Dissolved oxygen concentrations affect the function but not the relative abundance of nitrifying bacterial populations in full-scale municipal wastewater treatment bioreactors during cold weather

Highlights: • Nitrification was inhibited at low dissolved oxygen concentrations and temperatures. • Nitrification performance was continuous at higher dissolved oxygen concentrations. • Nitrifying biomass levels were sustained, independent of nitrification performance. • Nitrospira spp. were prominent year-round at higher dissolved oxygen concentrations. • Nitrotoga spp. were prominent during the summer but scarce during the winter. This study aimed to understand the effect of different dissolved oxygen (DO) concentrations on the abundance and performance of nitrifying bacteria in full-scale wastewater treatment bioreactors, particularly during the winter when nitrifying bacterial activity is often negligible. Biomass samples were collected from three parallel full-scale bioreactors with low DO concentrations (Nitrosomonas spp.) was curiously maintained. The relative abundance of nitrite oxidizing bacteria was similarly maintained, although there were substantial seasonal fluctuations in the relative abundance values of Nitrospira spp. versus Nitrotoga spp. This research suggests that nitrification activity can be controlled during the winter via DO to produce better effluent quality with high DO concentrations or to reduce aeration costs with a concomitant decline in nitrification activity.