TREATING THE SOURCE OF PERCHLORATE-CONTAINING GROUNDWATER IMPACTING LAKE MEAD USING A HIGH PERFORMANCE BIOREACTOR SYSTEM

Perchlorate manufacturing in Henderson, Nevada has resulted in releases of perchlorate into groundwater and Lake Mead via the Las Vegas Wash. The plant was built in the late 1940s and produced ammonium and sodium perchlorate for ultimate use by the U. S. Department of Defense and the National Aeronautics and Space Administration (NASA) until 1998.Kerr-McGee began removing perchlorate from the groundwater in 1999 near the Las Vegas Wash using a pumping system coupled to ion exchange technology. In 2002, they installed additional wells, pumping systems and additional ion exchange capacity to capture and treat additional water near the perchlorate manufacturing sources. In December 2002, due to the high operating cost of using disposable ion exchange resin, Kerr-McGee initiated the design of a biological treatment process to replace the ion exchange system. A laboratory pilot program was initiated in early January 2003 to confirm the process design, and installation of the full-scale treatment system was completed in December 2003.The biological treatment process for the Kerr-McGee Henderson site involves the use of two trains of fluidized bed reactors (FBRs) in series. The FBR is a highly efficient fixed-film reactor that utilizes stationary microbes on a hydraulically fluidized bed of media particles. Sand is used as the fluidized bed media in the front train of four FBRs (each 14 ft. in diameter and 30 ft. in height), while carbon is used in the back train of four FBRs (14 ft. in diameter and 26 ft. in height). Perchlorate laden feed water is introduced to the front train of four FBRs, operated in parallel, at a total flow rate of 1,000 gpm, with an upper inlet perchlorate concentration of 400 mg/l. Chlorate and nitrate contaminants also are present in the feed water. The system design influent conditions are shown in Table 1. An electron donor (ethanol) is pumped into each of the front-train FBRs where it is utilized in anoxic, biological reduction processes. The byproducts of the processes are chloride ions, heat generation and additional biomass growth. All water from the front train of FBRs, along with any biomass that is separated from the media, combines together and enters the back train of four FBRs operated in parallel. In these reactors, any remaining concentration of perchlorate is further biologically treated to meet regulatory discharge limits. Contaminant free water from the back train of FBRs proceeds through an aeration system, two dissolved air flotation units and an ultraviolet ray disinfection system, before being released to the Las Vegas Wash. Solids recovery and filter press systems also are employed at the plant. Figure 6 shows the FBR plant layout and Figure 1 is a photograph of the first stage sand FBRs and electron donor storage tank.Operation of all of the components of the plant, as well as performance results, will be presented in detail. The FBR plant successfully completed a 35-day composite sampling performance test on November 6, 2004 at approximately 93 percent of the system's maximum design load. During the test all design water quality effluent conditions were met (Table 2).