From Tech Trends September 1996

Electrocoagulation Removes Metals From Wastewater

By Dr. Ronald C. Sims, Utah State University

Research funded through the Great Plains/Rocky Mountain Hazardous Substance Research Center will help engineers in designing prepared bed treatment units for soil contaminated with polycyclic aromatic hydrocarbons (PAHs) and pentachlorophenol (PCP). The research found that using prepared bed bioremediation technology and maintaining a minimal supply of oxygen in contaminated soil (lifts) applied to the bed will make it possible to apply a new lift of contaminated soil more quickly. This will significantly reduce the time required to complete remediation in prepared bed treatment systems. The research used soils from the Champion International Site, a former wood preserving facility in Libby, Montana, which used creosote and PCP.

A treatment bed is prepared by lining it with clay or plastic to retard the transport of contaminants from the site or by adding uncontaminated soil to provide additional treatment medium. Treatment is generally enhanced with physical and chemical methods that stimulate the activity of indigenous microorganisms, including fertilizing, tilling, controlling moisture and adjusting pH. A prepared bed system functions as a treatment unit for unsaturated soil as well as an ultimate disposal site for the remediated soil.

A two-year microcosm evaluation of Libby soils was conducted by Dr. Ronald Sims of Utah State University to determine effects of oxygen concentration in the soil gas on biodegradation rates. This was followed by a field study at Libby where, after a lift of contaminated soil [an amount of soil generally 9 to 12 inches deep and applied across the land treatment unit (LTU)] was treated to target remediation levels, another lift of contaminated soil was added and treated until target remediation levels were reached. To manage the soil, nutrients and moisture were added periodically and the soil was tilled to increase oxygen transfer into the prepared bed. Chemical analyses indicated that the target remediation levels were achieved at each level of soil. In the evaluation, a new lift of contaminated soil was placed on top of another one in a LTU only after the first lift reached target remediation levels for individual chemicals. However, information about the decrease in oxygen, obtained during the evaluation, indicated that it might be possible to place a new lift on top of a lift before the latter has reached target remediation levels, but has been treated to some significant level. This led them back to the laboratory microcosm where it would be possible to measure only biodegradation in relation to a decrease in oxygen by measuring mineralization (which cannot be measured in the field) as an indicator of biodegradation. The microcosm study demonstrated that significant mineralization occurred once oxygen concentration was available at a concentration of 2 to 5%. Additional enrichments of the soil with oxygen did not produce significant increases in mineralization rates. The researchers confirmed that there is a relationship between the concentration of oxygen in the soil in buried lifts and the rate of PCP and PAH disappearance. They are now in the process of going back to the field to enhance buried lifts to 2 to 5% oxygen to increase the rate at which the PCPs and PAHs degrade. These results will be used to further define the potential for decreasing the time required to treat soil in LTUs.

In the full-scale prepared bed system, initial concentrations of contaminants and % reduction were: 101.4 milligrams per kilogram (mg/kg) PCP reduced by 58% by day 54; 84.9 mg/kg pyrene reduced by 49%; 204.0 total carcinogenic PAH reduced by 50%. Oxygen concentrations were 1-17% at 1 foot, 3-20% at 2 ft., 0-21% at 3 ft. In the second laboratory microcosm study average concentrations before application of another lift and after application of another lift were: 150 mg/kg PCP to 20 mg/kg 420 days after application (and 120 days after covering); 175 mg/kg pyrene to 3 mg/kg; 32 mg/kg PAH compound (benzo(b)fluoranthrene) to 5 mg/kg.

As part of the research effort, a guidance manual on the use of prepared beds for the bioremediation of contaminated soils will be developed. Additionally, an interactive computerized decision support system incorporating the information contained in the guidance manual will be developed.

For more information on the research results, call Dr. Ronald Sims at Utah State University at 801-797-2926 or send e-mail to: rcsims@cc.usu.edu. To get on a mailing list for information about availability of the manual and computer decision support system projected for May 1997, send a FAX to Dr. Sims at 801-797-3663 or an e-mail at address above.