svEPA
United States
Environmental Protection
Agency
Office of Emergency and
Remedial Response
Washington, DC 20460
Office of
Research and Development
Cincinnati, OH 46268
Superfund
EPA/540/S-92/010
October 1992
Engineering Bulletin
Pyrolysis Treatment
Purpose
Section 121(b) of the Comprehensive Environmental Re-
sponse, Compensation, and Liability Act (CERCLA) mandates
the Environmental Protection Agency (EPA) to select remedies
that "utilize permanent solutions and alternative treatment
technologies or resource recovery technologies to the maxi-
mum extent practicable" and to prefer remedial actions in
which treatment "permanently and significantly reduces the
volume, toxicity, or mobility of hazardous substances, pollut-
ants, and contaminants as a principal element." The Engineer-
ing Bulletins are a series of documents that summarize the latest
information available on selected treatment and site remedia-
tion technologies and related issues. They provide summaries
of and references for the latest information to help remedial
project managers, on-scene coordinators, contractors, and other
site cleanup managers understand the type of data and site
characteristics needed to evaluate a technology for potential
applicability to their Superfund or other hazardous waste site.
Those documents that describe individual treatment technolo-
gies focus on remedial investigation scoping needs. Addenda
will be issued periodically to update the original bulletins.
Abstract
Pyrolysis is formally defined as chemical decomposition
induced in organic materials by heat in the absence of oxygen.
In practice, it is not possible to achieve a completely oxygen-
free atmosphere; actual pyrolytic systems are operated with less
than stoichiometric quantities of oxygen. Because some oxy-
gen will be present in any pyrolytic system, nominal oxidation
will occur. If volatile or semivolatile materials are present in the
waste, thermal desorption will also occur.
Pyrolysis is a thermal process that transforms hazardous
organic materials into gaseous components and a solid residue
(coke) containing fixed carbon and ash. Upon cooling, the
gaseous components condense, leaving an oil/tar residue. Py-
rolysis typically occurs at operating temperatures above 800°F
[1, pp. 165,167] [2, p. 5].* This bulletin does not: address other
thermal processes that operate at lower temperatures or those
that operate at very high temperatures, such as a plasma arc.
Pyrolysis is applicable to a wide range of organic wastes and is
generally not used in treating wastes consisting primarily of
inorganics and metals.
Pyrolysis should be considered an emerging technology.
(An emerging technology is a technology for which perfor-
mance data have not been evaluated according to methods
approved by EPA and adhering to EPA quality assurance/quality
control standards, although the basic concepts of the process
have been validated [3, pp. 1-2].) Performance data are cur-
rently available only from vendors. In addition, existing data
are limited in scope and quantity and frequently of a propri-
etary nature.
This bulletin provides information on the technology appli-
cability, the types of residuals resulting from the use of the
technology, the latest performance data, site requirements, the
status of the technology, and where to go for further informa-
tion.
Technology Applicability
Pyrolysis systems may be applicable to a number of or-
ganic materials that "crack" or undergo a chemical decomposi-
tion in the presence of heat. Pyrolysis has shown promise in
treating organic contaminants in soils and oily sludges. Chemi-
cal contaminants for which treatment data exist include poly-
chlorinated biphenyls (PCBs), dioxins, polycyclic aromatic hy-
drocarbons, and many other organics. Treatment data discussed
in this bulletin were taken from treatability studies conducted
by three vendors.
Pyrolysis is not effective in either destroying or physically
separating inorganics from the contaminated medium. Volatile
metals may be desorbed as a result of the higher temperatures
associated with the process but are similarly not destroyed.
The probable effectiveness of pyrolysis on general con-
taminant groups for various matrices is shown in Table 1.
Examples of constituents within contaminant groups are pro-
vided in "Technology Screening Guide for Treatment of CERCLA
Soils and Sludges" [4, pp. 10-12]. Table 1 is based on current
available information or professional judgment where no infor-
mation was available [1, pp. 165,168] [2, pp. 9-14] [5, pp. 10-
15] [6, p. 9]. The proven effectiveness of the technology for a
particular site or waste does not ensure that it will be effective at
all sites or that the treatment efficiencies achieved will be
acceptable at other sites. For the ratings used for this table,
demonstrated effectiveness means that, at some scale, treat-
ment results indicated that the technology was effective for
[reference number, page number]
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Table 1
Effectiveness of Pyrolysis on General Contaminant
Groups for Soil and Sediment/Sludge
Contaminant Croups
Halogenated volatiles
Halogenated semivolatiles
Nonhalogenated volatiles
Effectiveness
Sediment/
Soil Sludge
T T
T T
T •
.0 Nonhalogenated semivolatiles O •
§, PCBs
° Pesticides (halogenated)
Dioxins/Furans
Organic cyanides
Organic corrosives
Volatile metals
Nonvolatile metals
§ Asbestos
fe Radioactive materials
Inorganic corrosives
Inorganic cyanides
§» Oxidizers
fcj Reducers
• Demonstrated Effectiveness: Succ
scale completed.
• •
T V
V •
T T
n n
ci n
O Cl
o n
0 0
a n
n o
Cl O
n n
essful treatability test at some
V Potential Effectiveness: Expert opinion that technology will work.
Q No Expected Effectiveness: Expert opinion that technology will
not work.
that particular contaminant and medium. The ratings of po-
tential effectiveness or no expected effectiveness are both based
upon expert judgment. Where potential effectiveness is indi-
cated, the technology is believed capable of successfully treat-
ing the contaminant group in a particular medium. When the
technology is not applicable or will probably not work for a
particular combination of contaminant group and medium,, a
no-expected-effectiveness rating is given.
Limitations
The primary technical factors affecting pyrolytic perfor-
mance are the temperature, residence time, and heat transfer
rate to the material. There are also several practical limitations
which should be considered.
As the medium is heated and passes through a pyrolytic
system, energy is consumed in heating moisture contained in
the contaminated medium. A very high moisture content
would result in lower throughput. High moisture content,
therefore, causes increased treatment costs. For some wastes,
dewatering prior to pyrolysis may be desirable.
The treated medium will typically contain less than one
percent moisture. Dust can easily form in the transfer of the
treated medium from the treatment unit, but this problem can
be mitigated by water sprays.
A very high pH (greater than 11) or very low pH (less than
5) may corrode the system components. The pyrolysis of
halogenated organics will yield hydrogen halides; the pyrolysis
of sulfur-containing organics will yield various sulfur compounds
including hydrogen sulfide (H2S). Because hydrogen halides
and hydrogen sulfide are corrosive chemicals, corrosion control
measures should be taken for any pyrolytic system which will be
processing wastes with high concentrations of halogenated or
sulfur-containing organics.
Technology Description
Pyrolysis is formally defined as chemical decomposition
induced in organic materials by heat in the absence of oxygen.
Pyrolysis is a thermal process that transforms organic materials
into gaseous components and a solid residue (coke) containing
fixed carbon and ash. The pyrolysis of organics yields combus-
tible gases including carbon monoxide, hydrogen, methane,
and other low molecular weight hydrocarbons [7, pp. 252-
253]. Pyrolysis occurs to some degree whenever heat is applied
to an organic material. The rate at which pyrolysis occurs
increases with temperature. At low temperatures and in the
presence of oxygen, the rates are typically negligible. In addi-
tion, the final percent weight loss for the treated material is
directly proportional to the operating temperature. Similarly,
the hydrogen fraction in the treated material is inversely pro-
portional to the temperature.
The primary cleanup mechanisms in pyrolytic systems are
destruction and removal. Destruction occurs when organics are
broken down into lower molecular weight compounds. Re-
moval occurs when pollutants are desorbed from the contami-
nated material and leave the pyrolysis portion of the system
without being destroyed.
Pyrolysis systems typically generate solid, liquid, and gas-
eous products. Solid products include the treated (and dried)
medium and the carbon residue (coke) formed from hydrocar-
bon decomposition. Various gases are produced during pyroly-
sis, and certain low-boiling compounds may volatilize rather
than decompose. This is not typically a problem. Gases may be
condensed, treated, incinerated in an afterburner, flared, or a
combination of the above. Depending on the specific compo-
nents, organic condensate may be reusable. Other liquid streams
will include process water used throughout the system. A
general schematic of a pyrolytic process is shown in Figure 1.
As shown in Figure 1, the first step in the treatment process
is the excavation of the contaminated soil, sludge, or sediment.
Oversized rejects such as large rocks or branches are removed
and the material is transferred to the pyrolysis unit. The treat-
ment system may include a desorption stage prior to pyrolysis.
If so, the desorbed gases flow to the gas treatment system for
treatment and/or recovery, and the contaminated matrix (mi-
nus any desorbed chemicals) is transferred to the pyrolysis
chamber [1, p. 166] [2, pp. 3-6].
Engineering Bulletin: Pyrolysis Treatment
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The temperature in the pyrolysis chamber is typically be-
tween 800 and 2,100° F, and the quantity of the oxygen
present is not sufficient for the complete oxidation of al! con-
taminants. In pyrolysis, organic materials are transformed into
coke and gaseous components. Gas treatment options include:
1) condensation plus gas cleaning and 2) incineration plus gas
cleaning.
Pyrolysis forms new compounds whose presence could
impact the design of the offgas management system. For
example, compounds such as hydrogen halides and sulfur-
containing compounds may be formed. These must be ac-
counted for within the design of the Air Pollution Control (APC)
system.
There are three pyrolytic systems which will be discussed in
this bulletin. These systems are: the HT-V system marketed by
TDI Thermal Dynamics (formerly Southdown Thermal Dynam-
ics), a process developed by Deutsche Babcock Anlagen AC,
and an "anaerobic thermal processor" (ATP) marketed by
SoilTech, Inc.
The HT-V Thermal Distillation System is a mobile thermal
desorption system which may be operated in a pyrolytic mode.
The Thermal Distillation System processes waste by applying
heat in a nitrogen atmosphere. Gravity and a system of annular
augers are used to transfer waste through a series of three
electrically heated distillation chambers. The temperature is
ambient at the entrance to the distillation chambers and in-
creases to full operating temperature (up to 2,100°F) as the
waste progresses through the chambers. The continuous intro-
duction of a nitrogen sweep gas removes and separates the
volatile contaminants [8, p. 3]. The sweep gas must be periodi-
cally sent to a flare to reduce the noncondensible combustible
portion.
TDI is currently conducting bench-scale tests on the Ther-
mal Degradation System, which was developed for use in con-
junction with the Thermal Distillation System. The full-scale
design of the system is currently theoretical, but TDI envisions
that Thermal Degradation will follow Thermal Distillation and
will be used primarily for pyrolysis. In recent bench-scale tests,
the Thermal Degradation System was operated at approxi-
mately 2/000°F and a copper catalyst was used to enhance the
pyrolysis of halogenated organics [2, pp. 3-6] [5, pp. 3-7].
A German company, Deutsche Babcock Anlagen AG, de-
veloped a pyrolytic process which utilizes an indirectly heated
rotary kiln. In the first step of the Deutsche Babcock system,
pyrolysis occurs at a temperature of 1,100 to 1,200°F. If volatile
or semivolatile organics are present, they will be desorbed in
this step. In the second step, the gases produced by pyrolysis
(as well as other volatilized organics) are combusted in an
afterburner at a high temperature (1,800 to 2,400°F). Heat
produced during the second step may provide at least a portion
of the energy for the first step, which is endothermic. Prior to
discharge, effluent gases from the second step are scrubbed to
remove various pollutants including hydrogen halides and sul-
fur oxides [1, p. 166].
The pyrolysis systems marketed by Deutsche Babcock are
not currently available in mobile or transportable configura-
tions and are therefore not directly applicable to onsite
remediation of Superfund sites. These systems were included in
this discussion to provide additional data and to indicate the
potential viability of pyrolysis. In addition, full-scale applica-
tions and testing of the Deutsche Babcock system have in-
cluded the cleanup of contaminated soils [1, pp. 165-168].
Figure 1. Schematic Diagram of Pyrolysis
Gas Treatment
System
Rejects
Clean Offgas
Condensed Vblatiles
•^- Spent Carbon
-^- Water
E 1
Excavate
-*•
Material
Handling
^
-^
Desorption
(optional)
^
Pyrolysis
ireaieu
Medium
^
^ Oversized
Engineering Bulletin: Pyrolysis Treatment
-------
ana* S'h f ' nC (Can°nie Environ™ntal) markets an
anaerobic thermal processor (ATP) which may be operated in a
pyrolyt,c mode. The ATP is also known as the AOSTRA-Taduk
process and is essentially an indirectly-heated rotary kiln A
transportable ATP with a nominal processing rate of10 tons per
hoy, is available for onsite demonstrations and remediation
\y, p. 3j.
The ATP unit includes four chambers: preheat, reaction
combustion, and cooling. In the preheat chamber, volatile
materials are desorbed at temperatures up to 500°F Pyrolvtic
conditions and temperatures between 700 and 1 ISoVare
ZTrl Hn thC reaCtl'°n Chamber The desorPt;°n and/or
pyrolysis of heavier organics will occur in this chamber Coke
and noncombustible hydrocarbons produced by pyrolysis are
transferred to the combustion chamber and burned [9, pp A-l
to A-2J. Additional fuels such as gas or oil must be available for
start-up, for control, and to supplement the pyrolysis products
when they do not provide adequate fuel. Solids and gases from
the combustion chamber proceed into the cooling zone The
cooling zone and the preheat zone function as a heat ex-
changer in which heat is transferred from the combustion
residuals to the feed [10, p. 3].
Process Residuals
H Hr 9enerated by Pyolytic systems typically in-
elude solid, liquid, and gaseous residuals. Solid products in-
clude debris, oversized rejects, dust, ash, and the treated me-
dium Dust collected from particulate control devices may be
combined with the treated medium or, depending on analyses
C°ntamination' recycled throu9h the treatment
Depending on the individual system, the flue gases from
the pyrolysis unit will generally be treated by wet or dry AP(~
systems before discharge through a stack. In the Deutsche
Babcock System, offgases are treated by incineration
[1, p. 1 66].
Ash and treated soil/solids from pyrolysis may be contami-
nated with heavy metals. APC system solids, such as fly ash
may contain high concentrations of volatile metals If these
residues fail required leachate toxicity tests, they can be treated
by a process such as solidification/stabilization and disposed of
onsite or in an approved landfill [1 1, p. 8.97]. If the treated
medium and ash pass all required tests, they may be disposed
of onsite without further treatment.
Depending on the specific pyrolysis system, liquid streams
may include condensed organics or water from the APC sys-
tem After organics are removed, condensed water may be
used as a dust suppressant for the treated medium. Scrubber
purge water can be purified and returned to the site wastewa-
ter treatment facility (if available), discharged to the sewer or
used for rehumidification and cooling of the hot, dusty media.
Liquid waste from the APC system may contain excess
alkali, high chlorides, volatile metals, organics, metals particu-
lates, and inorganic particulates. Treatment may require neu-
tralization, chemical precipitation, settling, filtration, or carbon
adsorption before discharge.
Site Requirements
Pyrolytic treatment processes are not expected to have
significantly different site requirements than those for thermal
desorption or incineration processes.
Note that the pyrolytic systems marketed by Deutsche
Babcock are not currently available in mobile or transportable
configurations. The HT-V system and the ATP are transport-
able, and vendors claim that they can be set up in a matter of
days.
4«n v/ rec*uirements incl"de electric power (440 or
480 V, 3-phase) and water. The quantity of water required is
design- and site-specific. M
Treatment of contaminated soils or other waste materials
require that a site safety plan be developed to provide for
personnel protection and special handling measures. Storage
should be provided to hold the process product streams until
they have been tested to determine their acceptability for
disposal or release. Depending upon the site, a method to
store waste that has been prepared for treatment may be
necessary. Storage capacity will depend on waste volume
Onsite analytical equipment capable of monitoring site-
specific organic compounds for performance assessment make
the operation more efficient and provide better information for
process control.
Performance Data
Limited performance data are available for pyrolytic sys-
tems treating hazardous wastes containing PCBs, dioxins and
other organics [1, pp. 165,168] [2, pp. 9-14] [5, pp. 10-15] [6,
p. 9]. The quality of this information has not been determined
These data are included as a general indication of the perfor-
mance of pyrolysis equipment and may not be directly transfer-
able to a specific Superfund site. Good site characterization
and treatability studies are essential in further refining and
screening the pyrolysis technology.
The HT-V system's performance on oily sludges contami-
nated with dioxins and PCBs was evaluated in bench-scale
readability tests conducted by Law Environmental on April 25
V L2' PP> 9"14] [5' PP"1 °-15]' The Sl'm"lated waste used in
the dioxm test was contaminated with 2378-
tetrachlorodibenzo-p-dioxin (TCDD). A decontamination effi-
ciency of over 99.99% was calculated, as no 2,3,7,8-TCDD was
detected ,n the treated residue, offgases, or condensate In
addition, the test report claims that no significant quantities of
new toxic compounds were synthesized by the process [2, pp. 9-
A second bench-scale treatability study was conducted on
a rruxture of PCB-contaminated soil, PCB-contaminated oil, and
Engineering Bulletin: Pyrolysis Treatment
-------
water. All process streams were sampled and analyses indi-
cated a decontamination efficiency of over 99.99%. PCB levels
were below the detection limits in all effluent streams and the
test report claims that no significant quantities of new toxic
compounds were synthesized by the process [5, pp. 10-15].
Although these results appear promising, complete closures of
mass balances are not possible with the information collected
during the HT-V treatability tests.
The Deutsche Babcock system was tested in an industrial-
scale demonstration in May and |une 1988. Prior to this
demonstration, the same system was used to treat 35,000 tons
of soil. The plant is located in Unna-Bonen, West Germany, at a
former coke oven site. The unit had a design rate of 7 tons/
hour with a soil moisture content of 21 percent and 5 percent
volatile compounds. The destruction of 17 polycyclic aromat-
ics was measured. A system decontamination efficiency of
99.77 percent was achieved. The results are summarized in
Table 2 [1, p. 168]. Note that this test was conducted in
Table 2
Deutsche Babcock Pyrolytic Rotary Kiln
Contaminated Soil Results
Date
Pollutant
Naphthalene
2-methylnaphthalene
1 -methylnaphthalene
Dimethylnaphthalene
Acenaphthylene
Acenaphthene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Benz[a]anthracene
Chrysene
Benzo[e]pyrene
Benzo[b]fluoranthene
Benzo[k]fluoranthene
Benzo[a]pyrene
Dibenz[a, h]anthracene
Benzo[g, h, ijperylene
March 8,
Input
mg/kg
101.00
40.20
23.40
n.d.
n.d.
n.d.
156.00
686.00
281.00
n.d.
236.00
155.00
214.00
66.60
112.00
43.70
86.60
16.80
14.00
Indenop, 2, 3-cd]pyrene 33.80
Sum
n.d. = not detectable
2266.10
Decontamination efficiency in %
1989
Output
mg/kg
1.7
0.5
0.3
n.d.
n.d.
n.d.
0.1
0.6
0.1
n.d.
0.1
0.2
0.5
0.4
0.1
0.1
0.2
0.1
0.1
0.1
5.2
99.77
January 27, 1989
Input
mg/kg
161.60
73.80
42.90
93.20
68.20
42.30
238.00
1055.30
226.00
688.60
398.20
2259.20
1 34,60
111.50
168.50
81.90
138.10
23.20
60.20
69.50
61 34.80
Output
mg/kg
0.5
0.1
0.1
0.3
0.1
0.1
0.1
1.4
0.3
1.3
0.6
0.3
0.9
1.1
5.2
0.3
0.4
0.1
0.1
0.1
13.4
99.78
Germany and that the majority of the applications of the
Deutsche Babcock system have been in Germany. German
requirements regarding incineration were not researched and
may differ significantly from US requirements.
The Soiltech ATP is being used in conjunction with chemi-
cal dehalogenation to remediate the Wide Beach Superfund
site. Much of the soil in the small community of Wide Beach,
New York is contaminated with PCBs from road oils. PCB levels
range from approximately 10 ppm to over 5,000 ppm; the
primary cleanup requirement is to reduce PCB concentrations
to less than 2 ppm [6, pp. 2-3].
The system used at Wide Beach is similar to the ATP
described previously but also includes a reagent mix system.
The reagent mix system adds dechlorination chemicals (potas-
sium hydroxide and polyethylene glycol) to a stream of oils
recycled from the system effluent [6, p. 4] [12, p. 45].
PCB concentrations in the treated soil were below the
reporting limit of 70 ppb, which is significantly below the
required level. In addition, the process water contained no
more than 1 ppb PCBs, stack gas PCB levels were less than 33
percent of the New York State Department of Environmental
Conservation (NYDEC) limits, fugitive emissions were within
NYDEC limits, and treated soils passed the toxicity characteris-
tic leaching procedure (TCLP) [6, pp. 2,9]. At the beginning of
the cleanup effort, treated soil was returned to local sites. The
treated soil, however, does not have the same consistency as
untreated soil, and current plans are to landfill the soil rather
than returning it to the original sites [12, p. 45].
Technology Status
Pyrolysis has been used to treat various hazardous wastes
as documented in the Performance Data section of this bulle-
tin. In particular, pyrolysis has been applied to the remediation
of the Wide Beach Superfund site (in conjunction with chemi-
cal dehalogenation) [6, pp. 1 -2] and to the cleanup of contami-
nated soils in Germany [1, pp. 165-168].
EPA Contact
Technology-specific questions regarding pyrolysis may be
directed to:
Mr. Donald Oberacker
U.S. Environmental Protection Agency
Risk Reduction Engineering Laboratory
26 West Martin Luther King Drive
Cincinnati, Ohio 45268
Telephone: (513) 569-7510.
Acknolwedgments
This bulletin was prepared for the U.S. Environmental
Protection Agency, Office of Research and Development (ORD),
Risk Reduction Engineering Laboratory (RREL), Cincinnati, Ohio,
Engineering Bulletin: Pyrolysis Treatment
-------
by Science Applications International Corporation (SAIC) under
contract no. 68-C8-0062. Mr. Eugene Harris served as the EPA
Technical Project Monitor. Mr. Gary Baker (SAIC) was the Work
Assignment Manager, and Ms. Sharon Krietemeyer and Mr.
Richard Gardner (SAIC) were co-authors of this bulletin. The
authors are especially grateful to Mr. Donald Oberacker and
Mr. Paul de Percin of EPA, RREL, who have contributed signifi-
cantly by serving as technical consultants during the develop-
ment of this document.
The following other contractor personnel have contributed
their time and comments by participating in the expert review
meetings and/or peer reviewing the document-
Mr. James Cudahy
Dr. Steve Lanier
Focus Environmental, Inc.
Energy and Environmental
Research Corp.
REFERENCES
i.
2.
3.
4.
5.
6.
Schneider, D., and B.D. Beckstrom. Cleanup of Contami-
nated Soils by Pyrolysis in an Indirectly Heated Rotary
Kiln. Environmental Progress (Volume 9, No. 3), pp 165-
168. August 1990.
Test Report of Bench Scale Unit (BSU) Treatability Test for
Dioxin Contaminated Oily Sludge. Test Date: April 25,
1991. Prepared by Law Environmental, Inc. for South-'
down Thermal Dynamics. June 1991.
The Superfund Innovative Technology Evaluation
Program: Technology Profiles. U.S. Environmental
Protection Agency, Office of Solid Waste and Emergency
Response and Office of Research and Development,
Washington, D.C. EPA/540/5-90/006. November 1990.
Technology Screening Guide for Treatment of CERCLA
Soils and Sludges. EPA/540/2-88/004, U.S. Environmen-
tal Protection Agency, 1988.
Test Report of Bench Scale Unit (BSU) Treatability Test for
PCB Contaminated Oily Sludge. Test Date: April 25,
1991. Prepared by Law Environmental, Inc. for South-
down Thermal Dynamics. June 1991.
Vorum, M. PCB-Soil Dechlorination at the Wide Beach
Superfund Site: The Commercial Experience of SoilTech
Inc. May 1991.
7. Incinerating Hazardous Wastes, H. M. Freeman, Editor.
Technomic Publishing Co., Lancaster, PA 1988.
8. Southdown Thermal Dynamics, Marketing Brochures
circa 1990.
9. The Taciuk Process Technology: Thermal Remediation of
Solid Wastes and Sludges. Technical Information.
Submitted by SoilTech, Inc.
10. Ritcey, R. and F. Schwartz. Anaerobic Pyrolysis of Waste
Solids and Sludges: The AOSTRA Taciuk Process System.
Presented to the Environmental Hazards Conference &
Exposition, Environmental Hazards Management
Institute, Seattle. May 1990.
11. Standard Handbook of Hazardous Waste Treatment and
Disposal. H. M. Freeman, Editor. U.S. Environmental
Protection Agency, Hazardous Waste Engineering
Research Laboratory. McGraw-Hill Book Company New
York, pp. 8.91-8.104.
12. Turning "Dirty" Soil into "Clean" Mush. Soils. September-
October 1991.
'U.S. Government Printing Office: 1992— 648-080/60093
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