Description:

Figure 4-10a: Typical Six-Phase Soil Heating System.

Thermally enhanced SVE is normally a short- to medium-term technology.

Electrical Resistance Heating

Electrical resistance heating uses an electrical current to heat less permeable soils such as clays and fine-grained sediments so that water and contaminants trapped in these relatively conductive regions are vaporized and ready for vacuum extraction. Electrodes are placed directly into the less permeable soil matrix and activated so that electrical current passes through the soil, creating a resistance which then heats the soil. The heat dries out the soil causing it to fracture. These fractures make the soil more permeable allowing the use of SVE to remove the contaminants. The heat created by electrical resistance heating also forces trapped liquids to vaporize and move to the steam zone for removal by SVE. Six-phase soil heating (SPSH) is a typical electrical resistance heating which uses low-frequency electricity delivered to six electrodes in a circular array to heat soils. With SPSH, the temperature of the soil and contaminant is increased, thereby increasing the contaminant's vapor pressure and its removal rate. SPSH also creates an in situ source of steam to strip contaminants from soil.  At this time SPSH is in the demonstration phase, and all large scale in situ projects utilize three-phase soil heating.

Radio Frequency/Electromagnetic Heating

Radio frequency heating (RFH) is an in situ process that uses electromagnetic energy to heat soil and enhance soil vapor extraction (SVE). RFH 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 300 °C.

RFH enhances SVE in four ways: (1) contaminant vapor pressure and diffusivity are increased by heating, (2) the soil permeability is increased by drying, (3) an increase in the volatility of the contaminant from in situ steam stripping by the water vapor; and, (4) a decrease in the viscosity which improves mobility. The technology is self limiting; as the soil heats and dries, current will stop flowing. Extracted vapor can then be treated by a variety of existing technologies, such as granular activated carbon or incineration.

Hot Air/Steam Injection

Hot air or steam is injected below the contaminated zone to heat up contaminated soil. The heating enhances the release of contaminants form soil matrix. Some VOCs and SVOCs are stripped from contaminated zone and brought to the surface through soil vapor extraction.

Synonyms:

Applicability:

Limitations:

• Debris or other large objects buried in the media can cause operating difficulties.
• Performance in extracting certain contaminants varies depending upon the maximum temperature achieved in the process selected.
• Soil that is tight or has high moisture content has a reduced permeability to air, hindering the operation of thermally enhanced SVE and requiring more energy input to increase vacuum and temperature.
• Soil with highly variable permeabilities may result in uneven delivery of gas flow to the contaminated regions.
• Soil that has a high organic content has a high sorption capacity of VOCs, which results in reduced removal rates.
• Air emissions may need to be regulated to eliminate possible harm to the public and the environment. Air treatment and permitting will increase project costs.
• Residual liquids and spent activated carbon may require further treatment.
• Thermally enhanced SVE is not effective in the saturated zone; however, lowering the aquifer can expose more media to SVE (this may address concerns regarding LNAPLs).
• Hot air injection has limitations due to low heat capacity of air.

Data Needs:

Performance Data:

As with SVE, remediation projects using thermally enhanced SVE systems are highly dependent upon the specific soil and chemical properties of the contaminated media. The typical site consisting of 18,200 metric tons (20,000 tons) of contaminated media would require approximately 9 months.

DOE has developed and tested several thermally enhanced SVE processes. Dynamic underground stripping integrates steam injection and direct electric heating. Six phase soil heating is a pilot-scale technology that delivers six separate electric phases through electrodes placed in a circle around a soil vent. Thermally enhanced vapor extraction system combines conventional SVE with both powerline frequency and radio frequency soil heating.

Cost:

The key cost driver information and cost analysis was developed using the 2006 version of the Remedial Action Cost Engineering and Requirements (RACER) software.

Key Cost Drivers 

·        Soil Type

o       The primary cost driver is soil type, which once again determines soil permeability.  For thermal treatment, soils of lower permeability (silts/silty-clays) are less expensive to remediate as they require less gas flow.

·        Depth to Top/Thickness of Contaminated Area

o       The secondary cost drivers are depth to the top and thickness of the contaminated zone.  A deeper and thicker region of contaminated soils has higher remedial costs.

Cost Analysis

The following table represents estimated costs (by common unit of measure) to apply thermal treatment technology at sites of varying size and complexity.   A more detailed cost estimate table which includes specific site characteristics and significant cost elements that contributed to the final costs can be viewed by clicking on the link below.

Detailed Cost Estimate

References:

Treatment Technologies for Site Cleanup: Annual Status Report (ASR), Tenth Edition, EPA 542-R-01-004

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Remediation Technology Cost Compendium - Year 2000

Groundwater Cleanup: Overview of Operating Experience at 28 Sites, September 1999, EPA 542-R-99-006,

Treatment Experiences at RCRA Corrective Actions, December 2000, EPA 542-F-00-020

Abstracts of Remediation Case Studies, Volume 4,  June, 2000, EPA 542-R-00-006

Guide to Documenting and Managing Cost and Performance Information for Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007

A Process Description of Terra Therm's In Situ Thermal Desorption Process

U.S. Environmental Protection Agency (EPA) In Situ Thermal Treatment Site Profile Database

Dev, H., G.C. Sresty, J. Enk, N. Mshaiel, and M. Love, 1989. Radiofrequency Enhanced Decontamination of Soils Contaminated with Halogenated Hydrocarbons, EPA RREL, ORD, Cincinnati, OH, EPA Report EPA/600/2-89/008.

DOE, 2 October 1992. RCRA Research, Development and Demonstration Permit Application for a Thermal Enhanced Vapor Extraction System, Sandia National Laboratories, Environmental Restoration Technology Department, Albuquerque, NM.

DOE, 26 February 1993. "Technology Name: Thermal Enhanced Vapor Extraction System," in Technology Information Profile (Rev. 2) for ProTech, DOE ProTech Database, TTP Reference No.: AL-221121.

DOE, April 1995. Technology Catalogue, Second Edition, Office of Environmental Management & Office of Technology Development, DOE/EM-0235.

EPA, 1990. Toxic Treatments (USA) In-Situ Steam/Hot Air Stripping, EPA RREL, series includes Application Analysis, EPA/540/A5-90/008, and Demonstration Bulletin, EPA/540/M5-90/003.

EPA, 1995. In Situ Remediation Technology Status Report: Thermal Enhancement, EPA, OSWER, Washington, DC, EPA/542/K-94/009.

EPA, 1995. Soil Vapor Extraction (SVE) Enhancement Technology Resource Guide, EPA, EPA, OSWER, Washington, DC, EPA/542/K-95/003.

EPA, 1997. Analysis of Selected Enhancements for Soil Vapor Extraction, EPA OSWER, EPA/542/R-97/007.

EPA, 1997. Best Management Practices (BMPs) for Soil Treatment Technologies: Suggested Operational Guidelines to Prevent Cross-media Transfer of Contaminants During Clean-UP Activities, EPA OSWER, EPA/530/R-97/007.

EPA, 1997. How Heat Can Enhance In situ Soil and Aquifer Remediation, EPA/540/S-97/502.

EPA, 1997. Steam Injection for Soil and Aquifer Remediation, EPA/540/S-97/505.

Federal Remediation Technologies Roundtable, 1997. Remediation Case Studies: Soil Vapor Extraction and Other In Situ Technologies, EPA/542/R-97/009.

• In Situ Enhanced Soil Mixing at the U.S. Department of Energy's Portsmouth Gaseous Diffusion Plant, X-231B Unit, Piketon, Ohio
• Six Phase Soil Heating at the U.S. Department of Energy's Savannah River Site, M Area, Aiken, South Carolina; and Hanford Site, 300-Area, Richland, Washington

Pedersen, T.A., and J.T. Curtis, 1991. Soil Vapor Extraction Technology Reference Handbook, CDM, Inc. Cambridge, MA, for EPA RREL, ORD, Cincinnati, OH, EPA Report EPA/540/2-91/003.

Ramirez, A.L and W.D. Daily, 1995. Monitoring Six-Phase Ohmic Heating of Contaminated Soils Using Electrical Resistance Tomography, UCRL-ID-118418, Lawrence Livermore National Laboratory, Livermore, CA.

WESTON, IIT Research Institute, November 1992. Final Rocky Mountain Arsenal In Situ Radio Frequency Heating/Vapor Extraction Pilot Test Report, Vol. I, U.S. Army Report 5300-01-12-AAFP.

• DOE Demo: LLNL, CA
• Air Force Demo: Volk Field ANGB, WI
• DOE Demo: Hanford Reservation, WA
• DOE Demo: Sandia National Laboratory, NM
• EPA Demo: Annex Terminal, San Pedro, CA
• EPA Demo: LeMoore NAS, CA
• EPA Demo: San Fernando Valley Ground Water Basin Superfund Site, CA
• EPA Demo: Huntington Beach, CA
• EPA Demo: Pennsylvania Power and Light, PA
• Air Force & EPA Demo: Kelly AFB, TX
• Lockheed Aeronautical Systems Burbank, CA
• Kelly AFB, TX
• U.S. DOE's Savannah River Site, M Area, Aiken, SC
• DOE's Portsmouth Gaseous Diffusion Plant, X-231B Unit, Piketon, OH
• 

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A list of vendors offering In Situ Thermal Soil Treatment is available from EPA REACH IT which combines information from three established EPA databases, the Vendor Information System for Innovative Treatment Technologies (VISITT), the Vendor Field Analytical and Characterization Technologies System (Vendor FACTS), and the Innovative Treatment Technologies (ITT), to give users access to comprehensive information about treatment and characterization technologies and their applications.

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