United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington DC 20460
Research and Development
EPA/600/S6-91/001 July 1991
Project Summary
Preliminary Risk Assessment for
Parasites in Municipal Sewage
Sludge Applied to Land
This preliminary risk assessment fo-
cuses on the probability of human In-
fection from protozoa and helminths,
usually referred to as parasites, in mu-
nicipal sewage sludge applied to land.
It is based on the Pathogen Risk As-
sessment computer model and meth-
odology described in Pathogen Risk
Assessment for Land Application of
Municipal Sludge. The report docu-
ments (1) the results of a literature re-
view designed to find data on parasites
required by the pathogens methodol-
ogy, and (2) the results of numerous
site-specific computer simulations,
running the Pathogen Risk Assessment
model with a wide range of values for
the parameters required. The param-
eters required for parasites are density
in sludge, transport and die-off rates in
environmental media, and minimum in-
fective dose. Locations selected for site-
specific application of the model in-
cluded counties in California, Florida,
Iowa, New Mexico, Tennessee, and
Washington. Model runs predicted
probabilities of infection of a human
receptor exposed to pathogenic para-
sites by a variety of pathways arising
from the use of sludge-amended soil to
grow vegetable crops, lawns, or forage
for cattle used for meat or milk. Data
gaps are identified and research priori-
ties are recommended.
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 fo-
cuses on the probability of human infection
from protozoa and helminths, usually re-
ferred to as parasites, in municipal sludge
applied to land. It is based on the Patho-
gen Risk Assessment computer model and
methodology described in Pathogen Risk
Assessment for Land Application of Mu-
nicipal Sludge.
Parasites are of health significance in
land application practices because they
tend to become concentrated in sludge
during sewage treatment processes and
because they can remain viable as envi-
ronmentally stable protozoan cysts or hel-
minth ova for months or years under fa-
vorable conditions. Although epidemiologi-
cal studies suggest little risk to human
health from parasites in treated municipal
sludge or wastewater applied to land, their
low minimum infective dose and persis-
tence in soil mean that the issue cannot be
dismissed.
Procedure
The report documents (1) the results of
a literature review designed to find data on
parasites required by the pathogens
methodology, and (2) the results of numer-
ous site-specific computer simulations,
running the Pathogen Risk Assessment
model with a wide range of values for the
parameters required. The most important
parameters required for parasites are (1)
density of viable parasites in treated sludge
Printed on Recycled Paper
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destined for land application; (2) die-off
rates in soil, dry participates, liquid aero-
sols, and water; (3) dispersion in the envi-
ronment, i.e., transport in water, soil and
air; and (4) minimum infective dose, which
for parasites is assumed to be MID « 1
since single eggs of helminths and single
cysts of protozoa have produced infections
in humans. Of these parameters, density
is site-specific and requires a standard
method for enumerating parasites, die-off
rate data are very limited, transport data are
essentially nonexistent, and infective dose
has been determined to be MID * 1.
Locations selected for site-specific ap-
plication of the model include Anderson
County, Tennessee; Chaves County, New
Mexico; Clinton County, Iowa; Highlands
County, Florida; Kern County, California;
and Yakima County, Washington. The sites
were chosen to provide diversity in geo-
graphic location, topography, soil type,
rainfall pattern and temperature.
Results and Discussion
Density and viability of parasites in
sludge are site-specific, based on source
of wastes, species of parasites present,
climate, and efficacy of sludge treatment.
Densities of parasites have been reported
to be generally higher in sludges from
southern than from northern states. How-
ever, accurate risk assessment would re-
quire site-specific analysis of density levels
by standard methods for enumerating
parasites in sludges and soil. Parasite
densities reported in the literature range
from 100-2000 ova/kg dry wt in dried sludge
and 0-30,000 cysts or ova/kg in liquid
sludge; however, the values are highly
dispersed and geometric means are in the
range of 200-2000 ova/kg dry wt. Accord-
ing to EPA regulations, composted sludge
for distribution and marketing (D&M) must
have no more than 1 ovum/g or cyst/g
volatile sludge solids. Based on the litera-
ture ranges, values suggested for use in
the Pathogen Risk Assessment model are
5000 ova or cysts/kg for liquid sludge, 500
ova or cysts/kg for dried sludge and 1000
ova or cysts/kg for composted (D&M)
sludge. The value used in the risk assess-
ment was 5000 ova or cysts/kg for all
practices
During storage under unfavorable con-
ditions, ova and cysts may become inactive
(non-infective) before they die. Death may
be followed by disintegration. Although
some of the studies discussed include in-
formation on infectivity, in many cases only
viability of eggs and cysts was reported,
and some studies reported only occurrence,
not viability.
Inactivation of parasites appears to be
most closely tied to temperature during
treatment or storage, with higher tempera-
tures contributing to increased inactiva-
tion. Temperatures in the 45 -55°C range
are likely to kill resistant parasites within a
few hours. Alternate freezing and thawing
reduce viability more rapidly and to a
greater extent than constant above- or
below-freezing temperatures.
Field studies of parasite-contaminated
sludge applied to agricultural plots, how-
ever, have not produced a direct statistical
correlation between viable Ascaris ova
concentration and solar radiation, relative
humidity or soil temperatures. In fact, no
statistical correlation was found between
parasite egg concentration and chemical,
physical or biological parameters.
Data on die-off rates are very limited,
but a published 90% die-off time of 270
days implies an exponential rate of
10<-ooooi54)/nour published ranges for die-
off are approximately 10('0001>/hour to
10(-°0005>/hour at ambient temperature.
Based on these ranges, suggested values
in the model for die-off of ova and cysts
are:
During application/incorporation
0 for Temp <20°C;
10(-o.oooi78) or 0.00041/hour for
2040;
In moist soil
0 for Temp < 20°C or for 8 hours after
irrigation;
10(-o.ooo23) or o.000533/hour for 20 <
Temp < 40;
10<-°«67Vhour or 0.7845/hour for 40 <
Temp < 50;
10(-oi25> or o.25/hour for Temp>50;
On crop surfaces
10<-°667> or 0.7845/hour at all tempera-
tures;
In water
10K> 00023) or o.000533/hour at all tem-
peratures.
Model runs showed that within narrow
limits, the probability of human infection by
parasites as a result of exposure to soil
contaminated with sewage sludge is pro-
portional to the concentration of organisms
in the sludge, the amount of sludge applied
and the amount of contaminated soil to
which the individual is exposed, either by
casual contact or ingestion of food grown
in the contaminated soil. Many of the pa-
rameters of the model seemed to have
little bearing on the probability of infection,
apparently because they had no effect on
the number of organisms to which the
human receptor was exposed in each ex-
posure compartment or they exerted their
effect after the time of maximum exposure.
The probability of infection was sensitive
to the rate of inactivation or die-off of the
parasite ova or cysts and to the method of
application. According to the model, human
exposure via subsurface application of
sewage sludge would be unlikely because
it is believed that ova or cysts cannot move
significant distances through soil.
The model predicted that the most sig-
nificant potential source of infection would
be exposure to runoff water and sediment
transported to an onsite pond after rainfall.
Model runs indicated that it would be pru-
dent to limit access to runoff water and
sediment from a sludge-amended field, ei-
ther by mulching to reduce runoff, ditching
and diking to contain the runoff or restrict-
ing access to any onsite ponds receiving
runoff.
Various model runs predicted that it
was unlikely that a significant number of
ova or cysts would be transported off-site
either by droplet aerosols or wind-blown
dusts. The model also predicted that
parasite, moving through the soil column
into groundwater was unlikely. Therefore,
one can infer, based on the model param-
eters used, that there is relatively little risk
to human health from parasitic infection
via inhalation of contaminated fugitive dust
emissions or ingestion of contaminated
groundwater.
Using a benchmark probability of in-
fection of 1x10-* as an indicator of suffi-
cient protection of human hearth, a waiting
period appeared to be unnecessary for
consumption of aboveground crops con-
taminated with 0.1 g soil/crop unit. A wait-
ing period of 18 months appeared to be
adequate for belowground crops, whose
consumption is currently forbidden for 5
years after sludge application.
The current version of the Pathogen
Risk Assessment model does not address
some of the properties of parasite survival
in soil. Mathematical descriptions of the
die-off of parasite ova, cysts and oocysts
as a function of temperature and moisture
are not yet adequate to allow construction
of algorithms for die-off rates. It may be
appropriate to add a diurnal cycle to the
model's temperature algorithm. Other
changes may be limited by the constraint
that the model should run on a personal
computer.
Conclusions and
Recommendations
Although detailed data on survival and
transport of parasites in soil are lacking,
the Pathogen Risk Assessment model ap-
pears to confirm the general observations
in the literature that parasites are persis-
tent, justifying land-use restrictions. Model
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runs implied that restrictions on the con-
sumption of belowground crops may be
overly conservative, and they indicated that
the most significant potential source of
infection would be exposure to parasites in
runoff water and sediment transported to
an onsrte pond after rainfall.
The following research priorities are
recommended to allow development of a
definitive risk assessment for parasites in
land-applied sludge:
For Helminths:
Standard quantitative methods for
examining helminths in sludge and
soil samples;
Data on transport in water, soil and
aerosols;
Die-off rates in water, soil and aero-
sols;
The relationship of those decay rates
to environmental conditions, previous
sludge treatment, method of sludge
application and various effects of crop
cover.
For Protozoa:
Quantitative methods for examining
protozoa in sludge and soil samples;
and
Quantitative data on occurrence and
survivability of protozoa in treated
sludge.
If results indicate that protozoa survive in
sludge, the following additional research
needs to become a priority:
Data on transport in water, soil and
aerosols;
Die-off rates in water, soil and aero-
sols;
The relationship of hose decay rates
to environmental conditions, previous
sludge treatment, method of sludge
application and various effects of crop
cover.
3 &U.S. GOVERNMENT PRINTING OFFICE: 1»9I - 548-028/40016
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Norm Kowal is the EPA Project Officer (see below).
The complete report, entitled "Preliminary Risk Assessment for Parasites in
Municipal Sewage Sludge Applied to Land," (Order No. PB91-182352/AS; Cost:
$23.00, 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
EPA/600/S6-91/001
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