Long‐Term Low‐DO Operation Changes the Ammonia Oxidizer Community and Oxygen Kinetics

ABSTRACT

Low–dissolved oxygen (DO) biological nutrient removal is an attractive option for energy and cost reduction, but it is not common practice as critical outcomes remain uncertain. Open questions include whether ammonia‐oxidizer oxygen kinetics are fixed or can adapt to operating conditions; whether low DO increases simultaneous nitrification and denitrification (SND); whether good settling can be maintained; and whether N ₂ O emissions rise or fall at low DO. In this study, a continuously fed pilot anaerobic/oxic (A/O) process was run with 6.5‐ to 8.5‐day solids retention time (SRT), stable temperature, and constant influent flow, and the operating DO was decreased from 2.0 to 0.2 mg O ₂ /L over multiple months while maintaining complete nitrification. In situ ammonia uptake rate, ammonia oxidizer oxygen affinity (K _O ) (via two independent batch methods), quantitative PCR (qPCR)‐based community composition, floc size, settling, and SND were monitored throughout the transition to low DO. N ₂ O was measured before and after the low DO transition. Ammonia oxidizer K _O decreased significantly with reduced operating DO, driven primarily by a shift toward comammox Nitrospira (CMX), particularly Clade A, which comprised > 80% of the nitrifier community at the lowest operating DO. SND, measured as total inorganic nitrogen (TIN) removal across the aerobic zone, did not increase at lower DO within this well‐mixed A/O configuration. Settling performance remained acceptable across setpoints. The low‐DO sampling campaign had lower N ₂ O emissions than the high‐DO campaign, coincident with CMX enrichment. Total supplied airflow was reduced by 44% at 0.2 relative to 2.0 mg O ₂ /L, indicating reduced aeration demand.