Dense Nonaqueous Phase Liquids (DNAPLs)
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Thermal Processes: In Situ
In situ vitrification (ISV) is a thermal treatment process that converts contaminated soil to stable glass and crystalline solids. The commercial method uses electrodes and electrical resistance to vitrify materials by applying high voltage to electrodes (typically four) placed in the soil. Starter frit (generally graphite) is placed on the soil surface and electrical current heats the soil from the top down to temperatures between 1,400 and 2,000°C. Typical melt sizes range from 200 to 1,200 tons, with a processing rate of four to six tons per hour (U.S. EPA 1995). Maximum treatment depth is approximately 20 feet in a single setup. The process depends upon the presence of 1.4 to 15 percent alkali metal oxides in the material to be treated to ensure a proper balance between electrical conductivity and melting temperature. Too much alkali metal content increases the conductivity to a point where insufficient heating occurs.
If the silica content of the soil is sufficiently high, contaminated soil can be converted into glass. Heating vaporizes or pyrolyzes organic contaminants. Most inorganic contaminants are encased in the glass-like monolith that results when the soil cools after treatment. The system requires a vapor hood that traps offgases and channels them to a treatment train that generally consists of a quencher to cool the 100 to 400ï¿½C gases and, depending upon what is being treated, a scrubber, activated carbon unit, or thermal oxidizer (U.S. EPA 1997a). The scrubber and quench water may require secondary treatment.
The ISV process can destroy or remove organics and immobilize most inorganics in contaminated soil, sludge, or other earthen materials. The process has been used on a broad range of VOCs and SVOCs, other organics including dioxins and PCBs, and on most priority pollutant metals and radionuclides (FRTR 2002). SVOCs and VOCs can be treated with this process, with about 97 percent of the VOCs destroyed and the remainder captured by the offgas treatment system (U.S. EPA 1997a). ISV is applicable to sites with high clay and moisture content, although treatment costs increase with increasing moisture content. Treatment of materials in a permeable aquifer may require dewatering, and if the treatment area is expected to contain large voids, dynamic compaction is recommended (U.S. EPA 1997a). ISV has been tested in the field several times, including a SITE Program demonstration (U.S. EPA 1995), a demonstration at DOE's Hanford Reservation (FRTR 2002), and Superfund cleanups at the Wasatch Chemical Company Lot 6 site (U.S. EPA 1997b) and General Electric Spokane Shop (U.S. EPA 2005). Costs are estimated at $400/ton (U.S. EPA 1997a). The technology is licensed to only one vendor.
Planar melting is a modification of the conventional ISV method. It differs in that the starter material is injected in a vertical plane between electrodes at depth. Generally, two electrode pairs are used with a starter plane between each pair. As the melt proceeds, it grows vertically and horizontally away from the starter planes. Because the melts are initially separated and only merge late in the process, the potential for driving gases down into the formation is greatly reduced as compared with conventional ISV. The maximum established treatment depth is 26 feet, but deeper melts are theoretically possible. The cost of the process is estimated at between $355 and $460 per ton (Thompson 2002). A successful field demonstration of the planar technique was carried out at the Los Alamos National Laboratory in 2000 (Coel-Roback et al. 2003).
This discussion is taken from Engineering Forum Issue Paper: In Situ Treatment Technologies for Contaminated Soil, EPA 542-F-06-013, 2006.
Engineering Bulletin: In Situ Vitrification Treatment
U.S. EPA, EPA 540-S-94-504, 8 pp, 1994a
Provides a general overview of the in situ vitrification technology; however, it is old and does not have the benefit of experiences since 1994.
Remediation Technologies Screening Matrix and Reference Guide, Fourth Edition.
Federal Remediation Technologies Roundtable (FRTR), 2002
Provides a general description of in situ vitrification, including applicability, limitations, and costs.
Innovative Site Remediation Design and Application, Volume 4: Stabilization/ Solidification
U.S. EPA. EPA 542-B-97-007, 234 pp, 1997a
A compendium of stabilization and solidification techniques that includes in situ vitrification.
Non-Traditional In Situ Vitrification: A Technology Demonstration at Los Alamos National Laboratory
B. Coel-Roback, P. Lowery, M. Springer, L. Thompson, and G. Huddleston.
Waste Management 2003, February 23 - 27, 2003, Tucson, AZ, 12 pp.
Discusses a demonstration of the planar vitrification technique at a Los Alamos Laboratory site.
NPL Sites in Region 10
U.S. EPA. 2005
Wasatch Chemical Company Superfund Site: Five-Year Review Report
U.S. EPA. 1997b
Provides an example of ISV for dioxins and other organics, including residual soils from the landfarming remedial action in excess of hydrocarbon cleanup levels. A clean soil cap was placed over the treatment zone and a clean soil berm was placed around the concrete evaporation pond. Thirty-seven melts occurred to complete the ISV process for the entire area of the former evaporation pond. Sample results showed that the ISV process had been effective in reducing chemical concentrations, primarily via thermal destruction, to levels that were below the risk-based remedial action goals established for the site. Approximately 5,200 tons of soil and sludge contaminated with various organics were remediated. The RA work for the ISV was completed on January 15, 1996.
No case studies of the vitrification of compounds from the classes of Halogenated Alkenes or Multi-Component Waste have been located.