A 146 VOLUME 110 | NUMBER 3 | March 2002 • Environmental Health Perspectives

Innovations

By heating the ground using electricity orsteam, contaminants are volatilized orotherwise mobilized so they can beremoved from the ground and destroyed,or even destroyed in place. Among thetargets for this method are solvents such ascreosote, tetrachloroethylene, andtrichloroethylene, many of which areknown or reasonably anticipated to behuman carcinogens, according to theNational Toxicology Program.

While soils containing these contami-nants can simply be dug up and carted offto landfills, that apparently cheap remedyis not without costs of another kind, saysRalph Baker, CEO and technology man-ager of TerraTherm, a firm specializing in

environmental remediation and decon-tamination in Fitchburg, Massachusetts.“Excavation removes the source, but it isvery intrusive,” he says. “It tends to meanyou are potentially exposing the communityto contaminants that they don’t want to[have] trucked through the area.” Otherconcerns include the possibility of con-taminants leaching into the environmentand exposure to workers handling thecontaminated soil.

In situ thermal technologies avoid theseproblems. They also offer the potential toaddress contamination not previouslyamenable to cleanup at all, such as contami-nation at the depth of or beneath structuresand contamination below the water table.

Roger Aines, a geochemist at LawrenceLivermore National Laboratory inLivermore, California, who helped developthe use of steam to decontaminate toxicwaste sites, notes that steam has been usedby oil companies to help extract oil fromdeep within the earth. And in the late1980s, Shell Exploration and Recoverybegan to use electricity as a way to enhanceoil recovery, Baker says.

The Shell researchers found that heat-ing oil-containing geologic formations withelectricity had a surprising effect. “Theyfound the [underground] soil was likebeach sand, it was so clean. They realizedthis might have application for environmen-tal cleanup,” says Baker, whose company

Scientists and engineers are exploring a new way to decontaminatesoil at toxic waste sites by literally turning up the heat on pollutants.

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uses this technology. “Almost every majortechnology that has been exploited for insitu remediation came out of the ‘oilpatch,’” he says. And such methods seem tobe proving their value in ridding sites oftoxic contamination.

Electrifying TechnologyTerraTherm uses a technique called insitu thermal destruction in which the soilis heated to well beyond the boiling pointof water using electrically powered heatingelements. Baker says the elements are sim-ilar to those found in a toaster oven, andthey heat target compounds enough thatthey burn. If burned in the absence ofoxygen, only carbon is left. If oxygen ispresent, carbon dioxide and water are left.

The elements are contained inside pipesthat are typically spaced 5–7 feet apart for acleanup that will take 1–3 months. The

heat flows 4–6 feet out from the heatersinto the soil. For treating heavy contami-nants with higher boiling points, such asheavy oils, the spacing would be closer. Forlighter contaminants with lower boilingpoints, such as gasoline, the spacing wouldbe farther apart.

In 1997–1998, Shell used the tech-nology on seven contaminated sites thatcontained a range of contaminants includ-ing polychlorinated biphenyls, chlorinatedsolvents, and diesel and gasoline fuel.Virtually all of the post-treatment confir-matory soil samples had no traces of con-taminants, and just a few had minimaltraces, Baker says.

Among TerraTherm’s current projectsis the cleanup of a 5,000-cubic-yard site inLake Charles, Louisiana. Owned byEntergy Gulf States, the former manufac-tured gas plant is contaminated with tar.

Other projects include cleaning up pesticidewastes on a 2,500-cubic-yard site at theDepartment of Defense’s Rocky MountainArsenal near Denver, Colorado, and remov-ing creosote from a 10,500-cubic-yard woodtreatment site for the electric utility SouthernCalifornia Edison near Los Angeles. All theseprojects are slated for 2002.

A second electric method, which alsouses electrodes in the ground, is known asSix-Phase Heating. Developed at PacificNorthwest National Laboratory inRichland, Washington, the process uses atransformer to split ordinary electric currentinto six different phases, or paths betweenelectrodes. This creates a six-sided “web” ofelectricity that provides uniform heatthroughout the section of earth to becleaned, says William Heath, chief operatingofficer of Current Environmental Solutionsof Richland, which uses this technique.

Environmental Health Perspectives • VOLUME 110 | NUMBER 3 | March 2002 A 147

Innovations • The Hottest Thing in Remediation

A 148 VOLUME 110 | NUMBER 3 | March 2002 • Environmental Health Perspectives

Innovations • The Hottest Thing in Remediation

Baked clean. In soils such as clay that are too dense to penetrate with steam (below), in situ thermal destruction can be an effective remediationalternative. At a typical site, a central heater well is surrounded by six wells that contain heaters and vacuum mechanisms. The high temperaturesproduced by the heater array literally burn most pollutants out of the soil. Others are volatilized along with any moisture present in the soil. All thegases that result from this process are sucked toward the vacuum wells, where they can be collected and treated.

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Steam cleaning. In porous soils, large polluted areas can be remediated with steam. In a typical dynamic underground stripping setup, injection wellsforce the steam into the ground, where it displaces or volatilizes pollutants, pushing them toward an extraction well. Because the process utilizeslower temperatures than in situ thermal destruction (above), fewer pollutants are destroyed in the soil. Rather, they are collected at the extraction welland further treated at the surface—for example, by controlled burning.

Even heating is important, he says, because itensures there will be no untreated spots.

The method successfully cleaned up soilcontaminated with the solvents tri-chloroethane and trichloroethene at an elec-tronics manufacturing facility in Skokie,Illinois, according to an October 1999 reportby the U.S. Environmental ProtectionAgency (EPA) Technology InnovationOffice titled Cost and Performance Report:Six-Phase Heating (SPH) at a FormerManufacturing Facility, Skokie, Illinois. Theprocess ran from June 1998 through April1999, with the exception of approximately amonth between November and December.Describing the process as an “emerging tech-nology,” Heath says Six-Phase Heating hasbeen used at about 15 sites around thecountry.

Jim Cummings, a technical expert in theTechnology and Markets Program of theTechnology Innovation Office, says electricalheating is particularly suited to sites with claysoils, which are not very permeable but have ahigher water content than other soils andthus conduct electricity more effectively. Inaddition, he says, “heating generates steam,which causes expansion and, in the process ofdeparting the clay matrix, results in dessica-tion. This further contributes to contaminantrecovery.”

Full-Steam-Ahead CleanupSteam is more effective in soils that aremuch more permeable. “The more perme-able the medium, the more sand and gravelyou have, [and] the better steam is going towork,” says Aines. The steam-cleaningprocess, known as dynamic undergroundstripping, involves simply generating steamby using steam boilers and injecting thesteam into the ground through pipes. Thesteam can then volatilize the contaminantsand move them toward extraction wells thatcreate a vacuum over the contaminants. Thewells remove the volatilized contaminants

and transfer them to aboveground facilities,where they are destroyed, says NormanBrown, vice president and chief scientificofficer of Integrated Water Resources, aSanta Barbara, California, firm that uses thesteam process under license from LawrenceLivermore National Laboratory. The systemcools and condenses whatever material isrecovered, and the waste is then burned in acontrolled setting.

This method removed more than 1.3 million pounds of creosote from a four-acre Superfund site in Visalia, California,between June 1997 and November 1999.The site was a pole yard, where woodenutility poles had been treated with creosoteto protect them from decay. Cummingsnotes that since 1976 the Visalia site hadbeen the subject of “pump and treat” reme-dial activity, in which contaminatedgroundwater is brought to the surface andtreated. Workers were recovering approxi-mately 10 pounds of contamination eachweek. At that rate, given the amount thathas since been recovered by steam injection,the utility company would have to havepumped and treated for over 3,000 years.

More recently, between September 2000and September 2001, the steam processremoved approximately 70,000 pounds oftrichloroethylene and perchloroethylenefrom a site of 61,000 cubic yards at theDepartment of Energy’s Savannah River Sitein South Carolina. “That is more than twicethe maximum estimate of contaminant thathad been thought to exist [at the site],” saysBrown. “On top of that, we know that someamount has been destroyed in place”—although he has no estimate of the amountdestroyed in the ground.

Bright Future for a Hot CommodityThe thermal technologies appear unexpect-edly to have brought a natural ally into thecleanup process: thermophilic bacteria suchas Thermus spp., common bacteria that

thrive in high-heat environments. “We allused to think you basically sterilized the soil[with thermal remediation],” says Heath. “Inreality, that’s not true. We found that whenwe heat the soil, it wakes these [bacteria] up.”The bacteria eat and digest contaminants,effectively destroying extremely difficultcompounds that most other bacteria can’t.

Although the major part of the work isdone by the heat, these thermophilic bacteriamay play an important role in achievingcleanup goals. Says EPA hydrologist EvaDavis, “There are likely to be small amountsof the contaminants remaining after thermalremediation, and the bugs that have beenactivated by the thermal processes can pro-vide a ‘polishing step’ of getting the last littlebit out of the soil. However, more research isneded to understand and optimize the rela-tionship between the thermal and microbialprocesses.”

Regardless of how significant a factor thebacteria are, heat treatments are likely tobecome an important remediation tool.Cummings estimates that 70% of Superfundsites have solvent contamination. And he saysthere are over 80 sites contaminated with thewood preservatives creosote and pen-tachlorophenol. Citing the Visalia project, hesays, “We know that steam technology willwork at wood treatment sites.”

Many contaminated sites are in devel-oped areas with subsurface structures such asgas, sewer, and electric lines and fiber opticcables. “People are concerned that by usingthermal processes, you will damage fiber opticcables, telephone lines, that sort of thing,”Cummings says. Although such damage hasnot happened, it is a possibility that thosewho use the technology should be aware of.This is not a technological limitation, headds, but rather an engineering issue thatwould need to be addressed on a site-specificbasis. He points to the successful use of Six-Phase Heating to restore the groundwater at adry cleaner site located in a strip mall inSeattle. Restoration was achieved despite thesignificant presence of such structures, and allremediation equipment was placed belowgrade, thus eliminating possible interferencewith vehicular or pedestrian traffic.

Looking at thermal technologies ingeneral, Brown, along with others in thefield, is optimistic about their future. Theyhave, he says, “potential to change the waywe look at certain kinds of contaminatedsites, particularly where there is a need forand goal to remove source mass of highconcentration.”

Harvey Black

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Sugge s t ed Read ingFountain JC. 1998. Technologies for dense nonaqueous phase liquid source zone remedia-

tion. Technology Evaluation Report TE-98-02. Pittsburgh, PA:Ground-WaterRemediation Technologies Analysis Center.

Technology Information Office. Hazardous Waste Clean-Up Information (CLU-IN).Washington, DC:U.S. Environmental Protection Agency. Available: http://www.clu-in.org/ [accessed 15 February 2002].

Stegemeier GL, Vinegar HJ. 2001. Thermal conduction heating for in-situ thermal desorp-tion of soils. In: Hazardous and Radioactive Waste Treatment Technologies Handbook(Oh CH, ed). Boca Raton, FL:CRC Press, chapter 4.6-1.

U.S. EPA, Office of Solid Waste and Emergency Response, Technology Innovation Office.1999. Cost and Performance Report: Six-Phase Heating™ (SPH) at a FormerManufacturing Facility, Skokie, Illinois. Washington, DC:U.S. EnvironmentalProtection Agency.