Low dissolved oxygen nitrification through kinetic selection

• Low DO operation enables energy-efficient nitrification without infrastructure upgrades. • Long-term low DO exposure shifts nitrifier communities toward comammox and AOA dominance. • Nitrate production closely matches ammonia removal under low DO, indicating complete nitrification by CMX. • Comammox enrichment correlates with high oxygen affinity and improved nitrification performance. • Lower N₂O emissions observed in low DO systems due to reduced AOB/NOB activity. Recent research in wastewater treatment demonstrates that activated sludge plants can be operated more efficiently in terms of energy and carbon utilization without the need for new infrastructure through the implementation of low dissolved oxygen (DO) operation. The aim of this study was to understand how microbial communities adapt to long-term low DO operations and the implications for nitrification. This study synthesized findings from bench-scale and full-scale experiments to assess the impact of low DO operation on nitrification rates, microbial community structure, and nitrous oxide (N 2 O) generation. Long-term exposure to low DO conditions led to a shift in the nitrifier community structure, favoring comammox bacteria (CMX) and, in some cases, ammonia-oxidizing archaea (AOA) over canonical ammonia-oxidizing and nitrite-oxidizing bacteria (AOB, NOB). In conventional high-DO systems, the ratio of nitrate production rate to ammonia removal rate is approximately 0.78, reflecting the lower growth rate of NOB compared to AOB. However, in the low DO facilities studied, this ratio approached 1.0, indicating that nearly all ammonia removed was directly converted to nitrate. This finding strongly supports the dominance of CMX which can perform complete ammonia oxidation in a single organism. The correlation between increased CMX abundance and increased nitrate production rates was consistent across facilities operating at different DO levels. These adapted communities demonstrated higher oxygen affinity compared to AOB and NOB from plants operated at high DO concentrations. Long-term exposure of biomass to low DO concentration may have resulted in a decrease in N 2 O emissions since there is a low relative abundance of AOB and NOB, limiting N 2 O production via the hydroxylamine oxidation pathway and nitrifier denitrification by AOB.