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 may accumulate. One strategy for NH removal is to treat aerobically the leachate outside the landfill to convert NH into NO. 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 N2. Nevertheless, in some cases we observed nitrate reduction without gaseous nitrogen production. Using a stepwise multiple regression models, H2S concentration was the unique parameter found to have a negative effect on N2 production. We then suspected that dissimilatory nitrate reduction to ammonium (DNRA) occurred in the presence of H2S. In order to verify this hypothesis, 15N nitrate injections were performed into microcosms containing different H2S concentrations. The ammonium 15N enrichment was measured using an elemental analyser coupled to an isotope ratio mass spectrometer. In the two microcosms containing the highest H2S concentrations, the ammonium was 15N enriched and at the end of the experiment all the added nitrate was converted into ammonium. For the two microcosms containing the lowest H2S concentrations, no 15N enrichment of ammonium was observed. This isotopic approach has allowed us to demonstrate that, in the presence of significant concentrations of H2S, denitrification is replaced by DNRA.