water is very high.
Vibration
Reports have shown that sludge dewatering by mechanical or sonic
vibration has been investigated in a number of waste water treat-
156
-------
ment plants in the U.S., as well as overseas (14,17,158).
Eckenfelder and Spohr (158) reported on sludge thickening using the
"Heymann" process. This process consists of a series of vibrating
screens and a rotary press. Although the simplicity and small size
of this system is very advantageous; the solids capture efficiency
is very low, the dewatering performance of the system is unsatis-
factory, and the maintenance costs of the screens are generally
high.
Kiess (159) has discussed the application of "Rhewm" sonic vibrating
screen filters. This system consists of a set of three screens in
series. A vibrating coarse screen (8 to 24 mm. mesh) removing only
2% of the sludge solids; sonic screen (1.2-2 mm. mesh) capable of
removing 13% of the sludge solids; and sonic filter (0.1-0.5 mm.
mesh) removing 55% of the total solids. Dewatering efficiency of
this process is believed to depend on such factors as solids loading,
sludge particles, size distribution, type of sludge and the angle of
sludge discharge to the filter cloth. The resulting dewatered sludge
contained 20 to 25% solids. The filtrate though containing a high
percentage of solids was found to have good settling characteristics.
Advantages and disadvantages of this system are similar to those
of the "Heymann" process.
Heat Drying
The main objective in heat drying of the sludge is to convert it from
an undesirable state to a useable form, such as fertilizer or soil
conditioner. Sludge is generally dewatered to a 10 to 20% moisture con-
tent utilizing heat drying facilities. Heat drying to a low moisture
content permits the end product to grind well, reduces sludge weight
and odor, destroys pathogenic microorganisms preventing further
biological action, and most important of all, provides an ultimate
means of sludge disposal by recycling the organic material back
into the land.
The various types of equipment used for sludge drying include: (1)
Hash-drying system, (2) multiple hearth dryer, (3) rotary kiln
dryer, and (4) atomized spray dryer. All of the above drying
facilities have been used to dewater the sludge to a 10% moisture
content, but the most common type used in wastewater treatment
sludge has been the flash-drying system. Because of the high costs
involved in sludge drying, sludge is generally dewatered by mech-
anical means prior to drying.
157
-------
The flash drying operation involves pulverizing the sludge in a cage
mill in the presence of a hot gas stream (1300 'F). The wet sludge
cake is generally blended and mixed with recycled drier sludge,
thus bringing the moisture content to about 50%. The mixed sludge
and the stream of hot gases are then forced up a duct where drying
takes place. The equipment is designed so that enough contact time
between the sludge and the hot gases is provided, thus accomplishing
mass transfer of moisture from the sludge to the gases. The
separation of the dried sludge from the gases and vapor then takes
place in a cyclon (14,14,156). Dried sludge having moisture content
as low as 8% can be obtained in this process.
Multiple hearth drying of sludge utilizes basically a multiple hearth
incinerator with a few modifications to the basic furnace design,
including a fuel burner at the top and bottom hearth plus parallel
downward flow of both sludge and gases. During operation the sludge
solids flow downward through the furnace, the gases become cooler
and solids become drier. According to Burd (17) at the point of
exit from the furnace the gas temperature reaches about 385°F and
the solids temperature about 100°F.
Rotary kiln driers have been used in municipal water treatment
plants for calcining of lime sludge. Dayton, Ohio; Miami, Florida;
and Lansing, Michigan are examples. A rotary kiln drier consists
of a cylindrical drum, cyclone dust collector, and a gas deodorizing
incinerator. The mixture of the wet and previously dried sludge is
entered into one end of the rotating drum through which the hot gases
are passed. As the sludge solids are transported towards the outlet
they come in contact with the hot gases and lose their moisture.
The dried sludge solids emerge at the opposite end of the drum where
hot gases are entering. This process employs a counter cur rent
flow principle (14). Burd has reported that the system would work as
well employing parallel non- counter cur rent flow principle, in
which case both sludge and hot gases are introduced into the drum
from the same end. Gases that are emerging from the drum contain
certain amounts of fine sludge particles. Separation of these
particles takes place in a cyclone precipitater.
The atomized drier uses a high speed centrifuge bowl located at the
top into which the liquid sludge is fed. The object of the centrifuge
is to atomize the sludge into fine particles and then spray them
over a drying chamber. Hot gases enter into the chamber from the
top* As the atomized particles come in contact with the hot gases,
they release moisture to the hot gases and then drop to the bottom
part of the drying chamber.
158
-------
Sludge drying temperature commonly used is about 700°F in compar-
ison to complete incineration temperature of about 1200-1500°F.
Odor control is an important factor associated with sludge drying. For
complete elimination of odor the exhaust gases must reach approx-
imately 1350 F. This is generally accomplished by reheating the
exhaust gases. At lower temperature oxidation of the odor producing
compound may occur in which case the product may have a very
offensive odor. (14)
It appears that the trend from sludge drying has been shifted to
sludge incineration, although incineration is not considered to be the
ultimate disposal means. The major reason for the shift appears to
be: (1) lack of existence of fertilizer market, and (2) capital and high
fuel costs. Combustion (incineration) of combined sludge and muni-
cipal refuse is under consideration and is believed to improve the
economics of sludge disposal.
SLUDGE TRANSPORTATION
Common methods for conveying the resulting liquid or dried sludge
from*the wastewater treatment plant to a final disposal site include
transportation by: (1) barge; (2) pipeline; (3) truck; and (4) railroad.
Barging of the sludge is frequently used in conjunction with the ocean
disposal of the sewage sludge. It is principally in coastal cities;
namely, New York, Baltimore, and Philadelphia. Wastewater sludge
disposed of at sea from the New York area is handled by a fleet of
five self-propelled barges operated by the City of New York, having
capacities ranging from 1200 to 3200 tons. Private firms also have
contracts with the City of New York for barging sludge. The largest
barge used is 226 ft. long, 56 ft. wide and has a 20 ft. draft. It has
a capacity of 6300 tons, and the entire load is discharged in 30 min-
utes (160). The distance for sludge barging depends on the location
of the treatment plant and the off-shore disposal area. The average
round-trip distance for New York is approximately 30 miles, while
the City of Philadelphia averages about 227 miles (160).
Sludge is generally stabilized and thickened to about 4 to 10% solids
content prior to barging in order to reduce the cost of barging, to
prevent pollution hazards and prevent septic conditions of sewage
sludge during barging.
Approximately 4. 5 million tons (on a wet basis) of wastewater sludge
was barged to the sea in 1968. It was also projected that barging will
increase significantly in the next ten years (160). Table 65 represents
a summary of the quantities and estimated costs of various wastes
159
-------
TABLE 65
Summary of Type, Amount and Estimated Costs of Wastes
Disposed of in Pacific, Atlantic, and Gulf Coast Waters
For the Year
Waste type
Dredging spoils
Industrial wastes
bulk
containerized
Refuse, garbage^5)
Sewage &ludge(c)
Miscellaneous
Construction and
demolition debris
Explosives
Total, all wastesW
Pacific Coast
Annual
tonnage
8,320,000
981.000
300
26,000
200
9,327,500
Estimated
cost$
3,608,000
991,000
16,000
392,000
3,000
5,010,000
Atlantic Coast
Annual
tonnage
30,880,000
3,011,000
2,200
4.477,000
574.000
15.200
38,959,400
Estimated
cost $
16,810,000
5.406.000
17,000
4,433,000
430,000
235.000
27,331,000
Gulf Coast
Annual
tonnage
13,000,000
690,000
6,000
13,696,000
Estimated
cost J
3,228.000
1.592.000
171,000
4.991,000
Total
Annual
tonnage
52.200,000
4.682,000
8,500
26,000
4.477,000
200
574,000
15,200
61,982,900
Estimated
cost $»>
23,646,000
7.989,000
204,000
392.000
4.433,000
3,000
430.000
235,000
37,332.000
Total
%
Tonnage
64
8
<1
<1
7
<1
1
<1
100
%
Cost
63.5
21.7
<1
1
12
<1
1
<1
100
(a) Includes 200,000 tons of fly ash.
(b) At San Diego 4700 tons vessel garbage at $280,000 per ycat were discontinued in November 1968.
(c) Tonnage on wet basis. Assuming average 4.5 percent dry solids, this amounts to approximately 200,000 tons dry solids pet year being barged to tea.
(d) Radioactive wastes omitted. There were no dumps during 1968. Average Annual disposal in 1969-1970 was 4.2 tons.
(e) Estimated costs were increased proportionately for each area from the original Tonnage/cost data.
• Revised and updated by James L-Verber, FDA.
-------
barged to the sea for disposal in the Atlantic, Pacific, and Gulf
coastal waters.
Barging of sludge is also used in Chicago, Illinois where the sludge
is barged from the West-Southwest Waste Treatment Plant down the
Illinois River to Fulton County, Illinois, approximately 200 miles
from Chicago. Here it is pumped from the barge through 11 miles
of pipeline to storage lagpons.
Pipeline transportation of liquid sludge is very common. It has been
used for a variety of purposes including: (1) collection and trans-
porting of sludge from a multiple wastewater treatment plants and
sludge treatment processes to a centralized one such as Phila-
delphia, Chicago, etc.; (2) transport to a farm where sludge is used
as a soil conditioner; (3) transportation of sludge for land recla-
mation, such as in San Diego, and New York; and (4) transportation
of sludge to a disposal site at sea, such as Los Angeles.
In large cities, the practice of combining dewatering and ultimate
disposal of wastewater sludge from various treatment plants into a
single or centralized location has shown definite advantages. Pipe-
line transportation of sludge to the land disposal area has been
practiced in major cities. San Diego hauled sludge by truck from
the treatment plant to the land disposal area at a cost of $1. 9 per
1000 Ibs. in I960. This cost was reduced to about one-fourth by build-
ing of a pipeline system (161,162). New York City uses pipeline
and barges to transport vast quantities of liquid sludge to several
hundred acres of sandy soil, where the sludge is applied to a depth
of 18" and allowed to dry and form a topsoil layer. (164) Data on
long distance transporting of sludge via pipelines is presented in
Table 66, and has been compiled from various sources appearing
in the literature (163,164,165,166,167).
Pipeline transportation of sludge has definite advantages, as it
allows flexibility in ultimate disposal techniques. It permits the
relocation of large quantities of sludge away from land-short
urban areas for final disposal. (Final disposal may be land
spreading of the sludge as fertilizer or soil conditioner, lagoon-
ing, reclamation of abandoned strip mines or ocean disposal.)
Since it is a closed system, it is a relatively trouble-free and
odor-free operation. Pipelining of raw primary sludge is not
recommended due to potential gases, grease and abrasion problems.
Friction loss is an important factor in design of sludge pipelines.
Reported values of friction loss for digested sludge varies from
1 to 5 times that of water. Digested sludge exhibits non-Newtonian
flow characteristics for solids concentration above 6 percent. In
161
-------
TABLE 66
Long Distance Sludge Transportation via Pipelines
Location
Houston, Tex.
Kansas City, Mo.
San Diego, Calif.
Jersey City, N. J.
Bay Park, N. Y.
Rahway Valley, N. J.
Austin, Tex.
Knoxville, Tenn.
Morgantown, W. Va.
Mogden , England
Birmingham, England
The Hague, Netherlands
Los Angeles , Calif*
Chicago, Illinois
Cleveland, Ohio
Philadelphia , Pa.
Columbus-, Ohio
Length
(ft)
4,300
10,000
12,000
21,000
36,000
35,000
13,150
8,000
16,800
7,000
4,000
7,000
7,500
17,000
5,000
13,000
5,000
5,000
Diameter
(in.)
8
4
8
6
8
12
8
twin 6
10
8
8
6
2
12
9 and 12
8
24
14
12
12
8
-
Sludge
Type
Activated
Activated
Activated
Activated
Activated
Raw primary
Digested secondary
Raw primary
Digested activated
and primary
Raw primary,
some digested
primary
Activated
Raw primary and
humus
Digested
Digested
Digested
Digested
Digested
Raw
Raw
Raw
Raw
-
Concentration
(%)
0.5-1
0.5-1
0.5-1
0.5-1
0.5-1
6.0
0.85-1.55
2.8
2.8
5.0
0.8
0.55
Varies
4.0-5.0
8.5-10.0
4.0-5.0
3.73
1.0-2.0
2.0-4.0
3.0-4.0
3.0-4.0
4.0-5.0
-------
non-Newtonian flow, friction head loss depends primarily on the
viscosity of the liquid sludge, which in turn is related to the velocity of the
flow; diameter of the pipe; and type, particle size distribution, and
solids concentration of the sludge. The effect of solids concentra-
tion of the sludge on Hagen-Williams co-efficient (HWC) and the
power to overcome the pipeline friction are shown in Table 67. The
critical velocities of a sludge are affected by its solids content.
Velocities greater than the critical will cause an excessive head loss
due to an increase in friction. A wide range of operating velocities
has been reported in the literature. However, velocities ranging
from 5 to 8 ft. /sec have been reported to be satisfactory. Exten-
sive study on critical velocities of pipelines was made by Chou (24).
Results of this study are shown in Table 68.
TABLE 67
Variation in Power Requirements with Concentration
C(%)
3.5
5.0
6.0
8.3
10.5
D(in)
24
20
18
16
14
Q(cfs)
10.5
7.3
6.1
4.4
3.5
CHW
160
160
160
110
100
Horsepower /mile
11.5
9.6
8.0
16.0
17.0
TABLE 68
elocities fo:
Pipe Diameters'
Critical Velocities for Various
(24)
Critical Velocity
Diameter
(in.)
8
10
14
20
Lower
3.58
3.55
3.45
3.40
(fps)
Upper
4.52
4.42
4.31
4.23
Brisbon (26) has reported that a decrease in temperature below
65 F caused a considerable increase in head loss, whereas a
slight temperature increase above 65 F appeared to have very
little influence in decreasing friction loss.
163
-------
Truck transportation of liquid and dried sludge from the treatment
plants to the final disposal site is a very common method, partic-
ularly in smaller treatment plants. Disposal areas are generally
landfill, sanitary landfill, or farmland. Where liquid sludge is
transported, tank trucks are usually equipped with a splash plate or
distribution manifold to spread the sludge.
Railroad transportation of the wastewater treatment plant sludge is not
very common. However, it has been practiced in Chicago, Illinois.
SLUDGE REDUCTION PROCESS
Sludge Incineration
A review of the literature indicated that incineration of wastewater
sludge is rapidly increasing, particularly in large urban cities.
Reasons for this rapid growth are primarily due to: (1) rapid
increase of the cost and lack of land available for the alternative
disposal methods; (2) the health aspects associated with conven-
tional disposal methods are of increasing concern to the general
public, consulting engineers, public health and municipal officials;
and (3) increase in sludge production,pollution standards and
growth of population has increased the pressure for complete or
nearly complete"on site" disposal of solids.
Sludge is incinerated for two primary reasons: (1) volume reduc-
tion; and (2) solids reduction and sterilization. The end product of
the sludge incinerator consists of water, gases such as CO2» SO2»
NOX; and inert ash. The problems involved in sludge incineration
are air pollution.and disposal of the ash. However, it is believed
that advancement in technology will eventually eliminate the air
pollution problem, and the growing concern over the utilization
of incinerator ashes and coal ashes will overcome the ash disposal
problem.
Sludge combustion is effected by a number of factors, including:
sludge moisture content; sludge calorific value; sludge volatile
content; and sludge inert content. The efficiency of incineration
increases as the sludge moisture content decreases. Sludge is
generally dewatered to about 70% moisture content prior to
incineration. Chemical conditioning agents are commonly employed
to improve sludge de watering facilities. It has been found that
organic polymers used as sludge conditioners not only enhance
sludge dewaterability, but also increase the efficiency of incinera-
tion operations as a result of increasing the calorific value of sludge
(168).
164
-------
Calorific value and volatile content of the sludge are very impor-
tant parameters. They reflect the degree to which the sludge could
maintain self-sustained combustion. As indicated in Table 9» raw
sludge contains higher calorific values (about 10,000 Btu/pound)
than digested sludge (about 5,000 Btu/pound). The volatile con-
tent of the sludge may be improved by a highly efficient degritting
system. Using hydrocyclones, Burger (169) has removed about
95% of the plus 200-270 mesh inorganics at a specific gravity of
2.67, thus increasing the volatile content of the sludge from 70-
75% to 80-85%. In general the higher the volatile content of the
sludge the lower will be the auxiliary fuel requirement for com-
bustion. The effect of the volatile content of the sludge on oper-
ating costs of incineration is shown in Figures 49 and 50.
Presence of compounds such as Ca CO3 or CaO increases the fuel
requirement. These compounds react endothermically at com-
bustion temperatures. The decomposition of CaO materially
increases the thermal burdens (156).
It was stated previously that the end products of sludge combus-
tion are water, gases and inert ash. Formation of water, CO2 and
SO2 releases significant amounts of heat, as shown in Table 69.
This indicates that the combustible elements of sewage sludge are
found in the organic part of the sludge, such as grease, carbo-
hydrates and proteins. Elemental composition of the sewage sludge
compiled by Balakrishnan et al (156), is presented in Table 70.
TABLE 69
Combustion Reactions of Sewage Sludge
Heat Release
Reaction (Btu/lb)
1. Carbon + oxygen carbon dioxide
C + O2 * CO2 14,500
(lib) (2.67 Ib) (3.67 Ib)
2. Hydrogen + oxygen water
2H2 + O2- - - -> 2H20 62,000
(lib) (7.941b) (8.941b)
3. Sulfur + oxygen sulfur dioxide
S O > SO, 4,500
(1 Ib) (1 Ib7 (2 Ibj
165
-------
Figure 49
Effect of Volatiles in Sludge on Fuel (Natural Gas) Cost
(156)
8
M
wJ
8
i
1.5
1.0
.5
$1.68/TON
96 VOLATILE VS AUXILIARY FUEL
65
SLUDGE @ 3W TS
SXIT TEMP. @ 15OO°P
EXCESS AIR 2096
NATURAL GAS @ 1,OOO BTU/CF & 40^/1,000 CP
70
75
80
$0.16/TON
85
% VOLATILE
@ 10,000 BTU/LB V.S.
166
-------
Figure 50
Effect of Volatiles in Sludge on Fuel (No. 2 Oil) Cost
(156)
s
8
8
B
PU
•o.
o
o
2
D
CO
O
$3.40
DESIGN CONDITIONS
SLUDGB FEED--
GAS EXIT TEMP-
EXCESS AIR
•3096 T.S.
•1500 P
•20%
<
M
iJ
H
X
D
m
o
O
03
$0.80
65
70
75
8O
% VOLATILE IN SLUDGB
Q 9500 BTU/LB. VOL
85
-------
TABLE 70
Elemental Composition of Sewage Sludge
(156)
Elemental
Composition
Carbon (%)
Hydrogen (%)
Oxygen (%)
Nitrogen (%)
Sulfur (%)
Volatalite (%)
V.S.S. (BTU/lb)
T.S.S. (BTU/lb)
Source*
No. !
64.3
8.2
21.0
4.3
2.2
47.9
12,840
6,160
No. 2
65.6
9.0
20.9
3.4
1.1
72.5
12,510
9,080
No. 3
55.0
7.4
33.4
3.1
1.1
51.4
10,940
5,620
No. 4
51.8
7.2
38.0
3.0
Trace
82.0
8,990
7,380
*Source No. 1
No. 2
No. 3
No. 4
Cleveland Southerly Plant, 1955
Detroit, Michigan, 1955-56
Minneapolis, Minnesota, 1955
NewRochelle, New York, 1960-62
168
-------
A complete incineration process involves two steps: (1) drying, and
(2) combustion. Fuel and air are continually utilized to complete
the process. The auxilliary fuel requirement depends on such factors
as sludge, Btu value, and heat required to insure odor control.
Minimum deodorizing temperature of the sludge for conventional
incinerators ranges from 1350 to 1400 F. The deodorizing temper-
ature depends largely on the nature of the sludge as shown in Figure
51.
According to Balakrishnan (156) heat requirement of an incinerator
system depends on efficiency of burning and excess air required.
Total heat requirement of a system includes:
1. Heat required to raise the temperature of the sludge to 21Z F;
evaporating water from sludge; increasing the water vapor
and air temperature of the gas; and increasing the tempera-
ture of dried volatiles to the ignition point.
2. Heat required to raise the temperature of the exhaust gas to
the deodorizing temperature.
3. Heat required to raise the temperature of the air supply re-
quired for burning plus excess air.
4. Heat losses due to radiation.
5. Cooling air losses.
6. Heat required for other endothermic reactions taking place.
The moisture content of the sludge together with its volatile solids
content dictates the autogenous combustion condition of an incinera-
tion operation. As shown in Figure 52 at 25% solids content the
combustion product and moisture temperature will be raised as high
as 900 F. This temperature, however, is not enough to deodorize
the stack gases.
On the other hand, a rotary kiln can operate over a wide range of 7 to
70% solids content, whereas a fluidized bed reactor and atomized
spray system require lower solids levels, ranging from 2 to 10
percent (17).
Drying and combustion has been practiced in separate pieces of
equipment or successively in the same unit. A wide variety of
sludge drying and combustion equipment are presently available.
These include: (1) traveling-grate furnaces; (2) rotary-kiln type
furnaces; (3) multiple hearth furnaces; (4) fluidized bed units;
(5) flash-drying units; (6) wet-oxidation units; and (7) atomized
suspension units. The principle variation between these systems
lies on the requirement for heating, excess air, and the efficiency
of utilizing the waste gases. A schematic of the two major types
of incinerators are presented in Figure 53.
169
-------
1150
Figure 51
Level in Si
Process Temperature Encountered'
Relationship of Odor Level in Stack Gases to Highest
,(156)
1.6
ANAEROBIC FLOATED
SLUDGE
RAW CHEMICALLY
CONDITIONED
RAW. SLUDGE
DIGESTED ELUTRIATED
2OO 125O 1300 1350 1400 145O
TEMPERATURE *P
170
-------
Figure 52
Equilibrium Curves Relating Combustion Temperature
to Cake Concentration
40
35
8
O
CO
&
H
30
25
20
NO HEAT
T3BCOVERY
PREHEAT
AIR
WITH
DP COMBUSTION
800 900 100O 1100 120O 13OO
TEMPERATURE - °F
1400
1500 160O
-------
Figure 53
Schematic Representation of Incinerator Types
(170)
DISCHARGE TO ATMOSPHERE
\ 1
SLUDGE — s»-
AIR-*-
\
-**— WAItK III
-*~ WATER OUT
\
^.FUEL SCRUBBER
ASH
(1a) Multiple Hearth Incinerator
INCINERATOR
SLUDGE
FUEL-
AIR
A DISCHARGE TO ATMOSPHERE
WATER IN
WATER OUT
IX
T SCRUBBER
ASK
Fliiiuized Bed Incinerator
172
-------
The literature reveals that sludge incineration is being adopted by
many municipalities and industries despite the high capital cost of
the system. The major types of incinerators that have been used
are multiple hearth and flash-drying units. However, since the
early 1960's wet-oxidation and fluidized bed systems have gained
much popularity.
Advantages claimed for sludge incineration include: (1) greater
reduction of sludge volume; (2) complete or nearly complete com-
bustion of organics; (3) complete destruction of pathogenic organisms;
and (4) an end product that is an inert ash creating no disposal
problem.
A survey conducted on the attitude of consulting engineers regard-
ing sludge incineration showed that the overall attitude of the con-
sulting engineers was acceptance and even eagerness to employ
incineration. Further, they felt that grease, oils, screenings, and
organic industrial wastes could best be disposed of by incineration
(156).
In regard to the continued use of sludge incineration, Burd (17) has
stated that " with or without design improvements, incinera-
tion is the one sludge disposal process that, without a doubt, has a
bright future because it meets future sludge disposal criteria. "
Recently an extensive observation was made by the EPA Task Force
on Sewage Sludge Incineration(170). Conclusions of these observa-
tions were:
1. The present incinerator systems which are equipped with
air pollution control devices to meet the existing air quality
standards have been shown to produce acceptable stack
emissions of particulate matter, nitrogen oxides, sulfur
oxides, and odor when operated properly. However, it was
found that most existing sludge incinerators did not incor-
porate high efficiency particulate matter control devices.
2. Small, but measurable, quantities of toxic materials were
found in input sludge, stack emissions, scrubber water, and
incinerator residue.
3. Small, but measurable, amounts of specific organic chem-
ical compounds, including various kinds of pesticides and
PCB (polychlorinated biphenyls) were found in all of the
sludge samples analyzed. These compounds may be emitted
from the stacks when combustion conditions are poor.
4. The potential for health effects associated with sewage sludge
incineration cannot be established accurately, because:
173
-------
(1) insufficient health effect data relating to low atmospheric
concentration of suspected pollutant material such as heavy
metals, organic pesticides and PCB is available, and (2) soph-
isticated methods for proper sampling and analysis of stack
gases are not available to produce accurate information regard-
ing the quantity, size, distribution, and constituents quality
related to size of the particulate matter emitted by sewage sludge
incinerators.
Pyrolysis
Pyrolysis of waste water sludge and refuse has recently been in-
vestigated as a substitute for incineration. It functions similarly to
an incinerator as far as reduction of volume and stabilization of
wastewater sludge is concerned. Its advantages over incineration
are: (1) a useful by-product is produced, and (2) air pollution is
eliminated. In general, the basic purpose in pyrolyzing the
sludge is to decompose complex organisms to simpler materials
(17). In the case of refuse solids, useful end products and by-
products, such as combustible gas for boiler furnace fuel, ele-
mental carbon, tar, resins and various acids could be obtained
at 1200°F temperature (17).
Beeckmans and Ng (171) have investigated pyrolysis of activated
sludge to produce a pyrolysate with an appreciably higher absorb-
ing capacity than fly ash when used for the reduction of residual
oxidizable material in the effluent of wastewater treatment plants.
The pyrolysis unit used consisted of a closed, vertical stainless
steel cylinder, with internal dimensions of 8 inches diameter by
18 inches high, containing six equally spaced hearths. The sludge
was introduced continuously at the top of the unit by means of a
feed screw, and the pyrolysate was continuously removed from
the bottom by a vertical exit tube immersed in water. The sludge
was processed through the unit by the raking action of off-center
pairs of rabble arms attached to a central rotating shaft, with one
pair of arms per hearth. The motion of the rabble arms pushed
the sludge towards the periphery of the unit and toward its center,
on alternate hearths. Suitably placed holes in the hearths then
permitted the sludge to fall onto the next hearth beneath. A con-
trolled flow of oxygen-deficient gas (generally below 2% 02) was
maintained in the furnace by drawing in the combustion products
of a propane flame through an opening near the top. This was
accomplished by a suction pump which was connected via a con-
densate trap and a condenser, to the bottom of the unit. The flow
of gases was thus concurrent with the sludge.
174
-------
Tables 71 to 74 present the results of their investigation. The con-
clusions drawn were: (1) for maximum carbon content in the pyrol-
sate, the oxygen content of the furnace gases should not exceed
2% at the top of the furnace; (2) sludge residence time should be
approximately 90 minutes; (3) sludge feed rate should be about
12* 5 g/min. (wet basis); (4) the input gas flow rate should be 25
liters/min.; and (5) pyrolyzed sewage sludge is intermediate in its
absorption capacity between fly ash and activated carbon.
175
-------
TABLE 71
Effect of Oxygen Concentration in the Purge Gas on
the Carbon Content of the Pyrolysate* (171)
o
6
in inlet
>
2.
4.
10.
gas, %
0
2
0
C in pyrolysate ,
b
10.2
4.6b
b
0.2
%
a
Input air flow, 25 liters/min.; residence time,
120 min.; sludge feed rate, 9.4 g./min.;
bottom temperature, 1290 F.
Average of five runs.
TABLE 72
Effect of Residence Time and Sludge Feed Rate on
Carbon Content of Pyrolysatea (171)
Input air
flow,
liter /min.
25
25
25
25
15
15
15
Residence
time , min .
50
90
120
150
90
120
150
Sludge feed
rate, g./min.
22.0
12.5
9.4
7.6
12.5
9.4
7.6
C in pyrolysate, %
Pyrolysis
incomplete
b
14.1°
b
10.2
8.7b
b
10.7
b
8.5
b
8.4
aOxygen in inlet gas, 2%; bottom temperature, 1290 F.
Average of four runs.
176
-------
TABLE 73
Effect of Purge Gas Flow Rate on Carbon Content
of the Pyrolysatea (171)
Input air
flow, liter /min.
15
25
30
60
C in pyrolysate, %
8.5b
b
10.2
8.4b
b
4.1
aOxygen in inlet gas, 2%; residence time, 120 min.;
bottom temperature, 1290 F.
Average of four runs.
TABLE 74
Results of COD Adsorption Tests, a . .
Initial COD 53. 5 mg. /liter ( '
Pvrolyzed product , % C
' ' c _ _ __
Activated - '
Adsorbent carbon 14.3 13.25 10.7 5.3 ash
Final COD, Q ft
mg/liter 10.2 32.6 32.9 36.1 41.4 49.8
Removal, % 81.0 39.1 38.5 32.5 22.5 6.8
Contact time, 24 hr
177
-------
ULTIMATE DISPOSAL AND/OR UTILIZATION OF MUNICIPAL
WASTEWATER SLUDGES
The ultimate disposal of the vast quantities of sludge being gener-
ated throughout the U.S. is one of the most complex problems
facing sanitary engineers today. The sludge disposal problem will
continue to grow because of: more demanding water, air, and land
pollution control standards; lack of land availability resulting from
rapid growth of urbanized communities; public awareness of the
health aspects associated with disposal of raw sludge; growth of
population; and advancement of the technology in manufacturing
more sophisticated wastewater treatment facilities to meet the
present and future standards. This will result in a high degree of
solids removal, and a larger quantity of solids requiring disposal.
Numerous methods for disposal or utilization of the municipal
sludge have been practiced. These methods are summarized in
Table 75.
Ultimate disposal of the sludge is the final step in the process. In
some instances this transforms the sludge and then places it at its
final site. The criteria to be followed for selection of the ultimate
disposal method have been discussed by Smith (172). These criteria
are: (1) the ultimate disposal method must be in accordance with
State, interstate, and Federal Water Quality Office requirements;
(2) the ultimate disposal method should not result in any signifi-
cant degradation of surface or ground water, air, or land surfaces;
(3) no sludge residue, grit, ash or other solids should be dischar-
ged into the receiving waters or plant effluents; (4) sludge disposal
in ocean waters is not recommended as toxic metals may be
transmitted to the aquatic food chain, and (5) sludge must be sta-
bilized prior to spreading on land.
The major objective in this part of the study is to: (1) review
briefly the conventional methods used for disposal and utilization
of municipal sludge; and (2) evaluate these methods from aesthetic,
economic and pollution hazards viewpoints.
Ocean Disposal of Wastewater Sludges
For most coastal cities discharge of wastewater sludge has been
found to be very economical. Sludge with solids content ranging
from 3 to 10% have been disposed of in the sea by barges from
New York, New Jersey and Philadelphia. Metropolitan New York
has been barging and dumping its wastewater sludge (about 5 million
178
-------
TABLE 75
PRESENT METHODS OF MUNICIPAL SLUDGE DISPOSAL AND/OR UTILIZATION
\D
Wastewater Treatment
1. Ocean dumping or discharging
2. Land spreading of liquid sludge.
3. Land reclamation.
4. Lagooning and landfilling.
5. Disposal of dried sludge as fertilizer or
soil conditioner.
6. Underground disposal.
7. Incineration and landfill of ash.
8. By-product recovery.
Water Treatment
1. Direct discharge to streams,
ocean or lake.
2. Lagooning of the sludge.
3. Agriculture utilization.
4. Discharge to sewers.
5. Disposal in sanitary landfill.
6. Application of sludge to strip
mines.
7. By-product recovery.
-------
cubic yards) for over 40 years at a disposal site a few miles from
the entrance to New York harbor and near the coasts of New Jersey
and Long Island (6). On the Pacific coast, barges are not used;
but, sludge is discharged into the sea through submarine outfalls.
Approximately 650,000 Ibs. per day of digested wastewater solids
are discharged to the sea from the Los Angeles Metropolitan area.
About 50 percent of 325,000 Ibs. per day is discharged from the
Hyperion Treatment Plant of the City of Los Angeles into Santa
Monica Bay in 300 ft. of water 7 miles from shore via a 22"
I.D. conduit. Wastewater sludge is also discharged to the sea
from West Point Plant, Seattle, Washington; the West Point out-
fall extends 3650 ft. offshore to the diffusers and a depth of 230
ft. (173)
Most coastal cities have selected ocean disposal of sludge because
of its favorable economics as compared with the alternative methods.
In general, the cost of ocean sludge disposal, including barging
and sludge treatment, ranges from 17 to 27 dollars per ton on a
dry solids basis.
Other advantages of ocean disposal of the sludge for coastal cities
include: (1) complete removal of the sludge from treatment plants;
(2) a nuisance-free sludge handling, assuming the sludge is diges-
ted; and (3) ocean disposal permits flexibility in plant operation,
assuming sludge is digested (17).
Ocean disposal of wastewater sludge is the most controversial
problem of the day. The. review of literature reveals that this
probably is one of the most economical methods of ultimate
sludge disposal. However, the long term environmental effects of
this method have not been established. Middelton of EPA at
Cincinnati, Ohio, viewed the ocean disposal of the sludge this
way, "There are no hard and fast rules on this. It is the one major
area of uncertainty. The ocean has a great capacity to receive
discharges-- in some cases it might benefit from them, but the
ocean should not be regarded as an infinite sink. However, it could
be the receptor of brines and brackish streams. Clean, mixed
salt stream should be accepted, but not DDT-laden stream or
toxic sludges. We are primitive in our knowledge of pollution.
Until we know more we should not side with one way or the other. "
(174)
The environmental effects of both barge and submarine disposal
of sewage sludge have been studied by a number of investigators.
A review of the investigations by Smith and Brown (160) reveal the
180
-------
following: (1) the sewage in large concentrations destroys the
marine habitat in the immediate vacinity of the sludge field; (2)
the sludge drifts slowly along the bottom because of currents; (3)
coliform and related toxic substances are potential threats to
shellfish within a radius of 5 to 10 miles of the site; (4) the toxic
substance and coliform bacteria associated with the sludge are
concentrated in bottom sediments; and (5) a great deal more
field and laboratory work is required to accurately predict the
detailed behavior of the sludge and the probable environmental
response.
In summary, ocean disposal of sludge is the best economical disposal
method for coastal cities and probably will be more attractive in
the future as the volume of sludge increases due to growth of population;
higher pollution control standards for water, air, and land; and con-
tinual increase in cost of the alternative methods. However, because
of the observed adverse environmental effects of this method, indications
are that the Federal Government may halt all ocean dumping in
the near future.
Land Spreading of Liquid Sludge
Land spreading of liquid sewage sludge and sewage effluent has been
practiced for centuries in Europe and Asia. It has also been used in
the U.S. , mostly in smaller communities where farm lands are
located close to the wastewater treatment plants. Recently, many of
the larger communities have also been seriously considering land
spreading of sewage sludge as one of the most promising sludge dis-
posal alternatives.
Land disposal of sewage sludge is generally characterized by applica-
tion of liquid, treated (digested) sludge on crop lands in pre-determined
rates and amounts. It is based on the "living filter" concept, letting
nature recycle nutrients and water beneficially on land rather than
detrimentally through streams and lakes (17 5). Land disposal could
be used as an economical means of sludge treatment and disposal. If
used as a means of sludge treatment and disposal rather than competition
for chemical fertilizer, it becomes economically very att ractive. In
addition, liquid digested sludge contains soil conditioning agents and the
elements essential to the growth of green plants. It has been reported
(176) that some 20 or more elements are beneficial to plant growth and
at least 16 are essential. All 16 elements and other nutrients and growth
stimulators have been found in digested sludge. The fertilizer constituents
of the sludge given in Table 16 indicate the sludge is not balanced fer-
tilizer because of its low nitrogen, phosphorus, and potassium contents.
181
-------
Heavy application of sludge may be a solution to this problem if
provisions are made to control plant toxicity due to accumulation of
toxic metals, such as copper, boron, lead, zinc, cadmium, etc. It is reported
(164) that under normal loading conditions for ordinary sewage sludge
it will take 50 years before accumulation of heavy metals reaches
toxic levels. Metal toxicity can be controlled using lime to neutralize
the acid soil, which then converts the soluble excess metal to an
insoluble state. The metals then precipitate out and will no longer
be available for the plant. If the metals are separated as hydroxides
under high pH conditions, they will mineralize and be restored to
their original condition in the soil (6).
The major factors effecting the economics of liquid sludge disposal
are size (population), of the community; transportation costs; and
land cost. Investigations of Riddle and Cormack on economics of liquid
sewage sludge transport are reported in the literature (177). Figure 54
shows the relative costs of various transport alternatives for a city
of 100,000. It appears that pipeline transport is most economical for
distances ranging from 25 to 200 miles. Figures 55, 56 and 57 show
comparative estimated costs of sludge disposal by alternative means
for cities of 10,000, 100,000, and 1,000,000 respectively. It appears
that for smaller cities land disposal is more economical where
disposal sites are located within a 30 mile radius. For larger cities
the economical distance for sludge disposal is increased to 100 miles.
Raynes (178) has compared the sludge disposal costs by filtration
and incineration methods versus land disposal of liquid sludge using
pipelines for cities ranging in population from 0.125 to 4 million. The
effect of population on unit cost of sludge disposal resulting from this
study is shown in Table 76.
TABLE 76
Effect of Population on Unit Cost of
(178)
Sludge Disposal
Populations in Incineration Land Disposal
Millions Cost ($ per dry ton) Cost ($ per dry ton)
0.125 67 30
0.25 57 17
0.5 49 11
1 42 8
2 35 5
4 30 4
182
-------
I 600
Figure 54
Cost of Transporting Sludge from City of 100,000
1
(177)
400
"o
g-200
O
100
60
40
o
cr
o
in
cr
h-
20
T
I ' I '' I
TANK TRUCK
RAILROAD
TANK CAR
PIPE LINE
1
20 40 60 100 200 400
DISTANCE TO DISPOSAL
POINT, miles
183
-------
Figure 55
Cost of Sludge Disposal by Various Methods for City
of 10,000
350
c
o
h-
o
o
300
250
200
150
100
50
0,
INCINERATOR
ASH TO
LANDFILL -\ /FERTILIZER
PRODUCTION
DEWATERED -I
SLUDGE TO
LANDFILL
LAND APPLICATION OF ~
LIQUID DIGESTED SLUDGE
i I I I
0 50 100 150 20O
DISTANCE TO DISPOSAL
POINT, miles
184
-------
Figure 56
Cost of Sludge Disposal by Various Methods by City of
ioo,ooo<177)
c
O
O
350
300
250
200
150
100
50
FERTILIZER
PRODUCTION
INCINERATOR ASH
TO LANDRLL
DEWATERED SLUDGE
TO LANDFILL
LAND APPLICATION OF LIQUID
DIGESTED SLUDGE ,
'0 50 100 150 200
DISTANCE TO DISPOSAL
POINT, miles
185
-------
Figure 57
Cost of Sludge Disposal by Various Methods for City of 1,000,000 ^
c
o
e
i-
O
o
70
60
50
40
DEWATERED SLUDGE
TO LANDFILL
r FERTILIZER
PRODUC-
TION
10
0.
INCINERATOR
ASH TO
LANDFILL
LAND APPLICATION OF
LIQUID DIGESTED
SLUDGE
0 50 100
DISTANCE TO
POINT, miles
150 200
DISPOSAL
186
-------
Land is a cost item for treatment plants that buy or lease land.
Land cost for small plants is not a factor since nearby farmland
may be used. Some plants even receive a small payment for sludge
that is spread on farmland.
Actual costs for land spreading of digested sludge compiled by
Dotson, et al., (179) are shown in Table 77.
Public acceptance of liquid sludge disposal appears to have been the
major problem for wide use of this method. There remains some un-
certainty about the survival of various pathogenic organisms and
viruses transmitted to soil from the application of the sludge, and
their effects on surface and ground water. A review of the literature
indicates that no serious incident of disease has been traced from sludge
spreading on soils. However, this does not mean that application of
liquid digested sludge on farmland is absolutely safe. More informa-
tion is needed to confirm its safety. Disinfection may be needed
where people come in contact with the sludge. Dotson, et al., (179),
have suggested several methods for destroying pathogens in sludge.
These include: (1) long term storage; (2) pasteurization at elevated
temperatures (above 70 C) for 30 minutes or longer; (3) adding
lime to raise the pH to at least 11. 5 and holding pH at above 11. 0 for
two hours or longer; (4) disinfecting by high energy radiation, although
costs are high; and (5) disinfecting with other chemicals, although
the side effects are likewise unknown.
The rate of application of the sludge to soil depends upon such
factors as soil type, climate, sludge characteristics, and the crop
or land use. A typical application rate for a cornfield varies from
5 to 15 tons of solids per acre per year and for strip mines, up to
500 tons per acre per year are permissable.
In summary, it appears that land disposal of liquid digested sludge
offers many advantages, including: (1) economy; (2) recycling of
water, nutrients, and organic materials; (3) a final treatment pro-
cess and ultimate disposal; and (4) nuisance-free if done properly.
When compared with alternative methods, it has certain limitations,
including possibilities of toxic metal accumulation, surface and
ground water contamination and, to a lesser extent, health hazards
and nuisance.
Land Reclamation
The successful disposal of wastewater sludge on land for land recla-
mation purposes has been practiced for centuries throughout the
187
-------
TABLE 77
Summary of Costs for Land Spreading of Digested Sludge
(179)
CO
00
New York, New York
Chicago, Illinois
San Diego, California
St. Marys, Pennsylvania
Little Miami
(Greene County, Ohio)
Piqua, Ohio
Franklin Regional Waste-
water Treatment Plant,
Franklin, Ohio
Montgomery County (Dayton),
Ohio
Approximate
plant size
(mgd)
1300
90
1.3
1.5
3.8
4.5
Estimated cost for land
spreading of digested sludge
(dollars per ton of solids)
$11.89
26.02*
10.57
19.92
22.00
17.50 to 30.00
5.00
18.00 to 21.00
*Expected costs after construction of pipeline. Present costs using barge transportation are
$62.32.
-------
world. Sewage sludge has been utilized to reclaim sandy soils and
strip mine spoils by converting them into valuable crop land or
recreation areas.
For several years New York City has used digested sewage sludge
to build a topsoil for new parks. Marsh areas designated as park
sites are first filled with garbage, then covered with two feet of
sand, followed by a topping of the sludge cake. Sludge was applied
at intervals until a 6-inch layer of dry sludge was built up. In
1967 the cost of sludge disposal in this method was about $7. 50
per ton on a dry solids basis (17). This method was considered a
suitable means for disposal of sludge.
The Golden Gate State Park in San Francisco consisting of 1013 acres
was originally a sand dune. Sewage effluent was discharged to the
sandy area for land reclamation purposes. In 1932 the irrigation
was changed from sewage effluent to excess activated sludge. The
land has been reclaimed and converted to a park area with dried
digested sludge used as a fertilizer (180).
In 1956, the Division of Water Supply and Pollution Control, Depart-
ment of HEW, experimented with the application of waste water
sludges to two strip mine sites in Stark County, Ohio. At one site
6 levels of terraces (18 ft. by 150 ft.) were constructed. Liquid
sewage sludge was flooded into the benches ranging from 3 to 15
ga. per sq. ft. The second site was a high acid soil (pH 2.8 to 3. 3),
which had remained barren for about 8 years since exposure by
strip, mining. The sites were then seeded with grass after the
sludge had been allowed to dry out. Results were extremely success-
ful. It was concluded that the sludge apparently had acted as a
germination aid, a provider of nutrients, and, to some extent, a
neutralizer of acid-producing minerals in the soil. (180)
The largest land reclamation project of its kind has been proposed
by the City of Cleveland, Ohio. It is proposed to transport sewage
sludge to strip mine areas in Harrison County, located in the
southeast part of the State of Ohio, through 97 miles of a 12 inch
diameter pipe. The estimated cost of disposal including collection
and transportation averages about $25 per ton of dry solids over
a 28 year period (1972-2000), a savings of $5 to $15 per ton since
some alternative methods are estimated to cost $30 to $40 per
ton of dry solids (181).
Other land reclamation projects include the Metropolitan Sanitary
District of Greater Chicago; Tucson, Arizona; Springfield, Illinois;
Kaukake, Illinois; Las Vegas, Nevada; Miami, Florida; and San
Diego, California; etc.
189
-------
Investigations of health hazards associated with some of these sludge
disposal projects have been made. Results of these investigations
showed no adverse environmental effects. However, long term
investigations may be required to confirm the results of these short
term investigations.
In summary, it appears that this is a satisfactory method of sludge
disposal to the extent that it does not cause pollution; it is relatively
economical; and it utilizes the organic content of the sludge for
beneficial uses.
Lagooning and Landfilling of Sewage Sludge
Lagooning of the wastewater sludge is a common method of sludge
treatment and disposal. In municipal and industrial wastewater
treatment plants lagoons are used for various purposes such as;
(1) aerobic digestion of raw sludge; (2) thickening, storage and drying
of digested sludge; and (3) permanent disposal of sludge or landfill.
Although lagooning'of the sewage sludge has been a popular treatment
and/or disposal method, because of its favorable economics, several
disadvantages are insect breeding and nuisance, odor production,
and ground water contamination.
Occasionally, dewatered sludge is discharged to landfill or a sanitary
landfill area. In some instances the aim has been to reclaim the
land for parks, playgrounds, etc. The cost of sludge disposal by
landfill is obviously more than lagooning, however, disadvantages
are practically the same as those for lagoons.
Sludge disposal by lagooning and landfilling is relatively common at
the present time, particularly in smaller communities due to its
favorable economics, flexibility and simplicity of operation. However,
strengthening of pollution standards, the unavailability of land in the
vicinity of treatment plants, and the adverse environmental effects
will probably* decrease its popularity and use.
Disposal of Dried Sludge as Fertilizer or Soil Conditioner
Disposal of dried wastewater sludge on land as fertilizer or soil
conditioner has been practiced very extensively in Europe and Asia*
Table 16 shows the fertilizer content of the sludge. It appears that
sludge is not a competitor of the chemical commercial fertilizer be-
cause of its lower nitrogen, phosphorous, and potash contents. How-
ever, sludge contains a number of trace elements essential to plant
growth that are not found in chemical fertilizers.
190
-------
The fertilizer value of a sludge depends on the sources of wastewater
and the type of treatment processes to which both wastewater and
sludge are subjected. Sludge produced from treatment of pure
domestic wastewater has a higher fertilizer value as compared with
sludge obtained from combined domestic and industrial wastewater.
Sludge resulting from an activated sludge process contains 40 to
50% more nitrogen and phosphorus than sludge resulting from an
anaerobic digester.
The application of raw sewage siudge on land involves potential health
hazards. Furthermore, its physical structure and high grease con-
tent is believed to have a detrimental effect on the soil structure and
growing plants (182). For these reasons sludge is generally treated
and dried on sand beds; mechanically dewatered; or dried by
artificial heat prior to disposal on land.
The review of literature reveals that much field and greenhouse
evaluation of wastewater sludge has been investigated. In general, appli-
cation rates ranging from 10 to 40 tons per acre have been recommen-
ded. Higher application rates are not recommended because of
possible toxicity due to the trace element build-up in the soil- Liming
of the sludge p"rior to dewatering or liming of the soil receiving the
sludge fertilizer has been found to have advantages which are: (1)
neutralizes excess acidity of the soil and precipitates some metals;
(2) encourages bacterial decomposition of organic solids, hence,
increasing the effectiveness of sewage sludge; (3) makes phosphorus
more available; and (4) improves the physical structure of heavy
soils and supplies a necessary element, calcium (182).
In general, the sludge fertilizer market has not been a profitable one*
Many treatment plants with heat drying equipment have changed from
fertilizer production to sludge incineration or landfilling. However,
several wastewater treatment plants, such as those in Milwaukee,
Chicago, and Houston, have been very successful in marketing dry
activated sludge. Burd (17) has reported that over 200,000 tons of
sludge fertilizer were solid annually at prices ranging from $12 to
$18 per ton by Milwaukee, Chicago, and Houston. Cost of sludge
disposal by this method for Chicago reaches approximately $45
per ton of dried solids.
In summary, sludge disposal as a fertilizer or soil conditioner de-
serves much more consideration. Most obstacles to date have been
the lack of a profitable market. However, it should be noted that
dried sludge which may be used as fertilizer or soil conditioner is a
191
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significant and potential ultimate disposal method. The profit
gained from the sale of dried sludge should not be emphasized
since it is not significant, but the demand for sludge utilization
is great today. It can effectively be used as fertilizer for parks
and individual lawns, and as a supplement or filler for chemical
fertilizer.
Underground Disposal
Disposal of well treated wastewater sludge in subsurface strata that
contains natural and man-made cavities, such as depleted mines,
wells, and caves has been reported by many investigators (17,183,
184). They have believed that subsurface disposal is competitive
with other disposal methods economically. However, several
problems involved with this method of disposal, include: (1) high
solids content of the sludge; (2) legislative policies in many states
prohibiting this method of disposal; (3) difficulty finding a proper
site; (4) high cost of excavating underground sites; and (5) possibil-
ity of permanent ground water contamination if the sludge should
leach into the water supplies.
Deep well injection of sludge has been practiced by Dow Chemical
Company for disposal of waste activated sludge at one of their
largest production facilities. It was noted that the economics of
sludge disposal was greatly improved using deep-well injection;
however, cost values are not available (184).
Deep-well injection of industrial sludges has been practiced quite
often, particularly for plating and radioactive wastes. In these
cases the sites are studied very carefully and all measures are
taken to prevent leaching.
By-Product Recovery
Non-fertilizer by-product recovery of sewage sludge has been
studied by many investigators. It appears possible to recover
several by-products.
The recovery of Vitamin B-12 from wastewater sludge has been
discussed by Hoover (185) and Leary (186) as a supplement to
animal feeds. Vitamin B-12 was successfully recovered from
dried undigested activated sludge in pilot plant studies in Milwau-
kee. It is believed that Vitamin B-12 is derived from raw sludge
and, in part, from biological synthesis in the activated sludge
192
-------
aeration tanks. The pilot plant studies indicated that 308 pounds
of pure Vitamin B-12 could be produced from the Milwaukee 70,000
ton per year sludge production.
The Metropolitan Sanitary District of Greater Chicago, in cooperation
with the University of Illinois, has investigated the value of heat
dried activated sludge as an additive to animal feed. Feeding tests
have indicated that activated sludge at levels of 0. 5% to 2% is a
satisfactory source of Vitamin B-12 for pigs and chickens (42).
Rudolfs (187) has reported that for a waste water sludge which runs
high in grease content and low in alkalinity, successful extraction
of grease from raw sludge solids was made using the process known
as "Miles Acid Process."
Thompson, et al (8), have discussed the possibilities for commercial
recovery of some of the metals found in the sludge. The amount of
metals that could be recovered from the Oklahoma City Southside
Treatment Plant producing 2 million pounds of air dried sludge in
1962 were estimated. It was noted that, assuming 100% recovery,
940 Ibs. of silver, 5640 Ibs. of copper, and 11,750 Ibs. of titanium
could be recovered annually.
Beeckman and Ng (171) have reported on prospects for pyrolyzing
sludge to obtain an absorbent with greater capacity than the fly ash
produced by sludge incineration. (A complete discussion of this
method and results obtained can be found in the previous chapter,
under sludge pyrolysis.)
Feasibility of hydrolysis of sludge using low pressure steam with
SC>2 as a hydrolytic adjunct and utilization of the resulting hydrol-
ysate was reported in the literature (188). Sulfurous acid, when
used as a hydrolytic adjunct in the treatment of activated sludge
under hydrolytic parameters of the sludge at 140 C for one hour,
increased the solubility of the solids by 20%. Concentration of the
solubilized extract produced a molasses-type syrup; 85% of the
solids content of the syrup was organic; over 20% of these solids
were proteinaceous material. Rat feeding studies showed that it
could be used as animal feed.
In summary, it appears that by-product recovery is possible from
municipal sludges. However, marketing and the value of product
and associated mazketing problems have not been worked out. In
general, the market place determines the by-product specifications,
and, inmost cases, these specifications are rigid, involving product
193
-------
purity and concentration. A great amount of development and addi-
tional research needs to be done before this appears to be practical.
WATER TREATMENT PLANT SLUDGE DISPOSAL AND/OR
UTILIZATION
In 1946, the American Water Works Association appointed a commi-
tee to survey and study the disposal of wastes from water supply
treatment plants. After six years of study a comprehensive report
was prepared. It was then noted that more than 96% of the 1530
reporting plants discharged sludge to the nearby watercourse or
lakes without treatment and only 3% used drying beds.
In 1968 a questionnaire survey of the 100 largest cities in the U.S.
was conducted by Washington Aqueduct Division to identify the method
of filter wash water and of sludge disposal used (190). This survey
indicated that b&sin sludge and filter washwater were still discharged
into surface waters. Only a small percentage of the plants used
lagoons and landfills. Filterwash waste, however, was noted to be
better controlled, 21% lagooned washwater and 18% returned to the
rapid mix.
Prior to 1968 no restrictions were imposed on discharge of these
sludges to streams and lakes. It was then that action was taken by
the FWPCA (now USEPA) to implement the Water Quality Act of
1965 and passage of stricter state laws to regulate disposal of these
wastes.
It appears that an intensified effort is in progress by the water
industry, namely, American Water Works Association, to put the
task of handling and disposal of these wastes in proper perspective.
Methods for disposal and utilization of these wastes that are presently
used are listed in Table 75. These methods shall be discussed
briefly in the following sections.
Direct Discharge to Streams.Lakes, and Ocean
Discharge of the sludge without treatment to nearby available water
is economical and simple. These have been the main reasons for its
popularity. However, the recent survey made by the AWWA Re-
search Foundation (52), has indicated that this method is no longer
accepted. Federal and State water laws have totally eliminated
direct discharge of these wastes into surface waters. At the present
time most municipalities have already been converted to other means
of disposal, or are in the process of doing so.
194
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Direct disposal of sludge from water treatment plants into surface
waters may not cause serious health hazards as compared with
wastewater and wastewater sludge, because of its low organic
content. However, the high solids content of these sludges may-
color the receiving water, increase its turbidity, and may settle
and form a sludge blanket interferring with the natural aquatic
life cycle.
Lagooning of the Sludge
Lagoons can serve as settling basins or sludge thickeners preceding
some other process (temporary lagoons), or can be used for final
disposal of sludge (permanent lagoons). Lagooning of sludge is a
relatively simple and inexpensive sludge disposal method. Land
requirements are large, but operating costs are generally low.
Problems associated with lagoon disposal of the sludge are land
availability in the vicinity of the plant and occasionally odors.
Lagooning of softening basin sludge is practically odor-free, due
to the high pH of the sludge which prevents biological activities. On
the other hand, lagooning of purification wastes does involve odor
and nuisance because of the higher organic content and biological
population.
Parameters given for design and operation of permanent lagoons
are shown in Table 78 (55).
Lagoons should be considered for small treatment plants, where
land is inexpensive. However, investigation of the sludge toxicity
and metal concentration must be made so that ground water con-
tamination is prevented. After the lagoon is filled it could be
reclaimed as useful land through application of an earth cover.
Agricultural Utilization
Disposal of softening sludge on farm land has been frequently prac-
ticed, particularly in smaller communities. Lime sludge from
softening plants has been reported to be as effective as the agricul-
tural limestone from the quarries used for improving soils. Lime
sludge's total neutralizing power ranges from 92 to 100% as com-
pared with 60 to 90% for commercial lime used in soil (55). Furchermore,
lime sludge particles are much finer than those of marketed lime.
Advantages of the finer particles are that they react faster and
more completely with the acid in the soil.
Lime sludge can be applied on farm land both in liquid (1 to 5% solids)
or dewatered (20 to 40% solids) states. An application rate of 3 tons
195
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TABLE 78
Design Criteria and Operational Guides for Lagoons
(55)
Thickening and
Storage Lagoons
Permanent
Lagoons
Number of subdivisions
in lagoon
Depth (feet)
Fill time (years)
Area (Acres/MGD/100 mg/1
hardness removed)*
Decanting Mechanism
Bottom Slope
Embankment Slope
Exterior Side
Interior Side
Depth of Applied Wet
Sludge (inches)
Final Solids Content (%)
Volume Reduction (%)
Sludge Distribution
System
At least 3 •,
preferably 6
5-10
At least 6
5-10
20
0.7*** 2.0
Stop plate outlet or telescope valve
0.5% to 1.0%
away from the inlets
3:1
2.5:1
Up to
12
25-40
80-90
Equal to or
less than 2
Equal to or
more than 70
95**
Piping and valving arranged (1) to
allow uniform distribution, (2) so
that flow distance in the bed is
less than 200', and (3) so that dis-
tance between the sludge inlet and
the decanting device is 100" or
greater to avoid short circuiting.
* Based on lagoon depth of five feet
** After compaction
**# Based on 85% volume reduction
196
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per acre per year has been recommended for Ohio soils (55).
Disposal of lime sludge on farm land appears to be an advantageous
disposal method. However, sludge must be analyzed for toxic
metal content prior to land application.
Discharge to Sewers
Disposal of sludge to both sanitary and storm sewers has been prac-
ticed. Discharge to storm sewers is no longer acceptable for
reasons given previously. Discharge of lime sludge from softening
plants into the sanitary sewers has increased very rapidly. Kras-
auskas (191) reported that lime sludge disposal to sanitary sewers
has increased from 0. 3% of the water treatment plants surveyed
in 1953 to 8. 3% in 1968.
A recently developed use of lime indicates that lime sludge may be
used in sewage treatment plants for wastewater sludge stabilization.
No adverse effects of lime sludge disposal into sanitary sewers
has been reported in the literature studied. However, it could cause
problems, including: (1) precipitation if flow velocity is below 2.5
ft. per second, or solids formation on walls of sewers, reducing
the hydraulic efficiency of the pipes; (2) in excess amounts it may
hamper the biological activities of the secondary treatment pro-
cess in the wastewater treatment plant; (3) in excess amounts it
may reduce the digester efficiency by increasing solids content
and by increasing the hydraulic loading of the digester; and (4) it
may reduce the efficiency of the secondary digester by increasing
the sludge density.
Disposal in Sanitary Landfill
Disposal of water treatment plant sludge in sanitary landfills
appears to have a number of advantages. Mixing of the sludge with
refuse could help the compaction of refuse in a landfill. Leaching
has been reported when liquid sludge was applied, but leaching
could be controlled if dewatered sludge is used. The precent of
sludge that can be absorbed in a landfill without producing a
lea chant is given in Table 79.
Application of Sludge to Strip Mine Spoils
Sludge from softening plants can be utilized at strip mine spoils for
land reclamation purposes. Lime sludge will obviously reduce the
acid drainage and will aid in reclaiming the acid bearing soils.
197
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TABLE 79
Percent of Sludge Produced That Can
(55)
Be Absorbed in Landfill
Percent Percent of Sludge Produced
Solids in Absorbed in Landfill With-
Sludge out Leaching*
4 16
10 45
20 100
* Based on (1) 1600 pounds refuse per capita per year,
(Z) 100 gallons per capita per day water usage,
(3) removal of 100 milligrams per liter hardness,
(4) absorption of 65 gallons of water per ton refuse,
(5) nine feet depth of fill, (6) density of refuse is 13
pounds per cubic foot.
198
-------
Application rates are not available, however, it is believed that
vast amounts of sludge can be absorbed by the strip mine areas.
Cost of the sludge transportation is generally the main problem, but
increase in the value of the reclaimed properties could justify
this cost.
By-Product Recovery
By-product recovery of the waste from water treatment plants in-
cludes lime recovery, magnesium carbonate recovery, and alum
recovery. The production of lime from softening plant sludge has
almost eliminated the disposal problem. Recalcination has prac-
ticed in large cities in the U.S. , including Dayton, Ohio; Miami,
Florida; and Lansing, Michigan. Recalcination is not only the
best ultimate method, it could also be profitable by selling the
excess lime produced. Dayton has reported a profit of $0.79 per
ton of lime produced, while Miami has reported this profit to be
$5.08 per ton of lime produced. In the recalcination process vast
quantities of CO^ are also produced from the burning of fuel and
conversion of CaCO3 to CaO. Carbon dioxide has been recovered
and used (in Dayton) in the recarbonation chamber to dissolve
the Mg(OH)o present in the sludge and subsequently precipitated
from the CaCO3 sludge.
Lime recovery, although it is a profitable and most efficient method
of sludge disposal, is generally limited to large plants. Capital
and operating costs become prohibitive as units get smaller.
Magnesium carbonate recovery is generally associated with lime
recovery. There are two possible methods for recovering mag-
nesium values, as MgfOH)^, by selective softening, or as MgCO3
from the carbonated lime sludges. Selective softening is not
practical primarily because it would require major changes in the
plant and complicate the softening operation (192).
Recovery of magnesium greatly enhances the quality of lime produced.
Carbonation of the softening sludge containing appreciable magnes-
ium hydroxide can dissolve the insoluble magnesium salt. The
soluble magnesium carbonate can be physically separated from the
calcium carbon precipitate. The utility of magnesium carbonate as
a purification plant coagulant to replace alum is being evaluated at
Montgomery, Alabama (52).
Aluminum sulphate coagulant has been recovered from the purifi-
cation wastes containing appreciable amounts of aluminum in both
Japan and France. Aluminum hydroxide wastes resulting from
199
-------
purification plants are basic in nature. Acidification of such
wastes produces a soluble reuseable coagulant and inert
material.
Although this process substantially reduces the volume of sludge
for final disposal, success of this method depends upon the
efficiency of separation of the recovered alum solution from the
inert material not affected by acidification; and also upon the
minimizing of impurities, such as the iron and manganese buildup
in the recovered solution.
ECONOMICS OF WASTEWATER SLUDGE DISPOSAL
It is apparent that there are many factors effecting the economics
of wastewater sludge disposal and utilization. These factors
include: plant location; type of treatment processes to which both
wastewater and the sludge are subjected; amount, nature and
ultimate disposal of the wastes; air, water and land pollution
standards and requirements; the availability of land and trans-
portation; the cost of construction, operation, and maintenance
of the necessary facilities; and the availability of a market place
for the sale of by-products.
Numerous papers (11,17,156,160,167,172,179,180,193,194,195,196,
197,198) have discussed the economics of sludge disposal and
utilization. It is interesting to note that almost every author
admitted, directly or indirectly, that sewage sludge is a liability
in any sewage treatment plant, despite the fact that it has some
fuel and fertilizer value. This general opinion is based upon the
fact that the benefit-cost ratio almost always falls below unity.
Also, the aesthetic and environmental benefits derived from a
specific disposal method are not considered in the benefit-cost
analysis.
Due to the fact that numerous factors effect the economics of
sludge disposal, it is not feasible to establish a general sludge
disposal and utilization guideline applicable to every treatment
plant. In Table 79 is presented the range of sewage sludge
disposal costs compiled from various literature sources. These
values give only an order of magnitude, but are useful as
general information. Further, in Table 79 it is suggested that
selection of the wastewater sludge disposal method for a par-
ticular plant requires a complete economic analysis of all
available methods.
200
-------
IsJ
O
TABLE 79
Sludge Disposal and Utilization Costs
1. Barging to Sea
2. Pipeline to Sea
3. Liquid Sludge Disposal on Farmland
4. Liquid Sludge Disposal to Strip Mines
5. Landfilling Dewatered Sludge
6. Heat Drying and Selling as Soil
Conditioner or Fertilizer
7. Lagooning
8. Wet Air Oxidation
9. Incineration
Raw Capital and Operating Costs
($ / Dry Ton)
Range Average
5
5
16
10
25
8
40
25
- 25
--
- 40
- 25
- 50
- 60
- 25
- 50
- 70
9
11
15
20
25
40
12
45
50
-------
Sludge disposal and utilization methods currently being practiced
can be- divided into three major groups. Group I includes disposal
and/or utilization of liquid digested sludge in the sea, or on land.
Ocean disposal of liquid sewage sludge by barges or through
submarine outfalls is practiced by many coastal cities. The
reported costs for New York and Philadelphia (160,167) are about
$9/ton of dried solids for barging the liquid sludge to the disposal
sites. Although ocean disposal of sewage sludge is economically
very attractive, ecologically it is not a satisfactory method.
Disposal of liquid digested sludge on farmland, and for reclamation
of strip mine spoils and sandy soils appears to be the most advan-
tageous disposal and utilization method. It satisfies both economi-
cal and ecological constraints. This method is definitely an ideal
disposal method for smaller communities (20,000 or less), where
farmlands are located in the proximity of the sewage treatment
facilities. It is also economically feasible for medium and large
size communities, where receiving lands or strip mine spoils
are located within a reasonable distance.
Extensive studies have been made on the transportation of liquid
digested sludge. Results of the study made by Riddell and Cormack
(200) are shown in Figures 54, and 58 thru 61. It apoears that
truck transportation is most economical for small communities
(10,000 pop.) for distances up to about 150 miles. Furthermore,
use of pipeline transportation does not become economical for a
distance of 25 miles until the population reaches 150,000. (The
cost of pipeline transportation has been based on a minimum pipe
diameter of 6 in., a maximum pumping head of 220 ft. , and a
sludge solids concentration of 3.5%).
The Metropolitan Sanitary District of Greater Chicago conducted
extensive research to determine the best possible method for
disposal of their sewage sludge. Results of this study (shown in
Figure 62) indicated that application of digested liquid sludge on
land for land reclamation purposes was not only the most econo-
mical disposal method, it also was compatible with environmental
standards and requirements; it conserved organic matter for
beneficial use; and it did not adversely effect the water treatment
cycle.
Raynes (193) in conjunction with a land reclamation project for the
United States Public Health Service has made an economic analysis
of pipeline transportation of digested liquid sludge. Results of the
costs, population, and distance are shown in Figure 63. The sludge
202
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$6000
4000
2000
Q
>
Ul
oc
tooo
600
400
QC
UJ
Q.
o
o
20°
100
60
40
Figure 58
Transportation Cost
Population 10, 000
PIPE LINE
TANK TRUCK
20 40 60 100 200 400 600
MILES TO POINT OF DISPOSAL
203
-------
Figure 59
Transportation Cost
(199)
Population 1,000, 000* '
f WV
4OO
200
O
UJ
§
2 |o°
QC
to
CO CO
? 40
K
m
CL
fc 20
O
u
10
6
4
R
S
/
/
/*
.R. '
^
/
>
/
f
TAr
^-^
y
/
/
r
TANK
IK
***
/
/
X
c
/
r
/
A
r-
/
/
R
*•
/
/
TRUCK —
r~
-*i
*•
/
^
N
/
H
^
/
^-p
^*
/
««=
/
IPE
/I
•P—
/
: L
JNE
20 40 60 100 20O 40O 6OO
MILES TO POINT OF DISPOSAL
204
-------
Figure 60
Transportation Cost
25 Miles<199>
O
Ul
O
2
UJ
K
CO
CO
U
0.
fe
O
O
6OO
4OO
20O
IOO
6O
4O
to
IO
6
4
PIPE LINE
R.R. TANK CAR
10 4O 100 40O
POPULATION IN THOUSANDS
1000
205
-------
Figure 6l
o
Ul
o
2
Ul
CO
UJ
N
k
,
E
V
>
k
X
^
Ss
s,
^
s
^
>
IO 40 100 400
POPULATION IN THOUSANDS
1000
206
-------
Figure 62
Costs of Disposal Methods for Activated Sludge
(180)
MTtM AND
•ALE AS FERTILIICR
mil AIR OXIDATION
(ZIMPROI*
OEWATERINO AND
INCINERATION *
DIGESTION AMD
PERMANENT LAGOONS
DIGESTION AND RECLAMATION
Of 'ARM LANDS
DIGESTION AND RECLAMATION
OF .STRIP. .KUMCS
.a— fc .' - - : A
1 1 '
1
1 1
V 1' '
1 I
:. .!. _i
'."1$I5
C i-
L ^Ins
*— — «j
It
1
i
?~ i
i
riw
< I1BI| -»»
i L ^
,
,-J1
ISO
• '3«
149
«60
a»e»
>7
$0
»IO $20 «30 «40 S30 $60
COST PER EQUIVALENT DRV TON
Figure 63
Pipeline transportation costs per dry ton sludge solids
($/ton -f 0.9 = $/metric ton; miles X 1.6 = km.)
1C
(193)
DOLLARS
KM DRY TON SLUDGE SOUOS
MO 400 COO I Z
THOUSANDS MM.LONS
POPULATION
to
207
-------
used for this analysis was assumed to have a solids concentration
of 5%.
Group II includes disposal and utilization of dewatered or dried
sewage sludge on farm land as soil conditioner or fertilizer. Sludge
dewatering on sand drying beds and the application of the dried
sludge on farmland have been shown to be both economically and
ecologically acceptable. Land availability and land costs in large
urban centers make this method economically unattractive; how-
ever, for small and medium size communities, it should be given
serious consideration when a sludge disposal method is to be
selected.
Multiple hearths, fluidized beds, and similar devices have been
employed for sludge drying purposes. The cost of sludge drying by
such devices is generally high, as opposed to complete combustion
(incineration) of the sludge. However, the return money from the
sale of the dried by-products may drastically reduce the net cost
of ultimate sludge disposal when compared with complete incinera-
tion. Marketing and the local market potential for the dried sludge
appear to be the main problems. An extensive study was made by
Quirk (ZOO), comparing the economic costs of wastewater sludge
incineration vs. sludge drying. Two different systems were
studied. These systems included: (1) only incineration equipment,
and (2) a dual purpose system, with incineration or drying equip-
ment. Results of the annual unit costs are presented in Table 80.
It is readily seen that the cost of drying is far greater than that of
incineration. Furthermore, the cost of deodorizing dried sludge
is quite substantial.
Figure 64 illustrates the results of an analytical technique using
unit costs presented in Table 80. It presents a picture of the effect
of fluctuating market conditions on the relative economy of incinera-
tion versus incineration/drying. The loss area indicates that net
total annual costs of an incineration/drying cycle are greater than
those associated with a simple incineration operation. The rela-
tive economy area indicates that net total annual costs for incin-
eration/drying are lower in that area than those associated with
simple incineration operation. The profit area indicates that
the product income is exceeding the annual costs. Figure 65
represents actual costs rather than cost ratios. The solid
curves assume a non-deodorizing cycle, where the dashed
curves are based on deodorizing to at least 1200°F. The cross-
hatched area represents the range in soil conditioner selling
price which can normally be anticipated. Analysis of this figure
208
-------
TABLE 80
Unit Cost Summary
(200)
Equipment Operation
Cycle
1 . Incineration Incineration Deodor.
it it
2. Incin. -Drying "
ii ii it
3. " " Drying
ii n ii
Non-Deodor.
Deodor.
Non-Deodor.
Deodor.
Non-Deodor.
Cost/Ton D. C.S.
ncine ration
$
18.97
15.51
21.94
18.19
37.02
20.09
Vacuum Fil-
tration $ T°tal
10.51
10.51
10.51
10.51
10.51
10.51
29.48
26.02
32.45
28.70
47.53
30.60
Figure 64
Comparative Economy of Incineration vs. Incineration-Drying
Based on a Flash-Drying System; Vacuum Filtration Excluded
(200)
3456
I/PERCENT ANNUAL TONNAGE DRIED
209
-------
Figure 65
Comparative Economy Curves for Incineration-Drying
100
•— OEOOORIIATION
- -o-— NON oeooomuTioN
.3 A A .6 .8 1.0 2X> 10 4.0 5.06.0 BD
SELLING PRICE - ft PER 65 POUND BA6
10.0
210
-------
indicates the following: (1) at an average selling price, full cost
recovery is not possible; (2) under the most favorable market
conditions, from 60 to 80% of annual production must be sold to
recover the total annual cost of the incineration/drying operation;
(3) under the least favorable market conditions, from 25 to 40%
of the annual production must be sold in order to obtain relative
economy; and (4) deodorization requirement exerts a significant
influence on the market conditions required to justify an
incineration/drying operation.
Group III includes sludge incineration. It is apparent that this is
the most expensive method of all. Furthermore, application of
this method does not allow recycling of the valuable organic
matter back to nature, and the resulting incinerator ash imposes
additional disposal costs. Table 81 presents approximate costs
of various types of incinerators. Table 82 presents the capital outlay
required for incinerator systems by population group. Sludge
incineration costs are affected by many factors (156), including:
(1) nature of the wastewater sludge; (2) sludge conditioning method
and the amount and type of chemicals used; (3) size and design of
incinerator system; (4) costs of utilities (fuel, power, water); and
(5) air pollution control facilities required. The economics of
sludge incineration were also studied by Raynes (193.). Results
are presented in Table 83.
An extensive study of the ultimate disposal of sewage sludge from
23 treatment plants in Montgomery County, Ohio, was made by
Seifert (201 ). This study was based on a regional sludge disposal
concept. The study proposed ten alternatives for sludge disposal
and utilization. A detailed cost analysis of the methods under
consideration was made. Based on their technical and economic
feasibility the alternatives were then compared. Table 84 presents the
various disposal systems studied, their costs and ranking. Results
of the study showed that pipeline transport of the liquid digested
sludge to 640 acres of farmland was the most economically viable
disposal method.
The reviews of literature indicate that the costs of disposal of
sludge from water treatment plants are not clearly reported or
recorded. Furthermore, the type of sludge, the processes em-
ployed prior to disposal, and the type of disposal method will
greatly influence the disposal cost* It appears that each sludge
disposal method is unique; consequently, the costs are variable.
In general, sludge disposal costs amount to about 5% of the total
cost of treating water. In some softening plants where recal-
211
-------
TABLE 81
Pricing Information on Incineration Systems
(156)
Type
Fluid Solid
Multiple
Hearth
Cyclonic
Reactor
Wet
Oxidation
Manufacturer
Dorr-Oliver
Nichols
Bert 1 e t t -Snow-Pacif ic
Dorr-Oliver
Sterling Drug
Zirapro Division
Size
Ub/hr}
200
40O
1,000
2,000
5OO
2,OOO
4,000
6,000
100
200
1,000
470
1968 Dollars
180,000
300,000
550,000
825,000
300,000
550,000
700,000
850,000
85,000
120,000
300,000
284,000
(Oxidation
unknown)
(High oxl-
da t Ion)
Plash Drye:
and Incin-
erator
Combustion Engineering
Raymond Division
Cyclo-Durner Sergent - Zurn
400
600
1,000
2,OOO
5,OOO
130
300,000*
330,000
375,000
460,000
700,000
70,000
*Pricea included drying but not dewetaring equipment.
Delate 20ft for special equipment and add 25% for
dewatering.
212
-------
TABLE 82
Capital Outlay Required for Incinerator
Systems by Population Group
Average Incinerator System
Capital Cost -
Population (1968 Dollars)
250 Not applicable
750 50,OOO1
1,750 70,0001
3,750 120, OOO1
7,500 130,000^
17,500 345, OOO2
Based on one shift operation
Based on two shift operation
TABLE 83
Disposal by Filtration and Incineration
Population Cost
Served ($/dry ton)
100,000 60-90
500,000 50-75
1,000,000 30-50
2,000,000 25-40
*
Activated sludge plants
Note: $/ton -7 0. 9 = $/metric ton
213
-------
TABLE 84
Total Disposal Systems- Comparison Table *
No.
1
2
3
4
5
6
7
8
9
10
Process
Farm
Lagoon
Dehydration
Rotary Kiln
M.H. Incin.
Porcupine
Fl. Bed. Incin.
AST
Compost
Flash Dry
$1000
Capital
2,
2,
3,
2,
3,
3,
3,
3,
4,
4.
294
820
900
334
471
887
887
750
997
640
$1000
o&o
262
353
419
479
534
537
551
634
687
781
Feasibility Tech.
Tech. Econ. Rank
Good
Fair
Good
Poor
Poor
Fair
Poor
Poor
Poor
Poor
£xc.
Good
Good
Fair
Fair
Fair
Fair
Poor
Poor
Poor
2
3
1
7
6
4
5
9
10
8
O&O = Owning and Operation
TABLE 85
Costs for Disposal of Water Treatment Sludges
(202)
Location
Austin, Texa*
Boca Raton, Fla.
Dayton, Ohio
Coleta, Calif.
Lansing, Mich.
Lompoc, Calif.
Miami, Fla.
Minot, N. D.
New Britain, Conn.
Somerville, N. J.
Sunol. Calif.
Willingboro. N. J.
Mode) Studies
Sludge Type
Lime
Lime
Lime
D. Earth
Lime
Lime tt
D. Earth
Lime
Lime
Alum
Alum
Alum
Alum
Alum
Lime
Lime
Prooeuo*
C
T. VF
T, C. R
VF
T, C, R
S
T, C, R
T, VF
L
L
S
L
T, VF
T.VF
T. C
Total DUpoal Cow
l/mt
25.10
16.00
2.20f
8.52
2.00
24.20
6.9St
21.80
3.30
0.09
1.18
4.90
2540
12.35
11.40
i/ifuo/
itynKdl
25.10
16.00
l.SOf
95.10
6.15
4.89
lOSSf
7.29
39.00
2.00
56.60
33.50
122.00
12.35
11.40
J/tono/
JlJM
0.79f
—
2.60
—
s.ost
—
—
—
—
—
^_
—
Symbol*: C - Ccnuif u«c. L - Laatauvljic, R
VF - Vacuum filtration.
t Profit Irani rtealdntd Urn*.
- KcaUcinaUoa. S - Send Bed*, T - TUckntai.
214
-------
cination is practiced, substantial profits have been reported.
Recalcination of lime sludge offers many advantages; however,
its application is limited to only large softening plants, where
ground water is low in magnesium. Results of the study made by
Adrian and Nebiker (202) on the costs of disposal of water treat-
ment sludges are presented in Table 85. It is apparent that
application of mechanical thickening and dewatering facilities adds
significantly to the cost of sludge disposal. However, their use
is justified where re calcination is employed.
In summary, it appears that transporting liquid digested sludge by
pipeline and its application on farmland as well as sandy soil and
strip mine spoils is the most economically and ecologically viable
disposal and utilization method. Application of dried sludge on farm-
land as a soil conditioner and fertilizer is a satisfactory disposal
method. It may not be economically feasible if a mechanical dryer
is not employed or an adequate local market for sale of the dried
product is not available. Finally, sludge from water treatment
plants is not too costly to dispose of; furthermore, recalcination
of the water softening plant sludge has been shown to be an attrac-
tive and ecoribmical disposal method, where applicable.
215
-------
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79. Mayo, E., "Industrial Application of Air Flotation", Paper
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80. Katz, W.J., "Sewage Sludge Thickening by Flotation", Public
Works, Vol. 89, No. 12, December 1958.
81. Bradney, j-,., and Bragstad, R.E., "Concentration of Activated
Sludge by Centrifuge", Sewage and Industrial Wastes, Vol. 27,
No. 4, 1955.
82. Ettelt, G.A., andKennedy, T.J., "Research and Operation
Experience in Sludge Dewatering", Journal WPCF, Vol. 38,
No. 2, February 1966.
83. Kormanik, R.A. , "A Resume of the Anerobic Digestion Process",
W. andS.W., Reference No., 1968pp. R-154.
84. McCarthy, D.L., and Borsseau, M.H. , "Toxic Effect of
Individual Volatile Acids in Anaerobic Treatment". Proc. of the
18th Purdue University Inds. Wastes Conf. , 1963.
85. McCarthy, D.L., "Anaerobic Waste Treatment Fundamentals,
Part 3, Toxic Material and Their Control", Public Works
95:91 (1964).
86. Eckenfelder, W.W., Jr., contributions by Jerris, J.S., Cardenas,
R., McCarthy, D.L., and Gloyna, E.F., "Biological Waste
Treatment", unpublished (1965).
87. Estrada, A.A., "Design and Cost Consideration in High-Rate
Sludge Digestion", ASCE Sanitary Engineering Division, Vol. 86
No. SA3, May I960.
88. Suhr, E.J., and Brown, J.M., "High-Rate Digestion Tamed",
Water Works and Wastes Engineering, Vol. 1, No. 8, 1964.
89. Loehr, R.E., "Aerobic Digestion: Factors Affecting Design" ,
W. and S.W. , Reference No. 1965, R-169.
90. Viraraghovan, T., "Digesting Sludge by Aeration", Water Works
and Wastes Engineering, Vol. 2, No. 9, September 1965.
91. Eckenfelder, W.W., Jr., "Studies of the Oxidation Kinetics of Bio-
logical Sludges", Sewage and Industrial Wastes, Vol. 28, pp. 983, 1956,
222
-------
92. Randal, C.W., and Kock, C.T., "Dewatering Characteristics
of Aerobically Digested Sludge", JWPCF, Vol.41, No.5, May 1969.
93. Randal, C.W., Moor, H.R., and King, P.H., "The Effect of
pH on Aerobic Sludge Digestion", Paper presented at the 5th
Mid-Atlantic Inds. Waste Conf., November 1971.
94. Wiley, J.S., Gartrell, F.E., Gray Smith, H. , "Concept and
Design of a 3-Way Composting Project", Compost-Science
Autumn, 1966.
95. Wiley, J.S., and Pearce, G.W., 'A Preliminary Study of High-
Rate Composting", Proc. Amer. Soc. of Civil Engs., Paper
No. 846, December 1955.
96. Wiley, J. S., andSpillane, J.T., "Refuse-Sludge Composting
in Windows and Bins", J. of the Sanitary Engineering Division,
ASCE, SAS, September 1961.
97. Wiley, J.S., and Kochtitzky, O.W., "Composting of Dewatered
Sewage Sludge", Compost Science, Summer 1965.
98. Shell, C.L., and Boyd, J.L., "Composting of Dewatered Sewage
Sludge", Compost Science, May-June, 1970.
99. Wiley, J.S., "A Discussion of Composting of Refuse with Sewage
Sludge", Compost Science , Spring- Summer , 1967.
100. Kneiss, I.F., "Combined Sludge-Garbage Composting," Compost
Science, Summer 1962.
101. Gothard, S.A., "Garbage Processing in Jersey, British, Isles",
Compost Science, Spring 1961.
102. Black, R.J., "Combined Disposal of Sewage Sludge and Refuse",
Compost Science, Winter 1962.
103. Jeffrey, E.A., "Dewatering Rates for Digested Sludge in Lagoons",
WPCF, Vol., 32 No. 11, 1960.
104. Howells, D.H., andJubois, D.P., "The Design and Cost of
Stabilization Ponds in the Midwest", Sewage and Industrial Wastes,
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223
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105. Bowers, M., "Tips on Sludge Drying Bed Care", Sewage and
Ind. Wastes, Vol. 29, No. 7, 1957.
106. Haseltin, T.R., "Measurements of Sludge Drying Bed Perform-
ance", Sewage and Ind. Wastes, Vol. 23, No. 9, September 1951.
107. Alberton, O.E., "Dewatering of Heat Treated Sludges", paper
presented at 42nd Annual Conf. WPCF, Dallas, Texas, Oct. 1969.
108. "Sludge Conditioning without Chemicals by Zimpro", Zimpro Inc.
Publication, 1970.
109. Mann,K.G., "Heat-Treatment of Sludge-Zimpro Process",
paper presented at 46th Annual Meeting, Ohio Water Pollution
Control Conf. , Dayton, Ohio , June 1972.
110. "The Porteous Process-Sludge Dewatering Without Chemicals",
Envirotech Publication, Jan. 1971.
111. Malina, J.F., Jr., "Sludge Filtration and Sludge Conditioning",
in "Water Quality Improvement by Physical and Chemical
Processes", Univ. of Texas Press, Austin, Texas, 1970.
112. Nelson, F.G. and Budd, W.E., "New Development in Sewage
Sludge Treatment", J. of the San. Eng. Div., ASCE, Nov. 1959.
113. Farrel, J.B., Smith, J.E., Jr., Hathaway, S.W., and Dean,
R.B., "Lime Stabilization of Chemical-Primary Sludges at
1.15 MGD", Ultimate Disposal Research Program, NERC, EPA,
Cincinnati, Ohio, Oct. 1972 (Unpublished).
114. Riehl, M.L., Werser, H.H. , andRheins, B.T., "Effect of
Lime Treated Water on Survival of Bacteria", J. AWWA, May
1952.
115. Buzzell, J.C., Jr., and Sawyer, C.N., "Removal of Algal
Nutrients from Raw Wastewater with Lime", J. WPCF, Vol. 39,
No. 10, 1967.
116. Doyle, C.B., "Effectiveness of High pH for Destruction of
Pathogens in Raw Filter Cake", J. WPCF, Vol. 39, No. 8, 1967.
117. Stallery, R.H. and Eauth, E.H., "Treatment of Sewage Sludge
by the McDonald Process", Public Works, March 1957.
118. Slagle, A.A. and Robert, E.M., "Treatment of Sewage and
Sludge by Electrodialysis", Sewage Works J., Vol. 14, No. 5,
1942.
224
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119. Cooling, L. F. and others, "Dewatering of Sewage Sludge by
Electro-Osmosis", Water and Sanitary Engr., Vol. 3, No. 7,
1952.
120. Beaudoin, R. E., "Reduction of Moisture in Activated Sludge
Filter Cake by Electro-Osmosis", Sewage Works Journal, Vol. 15,
No. 6, 1943.
121. Center, A. L. , "Elutriation - How it Aids in Dewatering Sludge",
Public Works, 1934.
122. Willey, B. J., Duke, C. M. , Wojcieszak, A. L. , "Atomic
Absorption Spectrophotometry - Simplifies Heavy Metals Analysis",
AWWA, May 1972.
123. Center, A. L., "Conditioning and Vacuum Filtration of Sludge",
Sewage and Industrial Wastes, Vol. 28, No. 7, 1956.
124. "Operation of Wastewater Treatment Plants", Manual of Practice
No. 11, WPCF, Washington, D. C. (1961).
125. Zack, S. I., "Sludge Dewatering and Disposal", Sewage and
Industrial Wastes, Vol. 22, No. 8, Aug. 1950.
126. Dahl, B. W. , Zelinski, J. W. , and Taylor, O. W. , "Polymer
Aids in Dewatering and Elutriation11, J. WPCF, Vol. 44, No. 2,
Feb. 1972.
127. Bugg, M. H. , King, P. H. and Randall, C. W. , "Polyelectrolyte
Conditioning of Alum Sludge", J. AWWA, Dec. 1970.
128. Sharman, Les, "Polyelectrolyte Conditioning of Sludge", Water
and Wastes Eng., Aug. 1967.
129. Dickert, C. T. , "Polymer - What They are and How They Work",
paper presented at the 38th Annual Tech. Conf., Michigan Water
Pollution Control Assoc., 1963.
130. Goodman, B. L. and Witcher, C. P., "Polymer-Aided Sludge
Elutriation and Filtration", J. WPCF, Vol. 37, No. 12, Dec. 1965.
131. Goodman, B. L. , "Chemical Conditioning of Sludges, Six Case
Histories", Water and Wastes Eng., Vol. 3, No. 2, Feb. 1966.
132. Morris, R. H. , "Polymer Conditioned Sludge Filtration", Water
Works and Wastes Eng., Vol. 2, No. 3, 1965.
225
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133. Bargman, R. D. , et al. , "Sludge Filtration and Use of Synthetic
Organic Coagulants at Hyperion", Sewage and Industrial Wastes,
Vol. 30, No. 9, Sept. 1958.
134. Sherbeck, J. M., "Synthetic Organic Flocculants Used for
Sludge Conditioning", J. WPCF, Vol. 37, No. 8, Aug. 1965.
135. Katz, W. J. and Mason, D. G., "Freezing Method Used to Con-
dition Activated Sludge", Water and Sewage Works, April 1970.
136. Farrel, J. B., Smith, J. E. , Jr., Dean, R. B. , Grossman III,
and Grant, C. L. , "Natural Freezing for Dewatering of Aluminum
Hydroxide Sludges", J. AWWA, Dec. 1970.
137. Chalmers, B., "How Water Freezes", Scientific American,
200:114, Feb. 1958.
138. Eye, D. J. and Basu, T. K. , "The Use of Fly Ash in Wastewater
Treatment and Sludge Conditioning", J. WPCF, Vol. 46, No. 5,
Part 2, 1970.
139. Tanney, M. W. and Cole, T. C. , "The Use of Flyash in Condi-
tioning of Biological Sludge for Vacuum Filteration:, J. WPCF,
Vol. 40, p. 281, 1968.
140. Thoman, J. R. and Jenkins, K. H., "Statistical Summary of
Sewage Works in the U.S.", U. S. Public Health Service, USPHS,
Pub. No. 609 (1958).
141. Jannet, J. C., and Harris, D. J., "Environmental Factors in
Design and Operation of Wastewater Sludge Drying Beds", Missouri
Water Resources Research Center, Univ. of Missouri, Rolla,
July, 1971.
142. Quon, J. E., Johnson, G. M. , "Drainage Characteristics of
Digested Sludge", J. San. Eng. Div. ASCE, Vol. 92, SA2, 67,
1966.
143. Quon, J. E. and Tamblyn, T. A., "Intensity of Radiation and
Rate of Sludge Drying", J. San. Eng. Div., ASCE, Vol. 91, SA2
17, 1965.
144. Jennett, J. C. and Santry, I. W. , "Characteristics of Sludge
Drying", J. San. Eng. Div. ASCE, Vol. 95, SA5, 849, 1968.
145. Swanwick, J. D., "Recent Work on the Treatment and Dewatering
of Sewage Sludge", J. WPCF, Vol. 34, No. 3, March 1962.
226
-------
146. Templeton, W. E., "Experiments with Small Scale Sludge Drying
Beds at Motherwall", J. and Proc. Inst. Sew. Purif. , Part 2,
pp. 223, 1959.
147. Volger, J. F. and Rudolfs, W., "Factors Involved in the Drainage
of Whitewater Sludge", Proc. of the 5th Annual Univ. of Purdue
Indus. Waste Conf. , 1949.
148. Haseltine, T. R. , "Measurement of Sludge Drying Bed Perform-
ance", Sewage and Industrial Wastes, Vol. 23, pp. 1065, 1951.
149. Nebiker, J. H., Sanders, T. J., and Adrian, D. D., "An Investi-
gation of Sludge Dewatering Rates", J. WPCF, Vol. 41, R. 155,
1969.
150. Nebiker, J. H., "Drying of Wastewater Sludge in the Open Air",
J. WPCF, Vol. 39, pp. 608, 1967.
151. "Recommended Standards for Sewage Works (10 State Standards)",
by Great Lakes Upper Mississippi River Board of State Sanitary
Engineers, I960.
152. Alberton, O. E. and Guidi, E. J., "Advances in the Centrifugal
Dewatering of Sludges", Water and Sewage Works, R 1, 1967.
153. Alberton, O. E. and Sherwood, R. J., "Centrifugation for
Dewatering Sludges", Water and Wastes Engineering, April 1968.
154. Schempman, B. A. and Cornell, C. I. , "Fundamental Operating
Variables in Sewage Sludge Filtration", Sewage and Industrial
Wastes, Vol. 28, No. 12, Dec. 1956.
155. Vater, W. G. A., "European Practice in Sludge Digestion and
Disposal", in "Water Quality Improvement by Physical and Chemi-
cal Processes", Ed. by Eckenfelder and Gloyna, Univ. of Texas
Press, Austin, Texas, 1970.
156. Balakrishnan, S., Williamson, D. E., and Okey, R. W. , "State
of the Art Review on Sludge Incineration Practice", FWQA, U. S.
Dept. of the Interior, #17070 DIV 04, April 1970.
157. Smith, E. G. , "Features of a Mechanical Sludge Concentrator for
Dewatering Sludges", Sewage and Industrial Wastes, Vol. 29,
No. 5, 1957.
158. Spohr, G. and Eckenfelder, W. W. , Jr., "Sewage Sludge Thicken-
ing by Mechanical Vibration", Public Works, March 1958.
227
-------
159. Kiess, F., "Sludge Dewatering by Vibrating Screens", Water and
Sewage Works, Nov. 1959.
160. Smith, D. and Brown, R. P., "Ocean Disposal of Barge-Delivered
Liquid and Solid Wastes From U. S. Coastal Cities", SW-19c,
USGPO, Washington, D. C.
161. Merz, R. C. , "Continued Study of Wastewater Reclamation and
Utilization", California State Water Pollution Control Board,
Publ. No. 15, 1956.
162. Merz, R. C. , ed., "Third Report on Study of Wastewater Reclama-
tion and Utilization", California Water Pollution Control Board,
Publ. No. 18, 1957.
163. Wirts, J. J. , "Pipeline Transportation and Disposal of Digested
Sludge", Sewage and Industrial Wastes, Vol. 28, No. 2, Feb. 1956.
164. Harza Engineering Co., "Land Reclamation Project, An Interim
Report", U. S. Dept. of HEW, Washington, D. C. , 1968.
165. Raynes, B. C. , "Economic Transport of Digested Sludge Slurries",
J. WPCF, Vol. 42, pp. 1379 (1970).
166. Sparr, A. E. , "Pumping Sludge Long Distances", J. WPCF, Vol.
43, No. 8, 1971.
167. Guarino, C. F. and Cameron, M. S. , "Sludge Processing in
Philadelphia", J. WPCF, Vol. 43, No. 8, Aug. 1971.
168. Millward, R. S. and Darby, W. A., "Sludge Disposal by Dewatering
and Combustion", Water and Wastes Engineering, Oct. 1967.
169. Burger, T. B. , and Chasick, A. H. , "Using Graded Sand to
Test Grit Removal Apparatus", J. WPCF, Vol. 36, No. 7, July,
1964.
170. "Sewage Sludge Incineration", by EPA Task Force, PB 211-323,
PEA Washington, D. C. , Aug. 1972.
171. Beeckmans, J. M. and Ng, P. C. , "Pyrolyzed Sewage Sludge:
Its Production and Possible Utility", Environmental Science and
Technology, Vol. 5, No. 1, Jan. 1971.
172. Smith, J. E. , Jr., "Ultimate Disposal of Sludges", a paper pre-
sented in the Technical Seminar/Workshop on Advance Waste
Treatment, Chapel Hill, North Carolina, Feb. 1971.
228
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173. Domenowski, R. S., and Matsuda, R. I., "Sludge Disposal and
the Marine Environment1', J. WPCF, Vol. 41, No. 9, Sept. 1969.
174. Heckroth, Charles, "Where Do We Go From Here?" Water and
Wastes Engineering, May 1971.
175. "Recycling Sludge and Sewage Effluent by Land Disposal", Editorial,
Environmental Science and Technology, Vol. 6, No. 10, Oct. 1972.
176. Evans, J. O. , "Ultimate Sludge Disposal and Soil Improvement",
Water and Wastes Engineering, June 1969.
177. Ben, B. E. and Drck, R. I. , "Disposal and Sludge on Land", in
"Water Quality Improvement by Physical and Chemical Processes,"
Ed. by Gloyna and Eckenfelder, Univ. of Texas Press, 1970.
178. Raynes, B. C. , "Transport of Digested Slurries for Economic
Disposal", A report prepared for FWPCA, cont. PH-86-65-21
(1966).
179. Dotson, G. K., Dean, R. B. , Stern, G. , "The Cost of Dewatering
and Disposal of Sludge on Land", Unpublished.
180. Dalton, F. E. , Stein, J. E. , Lynam, B. T. , "Land Reclamation -
A Complete Solution of the Sludge and Solids Disposal Problem",
J. WPCF, Vol. 40, No. 5, Part I, May 1968.
181. "Bulk Transport of Waste Slurries to Inland and Ocean Disposal
Sites", A report by Bechtel Corporation, Contract No. 14-12-156,
for FWPCA, Dec. 1969.
182. Vankleck, L. W., "Utilization of Sewage Sludge", Water and
Sewage Works, Reference Edition, 1953.
183. Koenig, L., "Ultimate Disposal of Advanced Treatment Waste",
Publ. No. 999-WP-10, USPHS, AWTR-8, Oct. 1963.
184. Shannon, E. S., "Underground Disposal of Activated Sludge",
J. FWPC, Vol. 40, No. 12, Dec. 1968.
185. Hoover, S. R., et al. , "Activated Sludge as a Source of Vitamin
B-12 for Animal Feeds", Sewage and Industrial Wastes, Vol. 24,
No. 1, Jan. 1952.
186. Leary, R. D., "Production of Vitamin B-12 from Milorganite",
Proc. of the 9th Industrial Waste Conference, Purdue Univ., 1954.
229
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187 Rudolfs, W. , Cleary, E. J. , "Sludge Disposal and Future Trends",
Sewage Works J., Vol. 5, No. 3, 1933.
188. "Feasibility of Hydrolysis of Sludge Using Low Pressure Steam with
SO as a Hydrolytic Adjunct and Utilization of the Resulting Hydro-
lysate", by Foster D. Snell, Inc. for FWPCA, Contract No. 14-12-
188, Dec. 1968.
•^
189. Faber, H. A., "Report on What's Known, Disposal of Wastes from
Water Treatment Plants, Sec. I", AWWA, Vol. 61, No. 10, Oct.
1968.
190. Durfer, C. N. and Becker, E. "Public Water Supplies of the 100
Largest Cities in the U.S. ", Dept. of Interior, Washington, D. C.
1964.
191. Krasauskas, J. W. , "Review of Sludge Disposal Practices", J.
AWAA, May 1968.
192. Black, A. P., Shuey, B. S. and Fleming, P. G., "Recovery of
Calcium and Magnesium Values", J. AWWA, p. 616.
193. Raynes, C. B. , "Economic Transport of Digested Sludge Slurries",
J. WPCF, Vol. 42, No. 7, July 1970.
194. Linsley, S. E. and Mick, K. L., "An Examination of Sewage Solids
Incineration Costs", Water and Sewage Works, 104, 479 (1957).
195, Kershaw, M. A., "Developments in Sludge Treatment and Disposal
at the Maple Lodge Works, England", J. WPCF, Vol. 37, 674 (1965).
196. Bacon, V. W. and Dalton, F. E. , "Professionalisms and Water
Pollution Control in Greater Chicago", J. WPCF, Vol. 40, 1586
(1968).
197. Bowerman, F. R., "Solid Waste Disposal", Chemical Engineering/
Deskbook Issue, April 27, 1970.
198. Baxter, S. S. , "Sludge Disposal in Philadelphia", Proceedings
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199. Riddell, M. D. R. and Cormack, J. W., "Selection of Disposal
Methods for Wastewater Treatment Plants", Proceedings of 10th
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200. Quirk, P. T., "Economic Aspects of Incineration vs. Incineration-
Drying", J. WPCF, Vol. 36, No. 11 (1964).
230
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201. Seifert, V. W. and Moulenbelt, S., "Montgomery County, Ohio
Waste-water Treatment Plant Solids Ultimate Disposal", Project
7114, Dec. 1971.
202. Adrian, D. D. and Nebiker, H. J., "Report on Current Technology
and Costs" in "Disposal of Wastes from Water Treatment Plants",
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231
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-670/2-75-033b
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
CHARACTERIZATION AND UTILIZATION OF MUNICIPAL AND
UTILITY SLUDGES AND ASHES. VOLUME II - MUNICIPAL
SLUDGES
5. REPORT DATE
May 1975; issuing Date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Hecht, N. L., Duvall, D. S., and Rashidi, A. S.
8. PERFORMING ORGANIZATION REPORT NO
i. PERFORMING ORGANIZATION NAME AND ADDRESS
University of Dayton Research Institute
300 College Park Drive
Dayton, Ohio 45469
10. PROGRAM ELEMENT NO.
1DB064; ROAP 24ALH: Task 008
NO.
R800432
12. SPONSORING AGENCY NAME AND ADDRESS
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final Report
14. SPONSORING AGENCY CODE
'LEW
ITARY NOTES
Project Officer: Richard Carnes, 513/684-4487
See also; Volumes I, in, and IV, EPA-670/2-75-033a, c
and d
A comprehensive characterization and evaluation was performed of disposal and
utilization practices for sludges from municipal wastewater and water treatment
plants. The nature and quantities of the sludges were discussed. Various sludee
handling and treatment techniques were detailed. Problems encountered in sludge
disposal were reviewed, and the economics of wastewater sludge disposal were dis-
^•UoSCCl *
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
*Disposal, Utilization, *Sludge,
Economic analysis, *Sludge
disposal, Sludge drying, *Waste
water
Treatment plants, Sludge
handling, Municipal waste
water, Sludge origin,
Ultimate disposal, Com-
prehensive report, Charac
terization of sludges
13B
STRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
240
!0. SECURITY CLASS (TMspage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
232
U.S. GOVERNMENT PIINTING OFFICE: 1975-657-592/5371 Region No. 5-(|
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