United States
                   Environmental Protection
                   Agency
National Center for
Environmental Assessment
Cincinnati OH 45268
                   Research and Development
EPA/600/SR-95/016   February 1996
4>EPA       Project  Summary
                   Pathogen  Risk Assessment
                   Methodology for  Municipal
                   Sewage  Sludge  Landfilling and
                   Surface  Disposal
                     This document describes a method-
                   ology and associated computer model,
                   sludge-only landfill or surface disposal
                   (SLDGFILL), for assessing the risk to
                   humans of pathogens from disposal of
                   treated municipal sewage sludge. The
                   purpose of the SLDGFILL model is to
                   determine the probability of infection
                   of a human receptor from pathogens in
                   a sludge-only landfill (monofill) or in a
                   surface disposal site. The ultimate ob-
                   jective for the model is to assist the
                   U.S. Environmental Protection Agency
                   (EPA) in developing  technical criteria
                   for regulatory activities. More immedi-
                   ate objectives include  the use of the
                   model to perform actual pathogen risk
                   assessments and to identify research
                   needs.
                     This Project Summary was developed
                   by EPA's  National Center for Environ-
                   mental Assessment, Cincinnati, OH, to
                   announce the availability of a software
                   program that is fully described in a sepa-
                   rate document of the same title (see
                   software package ordering information
                   at back).
                     Specific  enteric bacteria, viruses, proto-
                   zoa, and helminths are identified as the
                   pathogens  of concern  in sludge. The ex-
                   posure  pathway addressed  by the
                   SLDGFILL model  is  the ingestion  by  a
                   human receptor of groundwater from  a
                   drinking  water well that has been  infil-
                   trated by  microbial  pathogens from  a
                   sludge disposal site. Viruses were identi-
                   fied by the  sensitivity analysis as the prin-
                   cipal organism  for which the model
                   demonstrates a potentially  significant
                   health hazard.
                     Quantity of treated sludge, application
                   frequency,  and other parameters specific
                   to a sludge disposal site are entered ini-
                   tially by the user. Pathogen parameters
required for SLDGFILL include (1) density
of pathogens in treated municipal sewage
sludge destined for landfilling or surface
disposal; (2) infectivity;  (3) inactivation
rates in sludge,  soil, and groundwater;
and (4) dispersion or transport in the envi-
ronment. The parameters to which the
model proved to be most sensitive are
pathogen density in sludge, infective dose,
inactivation rate in water, and sludge-to-
soil resuspension coefficient.
  The model test results imply that the
total number of sludge pathogens in the
surface  disposal site is less important to
health risk than is the concentration in the
sludge. To limit risk offsite, it may be use-
ful to regulate the concentration of sludge
pathogens, either by monitoring  sludge
treatment or by diluting  highly contami-
nated sludge with  less contaminated
sludge, compost, clay, or other material.
  The range of reported minimum in-
fective  doses for pathogenic bacteria
is 10 - 1011 organisms; for viruses,
the range is 9 X 10'1 - 9  X  104 virus
particles, 2 X 10'1  - 5.5 X  106 PFU, or
1 - 1 X 1076 TCID50; for protozoa, the
range is 1 - 100 cysts;  and for helm-
inths, 1 egg has been known to cause
infection.
  For the model, survival of pathogens in
soil and water is presented in terms of
inactivation  rate  constants  (Iog10 day1),
which may differ by several orders of mag-
nitude even for a specific pathogen. Low
temperatures and median pH levels pro-
long pathogen survival in water, and those
factors as well as moisture  content con-
tribute to increased survival in sludge and
soil. The ranking of pathogen persistence
in the environment, from  longest to short-
est, is helminth eggs, viruses, bacteria,
and protozoan cysts.

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  The depth to the groundwater presents
the greatest  barrier  to  the  transport of
pathogens  and,  hence,  to exposure  and
risk. Filtration and adsorption are the  pro-
cesses responsible for limiting pathogen
transport through the unsaturated zone.
The size of the organism, therefore,  de-
termines  which  pathogen will be  trans-
ported the  greatest distance. In  general,
viruses, the smallest of the pathogens con-
sidered, have the potential to travel  far-
ther in the environment. Large particles
like helminth  eggs and protozoan cysts
typically do not migrate  into  groundwater
because of the physical  barrier provided
by the soil, unless there are vertical cracks
or fissures. Due to their persistence,  po-
tential for transport,  and low infectious
dose, viruses seem to represent the worst
case when estimating human health  risk
from landfilling or surface disposal of sew-
age sludge.
  The  SLDGFILL model for pathogen
risk assessment has  been run with many
combinations  of input  parameters to
simulate the transport of sewage sludge
pathogens from a landfill or surface  dis-
posal site  to  a nearby drinking  water
well. The subsequent risk of infection to
humans who  drink  from the well was
estimated for  each run. The probability
of infection is calculated using a beta-
Poisson model.  Conservative exposure
assumptions  include a drinking  water
consumption rate of 2 L/day and param-
eters describing highly infective patho-
gens. The model indicates that risk from
pathogens in  groundwater  near a sur-
face disposal  site or landfill is typically
below a level  of concern. However, if a
risk exists, viruses are  more likely to be
the source of  that risk  than  bacteria or
parasites.
  The parameters to which the SLDGFILL
model are most sensitive are resuspension
coefficients, which describe  the adsorp-
tion of pathogens to  sludge and soil par-
ticles.  Other  parameters to which  the
model is  sensitive  are infective  dose,
pathogen density in sludge, and inactiva-
tion rate in water. Data on infective doses
are scarce, making further research nec-
essary for  reliable use  of the  model to
predict health risks. It is  likely that viruses
present a greater health risk because they
are expected to have a  lower minimum
infective dose and are more readily trans-
ported through soil.
  Future research should be oriented to-
ward  satisfying the  following  information
needs to allow more realistic modeling of
human health  risk  from pathogens  in
landfilled and surface-disposed  municipal
sludge:
      field data on  subsurface transport,
       in  both the saturated and unsatur-
      ated zones, of bacteria and viruses;
       inactivation rates of pathogens un-
      der field conditions in sludge, soil,
      and water;
      solids-to-water suspension  factors
      applicable to sludge- and soil-bound
       pathogens;
       leaching characteristics  of sludge-
       bound pathogens;
       interaction of factors affecting patho-
      gen resuspension from sludge and
      soil; and
       parameters needed to describe in-
      fective doses  of selected indicator
      species and  strains  of  pathogens
       in sludge.

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   This Project Summary was written by the staff of Science Applications International
    Corporation, Oak Ridge,  TN 37831.
   Norman Kowal is the EPA Project Officer (see below).
   The complete document, consisting of paper copy and computer diskette, entitled
    "Pathogen Risk Assessment Methodology for Municipal Sewage Sludge
    Landfilling and Surface Disposal,"(OrderNo. PB96-501911; Cost:$X.OO, 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
          National Center for Environmental Assessment
          U.S. Environmental Protection Agency
          Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management
Research Laboratory (G-72)
Cincinnati, OH 45268
     BULK RATE
POSTAGE & FEES PAID
         EPA
   PERMIT No. G-35
Official Business
Penalty for Private Use
$300
EPA/600/SR-95/016

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