Mechanisms underlying differences in nitrogen removal characteristics of anammox granular sludge immobilization...

The anaerobic ammonium oxidation (anammox) process serves as a green, low-carbon, and sustainable wastewater nitrogen removal technology. However, anammox processes based on granular sludge systems face structural instability issues, resulting in the loss of anammox bacteria (AnAOB). In this study, an immobilized filler system for AnAOB based on hydrogel encapsulation was constructed using anammox granular sludge (AnGS) of varying particle sizes (G1: < 0.10 cm, G2: 0.10-0.20 cm, G3: 0.20-0.32 cm, and G4: > 0.32 cm). The differences in nitrogen removal characteristics, microstructures, and pathways of these AnGS immobilized fillers were thoroughly investigated. The large particles (G3 and G4) immobilized fillers showed the best nitrogen removal performance, with total nitrogen removal rates reaching 0.591 kgN·m^-3·d^-1 and 0.615 kgN·m^-3·d^-1, respectively. The G2 immobilized filler was more conducive to the self-growth and enrichment of AnAOB, and the absolute abundance and relative abundance of Candidatus Brocadia increased by 23-fold and 9.20-fold, respectively. Conversely, the G1 immobilized filler displayed the poorest nitrogen removal rate due to insufficient microbial growth. The G1-G3 immobilized fillers possessed uniform and dense small pores, whereas the G4 filler featured uniform large pores. Metagenomic analysis confirmed that the abundance of denitrification-related genes in G3 and G4 immobilized fillers was highest, indicating that anammox and denitrification synergistically achieve nitrogen removal, whereas G2 immobilized fillers mainly relied on anammox. This study provides a crucial theoretical basis and technical guidance for optimizing the application of anammox immobilized filler systems.