Effect of high electron donor supply on dissimilatory nitrate reduction pathways in a bioreactor for nitrate removal.

Abstract The possible shift of a bioreactor for NO 3 − removal from predominantly denitrification (DEN) to dissimilatory nitrate reduction to ammonium (DNRA) by elevated electron donor supply was investigated. By increasing the C/NO 3 − ratio in one of two initially identical reactors, the production of high sulfide concentrations was induced. The response of the dissimilatory NO 3 − reduction processes to the increased availability of organic carbon and sulfide was monitored in a batch incubation system. The expected shift from a DEN- towards a DNRA-dominated bioreactor was not observed, also not under conditions where DNRA would be thermodynamically favorable. Remarkably, the microbial community exposed to a high C/NO 3 − ratio and sulfide concentration did not use the most energy-gaining process.