United btates
Environmental Protection
Agency
Solid Waste And
Emergency Response
(OS-220)
Directive 9200.5-250FS
November 1989
Innovative Technoloav
Soil Washing
TECHNOLOGY DESCRIPTION
Soil washing is potentially effective in
treating various organic and inorganic waste
groups. It was designed for the separation/
segregation and volumetric reduction of
hazardous /naterials in soils, sludges, and
sediments-. The process involves high en-
contaminants from the solids. The treated"
solid fractions (less than 74 microns) are
then rinsed, dewatered,andredeposited. The
contaminated washing fluid, containing
highly contaminated fine fractions (greater
than 74 microns) is recycled through a con-
ventional wastewater treatment system and
is reintroduced into the treatment process.
Figure 1: Schematic Diagram of a Mobile Soil Washing
Treatment Facility
Ftocculation
Chemically Break Foam
Incinerate) if Possible
V Mobile Soil Washing
System (MSWS)
itation
Note: Adapted from EPA lor Booz, Allen a Mammon Inc
ergy contacting and mixing of excavated
contaminated soils with an aqueous-based
washing solution in a series of mobile wash-
ing units. A typical soil washing treatment
flow diagram is shown in Figure 1.
Before treatment, the contaminated soil
is passed through a coarse-mesh sieve to re-
move material greater than two inches (e.g.,
rocks, debris). The remaining material then
enters a soil scrubbing unit, where it is sprayed
with a washing fluid and subsequently rinsed.
Contaminants are primarily concentrated in
the fine-grained soil fraction (i.e., silt and
clay) and are less tenaciously sorbedonthe
coarser-grained particles (i.e., sand). Ac-
cordingly, the sand fraction of the soil usu-
ally requires only the initial rinsing treat-
ment to meet designated performance crite-
ria prior to redeposition. The remaining silt/
clay soil fraction enters a four-staged counter-
current contactor to further separate the
The fines are separated, removed, and dewa-
tered and are handled/disposed as a mani-
fested hazardous waste material.
Advantages of soil washing include a
closed treatment system that permits control
of ambient environmental conditions, po-
tential significant volume reduction of the
contaminant mass (depending on soil char-
acteristics), wide application to varied waste
groups, mobility of technology (hazardous
wastes remain on-site), and relatively low
cost compared to other multi-contaminant
treatment technologies. Disadvantages in-
clude little reduction of the contaminant tox-
icity, and potentially hazardous chemicals
(e.g., chelating washing solutions) may be
brought on-site to be used in the process, and
also may be difficult to remove from the
treated soil fraction. Applications and limi-
tations of soil washing are discussed in the
following sections.
CHARAUIbHISIIUS AFFECTING
TREATMENT FEASIBILITY
Soil washing has the potential to treat a
wide variety of contaminants such as heavy
metals, halogenated solvents, aromatics,
gasoline and fuel oils, polychlorinated
btphenyls (PCBs), and chlorinated phenols.
The projected effectiveness of this treatment
on general contaminant groups is provided
in Table 1; treatability tests are required to
determine the feasibility of soil washing for
specific target contaminants at a particular
site.
Factors limiting the effectiveness of soil
washing include complex waste mixtures,
high humic content in the soil, inhibiting
solvent-soil reactions, and a high fine-
grained clay particle fraction. Site-specific
characteristics and their potential impact on
the soil washing process are listed in Table 2.
Table 1
Effectiveness of Soil Washing
Treatment on General Contaminant
Groups for Soil and Debris
TrMtablllty Group*
i
I
Hatogenated volatlles
Norvhatogenated volatllei
Non-haloganated semi-volatilea
PCBs
Pasticidea
Dto«ins/Furans
Organic cyanides
Organic corroeives
Volatile mauls
Non-volatile mauls
Asbestos
Radioactive materials
Inorganic corrosives
Inorganic cyanides
Oxidizers
Reducers
OmvwMtld EHeclveneis £ NoEip.cH
P**AV*lft«ni
0
Q
Q
Q
O
O
Q
Q
C
0
e
Q
o
0
e
o
Q
id E*ee»»enn>
DsMnenld
O
X
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Table 2
Site-Specific Characteristics and Impacts on
Soil Washing Treatment
Characteristic*
Impacting Proc»»»
Feasibility
Unfavorable separation
coelliclent (or
contamination
Complex mixtures of
waste types (e.g., metals
with organlcs)
Unfavorable soil
characteristics:
• High humlc content
• Soil, solvent reactions
• Rne panicle size (silt
and clay)
• Clay soil containing
seml-volatllea
Unfavorable washing fluid
characteristics:
• Difficult recovery of
solvent or surfactant
• Poor treatablllty of
washing fluid
• High toxlclty of washing
fluid
Rsssons for
Potential
Impact
Excessive volumes of
leaching medium required
Formultaion of suitable
washing fluids difficult
Inhibition of desorptlon
May reduce contaminant '
mobility
Fine particles difficult to
remove from washing fluid
Low recovery rate because
organlcs are held more
tenaciously
High cost If recovery low
Requires replacement of
washing fluid
Fluid processing requires
caution, soil may require
detoxification
Actions to
Minimize
Impact*
Bench- and pilot-scale tests
to determine a suitable
washing solution
Employ secondary treatment
technology
Employ secondary treatment
technology
Pilot testing to determine a
suitable washing fluid
None: or longer dewatering
period
None: or longer washing
period
Bench-scale testing to
determine if technology is
economically feasible
Bench-scale testing to
determine If technology Is
economically feasible
Longer dewatering period:
post-treatment of soils;
bench- and pilot-scale tests
to determine an alternate
washing solution
TECHNOLOGY CONSIDERATIONS
Because soil washing is primarily a volume-reduction process
that does not reduce the inherent toxicity of a contaminant, the major
technology consideration is determining the initial composition and
post-treatment of the washing fluid and contaminated fines. An
ideal washing fluid should possess the following characteristics: a
favorable separation coefficient for extraction, low volatility, low
toxicity, safety and ease of handling, and efficientrecoverability and
treatability. Typical soil washing fluids may be composed of water
only, or water in combination with organic solvents, chela ting com-
pounds, surfactants, acids, or bases; the exact washing fluid compo-
sition depends upon the chemistry of the target contaminants).
The treatment of the washing fluid is contingent upon the
composition of the contaminants removed from the waste stream.
For expensive washing fluids (e.g., lead chelating agents), the re-
cyclability of the fluid is an important factor when determining the
economic feasibility of the soil washing process. Full-scale soil
washing units are projected to treat an average of 100 cubic yards of
soil per day.
TECHNOLOGY STATUS
The following vendors claim to have successfully applied soil
washing to various media and waste types and presently possess the
technology to conduct pilot- and/or full-scale operation:
• MTA Remedial Resources, Inc., (MTARRI) uses technologies de-
veloped for mining and enhancing oil recovery to remove and con-
centrate organic contaminants from soils and sludges. In addition,
MTA has treated various metallic compounds with acidic washing
solutions. They state that 5 tons (5 percent) of contaminated treat-
ment residue is generated per 100 tons of soil treated.
• BioTrol, Inc. employs soil washing as a pretreatment process in
conjunction with biodegradation. EPA is presently evaluating the
BioTrol Soil WashingTreatment System (BSTS) under the SITE
program. BSTS will be demonstrated on wood-treating chemicals
(i.e., PCP, PAHs, copper, chromium, and arsenic) at the MacGillis
and Gibbs Site, New Brighton, Minnesota, by Fall 1989.
• EPA developed a mobile soil washing treatment system designed
for water extraction of a broad range of hazardous materials from
contaminated soils. The normal processing rate is 4 to 18 cubic
yards of contaminated soil per hour depending on the average
particle size. Treatability costs range from approximately $20,000
to over $100,000.
Vendor names, contacts, and addresses are listed in Table 3.
EPA has selected soil washing as a component of the source
control remedy for five CERCL A sites. Site names, ROD sign dates,
target contaminants, and waste volumes are provided in Table 4.
OFFICE OF RESEARCH AND DEVELOPMENT CONTACTS
Supplemental information concerning soil washing may be ob-
tained from RichardP. Traver, P.E., U.S. EPA, Edison, New Jersey
08837, (201) 321-6677 or FTS 340-6677.
Tables
Vendor Information
Company
MTARRI
Ecova Corporation
BioTrol. Inc.
U.S. EPA. Risk
Reduction
Engineering
Laboratory
Soil Cleaning
Company of
America. Inc.
Contact
Paul Trost
Al Bourquin
Dale Pflug
Richard P. Traver
Verl Rothlisberger
Address
1 511 Washington Avenue
Golden. CO 8O4O1
(3O3) 279-4255
382O 159th Avenue ME
Redmond. WA 98O52
(2O6) SS3-19OO
11 Peavey Road
Chaska. MN 55318
(612)448-2515
Releases Control Branch
Rarltan Depot -
Woodbridge Avenue
Edison. NJ 08837-3679
(201)321-6677
753 Peralta Avenue
San Leandro, CA 94577
(415) 568-1234
Table 4
Soil Washing Status at CERCLA Sites
SELECTED:
Region 1 - Tinkham Garage, NH
9/66
Region 4 - Palmetto, SO
9/87
Region 5 - United Scrap, OH
9/88
Region 6 - Koppers/Texarkana, TX
9/88
Region 6 - South Cavalcade, TX
9/88
TCE. PCE in Soil
Arsenic, Chromium in Soil
Arsenic, Lead in Soil
Arsenic in Soil
PAHs in Soil
10,800 cubic yards
19,850 cubic yards
60,600 cubic yards
Not Provided
19,500 cubic yards
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