United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/046 Sept. 1987
&EPA Project Summary
Potential Emissions of Hazardous
Organic Compounds from Sewage
Sludge Incineration
Sueann Mazer, Philip H. Taylor, and Barry Dellinger
Very little field data is available con-
cerning organic emissions from sewage
sludge incinerators. This is of particular
concern because hydrophobic hazardous
organic compounds, such as certain
pesticides, polynuclear aromatic hydro-
carbons, and polychlorinated biphenyls,
have been shown to partition onto the
sludge during the wastewater treatment
process. The environmental fate of these
compounds during sewage sludge in-
cineration is largely unknown.
Laboratory thermal decomposition
studies were undertaken to evaluate
potential organic emissions from sew-
age sludge incinerators. Precisely con-
trolled thermal decomposition experi-
ments were conducted on sludge spiked
with mixtures of hazardous organic
compounds, on the mixtures of pure
compounds in absence of sludge, and
on unspiked sludge. These experiments
were conducted using laboratory flow-
reactor systems interfaced with a gas
chromatograph or gas chromatograph-
mass spectrometer for product analysis.
Issues which were addressed included
the effect of the sludge matrix on the
thermal decomposition behavior of the
hazardous sludge contaminants; poten-
tial emissions from incineration of con-
taminated and uncontaminated sludge;
the relative contributions of the biomass
and the contaminants to mass emissions
in sewage sludge incineration; and
potential emissions due to volatilization
of organics from sludge in the drying
zones of multiple hearth incinerators.
In addition to the global thermal
decomposition studies describe above,
elementary reaction kinetic studies were
conducted using a laser photolysis/
laser-induced fluorescence technique.
To permit an in-depth understanding of
some of the fundamental chemistry oc-
curring in sewage sludge incinerators,
the reaction rates of OH radicals with
model chlorinated hydrocarbons were
measured.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
Is fully documented In a separate report
of the same title (see Project Report
ordering Information at back).
Introduction
While incineration has been used to
dispose of sewage sludge since the
1930's, concern regarding potential en-
vironmental insult due to organic emis-
sions from this disposal method has
developed only recently. Very little is
known about organic emissions from
municipal sludge incinerators, and the
research program described herein was
designed to begin filling in such a data
base, and to support the development of
regulations for sewage sludge in-
cinerators.
The Domestic Sludge Exclusion re-
moves from hazardous waste status the
solvents, industrial effluents, process
wastes, etc., which are discharged to
publicly owned treatment works. As a
result, some 20,000 metric tons of priority
organic pollutants enter wastewater
treatment plants in the United States
each year. It is documented that priority
pollutants such as pesticides, polychlori-
nated biphenyls, phenols, phthalates,
polynuclear aromatic hydrocarbons, diazo
dyes, common solvents, and nitrosamines
-------
concentrate on sewage sludge. Most of
these contaminants are found in con-
centrations of 0.1 to 100 /itg/g (ppm),
based on the dry weight of the sludge.
Some potentially toxic organics found in
sewage sludge are biodegradation pro-
ducts from less toxic precursors. For
example, nonylphenol, a biodegradation
product from nonylphenol ethoxylates
(common nonionic surfactants), has been
detected at levels around 1000 ng/g in
sewage sludge.
While emissions of metals, particulates,
and gases such as SOX and NOX have
been characterized to some extent for
sludge incinerators, very little data exists
concerning organic emissions. Limited
field data indicates that polynuclear
aromatic hydrocarbons and chlorinated
dioxins and furans are emitted from
sludge incinerators.
Assessment of organic emissions from
municipal sludge incineration is further
complicated by the wide range of in-
cineration conditions which occur. In-
cineration parameters are not generally
as well characterized or controlled as
they are for hazardous waste incineration.
This is particularly true for the multiple
hearth incinerators, which constitute
about 80% of the incinerators used for
sewage sludge disposal. The other 20%
are primarily fluidized bed incinerators,
in which conditions are better defined
and controlled.
The laboratory studies described herein
were designed to generate a data base
on potential organic emissions from
sewage sludge incinerators. The research
is broken down into three major areas:
1. Potential emissions of hazardous
sludge contaminants and products
of incomplete combustion (PIC's)
from sludge incineration;
2. Potential emissions of organics due
to volatilization in the drying zone of
the multiple hearth incinerator; and
3. Elementary reaction rates of
hydroxyl radicals with model sludge
contaminants.
Thermal Decomposition of
Spiked Sludges and Spiking
Mixtures
Experimental
To study the thermal decomposition of
common sewage sludge contaminants,
and to identify products of incomplete
combustion (PIC's) from contaminated
sewage sludge, thermal decomposition
studies were conducted on the Thermal
Decomposition Unit-Gas Chromato-
graphic (TDU-GC) system and the Thermal
Decomposition Analytical System (TDAS).
Block diagrams of these systems are given
in Figures 1 and 2, respectively. Both
systems include thermal decomposition
units in which temperature, atmosphere,
and mean residence time can be precisely
controlled. In the case of the TDU-GC,
this thermal unit is interfaced with an in-
line gas chromatograph (GC), while in
the case of the TDAS, the analytical in-
strumentation is an in-line gas chromato-
graph-mass spectrometer (GC-MS).
Details concerning these systems are
given in the full report.
Thermal decomposition studies were
conducted on a relatively clean sludge
spiked with mixtures of contaminants, on
mixtures of pure contaminants (without
sludge), and on unspiked sludge. Con-
taminants were studied in three mixtures
rather than as pure compounds so that a
number of compound classes could be
investigated within the constraints of the
program. The contaminants which were
studied were heptachlor; pentachloro-
phenol; diphenylnitrosamine; pyrene;
butyl benzyl phthalate; 2,3',4,4',5-
pentachlorobiphenyl; azobenzene; and
technical-grade nonylphenol.
Sewage sludge used for these studies
was a filter-cake sludge containing 17%
solids, and was obtained from a local
wastewater treatment plant from a pri-
marily residential municipality. This
sludge was not adequately dewatered for
efficient incineration, but was otherwise
similar in composition to sludges which
are incinerated. The sludge was spiked at
about 1 mg/g with the organic compo-
nents, except nonylphenol, which was
spiked at 10 mg/g. These levels are at
least an order of magnitude greater than
those typically found in "real world"
sludges, but such spike levels were re
quired to minimize interferences on the
TDU-GC from the sludge matrix.
Variables investigated in these studies
included gas phase temperature (300°C
to 1000°C), oxygen level (air for combus-
tion studies and nitrogen for pyrolytic
studies), and sample matrix (spiked sludge
versus mixtures of pure spike compounds)
For spiked sludges and mixtures of pure
compounds, the TDU-GC was used to
generate thermal decomposition curves
as a function of reactor temperature and
oxygen level. The TDAS was then used to
identify products of incomplete combus-
tion (PIC's) by selecting reactor tempera-
tures that, based upon TDU-GC data,
would yield the majority of the PIC identifi
Thermal Decomposition Unit
Captt
Effluent
ire of
Products
SSSS&
«••
sssss
Controlled High
Temperature Exposure
SSSSS
«^
SSSS^sS
Sample Insertion
and Vaporization
\ 1
High Temperature Transfer
Pressure and
Flow Regulation
Compressed Gas
and Purification
Multifunctional
Gas Chromatographic
Instrumentation
Containment or Destruction of
Effluent Products
Figure 1. Block diagram of TDU-GC.
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High Temperature Transfer
Capture
of
Effluent
Products
77/7/77,
Controlled
High
Temperature
Exposure
Sample
Insertion
and
Vaporization
In-Line
Gas Chromatograph
(High Resolution)
Figure 2. Block diagram of TDAS.
Computer
System
NIH-EPA
Chemical
Information
System
Pressure and
Flow Regulation
Compressed Gas
and Purification
Coupled
Mass
Spectrometer
(Magnetic)
cations. The unspiked sludge was studied
using the TDAS only, since there were
no contaminant thermal decomposition
profiles to generate.
A thermal decomposition run was
initiated by inserting the sample, con-
tained in the sludge probe, into the sample
insertion region of the TDU-GC or TDAS.
After allowing laboratory air to be purged
from the system, the insertion region
was programmed from 50°C to 200°C at
15°C/min. During sample heating, the
vaporized sample components were con-
veyed into the reactor using either
nitrogen or air. The reactor temperatures
were varied over the range of 300°C to
1000°C, and a 2-second gas-phase mean
residence time was maintained in the
reactor. The reactor effluent (both un-
decomposed parents and products of
incomplete combustion) was trapped at
the head of the in-line gas chromato-
graphic column. After purging the system
with a helium carrier gas, the GC was
programmed to elute the trapped species,
which were detected using an FID detec-
tor for the TDU-GC, or a mass spectro-
meter for the TDAS. Frequent method
blanks and duplicates were run to ensure
the integrity of the data.
Results and Discussion
The thermal decomposition of nonyl-
phenol typifies the behavior observed for
the majority of the sludge contaminants.
Thermal decomposition profiles for
nonylphenol as a function of matrix
(spiked onto sludge versus run in a mix-
ture of pure compounds) and of atmo-
sphere (nitrogen versus air) are shown in
Figure 3. These curves indicate that for a
given matrix, the compound was more
stable in nitrogen than in air. This is
typical behavior for many organic com-
pounds and was not surprising here.
Figure 3 indicates that sample matrix
also affected the thermal decomposition,
with nonylphenol being more difficult to
destroy in the presence of sludge than in
the presence of the pure mixture.
While the sludge appeared to stabilize
most of the contaminants studied, there
were some cases where the sludge en-
hanced the thermal decomposition, and
some where the sludge exerted little
effect. Based on the limited data from
eight hazardous contaminants, the fol-
lowing trends were observed:
• The sludge exerted little if any effect
on the ease of destruction of very
fragile contaminants.
• If a contaminant was intermediate
in thermal stability, it was more dif-
ficult to destroy in the presence of
the sludge.
• If a contaminant was very stable in
the pure mixture, the sludge en-
hanced its ease of destruction.
Comparing the overall thermal stability
of the eight sludge contaminants, pyrene
was the most stable and butyl benzyl
phthalate was the least stable. The rela-
tive ease of incineration of the eight
hazardous contaminants increases in the
following order: pyrene < pentachloro-
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700
.1/0
.c
c
£ 1
0.1
0,01
1 \ r
Npnylphenol
~t - 2.0 sec
OH
HC-CH2- CH-
( t
CH3 CH3
O /Vg2, Mixture
Q AIR, Mixture
& N2. Sludge
O AIR, Sludge
J 1 1 I I I
J L_
700 200 300 400 500 500 700
Exposure Temperature, °C -~
800 900 WOO
Figure 3 • Thermal decomposition profiles for nonylphenol
biphenyl < pentachlorophenol < di-
phenylamine < azobenzene < nonyl-
phenol < heptachlor < butyl benzyl
phthalate.
A multitude of thermal reaction pro-
ducts were identified for spiked sludge,
and mixtures of pure spiking compounds.
Analysis of the data indicates that some
PIC's could be attributed to the sludge,
while others could be attributed to the
contaminants, and still others may have
formed through an interaction of the
sludge and the contaminants. Interaction
between the sludge and chlorinated con-
taminants may be the source of the trace
emissions of chlorinated dioxins and
furans from certain sludge incinerators.
The majority of the PIC's were aromatic,
and the remainder were primarily chlori-
nated or unchlorinated short-chained
unsaturated aliphatic hydrocarbons. The
reader is referred to the final report for a
complete discussion and tabulation of
PIC's, since only a few examples can be
given here. Pentachlorophenol, for ex-
ample, yielded chlorinated benzenes,
tetrachlorophenol, tetrachloropropyne,
hexachlorodihydro-naphthalene, and
other PIC's. Diphenylnitrosamine was
100% converted to diphenylamine at very
low temperatures; this primary PIC was
then the precursor for carbazole, ben-
zonitrile, biphenyldicarbonitrile, and
possibly other PIC's.
The sludge itself yielded a number of
PIC's which are of environmental concern.
At relatively low temperatures (~500°C),
desorption/decomposition products from
the sludge included primarily compounds
having the cholesterol-fused ring struc-
ture and alkenes or cycloalkanes. At
higher temperatures (~800°C), benzene,
styrene, toluene, benzaldehyde, ethyl-
benzene, xylene, and various polynuclear
aromatic hydrocarbons were formed as
PIC's. A number of these are on the
Appendix VIII list of hazardous compounds
and/or have produced positive results in
various toxicological tests. Since the
sludge used for these experiments origi-
nated in a relatively nonindustrialized
municipality, these PIC's are attributed to
the biomass rather than to environmental
contaminants.
These laboratory studies indicate that
PIC's from the sludge contributed much
more to the total mass emissions than
did PIC's from the contaminants. This is
primarily because the contaminants were
spiked at only parts-per-thousand levels
in the sludge, meaning that the relative
weight of the starting sludge was orders
of magnitude greater than that of the
starting contaminants. For "real world"
sludges, which generally contain con-
taminants as parts-per-million rather than
parts-per-thousand levels, the relative
contribution of the biomass to total mass
emissions is expected to be even greater.
The fact that potentially toxic com-
pounds were formed from the sludge,
combined with the fact that the total
levels of PIC's were many times greater
for the sludge than for the trace con-
taminants, indicates the importance of
incinerating all sludges under highly con-
trolled and environmentally safe
conditions.
Multiple Hearth Approximation
Experimental
Because both temperatures and resi-
dence times in the drying zone of the
multiple hearth incinerator are relatively
low, it was hypothesized that incineration
efficiency would be poor in this zone. The
temperature appeared to be high enough
to volatilize many organic species, but
not sufficiently high to initiate their
thermal decomposition. For this reason,
an experiment was designed to determine
the potential for emissions from the drying
zone of a multiple hearth incinerator.
The thermal exposure within the drying
zone of the multiple hearth incinerator
was best mimicked by gradually heating
the sludge in the insertion region of the
TDU-GC while maintaining the reactor at
a low temperature. Therefore, a single
sludge sample was placed in the sample
insertion region of the TDU-GC, and was
successively heated at 15°C/min in
nitrogen over four temperature ranges.
These temperature ranges were 50°C to
200°C, 200°C to 300°C, 300°C to 400°C
and 400°C to 500°C. The gas-phase
residence time of desorbed species in the
insertion region was less than 1 second.
During the entire experiment, the reactor
was maintained at a nondegradative
temperature (300°C). While the sample
was being heated over a given tempera-
ture range, any evolved species were
condensed at the head of the in-line gas
chromatographic column. After these
species were collected, they were
analyzed using programmed temperature
capillary gas chromatography (GC), with
flame ionization detection (FID). Once the
GC run was complete, the same sample
was then heated over the next tempera-
ture range and the analytical process
repeated. This procedure was repeated a
total of four times, until a final tempera-
ture of 500°C was achieved.
Results and Discussion
Results of this experiment indicate that
the majority of the volatile sludge compo-
nents desorbed during the initial program
step between 50°C and 200°C. Signifi-
cant residuals also desorbed in the 200°C
to 300°C and the 300°C to 400°C ranges,
but a marked decrease in desorbed or-
-------
ganics was seen between 400°C and
500°C. Overall this experiment clearly
illustrated the potential for major emis-
sions from volatilization and/or in-
complete combustion of sludge compo-
nents in the drying zone of the multiple
hearth incinerator.
A comparison of potential emission
levels from the multiple hearth versus
the fluidized bed incinerator is shown in
Figure 4. The top chromatogram depicts
volatile species desorbed from the sludge
and conveyed through the reactor at
300°C, while the bottom chromatogram
shows the remaining species after these
volatilized organics have been subjected
to an 800°C reactor temperature. While
the thermal conditions in the top chro-
matogram approximate those in the drying
(a) Reactor at 300° C
(transport temperature)
zone of the multiple hearth incinerator,
those in the bottom chromatogram are
more characteristic of the thermal regime
in the fluidized bed incinerator. This
indicates that the thermal regime of the
fluidized bed incinerator is much less
likely than that of multiple hearth to lead
to major gaseous organic emissions.
Hydroxyl Radical
Reactivity Studies
Experimental
For the combustion of hydrocarbons,
OH radicals are the dominant reactive
species under all but the most fuel-rich
environments. Although it is recognized
that chlorine atom reactivity may be
I li II
U.UUJU I III
-
116
important for chlorinated hydrocarbons
such as those produced as PIC's in the
thermal degradation of contaminated
sewage sludge, OH reaction kinetics are
considered most significant with respect
to the oxidative destruction of these
materials. Since there is a general paucitv
of data on OH-chlorocarbon reaction;.
and data are practically non-existent a:
the high temperatures which simulant
incinerator conditions, the high tempera-
ture elementary reaction rates of OH
radicals with select chlorocarbons were
determined for this program via a laser
photolysis/laser-induced fluorescence
technique. This experimental approach
allows the generation and detection of
OH radicals in a "clean" environment
where essentially only OH-chlorocarbon
reactions can occur. This eliminates the
complexities of rate constant determina-
tions in multireaction media such as
shock tubes, flames, and flow reactors.
The experimental test system consisted
of five inter-dependent components; (1) £
test cell and temperature control system,
(2) the carrier gas and sample delivery
system, (3) the pump laser system, (4) the
probe beam system, and (5) the detection/
measurement system. The test cell was a
specially designed pyrex cell which al
lowed gas reactants to be heated, ir-
radiated with laser beams, and observed
By monitoring relative concentrations of
OH over time, the reaction rates of OH
radicals with organic molecules were
calculated.
The chloromethanes were chosen as
model chlorocarbon compounds for this
initial study because chlorinated hydro
carbons are both known sludge con
taminats and stable PIC's produced from
decomposition of contaminated sludge.
Pseudo-first order rate constants were
generated for all the chloromethanes over
a temperature range of 296 to 782K.
(b) Reactor at 800° C
129
Figun 4 . Comparison of potential emissions levels from fa) drying zone of multiple hearth
versus fb) fluidized bed incinerator.
Results and Discussion
Table 1 presents bimolecular rate con-
stants for reactions of OH radicals with
the chloromethanes over a temperature
range of 571 to 782K. A statistical
propagation of error analysis was appliec
to the data collection procedure with the
results presented in Table 2. With minoi
exceptions, good precision was indicated
as standard deviations in the rate con-
stants were generally less than 10% of
the measured value. The temperature
dependence of the bimolecular rate con-
stants were evaluated with weighted
least-squares algorithms in relation tc
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the Arrhenius expression:
k = A exp [-Ea/RT]
and the three-parameter non-Arrhenius
expression:
k = BTn exp [E/T].
The data analyzed in this manner always
exhibited non-Arrhenius kinetics with the
results presented in Table 2.
Table 1.
Reactant
CH3C1
CH3C1
CH3C1
CH2C12
CH2C1 2
CH2C12
CHC13
CHC13
CHC13
CC14
CC14
CC14
Chloromethane-OH Rate
Constants as a Function of
Temperature (571 to 782 K)
kdO^crrf/molec-s) TfKJ
8.38±0.296* 578
15.96±1.31 687
24.97±2.25 776
11.12±0.590 580
19.83±2.94 674
33.26±4.09 773
14.07±1.60 571
20.00±1.46 680
31.73±1.81 772
J.65±0.056 614
3.54±0.050 773
5. 12±0.800 782
* Standard deviation in rate constant.
The rate data indicated relative OH
reactivities for the chloromethanes to be:
CHC13 ^ CH2C12 > CH3C1 » CC14.
The high temperature relative reactivities
for CH3C1, CH2C12, and CHC13 are in
accord with their gas-phase thermal
stabilities under oxidative conditions. In
other words, OH reactivity increases with
increasing C1 substitution for the
chloromethanes. This indicates that OH
reactivity is more sensitive to the ease of
hydrogen atom abstraction (which in-
creases as the C1 atom substitution
increases) than to the total number of
abstractable H atoms present. If we extend
this hypothesis to the limit of perchlori-
nated compounds, these results indicate
a much smaller OH reactivity as compared
to any of the hydrogen-containing
chloromethanes. Although the C-C1 bond
in CC14 is weaker than any C-H bond in
the remaining chloromethanes, the
endothermicity of this reaction due to the
low stability of HOC1 formation results in
a relatively high-energy reaction pathway.
Extrapolating these results to the types
of PIC's produced in the oxidative de-
gradation of contaminanted sewage
sludge, perchlorinated hydrocarbons such
as C2C12, C2C14, C3C14, and C4C140,
should prove more difficult to incinerate
than their hydrogen containing analogs
(i.e., C2HC1, C2HC13) due to the lack of
abstractable hydrogen atoms. Additional
PIC's believed to form from the sludge
matrix, such as simple aromatics (i.e.,
benzene, toluene, styrene), should prove
more difficult to incinerate than their
chlorinated analogs because of the larger
C-H bond energies.
Conclusions
Based on laboratory studies of the
thermal decomposition of hazardous
organic mixtures and sewage sludge
spiked with various hazardous organic
compounds, the following conclusions are
proposed:
• Some common hazardous organic
contaminants are relatively fragile
and easily destroyed, but their pro-
ducts of incomplete combustion pose
a greater incineration challenge.
• Contaminants of moderate-to-high
thermal stability may be volatilized
in the drying zone of multiple hearth
incinerators and emitted with great
efficiency.
• The thermal decomposition behavior
of common hazardous sludge con-
taminants differed when these
principal organic hazardous con-
stituents (POHC's) were decomposed
as mixtures in a sludge matrix versus
as mixtures of pure compounds. The
following trends were observed for
the eight POHC's tested:
• For very fragile POHC's, the sludge
exerted no effect on the inciner-
ability of the POHC.
• For POHC's of intermediate
thermal stability (most of the
compounds tested), the sludge
appeared to decrease the inciner-
ability of the POHC (i.e., compound
was more stable in sludge).
Table 2. Non-Arrhenius Parameters for Chloromethane-OH Reactions
Reactant BfcrrP/molec-s) n E(cal/mole)
T Range (K)
CH3C1
CH2C12
CHC13
CC14
5.89±1.22*xW2'
5.14±0.98x10'21
8.38±0.45x10'21
4.56±0.80xia22
3.0
3.0
3.0
3.0
-145±59*
75±59
•70±21
331 ±67
296 - 776
298 - 773
298 - 772
296 - 782
* One standard deviation in data.
• For very stable POHC's the sludge
increased the incinerabilityof the
POHC (i.e., POHC was more
readily destroyed in presence of
sludge).
• Thermal decomposition products
from the biomass itself included
potentially hazardous PIC's which
are on EPA's Appendix VIII list.
• Total mass emissions from the
biomass are expected to be much
greater than from trace con-
taminants.
• Interactions between the sludge and
chlorinated contaminants may pose
additional health risks in incineration
of sludge from industrial sites.
Recommendations
The results of this study have answered
some very basic questions; however,
some very important questions have been
raised. The following additional research
is recommended:
• Conduct additional laboratory studies
to explain the sludge stabilization
effect on hazardous organics.
• Extend laboratory testing to other
sludges and contaminants.
• Extend the laboratory analysis to
include identification of volatile
emissions.
• Conduct laboratory analysis of the
uncombusted sludge residue to
determine the chemical composition
of full-scale particulate emissions.
• Conduct additional laboratory studies
on the effects of sludge heating rate
in both multiple hearth and fluidized
bed systems.
• Utilize laboratory test data to guide
future full-scale emissions testing
and derive correlations between
predicted versus observed emissions.
• Based on laboratory data, establish
a "hit list" of suspected major in-
cinerator emissions that would be
the subject of full-scale compliance
testing.
• Determine health effects of these
emissions to compare with other
sources for the establishment of a
mass emissions regulatory standard.
The full report was submitted in ful-
fillment of Cooperative Agreement No.
CR-811777 by the University of Dayton
Research Institute under sponsorship of
the U.S. Environmental Protection
Agency.
-------
-------
Sueann Mazer, Philip H. Taylor, and Barry Del linger are with the University
of Dayton Research Institute, Dayton, OH 45469.
Richard A. Dobbs is the EPA Project Officer (see below).
The complete report entitled "Potential Emissions of Hazardous Organic
Compounds from Sewage Sludge Incineration," (Order No. PB 87-199 626/
AS; Cost: $18.95, subject to change)
will be available only from:
National Technical Informaiion Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
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POSTAGE & FEES PAID
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EPA/600/S2-87/046
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