Mechanisms of aerobic simultaneous nitrogen removal under low COD/N conditions: Diffusion-reaction coupling and...

Nitrogen removal from ammonium-rich, carbon-limited wastewater remains constrained in continuous-flow microgranular sludge systems. In this study, a three-stage up-flow self-recirculating microgranular sludge reactor was developed to investigate nitrogen removal mechanisms under low chemical oxygen demand to nitrogen ratios (COD/N < 2.5) and high influent total nitrogen (TN > 400 mg/L). During long-term operation, the system achieved stable removal efficiencies of ammonium (98%), TN (94%), and COD (95%). Under ammonium stress, particle size decreased to a mean diameter of 249.2 μm, forming stable, non-flocculent microaggregates. Microgranules < 0.2 mm exhibited pronounced simultaneous partial nitrification-denitrification (SPND) and simultaneous nitrification-denitrification (SND) activities under aerobic conditions. Simultaneous nitrogen removal (SNR) activity peaked at 0.52 g TN/(g VSS·d) at a DO of 2 mg/L. In contrast, microgranules > 0.2 mm primarily followed SND-dominated pathways. Their SNR activity increased with DO and reached a maximum of 0.46 g TN/(g VSS·d). Microbial community and metagenomic analyses revealed a redox-stratified functional structural organization. Rubrivivax (11.5%) dominated the surface layer, likely linking organic matter degradation with nitrogen oxide reduction. Hyphomicrobium (11.9%) was enriched in intermediate layers and was associated with SND. In the core, the co-enrichment of Hyphomicrobium (7.2%) and Methylotenera (6.1%) supported the coupling of SND and SPND processes. These findings provide a basis for improving nitrogen removal from ammonium-rich, carbon-limited wastewater.