From Tech Trends August 1994

Ex Situ Bioremediation of TNT, Dinoseb & Other Pesticides/Herbicides

By Wendy Davis-Hoover, Risk Reduction Engineering Laboratory

The J. R. Simplot Ex-situ Anaerobic Bioremediation System is a technology designed to destroy nitroaromatic compounds without forming any toxic intermediates.

The process can successfully operate with bioreactor temperatures as low as 18 degrees Centigrade, much below optimum. The EPA SITE (Superfund Innovative Technology Evaluation) Program has evaluated the Simplot system's destruction of the herbicide, dinoseb (2-sec-butyl-4,6 dinitrophenol), at Bowers Field, an airport in Ellensburg, Washington with a site contaminated by crop dusters. The SITE program has also evaluated the destruction of TNT (2,4,6-trinitro- toluene) at the Weldon Spring site in St. Louis, Missouri.

In the Bowers Field demonstration, the Simplot process reduced the levels of dinoseb by greater than 99.88%, to below detection limits, from an average pre-treatment concentration of 28 milligrams per kilogram (mg/kg) of feed soil. No known toxic products were found in the analysis of the post- treatment samples. Other pesticides (nitroanaline, malathion, atrizine and parathion) were also reduced in the treated slurry.

In the Weldon Spring demonstration, TNT was reduced by greater than 95% to below 1 ppm for most soil samples, from an average pretreatment soil concentration of 1,500 ppm on a dry weight basis. No known toxic metabolites were seen; and, toxicity tests are ongoing.

Usually under aerobic conditions, degradation of nitroaromatic compounds form products that are toxic. However, the Simplot system treats contaminated soils (or liquids) using an anaerobic consortium of soil microorganisms. Anaerobic degradation of nitroaromatics takes place without the formation of these toxic products.

The J. R. Simplot technology mixes a carbon source with contaminated soil and then adds water and a phosphate buffer to create a buffered slurry. This prompts aerobic microorganisms to consume the carbon source and oxygen, thus lowering the redox potential of the slurry and creating anaerobic conditions. Anaerobic microorganisms then consume the targeted toxins present in the slurry.

The mechanics of the process begin with excavated soil being sent through a vibrating screen to remove large rocks and other debris. The rocks and debris are washed to remove surface contamination. For the dinoseb process, this rinse water is combined with make-up water and added to the bioreactor. (TNT is not very water soluble; thus, these rocks are treated in a different manner.)

For both dinoseb and TNT, soil which has been seived is added to the bioreactor with water to provide one liter of water for each kg of soil to be treated. A phosphate buffer is added to the system to control the pH to around 7 pH. Batches of soil and potato starch (2% by weight) are homogenized together and added to the bioreactor until the system is 75% full. According to laboratory studies, optimum conditions for dinoseb degradation are temperatures of 30 and 35 degrees Centigrade with a pH between 7 and 7.5. The optimum temperature range for TNT is also 30 and 35 degrees Centigrade with an optimum pH of 6 to 7.

The technology is suitable for soils and liquids contaminated with nitroaromatic compounds. However, the medium to be treated must be free of toxic metals or any other compounds that may be detrimental to the appropriate microorganisms.

An Innovative Technology Evaluation Report describing the complete dinoseb demonstration and other pertinent information will be available in the fall of 1994. An Innovative Technology Evaluation Report describing the TNT demonstration will be available in the winter of 1995. For more information call Wendy Davis-Hoover at EPA's Risk Reduction Engineering Laboratory at 513-569-7206.