United States
                    Environmental Protection
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
  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

  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.

  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

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-
   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-
   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
   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-
   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.

  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-
   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
                                                                                        U.S. Government Printing Office: 1992 648-080/60017

 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
Center for Environmental
Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300