Research Thermal Blanket for In-Situ Remediation of Surficial Contamination: A Pilot Test I. R. E. S. G. G. H.
E. B. E. R. R. L. J.
T . I B E N , † W . A . E D E L S T E I N , * ,† SHELDON,† A. P. SHAPIRO,† UZGIRIS,† C. R. SCATENA,‡ BLAHA,§ W. B. SILVERSTEIN,| BROWN,| STEGEMEIER,⊥ AND VINEGAR4
GE Corporate Research and Development, P.O. Box 8, Schenectady, New York 12301, GE Real Estate and Construction Operation, P.O. Box 1561, King of Prussia, Pennsylvania 19406, Woodward-Clyde Consultants, 201 Willowbrook Boulevard, Wayne, New Jersey 07470, RT Environmental, 215 West Church Road, King of Prussia, Pennsylvania 19406, GLS Engineering, 5819 Queensloch Drive, Houston, Texas 77096, and Shell Development Co., P.O. Box 481, Houston, Texas 77001
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-m2 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 × 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
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this site would consist of 20 modules assembled into 288 m2 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.
Introduction Removal of organic contaminants from soils is an important environmental and economic issue. Ex-situ techniques such as excavation followed by landfill or incineration are expensive and are frequently met with public opposition. In-situ techniques are advantageous in that they minimize exposure of personnel to contaminants, offer complete onsite destruction of contaminants, and eliminate the need for transporting waste. Multistage processes such as soil washing are complicated and not robust. In-situ techniques include soil vapor extraction, bioremediation, and thermal desorption (1-7). PCBs are difficult contaminants to extract from soils. This is because PCBs are very insoluble in water, have a high boiling point (>300 °C), can be strongly adsorbed to the surface of soil particles, and can be trapped in the interstitial layers of expandable clays that are found in many soils (8). In this paper, we present data on a pilot study using a thermal heating blanket to remediate a near-surface PCBcontaminated soil. In the late 1970s, waste oil containing PCBs (principally Aroclor 1242) was sprayed on portions of a dragstrip site for dust control. Soil tests show that PCB concentrations greater than 2 mg/kg are found at a depth of 15 cm or less below surface grade. Maximum concentrations of PCBs in the surface soils were about 2000 mg/kg. The shallow depth of contamination makes this site ideal for treatment by thermal desorption using surface heating (1-3). Heat is applied to the soil surface using an electrically heated thermal blanket. Over time, the heat diffuses into the soil, first boiling off any water and then removing both the soil organics and the PCB contaminants as the heat propagates downward. In soils initially containing
