BIOREMEDIATION OF SELENIUM OXYANIONS IN SAN-JOAQUIN DRAINAGE WATER USING THAUERA-SELENATIS IN A BIOLOGICAL REACTOR SYSTEM

This report describes the use of a new selenate-respiring bacterium, T hauera selenatis, for the bioremediation of selenium (Se, as selenate) in drainage water from the Westlands Water District, San Joaquin Vall ey, The organism respires selenate anaerobically using acetate as the preferred electron donor. The reduction of selenate is not inhibited b y nitrate; both electron accepters are reduced concomitantly. T. selen atis was inoculated into, and was maintained in, a biological reactor system for anaerobic treatment of selenate-nitrate containing drainage water; a population of denitrifying bacteria was also present. When t he pH of inflowing water was 6.9, and 2 mM acetate plus 0.56 mM ammoni um chloride were fed into the reactor, selenate/selenite levels were r educed from 350-450 mu g Se/l to 5.39 +/- 3.6 mu g Se/l. The final pro duct of selenate reduction was elemental Se. Analysis of reactor conte nts revealed that T. selenatis was the only selenate-respiring organis m present in the system. Nitrate in the drainage water was also reduce d in the reactor system by 98%. The lab-scale biological reactor syste m consisted of recycled sludge-blanket (1 l; 400 g sand) and fluidized -bed (1 l; 300 g sand) reactors. At a system flow rate of 6.5 ml/min, the retention time was 140 min.