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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.

What are treatment walls?

Treatment walls are structures installed underground to treat contaminated ground water found

at hazardous waste sites. Treatment walls, also

called passive treatment walls or permeable barriers, 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 contaminants. As the contaminated ground water passes

through the treatment wall, the contaminants are

either trapped by the treatment wall or transformed 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 encourages 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

degradation.

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 “precipitate” 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 difficult to trap and treat. A precipitation barrier

filled with limestone placed across the path of the

acidic, lead-contaminated ground water neutralizes the acid. This causes the lead to change to a

United States Solid Waste and EPA 542-F-96-016

Environmental Protection Emergency Response September 1996

Agency (5102G)

A Citizen’s Guide to

Treatment Walls

Technology Innovation Office Technology Fact Sheet
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Figure 1. Schematic Diagram of a Treatment Wall

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 welldocumented 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-plating operations, is treated by precipitation barriers

in a similar way. It is changed to immobile chromium (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 example, 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 microorganisms found in the ground water. Healthy

microorganisms are important because they are

responsible for the biodegradation of contaminants. Biodegradation is the process that naturally

occurring microorganisms (yeast, fungi, or bacteria) 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 humans. The fact sheet entitled A 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 contaminants. The reaction that occurs when contaminants 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 contaminant (usually a chlorine atom) is removed

from the compound. The iron granules are discontaminated

ground water

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slowly that remediation barriers, engineers predict, 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 chlorinated organic compounds such as trichloroethylene (TCE), tetrachloroethylene (PCE),

dichloroethene (DCE) and 1,1,1-trichloroethane

(TCA). Mixing palladium, another metal, with

the iron granules enables the wall to treat contaminants that iron alone cannot treat.

Why use treatment walls?

The major advantage of treatment walls over traditional treatment methods such as pump-andtreat 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 water, there are no parts to break, no need for electricity, 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 Defense, 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 requires careful study of the underground

environment and an understanding of the contaminant and ground-water flow.

In lab studies, some clogging of wall materials

has been observed. So far, clogging has not occurred 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.

2a. SORPTION

Contaminants are pulled,

unchanged, from ground water

and held by the wall surface.

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Figure 2. Sorption, Precipitation, and Degradation Treatment Walls

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Contaminants are changed into

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Contaminants are broken down

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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.

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. Box 42419

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-96-010c.

• 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.

* 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.

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.