A loading rate switch strategy for stable nitritation in mainstream municipal wastewater

Implementation of biological ammonia-to-nitrite conversion (nitritation) is crucial to more sustainable municipal wastewater treatment. However, achieving stable nitritation, especially in low-temperature mainstream municipal wastewater, remains a global challenge. Here we develop a loading rate switch strategy to enable ultrastable nitritation in a pilot-scale (6.5 m³) reactor treating real mainstream municipal wastewater. This strategy consists of a stage with high organic loading rate to decrease both ammonia-oxidizing bacteria and nitrite-oxidizing bacteria amounts and a stage in which the organic loading rate sharply decreases to enhance ammonia-oxidizing bacteria and promote their bioactivity while containing nitrite-oxidizing bacteria. By implementing this strategy, nitritation initiated within 7 days and a high nitrite accumulation ratio of ~98.1% was maintained for >450 days, even at temperature as low as 5.9 °C. During the long-term operation, nitrite-oxidizing bacteria were below the detection level, whereas ammonia-oxidizing bacteria remained abundant. Moreover, the average concentrations of effluent ammonium and nitrite were kept at 8.3 and 13.8 mg N l⁻¹, respectively, favouring further nitrogen removal by anammox process, an inherently low-carbon and low-energy process. Overall, this work presents a viable and environmentally friendly strategy for nitrogen removal, paving the way to sustainable sewage management.