vvEPA
United States
Environmental Protection
Agency
Office of Water
Regulations and Standards
Washington, DC 20460
Water
TECHNICAL SUPPORT
DOCUMENT
Pathogen/Vector Attraction
Reduction in Sewage Sludge
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PREFACE
Section 405(d) of the Clean Water Act requires the U.S. Environmental
Protection Agency (EPA) to develop and issue regulations that identify:
• Uses for sludge, including various means of disposal
• Factors, including costs, which must be considered when determining
the measures and practices applicable to each use or disposal method
• Pollutant concentrations that interfere with each use or disposal
method
To comply with this statutory mandate, EPA has embarked on a program to
develop five major technical regulations: land application, distribution and
marketing; monofilling; surface disposal; incineration; and reduction of
pathogens and vector attraction. EPA has also proposed regulations governing
the establishment of State sludge management programs, which will implement
both existing and future criteria (40 CFR 501).
The principal goal of the proposed regulation for pathogen and vector
attraction reduction is to protect human health. These requirements apply co
sewage sludge that is applied to the land, distributed and marketed, or placed
in a monofill or impoundment. This document provides the technical background
and justification for the provisions contained in Subpart F of the proposed
regulation.
Public comment on the technical adequacy and scientific validity of this
document as well as the requirements contained in the proposed regulation
should be submitted during the public comment period. Any questions related
to this document may be directed to:
Dr. Joseph B. Farrell
U.S. Environmental Protection Agency
Risk Reduction Environmental Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7645
Martha Prothro, Acting Director
Office of Water Regulations and Standards
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TABLE OF CONTENTS
Page
LIST OF UNITS AND ACRONYMS v
1. INTRODUCTION 1-1
1.1 Sludge Use on Land Before Federal Regulation 1-1
1.2 Federal Regulations 1-3
2. COMPONENTS OF DISEASE RISK AND THEIR CONTROL 2-1
2.1 Pathogens of Concern 2-1
2.1.1 Bacteria 2-1
2.1.2 Viruses 2-6
2.1.3 Protozoa 2-11
2.1.4 Helminths 2-12
2.2 Transport of Organisms 2-14
2.2.1 Air Transport 2-15
2.2.2 Groundwater Transport 2-16
2.2.3 Surface Water Transport 2-17
2.2.4 Adherence to Objects 2-18
2.2.5 Transport by Vectors 2-18
2.3 Vector Attraction 2-19
2.4 Infective Dose 2-22
3. THE PROPOSED REGULATION 3-1
3.1 Background 3-1
3.2 Scientific Basis of the Proposed Regulation 3-3
3.2.1 Specialized Definitions (Section 503.51) 3-3
3.2.2 Class A Requirements 3-5
3.2.3 Class B Requirements 3-16
3.2.4 Class C Requirements 3-23
3.2.5 Septage 3-26
4. REFERENCES 4-1
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LIST OF UNITS AND ACRONYMS
CFU colony-forming units
cm centimeter
EPA U.S. Environmental Protection Agency
g gram
ha hectare
m meter
mg milligram
mL milliliter
mm millimeter
MPN most probable number
NP/LSA no primary/long-sludge age
no./g VSS number pathogens/gram volatile suspended solids
sludge
PFRP process to further reduce pathogens
PFU plaque-forming units
PSRP process to significantly reduce pathogens
VS volatile solids
VSS volatile suspended solids
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SECTION ONE
INTRODUCTION
Human bodily waste has been applied to soil for its fertilizer value and
organic content since the beginning of recorded history and, no doubt, before
that time. Land application of human wastes has many beneficial aspects;
however, most countries of the world now regulate this practice to minimize
the potential for the spread of disease by pathogens (disease-causing
microorganisms) in human wastes and the possibility that contaminants in
sludge may reduce soil productivity and crop wholesomeness.
This technical support document describes sludge utilization in the United
States and the risks of disease from sludge utilization. It summarizes the
existing Federal regulations for controlling pathogens in sludge, discusses
their shortcomings, and presents new knowledge relevant to creating an
improved regulation. Section 3 presents the principal aspects of the proposed
new regulations concerning sludge pathogen control and discusses the available
scientific information that supports these regulations, as well as data gaps
and areas in which additional information would be helpful.
1.1 SLUDGE USE ON LAND BEFORE FEDERAL REGULATION
Most cities and towns in the United States have processed their wastewater
in centralized facilities since the early 1900s. This processing produced
large volumes of sludge that had to be disposed of. Widely disparate disposal
practices developed, ranging from ocean disposal to incineration; each
municipality chose the practice that best suited its particular circumstances.
Frequently, muncipalities simply gave their sludge away to farmers or
processed it into a fertilizer product.
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Agricultural use of sludge was important enough that, in 1946, members of
the Federation of Sewage Works Associations (the forerunner of the Water
Pollution Control Federation) made it the subject.of their second manual of
practice: "Utilization of Sewage Sludge as Fertilizer" (FSWA, 1946). Their
manual details a variety of utilization practices in use at that time.
Several large municipalities were heat drying waste-activated sludge and
selling it as a fertilizer. Air-dried digested sludge, generally dried on
sand beds, was used on orchards. The manual recommended using the material in
the same manner as manure for vegetable crops, including root crops. It also
described use of municipal sludge on farms growing oats and wheat, and tank
truck delivery of sludge for use on lawns and parks. The manual cautions
about pathogenic organisms in sludge, but notes that only 1 of 33 states chat
responded to a survey had regulations to protect public health from this
potential threat.
Although the manual indicates some awareness of the potential health
hazard from sludge use (for example, the manual recommends that raw primary
sludge not be used on the land) , the land application practices in use at that
time were not uniform and, in some cases, apparently posed a high risk to
health. Burd (1968) listed similar uses of sludge in the late 1960s and noted
that the hygienic considerations relating to sludge use were primarily under
the authority of local health boards.
The 1972 Federal Water Pollution Control Act (PL 92-500) encouraged land
application of sludge by explicitly endorsing nutrient recycling and by
stimulating construction of wastewater treatment facilities. Large cities
became interested in land application. Chicago developed its "Prairie Plan"
for agricultural use of most of its sludge (Dalton and Murphy, 1973). The
great interest in and diverse approaches being suggested for land application
of sludge necessitated strong guidance to protect human health and the
environment.
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1.2 FEDERAL REGULATION
In 1976, Congress passed the Resource Conservation and Recovery Act, which
mandated the U.S. Environmental Protection Agency (EPA) to regulate the
application of solid waste to land. Sludge was defined as a solid waste to be
regulated under the Act. "Criteria for Classification of Solid Waste Disposal
of Municipal Wastewater Sludge" (hereafter, "Criteria") were published in
September 1979 (Federal Register, 1979). Requirements relevant to sludge
covered allowable loadings of certain toxicants and criteria to control risk
of disease. The major features of the regulation relating to control of
disease risk were:
• Unstabilized sludge could not be used on the land.
• Sludge could be used on land if it was treated by processes that
reduced pathogens and the sludge's attractiveness to disease vectors
(flies, rodents, etc.), and if certain restrictions relating to access,
grazing, and type of crop grown were observed.
• If pathogen levels were reduced to below detection levels and vector
attraction was reduced, there were no restrictions to sludge use.
• The rules for septage were similar to those for sludge, except that
unstabilized septage could be applied to soil if crops for direct human
consumption were not grown.
The regulations created a uniform approach for land application. They did
not appear to substantially affect the practice of land application. They
mostly impacted new construction where plans for sludge treatment would be
examined by State and Federal authorities. Although statistics on land
utilization have not been kept, the practice appears to have kept pace with
the increase in mass of sludge generated in this country. According to a 1984
EPA report (EPA, 1984), up to 40% of the sludge produced by municipal
wastewater treatment plants is used on the land or distributed and marketed to
the public.
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SECTION TWO
COMPONENTS OF DISEASE RISK AND THEIR CONTROL
2.1 PATHOGENS OF CONCERN
2.1.1 Bacteria
2.1.1.1 Types and Measurement
Pathogenic bacteria (i.e., bacteria chat create disease) are not normal
inhabitants of the human enteric (intestinal) system. They are present when.
an individual has contracted a bacterial disease or illness. Consequently,
they rarely occur in fecal waste, but when they do, high levels are found.
Since wastewaters from a sewage treatment plant contain Che wastes from many
people, pathogens are usually present, but their densities vary depending on
Che prevalence of bacterial disease in the local community. Frequently, there
are long periods when certain pathogenic bacteria are below detection limits
in municipal wastewater and in the sludges produced (Farrell et al., in
press). Bacterial species present in wastewater and cheir densities are
therefore highly unpredictable.
Kowal (1985) lists several pathogenic enteric bacteria and bacterial
species that are of major concern in sludge and a greater number that are of
minor concern. The selection was based on the density of the bacteria in
sludge, the extent of the disease, and the seriousness of the illness
produced. Among bacteria species singled out are Shigella spp., Salmonella
spp., and Yersinia spp. All cause enteric disease in large numbers of people
in the United States. Most research on bacterial survival in sludge has
focussed on Salmonella spp., which include over 1,000 serotypes. These
organisms may cause salmonellosis, an acute gastroenteritis. Salmonellae have
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been emphasized because they are more frequently identified in sludge than
other bacterial species, they cause severe illness with relative frequency,
and a reliable quantitation method exists that can detect them.
The densities of pathogenic bacteria in the stool of an infected person
may be 106/gram (g) (Kowal, 1985). Typical values in sludge are much lower.
For example, densities of Salmonella spp. over 103/g are encountered
infrequently. On the other hand, the normal bacterial inhabitants of the
lower intestines are found at much higher densities. Obligate anaerobic
bacteria have densities approaching 109/g in feces. Facultative bacteria,
such as the fecal coliform group, have densities around 108/g-
Monitoring sludge on a regular basis to determine the types and densities
of pathogenic bacteria present is desirable but, in practical terms,
unattainable. Quantitation methods for some bacterial species are difficult
and unreliable, and require high skill levels. Too many species would have to
be measured, even if measurement were restricted to bacteria of major concern.
The presence of one pathogenic species cannot be used as an indicator of
the presence of other pathogenic species because there is no reason for the
high incidence of one enteric bacterial disease in a community to correlate
with other enteric bacterial diseases. The best indicators of the potential
presence of bacterial enteric pathogens in sludge are the facultative enteric
organisms that normally inhabit the intestines, such as Escherichia coli. the
fecal coliforms, and fecal streptococci. Though these indicators do not
correlate well with any individual enteric bacterial pathogen species, they do
indicate the presence of human fecal waste, which is the carrier of the
pathogens. Thus, they are good long-term indicators of pathogenic bacteria,
but will not always correlate well in the short term.
Indicator organisms are frequently used as surrogates for the pathogenic
enteric bacteria to reflect their response to environmental exposure or sludge
treatment processes. This offers advantages over attempting to measure
effects on pathogens directly, because pathogens are generally present in low
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densities in sludge and are sometimes absent; consequently, unreasonably long
experimental programs are frequently needed to obtain definitive results,
especially when the exposure causes large reductions in pathogen densities.
The sludge or wastewater can be "spiked" with cultured pathogenic bacteria to
increase their densities; however, the responses of cultured organisms are not
always the same as those of naturally occurring organisms (Farrah et al.,
1986). Indicator organisms, on the other hand, are expected to behave
similarly to the pathogenic enteric bacteria because their life processes and
ecological niches are similar (they belong to the same class,
Enterobacteriaceae). Experimental results indicate that the responses of the
indicators and salmonellae to sludge processing do correlate satisfactorily
(Farrell et al., in press; Farrah et al., 1986).
2.1.1.2 Effects of Processing
The pathogenic bacteria in wastewater are insoluble solids. The bacteria
are small particles with densities only slightly greater than water.
Consequently, they settle poorly unless flocculated. They can be removed from
the wastewater by filtration. In primary wastewater treatment, bacteria are
associated with solids. These solids (and the associated bacteria) settle
fairly well, but many escape this step. Collection is greatly improved in the
secondary clarifier, where they become enmeshed in the biological solids.
These solids autoflocculate and settle, carrying with them most of the
bacteria that escaped collection in the primary clarifier.
Wastewater Treatment
Besides merely collecting solids, the wastewater treatment processes may
actually reduce numbers of pathogens. Simple sedimentation in a clarifier
causes little change (Farrell et al., in press), but biological processes,
such as the trickling filter and activated sludge process, may substantially
reduce the numbers of pathogens (Farrell et al., in press). These reductions
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may be caused by the time of contact, temperature, lytic effect of bacterial
enzyme systems, action of bacterial viruses (bacteriophages), and "grazing of
protozoa" (Farrah et al. , 1986). Farrell et al. (in press) and Lee et al. (in
press) report bacterial reductions (based on number of organisms per unit mass
of suspended solids) of 1.5 logs in extended aeration systems.
Digestion
The conventional processes of anaerobic and aerobic digestion cause
substantial bacterial reductions. -Farrell et al. (1985) show reductions of
indicator organism densities (colony-forming units [CFU]/100 mL) of about 1
log for salmonellae and 1.7 logs for indicator organisms. Martin (in press)
showed that aerobic digestion for 10 days at temperatures averaging 30°C
causes reductions of similar magnitude.
Storage
Storage time has an important effect on the density of bacterial pathogens
and indicator organisms. Stern and Farrell (1978) have shown that storage at
208C reduces Salmonella spp. by 2 logs in 3 months and indicator densities by
about the same amount. Storage at 58C was much less effective. Storage of
sludge cake would probably show the same effect.
Lime Stabilization
Lime is frequently used to condition or stabilize sludge before disposal.
A pH of about 12 essentially eliminates salmonellae and greatly reduces the
density of indicator organisms (Counts and Shuckrow, 1975).
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Elevated Temperature
The enteric bacteria of concern do not form heat-resistant spores, so they
are easily destroyed by elevated temperatures. Stabilization processes chat
elevate the sludge temperature to 53°C or above for a sufficient time
effectively eliminate pathogenic bacteria.
2.1.13 Environmental Effects
When sludge is applied to land, environmental conditions reduce bacterial
densities. Kowal (1985) reviewed the literature on the survival times of
pathogenic bacteria in the environment. Survival times vary widely depending
on the initial densities of the pathogens and on environmental factors such as
temperature, degree of desiccation, and amount of ultraviolet radiation.
Bacterial pathogens in sludge on plant surfaces die off more quickly than
those in the sludge on the soil surface, because of more exposure to sunlight
and greater desiccation. Kowal indicates that the risk from pathogens
deposited on plant surfaces during application should become minimal 1 month
after application; however, he recommends that low-growing food crops, such as
strawberries, that touch the soil or are splattered by soil aerialized by
rainfall not be harvested until 6 months after sludge use.
Bacteria in sludge are effectively filtered by the soil except in cases
where the soil is a coarse sand. Gerba et al. (1975) observed that over 91%
of the coliforms were trapped in the first centimeter of soil. Soils subject
to cracking, such as clay soil in dry weather, pose special problems. Sludge
application on such soils after a dry period could be unwise, depending on the
proximity of the surface to ground or surface water. Farm soils used to grow
row crops (e.g., feed corn, soybeans) are probably superior co forest soils
for protecting ground water. Root, insect, and animal holes are destroyed by
the soil preparation required for crop production, and soil layers are usually
much deeper on farms than in forests. If bacteria should reach ground water,
transport within that media is extremely slow and should give ample time for
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further reductions in pathogen density. In certain kinds of geology, such as
limestone or fractured structures, soil leachate can travel rapidly. Use of
sludge on soil where such geology is not protected by a thick soil layer poses
an elevated risk.
Bacteria are unique among sludge pathogens in their ability to regrow.
Enteric bacteria are facultative and can multiply under aerobic or anaerobic
conditions. If they encounter a suitable food source such as bruised or
cracked fruit, they can regrow to high densities. They can also regrow to
high densities when competition from other bacteria is reduced, even in a low-
energy substrate such as well-stabilized compost. Burge et al. (1986) showed
that salmonellae innoculated into sterilized compost grew to a higher level
for a longer time than salmonellae innoculated into unsterilized compost.
2.1.2 Viruses
2.1.2.1 Types and Measurement
Kowal's extensive review lists the human enteric viruses likely to be
present in'wastewater and sludge (Kowal, 1985). These viruses are not normal
inhabitants of the gastrointestinal tract and their presence indicates an
infection that may show no symptoms. Viruses are released from cells in the
gastrointestinal tract in which they are replicating into the intestines and,
consequently, are present in fecal discharges. They are generally adsorbed to
or enmeshed in solids.
Kowal lists five subclasses of enteroviruses (e.g., polio-, Coxsackie-,
Echo-) as well as hepatitis A virus and rotaviruses. These viruses cause a
wide variety of illnesses; for example, hepatitis A. virus causes approximately
40,000 to 50,000 cases of infectious hepatitis in the United States.
Rotavirus causes acute gastroenteritis, primarily in children. The
enteroviruses cause many diseases including paralysis, diarrhea, meningitis,
heart disease, and respiratory illness. Most infections are asymptomatic, so
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many more infected people shed viruses than indicated by disease incidence
numbers. A particular virus may or may not be present in the wastewater,
depending on the presence or absence of infected people in the community.
There are a variety of methods for identifying viruses in sludge. The
most common procedure, the plaque assay method, is described by Bitton (1970).
A viral suspension extracted from sludge is placed on the surface of an animal
cell monolayer that has been grown on the interior wall of a glass bottle.
After providing time for adsorption of the viruses to the cell layer, an
overlay of agar is poured over the monolayer to immobilize the system and
provide nutrient and moisture for the cells. Time is allowed for the viruses
to invade the cells and replicate. Each virus invasion leads to a zone of
infection where cells have been destroyed. This localized area is called a
plaque. Staining or other techniques are used to identify plaques.
Unfortunately this and other virus identification methods are expensive and
require skilled personnel.
The plaque-forming method currently in use identifies a wide variety of
enteroviruses (Bitton, 1980). The number depends on the types of cells used
in the test. Some important viruses -- Hepatitis A and rotavirus -- are not
enumerated by this method. Serological methods can be used to determine the
specific viruses that formed the plaques but this adds another level of
complexity to an already complex procedure.
The densities of viruses found in sludge range widely. Brashear and Ward
(1982) report 5-145 PFU/raL (plaque-forming units per milliliter) of a raw
sludge. Assuming 2% solids in the sludge, this is equivalent to 250-7,000/g
of solids. Other sludges could show higher or lower densities.
Less information is available on virus densities than on indicator
bacteria or Salmonella spp. densities because of the complexity of the method
for determining densities and the special skills and equipment needed.
Although the plaque-forming method does not indicate the presence of some
viruses, it can enumerate enteric viruses of several types. It could serve as
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a useful indicator test for all enteric viruses, except for its aforementioned
cost and complexity.
An alternative to the enteric virus test to indicate viral densities is to
use the fecal coliforms and/or fecal streptococci test for this purpose. As
with pathogenic bacteria (see above), the fecal indicators are not expected to
correlate over the short term with virus densities; however, since they
indicate the presence of fecal wastes, they will correlate well over the long
term. Indicator organism densities can be used to indicate the effect of
sludge processing conditions on viral densities if available data indicate a
satisfactory correlation between the effect of the condition on viral
densities and indicator organisms.
Such correlations appear to be adequate for some processing procedures but
not for others. For example, Berg and Herman (1980) demonstrated that, in
anaerobic digestion, the decline in viruses showed a reasonable relationship
to the decline in fecal coliforms and fecal streptococci. On the other hand,
irradiation of sludge by gamma rays or high-energy electrons requires 20-30
kilorads to reduce coliforms by 1 log (Ward, 1981), and ten times that dose to
achieve the same reduction in viruses. With irradiation, a relationship
exists between the declines in the two types of organisms, but it is
inappropriate. The coliforms could not be an indicator for viruses because
they could be reduced to negligible values while viruses were still present.
2.1.2.2 Effects of Processing
Wastewater Treatment
Viruses are subcolloidal in size and would not be expected to settle out
or to be filtered out in wastewater treatment; however, they have a strong
tendency to adsorb to solids. Consequently, most of them are removed with the
solids and are found in the sludge streams. The sedimentation processes in
clarifiers do little to reduce viral numbers. However, biological processes,
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such as the activated sludge process, do reduce viruses. Time of exposure is
a factor. Virus densities decline with time even in nonaggressive
environments such as river water (Clarke et al., 1964). Because sludges are
retained in wastewater treatment processes for periods ranging from a few days
to 30 days, declines based on time and temperature of exposure occur.
Bacterial enzyme systems may reduce the numbers of some viruses.
Digestion
Conventional anaerobic and aerobic digestion of sludge reduces viral
densities depending on the temperature, time, type of processing, and
operation of the system. Farrell et al. (1988) have shown chac che manner of
feeding and withdrawing che sludge (fill/draw versus draw/fill) has a small
effect on virus survival. Draw/fill feeding yields better viral reductions.
Berg and Berman (1980) and Farrell et al. (1985) show the reductions achieved
by digestion at a large wastewater treatment plant. Farrah et al. (1986) and
Martin (in press) show effects of aerobic digestion on viral survival.
Lime Treatment
Strauch (1982) presented data demonstrating that lime treatment to pH 12
produces rapid and large reductions in viral populations. Chlorine treatment
is expected to cause similar reductions. Acid treatment is less effective in
reducing virus densities. Exposure to pH 3.5 is typically a step in the
procedure for enumerating viruses.
Elevated Temperature
Stabilization processes that elevate the sludge temperature to greater
than 53°C reduce or even totally eliminate enteric viruses. These processes
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include thermophilic anaerobic and aerobic digestion, composting, and
pasteurization.
Storage
Storage for a period of time is effective in reducing viral densities.
The reduction is a function of temperature. Stern and Farrell (1978) found
that less than 3% of viruses survived in digested sludge stored for 4 months
at 20°C, whereas 33% survived at 5°C.
2.1.23 Environmental Effects
Adverse environmental effects reduce viral densities when sludge is
applied to land. The rapidity of reduction depends in part on the proximity
of the sludge to the land surface. Sorber and Moore (1986) compare data from
several investigators and show that the longest times for 1-log and 2-log
reductions in densities were 30 and 52 days when sludge was applied to or
within 5 centimeters (cm) of the soil surface. The average time for die-off
was generally much shorter than these figures. For deeper application (15
cm), they cite the" work of Damgaard-Larsen et al. (1977) which showed 56 and
100 days required for 1- and 2-log reductions.
Viruses in sludge adhering to the surface of crops die off even more
rapidly because exposure is more severe. Larkin et al. (1976) showed an over
2-log (base 10) reduction in poliovirus densities on lettuce and radishes
about 14 days after spray-irrigation with sludge.
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2.13 Protozoa
2.13.1 Types and Quantification
Numerous protozoa invade the human gastrointestinal system and cause
disease. Their cysts are found in wastewater and sludge. The three most
important noted by Kowal (1985) are Entamoeba histolvtica. Giardia Iambiia.
and Balantidium coli. These organisms are known to transmit human disease
through a water route and by direct contact. The characteristic illness is
diarrhea.
Protozoan cysts are excreted in great numbers from infected persons.
Infection rates in the population are low except for Giardia lamblia. where
the carrier rate may range from 1.5-20% (Benenson, 1975). Levels in
wastewater have been estimated to be 4 cysts/L for Entamoeba histolvtica
(Foster and Englebrecht, 1973) and 10" to 2 x 10s cysts/L for Giardia lamblia
(Jakubowski and Ericksen, 1979). The densities of protozoa in sludge are not
known. If, as expected, most are trapped in the sludge, densities on a volume
basis would be about 200 times these figures.
2.13.2 Effects of Processing
Little is known about the survival of protozoan cysts when sludge is
processed. Based on the lack of information in the literature, Pedersen
(1981) suggests it is unlikely that many survive anaerobic digestion. Recent
information from Seattle (Metro, 1983) indicates that G. lamblia is present in
raw sludge but absent from digested sludge. Yanko (1988) was unable to
recover protozoan cysts from composted sludges, heat-dried sludges, and some
air-dried sludges. However, processing conditions for these sludge products
were severe.
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The acknowledged transmission of protozoan cysts by water should not lead
to the conclusion that transmission by sludge is likely. Transport of cysts
in wastewater to a susceptible host is possible in a matter of hours after
discharge, whereas transport of cysts in sludge to a susceptible host may take
many days.
2.133 Environmental Effects
Kowal's review (Kowal, 1985) cites work by Rudolfs et al. (1951) that
shows very short survival of Entamoeba histolvtica on soil: 18-24 hours on
dry soil and 42-72 hours on moist soil. In dry soil, Beaver et al. (1949)
showed survival of 10 days.
Survival on plants should be shorter than on soil. However, Tay (1980)
reported isolation of both G. lamblia and E. histolvtica on fruits and
vegetables from farms irrigated with wastewater. No information was found in
the literature on the survival of G. lamblia.
2.1.4 Helminths
2.1.4.1 Types and Quantification
The helminths of concern are the nematodes, or roundworms, and the
cestodes, or tapeworms. The most common helminths pathogenic to man and
likely to be found in sludge are:
Ascaris lumbricoides (human roundwonn)
Ascaris suum (pig roundworm)
Trichuris trichiura (human whipworm)
Taenia saginata (beef tapeworm)
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Taenia solum (pork tapeworm)
Toxacara canis (dog roundworm)
Details of the intricate life cycles of these helminths and the diseases
they cause are discussed by Kowal (1985) and Faust et al. (1975). Ascaris
create pneumonitis when the ingested larvae migrate through the lungs. The
human roundworm develops in the small intestine with potential blockage if a
number of worms are present. Toxacara canis larvae migrate blindly in the
human body, where they can do serious damage to viscera and other organs,
including the eyes. The tapeworms can cause pain and digestive disturbances.
Tapeworm eggs primarily are a hazard to livestock. When eggs are ingested,
larvae are produced that eventually form cystercerci that cause damage to the
animal's organs. Humans ingest the cysts from poorly cooked meat, develop the
tapeworm, and release the eggs in the feces. Animals ingest eggs when they
graze, which completes the cycle.
Reimers et al. (1981) report that Ascaris spp. have the highest
concentration of any helminth in sludge. Ascaris spp. are the hardiest of all
helminths to sludge processing and environmental exposure.
2.1.4.2 Effects of Processing
Because of their high density and relatively large size (0.07 millimeters
(mm)), helminth eggs in wastewater are concentrated into the sludge by
wastewater treatment. Conventional sludge stabilization processes, such as
aerobic or mesophilic anaerobic digestion, have little effect on the number of
viable eggs. Even exposure to harsh conditions, such as treatment with lime
to pH 12, has little effect. The use of strong acid embryonates the larvae
but does not significantly reduce their viability. Thermal treatment above
53°C, such as is attained in thermophilic aerobic or anaerobic digestion,
composting, or pasteurization, is effective in destroying helminth eggs.
Combinations of processing steps, such as alkali treatment to high pH combined
with desiccation for an adequate time period, are effective in destroying
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helminths (Burnham, 1988). Long-term storage (2 years) in simulated lagoons
at 25°C has been shown to destroy the helminth eggs found in sewage sludge
(Kaneshiro and Stern, 1986). Reimers et al. (in press) have confirmed this
finding in a full-scale demonstration in Louisiana and Texas.
2.1.43 Environmental Effects
Kowal's review (Kowal, 1985) observes that the eggs of the hardier
helminths survive for long periods in the soil. Jakubowski (unpublished)
reported quantitative information showing that helminth eggs in surface-
applied sludge died off within a year after application, whereas helminth eggs
in sludge mixed with soil showed only a 50% reduction after 5 years. Periodic
cultivation of the soil increased the die-off for a porous soil but not for a
heavier soil.
Helminth survival on the soil surface or on crops is reduced by
desiccation and other adverse conditions such as freeze-thaw cycles and
sunlight. Densities appear to be reduced to negligible levels in less than a
year.
2.2 TRANSPORT OF ORGANISMS
The pathogens in sludge applied to land pose a disease risk only if there
are routes by which they can contaminate man. The principal means of
contamination are ingestion and inhalation. Absorption through the skin is
believed to be a minor risk. Sludge may be transported to man by many routes:
aerial, ground water, or surface water; adherence to objects that are inhaled
or ingested; surficial or internal contamination of crops eaten by humans; and
vectors. The vectors may be flies, mosquitoes, fleas, or rodents, as well as
other animals that transport disease organisms to humans either mechanically
or by biological processes.
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2.2.1 Air Transport
Exposure Co aerially transported sludge can occur if dust or spray is
inhaled. Sludge is frequently applied to land as a liquid using a splash
plate on the back of a truck; sometimes the sludge applicator is a high-
pressure nozzle with a range of over 100 meters (m). Heat- or air-dried
sludge may be dry enough to create a dust when handled and applied. Movement
of equipment may also create dust if the soil dries out after sludge
application. The dust or aerosol may be inhaled and usually ends up in the
gastrointestinal system.
Kowal (1985) notes that "aerosol shock" substantially reduces bacteria and
virus numbers when sprayed wastewater forms an aerosol; however, this
information may not be directly translatable to sludge spraying. Sludge
typically contains 2-5% suspended solids and, when sprayed, forms relatively
large particles. These particles settle to the ground faster than the drops
of a wastewater spray and are less likely to evaporate down to a fine aerosol
that can be transported long distances. Harding et al. (1981) found fecal
indicator organisms in air samples taken when sludge was applied by high-
pressure sprays, but at much lower levels than at wastewater application
sites. Tank truck application using low-intensity sprays produced little
elevation of fecal indicator densities. Sorber (1984) concluded that spraying
sludge with high-energy sprays would not represent a health threat to
individuals more than 100 m downwind. A buffer zone between a site using
high-energy spraying to apply sludge and any area open to the public seems
appropriate.
No precautions to reduce aerial transport appear to be necessary when
liquid sludge or sludge cake is applied using low-energy means. By the time,
this sludge has dried sufficiently to create dust, the pathogens have probably
been greatly reduced. In dusty environments, workers generally work in
enclosed cabs on equipment or wear dust masks. Travel of dust is generally
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limited (e.g., Che dust from plowing or disking on a farm field is primarily
local).
For sludge products that create dust as they are applied (as might occur
with a heat-dried sludge), the only precautions that can be taken to protect
the person applying the sludge are using a respirator or reducing the sludge
pathogens to insignificant levels.
Aerosols created by low-energy spraying create negligible effects. High-
energy spraying poses a hazard primarily to workers; however, this hazard can
be minimized or eliminated by equipping workers them with protective devices.
Other work practices, such as spraying only in the day time and ensuring that
distance from near neighbors exceeds 100 m, will also help reduce any
potential health hazard from sludge spraying to insignificant levels.
2.2.2 Groundwater Transport
When sludge is applied to the land surface, the soil and the sludge
particles form an effective filter mat. For the most part, only soluble and
colloidal particles enter the soil. The larger organisms, such as helminth
eggs, are retained on the land surface; however, virus particles and,
sometimes, bacteria are small enough to potentially pass through the soil to
ground water. The mechanisms of removal of these organisms during soil
transport are quite different: bacteria are primarily removed by filtration
processes whereas viruses are removed by adsorption.
The literature cited by Kowal (1985) shows that coarse sand is the soil
medium most conducive to pathogen transport; it does not provide a good filter
medium to remove bacteria and is a poor adsorbent for viruses. Fine-grained
soils, on the other hand, provide good removals for both. Cracks in soils
caused by desiccation and root, insect, and animal holes can allow substantial
transport of organisms to the subsoil. Similarly, fissured rock and limestone
beneath the soil can allow transport. However, because liquid is only
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occasionally present in soil - - as a result of sludge application or rainfall
-- the risk of potential transport of sludge or sludge pathogens to ground
water is minimized. By contrast, a septic tank leach field creates a far
greater risk of groundwater contamination because the leach field contains
flowing pathogen-laden water that directly encounters all the subsurface
pathways that may exist in the soil. Similarly, a wastewater application site
that receives the wastewater equivalent of 200 cm of rainfall per year
provides a far greater driving force for virus movement than sludge addition,
which ordinarily contributes only about 2 cm additional water loading to the
annual rainfall loading at a site.
Viruses in particular appear to have a potential to migrate to ground
water; however, their movement to and within ground water is slow because the
water itself moves slowly, and because the viruses adsorb and desorb on the
soil, further slowing their progress (Landry et al., 1980). A typical maximum
survival time for viruses is 170 days (see Table 12 in Kowal's 1985 review).
If, as is likely, movement to ground water is slow, and the movement of ground
water itself beyond the site boundary is also slow, the potential for virus
contamination of ground water beyond the site will be negligible. At sites
where sludge loadings are high, soil layers are thin, and/or subsurface
conditions are unknown (such as in forests), buffer strips should be provided
to avoid movement of viruses to ground water with subsequent potential
transmission off site.
2.2 J Surface Water Transport
Surface water can potentially be contaminated by runoff from a land
application site, or by rainwater moving pathogens transversely below the
ground surface through root holes, animal burrows, and fissures in rock
strata. Movement through fissures is likely only for sludge applied to forest
soil. Helminth eggs are transported by rainwater but, because of their high
density, they tend to drop out of moving streams and concentrate in deposits
in a manner roughly analogous to deposits of gold in stream beds. On the
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other hand, bacteria and viruses can be carried by fine solids wherever the
runoff goes. As noted earlier, bacteria and viruses generally die off about 1
month after application to the soil surface, so the potential health hazard
from runoff disappears after this time. Runoff can be controlled by using
buffer strips of vegetation to remove solids from runoff. Runoff can be
controlled more directly by collecting and holding it in ponds for a
sufficient time to reduce microbiological densities.
2.2.4 Adherence to Objects
Sludge will adhere to crops, soil, and equipment. Viruses and bacteria on
exposed surfaces die off in less than a month; some helminths probably die off
less rapidly. Risk can be controlled by restricting the movement of objects
from the site until at least 1 month following application. Equipment or
animals that must move on and off the site shortly after sludge application
should be washed to eliminate risk.
2.2.5 Transport by Vectors
Transport of sludge by vectors is difficult to quantify. Several
varieties of flies are attracted to and breed in sludge. Primary sludge
contains food wastes attractive to rodents and some birds.
Vectors carry disease in several ways: by complex means (such as in some
mosquito-borne illnesses), by becoming infected themselves (as with
salmonellosis) , or by mechanical transport. The only effective way to
eliminate vector transport is to make the sludge unattractive to vectors
(i.e., by treating the sludge prior to land application).
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23 VECTOR ATTRACTION
Sludge from wastewater treatment processes is generally noxious and
therefore attractive to vectors. Sludge produced in primary treatment (simple
settling) is especially noxious. It contains identifiable fecal material and
food scraps and is usually devoid of oxygen. Primary sludge putrefies rapidly
and releases odorous compounds, a process that eventually results in vector
attraction. Sludge from secondary treatment, such as waste-activated sludge,
contains suspended solids that escaped primary clarification, as veil as
biological solids. Though not as unpleasant as primary sludge, secondary
sludge also attracts vectors. Some sludges may be processed for such a long
time that they do not attract vectors. For example, an extended aeration
sludge may be so thoroughly stabilized by long-term aeration that it does not
putrefy and is unlikely to attract vectors.
The attractiveness of a sludge to vectors can be reduced in various ways,
some permanent and some temporary. For example, reduction of sludge food
value and odor by long digestion permanently reduces the sludge's
attractiveness to vectors. Drying temporarily reduces vector attractiveness,
but the sludge will attract vectors again if it is rewetted. Both permanent
and temporary methods are valid approaches to reducing vector attraction,
although for methods that offer only a stasis in vector attraction, the period
of stasis must last until the sludge has been utilized.
Scientific tests to measure the attractiveness of a sludge to vectors have
not been developed, although there appears to be no great technical difficulty
in developing appropriate tests. Essentially, there are three requirements:
a standardized population of vectors, a standard way of presenting the sludge
to the vectors, and a measure of the interest of the vectors in the sludge
(e.g., number of visits to the sludge by a "standard" group of flies).
Different tests would be needed for different vectors. If such tests were
developed, their results could be correlated to more easily measured
properties of the sludge, such as specific oxygen uptake rates after aerobic
digestion or reduction in percent volatile solids by anaerobic digestion.
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Interest of vectors would be expected to be inversely related to the specific
oxygen uptake rate or the percent reduction in volatile solids.
Any vector attractant test result would not be an absolute property of a
sludge but would be influenced by the sludge's temperature and solids content,
the test duration, ambient temperature and humidity, and even wind speed. For
example, vectors might not be attracted to a sludge at 15°C but might be drawn
to the same sludge at 25°C. Consequently, a satisfactory test result or a
satisfactory value of some correlated parameter such as percent volatile
solids reduction does not ensure that the sludge would not attract vectors
under adverse conditions.
"Criteria" (Federal Register, 1979) lists several processes that have been
determined to reduce vector attraction. Since tests to measure vector
attraction did not exist when the Criteria were developed, field experience
was used for some processes to determine whether the resulting sludge would
attract vectors. In this "cut and try" approach, sludges with a range of
values of a parameter thought to be associated with vector attraction were
placed in the field, and the degree of vector attraction was observed. This
route was formally followed for lime stabilization (Farrell et al. , 1974;
No land et al., 1978; Counts and Shuckrow, 1975), one of the processes listed
in "Criteria" (Federal Register, 1979). Prior field experience with other
processes, such as anaerobic and aerobic digestion, permitted specifying the
conditions for these processes that reduced vector attraction.
The processes listed in "Criteria" that reduce vector attraction are
listed below along with a brief note on the action that reduces vector
attraction.
• Aerobic digestion (mesophilic or thermophilic). Reduction in food
value.
• Anaerobic digestion (mesophilic or thermophilic). Reduction in food
value.
• Lime stabilization. Temporary stasis in bacterial activity caused by
high pH. The effect disappears when pH falls.
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• Air drying on sand beds. Reduction in food value and reduced moisture.
The sludge would have to be biologically stabilized before it would be
possible to expose it to the environment on sand beds. Additional
biological oxidation occurs as the sludge slowly dries on the sand
beds.
• Heat drying. Temporary stasis that disappears when sludge moisture
content increases.
• Composting (mesophilic or thermophilic). Reduction in food value.
• Heat treatment. Temporary. Vector attraction is unlikely only if the
sludge is kept sterile. The proposed regulation imposes certain
restrictions on this form of treatment in order to reduce vector
attraction.
The processes in "Criteria" that reduce vectors also"have to reduce
pathogens to either of two levels. The proposed regulation disentangles
vector attraction from pathogen reduction. Greater freedom is available to
develop new processes or combinations of processes to reduce pathogens,
because it has been possible to establish a common performance target for
pathogen reduction. No common target for vector attraction appears possible,
primarily because there is no standardized test to measure vector attraction.
The subjective target that "vector attraction be reduced" is unusable for
regulatory purposes. Consequently, the regulation proposes four alternative
performance standards and one method of application for demonstrating
reduction of vector attraction.
If a field test to quantify vector attraction is developed, future
regulations could conceivably establish a common performance target for
reduction of vector attraction. This would allow plant operators the freedom
to select any types or combinations of processes to reduce vector attraction
as long as the end point is met. As discussed above, the likelihood that such
a test can be developed is small.
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2.4 INFECTIVE DOSE
To establish a level of concern about the disease risk posed by a
particular sludge pathogen, the infective dose for that organism must be
known. Infective dose is the minimum dose of a pathogen needed to cause
infection.
Kowal (1985) critically reviewed the literature on infective dose for all
pathogen groups of concern. He concludes that although infective doses for
most species of bacteria are high (i.e., many thousands of organisms are
required to cause infection), they can be low in some circumstances. He cites
Blaser and Newman (1982), who indicate that the infective dose for Salmonella
spp. may be less than 1,000 organisms. For Shigella. the infective dose is
low -- 10 to 100 organisms (Keusch, 1970). Ward and Akin (1984) are even more
pessimistic, citing work by D'Aoust (1985) indicating that Salmonella spp. may
be infective at doses below 10 organisms.
Currently, the infective dose for viruses is thought to be low (Kowal,
1985) --on the order of 10 virus particles or less. For protozoa, the
infective dose is likewise low. Kowal (1985) observes that single cysts of
Entamoeba coli have produced infections. For helminths, single eggs are
infective to man. The extent of the infection is dose-related, since most of
the worms produced do not multiply in man. However, an infection may
sensitize individuals so that subsequent light infections cause allergic
reactions.
For all pathogens of concern, the infectious dose is small. It is
therefore prudent to minimize exposure. If the conditions of land application
make sludge ingestion probable (e.g., sludge is applied to food crops to be
harvested shortly after application), the sludge should be essentially devoid
of pathogens.
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SECTION THREE
THE PROPOSED REGULATION
3.1 BACKGROUND
Nine years have elapsed since the publication of the "Criteria" in 1979.
Experience in this period has demonstrated that the pathogen requirements have
posed no great burden for most large treatment plants, but some large and many
smaller treatment plants have encountered compliance problems. These plants
generally produced an adequately treated product and utilized it in a safe
manner, but they had difficulty consistently meeting the technology-based
requirements of the regulation. This experience suggested that the regulation
should be reviewed and adjustments made to address inequities, while
continuing to provide adequate protection from disease risk.
The major difficulties with the current regulation and the proposed
solutions are briefly described below:
(1) Shifting from Technology-based to Performance-based Standards. As
noted earlier, "Criteria" defines two classes of treatment: PSRP (Processes
to Significantly Reduce Pathogens) and PFRP (Processes to Further Reduce
Pathogens). These processes reduce pathogens and vector attraction. Some
pathogens survive PSRP treatment and remain in the sludge; for this reason,
the existing regulation imposes constraints on crops grown and access at sites
treated with sludge produced by a PSRP. With PFRPs, pathogens are reduced to
such a low level that constraints are not needed. The PSRP and PFRP processes
were "technology-based." The process descriptions (time, temperature, and
other process conditions) were given but performance was not specified. This
created an inflexible condition. New technology was difficult to introduce
because there was no identified goal. The proposed regulations replace all
technology-based standards with performance-based standards. This allows much
freer exercise of creativity because the goals for new processes or process
combinations are clearly identified.
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(2) Separating the Requirements for Pathogen Reduction and Reduction of
Vector Attraction. "Criteria" identifies a limited number of single-step
processes that accomplish the goals of pathogen reduction and reduction in
vector attraction. There is no benefit in connecting these goals or requiring
them to be accomplished in a single step. The proposed regulation separates
these requirements and allows for combinations of more than one process to
accomplish either or both goals.
(3) Incorporating the Contribution of Wastewater Treatment to Pathogen
Reduction. "Criteria" implicitly assumes that sludges from all wastewater
treatment units have about the same pathogen density; however, continuing
research (see below) has revealed that the pathogen burden in raw sludges
varies depending on the nature of the wastewater treatment process that
produces the sludge. It now appears to be much more likely that the pathogen
densities (number/gram volatile suspended solids sludge [no./g VSS]) in the
incoming wastewater to different treatment plants will be similar, than that
the sludges produced by wastewater treatment will have similar pathogen
densities (no./g VSS). In the proposed regulation, emphasis has therefore
shifted to require that the reduction in pathogens from untreated wastewater
to treated sludge be made equivalent rather than requiring that all raw
sludges get equivalent treatment after collection.
(4) New Requirements for Well-Stabilized Sludges. Operators of extended
aeration plants have frequently experienced difficulty in meeting PSRP
requirements for their sludges because their untreated sludges were well
stabilized and could not meet requirements for further stabilization. A new
classification has been created that allows these well-stabilized sludges to
be applied to land provided they meet tighter crop and access restrictions.
By making the changes outlined above, the proposed regulation effectively
addresses the difficulties that have been encountered with the existing
"Disease" section of 40 CFR 257 without compromising protection of human
health and the environment.
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3.2 SCIENTIFIC BASIS OF THE PROPOSED REGULATION
This section discusses the component parts of the proposed regulation
following the exact format of the regulation. It presents the underlying
logic supporting the approach as well as the scientific support for selecting
specific requirements. Each topic discussed is keyed into the numbering
system used in the regulation.
3.2.1 Specialized Definitions (§503.51)
Some of the definitions in the regulation need no explanation. Those
discussed below are unique or different from common usage and require
elaboration.
3.2.1.1 Average Density of Microbial Organisms
This term is defined as the number of organisms per unit volume of sludge
divided by mass of volatile suspended solids per unit volume of sludge. It is
the number of organisms per unit mass of volatile suspended solids in the
sludge. Number is a count which, for bacteria, may be predicted by an MPN
(most probable number) or a count of colony-forming units (CFU); for viruses,
it may be a plaque count; for helminth eggs or protozoan cysts, it may be an
actual one-for-one enumeration. The density is determined by measuring (1)
the number of pathogens in a given volume of sludge, and (2) the suspended
volatile solids in the same volume, and then dividing these two results to
obtain the number per unit mass. In the following discussion, density is
identified by its actual density units (e.g., CFU/g VSS) or as "density (mass
basis)."
The density as defined above (no./g VSS) is different from the organism
density used in drinking water technology, where density is appropriately
defined as the number of organisms per unit volume. This volume-based
definition would be of little value in sludge processing, where the interest
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is in numbers related to sludge solids. For example, in dewatering, where
little organism destruction occurs, density per unit volume increases
dramatically when the water is removed from the sludge. Comparing organism
densities on a volume basis before and after dewatering tells little about the
fate of microorganisms. Tracking the density per unit mass of volatile solids
gives us the information we desire, that is, that densities on the mass basis
show little change, indicating the minimal effect of the process on
microorganisms.
3.2.1.2 Pathogen Reduction
In the proposed regulation, pathogen reduction means the reduction in
densities of pathogens on a mass basis (number per unit mass of volatile
suspended solids). A quantitative measure of pathogen reduction is a concern
primarily for bacteria and viruses. The densities (mass basis) in the sludge
leaving the plant are compared to the densities (mass basis) in the incoming
wastewater. For a batch or a plug-flow process, reductions could be
determined from a measurement at the start (or inlet) and at the end (or
outlet) of the operation. In wastewater and sludge processing, flow is
usually continuous or periodic, flowing streams are usually mixed or
recirculated, and sludge residence times can exceed 20 days. Consequently, on
a given day, inlet and outlet concentrations may bear no relationship to one
another. Reductions in bacterial and viral densities (mass basis) are best
estimated by comparing the average of the logarithmic densities determined on
samples collected before and after treatment over several weeks of steady
operation.
3.2.13 Specific Oxygen Uptake Rate (SOUR)
This parameter is determined in a standard test that measures the rate at
which a sludge consumes oxygen per unit mass of solids present. A high SOUR
indicates a large and active bacterial mass and is likely to putrefy rapidly
A low SOUR indicates that bacteria present are not metabolically active. This
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generally indicates that the bacteria have consumed the available food
resources and that the sludge will not putrefy rapidly. The test is only
appropriate for sludges that have a high proportion of aerobic bacteria. It
is not appropriate for untreated, limed, or anaerobically digested sludges.
3.2.1.4 Volatile Suspended Solids
The volatile solids concentration and the changes that occur in it are
parameters of concern for some of the processes that reduce vector attraction,
such as digestion. Volatile solids include volatile matter in both suspended
and dissolved solids. Volatile suspended solids concentration is the
parameter of concern when quantifying microbial densities, because the
microbes are primarily associated with or are particulate solids.
Analytically, it is easier to determine volatile solids concentration than
volatile suspended solids concentration. When the dissolved volatile solids
concentration is less than 5% of the total volatile solids concentration,
volatile solids concentration can be used in place of volatile suspended
solids concentration without introducing serious error.
3.2.2 Class A Requirements
The regulation proposed performance-based standards for three classes of
pathogen reduction: Class A, Class B, and Class C. Classes B and C also
include sludge use and site access restrictions. The specific requirements
for these classes of pathogen reduction and the scientific basis for them are
discussed in the rest of this section.
3.2.2.1 Pathogen Reduction Requirements (§503.52)
Class A pathogen reduction requires that densities of pathogenic bacteria,
animal viruses, protozoa, and helminth ova be reduced to nondetectable levels.
This requirement is necessary because there are no restrictions on the use of
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sludge that has received Class A treatment. People can come into direct
bodily contact with sludge at any time after application, possibly inhaling
dust or ingesting small amounts of sludge. Consequently, the presence of
concentrations of pathogens that could cause disease must be avoided. As
observed earlier, ingestion of only one viable animal virus, protozoa, or
helminth egg can cause an infection (Kowal, 1985). For bacteria, the
infective doses are normally much higher, but may be less than 10 organisms
during outbreaks (D'Aoust, 1985). In view of these low minimum infective
doses, it appears appropriate to require all pathogens to be below detectable
limits in Class A sludges.
Use of Fecal Indicators for Thermal Processes
The regulation allows the use of reduction of fecal indicators to indicate
elimination of pathogens, if the mechanism that destroys the pathogens is
primarily thermal. If temperatures of 53°C or above are used to destroy the
pathogens, reduction of fecal coliforms and fecal streptococci to densities
below 100/g VSS will ensure that pathogens are eliminated. Exposure to 70°C
for one-half hour will produce reductions of this magnitude; exposure to 55°C
for three days or 53°C for five days is expected to produce similar results.
Both laboratory and field experience suggest that reduction of fecal
indicators to low levels satisfactorily indicates the reduction of pathogens
to insignificant levels by thermal processes. The fecal indicators are
reduced at rates generally comparable to many of the pathogens, particularly
to bacterial pathogens, but their initial concentrations are ordinarily many
orders of magnitude (i.e., factors of ten) higher than the pathogens. Even if
they should decline faster than some pathogens (and they do decline faster
than some viruses), appreciable numbers would still survive to serve as
indicators when the pathogens have long since diminished to negligible values.
Berg and Berman (1980) showed that when sludge was thermophilically
digested at 49°C in a full-scale digester, viruses were eliminated or reduced
to low values while substantial numbers of fecal coliforms and fecal
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streptococci survived. The ratios of fecal coliforms to viruses and fecal
streptococci to viruses in the final product averaged 12,000:1. Median viral
density (volume basis) in the raw sludge fed to the digester was 1,500 PFU/100
ml, which is a typical viral density in untreated sludge. The indicator
organisms would clearly serve as a good indicator: An indicator density of
100/g VSS would indicate a viral density of 0.01/g VSS, a factor of 30 below
the detectable limit. For the same thermophilic digester, Farrell et al.
(1985) reported results from 27 measurements of bacterial densities.
Salmonellae, which averaged in the normal range -- about 210/g VSS -- in the
undigested sludge, were reduced to below detectable levels in all cases.
Fecal coliform density (mass basis) was reduced 5 logs but final densities
averaged 390 CFU/g. Fecal streptococci densities (mass basis) were reduced
2.7 logs and fecal densities averaged 7,600 CFU/g.
For composting -- another process that destroys pathogens using thermal
means -- lacaboni et al. (1984) reported reductions in fecal and total
coliform densities and salmonellae densities during full-scale windrow
composting in Los Angeles. Their work was divided in three phases. In Phase
I, data were obtained with low windrows and relatively dry sludge cake. Phase
II data were obtained with wetter sludge cake, and temperatures above 55"C
were frequently hard to sustain. Salmonellae and coliform densities were
elevated and provided a good test of the utility of using coliforms to
indicate the presence of salmonellae. In a third phase, these investigators
used high windrows. During these tests, fecal indicators were generally less
than 100/g solids and salmonellae were rarely detected.
The coliform and salmonellae data are shown in Table 3-1. These results
show that total and fecal coliforms survive the thermal stress of composting
(temp, range 45-65°C) better than salmonellae. When their densities were low
(less than 100/g), salmonellae were absent. When coliform densities were
above 100/g solids, the likelihood that salmonellae were present was high.
Salmonella spp. densities in the incoming sludge were approximately 1.7 X 10'
MPN/g for Phase I and 5.8 X 104 MPN/g for Phase II studies. (In such
comparisons, it is important to know that ample salmonellae were entering. If
salmonellae are not present in the incoming sludge, their absence in the final
product is no evidence that the process kills salmonellae.)
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In recent work reported by Yanko (1988), complete microbiological analyses
was carried out on composts and other sludge products. The composts were
prepared at thermophilic conditions (40-70°C). Results showed substantial
survival of indicators but complete absence of viruses. In an appendix to the
cited report by lacaboni et al. (1984), Yanko does not recommend fecal
indicators as indicators of virus density, but his reason appears to be that
the fecal indicators may regrow. Thus, a high indicator density might not
indicate a correspondingly high viral density. In this case the indicators
might be excessively conservative indicators of the presence of viruses.
Nevertheless, the conclusion that low fecal indicators indicate the absence of
viruses is still valid.
Yanko's work included enumeration of Ascaris eggs in sludge products
intended for distribution and marketing. These products were either composted
at temperatures exceeding 53°C or were air-dried in hot climates. In 350
examinations, Ascaris were recovered but none were viable. It is therefore
reasonable to conclude that if the process destroying microorganisms utilizes
thermal means at temperatures above 53°C, Ascaris will also be destroyed.
Fecal indicators less than 100/g and temperatures above 53°C are good evidence
of their destruction. Yanko also observed that it was extremely unlikely that
protozoan cysts could survive conditions that would destroy viruses and
Ascaris spp.
The origin of the 53°C requirement requires discussion. This critical
temperature is derived from results obtained by Brannen et al. (1975). These
authors showed that Ascaris egg densities were reduced 2 logs (base 10) in
about 5 minutes at 55°C, in 60 minutes at 51°C, and showed no reduction at all
after 2 hours at 47°C. Thus, to ensure Ascaris destruction, the required
minimum sludge processing temperature must exceed this threshold for Ascaris
sensitivity. Based on Brannen et al.'s data, a temperature of 53°C was
selected as sufficient to cause a rapid reduction in Ascaris density.
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Nonthermal Processes
For processes Chat use nonthermal means to destroy pathogens, fecal
indicators are not adequate for indicating pathogen reduction. Generally,
pathogen reduction can be adequately gauged by focusing on the one pathogen
type least susceptible to the adverse conditions of the process. For example,
Ward (1981) points out that the radiation dose to inactivate 90% of viable
viruses in sludge is about 10 times the dose required to inactivate enteric
bacteria. Larger organisms, such as helminth ova, are even more susceptible
to radiation than bacteria. Consequently, for processes that use radiation 'to
destroy pathogens, adequate pathogen destruction can be ensured at doses
sufficient to reduce typical maximum viral densities to below detection
limits.
For processes that treat sludge by chemicals, helminth ova are likely to
be the most resistant organism type, since the shells of the ova are resistant
to penetration by chemicals.
Unique and Novel Processes
For unique and novel processes, data on all the various pathogen types are
necessary to demonstrate adequate destruction. If one organism type proves
hardier than the others it can then be used as an indicator. If test data
demonstrate that this organism was present initially at typical concentrations
and was reduced by treatment to below detection limits, then it can be safely
assumed that the other pathogens are likewise absent.
Protecting Against Regrowth
The regulation specifies that, to meet Class A requirements, vector
attraction must be reduced simultaneously with or after the pathogen
reduction. This requirement is necessary for the following reason. To meet
Class A requirements, pathogens must be reduced to below detectable levels.
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In achieving these reductions, the nonpathogenic bacteria in sludge are also
destroyed. These bacteria normally act as competitors with pathogenic
bacteria and help prevent regrowth. When they are absent, as in Class A
sludges, explosive regrowth can occur. Processes that reduce vector
attraction add nonpathogenic bacteria back into the sludge, so when vector
attraction occurs simultaneously with or shortly after pathogen reduction,
explosive regrowth is prevented.
Several sludge treatment processes simultaneously reduce pathogens and
vector attraction. This is not the case, however, with heat treatment.
Though sterilized, heat-treated sludges still provide a good medium for
bacterial regrowth. Clements (1983) describes experience in Switzerland where
pasteurization was used at approximately 70 plants to disinfect sludge before
land application. A subsequent investigation conducted under governmental
auspices showed that the majority of the pasteurized products were
contaminated with pathogenic bacteria. The presence of the bacterial
pathogens was attributed to contamination downstream with bacteria that grew
rapidly, even in sludge that had been thoroughly digested before
pasteurization. Since that time, most post-pasteurization operations have
been abandoned. Current practice in Germany and Switzerland is to pre-
pasteurize sludge before digestion or to use thermophilic aerobic digestion
for agricultural applications that require minimal pathogen densities in the
sludge. With sludge digested after pasteurization, bacteria do not rapidly
grow to high and sustained levels when contamination occurs.
Sludge injected into the ground is removed from potential contact with
humans, and its moisture is rapidly removed by the soil. Underground
injection could be a suitable way to reduce vector attraction in pasteurized
sludges if the sludge is injected very shortly after pasteurization. However,
it is difficult to define the allowable time between pasteurization and
injection. If a very fluid sludge becomes contaminated, the mixing caused by
pumping or transport could promote rapid growth of bacterial contaminants to
high densities.
Good housekeeping could reduce (but hardly eliminate) sources of
contamination. Rather than specifying some time period, it seems more
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reasonable to look for evidence of regrowth. Fecal indicators would very
likely still be present in the pasteurized sludge and would almost certainly
be among the contaminating organisms. If these organisms increased from their
acceptable level of below 100/g solids to over 1,000/g VSS, this would
indicate contamination that could pose a health concern. If densities are
below 1,000/g VSS at the time of disposal, the likelihood of high densities of
contaminating bacterial pathogens is considered unlikely.
Sludge cake is much less likely to become contaminated than liquid sludge
because its drier nature limits mixing of any contamination into the entire
sludge mass. The potential for contamination of sludge cake could be
controlled by establishing a reasonable time limit between pasteurization and
disposal; however, this might be difficult to establish. Therefore, the
requirement that regrowth of indicator organisms be limited to 1,000/g VSS at
the time of injection seems reasonable for protecting against regrowth in both
liquid sludge and sludge cake injected underground. This is the approach
taken in the regulations to ensure that substantial regrowth has not taken
place in Class A sludges prior to underground injection.
In the 1979 "Criteria," heat treatment was considered a process to further
reduce pathogens. In the present regulation, heat-treated sludge would need
to either (1) be treated by a process to reduce vector attraction, or (2) be
injected underground under the conditions stated above. This requirement
applies to both heat-treated liquid sludge and dewatered sludge cake. It may
be possible to establish that sludge cake produced from a heat-treated sludge,
with a solids content above a certain percentage, would not attract vectors.
Information supporting such a contention would have to be presented to EPA's
Pathogen Equivalency Committee for a determination.
3.2.2.2 Vector Attraction Reduction (§503.53)
The potential for sludge to attract vectors must be reduced to break an
important link in the disease cycle. Vectors can transmit pathogens from
sludge, and they can contaminate sludges with pathogens. Therefore, even
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sludges that have received Class A pathogen reduction treatment require
reduction in vector attraction. The regulation lists five means to reduce
vector attraction. These are discussed below.
Volatile Solids Reduction
If the sludge volatile solids content has been reduced 38% by anaerobic or
aerobic biological treatment or chemical oxidation, it is presumed to be
adequately reduced in vector attraction. This requirement, which is the same
as was used in "Criteria," was drawn from the Water Pollution Control
Federation Manual of Practice No. 8 (WPCF, 1967). The selection was largely
judgmental but has been reinforced by 9 years of usage under the present
regulation. Volatile solids reduction is calculated by a volatile solids
balance around the digester or by the Van Kleek formula (Fisher, 1984).
The proposed regulation allows use of an alternative means to determine
whether a 38% volatile solids reduction has been achieved. In many treatment
plants, treated sludge is recycled back to the aerator for more treatment or
back to the inlet of the digester to improve the fluidity of the incoming
sludge. The sludge entering the digester has already been partially digested
so it is extremely difficult to achieve an additional 38% volatile solids
digestion.
Jeris et al. (1985) experimented with several sludges and attempted
unsuccessfully to develop a single index that would characterize the stability
of a sludge. They did demonstrate that most of their anaerobically digested
sludges could be further digested to a modest degree, while a few of the
sludges showed much more ability to digest further. This "ability to digest
further" appears to be as close as we can come to an index of ability to
putrefy further and attract vectors. The only problem is that 20-40 days are
needed to complete the determination. The time requirement makes the method
useless if a sludge must be immediately evaluated, but is no obstacle to
evaluating an operating process. The test must simply be started 20-40 days
before the result is needed.
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Jeris et al.'s data showed the following additional percent volatile
solids reduction after 40 days of additional digestion for six sludges: 9,
10, 13, 22, 36, and 38. The three sludges with the lowest volatile solids
(VS) reduction also showed low volatile acid concentrations before the
additional digestion period commenced. Because the percent VS versus time
curves had essentially flattened out at 30 days, a VS reduction of 15% or less
on additional batch digestion for 30 days was selected as adequate evidence of
satisfactory vector attraction reduction. This reduction is stated to be
equivalent to the aforementioned 38% volatile solids based on volatile solids
entering and leaving the digester.
It is expected that a procedure for conducting this VS reduction test will
be described in forthcoming EPA guidance. Jeris et al. (1985) used an 18-L
digester with continuous mixing and gas release measurement. Because only
volatile solids concentration would need to be measured, a smaller digester
(e.g., 4-L) with no gas collection and intermittent stirring (once per shift)
would be adequate. In the interim between proposed and final issuance of
regulations, additional information should be obtained that supports this
approach.
Specific Oxygen Uptake Rates (SOUR)
If a sludge has been treated aerobically to the point at which the
biological organisms present are consuming very little oxygen, the value of
the sludge as a food source for microorganisms is evidently very low. The
likelihood that such a sludge will attract vectors when applied to the land
surface or injected shallowly into the ground is likewise low. Eikura and
Paulsrud (1977) have shown that both the odor index of aerobically digested
sludges and the oxygen uptake level decline at about the same rate with
increasing nominal residence time in a continuous flow (fed once a day)
digester. The relationship between odor intensity and oxygen uptake rate is
approximately in direct proportion. Eikum and Paulsrud's results indicate
that at 208C, an oxygen uptake rate of 1.5 g/hr/g VSS or less indicates a
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well-stabilized sludge. The oxygen uptake rate depends on the temperature of
digestion. Within a range of ± 5eC, the oxygen uptake rate obtained at
another temperature can be converted to the uptake rate of 20°C by the
following relationship:
SOUR (20°C) - SOUR (t"C) x 1.10*2*0
The oxygen uptake rate depends on the conditions of the test and, to some
degree, on the nature of the original sludge before aerobic treatment.
Similarly the temperature-correction also depends on the nature of the sludge.
EPA's forthcoming guidance will provide information on test procedures and
sludge-dependent factors.
Alkali Addition
Vector attraction can be reduced by alkali addition; however, the
reduction is not permanent. Alkali addition does not significantly change the
nature of the substances in the sludge but instead causes stasis in biological
activity. If the pH should drop, the surviving bacterial spores would become
biologically active, and the sludge would putrefy and attract vectors. The
regulation provides target conditions that, if met, will ensure that the
sludge can be stored briefly at the treatment plant, transported, and applied
to the soil without the pH falling to a point where putrefaction occurs and
vectors are attracted.
No land et al. (1978) have shown that with addition of quicklime or slaked
lime, sludge pH remains high for extended periods. His Figure 9 shows that
for a sludge raised to pH 12.5, pH did not fall to below 12 for 25 days. One
reason the pH stays high for such a long period is that a substantial portion
of the lime is still not dissolved; this excess lime dissolves as the pH
starts to drop below 12.5. Other alkalis, such as cement kiln dust or wood
ash, are more soluble, so at pH 12 or above little undissolved alkali may be
present to help maintain the pH as it starts to fall. The requirement for pH
to exceed 11.5 for 24 hours after lime addition is to ensure that it will be
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high long enough to dispose of the sludge even if a large proportion of the
alkali is soluble. Obviously, collection of additional data using soluble
alkalies to raise pH will be helpful in substantiating these time-pH
requirements.
Moisture Reduction
Drying sludge to near total dryness causes a stasis in biological
activity. Sludge products such as Milorganite (Milwaukee's heat-dried wasre-
activated sludge) contain less than 10% moisture. They exhibit no biological
activity when kept dry and resist recontamination unless water is added.
Yeager and Ward's (1981) data show that bacterial densities in sterilized raw
sludge inoculated with several bacterial species declined rapidly when the
solids contents were over 75%, indicating diminished bacterial activity
Thus, it can be concluded that significant biological activity will not occur
if sludge is maintained above 75% solids. However, the nature of the sludge
and the manner in which it is handled can influence the degree of vector
attraction. Most air- or heat-dried sludges do not contain raw sludge from a
primary clarifier. Raw sludge from a primary clarifier could contain
undegraded or partially degraded food fragments, including rancid fats It is
therefore prudent not to use the 75% dryness level as a measure of reduced
vector attractiveness in sludges that contain raw sludge from primary
clarifiers. Heat drying of such sludges to a much higher solids content will
further limit biological activity and will strip off or decompose volatile
compounds that attract vectors. Permit conditions may allow reduction in
vector attraction to be demonstrated by such means if supported by
experimental findings.
The conditions of handling dried sludge before disposal can create or
prevent vector attraction in dried sludge. For example, sludge that was air-
dried to 75% solids on a sand bed and stored in piles under a roof could
become a problem in periods of cool weather with high humidity (e.g., during a
prolonged rainy period or a change of seasons). The outer surface of the
sludge would equilibrate to a lower solids content and could attract vectors.
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This possibility can be avoided by devising appropriate permit conditions and
is not sufficient justification for either rejecting drying as a method to
reduce vector attraction or including a requirement to store the dried sludge
in climate-controlled storage or in bags.
Subsurface Injection
When sludge is injected under the soil surface and does not leak
substantially to the surface, vectors no longer have easy access to the
sludge, and the odors that might attract vectors are reduced. The sludge
intimately contacts the soil and, in ordinary circumstances, the soil extracts
water from the sludge and dewaters it. The method could fail if loading rates
(mg dry solids basis/hectare) were unusually high and the soil was saturated
with water. When sludge is applied at agronomic rates to soils that do not
have a high moisture content, problems are not expected. Permit conditions
should specify maximum liquid loading rates and should specify that sludge not
be injected into moist soil.
3.23 Class B Requirements
Class B requirements are based on the observation that wastewater sludges
produced by conventional treatment (including anaerobic digestion) and used in
a careful fashion on farmland pose a minimal disease risk. Class B
requirements specify that sludge be treated to produce log average reductions
in pathogenic bacteria and animal viruses equivalent to the reductions
achieved by this conventional treatment. They also require reduction of
vector attraction, and establish controls over site access, crops grown and
harvested, and animals grazed. Together, these requirements reduce the
disease risk to minimal levels.
Class B does not require reduction of protozoan cysts or helminth eggs.
Protozoan cysts are believed to be greatly reduced in numbers by sludge
processing and are relatively susceptible to adverse environmental exposure.
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Even if survival were substantial, they would be effectively controlled by che
access and use restrictions. Helminth eggs are hardly reduced at all by
processing, and densities decline slowly in the environment. The requirements
forbidding growth of certain crops for 18 months and restricting site access
for 12 months are designed to protect the public against possible ingestion of
helminth eggs.
3.23.1 Pathogen Reduction (§503.52)
Class B pathogen reduction requires that the logarithm (base 10) of the
average density of pathogenic bacteria per unit mass of volatile suspended
solids in the processed sludge is at least 2.0 lower than the logarithm (base
10) of the average density of pathogenic bacteria per unit mass of suspended
solids in the influent to the treatment works. For viruses, the difference in
these two densities must be 2.0 or greater.
The use of the difference in pathogen log densities in suspended solids in
influent wastewater and suspended solids in processed sludge is a new
development. In "Criteria," the objective of the processes to significantly
reduce pathogens was to ensure that all sludges received at least a certain
minimum degree of treatment. Farrell et al. (in press) and Lee et al. (in
press) established that this approach is inequitable for some treatment plants
because the sludge from certain wastewater processes is lower in fecal
indicators and bacterial pathogens than other processes. The implication of
their work is that the wastewater treatment process itself is part of the
sludge treatment. One is then forced to look upstream before wastewater
treatment starts to find a measurement against which residual pathogen
densities in sludge can be compared to measure performance.
Two upstream measurements have been selected as indices of pathogen
burden: the pathogen densities and the bacterial fecal indicator densities in
the entering wastewater as related to the mass of the volatile suspended
solids in the wastewater. The volatile suspended solids was selected as the
"solids" stream of concern because it contains the great bulk of the pathogens
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(including the viruses that are adsorbed on solids). The nonvolatile fraction
of sludge (dirt, grit) mechanically carries along pathogens, but the pathogens
are part of the volatile solids fraction of the sludge solids.
Selection of the pathogen densities as an index needs no explanation. As
for the fecal indicator densities, they are direct measures of fecal
contamination provided the wastewater has not yet been subjected to treatment
adverse to bacterial survival, which is a reasonable assumption at the
treatment plant inlet. Because fecal matter is directly related to pathogen
burden (at least in the long term), fecal indicators at the plant inlet will
be related to pathogen burden also, and their densities (mass basis) will
serve as an index of pathogen burden in the sludge.
For the purposes of evaluating Class B (and Class C) treatment to reduce
pathogens, wastewater treatment and sludge treatment are considered as steps
taken to reduce the pathogen density (no./g VSS) in the incoming wastewater
solids. For those processes for which fecal indicators fall in a manner that
correlates uniformly with the fall in pathogen content, the fall in fecal
indicator densities (mass basis) can be used to indicate the fall in pathogen
densities.
What is needed now is information on typical wastewater and sludge
treatment processes that shows that fecal indicators, bacterial pathogens, and
animal viruses all fall in about the same fashion. Information is available
that shows that this is true for aerobic treatment and mesophilic anaerobic
digestion. Martin (in press) obtained data on the relative declines in fecal
indicators and animal viruses at temperatures from 8-40"C. In the temperature
range of 15-31°C, the reductions did not show marked trends with temperature.
Ratios of log reductions for fecal indicators relative to log reductions for
viruses were calculated for seven temperatures in this temperature range.
Average ratios and standard error of the mean are shown in Table 3-2, and are
compared with Farrell et al.'s (1985) data for anaerobic digestion at 358C.
The differences are not great. The agreement is particularly good for
fecal streptococci. It is reasonable then to conclude that for aerobic and
anaerobic processes in the commonly used temperature ranges, a decline in
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fecal indicator densities can be used to indicate declines in viruses for both
cases. Because of the scarcity of data (there were insufficient numbers of
salmonellae in Martin's influent for him to observe the fate of this
pathogen), it is necessary to assume that the declines in salmonellae relative
to the indicators would be similar for both types of processes.
For processes conducted at temperatures higher than 35°C, available
information (Farrell et al., in press; Martin, in press; Berg and Berman,
1980) indicates that viruses fall faster than indicators. However, this issue
does not require discussion because bacterial and viral reductions are much
greater than two logs at thermophilic temperatures.
The situation for chemical treatment of sludge by lime or chlorine is
similar to that for biological treatment in the thermophilic range. Under
these treatments, bacteria and viruses fall to a much greater extent than the
minimum requirement for Class B treatment. For lime treatment, results by
Counts and Shuckrow (1975) and Strauch (1982) show that bacteria and viruses
are greatly reduced at pHs exceeding 12. When wastewater is treated by
chlorine to a pH near 7, the free chlorine inactivates polio virus and E. coli
to a similar extent (Table 5-11 in EPA, 1986). In sludge treatment the bulk
of the chlorine would be converted to chloramines. Reduction in viruses would
occur but more slowly. Adequate contact time with the sludge would be
necessary. Typically, contact times of fewer than 30 minutes provide adequate
virus destruction.
For those processes for which the fall in indicators correlates with the
fall in pathogenic bacteria and viruses, the proposed regulation sets a
criterion for pathogen reduction based on the absolute densities of fecal
indicators in treated sludge for certain processes. This criterion requires
that the average log fecal coliform densities (no./g VSS) shall be less than
6.0 and the average log fecal streptococci densities (no./g VSS) shall be less
than 6.0 in the processed sludge. The rationale for this is as follows.
Until now, reductions in pathogen densities have been estimated by correlating
them to reduction in indicator densities. Absolute densities of the fecal
indicators can be used as a criterion because the densities of the indicators
in entering wastewater are surprisingly similar for different wastewater
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treatment plants; as a result, indicator densities after adequate treatment to
reduce pathogens are also expected to be similar. Farrell et al. (in press)
found that the ranges in log densities for total coliforms, fecal coliforms,
and fecal streptococci were 0.55, 0.43, and 0.43, respectively, for four
NP/LSA (no primary/long sludge age) plants and one conventional plant.
Densities in this study were on a total solids basis. It is expected that
Olivieri and Sarai's results (1988, in press) will confirm this observation,
when their investigation is completed.
For unique processes that might not adequately destroy viruses or
pathogenic bacteria, it will be necessary to either (1) collect information to
show that fecal indicators can be used as indicators of bacteria and virus
destruction, or (2) measure either pathogenic bacteria or animal virus
densities (whichever is the least sensitive to the process) to indicate
performance.
3.23.2 Vector Attraction Reduction (§503.53)
All vector attraction processes for Class B are the same as for Class A,
except, for Class B, the vector attraction process may precede, follow, or
occur simultaneously with pathogen reduction. The Class A requirement that
the vector attraction reduction process cannot precede the pathogen reduction
process is unnecessary for Class B because regrowth of bacterial pathogens is
unlikely in Class B sludges, because some competitor bacteria remain in the
sludge following treatment. For the same reason, no requirement exists to
determine fecal indicator densities to ensure that regrowth has not occurred
when vector attraction is reduced by underground injection.
3.233 Access and Use Restrictions (§503.52)
Class B requirements reduce pathogenic bacteria and animal viruses, but
some of these organisms still survive in the sludge. Protozoan cysts,
although probably the most susceptible pathogen to wastewater and sludge
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treatment, may also be present. Helminth eggs, if present in the untreated
wastewater, are very likely to be present because they are the least affected
of the pathogen types by wastewater and sludge treatment. Access and use
restrictions have been imposed for Class B sludges that either limit exposure
or provide time for attenuation of pathogens so that the risk of disease is
minimal.
Food crops whose harvested parts are above the ground and touch the sludge
or soil-sludge mixture cannot be grown for 18 months after application of a
Class B sludge. Food crops whose harvested parts are above the ground and do
not touch the sludge may be grown at any time. The 18 months provides time
for attenuation of pathogens -- particularly helminth eggs. Kowal's (1985)
review points out that helminth eggs are degraded by exposure to sunlight and
desiccation but survive for years when protected by the soil. The long time
period allows time for sun and desiccation to inactivate the helminth eggs,
which are the pathogen type most resistant to environmental stress.
The proposed regulation requires that food crops whose harvested parts are
below the surface shall not be grown for 5 years after land application of
Class B sludge, or 18 months if it is demonstrated at that time that there are
no viable helminth eggs in the soil. Research has shown that helminth eggs
below the soil surface survive in some soils for periods well in excess of 18
months (Jakubowski, 1988). After 5 years their survival is expected to be
low. Some sludges are initially low in helminth eggs. Initial low densities
coupled with the declines that occur after 18 months may produce trivial
densities in soil. It is reasonable to allow use of the soil for root crops
after 18 months if helminth eggs are demonstrated to be absent. Measurement
of low densities of helminth eggs in soil requires proper methodology and
training. A demonstration that helminth eggs are absent will be accepted only
if the ability to recover small numbers of helminths in soil is also
demonstrated.
Any food crops grown on agricultural land where Class B sludge has been
applied shall not be harvested for 30 days after sludge application. This
protection is needed because animals and humans may contact these harvested
crops very soon after harvesting. A period of exposure to environmental
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conditions before harvesting allows wind action and rainfall to reduce the
amount of sludge adhering to the crops and to attenuate pathogens. Exposure
of pathogens on the plant surface is especially severe because the sludge
layers are thin and desiccate quickly. As noted earlier, viruses on low-
growing vegetables have been shown to decline to low values in less than 2
weeks (Larkin et al. , 1976). Consequently, prevention of harvesting for 1
month is considered sufficiently protective.
The proposed regulation requires that animals be prevented from grazing
for 30 days after sludge is applied to the land. This requirement is
primarily designed to protect human health. Animals can physically carry
sludge off the site where humans may inadvertently come in contact with it.
Prevention of any grazing for 1 month after sludge use to allow attenuation of
pathogens and removal of sludge from plant surfaces by rain and wind is
appropriate and is considered to be sufficiently protective. The requirement
also protects animals against bacterial or viral diseases, such as
salmonellosis, which can be transmitted to humans; however, it is less
protective against helminths such as Taenia saginata (beef tapeworm). These
organisms are somewhat attenuated within a month, and agricultural inspection
of meat for cysticercosis reduces the risk of creating a cycle of infestation.
The proposed regulations require that public access to a site be prevented
for 12 months after application of Class B sludge. This restriction does not
apply to farm owners and agricultural workers who are aware of the presence of
sludge; it does apply to the uninformed public. The 12-month period is fully
protective against viruses, bacteria, and helminths. The restriction on
access is shorter than the restriction against growing food crops because the
exposure to helminths by walking or sitting on the ground is assumed to be
less than exposure by ingesting food crops grown on soil containing sludge.
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3.2.4 Class C Requirements
3.2.4.1 Pathogen Reduction (§503.52)
Class C requirements stem from the observation by Farrell et al. (in
press) and Lee et al. (in press) that sludge produced by "no primary/long
sludge age" (NP/LSA) plants (i.e., by wastewater treatment plants that do not
use primary clarification but expose wastewater and sludge to aerobic
conditions for long periods) is more reduced in salmonellae and fecal
indicator densities than untreated sludge from a conventional treatment plant
that utilizes primary clarification and the conventional activated sludge
process. It is well known that NP/LSA sludges are better stabilized than the
typical mixed primary and waste-activated sludge from a conventional plant.
In the past, NP/LSA plants have frequently had difficulty meeting the volatile
solids reduction for aerobic digestion specified in "Criteria" because they
already had lost much of their volatile solids in the wastewater treatment
step. The "Criteria" treatment of these sludges was inequitable. Class C
acknowledges that these sludges are nearly but not quite as reduced in
pathogens as sludges that have received Class B treatment.
Estimation of the pathogen reduction across NP/LSA plants is approximate
because almost no experimental work has been done relating pathogens and
indicator densities (mass basis) in inlet wastewater solids to their densities
(mass basis) in the waste sludge at these plants. The pathogen reductions of
1.5 logs (base 10) shown in the proposed regulation are estimates based on
reductions in densities of fecal indicators achieved by processing.
Table 3-3 shows densities of indicator organisms in the processed sludge
from several conventional treatment plants that anaerobically digest their
sludge and from several NP/LSA plants. The data show suggested ranges and
standards for fecal indicators for these sludges. Considering only fecal
coliforms and fecal streptococci, the NP/LSA sludges show densities that
averaged about 0.5 logs higher than for the conventional sludges. Using the
estimates of the ratio of log viral to indicator reductions presented in Table
3-2, log viral reductions are estimated to be approximately 0.5 log poorer for
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NP/LSA processes. For bacteria, Che NP/LSA processes showed an overall log
reduction in fecal streptococci of about 1.4 and a Salmonella spp. reduction
of 1.2 (Farrell et al. , in press; Lee et al., in press). For bacteria, the
0.5 less reduction in fecal indicators will mean approximately 0.4 log less
reduction in pathogenic bacteria.
On this basis, the proposed regulation states that average log fecal
coliform densities (no./g VSS) shall be less than 6.3, and average log fecal
streptococci shall be less than 6.7. As noted in Section 3.2.3, it is
possible to use absolute densities (as opposed to reductions in densities) as
standards because densities of fecal indicators in the incoming wastewater
(no./g VSS) are nearly the same at wastewater treatment plants.
The fact that the proposed 1.5-log reduction in pathogenic bacterial and
viral densities for NP/LSA plants has been estimated is recognized. For the
most part, this creates no difficulties because most facilities will be able
to use absolute fecal indicator densities of the treated sludge to demonstrate
conformance with regulations. The primary objective is to show that the
intent of the regulation is to control pathogen densities and not indicator
organism densities.
3.2.4.2 Vector Attraction Reduction (§503.53)
The Class C vector attraction reduction requirements are exactly the same
as the Class B requirements. No further discussion is needed.
3.2.43 Access and Use Restrictions (§503.52)
Class C access and use restrictions are similar to but more rigorous than
Class B requirements. The 1-month time restrictions in the Class B
requirements, which are designed to protect primarily against risks from
pathogenic bacteria and viruses, are increased for Class C requirements. The
18-month and 5-year time restrictions in the Class B requirements are designed
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to protect against risk from helminth eggs. Sludge processing used to achieve
Type B and C pathogen reduction requirements will have a similar slight effect
on helminth eggs survival; consequently, the time periods- chosen to protect
human health from helminth risk associated with Type fi sludges will also be
protective for Type C sludges.
The proposed regulation requires that feed crops whose harvested parts
touch the sludge and are above ground cannot be grown for 18 months after
application of Class C sludge. Feed crops whose harvested parts are below the
surface shall not be grown for 5 years after application of Class C sludge to
the land, or 18 months if it is demonstrated that no viable helminth eggs can
be found in the soil. These requirements are the same as for Class B.
Any food crops grown on agricultural land where Class C sludge has been
applied shall not be harvested for 2 months after sludge has been applied.
This is an increase of 1 month over Class B requirements. Additional time is
required for attenuation because bacterial and viral pathogen levels are
slightly higher in Class C sludges than Class B sludges.
Animals must be prevented from grazing for 2 months after Class C sludge
is applied to land. This is an increase of 1 month over Class B requirements.
Additional time is needed for attenuation of viruses and pathogenic bacteria
because the initial pathogenic bacterial and viral content is higher in Class
C sludges.
The Class C access restrictions are more rigorous than the Class B
restrictions. Both classes require control of site access for 12 months in
both cases; however, for Class C, only agricultural workers are allowed access
to the site during the 12 months. For example, members of farm families who
were not directly involved in crop production activities could visit fields
where Class B sludge had been applied, but would not be allowed in areas where
Class C sludge had been applied. Only workers actively managing crops would
be allowed access to an area where Class C sludge had been applied.
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3.2.5 Septage
The proposed regulation defines septage as 'sewage sludge, so all the
requirements, including chose concerning control of disease risks, apply to
septage. Thus the regulation requires that septage cannot be disposed to the
land surface unless it meets Class A, B, or C requirements. This is a
departure from "Criteria," which allowed septage to be utilized on the land
without any treatment under a slightly different set of restrictions than
those for sewage sludge.
Septage pumped from a septic tank is ordinarily an extremely noxious
substance. However, if the septic tank has experienced little use,
particularly recent use, the septage could be low in pathogens and fecal
indicators and well stabilized. Except for the temporary burst of noxious
odor when such a material is applied to the soil, it might pose few problems
and little disease or vector attraction threat. Finding practical ways to
demonstrate that the typical 1,000-galIon tank truck load of septage poses
little disease risk is difficult. Almost any sampling and analysis scheme
would be more expensive than paying to have the sludge processed by a
wastewater treatment plant or a special facility for treating septage. The
regulation will probably move septage disposal in this direction.
Some question remains whether allowing septage to be applied after meeting
Class B or C requirements is wise. The nature of infection is that a person
is free of pathogens most of the time and only occasionally sheds pathogens in
feces. In the treatment of a community's wastes, an averaging effect comes
into play. Most of the time the sludge has low levels of pathogens that must
be lowered still more to be sure disposal is safe. On the other hand, most
septic tanks serve a single household. Consequently, most septage loads are
not contaminated, but occasionally, a load will be very "hot" with pathogens.
For example, it is not uncommon for several family members to be carriers of
salmonellae for several weeks after an infection in the family, or one or two
people in a family may have an Ascaris infestation. The methods used to meet
Type B and C requirements reduce pathogens by factors of 10 to 100.
Ordinarily, this provides ample protection, but it might not if densities are
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a thousand times higher than the ordinary maximum values encountered in an
untreated sludge from a wastewater treatment plant. Considered in this light,
septage disposal should be moved toward community treatment centers or
wastewater treatment plants.
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REFERENCES
Beaver, P.C. and'G. Deschamps . 1949. The viability of E. histolvtica
cysts in soil. Am. J. Trop. Med. 29: 181-191.
Benenson, A.S., ed. 1975. Control of communicable diseases in man.
Washington, DC: American Public Health Association.
Berg, G. and D. Barman. 1980. Destruction by anaerobic mesophilic and
thermophilic digestion of viruses and indicator bacteria indigenous to
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