Comammox Nitrospira-driven sustainable mainstream nitrification

Complete ammonium oxidation (Comammox) is recognized as a promising alternative to canonical two-step nitrification, owing to its inherent merits of decreased aeration demand and limited nitrous oxide (N₂O) generation. However, its sustainable advantages and disturbance resilience await full validation within complex microbial communities. Herein, we evaluated the nitrification performance and stability of activated sludge reactors hosting comammox Nitrospira at a typical relative abundance (2%-8%). The comammox oxygen kinetics were quantified (K_s=0.05 mg O₂/L) for the first time in mixed communities. Based on this, parallel long-term operations were conducted using comammox and canonical nitrification systems under a stepwise aeration reduction strategy (30 to 10 mL/min), with the influent ammonium concentration maintained at 50 mg NH₄⁺-N/L to mimic the mainstream conditions. The comammox-mediated system consistently outperformed the control (canonical nitrification), achieving 21%-74% higher ammonia removal, and 22%-29% higher oxygen utilization efficiency, as well as reducing the N₂O emission factors by 8%-27%. Notably, comammox Nitrospira remained the predominant nitrifying population, and successfully endured the dual stress of oxygen limitation and elevated ammonium exposure (up to 15 mg NH₄⁺-N/L), thereby extending its in-situ ammonium-specific niche breadth to 10 mg NH₄⁺-N/L in mainstream conditions. Molecular ecological network analysis further revealed its system-level robustness, i.e., under limited aeration, canonical nitrifying consortia suffered functional decoupling between ammonia- and nitrite-oxidizing bacteria, whereas comammox-mediated community sustained complete nitrification. These results demonstrate that comammox Nitrospira can sustain stable functional activity at a typical relative abundance within activated sludge under mainstream operational conditions. Such strong niche adaptability effectively alleviates the risk of competitive exclusion in nutrient-rich wastewater, establishing a theoretical foundation for mainstream nitrification technology towards energy-saving, low-carbon wastewater treatment.