EPA-670/2-75-033a
May 1975
Environmental Protection Technology Series
CHARACTERIZATION AND UTILIZATION OF
MUNICIPAL AND UTILITY SLUDGES AND ASHES
Volume I. Summary
National Environmental Research Center
Office of Research and Development
US. Environmental Protection Agency
Cincinnati, Ohio 45268
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EPA-670/2-75-033a
May 1975
CHARACTERIZATION AND UTILIZATION OF
MUNICIPAL AND UTILITY SLUDGES AND ASHES
Volume I
Summary
N. L. Hecht and D. S. Duvall
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 RESEARCH 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 re-
quire a focus that recognizes the interplay between the
components of our physical environment—air, water, and
land. The National Environmental Research Centers pro-
vide 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
and to recycle valuable resources.
Presented here is a series of studies describing
the nature and disposal practices for municipal and
utility sludges and ashes. The study was primarily con-
cerned with the sludges emanating from municipal waste-
water, and water treatment plants, coal ash from power
stations, and grate residue from municipal solid waste
incinerators. Each of these subject areas is presented
as a separate report. Volume I presents a summary of
the results and conclusions developed for each of the
subject areas.
Andrew W. Breidenbach, Ph.D.
Director
National Environmental
Research Center, Cincinnati
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ABSTRACT
The nature and disposal practices for municipal and utility sludges and
ashes were studied. The study was primarily concerned with the sludges
from municipal waste water, and water treatment plants, coal ash from
power stations and grate residue from municipal incinerators. Each of
these subject areas is presented in a separate report. Volume I of this
series presents the summary for the results and conclusions developed for
each of the subject areas.
IV
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TABLE OF CONTENTS
Page
INTRODUCTION 1
MUNICIPAL SLUDGES 2
UTILITY COAL ASHES 19
MUNICIPAL INCINERATOR RESIDUE 25
TABLE OF CONTENTS, VOLUME II - MUNICIPAL SLUDGES 29
TABLE OF CONTENTS, VOLUME in - UTILITY COAL ASH 32
TABLE OF CONTENTS, VOLUME IV - MUNICIPAL
INCINERATOR RESIDUES 33
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INTRODUCTION
Under EPA Grant R-80043Z a comprehensive evaluation of disposal and
utilization practices for municipal and utility sludges and ashes was
performed. This study was primarily concerned with the sludges from
municipal waste water and water treatment plants, coal ash from power
stations, and grate residue from municipal incinerators. The nature and
amounts of these wastes were established and the current disposal and/or
reuse practices were examined. To facilitate the presentation of this work
the subject matter was divided into three major sections: Municipal Sludges;
Utility Coal Ash; and Incinerator Residue. The reports from the studies
conducted in each of these subject areas are presented in separate volumes.
In this first volume, a summary of the results and conclusions developed
for each of the study areas is presented.
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MUNICIPAL SLUDGES
The primary objective of a waste-water treatment plant is to prevent pollution
of streams, lakes, and ground water supplies, by removing the solid pollu-
tants from the wastewater. The solids removed are in the form of a liquid
slurry, called sludge. Sludges are concentrated 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 useful material of the sludge back to nature; also, the disposal
method should be economically feasible.
Sludge handling, treatment and disposal requires many steps, including:
concentration, stabilization, conditioning, dewatering and drying, trans-
porting, and solids reduction.
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 subsequent steps are questions of concern.
Methods used for sludge concentration or thickening include: (1) gravity
thickening; (2) dissolved air-flotation; and (3) centrifugation. Gravity thick-
ening is the most widely used, simplest and least expensive sludge concen-
tration method. This process is basically the same as sedimentation settling,
but relatively slow in action. The degree of sludge concentration obtained
and the efficiency of thickener operation depends on such factors as: (1) initial
solids concentration and temperature of the sludge in the thickener; (2) type
of sludge and volatile content; and (3) addition of chemicals and inert weighing
agents. Typical concentrations 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 in-
creasingly popular. The basic principle in flotation thickening is to attach
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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.
Four methods of flotation thickening that are used in wastewater treatment
plants include: (1) dispersed air flotation; (2) dissolved air pressure flota-
tion; (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 hydraulic 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 concentration 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 pro-
vide food for microorganisms which grow in and upon them. These micro-
organisms are mostly of fecal origin and many of them axe pathogenic, which
may be definite health hazards. Therefore, decomposition of the organic
material, or sludge stabilization seems an obvious necessity. Methods
applied for stabilization of wastewater sludges include: (1) anaerobic diges-
tion; (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
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group of microorganisms (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 microorganisms (methane
forming bacteria) in the second stage is to convert the organic acids to
methane gas and CG^. 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
wastewater in sludges include: sludge type and volatile content; digester
temperature; digestion detention period; feed sludge concentration; 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 oxidation is
completed in two stages: (1) direct oxidation of any biodegradable matter
by biologically active masses of organisms, and (2) oxidation of microbial
cellular material by endogenous respiration. 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 tem-
perature; 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
S0% 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 (me-
chanical), and (2) outdoor (windows and bins) processes. In the mechanical
composting process there are three phases. These are: (1) dewatering of
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the sludge, (2) composting, and (3) final curing. The factors which affect
the composting process include: mixing moisture content; percent of re-
cycled compost; aeration; and temperature and pH. Volume reduction of
about 70% with solids reduction of 30% can be obtained. The heat generated
(140 + 5°F) is sufficient to destroy most of the pathogenic bacteria. Com-
posting of wastewater 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 fre-
quently, 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 temperatures under pressure,
the gel-like structure of the sludge is destroyed and the bound water is libera-
ted. 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 con-
centrated and dewatered to about 40 to 50% solids content without use of
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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 waste-
water 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 extraction, 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 characteristics.
Various methods of sludge conditioning that are currently being practiced
include: (1) elutriation, (2) chemical conditioning, (3) freezing, and (4) ad-
dition 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 constituents that
interfere with sludge thickening and dewatering processes. As the elutriation
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, 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
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types of chemicals used in conditioning of wastewater sludge include: ferric
chloride (either alone or combined with lime); a combination of ferric chlo-
ride 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 conditioning effect
produced by freezing is believed to result from dehydration, and the pressure
exerted on the sludge particles by the ice structure. A significant increase
in solids content and dewaterability 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, news-
prints, 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 filter ability 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 costs when sludge is dried or incinerated; and increasing
the solids content so that the sludge is easily handled and disposed of. The
methods commonly used for sludge dewatering include: (1) sand bed drying;
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(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 common de-
watering method presently in use. Open sand bed dewatering is probably
the least expensive method of all. It has been used extensively in small
(<10, 000 pop.) communities. Variables effecting dewatering rates include:
(1) climate and atmospheric variations; (2) 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 dewatering 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 dewatering 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, temperature, and
chemical aids. Total solids of the cake range 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 dewatering facilities
used today, particularly when incineration is used for final disposal. Various
types of vacuum filters used include: (1) drum-type filters; (2) string-dis-
charge falters; (3) belt-type filters; and (4) coil-type filters. Factors effecting
dewaterability of the sludge include: (1) sludge solids content; (2) sludge age
and temperature; (3) sludge and filtrate viscosity; (4) sludge compressibility;
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and (5) the nature of the sludge solids. Chemicals are frequently used for
conditioning the sludge. Cake solids contents for various types of sludges
range from 15 to 35%.
Filter presses and plug presses have also been used for sludge dewatering
purposes. However, their use is very limited in the U.S. because of the
high cost of manual labor and maintenance of the system.
Wastewater sludge has been heat dried particularly in conjunction with pro-
duction of fertilizer or soil conditioner, in which sludge is generally de-
watered 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
dewatered by mechanical means prior to drying. The sludge drying tem-
perature commonly used is about 700°F.
Common methods for conveying the resulting liquid or dried sludge from the
wastewater treatment plant to a final disposal site include: (1) barging trans-
port; (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 wastewater 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 purposes, 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.
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Sludge is generally incinerated for two main reasons: (1) volume reduction,
and (2) solids reduction and sterilization. Methods used for sludge incinera-
tion 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 incinera-
tion. 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) com-
bustion. 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 (CO2, SC>2, NOX), 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 market-
able 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 treat-
ment 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.
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Sludge consists of a mixture of organic, and inorganic solid phases, sus-
pended 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 charac-
teristics 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 treatment plants
throughout the U.S. is about 13 million tons/year on a dry solids basis. In-
tensified water quality enhancement programs, stringent water pollution
standards, and the construction of additional wastewater 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 treat-
ment plants are far less than those produced by wastewater 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 sep-
tic conditions it has the disagreeable odor of putrefaction, and the solids
content ranges from 3 to 8%. Trickling filter humus has a brownish color;
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the fresh sludge has relatively inoffensive odor, but when it undergoes de-
composition, 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 content
ranging from 0. 2 to 4. 0%.
The solids fraction of the wastewater sludge is primarily composed of bio-
degradeable material (30%), stable organic matter (35%), and inert material
(35%). Further, about 60% of the total solids are dissolved solids, 20% are
settle able solids, and 20% are colloidal solids.
The characteristics effecting ultimate disposal and effective utilization of the
wastewater sludges include: settling characteristics, specific resistance,
flow characteristics, 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 sedimentation. 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: wastewater characteristics;
type of biological treatment the wastewater 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 concentra-
tion of the sludge varies from 0. 5 to 10% for raw sludges and from 2 to 10%
after digestion.
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Another physical characteristic of the sludge that is important in the design
of treatment processes such as vacuum filters, centrifuges, 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 encountered in removing water or conversely, the ability of the
sludge to retain water. Specific resistance values for wastewater sludge
7 ^
range from 10 x 10 sec /gr, when chemical coagulants are employed, to
as high as 2800 x 10' sec^/gr for pure activated sludge. Low values are
indicative of a sludge with rapid draining or filtering 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 in most cases.
Flow characteristics of the sludge become a significant 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 suspen-
sion, and (2) plastic flow. Numerous methods for calculating critical veloc-
ities, 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 ulti-
mate 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
respectively.
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
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60 to 80% on a dry weight basis, for 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. How-
ever, more useful and implicit tools are the COD and TOG tests. Sludge
also contains a variety of metallic ions, including toxic metals. Their rela-
tive concentrations depend mainly upon the origin of the wastewater.
Wastewater sludge contains many fertilizing elements. In comparison 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 pro-
cesses 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;
settle ability; filter ability; 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.
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The ultimate disposal of the vast quantities of sludge being generated through-
out 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.
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 economically feasible; and it should assure public health safety.
Ocean disposal of wastewater sludge by barging or through submarine out-
falls 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 environ-
mental 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. Possibilities 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 purposes.
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TABLE I
PRESENT METHODS OF MUNICIPAL SLUDGE DISPOSAL AND/OR UTILIZATION
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.
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.
8. By-product recovery.
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Application of dried treated sludge on land as fertilizer, or soil conditioner
is a viable ultimate disposal method. From the standpoint 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 wastewater sludges has been
reported in the literature. However, marketing 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, in most
cases, the specifications are rigid concerning product purity and concentra-
tion. 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 dis-
charge 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 a water course may not cause serious
health hazards as compared with wastewater and wastewater 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, interferring 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 disadvan-
tages, 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
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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 recovery. 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 replacement 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 substantially reduces the volume of sludge for final disposal,
complete separation of the impurities, such as iron, manganese, etc. is a
difficult job.
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UTILITY COAL ASH
The burning of coal produces an ash residue which is derived from the
inorganic mineral constituents in the coal and the organic material not
completely burned. In coal burning utility boilers, the coal ash residue
is collected from the bottom of the boiler unit (bottom ash) and from the
air pollution equipment through which the stack gases pass (fly ash). Over
46 million tons of coal ash were collected in 1972 by some 500 power plants
in the United States. The distribution of power plants defines the ash pro-
ducing regions of the country. The largest concentration of power plants
is in the middle Atlantic and the east north central states. There are very
few coal burning power plants west of the Mississippi River.
The coal ash residues recovered from the boiler units are primarily iron
aluminum silicates with additional amounts of lime, magnesia, sulfur trioxide,
sodium oxide, potassium oxide, and carbon. About 12 percent of the coal
burned is recovered as coal ash residue. A high percent of that ash is in
the glass state (50-90 percent), with small quantities of quartz, mullite,
magnetite, and hematite mineral phases. An average chemical analysis
for coal ash would be:
Si02
A1203
Fe203
CaO
MgO
Ti02
K20
Na20
so3
C
B )
P!
45%
25%
14%
4%
2%
1%
2%
1%
2%
4%
Trace
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Mn
Mo
Zn
Cu
__ Trace
Hg
U
Th
The specific chemical composition of a coal ash is primarily dictated by
the geology of the coal deposit and the operating parameters of the boiler
unit.
About 70 percent of the coal ash residue is collected as fly ash. For any
specific boiler unit the fly ash and bottom ash will have essentially the same
chemical composition except that the bottom ash will be lower in carbon
content. Fly ash generally occurs as fine spherical particulates having an
average particle diameter of 7M. The fly ash will range in color from light
tan to black, depending on the carbon content, and have an average specific
gravity of 2. 3. The pH of the fly ash will vary from 6. 5 to 11.5 and will
average about 11. About 20 volume percent of the fly ash will be composed
of very lightweight particles which float on the surface of the ash lagoon.
These lightweight particles have a true density of about 0.5 g/cc and are
termed cenospheres. These cenospheres are carbon dioxide and nitrogen
filled microspheres of silicate glass ranging in size from 20^ to 200/4.
The bottom ash is collected either as an ash or a slag depending on the
particular boiler design. The ash material is grey to black in color, quite
angular and has a porous surface. The slag particles are normally black
angular particles having a glass appearance. The bottom ash particles will
have an average particle diameter size of 2-1/2 millimeters and an average
specific gravity of 2. 5.
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Advancing boiler design technology and the establishment of stricter air
pollution codes for boiler facilities may alter the nature of the coal ash
produced in future years. The various proposed desulfurization processes,
coal fractionation processes, and new designs for electric generating facili-
ties can result in coal ash and slag products considerably different from
those currently being produced.
The coal fractionation processes used for obtaining clean gas or liquid fuels
and the reconstitution of the coal to obtain a clean low-ash, low-sulfur fuel
results in the production of slag and char residues at the conversion facility
rather than at the power plant. The liquefaction process produces a filter
cake of inorganic materials. The fluidized-bed gasification generates a
powder waste composed of the fluid media, the coal residue, and a calcium
sulfate precipitate. In the high temperature gasification process the residue
is a glassy slag. The chemical composition and physical characteristics of
these residues have not been well defined due to the relative newness of these
processes.
Conversion of existing boiler units to fluidized bed units will result in a
change in the nature of the coal ash recovered. Ash from this process will
be less vitrified, due to the lower operating temperatures. Also, the quan-
tity of crystalline material increase (quartz, magnetite, alumina, and cal-
cium sulfate) and the alkaline content is likely to be higher.
Several processes have been developed for meeting the newly established
codes for control of SO, emissions from stationary sources. A number of
these processes completely alter the nature of the collected fly ash and others
add a new residue material to the solid waste stream. Most of these processes
require the wet or dry injection of an alkaline powder (limestone, dolomites,
etc.) to absorb the gaseous sulfur in the stack effluent. The wet injection or
scrubbing process (limestone) which appears to be more prevalent, in most
cases, results in the generation of a new waste (CaSO^) rather than modifying
the fly ash. Preliminary calculations indicate that these wastes will most
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likely result in a doubling of utility residue waste. Since these desulfuriza-
tion processes are still largely in the development or pilot state, it is not
possible to adequately define the chemical characteristics at this time.
Since 1966, coal ash utilization has fluctuated around 15 to 16 percent of the
total ash collected in the United States. From data supplied by the Edison
Electric Institute it is apparent that the single largest application for coal
ash is as mineral fill material for roads and other construction products.
Average European usage of bituminous coal ash for 1972 was almost 27 per-
cent and in Belgium, France, Poland, the United Kingdom, and West Germany,
usage exceeded 50 percent. The two largest applications for European coal
ash were filler on construction sites and for concrete block.
Although a multitude of technically sound applications have been developed
for the utilization of coal ash, usage has been very limited. Yearly fluctua-
tions in the quantities of coal ash used in the various applications developed,
would suggest that firm markets have not been established for these coal
ash uses. At the present time, appreciable quantities of coal ash are only
being used as fill material for roads and other construction projects. The
use of coal ash as a replacement for cement in concrete and concrete pro-
ducts is starting to increase and a more stable market is being established.
The use of fly ash in concrete offers a number of technical advantages, e. g.,
improved mechanical strength and improved resistance to sulfate leaching,
etc. Fly ash, and boiler slag are also being used to an appreciable extent
for road base stabilization and as filler in asphalt. Boiler slag is particularly
noted for increasing the skid resistance of asphalt pavement. The use of
coal ash as a raw material in the manufacture of Portland cement is another
application where usage has increased during the past several years. Recent
research results indicate that large quantities of coal ash can also be ef-
fectively used for agriculture, land, and water reclamation projects. Fly
ash has been effectively used in reclaiming surface mine spoil (high pH of
ash neutralizes mineral soil), as a soil nutrient, and as an aid in the treat-
ment of polluted waters.
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A number of the applications developed for coal ash have the potential to
utilize the entire quantity of ash generated. These include agriculture and
land recovery, road base stabilization, structural fill, and cement and
concrete products.
Effective utilization of coal ash in the many defined applications requires that
the potential user be favorably impressed with the product and the product be
economically advantageous. The economic competitiveness of coal ash is
impaired by the discriminatory federal practices that favor virgin materials
in freight rates and depletion allowances. Improved federal economic policy
toward secondary materials like coal ash would enhance their utilization
potential.
CONCLUSIONS AND RECOMMENDATIONS
In 1972, approximately 46 million tons of coal ash were collected from the
burning of some 350 million tons of coal in over 500 utilities. About 16%
of the ash collected was utilized. Therefore, over 38 million tons of ash
had to be removed to disposal sites at the expense of the utility. At the
present time, disposal costs are approaching $2. 00/ton of ash disposed.
By 1980, coal consumption by the utilities, to meet expanding energy needs,
is predicted to be almost 500 million tons. The projected increase in coal
consumption coupled with the decreasing quality of available coal (higher
ash content) will result in substantially increased quantities of coal ash.
Stricter air pollution codes (reduction of particle and sulfur emission) will
also result in an increase in the quantity of coal ash collected.
The technology for a diversity of applications, for coal ash, has been well
established. The potential market for most of these applications is quite
good and several of these applications have potential markets which can
utilize all the ash collected. The major need at this time is the initiation
of programs which will encourage greater use of the coal ash in these ap-
plications. With the anticipated increase in coal ash collected and the
increase in disposal costs, the need for programs to stimulate ash utilization
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becomes more important. Some study should be devoted toward determining
the types of programs best suited for effectively stimulating increased ash
utilization. Studies characterizing ash residue from fluidized bed boiler
units, gasification and liquefaction processes, and desulfurization processes
are needed if effective utilization technology for these wastes are to become
available. Further, implementation of these new processes will result in
the generation of new waste products that can significantly add to the dis-
posal problem unless applications for these materials are available.
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MUNICIPAL INCINERATOR RESIDUES
Incineration is utilized for the disposal of approximately ten percent of the
collected municipal refuse, on a national basis. Annually, from 16 to 18
million tons of refuse are incinerated. It is estimated that in 1972 about
193 incinerators were operating in the U.S. providing a total capacity for
approximately 71, 000 tons of refuse per 24 hour day. From the reported
data, it appears that most incinerator facilities operate at about 70% of
their rated capacity. Most of the incinerators are located in the eastern
U.S. with New York, Massachusetts, Connecticut, Florida, and Ohio having
the largest number of incinerators. Since 1969, construction of new incin-
erators or rebuilding of existing facilities has decreased significantly. It
appears that the major factors for this decrease are the higher costs of
incinerator construction, and higher operation costs due to the institution
of stricter pollution regulations for incinerator operations. Capital costs
for an incinerator range between $6, 000 and $10, 000 per daily ton and
operating costs range between $5 and $20 per daily ton.
During incineration, furnace temperatures are between 1800 F and 2000°F
with flame temperatures at approximately 2500°F. This process results in
the reduction of the refuse incinerated to between 25 to 35% of its original
weight; and, on the average, to less than 10% of its volume. The resultant
residue after quenching is a wet, complex mixture of metal, glass, slag,
charred and unburned paper, and ash. The typical range of values obtained
for the various residue components is presented below.
RESIDUE COMPOSITION (%)
Material Range
metals 20-40
glass 10-55
ceramics, stones 1-5
clinker 15-25
ash 10-20
organics 1-10
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On a national basis, 4 to 6-1/2 million tons of incinerated residue are generated
annually, containing about 1-1/2 to 2 million tons of ferrous metal, 100, 000
to 200, 000 tons of nonferrous metal and 2 to 3 million tons of glass. In ad-
dition to the residue, about 1% of the refuse exits with the exhaust gases
leaving the furnace chamber. The particulate matter (or fly ash) retained
is predominantly minus 200 microns in size, and consists of wood and paper
ash, aluminum foil, carbon particles, metal pins and wire, glass, sand and
iron scale. The chemical analysis of this material is very similar to fly
ash from coal burning boilers.
The majority of the incinerator residue and fly ash is disposed of by burying.
However, some problems are associated with this method of disposal because
of potential water pollution from the water soluble portion of the residue.
Depending on the specific residue, from 1 to 6% is water soluble. In addition
to land fill, some communities are using the residue as a filler for road con-
struction (road bed). The City of Baltimore is screening out the fine fraction
for use as aggregate in asphalt. Several cities are salvaging the metal cans
from the residue for the copper smelting industry and for use in the manufac-
ture of Rebar. Several studies are now in progress to develop the technology
for recovering the glass and metal fractions from incinerator residue. A
pilot project by the Bureau of Mines has been relatively successful in develop-
ing a system for recovering the glass, ferrous metal, aluminum and other
nonferrous metals from the residue. A breakdown of the various products
which would be recovered from a 250 ton per day facility is presented below:
QUANTITIES OF THE VARIOUS PRODUCTS RECOVERED
FROM THE BUREAU OF MINES' INCINERATOR RESIDUE RECOVERY
PROJECT*
Project Tons/Day
+4 mesh iron 41
-4 mesh iron 35
aluminum 4
copper and zinc 3
colorless glass 69
colored glass 50
waste solids 48
*for a plant processing 250 tons/day
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A demonstration facility for residue recovery is scheduled for operation by
1975, at Lowell, Mass. The quality of the products recovered from the
residue and the economics of recovery have not been well determined. Pre-
liminary estimates indicate that a plant to process 50 tons per day in an eight
hour shift would cost about 2 million dollars and operating costs would be
9 to 11 dollars per ton of residue processed.
The degradation of the metal and glass resulting from the incineration opera-
tion may limit the market acceptance of these materials. During incineration
the ferrous metal is contaminated by copper and tin and undergoes considerable
oxidation. The glass is subjected to slagging and contamination from metal
and other minerals. Estimates for the revenue from the products of a ton
of residue have varied from $6 to $15. For distant markets, freight rates
become a major factor in the economics of the recovery process; and this is
further compounded by the higher rates for secondary materials. In the final
analysis, the economic viability for these recovery processes has yet to be
firmly established and until an actual unit is in operation, it will not be
possible to make a final determination on this matter.
The high cost of incineration, the institution of stricter pollution codes, and
the increased need for the conservation of national resources suggests an
uncertain future for conventional incineration, as indicated by the reduction
in the construction of new facilities. The development of advanced combus-
tion processes for urban refuse would appear to have a more promising po-
tential. The advanced processes under development include: waste heat
recovery for steam generation; high temperature incineration; fluidized bed
incineration; pyrolysis and hydrogenation of refuse and the processing of
refuse for use as a low-sulfur fuel supplement for coal burning furnaces and
boilers. The residue from many of these processes will be considerably
different from that obtained by conventional incineration. In high temperature
incineration, combustion is more complete. All the organics are eliminated
and the glass and metal is melted forming a slag, which after quenching is
a good aggregrate material. In the fluidized bed process, the refuse is
27
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usually shredded and the metal removed prior to combustion. The residue
is a powdery inorganic ash. Waste heat recovery for steam generation can
be incorporated with conventional incineration as well as with high tempera-
ture and fluidized bed incineration. The nature of the residue will be deter-
mined by the precombustion processes (metal, glass removal, etc.) and the
temperature of combustion. In the various pyrolysis processes the refuse
is shredded and the metal and glass removed prior to the destructive distil-
lation of the organic materials. One ton of refuse will yield from 154 to
230 pounds of char residue by this process. The shredded refuse with the
glass and metal removed can also be effectively used as a low-sulfur fuel
supplement. The residue from the refuse in this case would be combined
with the coal ash and recovered from the pit (bottom ash) and from the air
pollution equipment (fly ash). In all of these advanced processes, the resi-
due produced is primarily recovered as ash which can be used as fill in
various construction applications. Removal of the glass and metal prior to
combustion results in a residue that is easier to utilize and provides metal
to glass fractions of higher quality. The economics for the different refuse
disposal and recovery processes have been compiled by Midwest Research
and are presented next for purposes of comparison. These data were com-
piled in 1972 and are based on the economic conditions at that time. Although
the specific numbers quoted are not out of date the economic ratio between
systems is still relatively valid.
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TABLE OF CONTENTS
VOLUME II - MUNICIPAL SLUDGES
SUMMARY 1
INTRODUCTION 15
SLUDGE ORIGIN AND TYPE 17
SLUDGE QUANTITIES 19
Wastewater 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
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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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 157
SLUDGE TRANSPORTATION 159
SLUDGE REDUCTION PROCESS 164
Sludge Incineration 164
Pyrolysis 174
ULTIMATE DISPOSAL AND/OR UTILIZATION OF
MUNICIPAL WASTE-WATER 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
30
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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
31
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TABLE OF CONTENTS
VOLUME III - UTILITY COAL ASH
Page
SUMMARY 1
CONCLUSIONS AND RECOMMENDATIONS 5
COAL ASH CHARACTERIZATION 6
COAL ASH UTILIZATION 34
REFERENCES 46
APPENDDC I REGIONAL FUEL USE BY SELECTED
ELECTRIC UTILITIES 53
APPENDIX II ANALYSES AND FUSIBILITY OF ASH
FROM VARIOUS U. S. COALS 58
APPENDIX III ASH UTILIZATION 64
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TABLE OF CONTENTS
VOLUME IV - MUNICIPAL INCINERATOR RESIDUES
Page
SUMMARY 1
INCINERATOR RESIDUE CHARACTERIZATION 8
INCINERATOR RESIDUE UTILIZATION 50
REFERENCES 54
33
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TECHNICAL REPOR F DATA
(I'll r .(• iczJ l^±i.- ;, ,'(. "..v un r/.'i' /-el cnv 6c /
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