How does In-Situ Thermal Desorption (ISTD) compare with other in-situ remediation technologies?
A major reason for the effectiveness of ISTD is its application of heat to the soil using thermal conduction. During conductive heating, heat moves out through the soil and waste material in a highly predictable fashion, regardless of how heterogeneous the soil is or its permeability. This is in sharp contrast to the movement of a fluid through the soil, which is the basis for nearly all other in-situ remediation technologies (e.g., groundwater pump-and-treat, soil vapor extraction, air sparging, steam injection, solvent and surfactant injection or chemical oxidant injection). Rates of fluid flow can vary over many orders of magnitude, depending on how permeable the soil is and on the degree of heterogeneity. Fluid-based technologies thus tend to bypass some contaminated zones, leading to poor efficiency, diffusion-limited mass transport and an extended duration of remediation. By contrast, the thermal conductivity of a wide range of soil types varies over less than a factor of plus or minus two.
ISTD has a wider applicability than other in-situ thermal technologies. For example, the process of electrical resistivity heating, also known as Six-Phase Heating or joule heating, relies on the flow of an electrical current through soil. Electrical conductivity can vary over two orders of magnitude. Since electrical currents ceases to flow in soils once the water has boiled off, moreover, electrical resistivity heating cannot heat the soil above the boiling point of water. As a result, it is not suitable in thetreatment of high boiling-point compounds such as pesticides, PCBs, and PAHs that require stringent soil cleanup levels. Similarly, steam injection in the shallow subsurface is limited to heating approximately to the boiling point of water.
