Thermal blanket for in-situ remediation of surficial contamination: A pilot test

Surficial PCB contamination has been successfully and safely removed from soils in a field test at the site of a former dragstrip where oil- containing PCBs had been sprayed to minimize airborne dust. Decontamination was achieved by electrically heating a 9.3-m ² area under a thermal blanket, and PCB concentrations were reduced from up to 2000 ppm to less than 2 ppm in 24 h of heating. Initial PCB concentration in some of the more contaminated areas averaged 700 mg/kg from 0 to 7.5 cm deep and 100 mg/kg from 7.5 to 15 cm, with maximum concentrations as high as 2000 mg/kg at the surface. The thermal blanket was operated at temperatures ranging from 815 to 925 °C. It took about 20-24 h for a 15-cm depth to reach 200 °C, which was sufficient to reduce the total PCB concentration to below the mandated 2 mg/kg cleanup levels. The variation of times to reach the desired temperature is principally related to soil water content. A vapor stream was drawn by vacuum from the thermal blanket at a rate of 550-1100 L(STP)/min. Vaporized groundwater constitutes from 40% to 70% of the vapor stream at the beginning of each heating cycle and therefore displaces a significant fraction of the air, but enough remains for oxidation of waste stream hydrocarbons in an external thermal oxidizer. We also tested 2.4 m x 6 m thermal blanket modules that could be assembled into large arrays to treat extensive areas. A full- scale thermal blanket system designed to treat this site would consist of 20 modules assembled into 288 m ² thermal treatment systems, which require 1.8 MW of power and operate on a three-part cycle: heat and treat soil cool, and move to a new treatment area. Emissions from the blanket consist of organics, which are destroyed by passing through a flameless thermal oxidizer.