United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington, DC 20460
Research and Development
EPA/600/S6-88/003 May 1988
Project Summary
Pathogen Risk Assessment
Feasibility Study
This report evaluates the
practicality of formulating guidelines
to assess the risk associated with
exposure to pathogens in sludge.
Risk assessment may be used to
determine the likelihood that an
environmental agent may cause
human disease (that is, potential to
cause human cancer or toxicity). On
the assumption that the agent
causes a particular disease, given
current and projected exposure
levels, a quantitative evaluation can
be made on the magnitude of the
likely impact of the agent on public
health. In this report, the feasibility of
performing a microbiological risk
assessment for pathogens in
municipal wastewater sludge by
various disposal options was
evaluated.
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 back).
Introduction
Pathogen risk assessment involves an
evaluation of the information available on
representative species of microbes and
their potential health effects, and
modeling, which includes fate,
persistence, and transport. The result of
risk assessment is a focused output that
addresses the potential for human health
impacts. It permits a decision about the
appropriate level of concern about
existing sludge treatment and disposal
options used to reduce the flow of
pathogens from sludges to the human
population, and whether other options
should be considered.
Pathogenic organisms found in sludge
of human origin include certain bacteria,
viruses, fungi, protozoa, and helminths.
After treatment by anaerobic, aerobic,
composting, lime stabilization or other
methods, the sludge and remaining
pathogens are disposed of in one of five
major ways disposal of sludge in
dedicated sanitary landfills; direct
application of sludge to agricultural,
pasture land, silviculture and reclamation
areas; distribution and marketing by
direct application of sludge to gardens
and municipal areas such as roadsides,
cemeteries and golf courses; dumping
sludge into the ocean from a barge or
tanker; and combustion of sludge in a
multiple hearth or fluidized bed
incinerator. Assessment of risk
associated with incineration is not
discussed because pathogens are
destroyed by this method.
Mechanism of Pathogen
Transmission
A number of possible pathways by
which sludge pathogens and other
constituents can be transferred through
the environment to exert potentially
adverse effects on humans have been
identified. The pathways begin at the
point where wastewater enters a
municipal treatment plant. The first three
stages represent wastewater treatment
and sludge processing procedures
necessary to dispose of sludge, or in the
case of distribution and marketing, where
a treatment plant, a retailer, or a broker
can distribute and market sludge
products.
Selection of Representative
Pathogens for Risk Assessment
Study
The number and types of microbes
found in municipal wastewater sludges
varies from community to community and
depend on several factors. These
include, but are not limited to, the degree
of urbanization, population chemistry,
sanitary habits, season of the year, and
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rate of disease in the community.
Microorganisms found in wastewater and
sludges and their potential health effects
are identified in the report.
Because of the limited data base on
these and other species, and the lack of
appropriate or simple measurement
methods, specific representative species
are tracked through the waste handling
and disposal pathways. Thus, these
representative pathogens may be used
to assess risk and are, in fact, pathogenic
surrogate organisms used for detection
of human health hazards. This latter fact
separates these organisms from
"indicator organisms" that are used to
monitor the microbiological quality of the
environment, but may not be pathogenic
or may only pose a minimal risk to
hurna.ns..
The criteria used to select these
representative pathogenic microbial
species include:
• Known demonstrated occurrence in
municipal sludge;
• Known pathogen in the general
population;
• More adequate information base for
the given species than for other
species of the principal pathogen
groups;
• Known infectious doses; and
• Relatively hardy species outside
the host.
Thus, in practice, species are selected
as examples from each of the principal
pathogen groups. Many studies have
used the following representative
species: Salmonella as an example of
enteric bacteria; enteroviruses as an
example of human enteric viruses;
Entamoeba histolytica (the cause of
amoebiasis) or Giardia lamblia (the
cause of Giardiasis) for protozoans;
Ascaris lumbricoides for helminths; and
Aspergillus fumigatus for fungi.
Because a model is an approximation
of reality, decisions have to be made
regarding which components of reality
can be relaxed and which cannot. It is
more feasible to model a few species as
opposed to hundreds of species.
Representative species are selected to
be modeled and substitutes are used
only when necessary The part of the
model that must approximate reality to
the greatest extent possible is the
tracking of these pathogens through the
treatment and disposal pathway to
human exposure sites. The available data
must describe the changes in viability
and concentration that occur in pathogen
populations along the pathways. The
goal of the model is to provide
reasonable predictions, within the
constraints of data uncertainties, of the
time-dependent concentrations and
locations of pathogens. The
concentrations of pathogens can provide
a basis to assess the likelihood and
consequences of infection, disease or
fatality.
Uncertainties and Major Data
Gaps
Some uncertainties exist in the
methodologies used to enumerate
pathogens in sludges, soils, groundwater
and surface water. The quantitative
assumptions used to model risk
exposure must take this into account
Many of these uncertainties can be
attributed to procedural differences
among laboratories, even though the
same "standard procedure" is followed.
For example, in reviewing the literature
on the efficiency of pathogen removal
from wastewater during treatment
processes, one may conclude that
quantitative information should be
compared on the basis of orders of
magnitude. This may be true for
detection of pathogens in general
because of the laboratory-to-
laboratory variability in methods, and the
differences in pathogen recovery within a
single laboratory depending on what
methods were chosen. Also, as new
methods are developed and older
methods improved, the numbers of
organisms typically isolated from
wastewater and sludges will probably
increase. Subsequent attempts to
compare the new results with older
results could be problematic.
Campylobacter sp., for example, can
contaminate drinking water supplies and
cause enteritis. With increased attention
focused on Campylobacter enteritis, new
methods resulting in greater recovery of
the organism are being evaluated
Recently, several methods were
evaluated for recovery of Campylobacter
from various specimens. Pretreatment,
growth medium type, incubation time and
temperature, and pre-enrichment
techniques used, all affected the
quantitative results. Results of standard
tests, even for representative species,
are subject to variability among different
laboratories.
One way to evaluate the suitability of
quantitative data among different
laboratories is round-robin testing that
involves simultaneous analyses of the
same sample by several different
laboratories. The results reported support
the conclusion that quantitative detection
of pathogens, especially viruses, is not
highly precise. For modeling purposf
the variabilities must be reported, i
order of magnitude, plus or minus, m
be the only reasonable starting point
lieu of rigorous interlaborato
development of standard methods 1
detecting pathogens in sludge.
Major data gaps for varioi
components of a risk assessment exi
The following are specific examples th
must be dealt with:
• Microbes—population dynamii
of important pathogen species a
not completely understoo-
especially in regard to interactioi
with other microbes or organisms
their ecosystem;
• Treatment and storage—th
survival rate of sludge-bour
pathogens needs to be more ful
clarified along with th
development of a bette
understanding of the importance
such survival to human health;
• Disposal, transport and fate
relationship of key environment
variables to pathogen survival ar
movement, especially as related i
pathogens being bound to sludgi
and
• Human exposure-relationship <
infection to disease (case historic;
needs to be more fully explored.
These issues are among the mo:
significant ones that warrant extensiv
research.
The dynamic nature of som
pathogens bound to sludge pose
questions of reduced die-off rate
during treatment and the development (
problems at the disposal or exposur
site. The rate of pathogen movemer
may decrease and allow for longe
retention at or in a given site. To dat«
these processes are not quantifiec
Finally, there is a lack of conclusiv
evidence (case histories) of disease
resulting from pathogens in treate
sludge disposed by any of the method
previously described.
Likelihood of Exposure
The probability of contact betwee
sludge-originated pathogens am
humans is never zero. Rather, certaii
likelihoods of exposure can be advancei
for pathogens as they move from thi
various sludge treatment and dispose
options through the exposure pathways
Treatment, sludge management practice;
at the treatment site, sludge dispose
methods, and pathogen survival am
mobility in soil, water and air greath
affect pathogens and limit exposure t<
humans. For example, helminth eggs ar<
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'arge, relative to viruses, and exhibit little
ownward movement in a soil profile.
Thus, helminth egg infiltration to
groundwater is unlikely unless the water
table is near the surface, the soil is very
porous, or a fissure exists that connects
the land surface with the saturated zone.
The likely contact between pathogens
and humans is discussed in the report.
Consideration is given to various
exposure routes as they relate to sludge
disposal options. This scoring of the
various exposure routes is meant to
guide and focus efforts for modeling and
data collection that will allow consistent
risk assessment of exposure to
pathogens in sludge.
Most infections from pathogens follow
a dose-response relationship.
Therefore, as the concentration of
consumed or inhaled pathogens
increases, there is a greater likelihood of
a population becoming infected. The
number of cases of the disease that
result is eventually expressed as an
incidence rate. The measured response
in humans to a microbial challenge could
be in the form of no infection, infection
without illness (such as subclinical, in
apparent infection), or infection with
illness (such as infectious disease in an
increasing proportion of test subjects).
Whether or not a response is noted
depends on the dose of the pathogen the
human is exposed to, the susceptibility
of the individual, and the virulence of the
pathogenic organisms. Infection is
detected by identifying progeny bacteria
in body products, such as nasal or oral
secretions, blood, urine and feces, or by
host response such as antibody
formation that results from infection.
Host response to an infectious agent
has also been measured in terms of
disease production, that is, visible signs
of illness. However, this is a much less
objective measure of response and does
not include infections in which no clinical
disease is produced. In this "carrier"
state the agent is still shed in body
products in a viable, communicable form.
Viral infectivity can be measured in
ways similar to those described for
bacteria. However, viruses are also
measured in cell cultures. Cell culture
methods require that the virus replicate
and kill the infected cell, and that
progeny virus, in turn, replicate and kill
other cells in the culture. The presence
of the infectious virus is detected by its
ability to cause destruction throughout
the cell monolayer (cytopathic effect) or
to cause cell destruction in restricted
egions of the monolayer (plaque
formation).
Infectivity of protozoans is measured
by the detection of cysts in feces of the
host. Depending on the strain, 1 to 10
cysts can produce an infection and many
of these infections are asymptomatic.
Similarly, single eggs (ova) of helminths
produce human infections, as measured
by the identification of the eggs in the
host's feces.
The terms "infective dose" (ID) and
"minimal infectious dose" (MID) are
actually a discrete part of the dose-
response. Generally, the MID is the dose
required to infect 50% of the population
(ID5o) though infectious doses such as
ID-| could be used for worst-case
scenarios.
Minimum infectious doses for bacteria
are on the order of 102 to 106. Even
though these doses are high, such
concentrations can be found in some
sludges. In contrast, a single viral unit
may initiate an infection. In this particular
case it was considered that about 1% of
the human population would become
infected from exposure to one viral unit.
If 50% of the population were to respond
to an infection, the MID would be 5 to 30
viral units. Similarly, for helminths and
protozoa, the MIDs are lower than for
bacteria. A single egg is considered
infectious to man, although some
researchers assume 10 cells or cysts to
be an infective dose. Much less is known
about infectious doses for fungi
Individuals predisposed to lung problems
may be at high risk from inhalation of
Aspergillus spores from composting
sludge. The actual infective dose for
Aspergillus is not known,but exposure to
the fungus seems to be much less
important than levels of abnormal human
susceptibility.
The information on minimal infectious
dose can be systematically integrated
with information on the number of
pathogens that are likely to be present in
the various exposure pathways. In this
matrix, consideration was given to the
survival and transport capabilities of each
of the principal pathogen groups as they
relate to the various exposure pathways.
For example, helminths move very little
in soil and their contamination of
groundwater is unlikely. In contrast,
however, viruses can move through a soil
profile and contaminate groundwater.
The infectious dose for viruses is also
very low. The integration of these facts
produces a high likelihood of occurrence
relative to the previously described
example with a helminth.
Relative to helminths and protozoa,
bacteria and viruses are more likely to
penetrate and move along exposure
pathways, and finally come in contact
with humans. This information, when
coupled with infectious doses for viruses
and bacteria directs risk assessment
efforts toward viruses because of the
large number of viruses in sludge, their
relative ease of mobility, and their low
infectious dose.
Conclusions
Pathogens in sludge, especially
pathogenic bacteria, viruses, protozoa,
helminths and fungi, have been studied
for many years. Studies range from
enumeration of microorganisms before
and after various treatments to
epidemiological documentation of the
role of aerosol pathogens in human
infection and disease. Priorities can be
set for what exposure situations should
be recognized and examined first.
Data available for microbiological risk
assessment for sludge pathogens varies
in quality and quantity for all parts of the
process for a limited number of pathogen
species. Uncertainties can be identified
and rational assumptions justified to
augment the evaluation. Risk assessment
of pathogens in sludge is a reasonable
activity to undertake at this time.
Although the models can be improved,
sufficient data are available that should
approximate reality
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This Project Summary was prepared by staff of the Environmental Criteria and
Assessment Office-Cincinnati, Cincinnati, OH 45268.
Larry Fradkin is the EPA Project Officer (see below).
The complete report, entitled "Pathogen Risk Assessment Feasibility Study,"
(Order No. PB 88-191 4401 AS; Cost: $25.95, subject 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
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S6-88/003
0600329 PS
U S
40604
GOVERNMENT PRINTING OFFICE. 1988—548-013/87051
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