Societal problems in siteing new landfills and the success of compost-
ing and marketing of sewage sludge has renewed public interest in the
INTRODUCTION
Agency's administrative review. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
1 This document has been subjected to the United States Environmental Protectior
Abstract
Efforts have not begun to regulate MSW-compost at the Federal level
nor in most states. However, municipal sewage sludge regulations under the
Clean Water ActSection503 have been proposed and are being finalized. The
CWA-503 regulations will be applicable to municipal sewage sludge products
including MS W-sludge composts. MSW-compost, being similar to munici-
pal sewage sludge-compost and having similar uses, will ultimately be
required to meet similar standards or at least similar methodology will be usec
to develop standards for MSW-compost. Therefore, an understanding of the
methodology and its data requirements will be beneficial in the developmen!
of appropriate data for MSW-composting. Further, voluntary compliance
with the CWA-503 regulations should enable the MSW-composting industry
to avoid waiting for state regulations. In particular, compliance with the nc
observed adverse effect level (NOAEL) sludge quality limitations, including
Class A pathogen reduction, should be acceptable for MSW-compost prod-
ucts. Therefore, a discussion of the proposed CWA-503 proposed regulatior
and its pathway analysis for agricultural utilization, non-agricultural lane
application and distribution and marketing of sludge is provided.
Environmental Chemistry Laboratory
Beltsville, Maryland
Rufus L. Chaney
USDA-Agricultural Research Service
Risk Reduction Engineering Laboratory
Cincinnati, Ohio
James A. Ryan
US Environmental Protection Agency
Risk Assessment for MS W-Compost1
Section 503: A Model for Exposure and
Regulation of Municipal Sewage Sludge
Under the Clean Water Act
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acceptability of change.
approach and how it might be used directly or modified for developing
guidance and regulations pertaining to MS W composting. We will argue that
it is better to clearly define policy-based issues, science-based issues, and
mixtures of the two and allow for an informed public to determine the
attempt to point out the science vs. policy decisions. We will attempt to show
the degree of protection that results in a reasonable-risk science-based
pollutant-content. We will critically examine the CWA-503 approach and
the envu-onment, differs markedly from the approach of no-change in soil-
and unless the policy is followed, unreasonable risk will be encountered. In
contrast, the CWA-503 proposed sludge rule, which is designed to allow the
use of sludge products in soils and at the same time protect human health and
An approach used by some European countries has been to start with
a policy decision that soils will not be allowed to change with respect to their
"pollutant" contents. If this policy based decision is not clearly defined the
public is erroneously led to believe that science has dictated the conclusion
424 Science and Engineering of Composting
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and organic compounds, including known or suspected carcinogens (EPA,
1985). In establishing standards for the disposal of sewage sludge, the Agency
each disposal option for 23 contaminants in sludges (Table 1). The contami-
nants considered were screened from a larger list of potentially harmful metals
assessment exercises which consider the health and environmental risk from
tural land application, agricultural land application, and distribution and
marketing (EPA, 1989a). Additionally it represents the Agency's first
comprehensive assessment including ecological as well as human health
effects. The contaminant loading limits are based on results of a series of risk
applicable to sludge monofills, surface disposal, incineration, non-agncul-
The proposed CWA-503 rule represents EPA's first attempt to estab-
lish comprehensive regulations for all sludge management options and is
PROPOSED RULE
tative of technical limits without consideration of policy issues.
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economic or social impacts are not critical. In fact it may be acceptable for
the decision maker to know trends and not values associated with a particular
426 Science and Engineering of Composting
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changes on the regulatory limits (EPA, 1990). We will explore the nature and
ramifications of these changes from a technical perspective and evaluate how
conservative the final limitations might be.
exposure models used to generate numerical limitationsfor the proposed rule
will be changed to reflect more up-to-date information and more realistic
scenarios describing the expected conditions in which sewage sludge will be
land applied." At the same time EPA gave an indication of the effect of these
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In response to the comments, the Agency, in their November 9, 1990
announcement in the Federal Register (U. S. Environmental Protection
EVALUATION OF POTENTIAL REVISIONS TO
THE PROPOSED STANDARD
metric tons per hectare.
(f) failed to take into consideration no-effect data, and
(g) arbitrarily limited sludge applications to no more than 50
ground water,
(e) used inappropriate and inadequate models to describe the
transfer of contaminant(s) from sludge source to surface and
conducted areview on the technical bases of the sludge incineration regulation
and the U.S. Department of Agriculture Cooperative State Research Service
Regional Technical Committee W-170, with assistance from EPA, academia,
environmental groups, and units of state and local government agencies
conducted a review of the technical bases for the sludge regulations on land
application, distribution and marketing, and monof ill and surface disposal. In
addition to these two reports, the Agency received in excess of 5500 pages of
comments from 656 respondents during the 180 day public comment period
on the proposed rule.
The public and scientific peer review groups provided a comprehen-
sive range of opinions, comments and recommendations which we will not
attempt to summarize. Rather, we will consider the USDA-CSRS W-170
Technical Committee report (1989). This Peer Review Committee's report
(PRC), although applauding the Agency'sattemptat using the risk assessment
methodology in establishing pollution control regulations, was critical of the
assumptions and data selections made by the Agency. The primary criticism
of the Agency's efforts of developing contaminant loading limits for agricul-
tural land may be outlined as follows:
(a) defined the MEI in an unrealistic manner,
(b) used a hypothetical and inappropriate diet scenario,
(c) used incorrect, incorrectly interpreted and inappropriate data
(d) used overly conservative relative effectiveness and dose
coefficients for the absorption of contaminant by humans and
animals,
430 Science and Engineering of Composting
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concern.
in sludge to land before exposure to the MEI has reached a level of regulatory
calculated RP represent the maximum allowable application of contaminants
this endpoint is worthwhile. The Food and Agricultural Organization and the
World Health Organization have defined ADI (acceptable daily intake) as
"the daily intake of a chemical which, during an entire lifetime, appears to be
without appreciable risk on the basis of all the known facts at the time. It is
expressed in milligrams of thechemical per kilogram of body weight (mg/kg)"
(Lu, 1983). It is apparent that this value is developed to protect the more
susceptible members of the population and thus allows greater protection for
the majority of the population (Ryan et.al.,1982; EPA.1988; Barnes and
Dourson, 1988). The Agency prefers the term reference dose (RfD) to avoid
the connotation of acceptability. The Agency's Integrated Risk Information
System (IRIS) has a list of RfDs for most noncarcinogenic chemicals. For
chemicals not listed, RfD values should be derived according to established
Agency procedures (EPA, 1988). Doses less than the RfD are not likely to be
associated with adverse health risks, and are therefore, less likely to be a
regulatory concern. As frequency and/or magnitude of the exposures exceed-
ing the RfD increases, the probability of adverse effects in the exposed
population increases and therefore becomes of regulatory concern (Hallenbeck
and Cunningham, 1986; EPA., 1988; Barnes and Dourson, 1988). Thus, the
measure of the potential for adverse effects. Therefore, a brief description of
utilized in pathways 1 & IF require a dietary intake of a contaminant as a
as dose (RfD) or exposure variables (all others). The risk reference dose (RfD)
The variables in the equation utilized to calculate RP can be classified
MS = 2x10' g/ha = assumed mass of soil in upper 15 cm
10"9 = conversion factor (kg/jig)
contaminant (ng/g DW)
RP = reference application rate of contaminant (kg/ha)
RLC = reference (allowed cumulative) soil concentration of
1
In theory, statistical tools can be used to enter the values as frequency
distributions and calculate the results in a frequency distribution. This
PointEstimatesof Exposure of the MEI '
as well as information on the exposed .population become critical. It is
apparent that not only is the definition of the MEI important, but also its
exposure and the two must be linked if the scenario approach is utilized.
modeling and scenario development approach is recommended and the link
between individuals and source is emphasized (EPA, 1991). Thus, informa-
tion on chemical concentration and time of contact data (duration of exposure)
1991). Further, when the exposure assessment is predictive in nature, a
between source and the exposed or potentially exposed population (EPA,
The purpose of an exposure assessment is to estimate exposure and
combine it with chemical specific dose response data to estimate risk. It is
important that assessments for specific chemical source demonstrate a link
MEI and Exposure must be Linked
its exposure.
corresponds to a very small, but statistically meaningful, percentage of the
population before it is appropriate to create algorithms to attempt to quantify
In defining exposure, the MEI is of critical importance. A MEI can be
human, plant or animal that is supposed to represent a living organism that,
because of individual circumstances, has the maximum exposure to a given
contaminant for a particular disposal practice. While this concept seems
simple, it presents severe methodological problems to a risk assessment. Risk
assessment is fundamentally a probabilistic analysis dealing with a random
variable. Traditionally, risk assessment has dealt with two extreme ends of
the risk scale. One is the low probability-high consequence risk (e.g., nuclear
reactor meltdown). The other is the high probability-low consequence risk
(e.g., car accidents). The MEI approach which is utilized by the Agency
represents another extreme, namely a low probability-low consequence risk.
That is, the probability that an MEI as defined actually exists is certainly very
small, and it may approach zero, The health consequence based on Agency
policy, if this hypothetical person does exist, is 10"4, or less for carcinogenic
chemicals or no greater than the RfD for noncarcinogenic chemicals. It is
possible to discuss the upper 99th percentile (or 90th or 95th), but an
improperly defined MEI (the individual with the greatest exposure) is a
concept without statistical relevance and represents a bounding estimate
whose exposure is irrelevant. When worst case assumptions about the MEI
are made, do they lead to the 95th percentile, the 99th percentile, the
99.99999th percentile? At a certain point, which is a function of the size of
the exposed population, there is apercentile which is not defined because there
are no individuals in the group. Thus, exposure to this undefinable group is
irrelevant as no one is at risk. Therefore, the MEI must be defined and
MEI must be Real
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further clarification. Therefore, we will examine the exposure variables and
evaluate their impact on the exposed population and its potential exposure.
in the 503 regulation and its impact on the size of the most exposed population
(MEI) as well as where on the exposure distribution the (MEI) falls needs
being the case an understanding of the distribution of each variable utilized
and its interactions mustbe known if the final distribution is to be known. This
product of several conservative assumptions is more conservative than any
single assumption alone, information about the distribution of each variable
"high end exposure", "reasonable worst case", "worst case" and "maximally
exposed individual" might fall (Figure 1). The question now being ask is how
does the 503 Proposed Rule fit in with these definitions? As the mathematical
to communicate where on the distribution these loosely defined terms such as
Descriptions of these point estimates: During the time the Agency has been
working on the C WA-503 exposure assessment, others have been attempting
the ranges of each variable is utilized to make the predictions. This approach
results in a less precise estimate that is described with ill-defined terms (e.g.
worst case, maximally exposed individual, etc.). As historically illustrated,
use of these exposure scenarios has been a source of controversy regarding
how conservative they are. In part conservatism can occur because of attempts
to account for data uncertainty by becoming more conservative in expression
of the data; without specifying what was done. A clear distinction between
the variability of exposures received by individuals in the population and the
uncertainty of the data would help resolve the controversy. In many cases
where estimates are termed "worst case", both a focus on the high end of the
exposed population and a selection of high end value from the data set (for
uncertainty) are used, leading to values that are quite conservative. By using
both the high end individuals (variability) and upper confidence bounds on
data (uncertainty), the estimates might be interpreted as approaching upper
bound exposures received by high end individuals.
is known. Thus, the alternative approach of selection of discrete values from
requires that the frequency distribution of the variables be known and that they
are independent. Unfortunately, it is only in rare cases that such information
434 Science and Engineering of Composting
PERCEKT OF GARD
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Answers to these questions not only impact the potential MEI population but
have significant impacts on its exposure.
application rate are unaware of good agronomic management practices?
sludge are at the maximum application rate (RP)? How many of these
subsistence lifetime gardeners who use sludge and are at the maximum
lifetime gardeners use sludge or have gardens that are located on previously
sludge soils? How many of these subsistence lifetime gardeners who use
population exists, the size of the population diminishes drastically as some of
the other assumptions are placed on them. For example how many subsistence
may be worst case or bounding estimate. While one might argue that this
lifetime gardener population would be classified as maximum exposure and
of them will continue to garden for a lifetime. It is evident that the subsistence
because the data on gardeners is short term information and only a small part
As previously discussed, pathway IF ( home gardening) represents the
most limiting of the pathways for human consumption of agricultural crops.
It is necessary to attempt to quantify the number of home gardeners and their
production. If the term "households" is equated to population, then 46% of
the population produces someof the food itconsumes(Kaitz,-1978). fhedata
onpercent of gardens vs garden size would suggest that the median garden size
was 800 ft2 and that less than 8% of the gardeners had a garden of greater than
21,000 ft2 (Figure 2). If one equates garden size to the amount of food
production and assumes that a 21,000 ft2 garden is required to produce all
garden foods consumed each year (Ryan et al., 1982), less than 12% of
gardeners can produce half their yearly consumption (Figure 2). Thus most
of the population of gardeners do not have a large enough area to produce a
large part of their annual consumption, and only (46 x .12) = 5.5% of the,
population would be in the defined subsistence home gardener category.
Further, demographics suggest that less than 2% of the population live in the
same county for a lifetime and thus a change in garden location would alter
their exposure (EPA, 1977). It would seem that the subsistence home gardener
is no more than 2% of the population (assuming all persons who do not move
are subsistence gardeners) nor less than 0.1% (5.5 x .02), but most likely less
than 1 %. This is a conservative estimate and the actual number is much less
Subsistence Home Gardeners
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Soil variables
Of all the soil variables which ha
<
e been reported to affect plant uptake
may appear to say that the UC value in t
value but as discussed above could be
O 3^
j exposure assessment is a mid range
onsidered a bounding estimate.
appears to represent a log normal dist
geometric mean which best represents
<"* ^f
bution, it is our contention that the
le distribution should be used. This
which doesn't appear to be justified. /
t^
Tn
: the UC data set for any food group
estimate of the distribution would onlj
ayer on another conservative factor
long term data set it is representing.
*~~3
'hereforc, the use of a conservative
revised UC data set to overestimate the
new set may represent upper bound ex
With this conservatism built in
representation of thedata set will also y
CT" ?r
> the data set, it is apparent that any
Id a conservative estimate of the true
•a ~
rue long term data set and in fact the
osures.
cause, but it could cause UC to be b
linearity and long term sludge equilibi
ofexposurebyRP/4 [(RP/20) x 5]. Itisl
eliminated the error caused by utilizati
ment of the UC data set, we have all
Q o sr
us apparent that even though we have
a of salt pot studies for the develop-
ed data uncertainties to cause the
P- 2
jnding estimates. Just considering
im could result in an overestimation
442 Science and Engineering of (
\ i
O
imposting
lldnot harvest 60% of their yearly consumption during ti
uming that the gardener may obtain 10% of his le<
: home garden, and allowing 60% for the other food grou;
irden produced leafy vegetables for one month each ye
s them throughout the year, except in extreme situatio:
lucky to harvest a few weeks production, which means tl
s the MEI an upper bond estimate of both the populati
ify vegetables are consumed in a fresh state and it is i
P. An issue which is not apparent is that the requiremi
ring little impact on RP, whereas reductions in FC lead
ly, changes in this FC have asignificant effect on RP (Ta
lie 60% FC would represent a high end value with furtl
isumptions and make for a small number of people wit
ipulation. Changes in the fraction of food originating fn
oil (FC) would alter the size of the exposed populat:
aroduced
y defined, the subsistence gardener is assumed to prodi
me consumption of potatoes and 59% of his lifeti
other food groups. As discussed in the MEI section, thi
r«f'>O«E3't
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002.
vironmental Protection Agency. 1985. Summary ofenvironm
profiles and hazard indices for constituents of municipal sludge
EPA, Office of Water Regulations and Standards, Waster
Critera Branch, 200pp.
ivironmental Protection Agency. 1986. Guidelines for Estim
Exposures. Federal Register 51:34042-34054.
ivironmental Protection Agency. 1988 Reference Dose (RfD)
scription and use in health risk assessments. Integrated Risk ]
mation Systems (IRIS). Online. Intra Agency Reference Dose (
Work Group, Office of Health and Environmental Assess:
Environmental Criteria and Assessment Office, Cincinnati O
vironmental Protection Agency. 1989.a. Standards for the Dis
of Sewage Sludge; Proposed Rule 40 CFR Parts 257 & 503. F<
Register 54:5746-5902.
vironmental Protection Agency. 1989.b. Development ofriska,
ment methodologyfor land application and distribution andmt
ing of municipal sludge. EPA/600/6-89/001.
nvironrtiental Protection Agency. 1990. National Sewage S
Survey: Availability of Information and Data and Anticipate
Ir, ,_..• r> /i-D,^aArtr°aaVart
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