Multiscale Characterization of Anammox Granules and Microbial Migration Under Variable Nitrogen Loading Rates

The sustainable restoration of river and lake ecosystems requires advanced wastewater treatment technologies to control nitrogen pollution, a key driver of aquatic degradation. This study explores the physiological responses of anammox granular sludge (AnGS) to varying nitrogen loading rates (NLRs), offering insights into microbial stability under environmental stress. AnGS samples with different particle sizes (<1.0 mm, 1–2 mm, >2 mm) were subjected to NLRs ranging from 0.9 to 3.6 gN/L/d. As the NLR increased, the NO2−-N/NH4⁺-N consumption ratio rose from 1.0 to 1.2, and the most active particle size shifted to 1–2 mm. Hydroxyapatite (HAP) crystals formed at higher NLRs, enhancing the settling and activity of 1–2 mm AnGS but inhibiting larger granules (>2 mm). Microbial analysis revealed that Candidatus Brocadia dominated at high NLRs (10.5%), outperforming Candidatus Kuenenia (2.47%). The enrichment of these key genera across granules indicates adaptive microbial migration under loading stress. These findings provide critical operational strategies for sustaining AnGS performance through particle size regulation, contributing to nitrogen control solutions vital for river and lake restoration efforts.