The combined effect of dissolved oxygen and nitrite on N₂O production by ammonia oxidizing bacteria in an enriched nitrifying sludge

Both nitrite (NO₂^-) and dissolved oxygen (DO) play important roles in nitrous oxide (N₂O) production by ammonia oxidizing bacteria (AOB). However, few studies focused on the combined effect of them on N₂O production by AOB as well as the corresponding mechanisms. In this study, N₂O production by an enriched nitrifying sludge, consisting of both AOB and nitrite-oxidizing bacteria (NOB), was investigated under various NO₂^- and DO concentrations. At each investigated DO level, both the biomass specific N₂O production rate and the N₂O emission factor (the ratio between N₂O nitrogen emitted and the ammonium nitrogen converted) increased as NO₂^- concentration increased from 3mgN/L to 50mgN/L. However, at each investigated NO₂^- level, the maximum biomass specific N₂O production rate occurred at DO of 0.85mgO₂/L, while the N₂O emission factor decreased as DO increased from 0.35 to 3.5mgO₂/L. The analysis of the process data using a mathematical N₂O model incorporating both the AOB denitrification and hydroxylamine (NH₂OH) oxidation pathways indicated that the contribution of AOB denitrification pathway increased as NO₂^- concentration increased, but decreased as DO concentration increased, accompanied by a corresponding change in the contribution of NH₂OH oxidation pathway to N₂O production. The AOB denitrification pathway was predominant in most cases, with the NH₂OH oxidation pathway making a comparable contribution only at high DO level (e.g. 3.5mgO₂/L). © 2015 Elsevier Ltd.