Effects of Anammox Sludge Succession on Nitrogen Removal Performance and Sludge Characteristics.

To investigate the optimization mechanism of nitrogen removal in anaerobic ammonium oxidation (Anammox) systems under high nitrogen loading, an up-flow reactor was operated continuously for 120 days. By stepwise increasing influent nitrogen loading and combining sludge characterization with high-throughput sequencing, we systematically analyzed nitrogen removal efficiency, sludge physicochemical properties, and microbial community succession. Results showed that at a volumetric nitrogen loading rate of 1.43 kg N/(m3·d), ammonia nitrogen, nitrite nitrogen, and total nitrogen removal efficiencies reached 82%, 95%, and 80%, respectively, with the nitrogen conversion ratio approaching the theoretical stoichiometry. High-throughput sequencing revealed that dominant phyla (Proteobacteria, Planctomycetota, and Chloroflexota) remained present throughout operation but shifted in relative abundance. Under progressively increasing nitrogen loading, the dominant AnAOB shifted from Ca. Brocadia (initial 21.3%) to the high-substrate-tolerant Ca. Kuenenia (final 28.7%). These results support a stage-wise conceptual understanding of ecology-function-performance coupling and provide engineering reference for optimizing high-load Anammox systems.