V-/EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-242 Dec. 1981
Project Summary
Physical and Chemical
Characteristics of Synthetic
Asphalt Produced from
Liquefaction of Sewage Sludge
J. M. Donovan, R. K. Miller, T. R. Batter, and R. P. Lottman
Direct thermochemical liquefaction
of primary undigested municipal
sewage sludge was carried out to
produce a low molecular weight
steam-volatile oil, a high molecular
weight synthetic asphalt, and a
residual char cake. The latter product
is capable of supplying the thermal
energy requirements of the
conversion process. The steam-
volatile oil has immediate value as a
synthetic fuel oil. The synthetic
asphalt may prove to be a useful
cement for paving with further
research, or it can be used as a fuel or
coking stock. It is outwardly similar to
petroleum asphalt, but chemically
different.
The thermochemical liquefaction
process should be capable of opera-
ting in a technical and environmentally
acceptable manner in conjunction
with many existing wastewater treat-
ment facilities. The overall feasibility
of the process depends on the value of
the oil and synthetic asphalt products
as petroleum replacements and on the
costs associated with disposal of
sludge. Projected economics indicate
that the process has considerable
promise for many potential sites in the
United States at the present time.
This Project Summary was develop-
ed by EPA's Municipal Environmental
Research Laboratory, Cincinnati. OH,
to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title fsee Project Report ordering
information at back).
Introduction
Disposal of sewage sludge is an
increasing problem for many munici-
palities in the United States. Currently,
there is a need to implement alternative
disposal technologies. The alternative
technologies need to be energy efficient
and,.if possible, some product of value
should be recovered from sludge. Direct
thermochemical liquefaction has the
capability of meeting these requirements.
Thermochemical liquefaction investi-
gations by various authors have shown
that organic biomass can be converted
to bitumen, heavy oil, and distillate oils
having combustion heats closely
approximating petroleum products.
The utility of the high molecular
weight fraction of the liquefaction
product (because it is a substantial part
of the liquefaction products) is often
questioned. To this end, the present
study was conducted to determine the
value of this fraction as synthetic
asphalt for use as a paving cement. A
synthetic asphalt product would be
valuable since petroleum asphalt has
increased in value with escalating
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petroleum costs and it is being increas-
ingly used (e.g., production of olefins) as
a petrochemical feed stock.
The low molecular weight product of
biomass liquefaction has always been
viewed as a valuable synthetic fuel. In
the case of sludge liquefaction, the low
molecular weight oil product was
conveniently separated during the
reaction period by steam auto-distilla-
tion. This steam-volatile oil was
obtained in significant quantities using
a relatively simple process. This process
also led to simplified separation of
excess water present because of the
use of wet sludge.
Thermochemical Liquefaction
of Primary Sewage Sludge
Previous work in the area of direct
thermochemical liquefaction of sewage
sludge and other forms of biomass has
resulted in the production of high
molecular weight fractions that are
similar in appearance to petroleum
asphalt. A high molecular weight
fraction derived from pure cellulose in
our Battelle-Northwest laboratory
showed promise as a replacement or
extender for petroleum asphalt.
Because of the high cost of pure cellu-
lose and other biomass forms, other
alternative feed materials were
suggested. Primary undigested sewage
sludge, because of its low or negative
cost, was a logical choice.
Experimental Procedure
Although the intended feed stock for
the experimental work was to have been
dried primary sludge from Honolulu, the
actual feed stock was fresh, primary
sludge dewatered with polymer. The
fresh sludge was preserved by adding
chloroform and, later, by freezing. It was
then sent by air to our laboratory at
Richland, Washington.
Fresh sludge received at our
laboratory was prepared for reaction by
adding a base (either Na2C03 or CaO).
This material was then loaded into an
Inconel linerand blanketed with an inert
gas (Argon). The liner was sealed and
placed into an autoclave. Water was
added between the liner and autoclave
to assist in heat transfer and to balance
the differential pressure across the liner
that would result from heating to
reaction temperature. Pressure inside
the liner was controlled to equal the
vapor pressure of water in the annular
space; this prevented either implosion
or explosion of the liner.
After being prepared in this manner,
the autoclave was heated to reaction
temperature (2 to 3 hours), held at
reaction temperature for 1 hour, and
cooled 4 to 6 hours. During the heating,
reaction, and cooling periods, gas and
steam were emitted from the liner
because of the pressure in the liner
caused by the gas generation accom-
panying thermochemical liquefaction.
The predominant gas formed and dis-
charged was carbon dioxide, but steam
and steam-volatile oil were also dis-
charged with the gas. These gases were
condensed in a water trap and saved for
later analysis. Although hydrogen
sulfide was monitored during the
reactions, none was detected.
After cooling, the autoclave was
opened, the liner removed and opened,
and the product taken out. This product
could be separated into an aqueous
supernatant liquid and a char cake
either by settling or centrifugation. In
our laboratory work, the latter was more
convenient and was used most often.
The supernatant liquid was analyzed for
its volatile constituents and-was sub-
jected to treatability analyses. The char
cake contained the high molecular
weight fraction of the liquefaction
product, which was intended to become
synthetic asphalt. As a result of being
intermixed with the char and ash, the
high molecular weight fraction was not
acceptable for use as a replacement or
extender for petroleum asphalt since it
would not melt when heated nor mix
with heated petroleum asphalt.
To overcome the problems caused by
the char and ash, the high molecular
weight fraction was separated by sol-
vent extraction. Previous liquefaction
work, and specifically work with lique-
faction products from cellulose, indi-
cated that acetone was an acceptable
solvent. Soxhlet extractors were used, and
extraction times were 8 to 24 hours. The
extracted char cake was crumbly after
the solvent was removed in a rotary
evaporator under vacuum. The resulting
material, whose appearance resembled
heavy crude oil, was not acceptable for
direct use as a petroleum asphalt
replacement or extender in preliminary
work because its viscosity was not high
enough.
The use of vacuum distillation to
remove residual low molecular weight
liquefaction products from the extracted
product solved the low viscosity
problem. Although only a very small
amount of low molecular weight
material was removed, the viscosity of
the product was substantially
increased.
Synthetic asphalt samples prepared
in this manner were sent to Dr. R.
Lottman at the University of Idaho for
testing and analysis as paving cements.
Liquefaction Test Results
Reaction conditions and resulting
product yields are given in Table 1.
Pressure may be estimated from the
vapor pressure - temperature relation-
ship for water. The mean total yield (oil +
asphalt) for conversions carried out at
320°C with Na2CO3 was 20.4% with a
standard deviation of 7.1%. Total yield
seemed to increase with temperature
since the yields at 295°C and 345°C
differ from the mean by 1.5 and 2.3
standard deviations, respectively.
Several runs at 320°C were done to
provide the University of Idaho with a
large, consistent sample for a complete
series of asphalt paving material
testing. Unfortunately, this limited the.
amount of data taken at other points|
which in turn limited our ability to des-
cribe yield as a function of temperature.
Lime (CaO) is apparently an excellent
liquefaction adjunct* having yields
approximately equal to those of
carbonate under the same experimental
conditions. Unfortunately, asphalt
testing results indicated that the one
sample we produced using lime was an
inferior product. Because of the
inherent variations in using sludge as a
raw material, perhaps additional trials
with lime would give better results.
Elemental analyses of the products
produced by liquefaction (Table 2)
allows a comparison with conventional
feeds and petroleum asphalt. The sulfur
concentration in the synthetic asphalt
and oil samples places them in approxi-
mately a grade four heating oil category.
Both the sulfur and nitrogen contained
in the synthetic asphalt samples and
oils result from the use of sludge. Our
analyses of volatiles (reported next)
show that sulfur and nitrogen are
"Previous liquefaction work has referred to alkaline
adjuncts as catalysts However, our previous work
has indicated that the alkali is a reactant, and that
the overall reaction is highly dependent on pH.
Because of this, alkalis used for liquefaction an
referred to as adjuncts in this report
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Table 1. Reaction Conditions and Yields
Experimental
Designation
HS-3
HS-4
HS-5
HS-6
HS-7
HS-8
HS-9
HS-10
Dry
Ash-Free
Sludge, Kg
2.61
2.52
2.41
3.99
2.01
2.28
2.49
1.33
Steam '
Volatile
Oil gm
330
480
-0-
450
-0-
250
450
250
Synthetic
Asphalt, gm
268
440
250
330
310
50
430
164
Total Yield
%*
23
37
10
20
15
13
35
31
Reaction
Temp. °C
320
345
295
320
320
320
320
320
5% by Weight
NatCOs
/Va2C03
/Va2CO3
/Va2CO3
Na2C03
Na2C03
CaO
NatC03
* Weight of light oil and synthetic asphalt as percent of dry, ash-free sludge.
Table 2. Elemental Analyses (Percent by WeightJ
Element
C
H
N
0
S
Petroleum
Asphalt
AC-10
87.
11.
0.4
1.
-
Synthetic*
Asphalt
74.
10.
4.
9.
0.8
Steam-Volatile^
Oil
77.
12.
3.
7.
0.9
Char Cake
HS-10
26.
3.
0.9
10.
-
* Average values from Experiments HS-4. HS-5, HS-6, HS-9, and HS-10.
t Average values from Experiments HS-9 and HS-10.
substituted into the aromatic and ali-
phatic constituents of the volatile
products. As a result of this, we suspect
that the synthetic asphalt and oil
products also contain a wide range of
substitutecLsulfur and nitrogen com-
pounds. For use as fuel, this is probably
of minor concern. For use as synthetic
asphalt, however, the presence of ni-
trogen may be limiting because of poten-
tial interactions between petroleum
asphalt and the more polar synthetic
asphalt (if synthetic asphalt is to be
blended with petroleum asphalt). Also,
since nitrogen ischemically substituted
and since the average molecular weight
of synthetic asphalt is lower than petro-
leum, there may be a more pronounced
tendency for synthetic asphalt to be
soluble in water and for water solubility
_m synthetic asphalt to be higher than
hat in petroleum asphalt.
Table 3 illustrates heats of
combustion for steam-volatile oils,
synthetic asphalt and char cake. The
low combustion heat of char cake (Table
3) is due to the large concentration of
ash in the char cake. The HS-10 char
cake contained- 60% ash before
combustion. Heats of combustion for
the synthetic asphalt and oil reported in
Table 3 are approximately 90% of the
values for petroleum equivalents.
Although not measured, the viscosity of
steam-volatile oil was approximately
that of No. 2 heating oil, judging from its
pouring properties at room
temperature.
Synthetic Asphalt Test Results
Data from testing the various
synthetic asphalts produced by sludge
liquefaction varied widely. The data
presented here are for sample HS-7,
one of the better samples.
Before proceeding with the testing,
the HS-7 synthetic asphalt sample was
melted and washed with hot water. The
sample lost 30% of its original weight
during washing; olive-brown solubles
were removed in the wash water. The
synthetic asphalt sample was then dried
at 60°C.
The washed sample began to melt at
50°C and became completely liquid at
80°C. It was sticky and adhered well to
cardboard when subjected to freezing
temperatures. When frozen, the sample
was brittle but no more so than petro-
leum asphalt. When exposed to room
temperature, it regained its putty-like
consistency much more rapidly than
petroleum asphalt.
Part of the sample was used directly
with aggregate, and another sample
was prepared from a mixture of 50%
high-grade (AC-10) petroleum asphalt
and 50% synthetic asphalt. During
blending, an adverse reaction between
the petroleum and synthetic asphalts
was noticed; there seemed to be a rapid
increase in apparent viscosity when the
two were mixed.
Both the 100% synthetic and the 50-
50 blend were mixed with hot aggregate
and oven cured at 60°C for 16 hours.
When the loose mixes were removed
from the oven, the blend appeared
duller in appearance.
After the two mixes were subjected to
heating arid compaction, they were
cooled to room temperature and bent
and pulled apart for a preliminary
assessment of adhesion. The 100%
synthetic asphalt showed good adhe-
sion whereas the 50-50 blend mix was
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Heats of Combustion
Synthetic Asphalt
HS-9
HS-10*
Calculation based on total mass including ash.
poor and crumbly. Further testing Table 3.
excluded the blend because of its poor
performance.
Compacted mix specimens were
made with petroleum asphalt and with Type
100% synthetic asphalt. Both were sub-
jected to dry and accelerated moisture
conditioning (vacuum saturation
followed by 0°C freezing and 60°C
water soaking). Mechanical properties
of tensile splitting strength and resilient
modulus were obtained for dry speci-
mens, moisture saturated specimens,
and specimens after accelerated
moisture conditioning.
The mechanical property values for
the 100% synthetic mix are close to
those for the petroleum asphalt mix. The
synthetic mix retained only 62% of its
dry tensile strength after accelerated
moisture conditioning, compared with
79% retained strength for petroleum
asphalt. Even though no stripping was
noted for the synthetic mix specimen, its
low retained tensile strength puts it on
the lower end of the scale for petroleum
asphalt. The resilient modulus,
however, was relatively unaffected by
moisture. The synthetic mix retained
96% of its dry modulus when saturated
and 100% of its dry modulus after accel-
erated conditioning Petroleum asphalt
under the same conditions retained
103% and 76%, respectively.
Although the synthetic asphalt
appeared to be duller and more putty-
like than petroleum asphalt, it showed
good bonding behavior with no strip-
ping. The synthetic asphalt was
inherently different from petroleum *100 Short ton/day
Table 4. Payback Period for Several Sludge Liquefaction Process Options
Steam-Volatile Oil Leachate Char Cake
HS-9 HS-10
HS-6
HS-10
cal/g
Btu/lb
8,730
15,700
9,060
16,300
9,380
16,900
9,410
14,000
7.760
5.400*
3.020*
asphalt but showed promise because of
its mechanical performance during
testing. Blends of synthetic asphalt and
petroleum asphalt will require more
investigation because of the adverse
effect they have on each other when
mixed.
Conceptual Design and Cost
Estimates for a 91 Tonne/Day*
Commercial Sludge Thermo-
chemical Liquefaction Plant
The conceptual plant design and cost
estimate were made to help determine
the overall feasibility of the process and
to encourage further research and
development on continuous liquefac-
tion of sewage sludge. Estimates, made
on the basis of the best available
information at this time, are tentative
since there are no pilot plant data, or
even bench-scale continuous process
data, to support equipment specifica-
tions.
In the flowsheet of the process (Figure
1), approximate product flows and
temperatures are given for reference. A
more detailed analysis of heat and mass
balance is not currently warranted since
heat of reaction, product yields, and
processability of some of the streams in
a continuous process are unknown.
Process Design
A plant capacity of 91 dry tonne/day
(100 short ton/day) was chosen as being
representative of the volume of primary
sludge produced in large wastewater
treatment plants around the United
States. Therefore, the estimates are for
commercial plants, not for pilot or dem-
onstration plants.
The process would operate on I
primary sludge dewatered to at least"
30% solids. Other sludges could be
used, but for these estimates, only
primary sludge was considered.
Dewatering to 30% solids is already
practiced in many wastewater treat-
ment plants. Many of these plants
already use lime to condition the sludge
$ Mil/ion
Process
With
Dewatering
Vith Centrifuge
1 vnha/t
\&frjl IGIL
Recovery Without
Dewatering
Centrifuge
With
Dewatering
Without Centrifuge
1 cn/i^/f
i ofJf Idlt
Recover Without
Dewatering
Centrifuge
Facility
Cost
9.8
7.8
7.8
5.8
Manufacturing
Costs
2.97
2.68
2.58
2.29
Sludge
Disposal
Credit
1.98
1.98
1.98
1.98
Oil/Asphalt
Revenue
1.79
1.79
1.36
1.36
Total
Revenue
3.77
3.77
3.34
3.34
Simple
Payback
(Years)
12
7
10
6
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Primary
Sludge
91
Tonne/day
Dry Basis
Centrifuge 25°l/min
t
Vapor-Liquid Separator
J[ 1 129 l/min
Waste
Water
Gas
121 l/min | to
Vent
High Pressure
Slurry Feeder
Water
to Secondary
Treatment
Light Oil
Centrifuge
Waste Water
Char Cake
Dowtherm A
Condensate
X
Dowtherm A
Vapor
350°C
Cake
Desolventizer
4
Dowtherm
Vaporizer
Solvent
Recovery
Ash
to Disposal
Heavy Oil
Washer
Light Oil
to Storage
Waste Water
Synthetic
Asphalt
to Storage
6.4 kg/min
Figure 1. Preliminary schematic for a sludge liquefaction plant
Waste Water
for dewatering, and, in many cases, add
10% or more lime on a dry weight basis.
In experiment HS-9, lime proved to be
an effective adjunct for producing
steam-volatile oil, but rated poorly for
production of synthetic asphalt. There-
fore, lime-treated sludge can be used
directly without further treatment if the
primary goal is fuel production. Syn-
thetic asphalt from lime-treated sludge
would require more investigation based
an current results.
Because the thermochemical lique-
faction yields varied significantly, in the
design and estimation work, we used
the sludge characteristics and yields
obtained from HS-10 as representative.
With the use of these data, the yields
from a plant processing 91 tonne/day
would be:
Synthetic asphalt:
Steam-volatile oil:
Char cake:
9,000 Kg/day
13,800 Kg/day
37,000 Kg/day
At a density of a pproxi mately 830 g m / L,
the light oil would amount to approxi-
mately 16,300 L, or just slightly greater
than 100 petroleum barrels/day.
Process heat requirements for this
plant would be supplied by combustion
of the residual char cake. The extracted
cake, with heat of combustion of
approximately 3,020 cal/gm, would be
produced at the rate of 26 Kg/min and,
therefore, would be capable of
supplying 1.56 X 10'° J/hr (1.5 X 107
-------
Btu/hr) of process heat at a combustion
efficiency of 80%. This amount would be
more than adequate to supply process
heat requirements, especially if heat
recovery were used on the main stream
coming out of the liquefaction reactor.
The greatest heat requirement by far
would be to heat the liquefaction
reactor, even with heat recovery. For the
flow design presented in Figure 1, the
liquefaction reactor would require heat
input of 1.17 X 1010 J/hr. Other minor
heat requirements for solvent
extraction and recycle are estimated to
be less than 04 X 10'° J/hr. Total
energy requirements therefore are
balanced by the available energy in the
residual char cake. Energy from com-
bustion of the char cake would be
supplied to the liquefaction reactor by
m Dowtherm* vaporizer. Other process
utility requirements would be limited to
electrical power of about 1.8 X 106
KWH/yr and a small amount of cooling
water.
Estimates given for capital equipment
and manufacturing costs are derived
from a plant that would already include
sludge dewatering equipment (a centri-
fuge) and solvent extraction to recover
synthetic asphalt, since many sites have
dewatering equipment m place and
since some plants may elect not to
recover synthetic asphalt.
Simple Payback Period for
Different Process Options
A major cost in this process would be
associated with primary sludge de-
watering. Estimates given in Table 4
show that if investment for this process
is not required, the payback period for
the plant will decrease from 10 to 12
years to 6 to 7 years.
Although synthetic asphalt may be a
valuable product when produced from
sewage sludge, these payback esti-
mates (Table 4) show that the payback
period is actually shorter if asphalt were
not recovered because the solvent
extraction process adds more capital
and operating cost than could currently
be recovered by sale of synthetic
asphalt. If synthetic asphalt were not
recovered, it would be contained in the
char cake and simply burned. The viabil-
ity of synthetic asphalt recovery will
change depending on the yield of syn-
thetic asphalt (as yet to be determined in
•Mention of trade names or commercial products
does not constitute endorsement or recommen-
dation for use.
a continuous process for which these
estimates were made) and depending
on its value as an alternative to petro-
leum asphalt. Either or both of these
factors could significantly change the
economics of synthetic asphalt recovery
from sludge in future years.
In Table 4, credit for sludge disposal
was estimated at $66 per dry tonne
($60/short ton). Revenue from sale of
oil was calculated at S0.25/L ($40/bbl)
and for the synthetic asphalt,
$143/tonne ($130/short ton). These
prices are representative of 1980 prices
for equivalent petroleum products.
At current petroleum prices and with
the capital and operating costs shown, it
is necessary to take some credit for
sludge disposal to make the process
economically viable. As envisioned, the
liquefaction plant would be adjacent to a
wastewater treatment plant and would
take all the primary sludge generated by
the wastewater plant—so, in fact, some
credit is due since the sludge would
otherwise have to be disposed of.
Because the cost of disposal will
certainly rise in the future, as will petro-
leum prices, the liquefaction process
should become viable in future years
even should it not be thought to be
viable at current projected payback
periods of 6 to 12 years.
Recommendations
The potential for direct liquefaction of
sludge to produce liquid fuel at a
reasonable cost is very real based on the
data presented m this report. Unlike
other thermal processes, such as incin-
eration with heat recovery or gasifica-
tion, liquefaction produces a fuel that is
eminently storable and transportable.
The quantity of net energy produced by
liquefaction (i.e., steam-volatile oil and
synthetic asphalt) is likely to be greater
than the net energy produced by
incineration with heat recovery.
Batch reaction methods used for this
study are not acceptable for design and
scale-up of even a modest pilot plant.
Therefore, a bench-scale continuous
reaction system is needed to:
• determine physical properties of
intermediate products;
• determine necessary designs for
ancillary equipment;
• obtain larger samples for more
detailed testing;
• determine liquefaction product
characteristics as a function of
temperature, time, alkali, and
incoming sludge composition; and
• determine the technical feasibility
of a continuous reactor for sludge
liquefaction.
Reliable data for design, scale-up, and
economic evaluation would result from
a continuous bench-scale unit. In addi-
tion, if this unit were located at the site
of a wastewater treatment plant,
product characteristics as a function of
sludge composition could be measured
and the treatability of the residual
aqueous phase could be determined.
Therefore, we recommended that a
small lab-scale continuous liquefaction
facility be built near a wastewater treat-
ment plant. With this system, emphasis
should be placed on developing fuel oil
from sludge because this option
appears to be more cost-effective in the
near term. However, since the high
molecular weight fraction is made along
with the steam-volatile oil, investigating
its utility as a petroleum asphalt re-
placement or as a coking stock or fuel
can continue at minimum additional
R&D cost.
Conclusions
Seventy percent of the combustion \
energy available in sewage sludge
(approximately 5000 cal/g or 9000
Btu/lb) can be converted to liquid and
solid fuels analogous to petroleum by
direct thermochemical liquefaction The
solid fuel can be burned to provide all
the necessary process heat requirements
so that the process is a net energy
producer.
the high molecular weight product of
liquefaction is, in some cases, an
acceptable replacement for petroleum
asphalt based on the results obtained so
far. Synthetic asphalt samples desig-
nated HS-1, 7, and 10 were ranked as
satisfactory by our preliminary asphalt
testing procedures; most others were
ranked as unsatisfactory. Since the
conversion procedure for the satisfac-
tory and unsatisfactory samples was the
same in most cases, the only plausible
explanation is that inherent differences
in the composition of the various
samples of sludge used led to physical
differences in the synthetic asphalt
products.
The synthetic asphalt can also be
used as a potential fuel or coking stock
in the event it is not immediately accep-
table as a paving binder. In addition, the
steam-volatile oil produced during^
-------
liquefaction has significant value as a
synfuel, having 90% of the heating
value of No. 2 fuel oil. Based on the
current value of petroleum asphalt and
heating oils, the liquefaction process
would be economically viable in many
existing situations, with expected pay-
back periods of less than 12 years.
If the process objective were changed
to production of fuels rather than
asphalt, a reduction in processing cost
would be likely—the economics would
probably be more favorable and, in addi-
tion, marketing of the products would be
simplified.
The full report was submitted in ful-
fillment of Grant No. R-806790-01 by
Battelle-Northwest, Richland, WA,
under the sponsorship of the U.S.
Environmental Protection Agency.
J. M. Donovan, R. K. Miller, and T. R. Batter are with Battelle-Northwest,
Richland, WA 99352; R. P. Lottman is with the University of Idaho, Moscow, ID
83843.
Howard Wall is the EPA Project Officer (see below).
The complete report, entitled "Physical and Chemical Characteristics of
Synthetic Asphalt Produced from Liquefaction of Sewage Sludge. "(Order No.
PB 82-119 082; Cost: $9.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
•frU.S. GOVERNMENT PRINTING OFFICE:1982--559-092/3364
-------
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Agency Cincinnati OH 45268 Protection
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