Soil Vapor Extraction (SVE)
Soil vapor extraction
(SVE) uses vacuum pressure to remove volatile
some semi-volatile contaminants
soil. The gas leaving the soil may be treated or destroyed, depending
and state air discharge regulations. Extraction wells are typically
depths of 5 feet or greater, and have been successfully applied as deep
meters (300 feet). Groundwater
pumps may be used in conjunction with SVE to keep groundwater from
zone as a result of vacuum pressure, or to increase the depth of
unsaturated zone. This area, called the capillary
fringe, is often highly contaminated, as it holds un-dissolved
chemicals that are lighter than water, and vapors that have escaped
dissolved groundwater below. In soils where the contamination is deep
there is low permeability,
injecting air into the soil helps extraction. During full-scale
can be run intermittently (pulsed operation) once the extracted mass
rate has reached a steady state level. Because the process involves the
continuous flow of air through the soil, it often promotes biodegradation
of low-volatility organic
compounds that may be present.
At some locations
where the compounds have low volatility, 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. These methods are also
described under Six-phase
Soil Heating, Thermal
Enhanced Vapor Extraction System, and Electrical
SVE can also be used on piles of excavated soil. A vacuum is applied to a network of aboveground piping to encourage volatilization of organics from the excavated media. The process includes a system for handling off-gases.
Limitations and Concerns
Soil with high moisture requires higher vacuums, hindering the operation of the SVE system. Soils with high organic content or soils that are extremely dry have a high sorption capacity of VOCs. These conditions limit the effectiveness of SVE. Soils with low permeability also limit the effectiveness of SVE.
Because SVE applies vacuum pressure to subsurface soils, it can raise groundwater levels. As soil becomes saturated, some contaminants may dissolve into the water. As a result, groundwater could show increases in contamination levels, especially when this process begins.
Exhaust air from in-situ SVE system may require treatment. Off-gas treatment usually involves vapor-phase Granular Activated Carbon (GAC).
SVE is not effective in the saturated zone.
A field pilot
necessary to establish the feasibility of the method as well as to
information necessary to design and configure the system.
The effectiveness of enhancements by 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.
Injection of hot air may require long injection times, large blowers, and significant pumping costs.
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.
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.
The technology is typically applicable for soil with VOC and/or fuels contamination. SVE works only on compounds that readily vaporize (i.e., with a high Henry's law constant). Although the technology is primarily used in situ, there have been cases where it was used on excavated soils.
Technology Development Status
SVE is commercially available and widely used.
See https://portal.navfac.navy.mil/portal/page/portal/NAVFAC/NAVFAC_WW_PP/NAVFAC_NFESC_PP/ENVIRONMENTAL/ERB/SVE-EXSITU for a description of an ex-situ SVE system.