SUCCESSFUL FULL SCALE BIOREMEDIATION OF A 500 ppm 1,1,1 TCA SOURCE
Sami A. Fam, Ph.D., P.E. L.S.P., David Falatko, P.E., Innovative Engineering Solutions, Inc.,
George Pon, Ph.D., Bioremediation & Treatability Center. Louis J. Burkhardt, P.G., Jonathan Hone, Ronald Slager, Raytheon Corporation
Abstract
Bioremediation of 1,1,1 TCA (TCA) is significantly more challenging than bioremediation of tetrachloroethene (PCE) and trichloroethene (TCE) series compounds. TCA transformation often occurs under complex methanogenic and sulfate reducing conditions (1). The source area at the project site (moderately permeable medium sand w/ a low hydraulic gradient) is approximately 0.5 acres and several wells contained 300 to 600 ppm of TCA prior to bioremediation. TCA contamination generally extends to 35 feet and decreases with depth. The area of treatment also contained 2-30 ppm of TCE from an upgradient source. Initial site groundwater conditions indicated minimal biotic dechlorination and the presence of up to 20 ppm nitrate and 90 ppm sulfate. Our microcosm testing indicated that TCA dechlorination is inhibited by the site's relatively low pH (5 to 5.5) and high TCA concentration. Once pH was adjusted and TCA concentrations were reduced to less than 35 ppm (by dilution with site water), dechlorination proceed rapidly using whey (or slower with sodium lactate) as an electron donor.
A full scale groundwater enhanced anaerobic dechlorination (EAD) recirculation (15-30 gpm; 6 extraction and 21 injection wells) system was installed in 2006 and has operated for approximately 1 year. Sodium lactate and whey have been used as electron donors. Groundwater pH was initially adjusted by addition of sodium bicarbonate and sodium hydroxide. We installed 4,000 lbs of granular activated carbon (GAC) within the recirculation stream. The GAC acted as a bioreactor, adsorbing TCA for sufficient time to allow for dechlorination on the carbon beds (recirculated groundwater contained amendments). The above ground bioreactor allowed injection of relatively clean (0.5 ppm) groundwater into the source area so as to flush it out and allow dechlorination to take hold. The GAC was never consumed (VOCs biologically degraded) and only operated for 5 months until a point where extracted groundwater concentrations were reduced to approximately 40 ppm (and then the GAC was removed). Currently (after 1 year), mixing low contaminant concentration groundwater with high contaminant concentration groundwater is the primary method of inhibition reduction. The EAD program has been quite successful to date and has reduced VOC concentrations in most wells and generated significant concentrations of chloroethane (CA) and 1,1 dichlorethane (DCA). Even in the most contaminated segment (well grouping), molar concentrations of CA and DCA now exceed TCA molar concentrations, whereas at the beginning of site remediation negligible CA and DCA concentrations were observed. To date, we have added approximately 30,000 lbs of carbon source electron donor. The project demonstrates techniques to effectively remove inhibition and the value of whey as an electron donor for TCA dechlorination.
(1) Kinetic of 1,1,1-Trichloroethane Transformation by Iron Sulfide and a Methanogenic Consortium.
Environ. Sci. Technol. 2002, 36, 4540-4546.
