United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington, D.C. 20460
Research and Development
EPA/600/S6-90/002 Apr. 1990
&EPA Project Summary
Pathogen Risk Assessment for
Land Application of Municipal
Sludge
Volume I: Methodology and
Computer Model
Volume II: User's Manual
This document describes a
methodology and associated com-
puter model for assessing the risk to
humans of pathogens in treated
municipal sewage sludge applied to
land. Land application of sludge in
this methodology refers to the dis-
tribution of sludge on or just below
the soil surface where it is employed
as a fertilizer or soil conditioner for
growing human food-chain and non-
food-chain crops. The two categories
of land application addressed in this
model are (1) agricultural utilization
and (2) distribution and marketing
(D&M), and the source of microbial
pathogens is (1) liquid or (2) dried or
composted municipal sewage sludge.
This Project Summary was
developed by EPA's Environmental
Criteria and Assessment Office,
Cincinnati, OH, to announce key
findings of the research project that is
fully documented in two separate
volumes of the same title (see Project
Report ordering information at back).
Introduction
Section 405 of the Clean Water Act
requires the U.S. Environmental Protec-
tion Agency to develop and issue
regulations that identify: (1) uses for
sludge including disposal; (2) specific
factors (including costs) to be taken into
account in determining the measures and
practices applicable for each use or
disposal; and (3) concentrations of
pollutants that interfere with each use or
disposal. To comply with this mandate,
the U.S. EPA has embarked on a
program to develop four major technical
regulations: land application, including
distribution and marketing; landfilling;
incineration and surface disposal. The
development of these technical regula-
tions requires a consideration of
pathogens as well as chemical constitu-
ents of sludge. Public concern related to
the reuse and disposal of municipal
sludge often focuses on the issue of
pathogenic organisms. The purpose of
this report is to describe a proposed
methodology and associated computer
model designed to assess the potential
risks to human health posed by
pathogens in municipal sewage sludge
applied to land as fertilizer or soil
conditioner.
Volume I: Methodology and Computer
Model describes the conceptual frame-
work of the risk assessment methodology
and the structural organization, including
assumptions and components, of the
computer model. Volume II: User's
Manual contains background information
to provide the user with an understanding
of the actual functioning of the model.
This information includes descriptions of
operating variables and their default
values, explanations of the various
subroutines, and the mathematical basis
for process and transfer functions.
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Approach
The approach used for the model
provides a structure capable of support-
ing both stochastic and deterministic
mathematical relationships, i.e., it is a
dynamic model that can incorporate site-
specific data while allowing process
functions to be dependent on environ-
mental factors, such as temperature and
rainfall. The model structure provides a
flexibility that permits addition and/or
deletion of sludge management practice
compartments as well as modifications in
process and transfer functions. The
model is designed to run on a personal
computer with a minimum of 540 KB of
free memory. Currently limited by a lack
of data, the model will be able to utilize
data gathered in the future to enhance its
predictive accuracy.
The purpose of the model is to
determine the probability of infection of
the human receptor from pathogens in
the land-applied sludge. The ultimate
objective is to use the model to assist
EPA in its regulatory activities, but the
immediate uses include (1) further
development of the pathogen model as a
research and risk assessment tool and (2)
the application of the methodology in the
performance of actual pathogen risk
assessments.
The five municipal sewage sludge
management practices addressed by the
model are: application of liquid treated
sludge (1) for production of commercial
crops for human consumption, (2) to
grazed pastures, and (3) for production of
crops processed before animal consump-
tion; and application of dried or
composted sludge, (4) to residential
vegetable gardens, and (5) to residential
lawns.
Model Structure
The computer model represents the
compartments and transfers among
compartments of the five management
practices. The compartments are the
various locations, states, or activities in
which sludge or sludge-associated path-
ogens exist; they vary to some extent
among practices. In each compartment,
pathogens either increase, decrease, or
remain the same in number with time, as
specified by "process functions" (growth,
die-off or no population changes) and
"transfer functions" (movement between
compartments). The population in each
compartment, therefore, generally varies
with time and is determined by a
combination of initial pathogen input,
"transfer functions" and "process
functions." The populations of pathogens
in the compartments representing human
exposure locations, together with ap-
propriate intake and infective dose data,
are used to estimate human health risk.
Considering modern disposal prac-
tices, almost any pathogenic organism
can be found in municipal sewage.
Because of the difficulty of designing a
model that could accurately simulate the
survival arid environmental movement of
more than a few microbial species,
organisms or organism groups were se-
lected to represent the enteric pathogens
most commonly found in sludge. The
current version of the model deals with
only three of these selections: Salmonella
spp. representing the bacteria; Ascaris
lumbricoides the parasites (both hel-
minth worms and protozoa); and
enteroviruses (a grouping of several
animal viruses), the enteric viruses.
Exposure of an individual to enteric
pathogens can lead to (1) no effect, (2) a
subclinical (asymptomatic) infection or (3)
a clinical (symptomatic) infection.
Although subclinical infections are not
clinically detectable, that individual by
either direct or indirect transmission of
the pathogenic organisms may cause
disease to develop in others. In this
methodology, infection rather than
disease is used to measure risk.
Exposure pathways, i.e., migration
routes of pathogens from or within the
application or disposal site to a target
organism or receptor, for sludge applied
to land include the following:
• inhalation or ingestion of emissions
from application of sludge or tilling of
sludge/soil;
• inhalation or ingestion of windblown or
mechanically generated particulates;
• swimming in a pond fed by surface
water runoff;
• direct contact with sludge-contam-
inated soil or crops (including grass,
vegetables, or forage crops);
• drinking water from an offsite well;
inhalation and subsequent ingestion of
aerosols from irrigation;
• consumption of vegetables grown in
sludge-amended soil;
• consumption of meat or milk from
cattle grazing on or consuming forage
from sludge-amended fields.
Since the model provides only an
approximation of environmental transport
mechanisms, it does not represent every
possible exposure pathway. It does,
however, trace the flow of pathogens
through the major routes leading to
possible human exposure.
Infection
The dose required to cause infection
is based on the virulence or infectivity of
the pathogenic organism and on the
susceptibility of the exposed population
or individual receptor. The "minimum
infective dose" or MID is typically the
dose required to infect 50% of the
population. The uncertainty in measuring
infectious doses greatly weakens the
power of any quantitative risk assess-
ment. The model is designed, therefore,
so that the user can supply a best
estimate of infectious dose for the
particular pathogen and practice being
modeled.
Risk assessments ordinarily proceed
from source to receptor. That is, the
source, or sludge disposal/reuse practice,
is first characterized, and contaminant
movement away from the source is then
modeled to estimate the degree of
exposure to the human receptor. Health
effects are then predicted based on the
estimated exposure and dose-response
relationships. This computer model sums
the hourly exposures of a human
receptor to pathogens in each exposure
compartment and computes the daily
(24-hour) probability of the human recep-
tor receiving an exposure exceeding an
infective dose (e.g., for Salmonella the
default MID = 10).
Uncertainty Analysis
Many factors contribute to the
uncertainties associated with the present
risk assessment model. Chief among
these is the lack of quantitative data
describing the processes involved. Even
when available, data are highly variable
with regard to (1) the initial
concentrations of microbial pathogens in
wastewater and sludge; (2) processes of
microbial transport and inactivation; (3)
dose-response relationships; and (4)
exposure levels and receptor
susceptibility.
A sensitivity analysis was performed,
but because of the large number of input
parameters and the uncertainty related to
the values of parameters, it should be
viewed as preliminary. However, the
analysis does indicate that the model is
very sensitive to the inactivation rate of
microorganisms in soil, as well as to the
parameters used to calculate the frac-
tions of pathogens transferred from
surface soil to subsurface soil, from
subsurface soil to groundwater and from
surface soil to surface runoff water.
Accordingly, these parameters should be
selected with great care, especially as
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they are all likely to be site-specific.
Because the data available to support
choices of the values are limited,
research efforts should be directed to
these areas in order to increase the
accuracy of the model.
Model Use
The user of the model has the option
of modifying many of the operating
variables used by the program to
calculate the number of organisms
present in the compartments. These
variables include initial pathogen concen-
tration; organism type; environmental
parameters such as soil temperature and
moisture, air temperature, windspeed,
rainfall amount and frequency; and
parameters descriptive of the land
application practice modeled such as size
of field or garden, type of crop, irrigation
method and frequency, timing and yield
of harvest or size of cattle herd. There
are also variables for the subroutines that
represent rainfall, groundwater and crop
preparation practices. Operation of the
model is governed by a series of
compartment differential equations, trans-
fer factors and times, and process
functions. These equations calculate the
increase or decrease of pathogens within
each compartment and the transfer of
pathogens from one compartment to
another at intervals during the model run.
Transfers may be conditional on a
particular applications option such as
spray irrigation, on time after initiation of
the practice or on physical processes
such as rainfall or dust storms. The more
complex transfer functions are those
resulting from generation of particulate
emissions by wind erosion or tilling, from
generation of liquid aerosols during spray
irrigation using wastewater, from transport
of soil-bound microorganisms by surface
runoff associated with rainfall or from
subsurface transport of pathogens
associated with groundwater movement.
Conclusions
Preliminary conclusions drawn from
sample runs of the model suggest that
the most likely candidate for infection is
an onsite human receptor who ingests
pathogens from direct contact or from
contaminated surface runoff confined
onsite; the probability of infection of an
offsite human receptor seems relatively
low if surface runoff does not move
offsite.
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Worm Kowal is the EPA Project Officer (see below).
The complete reports, entitled "Pathogen Risk Assessment for Land Application
of Municipal Sludge, Volume I: Methodology and Computer Model,"
(Order No. PB 90-171 901/AS; Cost: $23.00) and "Pathogen Risk
Assessment for Land Application of Municipal Sludge, Volume II: User's
Manual," (Order No. PB 90-171 919; Cost $31.00) (costs sub/ect to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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 Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S6-90/002
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