Soil Vapor Extraction Enhancements
This technology is an enhancement of soil vapor extraction (SVE), a proven technology for removing volatile organic contaminants (VOCs) from permeable soils. The effectiveness of conventional SVE is limited where contaminants cannot be easily vaporized, if the soil is too tight for air to pass, or if the contaminants are below the water table. Heating the soil while venting can increase the removal rate of SVE by raising the vapor pressure of contaminant. Heating is done by injecting hot air or steam into the soil, or by placing electrodes in the ground. This latter method is called six-phase soil heating and is described as a separate technology.
Limitations and Concerns
The effectiveness of directly injecting hot air or steam depends largely on characteristics of the soil. A highly permeable soil is required to ensure uniform heating, venting, and contaminant removal. Tightly bound soils such as clays and silts are not generally suited to hot air or steam injection.
In a complex underground geological system where impermeable layers of silt or clay are interspersed with permeable layers, injection of hot air or steam may be ineffective.
Injection of hot air suffers from the low heat capacity of air compared to soil, requiring long injection times, large blowers, and significant pumping costs.
Condensed steam may decrease the permeability of the soils to gas flow.
Steam adds significant amounts of water to the subsurface. Where the contaminated zone is close to the water table, precautions must be taken to avoid transferring contaminants from soil to groundwater.
Electrical heating is more applicable in tighter soils. Electrical heating not only raises the vapor pressure of the contaminants, but it also provides steam from soil moisture to accelerate the removal of contaminants from soils.
When electrical heating is sufficient to dry the soil, electrical conduction stops because dry soil is much more resistive. Water can be added to maintain conduction.
This technology is used to enhance the removal of VOCs and fuel from soil. It may work well for some dense non-aqueous phase liquid (DNAPL) removal.
Technology Development Status
Enhancements to soil vapor extraction are commercially available.
Other resources and Demonstrations
See Soil Vapor Extraction (SVE) Enhancement Technology Resource Guide, EPA, OSWER, Washington, DC, EPA/542/K-95/003.
See Analysis of Selected Enhancements for Soil Vapor Extraction, 1997, EPA 542-R-97-007. This report provides an engineering analysis of, and status report on, selected enhancements for the following soil vapor extraction (SVE) treatment technologies: air sparging, dual-phase extraction, directional drilling, pneumatic and hydraulic fracturing, and thermal enhancement.
See Steam Injection Used in Unsaturated Zone at German Landfill by H.P. Koschitzky, Ph.D., and T. Theurer, University of Stuttgart. Conventional soil vapor extraction (SVE) was used initially to remove chlorinated solvents from the unsaturated zone at a former hazardous waste disposal site near the City of Muehlacker, Germany, and a hydraulic pump and treat system was used to remediate ground water. Evaluation of SVE technology indicated that the low soil permeability in this region served as a limiting factor for ÒcoldÓ SVE. As a result, alternative technologies were considered, and thermally enhanced SVE by steam injection was selected in 1998 to address the unsaturated zone contaminants. After ten months of steam injection, nearly complete heating of the target zone has been achieved. 2,500 kilograms of trichloroethylene (TCE) were removed; approximately 95 percent were extracted in the gaseous phase and the remaining part as solute in water from the capillary barrier. For more information, e-mail Dr. Hans-Peter Koschitzky at firstname.lastname@example.org.