COMPLETION OF IN-SITU THERMAL REMEDIATION OF PAHs, PCP AND DIOXINS AT A FORMER WOOD TREATMENT FACILITY

摘要

The largest in-situ thermal conduction heating project ever undertaken at a wood treatment site was completed in March 2006. The site was a former utility pole treatment facility that Southern California Edison (SCE) operated from 1922 to 1957. The subsurface soils were contaminated primarily with polycyclic aromatic hydrocarbons (PAHs), pentachlorophenol (PCP), and polychlorinated dibenzodioxins and furans (PCDD/Fs), with soil treatment standards of 0.065 mg/kg Benzo(a)Pyrene Equivalents (B(a)P-E), 2.5 mg/kg PCP, and 1.0 μg/kg PCDD/Fs, expressed as 2,3,7,8-Tetrachlorodibenzodioxin (TCDD) Toxic Equivalents (TEQ), respectively. A feasibility study led to the selection of TerraTherm's patented In-Situ Thermal Destruction (ISTD) technology, also known as In-Situ Thermal Desorption, which utilizes simultaneous application of thermal conduction heating and vacuum to treat contaminated soil without excavation. The applied heat volatilizes organic contaminants within the soil, enabling them to be carried in the vapor stream toward heater-vacuum wells. Approximately 12,385 m~3 (16,200 cubic yards [CY]) of predominantly silty soil was treated to a maximum depth of 32 m (105 ft). TerraTherm installed 785 thermal wells, including 654 heater-only and 131 heater-vacuum wells, in a hexagonal pattern at 2.1-m (7.0-foot) spacing. Subsurface temperature monitoring tracked the progress of heating. The heating goal for inter-well temperatures was 335 °C (635 ℉) for three days, or 300 °C (570 ℉) sustained for thirty days. Gases emerging from the heated soil were collected under vacuum and conveyed to an Air Quality Control (AQC) system, permitted by the South Coast Air Quality Management District. The AQC system consisted of a thermal oxidizer, heat exchanger to cool the gases, and serial vessels of granular activated carbon. AQC system performance was gauged by a Continuous Emissions Monitoring (CEM) system operated by TerraTherm, vapor sampling, and four source tests conducted by an independent stack testing firm. Over the course of the project, TerraTherm reduced mean B(a)P-E and TEQ concentrations in soil from 30.6 mg/kg and 0.018 mg/kg (pre-treatment) to 0.059 mg/kg and 0.00011 mg/kg (post-treatment), respectively; thereby meeting the remedial goals, and resulting in a No Further Action letter from the Department of Toxic Substances Control. Attainment of such stringent soil treatment goals with an in-situ technology is unprecedented. Dioxin (PCDD/F) emissions based on four source tests averaged 0.0071 ng TEQ/dsm~3, compared to the standard of 0.2 ng TEQ/dsm~3. Averaged over the life of the project, this is equal to 0.00023 g TCDD, one-quarter of the projected (design) amount. Furthermore, this stack emission rate is equivalent to a TCDD TEQ concentration in the air of less than 2 parts per quadrillion, an extremely low emission rate for any remediation off-gas treatment system.