United States
                  Environmental Protection
                  Agency
Office of Health and
Environmental Assessment
Washington, D.C. 20460
                  Research and Development
EPA/600/S6-90/002 Apr. 1990
&EPA         Project Summary
                  Pathogen  Risk Assessment  for
                  Land  Application  of  Municipal
                  Sludge
                     Volume I:  Methodology and
                     Computer  Model
                     Volume II:  User's Manual
                    This  document describes  a
                  methodology and associated com-
                  puter model for assessing the risk to
                  humans of pathogens in  treated
                  municipal sewage sludge applied to
                  land. Land application  of sludge in
                  this methodology  refers to the dis-
                  tribution of sludge on or just below
                  the soil surface where it is employed
                  as a fertilizer or soil conditioner for
                  growing human food-chain and non-
                  food-chain crops. The two categories
                  of land application addressed in this
                  model are (1)  agricultural utilization
                  and (2)  distribution and marketing
                  (D&M),  and the source of microbial
                  pathogens is (1) liquid or (2) dried or
                  composted municipal sewage sludge.

                    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 two separate
                  volumes of the  same title (see Project
                  Report ordering information at back).
                 Introduction
                   Section 405 of the Clean Water Act
                 requires the U.S. Environmental Protec-
                 tion Agency to develop and issue
                 regulations that identify: (1) uses for
                 sludge including disposal; (2) specific
                 factors (including costs) to be taken into
account in determining the measures and
practices applicable  for each use or
disposal; and (3) concentrations of
pollutants that interfere with each use or
disposal. To comply with this mandate,
the  U.S. EPA  has  embarked  on  a
program to develop four major technical
regulations: land application, including
distribution and marketing; landfilling;
incineration and surface disposal. The
development of  these technical regula-
tions  requires a consideration  of
pathogens as  well as  chemical constitu-
ents of sludge. Public  concern related to
the  reuse  and  disposal of municipal
sludge often focuses on the issue of
pathogenic  organisms. The purpose of
this  report  is  to describe a proposed
methodology and associated computer
model  designed  to assess the potential
risks  to  human  health posed by
pathogens in  municipal  sewage sludge
applied to land  as  fertilizer or soil
conditioner.
  Volume I: Methodology and Computer
Model  describes the conceptual frame-
work of the risk assessment methodology
and the structural organization, including
assumptions and components, of the
computer model.  Volume II: User's
Manual contains  background information
to provide the user with an understanding
of the  actual  functioning of the model.
This information includes descriptions of
operating variables and their default
values, explanations of the  various
subroutines, and the mathematical basis
for process and transfer functions.

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Approach

   The  approach  used for the model
provides a structure capable of support-
ing both stochastic and  deterministic
mathematical relationships,  i.e.,  it is  a
dynamic model that can incorporate site-
specific  data  while allowing process
functions to  be  dependent on  environ-
mental factors, such as temperature  and
rainfall.  The  model  structure provides  a
flexibility that  permits  addition and/or
deletion of sludge  management practice
compartments  as well as modifications in
process  and  transfer  functions. The
model is designed to run on a personal
computer with  a minimum  of  540 KB of
free memory. Currently limited  by a lack
of data,  the  model will  be able to utilize
data gathered in the future to enhance its
predictive accuracy.
   The  purpose  of the model is  to
determine the  probability of infection of
the human  receptor from  pathogens in
the land-applied  sludge. The  ultimate
objective is  to  use the  model to assist
EPA in  its  regulatory activities,  but the
immediate  uses  include  (1)   further
development of the pathogen model as a
research and risk assessment tool and (2)
the application  of the methodology in the
performance of actual pathogen risk
assessments.
   The  five  municipal  sewage  sludge
management practices addressed by the
model are:  application  of liquid  treated
sludge (1) for  production of commercial
crops for human  consumption, (2) to
grazed pastures, and (3) for production of
crops processed before animal consump-
tion;  and  application  of dried or
composted  sludge,  (4) to  residential
vegetable gardens, and (5) to residential
lawns.
Model Structure
   The computer model  represents the
compartments  and  transfers  among
compartments  of  the  five management
practices. The  compartments  are the
various  locations,  states,  or activities in
which sludge or sludge-associated  path-
ogens  exist; they vary to some extent
among practices.  In each compartment,
pathogens either increase, decrease, or
remain the same in number with  time, as
specified by "process functions"  (growth,
die-off  or no population  changes)  and
"transfer functions" (movement  between
compartments).  The population  in  each
compartment, therefore, generally varies
with  time  and  is  determined by  a
combination  of  initial  pathogen  input,
"transfer  functions"  and  "process
functions."  The populations of pathogens
in the compartments representing human
exposure  locations,  together with ap-
propriate intake and infective  dose data,
are used to estimate human health risk.
   Considering modern disposal  prac-
tices, almost any  pathogenic organism
can  be found in municipal sewage.
Because of the difficulty of designing a
model that  could accurately simulate the
survival  arid environmental movement of
more than  a few  microbial species,
organisms  or organism groups were se-
lected to represent the enteric pathogens
most commonly found  in sludge. The
current  version of  the model  deals with
only three of these selections:  Salmonella
spp. representing  the bacteria;  Ascaris
lumbricoides  the  parasites  (both hel-
minth   worms  and  protozoa);  and
enteroviruses (a grouping  of  several
animal viruses), the enteric viruses.
   Exposure of an individual  to  enteric
pathogens can lead to (1) no effect, (2) a
subclinical (asymptomatic) infection or  (3)
a  clinical  (symptomatic)   infection.
Although subclinical  infections  are not
clinically detectable,  that individual  by
either direct  or indirect transmission  of
the pathogenic organisms  may  cause
disease to  develop  in  others.  In this
methodology,  infection  rather  than
disease  is used to measure risk.
   Exposure  pathways, i.e.,  migration
routes of pathogens  from  or  within the
application  or disposal site to a  target
organism or receptor, for sludge applied
to land include the following:
•  inhalation or ingestion of  emissions
   from  application of sludge or tilling of
   sludge/soil;
•  inhalation or ingestion of windblown or
   mechanically generated particulates;
•  swimming  in a  pond fed  by surface
   water runoff;
•  direct contact with sludge-contam-
   inated soil or crops (including  grass,
   vegetables, or forage crops);
•  drinking  water from an offsite  well;
   inhalation and subsequent ingestion of
   aerosols from irrigation;
•  consumption of vegetables grown  in
   sludge-amended soil;
•  consumption of meat or  milk  from
   cattle grazing on or consuming forage
   from  sludge-amended fields.
   Since the model  provides only  an
approximation  of environmental transport
mechanisms, it does not represent every
possible exposure pathway.  It  does,
however, trace the flow of  pathogens
through  the  major routes leading  to
possible human exposure.
Infection

   The dose  required to  cause infection
is based on the virulence or infectivity of
the  pathogenic  organism  and  on  the
susceptibility  of  the  exposed population
or individual  receptor.  The  "minimum
infective  dose" or MID is  typically  the
dose required to infect 50%  of  the
population. The uncertainty  in measuring
infectious  doses greatly weakens  the
power of any quantitative  risk assess-
ment. The model is designed, therefore,
so  that  the  user can  supply  a best
estimate  of  infectious  dose for  the
particular  pathogen  and  practice being
modeled.
   Risk  assessments  ordinarily proceed
from  source  to  receptor.  That  is,  the
source, or sludge disposal/reuse practice,
is first characterized, and  contaminant
movement away  from the source is then
modeled  to  estimate  the degree  of
exposure  to the  human  receptor.  Health
effects are then  predicted based on  the
estimated exposure and  dose-response
relationships.  This computer model sums
the  hourly  exposures  of  a  human
receptor  to pathogens in  each exposure
compartment and computes  the  daily
(24-hour) probability of the human recep-
tor receiving  an  exposure exceeding an
infective  dose (e.g., for  Salmonella   the
default MID =  10).


Uncertainty Analysis
   Many  factors contribute to  the
uncertainties associated with the  present
risk  assessment  model.  Chief  among
these is  the  lack of quantitative data
describing the processes involved. Even
when available, data are  highly variable
with  regard  to  (1)  the  initial
concentrations of  microbial  pathogens in
wastewater and sludge; (2)  processes of
microbial  transport and  inactivation;  (3)
dose-response relationships; and  (4)
exposure   levels  and   receptor
susceptibility.
   A sensitivity analysis was  performed,
but because of the large number of input
parameters and the uncertainty related to
the values of parameters,  it  should  be
viewed  as preliminary.  However, the
analysis does indicate that  the model is
very  sensitive to the inactivation rate of
microorganisms in soil, as well as to  the
parameters used  to  calculate  the frac-
tions of  pathogens  transferred  from
surface soil  to  subsurface  soil,  from
subsurface soil to groundwater and from
surface soil  to  surface runoff water.
Accordingly, these parameters should be
selected  with great care,  especially  as

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they are all likely to  be site-specific.
Because the  data  available  to  support
choices  of the values  are limited,
research efforts  should  be directed to
these areas in  order  to  increase  the
accuracy of the model.

Model Use
   The user of the  model has the option
of modifying  many  of the  operating
variables used  by  the  program  to
calculate the number of  organisms
present  in the  compartments.  These
variables include  initial pathogen concen-
tration;  organism  type;  environmental
parameters such  as soil temperature  and
moisture, air temperature, windspeed,
rainfall  amount and  frequency;  and
parameters descriptive  of  the land
application practice modeled such as size
of field or garden, type  of crop, irrigation
method  and frequency, timing and yield
of harvest or size  of cattle herd. There
are also variables for the subroutines  that
represent rainfall, groundwater and  crop
preparation practices.  Operation  of  the
model  is  governed  by  a  series  of
compartment differential equations, trans-
fer factors  and times,  and process
functions. These  equations calculate the
increase  or decrease of pathogens within
each  compartment and  the  transfer of
pathogens  from one compartment  to
another at intervals during the  model  run.
Transfers  may  be  conditional  on  a
particular applications  option such  as
spray irrigation, on  time after initiation of
the practice or  on physical  processes
such as rainfall or dust storms. The more
complex transfer  functions are those
resulting from  generation  of  particulate
emissions by wind erosion  or tilling, from
generation of liquid aerosols during spray
irrigation using wastewater, from transport
of soil-bound microorganisms  by surface
runoff associated with  rainfall or from
subsurface transport  of pathogens
associated with groundwater movement.

Conclusions
   Preliminary  conclusions drawn from
sample  runs of  the model suggest  that
the most likely candidate for infection is
an  onsite human receptor who ingests
pathogens from  direct  contact or from
contaminated  surface  runoff  confined
onsite; the  probability  of  infection of an
offsite human  receptor seems relatively
low  if surface runoff  does  not move
offsite.

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   Worm Kowal is the EPA Project Officer (see below).
   The complete reports, entitled "Pathogen Risk Assessment for Land Application
        of Municipal  Sludge,  Volume I:  Methodology  and Computer  Model,"
        (Order No.  PB  90-171 901/AS; Cost: $23.00) and "Pathogen Risk
        Assessment for Land Application of Municipal Sludge, Volume II:  User's
        Manual," (Order No. PB 90-171 919;  Cost  $31.00)  (costs sub/ect to
        change) will be available only from:
            National Technical Information Service
            5285 Port Royal Road
            Springfield, VA22161
            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                   Center for Environmental Research
Environmental Protection         Information
Agency                         Cincinnati OH 45268
Official Business
Penalty for Private Use $300

EPA/600/S6-90/002

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