Rapid start-up and nitrogen loading response of single-stage partial nitritation-anammox system using multi-channel...

Partial nitritation-anammox (PN/A) faces engineering challenges, including prolonged start-up, difficult bacterial enrichment, and instability under fluctuating loads. In this study, a novel airlift multi-channel internal circulation bioreactor (AMICR) with a multi-stage internal circulation design was operated continuously for 253 days to investigate system response mechanisms during nitrogen loading rate increase after rapid start-up, focusing on nitrogen removal, sludge characteristics, and microbial community evolution. The AMICR reactor achieved rapid start-up within 80 days, establishing a stable PN/A pathway, and during stepwise ammonia increase from 200 to 1100 mg/L exhibited excellent load adaptability, achieving 86.52% total nitrogen removal efficiency and 2.835 kg N/(m³·d) nitrogen removal rate. High-throughput sequencing analysis revealed progressive enrichment of anaerobic ammonium-oxidizing bacteria (AnAOB) and synergistic coexistence of Nitrosomonas, Candidatus Brocadia, and Candidatus Jettenia. The multi-channel recirculation structure of the AMICR enabled rapid start-up, likely supported dissolved oxygen (DO) stratification and efficient sludge recirculation, and facilitated niche differentiation and metabolic complementarity among functional bacteria, thereby achieving robust nitrogen removal and stable granulation under increasing nitrogen loads for practical high-strength wastewater treatment.