United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(5102G)
EPA542-F-96-016
September 1996
&EPA A Citizen's
Treatment Walls
to
Technology Innovation Office
Technology Fact Sheet
What are treatment walls?
Treatment walls are structures installed under-
ground to treat contaminated ground water found
at hazardous waste sites. Treatment walls, also
called passive treatment walls or permeable bar-
riers, are put in place by constructing a giant
trench across the flow path of contaminated
ground water and filling it with one of a variety of
materials (reactive fillings) carefully selected for
the ability to clean up specific types of contami-
nants. As the contaminated ground water passes
through the treatment wall, the contaminants are
either trapped by the treatment wall or trans-
formed into harmless substances that flow out of
the wall (Figure 1).
How do they work?
The reactive filling of a treatment wall is often
mixed with sand or some other porous material to
make it less dense than the soil around it. This en-
courages ground water to flow through the wall
because it provides the "path of least resistance."
At some sites, an underground funnel system is
added to direct the contaminated ground water to
the wall.
The specific filling chosen for a wall is based on
the types of contaminants found at the site.
Different fillings do their job through different
chemical processes: sorption, precipitation, and
iation.
Sorption barriers contain fillings that remove
contaminants from ground water by physically
"grabbing" contaminants out of the ground water
and holding them on the barrier surface (Figure
2a). Examples of these adsorbents are zeolites—
tiny cage-like particles that trap molecules of
contaminants inside them—and activated carbon
which has a very rough surface that contaminant
molecules stick to as they pass.
Precipitation barriers contain fillings that react
with contaminants in ground water as they seep
through the wall (Figure 2b). The reaction causes
the contaminants dissolved in the ground water to
change so they are no longer dissolved and "pre-
cipitate" out. These "insoluble" products are left
trapped in the barrier and clean ground water
flows out the other side. For example, lead is a
common contaminant at industrial sites where
careless recycling of automobile batteries has
taken place. The lead-saturated battery acid that
seeped into the ground water at these sites is dif-
ficult to trap and treat. A precipitation barrier
filled with limestone placed across the path of the
acidic, lead-contaminated ground water neutral-
izes the acid. This causes the lead to change to a
A Quick Look at Treatment Walls
Are passive systems that require no mechanical equipment or energy source.
Allow the site to be put to productive use while being cleaned up.
Can be modified to treat different types of contaminants.
Completely break down some organic contaminants.
Printed on Recycled Paper
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What Is An Innovative Treatment
Technology?
Treatment technologies are processes
applied to the treatment of hazardous waste
or contaminated materials to permanently
alter their condition through chemical,
biological, or physical means.
Innovative treatment technologies are those
that have been tested, selected or used for
treatment of hazardous waste or
contaminated materials but lack well-
documented cost and performance data
under a variety of operating conditions.
solid form that is trapped in the barrier. Highly
toxic chromium (VI), a by-product of metal-plat-
ing operations, is treated by precipitation barriers
in a similar way. It is changed to immobile chro-
mium (III) which is trapped in the barrier.
Degradation barriers cause reactions that break
down or "degrade" the contaminants in the ground
water into harmless products (Figure 2c). For ex-
ample, fillings of iron granules degrade certain
volatile organic compounds. Walls also may be
filled with a mixture of nutrients and oxygen
sources which stimulate the activity of the micro-
organisms found in the ground water. Healthy
microorganisms are important because they are
responsible for the biodegradation of contami-
nants. Biodegradation is the process that naturally
occurring microorganisms (yeast, fungi, or bacte-
ria) use to break down, or degrade, hazardous
substances into less toxic or nontoxic substances.
Microorganisms, just like humans, eat and digest
organic substances for nutrition and energy. (In
chemical terms, "organic" compounds are those
that contain carbon and hydrogen atoms.) Certain
microorganisms can digest organic substances
such as fuels or solvents that are hazardous to hu-
mans. The fact sheet entitled^ Citizen's Guide to
Bioremediation describes the process in detail
(see page 4).
Much research and testing has been done on the
use of iron for the treatment of chlorinated con-
taminants. The reaction that occurs when con-
taminants come in contact with iron granules puts
to beneficial use the common chemical reaction
called oxidation that causes iron to rust. As the
iron is oxidized, the toxic component of the con-
taminant (usually a chlorine atom) is removed
from the compound. The iron granules are dis-
solved by the process, but the metal disappears so
Figure 1. Schematic Diagram of a Treatment Wall
ground surface
contaminated
ground water
treatment
direction of ground-water flow
clean ground water
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Figure 2. Sorption, Precipitation, and Degradation Treatment Walls
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2a. SORPTION 2b. PRECIPITATION
Contaminants are pulled, Contaminants are changed into
unchanged, from ground water solid forms that remain in the
and held by the wall surface. wall.
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2c. DEGRADATION
Contaminants are broken down
into harmless products that flow
through the wall.
slowly that remediation barriers, engineers pre-
dict, will remain effective for many years, even
decades. These iron granules are a by-product of
manufacturing processes so their use as a barrier
wall material has the added benefit of recycling
this material.
Iron can be used to degrade many common chlo-
rinated organic compounds such as trichloroeth-
ylene (TCE), tetrachloroethylene (PCE),
dichloroethene (DCE) and 1,1,1-trichloroethane
(TCA). Mixing palladium, another metal, with
the iron granules enables the wall to treat con-
taminants that iron alone cannot treat.
Why use treatment walls?
The major advantage of treatment walls over tra-
ditional treatment methods such as pump-and-
treat is that they are passive systems that treat the
contaminants in place. There is no need to dig up
contaminated soil or pump out contaminated wa-
ter, there are no parts to break, no need for elec-
tricity, and, since there is no equipment on the
surface, the property can be put to productive use
while it is being cleaned up. Engineers estimate
at least a 50% cost savings using treatment walls
instead of pumping out contaminated ground
water.
Will they work at every site?
The ideal site for a treatment wall is one having
porous sandy soil, contamination no deeper than
about 50 feet below ground, and a good, solid
flow of ground water.
There are an estimated 5,000 Department of De-
fense, Department of Energy, and Superfund sites
contaminated with chlorinated solvents. Probably
10 to 20 percent of these have the right conditions
to use treatment walls. Treatment walls also are
useful at sites contaminated with metals and
radioactive contaminants.
The successful application of a treatment wall re-
quires careful study of the underground
environment and an understanding of the con-
taminant and ground-water flow.
In lab studies, some clogging of wall materials
has been observed. So far, clogging has not oc-
curred in the field, but walls have only been in
place for a few years.
Where have they been used?
At a former semiconductor manufacturing site in
Sunnyvale, California, 220 tons of iron shavings
were used to fill a reactive treatment wall that has
been breaking down TCE since December 1994.
The above-ground equipment that was part of a
previously installed pump-and-treat system was
removed and the site has been leased to another
company that uses it as a parking lot. Some
Superfund sites that have chosen treatment
walls as a cleanup method are listed in Table 1
on page 4.
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Table 1. Some Superfund Sites that Plan to Use Treatment Walls*
Name of Site Type of Wall/Filling Contaminants Site Use
Brown's Battery Breaking Site, PA Precipitation/Limestone Lead Battery recycling & disposal
Tonolli Corporation, PA Precipitation/Limestone Lead Battery recycling & disposal
Somersworth Sanitary Landfill, NH Degradation/Iron Organics Municipal & industrial landfill
For a listing of Superfund sites at which innovative treatment technologies have been used or selected for use, contact
NCEPI at the address in the box below for a copy of the document entitled Innovative Treatment Technologies:
Annual Status Report (7th Ed.), EPA 542-R-95-008. Additional information about the sites listed in the Annual Status
Report is available in database format. The database can be downloaded free of charge from EPA's Cleanup
Information (CLU-IN) World Wide Web site (http://clu-in.com) or electronic bulletin board (301-589-8366). The CLU-IN
help line number is 301-589-8368. The database also is available for purchase on diskettes. Contact NCEPI for details.
'Not all waste types and site conditions are comparable. Each site must be individually investigated and tested.
Engineering and scientific judgment must be used to determine if a technology is appropriate for a site.
For More Information
The publications listed below can be ordered free of charge by faxing your request to NCEPI at 513-489-8695.
If NCEPI is out of stock of a document, you may be directed to other sources. You may write to NCEPI at:
National Center for Environmental Publications and Information (NCEPI)
P.O. Box42419
Cincinnati, OH 45242
• A Citizen's Guide to Bioremediation, April 1996, EPA 542-F-96-007.
• "Metal-Enhanced Abiotic Degradation of VOCs," Ground Water Currents (newsletter), July 1995, EPA 542-
N-95-004.
• "Funnel and Gate System Directs Plume," Ground Water Currents (newsletter), June 1993, EPA 542-N-93-
006.
• "In Situ Degradation of Halogenated Organics by Permeable Reaction Wall," Ground Water Currents
(newsletter), March 1993, EPA 542-N-93-003.
• Permeable Barriers Action Team, April 1996, EPA 542-F-96-01 Oc.
• In Situ Remediation Technology Status Report: Treatment Walls, April 1995, EPA 542-K-4-004.
"Zero-Valent Metals Provide Possible Solution to Groundwater Problems" by Elizabeth K. Wilson in
Chemical and Engineering News, July 23, 1995, pages 19-22.
• "When Toxics Meet Metal" by Virginia Fairweather in Civil Engineering, May 1996, pages 44-48.
NOTICE: This fact sheet is intended solely as general guidance and information. It is not intended, nor can it be relied upon, to create any rights enforceable
by any party in litigation with the United States. The Agency also reserves the right to change this guidance at any time without public notice.
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