&EPA
United States
Environmental Protection
Agency
Office of Health and Environmental
Assessment
Washington, DC 20460
Research and Development
EPA/600/S6-90/001 Sept. 1990
Project Summary
Development of Risk
Assessment Methodology for
Surface Disposal of Municipal
Sludge
This is one of a series of reports
that present methodologies for
assessing the potential risks to
humans or other organisms from the
disposal or reuse of municipal
sludge. The sludge management
practices addressed by this series
include land application practices,
distribution and marketing programs,
landfilling, surface disposal,
incineration and ocean disposal. In
particular, these reports provide
methods for evaluating potential
health and environmental risks from
toxic chemicals that may be present
in sludge. This document addresses
risks from chemicals associated with
surface disposal of municipal sludge.
These proposed risk assessment
procedures are designed as tools to
assist in the development of
regulations for sludge management
practices. The procedures are
structured to allow calculation of
technical criteria for sludge
disposal/reuse options based on the
potential for adverse health or
environmental impacts. The criteria
may address management practices
(such as site design or process
control specifications), limits on
sludge disposal rates or limits on
toxic chemical concentrations in the
sludge.
This Project Summary was
developed by EPA's Environmental
Criteria and Assessment Office,
Cincinnati, OH, to announce key
findings of the reseach project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
This document addresses health and
environmental risks associated with
human and non-human exposure to
chemical contaminants in sludge from the
surface disposal of municipal sludge, and
explains the development of a
methodology by which criteria for sludge
surface disposal may be derived. The
term "surface disposal" refers to the
permanent disposal or long-term storage
of sludge in uncovered lagoons or
impoundments. Included in the
methodology are facilities at which
sludge accumulates and is stored for
long periods as a result of the use of
lagoons for wastewater treatment. A
lagoon or impoundment is an earth basin
used to deposit untreated or digested
sludge. Anaerobic and aerobic digestion
stabilize organic solids in untreated-
sludge lagoons. Stabilized solids settle
and accumulate at the bottom of the
lagoon.
No national survey has yet been
calculated to identify the number of
plants using surface disposal or to
estimate the volumes of sludge stored or
disposed in this manner. However, 679
plants participating in the 1986 Needs
survey reported using forms of sludge
treatment and disposal other than
landfilling, distribution and marketing,
land application, incineration" and ocean
disposal. Some of these plants may have
engaged in practices that fit the definition
of surface disposal. Furthermore, 727
plants reported having sludge lagoons,
although some of these may have been
used for temporary storage or treatment
and are thus not included in the surface
disposal definition.
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Types of Impoundments
The impoundments to be considered
in this document can be categorized into
three groups: lagoons for long-term
storage, lagoons for permanent disposal,
and wastewater treatment lagoons in
which sludge is stored for long periods.
Each of these types of facilities is
discussed briefly below.
Wastewater treatment plants may
store sludge from primary, secondary, or
tertiary treatment processes in lagoons,
with the intention of later exhuming the
sludge for ultimate reuse or disposal
elsewhere. In most cases, sludge is
periodically removed from the facility,
although the frequency of such removal
will vary among facilities.
Long-term sludge storage may serve a
number of purposes. Sludge storage may
be an integral part of a plant's overall
sludge management plan. For example,
sludge may be stored in impoundments
over the winter until weather conditions
permit land application. Storage of
sludge will also tend to decrease water
content, thereby decreasing hauling costs
when the sludge is exhumed for
permanent disposal elsewhere. Smaller
plants may store sludge to accumulate
quantities that can be practically
disposed or reused. Storage
impoundments may also be used
intermittently, in emergency situations,
when normal sludge management
operations are overwhelmed.
Although they are not technically
"disposal" facilities, lagoons for long-
term storage of sludge have been
included in methodologies for deriving
criteria for "surface disposal" for two
reasons. First, this practice may pose
health and environmental risks as severe
as or worse than those associated with
permanent disposal. Second, future
plans for removing sludge from "storage"
impoundments may be uncertain. As
economic or regulatory conditions
change, plans for the removal of sludge
from "temporary" impoundments have
also been known to change. Since the
ultimate fate of sludge stored in such
impoundments cannot be known with
certainty, analysis of risks associated with
their continued or permanent existence is
appropriate.
In certain cases, sludge may be
deposited in on-site impoundments
without further planning for removal.
Permanent surface disposal may be the
only practical disposal option for plants
that have accumulated very large
quantities of sludge onsite over a number
of years. It may also become the de facto
disposal method for plants that fail to
identify satisfactory final disposal options
for the sludge currently stored in lagoons.
Wastewater treatment lagoons are often
used by small wastewater treatment
facilities because they are less expensive
than alternative treatment processes.
Because such lagoons typically receive
relatively low volumes of wastewater,
they can accumulate sludge in a bottom
layer for years (or even decades) before
it begins to interfere with the treatment
process. Although the main purpose of
these lagoons is to treat wastewater
before discharge, the lagoons also serve
as long-term storage for the sludge, and
may fall within the definition of surface
disposal of sludge. In addition, the
lagoons are sometimes aerated, leading-
to increased volatilization of sludge
contaminants
Assumptions Used in Risk
Assessment
The purpose of a sludge storage or
disposal impoundment influences the
manner in which the sites are managed.
For example, sludge in impoundments
used for storage between land application
seasons will be emptie'd more frequently
than sludge from wastewater treatment
lagoons at small plants. All of the surface
disposal practices described here pose
potential risks from human and non-
human exposure. Each practice also
possesses unique features that affect its
associated risks. Long-term storage
impoundments undergo periodic sludge
removal, which may increase or decrease
movement of sludge contaminants into
the environment. Permanent disposal
impoundments might present risks
through future land uses of the disposal
site (although these risks are not
considered by the present methodology).
There follows a brief listing ~6T
assumptions used for estimating risks
from these sludge management
practices:
1. Sludge storage or disposal
occurs on plant property, and
the public has no access to the
affected areas.
2. Sludge impoundments will be
required to have berms, dikes or
other surface runoff controls that
effectively eliminate significant
risks of exposure from flooding
or accidental releases.
3. Periodic removal of sludge from
long-term storage lagoons allows
their continuous use over
extended periods of time.
4. Sludge that accumulates in long-
term storage or wastewater
treatment lagoons is removed
prior to plant closure, and before
any conversion of the land to
other uses can take place.
5. Sludge deposited in permanent
disposal lagoons is never
exhumed.
Conditions Affecting Risk
A number of environmental conditions
and management practices affect the
risks posed by long-term sludge storage
impoundments. Management or design
requirements can be developed, based
on an evaluation of the following factors,
to minimize risks from surface disposal:
1. Concentrations of contaminants
in the sludge will influence the
magnitude of risk posed by long-
term sludge storage or
permanent disposal.
2. Physical characteristics, such as
distance to groundwater and
surface water, soil type and
other geohydrologic features of
the site, and proximity to human
or non-human populations will
influence the rate of migration of
contaminants and the potential
for exposure.
3. The length of time sludge is
stored (i.e., the frequency of
sludge removal from the
impoundments) may affect the
mobility of sludge contaminants.
Frequent removal may decrease
the length of time sludge
contaminants are available for
leaching, but could also disturb
the underlying stabilized sludge
layer that may inhibit
contaminant movement from
_..._,., ——unlined impoundments. •- —
4. The use of synthetic or clay
liners will reduce the potential
for contaminant movement into
groundwater.
5. The use of berms and proper
siting can reduce risks
associated with surface runoff
and flooding.
6. For permanent disposal facilities,
restrictions on future access and
land use through a notice in the
deed may be required once the
facility is closed in order to
prevent inadvertent direct human
contact with disposed sludge.
Alternatively, the sludge disposal
area may be encapsulated at
closure in order to prevent future
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direct contact with sludge if land
is converted to another use.
7. Aeration of wastewater treatment
lagoons will affect the
volatilization and biodegradation
rates of the organic sludge
contaminants.
Improper management practices at a
particular facility can result in additional
risks to human health or the environment;
the methodology presented here does
not include techniques for assessing risks
associated with such practices.
Differences in potential risk associated
with siting of surface disposal facilities
are considered by the methodologies
insofar as they can be represented by the
input parameters required for model
calculations.
Summary
The methodology presented in this
document is designed to derive criteria
for lagoons used for both long-term
storage and permanent disposal of
sludge. The methods are intended to
apply to all types of municipal wastewater
sludges, regardless of the extent of
industrial contribution to the treated
wastewater, and regardless of the type of
wastewater treatment processes that
generate the sludge. Risks from these
surface disposal facilities can occur
through a variety of pathways of potential
human and non-human exposure. This
document discusses the selection of
pathways for inclusion in this
methodology, and the identification of
"most exposed individuals" and "most
exposed populations" at greatest risk
through each pathway, and provides a
detailed discussion of methods for
assessing potential risks and deriving
criteria for the three exposure pathways
thought to be of most concern:
groundwater, surface water, and air.
it\). S. GOVERNMENT PRINTING OFFICE: 1990/748-012/20111
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Norman Kowal is the EPA Project Officer (see below).
The complete report, entitled "Development of Risk Assessment Methodology
for Surface Disposal of Municipal Sludge," (Order No. PB 90-261 561/AS;
Cost: $31.00, subfect to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S6-90/001
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