Occurrence of Pathogens in Distribution and Marketing Municipal Sludges
William A. Yanko, County Sanitation Districts of Los Angeles County,
"Although the use of sludge as a soil amendment is attractive, it is not without potential health risks. Toxic
chemicals, including heavy metals and industrial organics, may enter the food chain and present long-term
health risks."
The plague causing bacteria Yersinia pestis was consistently found in static pile compost. CDC authorities
state, "Outbreaks in people still occur in rural communities or in cities."
significant increases in bacterial populations, including salmonellae, occurred during subsequent
production of commercial s o i l amendment products.
But, what about pathogenic yeast and Viable, But Non-culturable pathogens (VBNC)????
|
United States
Environmental Protection
Agencv
Health Effects 0122
Research Laboratory
Research Triangle Park, NC 2771 1
Research and Development EPAl600/S1-871014 Mar. 1988
Project Summary
Occurrence of Pathogens in
Distribution and Marketing
Municipal Sludges
William A. Yanko
William A. Yanko is with County Sanitation Districts of Los Angeles County,
Whittier, CA 90607.
A study of the occurrence of
microorganisms in distributed and
marketed municipal sewage sludges
was conducted in order to determine
t h e l e v e l s of i n d i c a t o r and
pathogenic organisms that might be
present in these products. Samples
were analyzed for a variety of
bacteria, viruses, parasites and fungi
in the indicator and pathogen
categories. In the first part of this
study, seven municipal sewage
sludge compost products were
sampled weekly for one year. Five of
the sample products originated from
one windrow composting facility and
two products originated from one
aerated static pile composting
facility.
The products sampled at the
windrow facility included the final
compost produced at the municipal
composting f a c i l i t y , and four
commercially marketed, compost
based soil amendment products. The
two sampling points at the static pile
facility were the final screened
compost which was utilized in a
number of bulk distribution programs
and the " b i n " which
contained unscreened compost
available to the public for home use.
The indicator microorganisms
were frequently detected at high
c o n c e n t r a t i o n s . T r e m e n d o u s
indicator variability was observed
with some concentrations varying by
as much as ten orders of magnitude.
Data from the windrow site grouped
into two strata. The first stratum
included the final compost and the
commercial product containing only
screened final compost. Average
concentrations of organisms were
higher in the bagged compost than in
the field compost samples, but the
differences were not significant at
the 95% confidence limits (C.L.). The
second grouping of data consisted of
bagged commercial p r o d u c t s
containing additional amendments;
these p r o d u c t s c o n t a i n e d
significantly higher concentrations of
microorganisms than those in the
first stratum.
Analysis of the indicator data from
the static pile composting facility
indicated that these two sampling
points were also s i g n i f i c a n t l y
different. The screened compost
contained higher levels of bacteria
than the material in the giveaway bin.
The only potential pathogens
detected w i t h regularity were
bacterial. No protozoan cysts were
found. Helminth ova were regularly
detected but none could be shown to
be viable. The most common ova
observed were Trichuris and Ascaris.
Many of the Trichuris were probably
of non-human origin. Enteric viruses
were confirmed in only two samples
at very low levels.
The potential bacterial pathogens
regularly detected were Salmonella
and Yersinia. Salmonellae were
detected at both facilities. Yersinia
only occurred significantly at the
static pile facility and were isolated
in a pattern consistent with a
seasonal occurrence. At the windrow
facility, salmonellae were primarily
isolated from amended compost
products. Toxigenic E. coli were
randomly isolated. No Campylobacter
were detected.
Total and fecal coliforms and fecal
streptococci were shown to be good
predictors of the presence of
salmonellae.
In the second part of the study, 24
additional municipalities were
sampled bimonthly for pathogen and
indicator microorganism determinations.
Final sludge products
included composts, a i r - d r i e d
sludges and heat-treated sludges.
Results from the 24 facilities also
showed a great deal of variation in
microbial densities. Air dried sludges
often contained lower concentrations
of enteric bacteria than composted
sludges and as a group, were
significantly lower than static pile
composting sites. Salmonellae,
Yersinia and toxigenic E. coli were
randomly isolated, generally at low
levels.
This study disclosed essentially
no hazard associated with treated
sludges from parasites or viruses. A
potential health hazard associated
with salmonellae was detected at
both facilities sampled weekly.
Results indicated t h a t current
composting practices may not insure
complete elimination of pathogenic
bacteria. The significance of Yersinia
populations at the static pile facility
was uncertain, but isolation patterns
suggested a seasonal occurrence.
Relatively few salmonellae were
detected in final compost from the
windrow facility, but significant
increases in bacterial populations,
including salmonellae, occurred
during subsequent production of
commercial s o i l amendment
products. These increases were
c o n s i s t e n t w i t h a r e g r o w t h
phenomenon. Conventional indicator
o r g a n i s m s appeared to be
reasonable predictors for the
presence of salmonellae. Final
sludge products derived from the
presence of salmonellae. Final
sludge products derived from the
various treatment processes often
did not contain detectable levels of
the tested enteric pathogens, but the
data suggest monitoring may be
necessary to ensure consistent
quality of sludge-based products
destined for home use.
This Project Summary was
developed by EPA' Health Effects
Research Laboratory, Research
Triangle Park, NC, 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
The Clean Water Act of 1972 (PL 92-
500) was enacted to improve the quality
of the nation' water supplies. As a
result, a by-product of sewage
treatment, known as sludge, has
increased in quantity as wastewater
treatment improved and expanded. An
estimated 7 to 9 million dry tons of
sludge are produced annually, and this
amount is predicted to increase
significantly in the future. Disposal of this
sludge has become a major function of
publicly owned treatment works (POTW);
disposal options, however, are often
limited. Ocean disposal of sludge has
been greatly reduced. Cost and air
pollution considerations have curtailed
the use of incineration, and transportation
costs and the lack of available sites have
lessened the popularity of landfilling as
sludge disposal options.
An attractive alternative, rapidly
gaining in popularity, is the beneficial use
of sludge as a soil amendment. Sludge
has been shown to be an excellent
organic amendment for soils; it is also a
source of nutrients and minerals for
plants. Land application of sludge
represents a significant and rapidly
increasing option for disposal of sludge
produced in the U.S.
EPA has promulgated regulations (40
CFR 257) specifying acceptable sludge
treatments depending on the ultimate
use of the sludge and the amount of
public exposure. These treatments are
termed Processes to Significantly
Reduce Pathogens (PSRP) and
Processes to Further Reduce Pathogens
(PFRP). PSRP are generally standard
treatment processes that incidentally
reduce pathogens, such as anaerobic
digestion. PFRP are processes
specifically intended to reduce
pathogens, as well as provide sludge
stabilization, such as high temperature
composting. Various restrictions are
placed on the land disposal of sludges
that have received only a PSRP
treatment. Under current guidelines, a
sludge subjected to PSRP followed by a
PFRP may be used without restrictions.
Although the use of sludge as a soil
amendment is attractive, it is not without
potential health risks. Toxic chemicals,
including heavy metals and industrial
organics, may enter the food chain and
present long-term health risks.
Pathogenic microorganisms present in
sewage and the resulting sludges
increase the potential for disease
transmission. These concerns must be
mitigated if the full resource potential of
sludge is to be realized.
In many areas, toxic chemicals are not
present or their levels may be reduced to
acceptable levels through source control
programs. Pathogenic microorganisms,
however, enter the wastewater from
infected individuals. These organisms
often concentrate in the resulting sludges
due to their density or through adsorption
to larger particles. The pathogens are
thus a normal component of sludge and
cannot be reduced by source control.
The treatment processes employed at
the POTW must effectively eliminate, or
reduce to acceptable levels, the
pathogenic microorganisms present in
sludge before the material can be
released for use.
The relative public health risk
associated with the beneficial use of
sludge is directly related to the extent of
public exposure. Agricultural use for feed
crops and silviculture in limited access
areas present minimal risks. Risk
increases if the sludge is used on food
chain crops or public access areas.
Many programs are currently
distributing and marketing (D & M)
PFRP-treated sludges for home use on
lawns and ornamental and vegetable
gardens. Of the various reuse options,
home use of treated sludge via some
form of marketing or distribution program
would appear to present the greatest
potential for significant health effects due
to increased exposure. The routes of
exposure may take various forms,
including handwork in gardens and eating
uncooked vegetables grown in sludge
amended soils. Perhaps at the highest
risk of ingesting pathogenic organisms
are very young children playing in yards
and gardens that have been treated with
sludge products.
A number of review articles have
discussed the occurrence and
significance of pathogens in sewage and
sludge and the effectiveness of various
treatment options. Some pathogenic or
potentially pathogenic microorganisms
have been shown to regrow in treated
sludges, but the significance of this
phenomenon has not been determined.
The pathogenic microorganisms of
concern in sewage and sludge are
members of four basic groups; these are
the bacteria, fungi, parasites, and viruses.
Many different bacterial pathogens
may be present in sewage and sludge. In
the major concern category, E. Coli
pathogenic strains), Salmonella sp.,
Campylobacter jejuni, Yersinia
enterocolitica. Leptospira spp., Shigella
spp., and Vibrio cholerae.
The pathogenic strains of E. coli are
often the cause of "travelers' diarrhea"
and may cause serious gastroenteritis
and diarrhea in children under five years
of age. These strains have also been
involved in outbreaks of gastroenteritis
resulting from contaminated water
supplies. Three types of pathogenic E.
coli have been recognized; they are
enterotoxigenic, enteropathogenic and
enteroinvasive. It has been estimated
that pathogenic E. coli represent less
than 1% of the fecal coliform population.
Little is known about the occurrence or
fate of enteropathogenic E. coli in sludge
and sludge treatment processes. The
potential for pathogenic E. coli regrowth
in sludge products is unknown, but
certainly possible.
In contrast to the paucity of data
concerning pathogenic E. coli in sludges,
the salmonellae have been widely
studied. The previously cited reviews
summarize much of this work. It has
been estimated that up to 2 million
people per year acquire Salmonella
infections, and the rate has been
increasing in recent years. For this
reason one of the prime concerns is the
potential for salmonellae regrowth.
Studies have found that salmonellae
would grow to high levels in sterilized
sludge. Other experiments demonstrated
regrowth of indigenous salmonellae
within the naturally occurring mixed
microbial population of compost.
Although the salmonellae increased more
than three orders of magnitude within
five days, the effect was transient, and
the salmonellae were returning to
background levels after three weeks.
Others have concluded that the active
microflora of moist compost would
eliminate contaminating salmonellae after
six weeks, and that once a sludge
product is applied to the soil, regrowth
would not likely be a problem.
Nevertheless, the salmonellae remain
an important concern with home use of
sludge products. Most authorities
indicate that a fairly high infective dose is
required to initiate a Salmonella infection;
however, there is evidence that this may
be an over generalization. Salmonellosis
outbreaks that may have been initiated
by infective doses as low as 10-100
cells have been described in the
literature. It would certainly appear
imprudent to overlook moderate
salmonellae populations because of
conventional wisdom " concerning
infective doses.
Campylobacter and Yersinia have
been referred to as "pathogens of
emerging significance." Although much
work has focused on Yersinia. there is
very little information concerning these
organisms in sludges. One study
reported 2 X 105 Yersinia enterocolitica
per gram total suspended solids in one
digested sample. A study in Seattle
reported Yersinia enterocolitica levels of
107 to 109 per gram in various sludge
samples. The significance of these
numbers i s unknown. Many Y.
enterocolitica-like .organisms, unusual
Y. enterocolitica or atypical Y.
enterocolitica have been reported. The
pathogenicity of individual strains is
unknown, and the role of these
organisms in human disease has not
been completely discerned. Yersiniosis
occurs only sporadically in the United
States.
Gastroenteritis caused b y
Campylobacter jejuni is much more
common than previously thought and
may approach the incidence of
salmonellosis. C. jejuni is commonly
found in the intestines of many animals,
but the fate of these organisms in the
environment is generally unknown.
Waterborne campylobacteriosis has been
documented, and the survival of the
organisms in water has been studied to a
limited extent. Limited experiments on
the survival of Campylobacter in sludges
from four wastewater treatment plants in
Ohio found that seeded Campylobacter
survived for seven days in sludge stored
at 4"C, however, no indigenous
Campylobacter were isolated from any
sludge samples
Although the other bacterial
pathogens listed as significant are
responsible for a substantial amount of
disease, they either have not been
demonstrated in sludges, or sludge
applied to land is not considered an
agent of transmission.
A number of pathogenic or allergic
fungi can be isolated from sludge. These
include yeasts, such as certain specious
of Candida, Cryptocuccus and
Trichosporon, and pathogenic members
of some filamentous genera, such as
Aspergillus, Phialophora, Geotrichum,
Trichophyton and Epidermophyton.
Aspergillus fumigatus, an opportunistic
pathogen to individuals with pulmonary
problems and a strong allergen to many,
may proliferate in some composting
systems. This may be a consideration
when selecting prospective composting
sites. The general consensus, however,
is that fungi in treated sludges present a
minimal hazard. With the exception of the
aspergilli, little work has been done to
define the relationships of fungi in
polluted environments or sludges. The
significance, if any, of fungal types and
diversity in compost is unknown. A highly
diversified mycobiota probably indicates
a stabilized environment. A lack of fungal
diversity in treated sludge would not
present a particular problem unless the
fungus was potentially harmful.
Dermatophytic fungi, for example, may
be present in sludge at detectable levels.
It i s not known i f the common
dermatophytes can survive or proliferate
in sludges. Conventional thought
considers the dermatophytes to be
parasitic, although there is evidence that
some dermatophytes live a saprophytic
existence. Adding large numbers of these
organisms to home soils would be
undesirable.
Parasitic infections present a potential
health risk associated with home use of
sludge due to the existence of highly
resistant stages of the organisms and low
infective doses. Two groups of parasites
are of concern, the protozoa and the
helminths. The common protozoan
parasites include organisms such as
Entamoeba histolytica and Giardia
lamblia. Giardia infection has become
endemic in some areas of the country
and numerous outbreaks of giardiasis
have occurred. Ascaris ova are the most
commonly isolated nematode ova in
sludge. Others include Trichuris,
Toxocara, Hymenolepis and Taenia, to
mention a few. In 1973, ascariasis was
estimated to affect four million people in
the United States.
Sludge digestion destroys protozoan
cysts but not metazoan eggs. This
observation was confirmed by recent
studies in Seattle where Giardia was
isolated from raw and waste activated
sludges but not from digested or
digested dewatered sludges. The Seattle
study concluded that Giardia pose a
negligible health risk from land
application of digested sludges. In
general, available evidence indicates that
helminth ova are more resistant to
environmental stresses than are
protozoan cysts. The ova of Ascaris sp.
are possibly the most resistant of the
eggs or cysts commonly found in
sewage. It has been proposed that
Ascaris ova be used as an indicator of
other parasites, however, the assessment
of viability is important because intact
non-viable ova may be detected. The
inactivation rates of ova from three
species of roundworms and a tapeworm
when stored in sludges have been
studied. Both viability and infectivity of
the ova were evaluated, and the
conclusion was that long-term storage
may be an effective method of
eliminating parasite eggs.
A number of
studies have shown that composting is
an effective method of eliminating
parasite ova. In this context, EPA permits
unrestricted use of sludges subjected to
PFRP treatment such as composting.
The World Health Organization has
concluded that the risk of infection to
man from parasites associated with the
use of (untreated) sludge needs
evaluation.
More than 110 enteric viruses are
recognized and may be present in
sewage. The list of enteric viruses is
increasing and now includes rotaviruses
and the Norwalk viral agent. Most viruses
probably adsorb to the solids in sewage
although the adsorption process has
been shown to be type- and strain dependent.
The potential for
transmission of viral diseases through
sludge reuse programs does exist. Virus
data collected during a compost monitoring
program indicated that
composting was an effective means of
reducing viral levels. The results also
suggested that the composting
procedures must be well-defined, with
monitoring or process assurances that all
of the composting mass is exposed to
adequate temperature. If not properly
controlled, virus survival could occur.
Infective doses for the viruses are
thought to be low. Therefore, reasonably
thorough virus inactivation is necessary
for home use of sludge products. Once
destroyed, enteric viruses cannot reestablish
themselves in sludge; regrowth
is not a concern.
The World Health Organization and
others have addressed the question of
risk to health from use of sludge on land.
Appropriate measures are available to
manage the risk. A group of scientists
meeting in Denver in 1983 pointed out
that there appear to be no published
records of adverse health effects on man
associated with the land application of
sludge.
The lack of information on health
problems associated with D & M sludges
may reflect the absence of a problem,
the lack of intensive surveillance, or the
inability to detect recurrent small-scale
incidents of disease. In general, the
levels of enteric disease in the U.S. .are
low because of good sanitation, personal
hygiene, and extensive public treatment
works. In recent years, however,
waterborne outbreaks of disease and the
rates of certain enteric diseases, such as
salmonellosis, have been increasing.
Conclusions
All of the sewage sludge products
examined were found to contain variable
densities of indicator microorganisms.
Some products contained bacterial
pathogens at high frequencies and
levels. Variability of microorganism
concentrations was often great between
different facilities and between different
samples from the same facility. Many of
the observed trends would not have been
detected without a large number of
samples collected over a long period of
time.
Overall, the highest concentrations of
microorganisms occurred in samples
from static pile composting systems; the
lowest concentrations were found in
pelletized sludge from a heat drying
process. Microorganism densities in
aged anaerobically digested, air-dried
sludges were as low as, or lower, than
most of the composted sludges.
Composts modified with various
materials to produce commercial soil
amendments contained significantly
higher concentrations of bacteria and
fungi than the base compost material.
The data suggested a nutrient-related
regrowth phenomenon.
Potentially pathogenic bacteria,
including Salmonella sp., Yersinia
enterocolitica and toxigenic E. coli were
detected. Salmonella sp. were the most
frequent pathogen detected. The
quantitative test for toxigenic E. coli
indicated that these strains, when
present, occurred at very low levels.
However, the percentage of colonies that
were toxigenic strongly suggested that
the concentration of toxigenic strains was
much higher than indicated. Yersinia
enterocolitica occurred at very high
densities in some samples. The isolation
of Yersinia was consistent with a
seasonal occurrence. The prevalence
and density was higher in colder months.
Based on a small number of tests, the
Yersinia appeared to be avirulent (not
causing disease).
No significant health hazard was
associated with respect to
Campylobacfer, parasitic helminth ova or
enteric viruses. The test for
campylobacters in compost was
relatively ineffective, but other available
data suggest these bacteria would not
survive composting or air drying.
Helminth ova were detected regularly,
but no indications of viability were
observed. No protozoan cysts were
found.
The fungus Aspergillus fumigatus was
detected in products from most sample
sites but usually at low densities. The
highest concentrations of A. fumigatus
occurred in composts from static pile
composting facilities.
Given the considerable variation
observed in microbial densities and the
reasonably frequent isolation of
salmonellae, bacterial monitoring to
assure product quality may be of value
for the home use of sludge and compost
soil amendments. Regression analysis
suggested that total or fecal coliforms or
fecal streptococci may be suitable
indicators for monitoring.
The occurrence of pathogenic bacteria
in distributed and marketed ,municipal
sewage sludge products represents a
potential health hazard. However, the
extent of risk associated with use of such
products remains to be determined.
Recommendations
Factors associated with the extensive
variability observed in the microbial
populations need to be better delineated
in order to institute appropriate control
measures.
Significance of the relatively high
microbial concentrations in static pile
compost products should be determined.
The influence of recycling wood chips
should be further evaluated.
Additional studies on Salmonella
regrowth are recommended. The effects
of substrate additions should be
evaluated. Laboratory regrowth
experiments to date may not have
adequately simulated field conditions.
Consideration should be given to
establishing criteria and conducting
research necessary for qualifying
digested, air dried sludges as equivalent
to PFRP treated sludges.
Further studies are recommended to
quantitate toxigenic E. coli populations.
Gene probe techniques may be
applicable to this task.
The potential for sludge and compost
to serve as a reservoir of pathogenic
Yersinia in certain locations needs
additional evaluation.
Bacterial limits may need to be
established for the uncontrolled home
use of sludge and compost products or
appropriate educational material should
be supplied to users of the products.
Studies should be conducted to
determine the extent of risk, if any, of
bacterial infections from the use of
distributed and marketed municipal
sewage sludge products.
William A. Yanko is with County Sanitation Districts of Los Angeles County,
Whittier, CA 90607.
Walter Jakubowski is the EPA Project Officer (see below).
The complete report, entitled "Occurrence of Pathogens in Distribution and
Marketing Municipal Sludges, " (Order No. PB88- 154 273iAS; Cost: $25.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22 16 1
Telephone: 703-487-4650
The Project Officer can be contacted at:
Health Effects Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 2771 1
United States Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268