Six Phase Soil Heating
Six Phase Soil Heating (SPSH) enhances the recovery of soils contaminated with volatile and semi-volatile organic compounds (VOCs and SVOCs). SPSH assists soil vapor extraction (SVE) by heating the contaminants in the soil. With SVE, a vacuum created in the soil pulls contaminated vapor into a well and then to the surface where the vapors are treated to remove the contaminants. However, its effectiveness is limited if the contaminants cannot be easily vaporized or if the soil is too tight for air to pass through readily. SPSH raises the vapor pressure of VOCs and SVOCs, increasing volatilization and removal via SVE. As SPSH dries the soil, it also creates a source of steam that strips contaminants from soils.
SPSH splits conventional three-phase electricity into six separate electrical phases, producing an improved subsurface heat distribution. Each phase is delivered to a single electrode, which is placed in a hexagonal pattern at up to 100 feet deep. Each electrode conducts electricity to all the others and to a central neutral electrode. At one demonstration, temperatures within the array were elevated to 100 degrees Celsius after 8 days of heating. The vapor extraction well is located in the center of the hexagon. An applied electrical field heats soils internally, and the soil itself acts as a heat source. Thus SPSH is more effective than other soil-heating techniques such as using heating elements, which has a much smaller radius of influence. Because conventional SVE relies on the ability to draw vapors out of the soil, SPSH is especially suited to sites where contaminants are tightly bound to clays and are thus difficult to remove.
Limitations and Concerns
Engineered barriers to prevent worker exposure to high voltages are recommended.
An off-gas treatment system for contaminated vapors removed from the subsurface needs to be installed. The system should be sized to handle peak extraction rates and the mix of VOCs in extracted vapor and liquid streams.
The extraction well should be screened both above and below the target zone to ensure sufficient vacuum pressure in the subsurface. This extraction well design should also ensure total capture of contaminants released as a result of the heating.
The presence of buried metal objects presents a safety hazard. The subsurface should be mapped before the heating system is installed.
Questions remain about how quickly soil should be heated. When heating is sufficient to dry the soil, electrical conduction stops because dry soil is more resistive to electricity. Water can be added to maintain conduction.
A potential explosive hazard exists from concentrated fumes released from the vacuum unit.
SPSH has been demonstrated to enhance removal of VOCs and SVOCs in unsaturated clay-rich soils. Bench-scale tests have demonstrated that SPSH is effective on lower volatility compounds.
Technology Development Status
SPSH is commercially available.
See http://www.frtr.gov/matrix2/section4/D01-4-9a.html for a general diagram.
R.A. Gauglitz, J.S. Roberts, T.M. Bergsman, R. Schalla, S.M.Caley, M.H. Schlender, W.O. Heath, T.R. Jarosch, M.C. Miller, C.A. Eddy Dilek, R.W. Moss, B.B. Looney, Six-Phase Soil Heating for Enhanced Removal of Contaminants: Volatile Organic Compounds in Non-Arid Soils Integrated Demonstration, Savannah River Site, PNL-101 84, Battelle Pacific Northwest Laboratory, 1994.
See http://costperformance.org/remediation/pdf/Navy-ERH_Review.pdf for a description of demonstrations using electrical resistance heating.