v'/EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-82-092 Jan. 1983
Project Summary
A Critical Review of
Wastewater Treatment Plant
Sludge Disposal by Landfilling
James C. S. Lu, Robert J. Stearns, Robert D. Morrison,
and Bert A. Eichenberger
Current landfilling practices are
reviewed for the disposal of sludges
from wastewater treatment plants
(WWTP). Major emphases of the report
include (1) assessments of the possible
impacts of sludge landfilling on the
environment and the public health, (2)
available control technologies and
management options, (3) sludge
treatment processes and the charac-
teristics of WWTP sludges, and (4)
design approaches and operational
procedures for current sludge land-
filling practices.
Codisposal landfills (which dispose
of sludge along with refuse or soil) are
generally favored over sludge-only
methods (which dispose of sludge
alone in either trenches or area fills).
Advantages of the codisposal landfills
are that they are the least costly
alternative, they generate the least
public opposition, and they provide for
more effective land use. However,
codisposal sites do have greater
potentials for increased odor, aerosol,
noise, and public health risks.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Disposal of wastewater treatment
plant (WWTP) sludge has historically
occurred by land burial, ocean dumping,
internment in evaporation (or percola-
tion) lagoons and ponds, incineration,
agricultural use, and land spreading.
But annual increases in sludge produc-
tion, current bans on ocean dumping of
sludge, and the high costs of incineration
have intensified the need for sludge
landfilling. Recent sludge disposal
research assesses the site-specific
problems of sludge landfilling as well as
the environmental benefits.
Sludge landfilling (with or without the
addition of municipal refuse) presents
many environmental and public health
risks and managerial options. Environ-
mental and public health risks include
leachate contamination of water and
soil resources, destruction of native
fauna and flora, obnoxious odors,
aerosol and dust generation, pathogen
transmission, and other related nuisances.
Management decisions about sludge
landfills encompass a range of topics
uncommon to conventional refuse
landfills, including acceptable types of
sludge for landfilling, the degree of
sludge stabilization or dewatering
required, and appropriate sludge/refuse
mixing ratios for codisposal. The degree
of environmental and public health risks
posed by sludge landfills will be greatly
influenced by such management deci-
sions.
This study reviews current landfilling
practices for the disposal of sludges
from wastewater treatment plants.
Major subjects covered in the report are
as follows:
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1. Sludge treatment processes and
the physical, chemical, and micro-
biological characteristics of WWTP
sludges;
2. Design approaches and opera-
tional procedures for current
sludge landfilling practices;
3. Potential environmental and public
health impacts of sludge landfilling
procedures; and
4. Available pollution control tech-
nologies and management options.
Characteristics of WWTP
Sludge
The physical, chemical, and micro-
biological quality of sludges from
municipal wastewater treatment plants
depends on the nature of incoming raw
wastewater and the effectiveness of the
treatment plant process. Most of the
sludge is readily biodegradable, though
measurable quantities of persistent
organics, metals, and minerals are
invariably present. The sludge may also
contain pathogenicorganismsthat have
survived the wastewater treatment
process. Without wastewater pretreat-
ment, industries may be the major
source for contributing trace metals,
refractory organics, or salts to the
WWTP sludges.
WWTP sludge is generally classified
according to the stage of wastewater
treatment (primary, secondary, or
tertiary). In addition, the sludge may be
categorized by the various sludge
treatment processes, including thicken-
ing, stabilization, disinfection, condi-
tioning, dewatering, drying, and high-
temperature processing. Within each of
these categories are numerous process
alternatives that ultimately affect the
disposal options for the sludge.
Sludge moisture content and per-
meability are critical physical charac-
teristics that can have pronounced
impacts on leachate generation and
sludge engineering properties. Excessive
residual moisture will complicate hand-
ling and landfill operations and hasten
the formation of landfill leachates. Even
dewatered sludge has poor engineering
properties because it is largely non-
compressible and has little bearing
strength.
The chemical characteristics of WWTP
sludges often impose limitations on
sludge disposal options. Though the
concentration and mobility of chemical
constituents vary greatly, sludge can be
highly contaminated. Sludge consti-
tuents that are potentially harmful to
the environment include nitrogen,
phosphorus, trace metals, major inor-
ganic ions, and certain organic com-
pounds. Industrial wastewater can be a
particularly important source of trace
metals and synthetic organic compounds.
Sludge microorganisms that may be
of health concern are bacteria, viruses,
parasites, and fungi. Bacteria, viruses,
and parasites (including protozoa,
helminths, and nematodes) are con-
sidered primary pathogens and are
incorporated into the sludge during its
formation in wastewater processing.
Fungi are considered secondary sludge
pathogens because they are only
numerous in sludge when given the
opportunity to grow during some treat-
ment or storage process. The concen-
trations of these microbes can be
diminished through various sludge
processing practices such as anaerobic
digestion, aerobic digestion, lime stabi-
lization, composting, and heat and
radiation treatment. But, the literature
reviewed in this study clearly indicates
that many pathogens actually survive
wastewater and sludge treatment
processes. Thus WWTP sludges can
pose various health hazards.
Current Sludge Landfill
Practices
Current landfilling practices for
WWTP sludge disposal fall into three
major categories:
• Sludge-only trench
narrow trench
wide trench
• Sludge-only area fill
area-fill mound
area-fill layer
diked containment
• Codisposal
sludge/refuse mixture
sludge/soil mixture
Each of these landfilling methods can
demonstrate site-specific advantages in
terms of design approach, operational
procedures, or effectiveness in protect-
ing the environment and the public
health.
Sludge-Only Trench
Disposal of sludge in a sludge-only
trench involves the excavation of a
trench into which the sludge is placed.
The excavated soil is then placed over
the sludge as a cover, thereby eliminating
the need for substantial soil import. The
practice is usually limited to areas with
suitable hydrogeological conditions or
in locations where surface application
or other alternatives are not viable.
Sludge-only trenching represents
potential despoiling of prime agriculture
soils, since the nutrient rich topsoil i
mixed and buried in a subsoil/surfac
soil mixture. Another disadvantage i
the occasional need for sludge limin
before placement in the trench. Thi
approach can have its advantage!
however, such as reduction of odor:
immobilization of heavy metals, an
reduced pathogen levels.
Methods for sludge-only trenchin
are generally designated as wid
trenches (greater than 3.0 m, or 10ft) c
narrow trenches (less than 3.0 m
Variables affecting the depth and lengt
of trenches include the followim
depth to bedrock, high water table, sc
stability, and operational limitatior
(i.e., site configuration, equipment, ar
physiographic characteristics). Thoug
the narrow-trench operation is usual
land-intensive, associated enviror
mental and public health risks ai
relatively minimal. Wide-trench sludc
disposal often creates a greater potenti
for leachate generation, vectors, odor
and possible'public health impact
Liners can minimize the risks of leachai
generation. Odors, vectors, and publ
health impacts are related to the largi
sludge exposure periods and greati
sludge application rates.
Sludge-Only Area Fill
Sludge-only area fill is a method I
which WWTP sludge is disposed <
above the original ground surfac
Three methods may be used for area-f
sludge applications—area-fill moun
area-fill layer, and diked containmer
The sludge-only area fills require mixir
with soil as a bulking agent to provii
sludge stability. Since sludge-only an
fills are usually located where a hi;
groundwater table or near-the-surfai
bedrock prevails, difficulties often arii
in obtaining sufficient onsite soils.
common design feature of sludge an
fills is the inclusion of surface wat
drainage control measures. Leacha
from the sludge and surface runoff fro
the site require control measures sui
as upland runoff diversion ditches ai
onsite leachate collection systems.
Codisposal
Codisposal is the practice of disposi
of sludge in a conventional refu
landfill. This method can offer signifies
cost savings through economies of sc<
(e.g., additional land acquisition a
equipment costs are expected to
low). Codisposal landfilling methods £
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viewed favorably when compared with
sludge-only alternatives because of
advantages such as lower capital costs,
less public opposition, and more effective
land use. Conventional codisposal
approaches include the use of both
sludge with refuse mixtures and sludge
with soil combinations. Both practices
are increasingly used as viable disposal
options.
Municipal refuse can absorb large
volumes of sludge moisture and prevent
or delay attainment of landfill field
capacity. Thus the sludge/refuse mix
ratio is a critical operational specification
for codisposal practices. A proper mix
ratio ensures that water released from
the sludge can be intercepted by the
refuse without depleting the refuse
adsorption capacity. Major factors
governing sludge/refuse mix ratios are
sludge solids content (or moisture
content), refuse composition, and depth
of d'isposal.
In a sludge/soil operation, sludge is
mixed with soil and the mixture is
applied as a cover for areas of a refuse
landfill. In this operation, the sludge is
disked or mixed into indigenous soil and
later retrieved for use as an intermediate
or final cover. Use of sludge/soil
mixtures requires greater manpower
and equipment and may have greater
environmental and public health impacts
than mixing sludge with refuse.
Environmental and Public
Health Impacts
Various environmental impacts are
associated with the landfilling of WWTP
sludges. Sludge landfilling can adversely
affect leachate composition, leachate
volume production, air quality, and land
quality. Because of the toxic and
pathogenic nature of many WWTP
sludges, their placement in a landfill
may lead to the formation of highly
contaminated leachates or surface
runoff that could migrate from the
landfill site. But the critical factor
influencing environmental impacts is
the method of sludge application.
The risk of transmitting disease is of
major concern for the various sludge
disposal practices. The direct pathways
for disease transmission from sludge
landfilling operations include aerosols,
vector transport, direct contact, ground-
water, and surface water runoff. The
groundwater and surface water runoff
pathways present the greatest risks for
disease transmission. Although the po-
tential health effects and virulence of
sludge and refuse contaminants have
been well documented, no known out-
breaks of disease have occurred among
persons exposed to sludge landfilling
operations.
The literature suggests that good
design and operation can mitigate many
of the potential environmental and
public health problems. For instance,
codisposal practices were found to
produce weaker leachates than munici-
pal refuse landfills. But the potentials
for increased odor, aerosol, and noise
are greater at codisposal sites. Possible
risks to the public health are also
greater because more viable contamin-
ants are available to enter the ground-
water. An acceptable sludge disposal
Table 1. Suitability of WWTP Sludges for Landfilling According to Potential Odor and Operation Problems
Type of Sludge
Sludge Only Landfilling Codisposal Landfilling
Suitability Reason Suitability Reason
Liquid - Unstabilized
Gravity-thickened primary, WAS* and primary, and WAS
Flotation-thickened primary and WAS, and WAS
without chemicals
Flotation-thickened WAS with chemicals
Thermal-conditioned primary or WAS
Liquid - Stabilized
Thickened anaerobic digested primary and primary,
and WAS
Thickened aerobic digested primary and primary,
and WAS
Thickened lime stabilized primary and primary, and WAS
Dewatered - Unstabilized
Vacuum-filtered, lime-conditioned primary
Dewatered - Stabilized
Drying-bed-digested and lime-stabilized
Vacuum-filtered, lime-conditioned digested
Pressure-filtered, lime-conditioned digested
Centrifuged, digested, and lime-conditioned digested
Heat-Dried
Heat-dried digested
NS
NS
NS
NS
NS
NS
NS
S
S
S
S
OD, OP
OD. OP
OP
OD, OP
OP
OP
OP
NS
NS
NS
MS.
MS
MS
MS
S
S
S
S
OD, OP
OD, OP
OD, OP
OD, OP
OP
OP
OP
High-Temperature-Processed
Incinerated dewatered primary and primary, and WAS
Wet—Air Oxidized Primary and Primary, and WAS
S
NS
—
OD, OP
S
MS
—
OD, OP
* Abbreviations: WAS - Waste-activated sludge
NS = Not suitable
MS = Marginally suitable
S = Suitable
OD = Odor problems
OP = Operational problems
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alternative must include measures for
minimizing the environmental and
public health impacts.
Control Technology and
Management Options
Landfill ing of WWTP sludge can be an
acceptable disposal method, providing
environmental and public health impacts
are controlled. Control technologies
that can be implemented are as follows:
1. Leachate and groundwater control,
including collection, treatment,
ultimate disposal, and monitoring.
2. Surface water control, including
run-on control, and runoff reten-
tion and treatment.
3. Air emissions control, including
control of gases, odor, dust, and
volatile sludge constituents.
4. Erosion control to minimize wind
and water erosion.
5. Mud control to prevent equipment
from slipping onsite or tracking
mud offsite.
6. Vector control to limit vector
access and to discourage burrow-
ing animals.
7. Aesthetic considerations, including
site closure planning and visual
barriers.
8. Noise control to prevent excessive
noise.
9. Personal health and safety control
for onsite workers and others near
the facility.
Management options depend on
current WWTP sludge landfill practices,
the types of WWTP sludges placed in the
fill, related environmental and health
impacts, and other special constraints
of WWTP sludge la ndfilling. Appropriate
control technologies should be applied
for anticipated environmental impacts.
Consideration should be given to
operational factors such as sludge
dewatering and stabilization or special
regulatory constraints that exist in most
states.
The feasibil ity of various management
options is primarily influenced by the
selected landfill method and the type of
incoming sludge (Table 1). Additional
considerations include cost evaluations,
regulatory limitations, and geographical
distinctions.
Conclusions
Codisposal landfills are generally
favored over sludge-only alternatives
because they are the least costly,
generate the least public opposition,
and provide for more effective land use.
The potentials for increased odor,
aerosol, noise, and public health risks
are greater at codisposal sites.
The need for sludge stabilization
depends strictly on the landfill site and
the engineering design. Sludge de-
watering, .however, is imperative for all
landfilling operations. Available control
technologies such as leachate and
methane gas control must be applied at
sludge landfill sites to prevent odor and
operational problems and to ensure
overall public acceptance.
The full report was submitted in
fulfillment of contract No. 68-03-2886
by Calscience Research, Inc., under the
sponsorship of the U.S. Environmental
Protection Agency.
James C. S. Lu, Robert J. Stearns, Robert D. Morrison, and Bert A. Eichenberger
are with Calscience Research, Inc., Huntington Beach, CA 92647.
Wendy J. Davis-Hoover and Laura Ringenbach were the EPA Project Officers
(see below for present contact).
The complete report, entitled "A Critical Review of Waste water Treatment
Plant Sludge Disposal by Landfilling," (Order No. PB 83-111 518; Cost:
$25.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
For information, contact Norms Lewis:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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