United States
Environmental Protection
Agency
Off ice of Health'and
Environmental Assessment
Washington, DC 20460
Research and Development
EPA/600/SR-92/064 June 1991
& EPA Project Summary
Preliminary Risk Assessment for
Viruses in Municipal Sewage
Sludge Applied to Land
This preliminary risk assessment fo-
cuses on the probability of human in-
fection from enteric viral pathogens in
municipal sewage sludge applied to
land. Based on the Pathogen Risk As-
sessment computer model and meth-
odology described in Pathogen Risk
Assessment for Land Application of
Municipal Sludge, this study reports
(1) the results of a literature review
designed to find the data required to
model human exposures to pathogenic
viruses in sewage sludge and (2) the
results of numerous site-specific com-
puter simulations run with the Patho-
gen Risk Assessment Model using a
wide range of values for the input pa-
rameters: minimum infective dose, den-
sity in sludge, die-off rates and trans-
port in environmental media.
Counties in California, Florida, Iowa,
New Mexico, Tennessee and Washing-
ton were selected for site-specific ap-
plication of the model. Model runs pre-
dicted probabilities of infection of a
human receptor exposed to pathogenic
viruses by a variety of pathways aris-
ing from using sludge-amended soil to
grow vegetable crops, lawns, or forage
for cattle used for meat or milk.
Information needs are identified to
guide further research, and model modi-
fications are recommended.
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 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 viral pathogens in mu-
nicipal sludge applied to land. Based on
the Pathogen Risk Assessment computer
model and methodology described in
Pathogen Risk Assessment for Land Ap-
plication of Municipal Sludge, this study
reports (1) the results of a literature re-
view designed to find the data on patho-
genic viruses required by the pathogens
methodology and (2) the results of numer-
ous site-specific computer simulations, run-
ning the Pathogen Risk Assessment Model
with a wide range of values for the param-
eters required. The parameters required
for viruses are (1) minimum infective dose
(MID); (2) density of infective viruses in
treated sludge destined for land applica-
tion; (3) inactivation rates in soil, dry par-
ticulates, liquid aerosols and water; and
(4) dispersion in the environment, i.e.,
transport in water, soil and air. Human
receptors whose probability of infection by
viruses is calculated by this model in-
clude: (1) an onsite person exposed by
ingestion of soil, vegetables or forage, or
by inhalation and subsequent ingestion of
aerosols; (2) an offsite person exposed to
particulate or liquid aerosols carried by
wind; (3) a food consumer who eats veg-
etable crops, meat or milk produced on
sludge-amended soil; (4) a groundwater
drinker who consumes water from a well
near but not on the sludge application
site; and (5) a pond swimmer who ingests
a small amount of water while swimming
in an onsite pond that receives the sur-
face runoff from the application site.
Printed on Recycled Paper
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To provide diversity in geographic loca-
tion, topography, soil type, rainfall pattern
and temperature, six locations were se-
lected for site-specific application of the
model: Anderson County, TN; Chaves
County, NM; Clinton County, IA; Highlands
County, FL; Kem County, CA; and Yakima
County, WA.
Procedure
This analysis assumes that viruses are
transported into subsurface soil and sub-
sequently into groundwater and are in-
cluded in any droplet aerosols formed by
spray application, as well as in any par-
ticulate aerosols formed by disturbance of
the soil by wind or by cultivation. It also
assumes that the viruses are inactivated
at a characteristic rate that depends on
the ambient temperature and the medium
in which they are found.
An initial sensitivity analysis was per-
formed using site-specific data for Ander-
son County, TN. Main program variables
used in the model run were varied over a
range of values. In general, the default
value of a given parameter was compared
w'rth a reasonable higher and a reason-
able lower value. Many of the parameters
of the model seemed to have little bearing
on the probability of infection, apparently
because they ultimately had no effect on
the number of viral particles to which the
human receptor was exposed in each ex-
posure compartment or because they ex-
erted their effect on survival or transport
after the maximum probability of infection
had occurred. In model runs using data
from all sites, variables showing no effect
on maximum probability of infection were
eliminated from further consideration.
Infective doses have been reported in
the literature to be as low as 1 infective
particle, although this number varies ac-
cording to the type of virus and the labo-
ratory method used for detection. As a
conservative assumption, this minimum
value was used for the model runs. Litera-
ture values for virus density in treated
sludge were so variable that no single
number could be selected as typical. How-
ever, 2000 virus particles/kg was chosen
as representative of viral density in
composted sludge and 100,000 particles/
kg in digested sludge. Inactivation rates
reported in the literature (often given as
log reductions in numbers of virus par-
ticles per hour) range from 7.1 xlO* to
1.6x10-' logs/hour in soil, 1.6X10"4 to
1.4x10-l tags/hour in water, and 4.9x10'5
to 8x10'7 togs/second in aerosols. Like the
density values, these rates are quite vari-
able. Because these literature rates were
lower than the default values for the model
runs, inactivation rates were decreased in
soil, water and droplet aerosols for many
of the computer simulations. Information
on dispersion of viruses in the environ-
ment is limited in its applicability to gener-
ating a rate of transport in environmental
media.
Results and Discussion
Using baseline values for parameters,
the maximum probabilities of infection were
evaluated. Results show that the inactiva-
tion rate of virus particles is extremely
important in determining whether a ground-
water well is likely to become contami-
nated and in determining how long sur-
face soil or surface water is likely to re-
main infectious. The results also demon-
strate the importance of accurate charac-
terization of inactivation rates for viruses
of different kinds in the various transport
and exposure media.
In all model runs, the probability of in-
fection offsrte was calculated as zero, in-
dicating that the calculated quantities of
liquid and dry particulate aerosols and con-
centrations of viruses in the aerosols were
too low to provide an infective dose to the
modeled receptor.
Consumption of contaminated vegetable
crops was shown by model calculations to
be a potential source of human infection,
provided that inactivation rates were suffi-
ciently low or harvesting times were suffi-
ciently close to application of the sludge.
In addition, infection via food crops was
sensitive to the relative fractions of patho-
gens transferred among surface soil, sub-
surface soil, and crop surface and to the
type of crop or fraction of the total crop
grown above-ground, below-ground, or on-
ground.
Contamination of meat or milk by vi-
ruses from sewage sludge did not appear
to pose a significant risk to human health.
Transport of viruses via groundwater to
an offsite well was not shown by this model
to be a major risk, but exposure by con-
taminated groundwater was shown to be
likely if the rate of inactivation of viruses in
water was less than the default values.
The probability of infection was related to
the periodic introduction of pathogens to
groundwater by the infiltration of rainwa-
ter. The most important parameter related
to subsurface transport of viruses ap-
peared to be the inactivation rate of vi-
ruses in water. The results also showed
an increase in probability of infection at
the offsite well whenever the time required
for the viruses to reach the well was de-
creased.
Exposure to contaminated surface wa-
ter, represented by the swimmer in an
onsite pond, was the most significant
source of infection. A peak in probability
of infection occurred after each rainfall,
when additional contaminated surface wa-
ter and soil were washed into the pond.
Conclusions and
Recommendations
Although detailed data on survival and
transport of viruses in soil are limited, the
model appears to confirm the general ob-
servations in the literature that viruses in
treated sewage sludge present a potential
health risk, justifying land-use restrictions.
However, model runs implied that restric-
tions may be overly conservative.
Model runs show significant onsite ex-
posures. A probability of infection greater
than 1x1 QA (tentatively chosen as a bench-
mark for sufficient protection of human
health) is likely during application and in-
corporation of liquid treated sludge for ag-
ricultural practices. If the initial viral con-
centrations in composted sludge are higher
than about 50/kg, the user is likely to be
at risk of infection. A person applying
sludge or present at the application site
during or soon after sludge application
could probably reduce the risk by wearing
a protective mask and washing thoroughly
before handling food.
The most significant potential source of
infection was exposure to runoff water and
transported sediment after rainfall. Model
runs indicated that it would be prudent to
limit access to runoff water and sediment
from a sludge-amended field, either by
mulching to reduce runoff, ditching and/or
diking to contain the runoff or restricting
access to any onsite ponds receiving run-
off.
Reports of offsite infection by viruses in
sludge-amended soil (particulate aerosols)
or in aerosols from liquid treated sludge
were not found. Mode] runs confirm the
low probability of offsite infection.
U.S. EPA restrictions on growing food
crops in sludge-amended soil, while nec-
essary for protection against potential
health hazards from parasites, appear to
be more stringent than required by typical
or even worst-case inactivation rates for
. viruses on crops. Model results suggest
that the appropriate waiting period before
access to sludge-amended land or con-
sumption of crops grown thereon should
probably be variable, depending not only
on intended land use, as is currently the
case, but also on sludge application rate
and pathogen concentration. In calculat-
ing a safe waiting period, conservative
assumptions should be made about
amounts of soil ingested with crops.
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The following information is needed to
improve the usefulness of the Pathogen
Risk Assessment Model and to allow for a
more reliable risk assessment of the land
application of sewage sludge:
• Simple and accurate standardized
methods for detecting and quantify-
ing, by type, pathogenic viruses in
treated sludge destined for land ap-
plication, in final distributed and mar-
keted sludge products, and in envi-
ronmental media;
• Improved understanding of minimum
infective doses, particularly low-dose
effects and minimum infective doses
for sensitive subjects;
• More accurate persistence andjrans-
port data on all pathogenic viruses of
major concern in sludge;
• Development of an index of soil types
that would correlate capacity for sol-
ute transport with 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
virus transport in groundwater; and
• Epidemiologic studies evaluating en-
teric viral transmission.
The following revisions would improve
the accuracy of the model:
• Revision of default parameter values,
especially for inactivation rates in
aerosols and temperature-dependent
inactivation rates in soil and water;
• Revision of temperature-dependent
inactivation algorithms;
• Incorporation of factors for humidity
and temperature in inactivation equa-
tions for aerosols;
• Incorporation of subroutines for sub-
surface transport under conditions of
transient flow; and
• Incorporation of factors to allow for
subsurface transport through solution
channels, cracks, etc.
In addition, field validation of the model's
predictions is necessary before the Patho-
gen Risk Assessment Model can be con-
sidered an accurate predictor of health
risk.
•U.S. Government Printing Office: 1992— 648-080/60017
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Norm Kowal is the EPA Project Officer (see below).
The complete report, entitled "Preliminary Risk Assessment for Viruses in Municipal
Sewage Sludge Applied to Land,"(Order No. PB92-182336/AS; Cost: $26.00;
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 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
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PERMIT No. G-35
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Penalty for Private Use $300
EPA/600/SR-92/064
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