&EFA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-170 Oct. 1981
Project Summary
Density Levels of Pathogenic
Organisms in Municipal
Wastewater Sludge—
A Literature Review
Dana C. Pederson
This report discusses a critical
review of the literature from 1940 to
1980 of laboratory and full-scale
studies on density levels of indicator
and pathogenic organisms in munici-
pal wastewater sludges and septage.
The effectiveness of conventional
municipal sludge stabilization processes
(mesophilic anaerobic and aerobic
digestion, composting and lime stabil-
ization) and dewatering processes
(drying beds, lagooning/storage, and
sludge conditioning/mechanical de-
watering) was evaluated for reducing
density levels of indicator and patho-
genic organisms. An annotated bibli-
ography presents all citations reviewed,
with pertinent abstracts and methods
used by researchers.
This Project Summary was devel-
oped by EPA's Municipal Environmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Sludges originating from municipal
wastewater treatment plants harbor a
multitude of microorganisms, many of
which present a potential health hazard.
Risk of public exposure to these or-
ganisms is possible when sludges are
applied to land as a means of disposal. In
recognition of this problem, and as
required by Section 405 of the Clean
Water Act of 1977 (PL 95-217), criteria
for the control of infectious disease in
the land application of sewage sludge
and septic tank pumpings were issued
by the U.S. Environmental Protection
Agency (EPA) in 40 CFR Part 257
(Federal Register Vol. 44, No. 179,
September 13, 1979).
The "Part 257 criteria" specify what
minimum treatment of municipal waste-
water treatment plant sludges is re-
quired prior to land application of the
residue. Acceptable treatment methods,
termed "Processes to Significantly
Reduce Pathogens," are as follows:
• Aerobic digestion—Agitation of
sludge in aerobic conditions at
residence times ranging from 60
days at 15 °C to 40 days at 20 °C,
with a volati le sol ids red uction of at
least 38%.
• Air drying—Draining and/or drying
of liquid sludge on underdrained
sand beds, or on paved or unpaved
basins in which the sludge is at a
depth of 9 inches (22.9 cm). A
minimum of three months is needed,
two months of which temperature
average on a daily basis is above 0
°C.
• Anaerobic digestion—Maintenance
of sludge in the absence of air at
residence times ranging from 60
days at 20°C to 15 days at 35°C to
-------
55°C, with a volatile solids reduc-
tion of at least 38%.
• Composting—Using the within-
vessel, static aerated pile or wind-
row composting methods, the
sludge is maintained at minimum
operating conditions of 40°C for
five days. For four hours during this
period, the temperature exceeds
55°C.
• Lime stabilization—Application of
lime to sludge in quantities suf-
ficient to produce a pH of 12 after
two hours of contact.
• Techniques demonstrated to be the
equivalent of the above on the
basis of pathogen removals and
volatile solids reduction.
An additional category of treatment
processes, termed "Processes to Further
Reduce Pathogens", was designated in
Appendix II of 40 CFR Part 257 as
required if (1) affected land is to be used
within 18 months of sludge application
for the cultivation of food crops and (2)
the edible portion of the crop is likely to
be exposed to the sludge. These addi-
tional processes are:
• High temperature composting
• Heat drying
• Heat treatment
• Thermophilic aerobic digestion
* Irradiation
In 1980, Camp, Dresser and McKee,
Inc. (COM) undertook a literature review
of available domestic and foreign data,
from 1940 to 1980, of bacteria, viruses
and parasites densities in raw municipal
wastewater sludges, on the effectiveness
of the "Processes to Significantly
Reduce Pathogens" and of conventional
sludge dewatering techniques (mechan-
ical dewatering/sludge conditioning
and sludge storage/lagooning) to reduce
levels of these organisms.
The following organisms, categorized
into four groupings, were emphasized:
• Indicators—Total coliform, fecal
coliform, and fecal streptococcus
bacteria; Clostridum perfringens
(welchii); bacteriophage
• Pathogenic bacteria—Salmonellae,
Shigellae, Pseudomonas sp.,
Mycobacterium spp., Candida
albicans, Aspergillus fumigatus
• Enteric viruses—Enterovirus and
its subgroups (polioviruses, echo-
viruses and coxsackieviruses),
reovirus and adenovirus
• Parasites—Entamoeba histolytica,
Ascaris lumbricoides, Taenia spp.,
Schistosoma spp., and others
In addition to reporting density levels
in raw sludge and septage, and the
effectiveness of conventional sludge
treatment processes in reducing density
levels, this review also identified design
and operating variables that affect
process efficiency, compared results of
laboratory pilot-scale studies to those of
full-scale plants, and contrasted survival
of indicator organisms to that of
pathogens. Methods used by each
researcher to enumerate organisms
were also described, and brief sum-
maries were provided of related cita-
tions that were encountered but were
not actually used in this report.
Density Levels in Raw Sludge
Levels of bacteria, viruses and para-
sites in raw sludge are presented in
Table 1. Note that the densities of
pathogenic organisms are several logs
less than indicator organisms. Also,
there is a noticeable lack of information
on the densities of select pathogenic
organisms in raw sludges and septages
(i.e., lack of parasite organisms data in
septages).
Anaerobic Digestion
This process involves biological
degradation of complex organic sub-
stances present in wastewater sludges
in the absence of free oxygen. Primary
or secondary sludge, or a mixture of
both, is fed continuously or intermittently
into an airtight vessel and retained for
varying periods of time.
Retention times can vary from 30 to
60 days in low-rate (unmixed) reactors
and from 10 to 20 days in high-rate
reactors which are mixed and heated to
either mesophilic—30 to 38 °C—or
thermophilic—50 to 60 °C—tempera-
tures. The digester's performance is
indicated by the percent of volatile
solids (VS) destroyed. Reduction of VS
usually ranges between 35% and 60%,
depending on the character of the
sludge, detention time and tempera-
ture.
Only limited information was found
on levels and reductions of densities of
organisms in low-rate digesters. Longer
detention times and higher temperatures
are correlated with greater density
reductions. In high-rate digesters at
full-scale plants, reductions of greater
than 1 log occur in densities of bacteria
and viruses, with the exception of
Pseudomonas aeruginosa (Table 2). Ova
and cysts of parasitic tapeworms,
flatworms and roundworms (with the
exception of Trichinelta spiralis) were
able to survive this digestion process,
while parasitic protozoans were reduced
to non-detectable levels.
Comparison of laboratory/pilot-scale
data to those of full-scale plants
generally indicated that greater density
reductions are accomplished in the
smaller-scale studies. The larger density
reductions are attributed to (1) the
ability to achieve optimum digestion
conditions on a smaller scale; (2) the
absence of short circuiting—when fresh
sludge (and, with it, high levels of
organisms) is allowed to exit—in labora-
tory/pilot-scale studies; (3) the dif-
ferences in sensitivity to the effects of
anaerobic digestion of laboratory-
Table 1. Density Levels of Organisms in Raw Sludge and Septage (Average
Geometric Mean of Organisms Per Gram Dry Weight)
Organism
Primary Secondary
Mixed
Septage
Total coliform bacteria
Fecal coliform bacteria
Fecal streptococci
Bacteriophage
Salmonella sp.
Shigella sp.
Pseudomas aeruginosa
Parasite ova/cysts (total)
Ascaris sp.
Trichiuris trichiura
Trichiuris vulpis
Toxocara sp.
Hymenolepsis diminuta
Enteric viruses*
1.2 x 10s
2.0 x W7
8.9 x 10s
1.3 x 10s
4. 1 x 102
NR
2.8 x JO3
2. 1 x 102
7.2 x JO2
1.0 x W
1.1 x 1O2
2.4 x 102
6. x 10°
3.9 x 102
7.1 x 10*
8.3 x 10s
1.7 x 10s
NR*
8.8 x 102
NR
1.1 x 104
NR
1.4 x 103
<1.0x 10'
<1.0x W
2.8 x 102
2.0 x W
3.2 x 102
1.1 x 10s
1.9 x 10s
3.7 x 106
NR
2.9 x 102
ND*
3.3 x 103
<5.0x 10'
2.9 x 102
0
1.4 x 102
1.3 x 103
0
3.6 x 102"
1.4 x 10s
1.2 x 10s
6.6 x 10s
NR
5.1 x 10''
NR
2.6 x 10'
NR
NR
NR
NR
NR
NR
NR
* NR = No data available
" ND = None detected
c Plaque forming units per gram dry weight (PFU/gdw)
d TCIDso = 50 percent tissue culture infectious dose
-------
Table 2.
Density Levels of Indicator Bacteria, Pathogenic Bacteria and Enterovirus
Following High Rate Anaerobic Digestion at 35°C for 14-15 to 21 Days
Log Reduction
Organism
Total coliform
Fecal coliform
Fecal streptococcus
Salmonella sp.
Ps. aeruginosa
Enterovirus
Density Level"
per 1OO ml
3x 10^
2 x Wec
9 x W5C
3.7 x W"
6x 10s"
7.9 x 10
-------
(f2) when added to sludge was found to
be far more resistant to mesophilic
temperatures and, therefore, reductions
in levels of this non-pathogenic and
easily cultured virus could provide a
useful indication of enterovirus inacti-
vation.
Ova of the roundworm, Ascaris
lumbricoides, can survive at tempera-
tures higher than those specified for
mesophilic composting, presenting a
potential problem in sludge treated by
mesophilic composting.
Lime Stabilization
Lime is mixed with sludge in quantities
sufficient to raise the pH to 12.0 for at
least two hours. Lime may be added (1)
to liquid sludge prior to dewatering, (2)
directly to a mixed-sludge storage tank,
followed by land application; or (3) to a
dewatered sludge cake. The technique
most commonly used by the researchers
whose data were utilized in this review
involved the addition of lime to liquid
sludge.
Lime stabilization can effect signifi-
cant reductions in levels of some
indicator and pathogenic bacteria and,
possibly, of poliovirus. The effective-
ness has been shown to be contingent
upon the pH achieved in the stabilization
protocol. It appears that different
bacteria respond differently to increas-
ing levels of pH achieved in the process.
Even after an effective pH level is
achieved in sludge, the decrease in pH
level that occurs after the initial
exposure and minimum contact time
can create an environment favorable to
regrowth of some bacteria.
Fecal coliform, Salmonella spp. and
Pseudomonas aeruginosa density levels
all appear to be reduced by 2 logs or
greater at pH 11 or above. There is no
apparent tendency for these micro-
organisms to regrow. Fecal strepto-
coccus, however, are more resistant to
lime inactivation and are able to regrow
quickly with decreasing pH to near
original densities or greater within 24
hours.
In one study, lime treatment of
sludges inactivated Ascaris eggs; how-
ever, the lime concentrations and time
needed were substantially greater than
is normally used for sludge condition-
ing. Also, this inactivation of Ascaris
eggs was not always consistent and
therefore cannot be relied upon.
Conventional Sludge
Dewatering Processes
Drying Beds
In general practice, digested sludge is
placed on sand beds or paved beds that
have been provided with drainage. The
sludge is allowed to dry to approxi-
mately 40% solids content, over a period
of about 10 to 15 days (under favorable
conditions)—a significantly shorter time
period than the minimum of three
months delineated by the EPA.
Information was found on the in-
activation of bacteria, viruses and
parasites during drying, but none of the
data conformed to the criteria specified
by the EPA. The research conducted
does, however, focus attention on the
solids level achieved during drying. This
parameter could be useful, in addition to
time, temperature, and sludge depth, as
an additional criterion for defining air
drying of sludge.
Sludge Storage/Lagooning
In this process, anaerobically or
aerobically digested sludge is stored in
earth- or concrete-lined lagoons at
depths of from 60 to 600 cm for periods
ranging from several months to years.
The performance of the lagoons is
affected by climate; both precipitation
and low temperatures will inhibit
dewatering and the rate of volatile
solids reduction. The two factors that
have been studied with regard to
survival of indicators, pathogens and
parasites are temperature and length of
storage time. At lower temperatures, a
longer detention time is required to
achieve reduction of density levels.
It was concluded, based on trends
indicated by the data reviewed, that at
temperatures of 20°C or greater, the
minimum storage time required to
achieve a 1 -log density reduction is one
month for bacteria, two months for
viruses, and greater than six months for
parasitic ova. At temperatures of less
than 20°C, more than six months,
storage is required to reduce density
levels of pathogenic bacteria by 1 log,
more than eight months for viruses, and
at least three years for parasitic ova.
Sludge Conditioning/
Mechanical Dewatering
For purposes of this review (and as
commonly practiced), dewatering in-
volves use of vacuum filter, chamber
filter press, belt filter press, or centrifuge
to separate the liquid and solid com-
ponents of sludge. Typically sludge cake
solids content of 15 to 40% are achieved.
Chemical conditioners used to aid
sludge dewatering include lime (CaO),
ferric chloride (FeCI3), ferrous sulfate
(FeS04), and polyelectrolytes or poly-
mers.
The process of mechanical dewatering
of municipal wastewater sludges alone
has little effect on the density levels of
pathogens. The conditioners that are
commonly used in combination with
mechanical dewatering vary in effects.
Polymer has no apparent effect on
density levels of pathogens. Lime,
added in concentrations to optimize
dewatering, cannot be relied on to
reduce pathogen levels because of the
variations in pH levels obtained. Ferric
chloride, often used in conjunction with
lime, appears to reduce whatever
virucidal and bactericidal effects the
lime normally has when applied to
sludge.
Conclusions and
Recommendations
Because a large body of literature
containing comparable data is not
available, it is recommended that
additional research be conducted on the
effectiveness of sludge treatment
processes in reducing density levels of
organisms. It is recommended, further,
that researchers document carefully all
pertinent aspects of their experimental
design.
The following conclusions appear to
be valid based on the literature reviewed:
• Anaerobic digestion and lime
stabilization consistently produce
reductions of about 1 to 2 logs in j
densities of indicator and patho- [
genie bacteria and, in the case of |
anaerobic digestion, in densities of i
viruses as well. At a minimum, \
effectiveness depends on the
processes being carried out under'
the conditions specified in 40 CFR
Part 257. Neither sludge stabiliza-
tion process appears to be particu-
larly effective for inactivating
parasite organisms.
• Conditions of mesophilic composting
may inactivate common indicator
and pathogenic bacteria and viruses,
provided that specified tempera-
tures are attained uniformly
throughout the compost mass for
over the specified time period. The
pathogenic fungus Aspergillus
fumigatus thrives under conditions
4
-------
of mesophilic composting, how-
ever, and parasite ova appear to
survive this process.
• Density reductions of bacteria by
aerobic digestion are variable and
of relatively small magnitude.
However, there is a lack of data on
the performance of this process
and also of air drying in reducing
densities of microorganisms.
• Sludge lagoons can achieve 1-log
reductions in densities of bacteria
and viable parasite ova, but, de-
pending on conditions, storage of
one month to more than three
years may be required.
• Mechanical dewatering of sludge,
with or without the use of chemical
conditioners, has little reliable
effect on densities of pathogens.
Few of the laboratory-scale studies
reviewed could be related to results
obtained at full-scale treatment plants.
Operating parameters used in laboratory
experiments differed radically from
those at full-scale plants. For this
reason, comparing density levels was
seldom possible. In addition, laboratory
studies often used seeded bacteria,
viruses, or parasites and it is doubtful
whether their behavior mimics that of
naturally occurring organisms.
No single indicator organism (either
bacteria or bacteriophage) was found to
maintain a density level of a constant
relative value to that of pathogenic
organisms. The data available made it
impossible to determine whether this
inconsistency is due to the inability of
current techniques to enumerate patho-
genic bacteria and enteroviruses ac-
curately, or to the fact that densities
actually vary.
Of the traditional indicators, fecal
streptococci appear to be the most
conservative indicator of both the
density levels of pathogenic bacteria
and enterovirus in raw sludge and of
their inactivation during sludge treat-
ments. Additional research is required
to identify other indicator systems, both
bacterial and viral, whose numbers
better reflect both density and reduction
of density levels of pathogenic organisms.
A wide variety of methods were used
to enumerate all of the organisms
considered in this review. Although
standard methods are available for
quantifying the coliform and strepto-
coccus bacteria and for Salmonella sp.,
there are no standard techniques for
other pathogens, enteroviruses, or
parasites. It is recommended that this
area be addressed so that comparable
data can be produced in future studies.
The full report was submitted in ful-
fillment of Work Task 08 for Contract
No. 68-03-2803 by Camp Dresser &
McKee Inc., under the sponsorship of
the U.S. Environmental Protection
Agency.
Dana C. Pederson is with Camp, Dresser, and McKee, Inc., One Center Plaza,
Boston, MA 02108.
Gerald Stern is the EPA Project Officer (see below).
The complete report, entitled "Density Levels of Pathogenic Organisms in
Municipal Wastewater Sludge—A Literature Review," {Order No. PB 82-102 286;
Cost: $21.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
U.S. GOVERNMENT PRINTING OFFICE:1981--559-092/3317
------- |