United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington DC 20460
Research and Development
EPA/600/S6-91/006Jan. 1992
EPA Project Summary
Preliminary Risk Assessment for
Bacteria in Municipal Sewage
Sludge Applied to Land
Section 405 of the Clean Water Act
requires the U.S. Environmental Protec-
tion Agency to develop and issue regu-
lations that identify: (1) uses for sludge
Including disposal, (2) specific factors
(Including costs) to be taken into ac-
count in determining the measures and
practices applicable for each use or
disposal, and (3) concentrations of pol-
lutants that interfere with each use or
disposal. To comply with this mandate,
the U.S. EPA has embarked on a pro-
gram 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 patho-
gens as well as chemical constituents
of sludge. Public concern related to the
reuse and disposal of municipal sludge
often focuses on the Issue of patho-
genic organisms.
This report is one of a series whose
purpose is to use the methodology de-
scribed in Pathogen Risk Assessment
for Land Application of Municipal Sludge
(EPA/600/6-90/002a,b) to develop pre-
liminary assessments of risk to human
health posed by parasites, bacteria, and
viruses in municipal sewage sludge ap-
plied to land as fertilizer or soil condi-
tioner. The preliminary risk assessment
includes a description of the most criti-
cal data gaps that must be filled before
development of a definitive risk assess-
ment, and recommends research priori-
ties.
This Project Summary was devel-
oped by EPA's Environmental Criteria
and Assessment Office, Cincinnati, OH,
to announce key findings of the research
project that Is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
This preliminary risk assessment study
focuses on the probability of human infec-
tion from enteric bacterial pathogens in
municipal sludge applied to land. It is based
on the Pathogen Risk Assessment com-
puter model and methodology described in
Pathogen Risk Assessment for Land Ap-
plication of Municipal Sludge.
This document reports the results of a
literature review designed to find the data
on pathogenic bacteria required by the
pathogen's methodology, and the results
of numerous site-specific computer simu-
lations, running the Pathogen Risk As-
sessment Model with a wide range of values
for the parameters required. The param-
eters required for bacteria are (1) minimum
infective dose; (2) density of viable bacte-
ria in treated sludge destined for land ap-
plication; (3) die-off rates in soil, dry
particulates, liquid aerosols, and water; and
(4) dispersion in the environment, i.e., trans-
port in water, soil, and air.
Procedure
Six sites were chosen to provide diver-
sity in geographic location, topography,
soil type, rainfall pattern, and temperature.
Locations selected for site-specific appli-
cation of the model include Anderson
County, TN; Chaves County, NM; Clinton
-------
County, IA; Highlands County, FL; Kern
County, CA; and Yakima County, WA.
An initial sensitivity analysis was per-
formed using site-specific parameters for
Site 1, Anderson County, TN. Main pro-
gram variables used in the model run were
varied over a range of values to determine
the sensitivity of the model to variations in
conditions. In general, the default value of
a given parameter was compared with a
reasonable higher and a reasonable lower
value, where the high and low values were
taken from available literature or estimated
when literature values were not available.
In this analysis, it is assumed that bac-
teria are transported into subsurface soil
and subsequently into groundwater and
are included in any droplet aerosols formed
by spray application, as well as in any
paniculate aerosols formed by disturbance
of the soil by wind or by cultivation. It is
also assumed that the bacteria die at a
characteristic rate that depends on the
ambient temperature and the medium in
which they are found.
Conclusions
The risk of infection from bacterial
pathogens in treated sewage sludge ap-
pears to be small when judged by model
results, but there are a number of factors
the model does not address, including
regrowth in composted or D&M sludge.
The model runs indicated that significant
exposures are likely only if the number of
organisms is very high, either because the
concentration of bacteria in the treated
sludge is unrealistically high or because a
high application rate is used. Significant
exposures occurred only onsite, either by
direct contact or by swimming in a pond
containing runoff. However, runoff and sur-
face transport of bacterial pathogens to
the onsite pond do not appear to present a
major health risk. Exposure by direct con-
tact immediately after sludge application
could be a source of infection, although
the risk of infection decreases very rapidly,
so that the cumulative risk of infection from
a single application is typically only slightly
higher than the maximum daily risk. The
results suggest that if the infective dose is
>20, the probability of infection becomes
minimal.
The results of the model runs clearly
indicate that the highest risk of infection
should occur during and immediately after
application of the sludge. Die-off and dilu-
tion by soil should subsequently reduce
the number of infectious organisms very
rapidly. Aside from the expected depen-
dence of exposure on total numbers of
pathogens present and the infectious dose,
the most significant effects on exposure
appear to be related to die-off rates and to
dry particulate aerosol formation. Fractional
transfers of pathogens from soil to subsoil
and to soil surface water were also signifi-
cant, as was the volume of the onsite pond
in which the contaminated soil surface wa-
ter was diluted.
The results described above do not
support the requirement for an extended
waiting period before use of sludge-
amended soils. Bacterial concentrations in
all of the exposure media decreased so
rapidly that a waiting period of a few days
at most should be sufficiently protective.
However, compost and D&M sludge prod-
ucts designed for use in the home garden
can allow multiplication or regrowth of bac-
teria, such as Salmonella, resulting in ex-
tremely variable pathogen densities and
the possibility of a higher dose of patho-
gens upon ingestion of crops.
Recommendations
The following information is needed to
improve the usefulness of the Pathogen
Risk Assessment Model and to allow for
more reliable risk assessment of land ap-
plication of sewage sludge:
Simple and accurate standardized
methods for quantifying, by species
and strain, pathogenic bacteria in
treated sludge destined for land ap-
plication, in final distributed and mar-
keted (D&M) sludge products, and in
environmental media;
• Improved understanding of minir
infective doses, particularly low-di
effects and MIDs for sensitive sub-
jects;
Additional information on regrowth of
bacteria in compost and D&M sludge,
including factors enhancing or limit-
ing regrowth;
More accurate survival and transport
data on all pathogenic bacteria of
major concern in sludge, especially
retardation coefficients for. transport
of bacteria in saturated soil;
Development of an index of soil types
that would correlate capacity for sol-
ute transport and suitability for sludge
application (also valuable for onsite
waste disposal or solid waste dis-
posal);
• Research on subsurface injection of
sludge and the relative probability of
bacterial transport in groundwater;
and
Epidemiologic studies evaluating
whether there is a correlation be-
tween bacterial infections (not nec-
essarily disease) and bacterial
aerosols.
Future modifications of the Pathogen
Risk Assessment Model that may improve
its accuracy include the following:
The model should be changec
consider runoff of pathogens fa.
unincorporated sludge when rainfall
occurs in the first 24 hours.
A transfer factor could be added to
the model to allow for redistribution
of pathogens from subsurface to sur-
face soil when the field is plowed.
For a better description of sludge
use on public parks and golf courses,
which are more likely to have ponds,
it might be beneficial to add the op-
tion for existence of a pond onsite.
The limits of Subroutine RAINS
should be further characterized to
establish operating boundaries for
input variables.
-------
Norm Kowal is the EPA Project Officer, (see below).
The complete report, entitled "Preliminary Risk Assessment for Bacteria in Municipal
Sewage Sludge Applied to Land," (Order No. PB92-126820/AS; Cost: $35.00, 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:
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
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
------- |