Take control of soil vapor extraction systems
According to the EPA, both soil vapor extraction, or “SVE,” and air sparging extract (remove) contaminant vapors from below ground for treatment above ground. Vapors are the gases that form when chemicals evaporate. SVE extracts vapors from the soil above the water table by applying a vacuum to pull the vapors out. Air sparging, on the other hand, pumps air underground to help extract vapors from groundwater and wet soil found beneath the water table.
The addition of air makes the chemicals evaporate faster, which makes them easier to extract with another technology, such as SVE. Both methods are used for chemicals that evaporate easily─like those found in solvents and gasoline. These chemicals are known as “volatile organic compounds,” or “VOCs”.
Soil Vapor Extraction Design
Soil Vapor Extraction Systems are designed on the findings of field investigations and representative of the site conditions. Pilot testing is executed to establish the number and location of the extraction wells. The system must be capable of providing sufficient venting to remove VOCs in a reasonable time frame.
The Soil Vapor Extraction Process
SVE involves drilling one or more extraction wells into the contaminated soil to a depth above the water table, which must be deeper than 3 feet below the ground surface. Attached to the wells is equipment (such as a blower or vacuum pump) that creates a vacuum. The vacuum pulls air and vapors through the soil and up the well to the ground surface for treatment. Sometimes the ground must be paved or covered with a tarp to make sure that the vacuum does not pull air from above into the system. Pulling in clean air would reduce the efficiency of the cleanup. The cover also prevents any vapors from escaping from the ground to the air above.
Involves drilling one or more injection wells into the groundwater-soaked soil below the water table. An air compressor at the surface pumps air underground through the wells. As air bubbles through the groundwater, it carries contaminant vapors upward into the soil above the water table. The mixture of air and vapors is then pulled out of the ground for treatment using SVE.
Extracted air and contaminant vapors, sometimes referred to as “off-gases,” are treated to remove any harmful levels of contaminants. The off-gases are first piped from the extraction wells to an air-water separator to remove moisture, which interferes with treatment.
The vapors are then separated from the air, usually by pumping them through containers of activated carbon. The chemicals are captured by the carbon while clean air exits to the atmosphere. Filter materials other than activated carbon may be used. In a process called “biofiltration,” tiny microbes (bacteria) are added to break down the vapors into gases, such as carbon dioxide and water vapor. Another option is to destroy vapors by heating them to high temperatures.
Soil Vapor Extraction Advantages & Disadvantages
Some advantages of SVE are ease of installation due to construction materials being readily available, remedial technology is easily coupled with techniques such as air sparging, and the SVE system is easily monitored compared to other techniques.
Some disadvantages of SVE are the system is applicable for only unsaturated soil and soils with high permeability, during excavation contaminated air may escape the atmosphere, and the SVE may require a large amount of land and permits and therefore may increase cost.