In Situ Soil Vapor Stripping Or Soil Vacuum Extraction

Soil vapor stripping (SVS), soil vapor extraction (SVE), soil venting (SV), vacuum extraction (VE), and soil vacuum extraction (SVE) are the terms used interchangeably to describe a process that removes volatile organic compounds (VOC) from the vadose, or unsaturated soil zone, by vacuum stripping. These compounds can often be removed from the vadose zone before they contaminate groundwater. The extraction process uses readily available equipment, including extraction and monitoring wells, manifold piping, a vapor and liquid separator, a vacuum pump, and an emission control device, such as an activated carbon adsorption filter. After the contaminated area is completely defined, extraction wells are installed and connected by piping to the vacuum extraction and treatment system.

First, a vacuum pump draws the subsurface contaminants from the extraction wells to the liquid/gas separator. The vapor-phase contaminants are then treated with an activated carbon adsorption filter or a catalytic oxidizer before the gases are discharged to the atmosphere. Subsurface vacuum and soil vapor concentrations are monitored with vadose zone monitoring wells.

The technology is effective in most hydrogeological settings, and can reduce soil contaminant levels from saturated conditions to a nondetectable level. The process even works in less permeable soils (clays) with sufficient porosity. Dual vacuum extraction of groundwater and vapor quickly restores groundwater quality to drinking water standards. In addition, the technology is less expensive than other remediation methods, such as incineration. Figure 2 illustrates the SVS or VE process. Typical contaminant recovery rates range from 20 to 2500 lb/ day (1 lb=454 g), depending on the degree of site contamination and the VOCs to be removed.

The VE or SVS technology effectively treats soils containing virtually any VOCs and has successfully removed over 40 types of chemicals from soils and groundwater, including toxic organic solvents and gasoline-and diesel-range hydrocarbons. Nevertheless, the range of applicability of VE or SVS processes is bounded by the following constraints [34]:

1. The hazardous substances to be removed must be volatile or at least semivolatile (a vapor pressure of 0.5 torr or greater);

2. The hazardous substances to be removed must have relatively low water solubility or the soil moisture content must be quite low;

3. The hazardous substances to be removed must be in the vadose zone (above the groundwater table) or, in the case of LNAPLs, floating on it;

4. The soil must be sufficiently permeable to permit the vapor extraction wells to draw air through all of the contaminated domains at a reasonable rate.

The S VS or VE process cannot remove heavy metals, most pesticides, water-soluble solvents (acetone, alcohols, etc.), and PCBs because their vapor pressures in moist soils are too low.

Cercla Process Diagram
Figure 2 In situ vacuum extraction process diagram. (Courtesy of USEPA.)

The technology is relatively cheap and rapid, has a comparatively low environmental impact, and results in elimination of the contaminated hazardous substances or its concentration into a small volume of highly concentrated, easily handled waste that may be disposed of by incineration or recycled for reuse.

The SVS or VE process was first demonstrated at a Superfund site in Puerto Rico. Terra Vac has since applied the technology at 15 additional Superfund sites and at more than 400 other waste sites throughout the United States, Europe, and Japan.

The process (Fig. 2) was demonstrated under USEPA supervision at the Groveland Wells Superfund site in Groveland, MA, United States, in 1987-1988. The technology successfully remediated soils contaminated by trichloroethene (TCE). The USEPA Technology Evaluation Report [8] and the USEPA Applications Analysis Report [7] have been published. During the Groveland Wells demonstration, four extraction wells pumped contaminants to the process system. During a 56-day operational period, 1300 lb (1 lb=454 g) of VOCs, mainly TCE, were extracted from both highly permeable strata and less permeable clays. The vacuum extraction process achieved nondetectable VOC

levels at some locations, and reduced the VOC concentration in soil gas by 95%. Average reductions were 92% for sandy soils and 90% for clays. Field evaluations have yielded the following conclusions:

1. VOCs can be reduced to nondetectable levels; however, some residual VOC concentrations usually remained in the treated soils.

2. Volatility of the contaminants and site soils is a major consideration when applying this technology.

3. Pilot demonstrations are necessary at sites with complex geology or contaminant distributions.

4. Treatment costs are typically $40 per ton of soil, but can range from $10 to $150 per ton of soil, depending on requirements for gas effluent or wastewater treatment (1989 costs).

5. Contaminants should have a Henry's constant of 0.001 or higher.

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