Elucidation of nitrate reduction pathways in anaerobic bioreactors using a stable isotope approach

Leachate recirculation allows an increase of moisture content and the enhancement of the anaerobic digestion of wastes in landfill. Since there is no ammonia elimination process in landfill when leachate is recirculated, NH(4) (+) may accumulate. One strategy for NH(4) (+) removal is to treat aerobically the leachate outside the landfill to convert NH(4) (+) into NO(3) (-). When nitrified leachate is recirculated, denitrification should occur in the waste. We have previously shown that wastes have a large capacity to convert nitrate into N(2). Nevertheless, in some cases we observed nitrate reduction without gaseous nitrogen production. Using a stepwise multiple regression models, H(2)S concentration was the unique parameter found to have a negative effect on N(2) production. We then suspected that dissimilatory nitrate reduction to ammonium (DNRA) occurred in the presence of H(2)S. In order to verify this hypothesis, (15)N nitrate injections were performed into microcosms containing different H(2)S concentrations. The ammonium (15)N enrichment was measured using an elemental analyser coupled to an isotope ratio mass spectrometer. In the two microcosms containing the highest H(2)S concentrations, the ammonium was (15)N enriched and at the end of the experiment all the added nitrate was converted into ammonium. For the two microcosms containing the lowest H(2)S concentrations, no (15)N enrichment of ammonium was observed. This isotopic approach has allowed us to demonstrate that, in the presence of significant concentrations of H(2)S, denitrification is replaced by DNRA.