United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington, DC 20460
Research and Development
EPA/600/S6-88/006 July 1988
Project Summary
Development of a Qualitative
Pathogen Risk Assessment
Methodology for Municipal
Sludge Landfilling
This report addresses potential
risks from microbiological pathogens
present in municipal sludge
disposed of in landfills. Municipal
sludge landfilling is defined for
purposes of this assessment as the
application of sludge to the land and
subsequent internment by applying a
layer of cover soil over the sludge
that is thicker than the depth of the
plow zone.
Municipal sludges contain a wide
variety of bacteria, viruses, protozoa,
helminths and fungi. Although
humans may potentially be exposed
to pathogens from municipal sludge
via aerosols and direct contact,
surface water and runoff, plants and
animals, and ground water, proper
landfill management techniques
make transport of significant
amounts of pathogenic micro-
organisms from landfilled sludge by
the first three routes unlikely.
Survival characteristics of path-
ogens are critical factors in
assessing the risks associated with
potential transport of microorgan-
isms from the sludge-soil matrix to
the groundwater environment of
landfills. Various models are dis-
cussed for predicting microbial die-
off. The order of persistence in the
environment from longest to shortest
survival time appears to be the
helminth eggs > viruses > bacteria
> protozoan cysts.
Whether or not a pathogen
reaches ground water and is trans-
ported to drinking-water wells de-
pends on a number of factors,
Including initial concentration of the
pathogen, survival of the pathogen,
number of pathogens that reach the
sludge-soil interface, degree of re-
moval through the unsaturated and
saturated soil zones, and the
hydraulic gradient. The degree to
which each of these factors will
influence the probability of path-
ogens entering ground water cannot
be determined precisely. Viruses,
because of their small size, probably
have the greatest potential of all the
pathogens of actually reaching the
ground water and being transported
from the site. Laboratory studies
suggest that at least 0.1-1% of the
viruses present may be leached from
a municipal sludge landfill.
Information on the fate of path-
ogens at existing landfills is sorely
lacking. Additional laboratory and
field studies are needed to determine
the degree of pathogen leaching,
survival and transport in ground
water in order to estimate potential
risks from pathogens at sludge
landfills with reasonable validity.
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
Municipal sludge landfilling is
defined for purposes of this assessment
as the burying of sludge (that is, the
application of sludge to the land and
subsequent treatment by applying a layer
of cover soil over sludge). To be defined
as a landfill, an area must have soil
thicker than the depth of the plow zone
(e.g , 15 cm). Several different methods
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of disposal are used at sludge-only
landfills. The type of method utilized is
dependent on the characteristics of the
sludge and the nature of the site. The
different methods of disposal may have
different risks associated with them. The
presence of infectious microorganisms in
sludges necessitates the placement of
certain constraints on landfilling of
municipal sludge.
Sludge Characteristics and
Landfilling Methods
Municipal sludge is a complex
mixture of solids of biological and
mineral origin that are removed from
wastewater in sewage treatment plants.
Sludge is a by-product of physical
(primary treatment), biological (activated
sludge, trickling filters) and physio-
chemical (precipitation with lime, ferric
chloride or alum) treatment of
wastewater. Many of the pathogenic
microorganisms present in raw waste-
waters will find their way into municipal
sludges. Treatment of these sludges by
anaerobic or aerobic digestion and/or
dewatering will reduce the number of
pathogens, but some numbers may
remain. Only dewatered sludges with
solids contents >15% are considered
suitable for disposal in sludge-only
landfills. The type of treatment will
determine the concentration of
pathogens and the relative risks of
disposal. Stabilization of sludges may be
accomplished by either aerobic or
anaerobic digestion, lime addition, heat
or wet oxidation. In general, only
stabilized sludges are recommended for
landfilling
Sludge is commonly landfilled either
by subsurface excavation (trenches) or
by area fill above the original ground
surface. Because filling proceeds above
the ground surface, liners can be
installed more readily at area fill
operations than at trench sites. With or
without liners, surface runoff of moisture
from the sludge and contaminated
rainwater should be expected in rel-
atively greater quantities at area fills.
Diked containment area fill sites are
relatively large, with typical dimensions
of 50-100 ft (15-30 m) wide, 100-200
ft (30-60 m) long and 10-30 ft (3-9
m) deep. The depth of the fill in
conjunction with the weight of the sludge
and cover fill results in much of the
sludge moisture being squeezed into the
surrounding dikes and into the floor of
the containment; thus, the potential for
leachate emissions is present.
Pathogens
Raw sewage may contain a wide
variety of pathogenic microorganisms,
including bacteria, viruses and parasites
such as protozoa, helminths and fungi.
All of these types of pathogens can be
expected to be present in raw, primary
and secondary sludges. Pathogens of
primary concern and the associated
diseases they can induce in humans are
listed in Tables 1 and 2.
Salmonella bacteria are the most
widely recognized enteric pathogens.
Often associated with food and water-
borne outbreaks of illness, Salmonella
are responsible annually for 1-2 million
human disease cases in the United
States. Shigella, Campylobacter, Vibrio
cholerae, Yersinia enterocolitica and
even Escherichia coli have all been
recognized as etiological agents of acute
enteritis, but information is lacking on the
concentrations of these organisms in
sludge or their removal by sewage
treatment processes.
The most commonly studied enteric
viruses in sewage and sludge are the
enteroviruses, which include polioviruses,
coxsackie A and B viruses, echoviruses
and hepatitis A virus. Much information is
available on removal of enteroviruses by
sewage treatment, and many studies
have been conducted on their occurrence
in sludges. Rotaviruses, Norwalk viruses
and adenoviruses have also been
associated with human gastroenteritis
but little is known about the concentration
in or removal of these viruses from
sewage or sludge.
Of the common protozoa found in
sewage, only Entamoeba histolytica,
Giardia lamblia, Balantidium coli and
Cryptosporidium so. are believed to be
of major significance for transmission of
disease to humans (see Table 1). All four
species have been linked to waterborne
Table 1. Bacteria, Parasites and Fungi Pathogenic to Man That May Be Present m Sewage and Sludge
Group
Pathogen
Symptoms and/or Disease Caused
Bacteria
Protozoa
Helminths
Fungi
Salmonella (1700 types)
Shigella (4 spp.)
Enteropathogenic Escherichia coli
Yersinia enterocolitica
Campylobacter jejuni
Vibrio cholerae
Leptospira
Entamoeba histolytica
Giardia lamblia
Balantidium coli
Cryptosporidium
Ascans lumbricoides (Roundworm)
Ancyclostoma duodenale (Hookworm)
Necator amencanus (Hookworm)
Taenia saginata (Tapeworm)
Tnchuns (Whipworm)
Toxocara (Roundworm)
Strongyloides (Threadworm)
Aspergillus fumigatus
Candida albicans
Cryptococcus neopormans
Epidermophyton spp. and Tricophyton spp.
Tnchosporon spp.
Phialophora spp.
Typhoid, paratyphoid, salmonellosis
Bacillary dysentery
Gastroenteritis
Gastroenteritis
Gastroenteritis
Cholera
Weil's disease
Amoebic dysentery, liver abscess, colonic ulceration
Diarrhea, malabsorption
Mild diarrhea, colonic ulceration
Diarrhea
Ascariasis
Anemia
Anemia
Taeniasis
Abdominal pain, diarrhea
Fever, abdominal pain
Abdominal pain, nausea, diarrhea
Respiratory disease, otomycosis
Candidiasis
Subacute chronic meningitis
Ringworm and athlete's foot
Infection of hair follicles
Deep tissue infections
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Table 2. Enteric Viruses That May Be Present in Sewage and Sludge
Viruses Type
Symptoms and/or Disease Caused
Enteroviruses.
Poliovirus 3
Echovirus 31
Coxsackievirus A 23
Coxsackievirus B 6
New enteroviruses (Types 68-71) 4
Hepatitis Type A (Enterovirus 72) 1
Norwalk virus 1
Calicivirus 1
Astrovirus 1
Reovirus 3
Rotavirus 2
Adenovirus 41
Pararotavirus unknown
Snow Mountain Agent unknown
Epidemic non-A non-8 hepatitis unknown
Meningitis, paralysis, fever
Meningitis, diarrhea, rash, fever, respiratory disease
Meningitis, herpangina, fever, respiratory disease
Myocarditis, congenital heart anomalies, pleurodynia, respiratory disease, fever, rash,
meningitis
Meningitis, encephalitis, acute hemorrhagic coniunctivitis, fever, respiratory disease
Infectious hepatitis
Diarrhea, vomiting, fever
Gastroenteritis
Gastroenteritis
Not clearly established
Diarrhea, vomiting
Respiratory disease, eye infections, gastroenteritis
Gastroenteritis
Gastroenteritis
Hepatitis
outbreaks of mild to severe diarrhea.
Limited information is available on the
occurrence of protozoa in sewage, and
even less is known about their
concentration in sludges.
A wide variety of helminths and their
eggs may occur in domestic sludges.
Those of primary concern include
nematodes (roundworms) such as
Ascaris lumbricoides and Toxocara
cestodes (tapeworms) such as Taenia
saginata, as well as hookworms,
whipworms and threadworms (see Table
1) Helminth eggs have been found in
municipal wastewater sludge in the
southeastern and northern United States.
Many common helminths are pathogenic
to domestic animals (e.g., cats and dogs)
but cause only mild or asymptomatic
infections in humans.
Fungi are usually considered to be
of minimal health risk in the application
of municipal sludge, although yeasts
(Candida albicans, Cryptococcus
neopormans and Trichosporon spp.) and
filamentous molds Aspergillus fumigatus,
Epidermophoyton spp., Phialophora spp.
and Trichophyton spp.) have been
reported to be present in sewage and in
all stages of sludge treatment.
Exposure Pathways
Possible exposure pathways by
which infectious microorganisms may
come into contact with humans during
the operation of sludge landfills are
shown in Figure 1. Exposure to
personnel may occur through direct
contact with the sludge or through
exposure to aerosols generated during
burial. Aerosols containing viable
microorganisms could also be trans-
ported downwind to exposure areas
distant from the disposal site. Pathogens
may leach from the buried sludge with
infiltrating water to contaminate the
ground water. Surface runoff could also
contaminate nearby bodies of water
Burrowing animals or birds could serve
to transport exposed sludge offsite
before burial.
Aerosols of enteric pathogens are
generated during sewage treatment and
during the spraying of sewage effluents
and sludges onto land. The micro-
organisms in such aerosols can be
transmitted by inhalation or through the
settling of the organisms onto surfaces
that come into contact with humans. The
greatest chance for transport of aerosols
offsite could be expected to occur with
area fill operations involving primary
sludges However, through proper landfill
management and the use of a buffer
zone, significant microbial aerosols are
not expected to occur offsite.
Based on the assumption of good
operating practices involving the use of
drainage ditches at sludge landfills,
surface runoff becomes a part of the
groundwater pathway or is eliminated.
Transport of significant amounts of
pathogenic microorganisms from landfill
sites by plants and animals also appears
unlikely.
Contamination of ground water that
is used for domestic purposes appears
be the most likely route of significant
human exposure to pathogens from
sludge burial. The risk assessment
methodology developed in this report
considers the ground-water
contamination pathway in the greatest
detail
Expected Concentrations of
Pathogens in Sludge
Concentrations and types of
pathogens m sludges depend on the
incidence of infection within a community
and the type of treatment the sludge
receives. Various sludge treatment
processes, such as anaerobic digestion
and dewatenng, reduce the numbers of
some pathogens initially present
Primary sludge, obtained after
gravity sedimentation of solids in raw
wastewater, has remaining ~60% of the
total suspended solids from sewage.
Primary sludge is a semisolid substance
that typically contains ~5% solids by
weight and has a pH of ~6
Secondary sludges are obtained
from wastewater treated by the activated
sludge process, trickling filters or rotating
biological contactors Secondary sludges
obtained following such biological treat-
ment commonly have low percentages of
solids and may be thickened by flotation,
centrifugation or other means. Before
disposal, both primary and secondary
sludges must be stabilized and
dewatered to reduce volatile solids
Most pathogens contained in raw
sewage are concentrated in sludge
during primary sedimentation. Microbial
densities range from 10-103/g dry
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Sludge •
Direct Contact
Burial
Aerosol
Surface^
Runoff
Surface
Exposure"
Subsurface
Animals
Plants
Subsurface Soil
\
Groundwater
Figure 1. Pathways of microbial transport from sludge landfills.
weight primary sludge for parasites to
106-107 for coliform bacteria. Viral and
bacterial pathogens have been shown to
be reduced in concentration by ac-
tivated-sludge treatment, but microbial
densities in most secondary sludges still
range from 10-103/g dry weight for
typical parasites to 8x106-7x108 for
coliforms.
Reductions in microbial concentra-
tions of sludge after stabilization, de-
watering and disinfection are estimated
to range from 0-3 orders of magnitude.
Survival Characteristics of
Pathogens
Although most pathogenic micro-
organisms have a finite lifetime in the
environment once they have left the host
organism, under the proper conditions
microbes may actually increase in num-
bers. To determine risks of disease
associated with landfilling of sludge, it is
necessary to be able to predict the
persistence of pathogens in the soil-
sludge and ground-water environments.
Field data on pathogen survival in
sludge landfills and leachate are virtually
nonexistent; however, laboratory lysim-
eter studies suggest that total coliforms
may persist for at least 100 weeks in
buried sewage sludge. A review of the
literature on pathogen survival in water,
sludge and soil was undertaken to
ascertain significant factors controlling
microbial survival and to aid in devel-
oping mathematical models for predicting
pathogen die-off or inactivation.
Temperature, pH, moisture and
nutrient supply (except for viruses) are
key factors governing the survival of
microorganisms in the sludge landfill
environment. Acidic conditions can
greatly increase bacterial die-off rates.
While more resistant to inactivation under
acidic conditions, both viruses and
parasites are inactivated at extremes of
pH. Survival times of all enteric
pathogens are increased at lower tem-
peratures. Although freezing tempera-
tures may kill bacteria and protozoa, they
have little effect on viruses and may
actually increase their survival. Low
nutrient availability will decrease bacterial
persistence.
In soil flooded with inoculated
sewage sludge, poliovirus 1 was found to
survive for at least 96 days during the
winter and 36 days during the summer,
but in a study using seeded effluent
applied to soil columns, a 99% die-off
of poliovirus 1 occurred in clay soil after
10 days at 30°C. At 4°C, a comparable
die-off did not occur even after 134
days Hepatitis A virus (HAV) appears to
be more resistant than other entero-
viruses to thermal inactivation in sewage
effluents and soils
Bacterial die-off approximately
doubles with each 10° rise in temper-
ature between 5 and 30°C. Salmonella
applied to arid land in summer persisted
for 6-7 weeks, while Salmonella on
grass treated with sludge sur-vived for
<16 months in Switzerland. Organic
content present in sludge is thought to
enhance bacterial survival Vibrio
cholerae appears capable of surviving for
4-10 days in soils moistened with
sewage at 20-28°C. No studies could
be located on survival of Shigella in soils
or sludge, but a literature review
suggests that at temperatures <30°C,
Shigella survival is less than that of
Salmonella
Protozoan cysts are more sus-
ceptible to adverse environmental effects,
such as drying and elevated temper-
atures, than are eggs of helminths.
Entamoeba histolytica cysts died within 5
minutes after drying in the laboratory, but
under agricultural field conditions they
survived 42 hours in wet soil and 18
hours in dry soil. Ascaris (roundworm)
eggs have survived <4 years in soil, and
Trichuris (whipworm) eggs may remain
viable on soil for 6 years In a U S EPA-
sponsored study on the presence ol
parasites in land-applied sludges at 12
sites nationwide, 8 sites reported Ascaris,
Toxocara, Trichuris or hookworms
present in soil or sludge samples.
The order of pathogen persistence in
the sludge landfill environment from
longest to shortest survival time appears
to be as follows: helminth eggs > viruses
> bacteria > protozoan cysts Quanti-
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tative models based on first-order
reaction kinetics have been attempted for
predicting viral and bacterial decay rates
in water and soil; however, insufficient
information is available at present to
develop models for predicting survival of
helminths and protozoan cysts. Tem-
perature is by far the most useful factor
for mathematical prediction of pathogen
survival time. In studies of viral decay in
ground water, 77.5% of the variation in
decay rates among samples could be
explained by temperature. As ground-
water temperatures approach -8°C, viral
decay becomes negligible.
Insufficient data are currently avail-
able on viral and bacterial decay in
sludges at different temperatures to be
used in a predictive model. With
additional data, models could be
developed for pathogen decay in sludge
landfills, soil and ground water. Microbial
survival times could be predicted on the
basis of sludge-soil type, pH, temper-
ature and moisture.
Transport of Pathogens in the
Subsurface
In conjunction with pathogen survival
rates, knowledge of microbial movement
through the sludge-soil matrix is critical
for assessing potential risks posed by
microorganisms at sludge landfills.
Microbial movement in soil is governed
in part by physical characteristics of the
soil, such as texture, particle size, clay
content, organic matter content, pH,
cation exchange capacity and pore size
distribution. Environmental and chemical
factors related to the soil, such as
temperature, moisture content, water
flux, ionic content, and microbial density
and dimensions, are also important
factors affecting movement of pathogens
through the subsurface.
Retention by soil particles is great
for soils with a high clay content, and
movement of pathogens through the soil
matrix is substantially reduced so that
ground-water contamination is not
considered a major exposure route with
clay soil unless cracks or fissures are
present. Conversely, sand and gravel
permit greater and more rapid microbial
movement. Size of the microbes them-
selves, however, is probably the most
important factor. In most soils, viruses
could be expected to travel the greatest
distances because of their small size
(0.02-0.08nm), while the movement of
protozoa and helminths would be limited
because of their large size (5-38 urn).
Published data indicate that viruses
can travel at least 67 m vertically and
408 m laterally in soil. In gravel and karst
substrata, viruses have been observed to
travel as far as 1600 m at s 1000 m/hour.
Removal of viruses from soil is primarily
by adsorption to soil particles, whereas
bacteria, protozoa and helminths are
removed mainly by filtration and
straining. Poliovirus type 1 and
coxsackievirus 63 appear to adsorb to a
much greater degree on sludge and soils
than many of the other enteroviruses.
Viral binding to sludge is also
significantly influenced by pH. Binding of
poliovirus is ~42% at pH 5-7 but
decreases rapidly at pH >7.0 and is
<10% at pH >9. Thus, when high pH
sludges are disposed of, viruses may be
more mobile.
Viral transport in the subsurface has
been modeled assuming instantaneous
equilibrium between suspended and ad-
sorbed virus concentrations and assum-
ing a mass-transfer or "rate-con-
trolled" model to account for distribution
of viruses between the fluid and solid
phases. In both cases, the model form-
ulation leads to linear partial differential
equations that can be solved by a variety
of methods depending upon the problem
domain, heterogeneities in aquifer prop-
erties and boundary conditions. The
mathematical capabilities of the methods
far exceed current basic understanding
of the behavior of viruses in soil and
ground-water systems, and further
laboratory experimentation and field
verification with different substrata are
needed to validate the usefulness of the
models. A comparison of previous field
and laboratory studies suggests that
laboratory evaluations tend to over-
estimate virus removal, possibly because
they do not adequately take into account
soil inhomogeneities and rainfall in the
field.
Filtration is the key mechanism for
bacterial removal from soil, although
adsorption also plays a role. Bacterial
movement appears to be limited to
depths of 10-50 cm in most soils, but
travel distances of 3-122 m have been
observed in sandy soils, and bacterial
transport as great as 920 m has been
reported for gravel. Rainfall can have a
major effect on bacterial migration
through the unsaturated zone by
lowering ionic concentration and in-
creasing infiltration rates. If bacteria are
able to penetrate to the saturated zone,
they appear capable of being transmitted
significant distances in sandy and gravel
soils. Rate of water flowing through the
soil is highly correlated (r = 0.88) with
the degree of removal of both bacteria
and viruses
Because of their large size,
protozoan cysts and helminth eggs are
expected to exhibit even less movement
through sludge than bacteria. Limited
laboratory and field studies involving
Ascaris, hookworm and Taenia saginata
eggs and Entamoeba histolytica cysts
have confirmed <2 cm of vertical
movement through soil, but Giardia cysts
have been reported to penetrate a sand
column to a depth of 96 cm at
operational flow rates of 0.04-0.4 ml
hour. No studies could be found on the
expected removal of parasites by soils.
None of the several models
developed for predicting microbial
transport through the saturated zone has
been verified by laboratory or field
studies. A comparison of field and
laboratory studies on viral and bacterial
movement through solids suggests that
travel of microorganisms in the sub-
surface is greater in the field than
laboratory studies indicate. Only through
field studies at actual sludge landfills will
the real potential for transport of
pathogens be fully revealed.
Infective Dose and Risk of
Disease from Microorganisms
Estimation of minimum infectious
doses (MIDs) for various pathogens is
difficult because of uncertainties m
immune status of host, assay technique,
sensitivity of host, virulence of pathogen,
use of upper 95% confidence limit, route
of exposure, choice of dose-response
model, synergism/antagonism, dietary
considerations and distribution of
subjects among doses and number used.
In many studies, small numbers of
viruses (as few as 1 or 2 tissue culture
plaque-forming units), primarily vaccine
strains, have produced infection in human
subjects The infective dose of protozoan
cysts such as Giardia lamblia and
Entamoeba by the oral route appears to
be as low as between 1 and 10 cysts
Essentially one helminth egg can be
considered to be infectious, although
symptoms may be dose-related.
MIDs for bacteria are generally
higher than those for viruses and
parasites. The number of ingested
bacteria required to cause illness appears
to range from 102-105 although recent
studies suggest that during outbreaks the
infective dose for Salmonella may be
<10 organisms. Virulence of the par-
ticular type and strain of microorganism
and host factors may play roles in
determining the actual number of
microbes required to cause infection.
Individuals who do not actually
consume or come into contact with
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contaminated water or sludge are also
potentially at risk, because micro-
organisms may be spread by person-
to-person contact or by subsequent
contamination of other materials with
which noninfected individuals may come
into contact. Conversely, not everyone
who may become infected with enteric
viruses or parasites will become clinically
ill. Asymptomatic infections are par-
ticularly common with some of the
enteroviruses. The development of clin-
ical illness depends on many factors,
including the immune status and age of
the host; virulence, type and strain of the
microorganisms; and route of infection.
For hepatitis A virus, the percentage of
individuals with clinically observed illness
is low for children (usually <5%) but
increases greatly with age. In contrast,
the frequency of clinical symptoms for
rotavirus is greatest in childhood and
lowest in adulthood. Frequency of clinical
hepatitis A virus in adults is estimated at
75%, but during waterborne outbreaks it
has been observed as high as 97%
Ground-water Pathway Risk
Assessment Methodology
A major difficulty in assessing risks
of ground-water contamination from
municipal sludge landfills is the absence
of any field or laboratory studies
concerning survival of microorganisms
and transport of pathogens into ground
water from disposal sites. Previous
studies on land application of sludge
have concerned only its application to
the soil surface or within several
centimeters of the soil surface. Ap-
plication rates at such sites are on the
order of 22 metric tons/hectare. Disposal
rates at sludge landfills range upwards of
22,000 metric tons/hectare, generating a
much larger concentration of pathogens
A literature review suggests that
significant concentrations of pathogens
can be expected in the sludges that
landfills receive. Many pathogens are
capable of prolonged survival in sludges,
especially at low temperatures under
high moisture conditions. Cohforms have
been observed to survive for years in
sludge and codisposal landfills. Under
ideal conditions, viruses and parasites
may be expected to survive for months
to years, especially if subsurface
temperatures approach 10°C
Transport of pathogens from sludge
landfills to ground water is more difficult
to assess, but experimental results sug-
gest that significant leaching of path-
ogenic bacteria and viruses can occur
The amount of rainfall is probably a
major factor governing microbial release
from sludge Most of the landfills
described in the U.S. EPA's Process
Design Manual for Municipal Sludge
Landfills are situated within 3 m of
ground water, and although laboratory
studies suggest substantial removal of
microorganisms through the unsaturated
zone, field studies indicate that
penetration of enteric bacteria and vi-
ruses is possible. Quantitative infor-
mation on pathogen removal through the
unsaturated zone is almost nonexistent.
Based on a review of the literature,
an ideal landfill site (i.e., one that poses a
minimum risk of ground-water contam-
ination) would utilize digested secondary
sludge with a solids content of s20%
The substrata would be a clayey soil with
a deep ground-water table in an area of
low rainfall With a clayish soil and a clay
lining, no enteric pathogens would be
expected to leach into the ground water.
A worst-case landfill would dispose of
raw or primary sludge with a solids con-
tent of 15%. The site would lay within 1
m of the ground-water table, be unlined
with sand or gravel substrata and would
experience high rainfall.
Two example sludge landfill sites
with characteristics shown in Table 3
were evaluated by the micro-DRASTIC
rating system to assess the likelihood of
ground-water contamination It was
determined that microbial contamination
was probable directly beneath site A and
possible beneath site B. At both sites,
contamination was judged to be possible
at distances of 100 and 200 m. Although
the rating system has not been verified in
the field, it provides a mechanism for
evaluating the many interacting factors
that control microbial survival and
transport m the subsurface Micro-
DRASTIC could potentially be used as a
first step in evaluating the potential for
microbial contamination at a particular
site, based upon hydrogeologic settings
Other approaches to determine the
likelihood of ground-water contamina-
tion have been based on estimating the
leaching of pathogens from sludge landfill
and plotting the concentrations of
microorganisms at various distances from
a given site. From this information, it is
possible to estimate the risk of illness
from using the ground water for drinking
Of course, whether or not a pathogen
reaches ground water and is transported
to drinking-water wells depends on
many factors, including initial concen-
tration of the pathogens, survival of the
microbes, number of pathogens that
reach the sludge-soil interface, degree
of removaj through the unsaturated and
saturated soil zones, and the hydraulic
gradient. Under most favorable con-
ditions, it is estimated that no more than
0.1% of viruses are released from
sludge; for most probable conditions, 1%
is estimated; and under worst possible
conditions, a 10% release is estimated
Recommendations
It is clear that information on the fate
of pathogens at existing landfills is
essentially nonexistent. Laboratory and
field studies are needed to determine the
degree of pathogen leaching, survival
and transport to ground water Ap-
proaches are available to estimate
potential risks from pathogens at sludge
landfills, but without adequate infor-
mation, the reliability of the conclusions
is weakened The availability of neces-
sary information to perform a risk as-
sessment and research needs are shown
in Table 4.
Table 3 Characteristics of Selected Municipal Sludge Landfills
Depth to Water Net Recharge Hydraulic Conductivity Temperature
Site (m) (m) (€ /d-rn2) (°C)
Soil Type Medium Aquifer Type Sludge
A 3-4
B 10-12
0-1
7-8
0.35
0.35-0 035
10
14
clay, sand, and gravel
silty clay
silt loam
silty
secondary
primary
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Table 4.
Status of Information for Ground-water Risk Assessment and Research Needs
Item
Adequate Data to Make a Risk Assessment
Protozoan Helminth
Bacteria Viruses Cysts Eggs
Research Needs
Concentration in sludge
Concentration of organisms leached
yes
limited
yes
no
yes
no
yes
no
Data for emerging pathogens and better detection
methods needed
No data on pathogens. Field and laboratory studies
needed
Survival (decay rate) in leachate no
Survival (decay rate) in ground water yes
Transport through unsaturated zone limited
Transport through saturated zone yes
Risk of illness yes
no
yes
limited
limited
yes
no
limited
limited
limited
yes
no
limited
limited
yes
yes
No data on pathogens
Data for emerging pathogens would be useful
Limited data available. Information on effect of rainfall
needed
Proposed models need laboratory and field
verification
Data needed on nature of distribution of pathogens
in water
Larry Fradkin is the EPA Project Officer (see below).
The complete report, entitled "Development of a Qualitative Pathogen Risk
Assessment Methodology for Municipal Sludge Landfilling," (Order No.
PB 88-198 544/AS; Cost: $19.95, subject to change) will be available
only from:
National Technical Information Service
5285 Porf Royal Road
Springfield, V'A 221'61
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
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6250109 i
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S6-88/006
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