Thermal Treatment: In Situ

For more information on Thermal Treatment - In Situ, please contact:

Jim Cummings
Technology Assessment Branch
(703) 603-7197
cummings.james@epa.gov

Overview Guidance Application Training Additional Resources

Overview

Many different methods and combinations of techniques can be used to apply heat to polluted soil and/or groundwater in situ. The heat can destroy or volatilize organic chemicals. As the chemicals change into gases, their mobility increases, and the gases can be extracted via collection wells for capture and cleanup in an ex situ treatment unit. Thermal methods can be particularly useful for dense or light nonaqueous phase liquids (DNAPLs or LNAPLs). Heat can be introduced to the subsurface by electrical resistance heating, radio frequency heating, dynamic underground stripping, thermal conduction, or injection of hot water, hot air, or steam.

The main advantage of in situ thermal methods is that they allow soil to be treated without being excavated and transported, resulting in significant cost savings; however, in situ treatment generally requires longer time periods than ex situ treatment, and there is less certainty about the uniformity of treatment because of the variability in soil and aquifer characteristics and because the efficacy of the process is more difficult to verify.

ELECTRICAL RESISTANCE HEATING uses arrays of electrodes installed around a central neutral electrode to create a concentrated flow of current toward the central point. Resistance to flow in the soils generates heat greater than 100ºC, producing steam and readily mobile contaminants that are recovered via vacuum extraction and processed at the surface. Electrical resistance heating is an extremely rapid form of remediation with case studies of effective treatment of soil and groundwater in less than 40 days. Three-phase heating and six-phase soil heating are varieties of this technology.

INJECTION OF HOT AIR can volatilize organic contaminants (e.g., fuel hydrocarbons) in soils or sediments. With deeper subsurface applications, hot air is introduced at high pressure through wells or soil fractures. In surface soils, hot air is usually applied in combination with soil mixing or tilling, either in situ or ex situ.

INJECTION OF HOT WATER via injection wells heats the soil and ground water and enhances contaminant release. Hot water injection also displaces fluids (including LNAPL and DNAPL free product) and decreases contaminant viscosity in the subsurface to accelerate remediation through enhanced recovery.

INJECTION OF STEAM heats the soil and groundwater and enhances the release of contaminants from the soil matrix by decreasing viscosity and accelerating volatilization. Steam injection may also destroy some contaminants. As steam is injected through a series of wells within and around a source area, the steam zone grows radially around each injection well. The steam front drives the contamination to a system of ground-water pumping wells in the saturated zone and soil vapor extraction wells in the vadose zone.

RADIO FREQUENCY HEATING is an in situ process that uses electromagnetic energy to heat soil and enhance soil vapor extraction. The technique heats a discrete volume of soil using rows of vertical electrodes embedded in soil or other media. Heated soil volumes are bounded by two rows of ground electrodes with energy applied to a third row midway between the ground rows. The three rows act as a buried triplate capacitor. When energy is applied to the electrode array, heating begins at the top center and proceeds vertically downward and laterally outward through the soil volume. The technique can heat soils to over 300ºC.

THERMAL CONDUCTION (also referred to as electrical conductive heating or in situ thermal desorption) supplies heat to the soil through steel wells or with a blanket that covers the ground surface. As the polluted area is heated, the contaminants are destroyed or evaporated. Steel wells are used when the polluted soil is deep. The blanket is used where the polluted soil is shallow. Typically, a carrier gas or vacuum system transports the volatilized water and organics to a treatment system.

VITRIFICATION uses an electric current to melt contaminated soil at elevated temperatures (1,600 to 2,000ºC or 2,900 to 3,650ºF). Upon cooling, the vitrification product is a chemically stable, leach-resistant, glass and crystalline material similar to obsidian or basalt rock. The high temperature component of the process destroys or removes organic materials. Radionuclides and heavy metals are retained within the vitrified product. Vitrification can be conducted in situ or ex situ.

A Citizen's Guide to In Situ Thermal Treatment Methods
EPA 542-F-01-012, 2001

The Citizen's Guide Series are 2-page fact sheets that provide a general description on approaches to clean up contaminated waste sites, soil, and groundwater. Each fact sheet answers the questions, What is it? How does it work? Is it safe? How long will it take? and Why use it?

Guía para el Ciudadano sobre Métodos de Tratamiento Térmico In Situ (Spanish Translation)
EPA 542-F-01-012S, 2001

La Serie de Guías del Ciudadano son boletines de dos páginas con datos que proveen una descripción general en cómo sanear sitios contaminados. Estos boletines con datos que cubren cinco preguntas acerca de cada procedimiento de saneamiento: ¿Qúe es?, ¿Cómo trabaja?, ¿Es seguro?, ¿Cúanto tiempo requerirá?, y ¿Por qué debe de usarse?

Remediation Technologies Screening Matrix and Reference Guide, Version 4.0.
Federal Remediation Technologies Roundtable.

Treatment Technologies for Site Cleanup: Annual Status Report (ASR), Twelfth Edition
EPA 542-R-07-012, 2007

The Twelfth Edition of this report, published by the EPA Office of Superfund Remediation and Technology Innovation (OSRTI) in September 2007, documents treatment technology applications at more than 1,900 soil and groundwater cleanup projects at National Priorities List (NPL) sites. The status of more than 1,200 projects included in the ASR Eleventh Edition is updated, and information about 192 new projects derived from Records of Decision (ROD) signed from 2002 through 2005 is added. The report also includes a special section about on-site containment remedies. The ASR is based on the analysis of nearly 3,000 RODs signed since 1982 at 1,536 NPL sites. The online version includes new downloadable spreadsheets with the data for several of the key tables and figures in the report. Specific information about each technology application included in the ASR Twelfth Edition is available in the ASR Remediation Database.

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Last updated on Wednesday, August 20, 2008