Superfund Innovative Technology Evaluation (SITE)
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This report summarizes the U.S. Environmental Protection Agency (EPA) Superfund Innovative Technology Evaluation (SITE) Program evaluation of the In Situ Thermal Destruction (ISTD) technology, developed by others, was refined by TerraTherm, Inc. The demonstration was designed to evaluate the technology's ability to treat soil-and-waste material contaminated with hexachlorocyclopentadiene (hex) and chlorinated pesticides at a former disposal pit (the Hex Pit) located at the Rocky Mountain Arsenal in Commerce City, Colorado. Operation of the system was terminated soon after initial startup and before the SITE demonstration could be completed, due to the destruction of system components from highly corrosive vapors and liquids.
ISTD is a soil remediation process that applies heat and vacuum simultaneously to contaminated soils, either with surface heater blankets or with an array of vertical heater and vacuum extraction wells. The ISTD system at the Hex Pit used an array of vertical heater and combination heater and vacuum extraction wells. According to the developer, as the soil is heated, volatile contaminants are vaporized or destroyed by a number of mechanisms, including the following:
- evaporation into the vapor stream,
- steam distillation into the vapor stream,
- oxidation, and
- pyrolysis (Stegemeier and Vinegar 2001).
Most of the contaminants are expected to be destroyed in the soil before the vapor stream is removed by vacuum extraction. Contaminants that have not been destroyed in situ and remain in the vapor stream are destroyed by an off-gas treatment system.
Evaluation of the ISTD technology as part of this SITE demonstration included extensive sampling to characterize soil-and-waste material in the Hex Pit before construction and startup of the ISTD system. In general, the Hex Pit contains layers or bands of virtually pure, tar-like waste material interlayered with soil that was used to cover the waste. Due to the early termination of the treatment process, SITE’s project objectives and post-treatment sampling were modified from the original plan. For post-treatment sampling, the revised demonstration objective was to evaluate potential contaminant destruction or removal resulting from short-term operation of the system in the near vicinity of combination heater and vacuum extraction wells. Sampling results were inconclusive regarding evidence of contaminant destruction or removal from short-term operation of the system.
ISTD treatment at the Hex Pit was terminated 12 days after initial startup of the system due to the destruction of system components, likely from higher-than-anticipated production of hydrogen chloride (HCl). In addition, vapor-phase HCl condensed to the more corrosive liquid form in the system piping. Corrosion occurred in both aboveground and subsurface piping components constructed of 304 stainless steel. Destruction of the system components appeared to result from a combination of circumstances, including
- the occurrence of layers of virtually pure, tar-like waste material that were not destroyed in situ;
- the generation of HCl that was not adequately neutralized by in situ materials;
- the choice of 304 stainless steel for system components, which was insufficiently resistant to corrosion; and
- the inability of the system to maintain extracted vapors in the vapor phase for transport to the off-gas treatment system.