Enhancing the relative abundance of comammox nitrospira in ammonia oxidizer community decreases NO emission in nitrification exponentially.

Comammox Nitrospira and canonical ammonia-oxidizing bacteria (cAOB) generally coexist in activated sludge. In present study, the effect of comammox Nitrospira on NO production during nitrification of activated sludge was investigated. Comammox Nitrospira and cAOB were separately enriched in two nitrifying reactors, with respective relative abundance of approximately 98% in ammonia oxidizer community. The NO emission factor (EF) of nitrification in comammox Nitrospira dominated reactor was 0.35%, consistently lower than that (2.2%) in cAOB dominated reactor. When increasing the relative abundance of comammox Nitrospira in ammonia oxidizer community, the NO EF of nitrification decreased exponentially, which suggested that comammox Nitrospira not only decreased NO production directly but also might have reduced NO yield by cAOB. When cAOB dominated the ammonia oxidizer community of sludge, decreasing pH to 6.3, lowering DO to less than 0.5 mg/L, and increasing nitrite concentration enhanced NO EF dramatically. When comammox Nitrospira became the dominant ammonia oxidizer, however, the NO EF correlated to nitrite insignificantly and a low DO of 0.2 mg/L and weakly acidic pH (6.3) decreased NO EF by approximately 70% and 60%, respectively. These results imply that enhancing the relative abundance of comammox Nitrospira in sludge is an effective way to reducing NO emissions and can also offset the promoting effects of acidic pH, low DO, and high nitrite concentration on NO production during nitrification.