United States
Environmental Protection
Agency
National Center for
Environmental Assessment
Cincinnati OH 45268
Research and Development
EPA/600/SR-95/016 February 1996
4>EPA Project Summary
Pathogen Risk Assessment
Methodology for Municipal
Sewage Sludge Landfilling and
Surface Disposal
This document describes a method-
ology and associated computer model,
sludge-only landfill or surface disposal
(SLDGFILL), for assessing the risk to
humans of pathogens from disposal of
treated municipal sewage sludge. The
purpose of the SLDGFILL model is to
determine the probability of infection
of a human receptor from pathogens in
a sludge-only landfill (monofill) or in a
surface disposal site. The ultimate ob-
jective for the model is to assist the
U.S. Environmental Protection Agency
(EPA) in developing technical criteria
for regulatory activities. More immedi-
ate objectives include the use of the
model to perform actual pathogen risk
assessments and to identify research
needs.
This Project Summary was developed
by EPA's National Center for Environ-
mental Assessment, Cincinnati, OH, to
announce the availability of a software
program that is fully described in a sepa-
rate document of the same title (see
software package ordering information
at back).
Specific enteric bacteria, viruses, proto-
zoa, and helminths are identified as the
pathogens of concern in sludge. The ex-
posure pathway addressed by the
SLDGFILL model is the ingestion by a
human receptor of groundwater from a
drinking water well that has been infil-
trated by microbial pathogens from a
sludge disposal site. Viruses were identi-
fied by the sensitivity analysis as the prin-
cipal organism for which the model
demonstrates a potentially significant
health hazard.
Quantity of treated sludge, application
frequency, and other parameters specific
to a sludge disposal site are entered ini-
tially by the user. Pathogen parameters
required for SLDGFILL include (1) density
of pathogens in treated municipal sewage
sludge destined for landfilling or surface
disposal; (2) infectivity; (3) inactivation
rates in sludge, soil, and groundwater;
and (4) dispersion or transport in the envi-
ronment. The parameters to which the
model proved to be most sensitive are
pathogen density in sludge, infective dose,
inactivation rate in water, and sludge-to-
soil resuspension coefficient.
The model test results imply that the
total number of sludge pathogens in the
surface disposal site is less important to
health risk than is the concentration in the
sludge. To limit risk offsite, it may be use-
ful to regulate the concentration of sludge
pathogens, either by monitoring sludge
treatment or by diluting highly contami-
nated sludge with less contaminated
sludge, compost, clay, or other material.
The range of reported minimum in-
fective doses for pathogenic bacteria
is 10 - 1011 organisms; for viruses,
the range is 9 X 10'1 - 9 X 104 virus
particles, 2 X 10'1 - 5.5 X 106 PFU, or
1 - 1 X 1076 TCID50; for protozoa, the
range is 1 - 100 cysts; and for helm-
inths, 1 egg has been known to cause
infection.
For the model, survival of pathogens in
soil and water is presented in terms of
inactivation rate constants (Iog10 day1),
which may differ by several orders of mag-
nitude even for a specific pathogen. Low
temperatures and median pH levels pro-
long pathogen survival in water, and those
factors as well as moisture content con-
tribute to increased survival in sludge and
soil. The ranking of pathogen persistence
in the environment, from longest to short-
est, is helminth eggs, viruses, bacteria,
and protozoan cysts.
-------
The depth to the groundwater presents
the greatest barrier to the transport of
pathogens and, hence, to exposure and
risk. Filtration and adsorption are the pro-
cesses responsible for limiting pathogen
transport through the unsaturated zone.
The size of the organism, therefore, de-
termines which pathogen will be trans-
ported the greatest distance. In general,
viruses, the smallest of the pathogens con-
sidered, have the potential to travel far-
ther in the environment. Large particles
like helminth eggs and protozoan cysts
typically do not migrate into groundwater
because of the physical barrier provided
by the soil, unless there are vertical cracks
or fissures. Due to their persistence, po-
tential for transport, and low infectious
dose, viruses seem to represent the worst
case when estimating human health risk
from landfilling or surface disposal of sew-
age sludge.
The SLDGFILL model for pathogen
risk assessment has been run with many
combinations of input parameters to
simulate the transport of sewage sludge
pathogens from a landfill or surface dis-
posal site to a nearby drinking water
well. The subsequent risk of infection to
humans who drink from the well was
estimated for each run. The probability
of infection is calculated using a beta-
Poisson model. Conservative exposure
assumptions include a drinking water
consumption rate of 2 L/day and param-
eters describing highly infective patho-
gens. The model indicates that risk from
pathogens in groundwater near a sur-
face disposal site or landfill is typically
below a level of concern. However, if a
risk exists, viruses are more likely to be
the source of that risk than bacteria or
parasites.
The parameters to which the SLDGFILL
model are most sensitive are resuspension
coefficients, which describe the adsorp-
tion of pathogens to sludge and soil par-
ticles. Other parameters to which the
model is sensitive are infective dose,
pathogen density in sludge, and inactiva-
tion rate in water. Data on infective doses
are scarce, making further research nec-
essary for reliable use of the model to
predict health risks. It is likely that viruses
present a greater health risk because they
are expected to have a lower minimum
infective dose and are more readily trans-
ported through soil.
Future research should be oriented to-
ward satisfying the following information
needs to allow more realistic modeling of
human health risk from pathogens in
landfilled and surface-disposed municipal
sludge:
field data on subsurface transport,
in both the saturated and unsatur-
ated zones, of bacteria and viruses;
inactivation rates of pathogens un-
der field conditions in sludge, soil,
and water;
solids-to-water suspension factors
applicable to sludge- and soil-bound
pathogens;
leaching characteristics of sludge-
bound pathogens;
interaction of factors affecting patho-
gen resuspension from sludge and
soil; and
parameters needed to describe in-
fective doses of selected indicator
species and strains of pathogens
in sludge.
-------
This Project Summary was written by the staff of Science Applications International
Corporation, Oak Ridge, TN 37831.
Norman Kowal is the EPA Project Officer (see below).
The complete document, consisting of paper copy and computer diskette, entitled
"Pathogen Risk Assessment Methodology for Municipal Sewage Sludge
Landfilling and Surface Disposal,"(OrderNo. PB96-501911; Cost:$X.OO, subject
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
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management
Research Laboratory (G-72)
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use
$300
EPA/600/SR-95/016
------- |