Functional and compositional characteristics of nitrifiers reveal the failure of achieving mainstream nitritation under limited oxygen or ammonia conditions

[Display omitted] •Oxygen or ammonia limited condition was adverse to achieve nitrite accumulation.•Different AOB with similar oxygen sensitivity were enriched in the two reactors.•Nitrospira defluvii dominated both reactors but had different oxygen affinity.•Ammonia and nitrite levels were the key factors for selection of the nitrifiers. For understanding the potential of achieving nitritation under different oxygen and ammonia levels, two activated sludge reactors with high (RH) and low (RL) dissolved oxygen (DO) were parallelly operated. During over two months continuous operation, rare nitrite accumulation was observed in both reactors. K-strategists Nitrosomonas oligotropha and r-strategists Nitrosomonas europaea were enriched in the RH and RL, respectively, yet their response to DO variations was almost identical. Although K-strategists Nitrospira defluvii dominated both reactors, species cultured with low-DO exhibited higher oxygen affinity. Instead of DO, ammonia and nitrite availability should be the key factor for the selective enrichment of these nitrifiers. Taken together, the limiting ammonia for ammonia oxidizing bacteria and the better oxygen-uptake capacity of nitrite oxidizing bacteria wasrespectively responsible for the failure of nitrite accumulation in the RH and RL. This study supported that high DO coupled with excess ammonia would favor the achievement of mainstream nitritation.