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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