CLEANING CONTAMINATED SOIL USING ELECTRICAL HEATING AND AIR STRIPPING

In the summer of 1992, a proof-of-concept demonstration of direct electrical heating and air stripping was conducted for enhancing the removal of a volatile organic contaminant, trichloroethylene (TCE), from soil. Six electrodes were buried in shallow boreholes so that a target region 6.1 m in diameter and 3.05 m in height was heated by ohmic dissipation of power-line-frequency electrical currents supplied by a diesel generator. Air stripping of TCE contamination from the same region was accomplished from a single well at the center of the heated volume. The electrical energy used during the demonstration was 3.46 x 10(10) J (9,600 kW . h), and the temperature of the extracted air rose from 16 degrees C to 38 degrees C. An energy balance shows that the input energy is consistent with the temperature rise in the target volume and the amount of water vaporized at the electrodes. Prior to heating, the TCE concentration in the vapor decreased from about 80 parts per million by volume (ppm(v)) to around 60 ppm(v). As soon as electrical heating started, TCE concentrations began to increase. Some concentration data were lost shortly after electrical heating began. After the system was repaired, the TCE concentration fell rapidly from about 140 ppm(v) to 5 ppm(v) over a period of about 25 days. A simple two-dimensional model for calculation of hearing rates is also presented and verified experimentally. Finally some of the operational and safety issues associated with electrical heating are discussed.