From Tech Trends January 1993

Organics Desorbed From Soil with Low Temperature Thermal Treatment

By Paul dePercin, Risk Reduction Engineering Laboratory

The Roy F. Weston, Inc., low temperature thermal treatment system (LT3) thermally desorbs organic compounds from contaminated soil without treating the soil to combustion temperatures. The LT3 can process a wide variety of soils with differing moisture and contaminant concentrations. Bench, pilot or full scale systems have been used to treat soil contaminated with coal tar, drill cuttings (oil-based mud), petroleum hydrocarbons, halogenated and nonhalogenated volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs), including polynuclear aromatic hydrocarbons. The LT3 was demonstrated under the Superfund Innovative Technology Evaluation Program (SITE) at the Anderson Development Company (ADC) site in Adrian, Michigan, where the soil was contaminated with VOCs and SVOCs, including 4,4-methylene-bis(2-chloroaniline) (MBOCA).

The system is divided into three main areas of treatment: soil treatment, emissions control and water treatment. The thermal processor for soil treatment consists of two jacketed troughs, one above the other. Each trough houses four intermeshed screw conveyors. A front-end loader transports feed soil (or sludge) to a weigh scale and deposits the material onto a conveyor that discharges into a surge feed hopper located above the thermal processor. The surge hopper is equipped with level sensors and provides a seal over the thermal processor to minimize air infiltration and contaminant loss. Heat transfer fluid (typically hot oil) from the burner circulates through the hollow screws and trough jackets as the soil moves across the upper trough, drops to the second trough and exits the processor at the same end that it entered. Thus, each screw conveyor mixes, conveys and heats the contaminated soil during treatment. Soil is discharged from the thermal processor into a conditioner where it is sprayed with water to reduce the temperature and to minimize fugitive dust emissions. An inclined belt conveys the treated soil to a truck or pile.

The hot oil, or heat transfer fluid, used above is heated in the burner to an operating temperature of 400 to 850 F (about 100 F higher than desired soil temperature). Combustion gases released from the burner are used as sweep gas in the thermal processor, where a fan draws the sweep gases and desorbed organics from the thermal processor through a fabric filter baghouse. Exhaust gas from the filter is drawn into an air cooled condensor to remove most of the water vapor and organics and then through a second, refrigerated condensor to further lower the temperature and reduce the moisture and organic content of the off-gases. Electric resistance heaters then increase the off-gas temperature to approximately 70 F to optimize the performance of the vapor-phase activated carbon column which removes any remaining organics. The condensate stream is typically treated in a three-phase oil-water separator to remove light and heavy organics from the water, which is then treated in a carbon adsorption system to remove residual organic contaminants.

At the ADC SITE demonstration, the LT3 removed VOCs to below method detection limits (less than 0.060 milligrams per kilogram [mg/kg]) for most compounds, from initial concentrations of about 50 mg/kg. MBOCA removal efficiency was greater than 88%; concentrations in the treated sludge ranged from 3.0 to 9.6 mg/kg gas opposed to 43.6 to 860 mg/kg in untreated sludge. All SVOCs, with two exceptions, decreased in concentration in the sludge. Low levels of dioxins and furans were formed in the LT3 system. The majority of these were recovered or treated by the gas and liquid residuals treatment system. The sampling results will be discussed in the Technical Evaluation Report and the Application Analysis Report which will be available in early 1993. For more information, contact Paul dePercin at EPA's Risk Reduction Engineering Laboratory at 513-569-7797.