Efficient Nitrification and Low-Level NO Emission in a Weakly Acidic Bioreactor at Low Dissolved-Oxygen Levels Are Due to Comammox.

Nitrification is an essential process for nutrient removal from wastewater and an important emission source of nitrous oxide (NO), which is a powerful greenhouse gas and a dominant ozone-depleting substance. In this study, nitrification and NO emissions were tested in two weakly acidic (pH 6.3 to 6.8) reactors: one with dissolved oxygen (DO) at over 2.0 mg/liter and the other with DO at approximately 0.5 mg/liter. Efficient nitrification was achieved in both reactors. Compared to that in the high-DO reactor, NO emission in the low-DO reactor decreased slightly, by 20%, and had insignificant correlation with the fluctuations of DO ( = 0.935) and nitrite ( = 0.713), indicating that NO might not be produced mainly via nitrifier denitrification. Based on quantitative PCR (qPCR), quantitative fluorescent hybridization (qFISH), and functional gene amplicon and metagenome sequencing, it was found that complete ammonia oxidizers (comammox), i.e., organisms, significantly outnumbered canonical ammonia-oxidizing bacteria (AOB) in both weakly acidic reactors, especially in the low-DO reactor with the comammox/AOB gene ratio increasing from 6.6 to 17.1. Therefore, it was speculated that the enriched comammox was the primary cause for the slightly decreased NO emission under long-term low DO in the weakly acidic reactor. This study demonstrated that the comammox can survive well under the weakly acidic and low-DO conditions, implying that achieving efficient nitrification with low NO emission as well as low energy and alkalinity consumption is feasible for wastewater treatment. Nitrification in wastewater treatment is an important process for eutrophication control and an emission source for the greenhouse gas NO. The nitrifying process is usually operated at a slightly alkaline pH and high DO (>2 mg/liter) to ensure efficient nitrification. However, it consumes a large amount of energy and chemicals, especially for wastewater without sufficient alkalinity. This paper demonstrates that comammox can adapt well to the weakly acidic and low-DO bioreactors, with a result of efficient nitrification and low NO emission. These findings indicate that comammox organisms are significant for sustainable wastewater treatment, which provides an opportunity to achieve efficient nitrification with low NO production as well as low energy and chemical consumption simultaneously.