(Fluid Extraction-Biological Degradation Process)
The three-step fluid extraction-biological degradation (FEBD) process removes organic contaminants from soil (see figure below). The process combines three distinct technologies: (1) fluid extraction, which removes the organics from contaminated solids; (2) separation, which transfers the pollutants from the extract to a biologically compatible solvent or activated carbon carrier; and (3) biological degradation, which destroys the pollutants and leaves innocuous end-products.
In the fluid extraction step, excavated soils are placed in a pressure vessel and extracted with a recirculated stream of supercritical or near-supercritical carbon dioxide. An extraction cosolvent may be added to enhance the removal of additional contaminants.
During separation, organic contaminants are transferred to a biologically compatible separation solvent such as water or a water-methanol mixture. The separation solvent is then sent to the final stage of the process, where bacteria degrade the waste to carbon dioxide and water. Clean extraction solvent is then recycled for use in the extraction stage.
Organic contaminants are biodegraded in aboveground aerobic bioreactors, using mixtures of bacterial cultures capable of degrading the contaminants. Selection of cultures is based on site contaminant characteristics. For example, if a site is mainly contaminated with polynuclear aromatic hydrocarbons (PAH), cultures able to metabolize or cometabolize these hydrocarbons are used. The bioreactors can be configured to enhance the rate and extent of biodegradation.
Research continues on using bound activated carbon in a carrier system during the separation step. Bound activated carbon should allow high-pressure conditions to be maintained in the fluid extraction step, enhancing extraction efficiency and decreasing extraction time. Bound activated carbon should also limit the loss of carbon dioxide, thereby decreasing costs. The activated carbon containing the bound PAHs could then be treated in the biodegradation step by converting the carrier system to a biofilm reactor. These activated carbon carrier systems could then be recycled into the high-pressure system of the extraction and separation steps.
This technology removes organic compounds from contaminated solids. It is more effective on some classes of organics, such as hydrocarbons (for example, gasoline and fuel oils) than on others, such as halogenated solvents and polychlorinated biphenyls. The process has also been effective in treating nonhalogenated aliphatic hydrocarbons and PAHs.
This technology was accepted into the SITE Emerging Technology Program in June 1990. The Institute of Gas Technology has evaluated all three stages of the technology with soils from a Superfund site and from three town gas sites. These soils exhibited a variety of physical and chemical characteristics. Approximately 85 to 99 percent of detectable PAHs, including two- to six-ring compounds, were removed from the soils.
The measurable PAHs were biologically converted in both batch-fed and continuously fed, constantly stirred tank reactors. The conversion rate and removal efficiency were high in all systems. The PAHs were biologically removed or transformed at short hydraulic retention times. All PAHs, including four- to six-ring compounds, were susceptible to biological removal.
Results from this project were published in the Emerging Technology Bulletin (EPA/540/F-94/501), which is available from EPA. An article was submitted to the Journal of Air and Waste Management.
Potential users of this technology have expressed interest in continuing research. This technology has been invited to participate in the SITE Demonstration Program. The technology would be able to remediate town gas sites, wood treatment sites, and other contaminated soils and sediments.
EPA PROJECT MANAGER:
Annette Gatchett
U.S. EPA
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7697
Fax: 513-569-7620
TECHNOLOGY DEVELOPER CONTACT:
Robert Paterek
Institute of Gas Technology
1700 South Mount Prospect Road
Des Plaines, IL 60018-1804
847-768-0720
Fax: 847-768-0546