United States
Environmental Protection
Agency
Health Effects Research
Laboratory
Cincinnati OH 45268
svEPA
Research and Development
EPA-600/S1-81-049 Oct. 1981
Project Summary
Sewage Sludge Pathogen
Transport Model Project
J. F. Dawson, K. E. Main, B. McClure, R. E. Sheridan, and J. G. Yeager
The sewage sludge pathogen trans-
port model predicts the numbers of
salmonella, ascaris, and polioviruses
which might be expected to occur at
various points in the environment
along 13 defined pathways. These
pathways describe the use of dried or
liquid, raw or anaerobically digested
sludge as a cropland fertilizer, dried
raw sludge as an animal feed supple-
ment, and composted sludge as a resi-
dential soil amendment.
The model uses a compartment-
vector approach in which a mathe-
matical state represents a discrete
point in a treatment or application
pathway where pathogen populations
are computed as a function of time.
Within these compartments, mathe-
matical process functions describe
population changes due to environ-
mental factors. Pathogen exchanges
between compartments are described
by transfer functions. The model
permits user specification of various
parameters in both process and trans-
fer functions, enabling him to simulate
a unique set of environmental condi-
tions. The model has the additional
capability of performing exposure risk
calculations for environmental com-
partments specified by the model user
at the time of model variable initializa-
tion. These calculations are then per-
formed automatically at each hour
interval using the pathogen population
calculated for the compartment at
that time.
The five separate exposure risk
calculations provide risk assessment
determinations for pathogens associ-
ated with airborne particulates, resi-
due and soil, vegetable crops, meat,
and milk. Certain of the exposure risk
calculations can be modified by the
model user to simulate unique expo-
sure conditions. The model can be
progressively modified to accommo-
date new information, thus constantly
enhancing its predictive accuracy.
This Project Summary was devel-
oped by EPA's Health Effects Re-
search Laboratory, Cincinnati. Ohio,
to announce key findings of the re-
search project that is fully document-
ed in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The constraints imposed on sewage
treatment and disposal by clean air and
water legislation are increasing the
amounts of sewage sludge which a
municipality must dispose of while
reducing disposal alternatives. Legisla-
tion has encouraged waste manage-
ment procedures which emphasize the
recycling and beneficial use of waste
materials. At the same time, the conse-
quences of such use are being ques-
tioned. Before municipal officials can
consider using sludge for beneficial
purposes they must decide whether the
environmental release of human patho-
gens inherent in sewage sludge poses
an unacceptable risk.
One mission of the Applied Biology
and Isotope Utilization Division of Sand-
ia Laboratories is the development of
cost-beneficial uses for existing and
future supplies of radioactive isotopes.
The Waste Resources Utilization Pro-
gram, a major subprogram of the Bene-
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ficial Uses Program,has developed
methods, using cesium-137, to reduce
the pathogen content of municipal
sewage sludge so that the organic and
nutrient value of sludge can be used
safely and productively. As a result of
this effort, a prototype sewage sludge
irradiator has been developed which
may make this treatment technology
available to municipalities.
The extent of the health hazard posed
by enteric pathogens in sludge, how-
ever, remains an unanswered question.
In an attempt to evaluate and possibly
quantify this hazard, Sandia Laborato-
ries contracted with the BDM Corpora-
tion to accumulate an information base
that would include available, relevant
data regarding pathogen occurrence in
sewage, pathogen survival during com-
mon sewage treatment processes, and
pathogen movement through the envi-
ronment as a result of sludge applica-
tion and use. A computerized library
storage and retrieval system was de-
signed to classify and store the ab-
stracts of reviewed literature which
constitute the information base. This
information base was used to develop a
computer model which predicts the
numbers of specific pathogens that
would be found in sewage sludge at
various points during sludge treatment
and application. Once the pathogen
population estimates are determined for
points in the environment, the model
has the added capability of assessing
the risk to health resulting from human
exposure to the pathogens. The model is
a first step towards evaluating the
potential risk to human health from
beneficial uses of municipal sewage
sludge.
Results
The sludge treatment and application
alternatives described in the computer
model were specified by Sandia Labora-
tories. These alternatives include the
use of cesium-137 gamma irradiation in
conjunction with the following sludge
uses:
a. Dried raw sludge applied to crop-
land as a fertilizer.
b. Dried, anaerobically digested sludge
applied to cropland as a fertilizer.
c. Dried raw sludge used as a feed
supplement for ruminant animals.
d. Composted sludge used as a soil
conditioner by the general public.
e. Liquid raw or anaerobically digested
sludge applied to cropland as a
fertilizer.
In designing the computer model,
these treatments and applications were
divided into 13 separate pathways
which describe the major steps in the
flow of pathogens through the environ-
ment. Figure 1 is a sample illustration of
Pathway 6.
Four pathways were developed which
describe sludge treatments. Pathway 1,
General Sludge Treatment Pathway,
describes the treatment of raw or di-
gested sludge for application as cropland
fertilizer. Pathway 2, Sludge Treatment
Pathway - Animal Feed Supplement,
describes the treatment of raw sludge
necessary to prepare an animal feed
supplement. Pathway 3, Sludge Treat-
ment Pathway - Composting of Raw or
Digested Sludge, describes a general
composting cycle to render sludge an
inoffensive residential soil amendment.
Pathway 10, Sludge Treatment Path-
way - Liquid Sludge, describes the
treatment of either raw or anaerobically
digested, liquid sludge intended for
application to cropland.
Nine pathways describe the applica-
tions of the sludge that could follow the
defined treatment alternatives. Pathway
4, Sludge Application Pathway - Fertili-
zer for Crops Destined for Human Con-
sumption, and Pathway 11, Sludge
Application Pathway - Liquid Sludge
Used as a Fertilizer for Crops Destined
for Human Consumption, describe the
environmental flow of pathogens after
sludge is applied to cultivated farm land
producing common vegetable crops.
Pathway 5, Sludge Application Pathway
- Fertilizer for Pasture Crops, and Path-
way 12, Sludge Application Pathway -
Liquid Sludge Used as Fertilizer for a
Grazed Pasture, describe the use of
treated sludge on a grazed field, while
Pathway 6, Sludge Application Pathway
- Fertilizer for Crops that Are Processed
Prior to Animal Consumption, and Path-
way 13, Sludge Application Pathway -
Liquid Sludge Used as a Fertilizer on
Crops That Are Processed Prior to
Animal Consumption, describe the use
of treated sludge on field crops harvested
for animal consumption. Pathway 7,
Sludge Application Pathway - Animal
Feed Supplement, is designed to evalu-
ate the use of herbivore feeds contain-
ing sludge such as those prepared in
Pathway 2. Pathways 8 and 9, Sludge
Application Pathway - Residential Gar-
den Soil Amendment and Sludge Appli-
cation Pathway - Residential Lawn Soil
Amendment, respectively, describe the
application of composted sludge to
home vegetable gardens and in the
establishment of a new lawn.
Considering modern disposal practices,
any pathogenic organism can be found
in municipal sewage. It would be difficult
to design a model which could accurate-
ly simulate the survival and environ-
mental movement of more than a few
specific organisms. Three organisms
were selected to represent the enteric
pathogens most commonly found in
sludge. These organisms were chosen
because each causes significantdisease
in the general population; there was
more information available concerning
these organisms than for other members
of the principal opathogen groups; and
each is exceptionally hardy, surviving
longer than average outside the human
body. Salmonella species were used to
represent the bacteria; Ascaris species
were used to represent parasites, both
the helminths and the protozoa; and
polioviruses were used to represent all
enteric viruses. No representative for
the fungi was selected because not
enough information is available on this
group as a whole or on any individual
member of the group to support the type
of decision making which is the founda-
tion of environmental modeling.
The possible growth, inactivation, and
movement of each of the three selected
pathogens, Salmonella, Ascaris, and
poliovirus, were analyzed for each
compartment and transfer of the 13
defined pathways which constitute the
sewage sludge pathogen transport
model. The model uses the state-vector
approach. A state or compartment is a
discrete point along a pathway where
the pathogen population is computed as
a function of time. Compartments are
indicated in the pathway illustrations as
boxes labeled to indicate what processes
are acting on the pathogen populations.
Within each compartment,first-order,
linear, differential equations are derived
to compute the rate and direction of
change in the pathogen population and
the effect that any subprocesses in a
compartment may have on that popula-
tion. The process functions which de-
scribe population changes are vectors
including parameters to account for
time, temperature, moisture content,
nutrients, etc. The transfer of pathogens
between compartments is described by
a set of ordinary differential equations
derived using conservation principles,
environmental parameters, and rela-
tionships developed from data obtained
in the literature review. These equations
are then integrated to determine the
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Dried Raw or
Digested Sludge
External
Source
Harvesting,
Preparation
and Storage
Soil
Sink
Figure 1. Sludge application pathway—fertilizer for crops that are processed prior to animal consumption (Pathway 6)
pathogen populations in each compart-
ment.
The state-vector approach used to
develop the sewage sludge pathogen
transport model provides a structure
with the capability for supporting both
stochastic and deterministic mathematic-
al relationships. It provides a flexibility
of model structure which permits addi-
tion and/or deletion of pathway com-
partments and modifications in process
and transfer functions. The modeling
effort itself has served as a tool to
identify areas where data is currently
nonexistent or incomplete and where
further research is needed. Informed
estimates were incorporated into the
model in these less studied areas. As
new information becomes available
these estimates can be removed from
the model and can be replaced with
supported data. The model can be pro-
gressively modified, thus constantly
enhancing its predictive accuracy.
Each of the selected pathogens differs
in its response to treatment and envi-
ronmental processes. Process and
transfer functions were developed to
describe each pathogen population
separately. The user selects which
pathogen will be considered in any
particular pathway run. The user may
also control the environmental conditions
to be simulated in the model by inputing
user-selected variables to define dura-
tion of compartments, temperature,
rainfall, and other parameters. Default
values for each parameter are incorpor-
ated into the model. These values are
based on the extensive literature review
which preceded the modeling effort.
The decisions which resulted in variable
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value determinations are documented
in the report. The user may compare the
final pathogen populations of one path-
ogen against another for a particular
pathway, or of one pathogen against
itself for several different pathways, or
may compare the effects of different
parameter values on pathogen popula-
tions within and between pathways. For
example, the default value for ambient
temperature in all 13 pathways is 20°C.
It is possible to follow all three pathogens
through a particular pathway at this
temperature. The user may then specify
an ambient temperature of 30°C and
follow the pathogens through again.
Comparison of the six print-outs will
show the effects of temperature.
The model pathways were developed
using relationships that describe the
behavior of representatives of three
pathogen classes (bacterial, viral, para-
sitic) during sewage treatment and in
the environment following their dis-
semination in sludge. The flexible struc-
ture of the model and the ample supply
of user-specifiable variables will allow
an informed user to adapt compart-
ments in any of the pathways to describe
the fate of other members of each
pathogen class. It must be remembered,
however, that even pathogens in the
same class may differ widely in their
responses to stresses encountered
during treatment or in the environment.
For example, one might incorporate
data describing Mycobacterium tuber-
culosis into the process function associ-
ated with one of the compartments in a
pathway. The model will function nor-
mally with these data incorporated, but
it must be remembered when interpret-
ing model output that the relationships
in the other compartments of the path-
way describe the behavior of salmonella.
In addition to the 13 sludge treatment
and application pathways just described,
the model contains five exposure risk
calculations. These calculations provide
estimates of the risk to human health
resulting from exposure to sludge path-
ogens associated with airborne particu-
lates, soil or residue, vegetable crops,
meat, and milk. These calculations are
executed automatically when the model
user elects to determine the risk associ-
ated with the environmental compart-
ment to which the exposure risk calcu-
lations are coupled. The exposure risk
calculations begin with the number of
pathogens present in the selected com-
partment (e.g., RESIDUE). The pathogen
population is then modified based on a
series of events that would normally
occur during a human exposure. The
final pathogen number is compared
with human-dose-response data to
provide a n esti mate of the circu msta nces
that would lead to a human infection.
For example, the model would provide
the number of vegetable units that
would have to be consumed to provide
an infectious dose. Certain aspects of
these calculations may be modified at
the time of model initialization by the
model user to enable him to tailor the
exposure scenario according to unique
conditions.
Conclusions
The accuracy of this or any other
predictive model is directly related to the
quality of the quantitative data that is
used in the development of the mathe-
matical relationships in the model. Data
are available on most aspects of micro-
bial survival during sewage treatment
processes. However, most of these data
are qualitative in nature and cannot be
used directly in the development of
quantitative mathematical relation-
ships. Quantitative data available for
bacteria are often concerned with the
fate of indicator organisms. It was often
necessary to extrapolate these data to
the less numerous and usually more
fastidious pathogenic bacteria. The
validity of this extrapolation issubjectto
the similarities between the pathogens
and indicators. Data concerning enteric
parasites are usually qualitative in
nature, and problems exist in assessing
the viability of resistant forms. There is
much recent data concerning the sur-
vival of viruses during sewage treatment.
Most of this information pertains to
poliovirus and may not be applicable to
other viral pathogens, especially hepati-
tis A and rotavirus.
Much of the current information on
the persistence of pathogens in the
environment is also qualitative in nature.
Most of the data from laboratory studies
lacks confirmation under field condi-
tions. Conversely, descriptivefield
studies generally do not include labora-
tory investigation of underlying basic
principles. The lack of quantitative
information on the relative contributions
of interacting environmental parameters
such as pH, temperature, microbial
antagonism, moisture, toxic substances,
ultraviolet radiation, etc. makes it im-
possible to model the discrete micro-
processes that affect environmental
survival of pathogens.
The Sewage Sludge Pathogen Trans-
port Model uses general mathematical
descriptions of pathogen behaviorduring
treatment and in the environment. The
modular structure of the model allows
the easy incorporation of any desired
microprocess data as it becomes availa-
ble. The mathematical relationships in
the model were developed using data
from three representative pathogens,
salmonella, ascaris, and poliovirus. If
this caveat is kept in mind, the model
can be modified by an informed user to
predict the survival and health risks
associated with other pathogens.
The risk assessment portion of the
model was developed in the absence of
information concerning the probability
of exposure of individuals to pathogens
originating from sludge. Because these
data are not available, the risk assess-
ment portion of the model assumes that
an individual is exposed to sludge
pathogens. The model then provides the
user with information about the condi-
tions of exposure that would lead to
infection or disease.
As developed, the model will be a
useful tool for exploring the relative
health risks associated with a variety of
user-defined sludge treatment and
application scenarios. The value of the
model will be its ability to provide rapid
comparisons of relative health risks
rather than its ability to accurately
quantify pathogen survival in the envi-
ronment.
The Sewage Sludge Pathogen Trans*
port Model was developed on the CDC?
(Control Data Corporation) 6600 com-
puter at Sandia National Laboratories in
Albuquerque, New Mexico. The trans-
portability of the model's FORTRAN
code maybe limited by characteristics of
this computer and its operating system
and by the use of several unique system
programs in the model.
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J. F Dawson, K. E. Hain, B. McClure, R. E. Sheridan, and J. G. yeager are with
The BDM Corporation. Albuquerque, NM 87106.
Norman E. Kowal and Gerald Stern are the EPA Project Officers (see below).
The complete report is in two parts, entitled "Sewage Sludge Pathogen Trans-
port Model Project," (Order No. PB 82-109 000; Cost: $33.50, subject to change)
Magnetic Tape (Order No. PB 82-108 994; Cost: $540.00, 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 Officers can be contacted at:
Health Effects Research Laboratory (Kowal)
Municipal Environmental Research Laboratory (Stern)
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Center for Environmental Research
Information
Postage and
Fees Paid
Environmental
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