Soil vapor extraction

Soil vapor extraction

Soil Vapor Extraction (SVE) is an "in situ" process for soil remediation where contamination is removed from soil by carrying it out through a medium such as air or steam. The extracted soil vapors are separated into liquids and vapors, and each stream is treated as necessary. SVE is suitable for removing a variety of contaminants that have a high vapor pressure or a low boiling point compared to water, such as chlorinated solvents. SVE can handle high concentrations of contaminant, including contaminants in the form of a non-aqueous phase liquid (NAPL). SVE is characterized as a quick and effective remediation technique.

Medium for Contaminant Transport

The medium in which the contaminants are removed is typically air or steam. It can be injected into the ground, or created "in situ" in limited cases (see next section). Air sparging is the process of injecting air into the ground using a system of blowers and injection wells.

Effect of temperature

Heating the soil is desirable because it volatilizes the contaminants and increases their vapor pressures. Heating can significantly reduce the time needed to achieve the target level of remediation. Moderate heating can be achieved using an underground heat exchange system. Heating to higher temperatures can be achieved by a process like three-phase or six-phase heating (SPH). This involves installing electrodes in the area of treatment and applying 3-phase or 6-phase electricity to them. Electrical current flowing through the soil generates heat (through electro-resistive heating), causing contaminants to volatilize. The vapor-phase contaminants (as well as any other volatilized compounds in the soil, such as groundwater) are then transported through the SVE conveyance to the surface, where they can be treated and discharged (see next section).

Treatment of Soil Vapor

Extracted soil vapor is cooled if necessary and separated into liquids and vapors. The design of the treatment system should take into account the expected concentration of contaminants in each phase. For the liquid phase, a common treatment is to run it through a granulated activated carbon (GAC) system, and then to send it to the sewer or discharge it to a local body of water. For the vapor phase, these are common treatments: 1. running the vapor through a GAC system (separate from the one for the liquid phase), 2. burning it in a thermal oxidizer or a catalytic oxidizer, or 3. condensing the vapors into NAPL form for reuse or recycling. Generally, GAC is used for low VOC vapor concentrations (<500 ppmV), oxidation is used for vapor concentrations between 500 and 2,000 ppmV because of the need for dilution of the vapor stream which slows the process of remediation, and vapor condensation and chemical recovery for vapor concentrations in excess of 1,000 ppmV of VOCs.

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