Kinetics of microbial bromate reduction in a hydrogen-oxidizing, denitrifying biofilm reactor

Bromate (BrO(3)(-)) is an oxidized contaminant produced from bromide (Br(-)) during ozonation and advanced oxidation of drinking water. Previous research shows that denitrifying bioreactors can reduce bromate to innocuous bromide. We studied a hydrogen-based, denitrifying membrane-biofilm reactor (MBfR) for bromate reduction, and report the first kinetics for a hydrogen-based bromate reduction process. A mixed-culture MBfR reduced up to 1,500 microg/L bromate to below 10 microg/L with a 50-min hydraulic residence time. Kinetics were determined using short-term tests on a completely mixed MBfR at steady state with an influent of 5 mg N/L nitrate plus 100 microg/L bromate. Short-term tests examined the impact of pH, nitrite, nitrate, and bromate on bromate reduction rates in the MBfR. Kinetic parameters for the process were estimated based on the short-term bromate tests. The q(max) for bromate reduction was 0.12 mg BrO(3)(-) x mg(x)(-1) x day(-1), and the K was 1.2 mg BrO(3)(-)/L. This q(max) is 2-3 times higher than reported for heterotrophic enrichments, and the K is the first reported in the literature. Nitrite and nitrate partially inhibited bromate reduction, with nitrite exerting a stronger inhibitory effect. Bromate was self-inhibitory at concentrations above 15 mg/L, but up to 50 mg/L of bromate had no inhibitory effect on denitrification. The optimum pH was approximately 7. We also examined the performance of an MBfR containing pure culture of the denitrifying bacterium Ralstonia eutropha. Under conditions similar to the mixed-culture tests, no bromate reduction was detected, showing that not all denitrifying bacteria are active in bromate reduction. Our results suggest the presence of specialized, dissimilatory bromate-reducing bacteria in the mixed-culture MBfR.