Enhancing vacuum extraction of volatile organics using electrical heating

Vacuum extraction is an effective tool for the in situ removal of liquid, residual and vapor phase volatile hydrocarbons from subsurface soils (Trowbridge, 1990). The vacuum extraction process creates air flow through soils by decreasing the gas phase pressure in the soil matrix. As the air flows through the pore spaces, volatile organic compounds (VOC`s) are volatilized and moved from the soil towards an extraction well. The effectiveness of the process varies with the permeability of the soil. For a given vacuum pressure applied to a well, higher air flow rates will be observed in coarser-grained sediments which have higher gas permeabilities than fine-grained sediments. Soils with lower gas permeabilities such as silts and clays, require a stronger vacuum to induce air flow through the soil. The capacity to induce air flow through fine-grained materials reaches an upper limit when the required vacuum capacity cannot be achieved. Remediation of fine-grained soils using vacuum extraction may be ineffective because a closer spacing between extraction wells will be required, or in fact may become impossible for soils with very low permeabilities.