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Treatment of Petroleum-Contaminated Soils

Treatment Options (continued)

Soil Washing

Soil washing is a water-based process for scrubbing soils ex situ to remove contaminants. Two mechanisms remove contaminants: particle size separation and dissolution into the wash water. As discussed previously, some metals and some organics have a tendency to bind to the clay and silt fraction in soils (table 5). A large proportion of the contaminants will be removed by simple particle separation and subsequent treatment or disposal. Contaminants can also be removed from the soil by adding different compounds, such as complexing agents, leaching agents, or surfactants to the wash water, or by adjusting the pH of the wash water. Treatment of the wash water will be required to remove the dissolved contaminants desorbed from the soil. A likely treatment technique for the wash water is granular activated carbon adsorption. Soil washing has been effective on petroleum-contaminated soils.

While commercial use of soil washing is uncommon, several companies have successfully used this technology to clean contaminated soils. The process is carried out in a reactor that provides proper mixing and settling. Figure 13 shows a typical soil washing reactor. Soil washing units may be configured in several ways. Specifications given in this report apply to a particular soil washing unit marketed by Biogenesis Enterprises (appendix C). The equipment weighs about 18,000 pounds. The unit is skid mounted and requires a grizzly (coarse screen) to remove the larger soil particles. The process requires three-phase electrical power. About 1,000 gallons of water per day (assuming an 8-hour workday) is used in the treatment process. Unlike the previous treatment technologies, no foundation is required for the soil washing unit. However, the unit must be placed on level ground.

Figure 13—Typical soil washing unit.

Use in Cold, Wet, Remote Regions

Because the washing process generates heat, soil can be washed at temperatures below 0 °C. If the unit is not operating, it could be damaged by water freezing inside it. Low temperatures should have only a minimal effect on the solubility of petroleum hydrocarbons. Cold water temperatures should not greatly reduce the performance of these systems.

In comparison to incinerators and thermal desorption units, soil washing devices are small and transportable. A backhoe will be required for loading the soil into the unit. Other than the difficulties of excavating wet soils, high precipitation should not hinder washing of contaminated soils.

Seawater is readily available in some remote regions of Alaska. It may be possible to use seawater for treating petroleum-contaminated soils by particle separation. Solids settle by gravity in liquids according to Stoke's Law (Fox and McDonald 1978). Stoke's Law was used to compare the settling characteristics of soil particles in seawater (ion concentration of 35 parts per million) to those in freshwater at 20 °C. Results from this simple calculation show that the settling velocity of soil particles in seawater is 96 percent of the settling velocity of the same diameter particle in freshwater. A longer detention time is required in the reactor when seawater is used. Soil washing reduces the mass of soil contamination by both particle separation and dissolution into the wash water. A limited review of the soil washing literature indicates that little is known about soil washing using seawater as the washing fluid. The use of seawater for dissolution treatment will require theoretical analysis and laboratory tests before conclusions can be drawn about its use for treating petroleum-contaminated soil.

Soil washing has been successfully used in Kotzebue, Kenai, and Anchorage, AK, with minimal problems (information provided by Biogenesis Enterprises, appendix C).

Cost Estimate

Soil washing units can be purchased or rented. Items to be included in a cost estimate for soil washing are similar to those for incineration and thermal desorption (table 11). Assumptions include:

• After soil washing, the treated soil can be used as fill material.
• The Forest Service owns the backhoe.
• The backhoe used for soil excavation can be used for loading soil into the soil washing unit and to remove treated material for fill.

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This page last modified September 23, 2002
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