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Treatment of Petroleum-Contaminated Soils

Treatment Options (continued)

Thermal Desorption

While the goal of incineration is to oxidize contaminants in the soil, the goal of thermal desorption is to volatilize the contaminants from the soil. Figures 12a and 12b show a typical portable thermal desorption unit. After volatilization, contaminants in the gas phase are removed by a gas treatment system such as granular-activated carbon adsorption. Thermal desorption systems are classified by temperature. Low-temperature thermal desorption (LTTD) units operate between 90 and 320 °C. High-temperature thermal desorption (HTTD) units operate between 320 and 560 °C. Petroleum-contaminated soils are best treated with LTTD.

Figure 12a—Two views of a typical
portable thermal desorption unit.

Figure 12b.

Thermal desorption units can be purchased or rented. Units are shipped on several trailers (typically four trailers) and assembled on the site. The equipment weighs about 95,000 pounds. Thermal desorption vendors contacted for this study indicated that the units would require some type of foundation. Compacted soil or a gravel pad may be sufficient. The units require natural gas, diesel, or propane for combustion and three-phase electrical power for operation. Some units include three-phase generators, eliminating the need for an external power source. Typical fuel requirements are 18 to 35 gallons of diesel per hour. Water is also required during operation to rehydrate cuttings, reducing dust. Typically, 120 gallons of water per hour is needed to increase the moisture content of the cuttings to 15 percent. Collectively, vendors contacted for this study indicate that this process is very effective on diesel-contaminated soil and that thermal desorption units generate very little or no hazardous wastes (information provided by Enviro-Klean and On-Site Technology, appendix C).

Use in Cold, Wet, Remote Regions

The problems of operating thermal desorption units in cold regions seem to be minimal and correctable. Condensed water and water used for rehydration may freeze during operation. A professional engineer contacted during this study experienced soil heating problems while operating one of these units in Alaska. "Tar balls" formed at the bottom of the thermal desorption unit (information provided by Nortech Environmental & Engineering Consultants, appendix C). This problem could have been the result of heat lost out the bottom of the unit into the underlying geologic formation, resulting in a lower operating temperature. Insulation between the unit and the underlying formation may have reduced the heat transfer and the formation of "tar balls."

Thermal desorption is less efficient on soils with high water content. Extra effort is needed to keep the soil dry in regions with high precipitation. Thermal desorption units have been used near Ketchikan (an area of high rainfall) with relative success.

Because of the complexity of shipping these units, this type of treatment is only practical for accessible sites with large volumes of soil.

Cost Estimate

Items to be included in a cost estimate for thermal desorption are similar to those included for incineration. Table 10 lists the items for the cost estimate. The majority of the cost will probably be in the shipping and the pro-vided service. Table 10 was produced using the same assumptions as for table 9.

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