Exploring Community and Kinetic Shifts in Nitrifying Microbial Communities in Low Dissolved Oxygen Activated Sludge Facilities for Energy-Efficient Biological Nitrogen Removal

The discovery of the complete ammonia oxidation process (comammox) has challenged conventional nitrification theory, showing microbial adaption to very low dissolved oxygen (DO) concentrations. This study aimed at investigating the effects of different DO concentrations using a series of bioreactors inoculated with biomass from three operationally diverse water resource recovery facilities. Results show that microbial populations adapted to low DO environments can maintain high rates across a range of DO concentrations, indicating their ability to function well even at high DO concentrations. Additionally, long solids retention times (>10 days) can encourage the persistence of comammox populations adapted to different DO concentrations. Molecular analyses revealed that the low DO-facility had a nitrifying population with similar ratios of comammox clades A and B, while the high DO facility was dominated by clade A. Modeling results suggest that the nitrifying population including comammox bacteria from the low DO facility has a different half-saturation coefficient for DO (e.g., 0.05 mg L^–1) and possible intrapopulation diversity within clades A and B. This study highlights that a changing nitrification community can enable the activated sludge process to operate effectively at low DO concentrations, leading to low-energy biological nitrogen removal.