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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Environmental Protection        Information
Agency                     Cincinnati OH 45268
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