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

Dissolved oxygen (DO) is commonly recognized as an important factor influencing nitrous oxide (N₂O) production by ammonia-oxidizing bacteria (AOB). However, it has been difficult to separate the true effect of DO from that of nitrite, as DO variation often affects nitrite accumulation. The effect of DO on N₂O production by an enriched nitrifying sludge, consisting of both AOB and nitrite-oxidizing bacteria (NOB), was investigated in this study. Nitrite accumulation was minimised by augmenting nitrite oxidation through the addition of an enriched NOB sludge. It was demonstrated that the specific N₂O production rate increased from 0 to 1.9 ± 0.09 (n = 3) mg N₂O–N/hr/g VSS with an increase of DO concentration from 0 to 3.0 mg O₂/L, whereas N₂O emission factor (the ratio between N₂O nitrogen emitted and the ammonium nitrogen converted) decreased from 10.6 ± 1.7% (n = 3) at DO = 0.2 mg O₂/L to 2.4 ± 0.1% (n = 3) at DO = 3.0 mg O₂/L. The site preference measurements indicated that both the AOB denitrification and hydroxylamine (NH₂OH) oxidation pathways contributed to N₂O production, and DO had an important effect on the relative contributions of the two pathways. This finding is supported by analysis of the process data using an N₂O model describing both pathways. As DO increased from 0.2 to 3.0 mg O₂/L, the contribution of AOB denitrification decreased from 92% − 95%–66% − 73%, accompanied by a corresponding increase in the contribution by the NH₂OH oxidation pathway.