Deciphering EPS and microbial roles in nitrogen removal and sludge characteristics in AGS systems under different...

The optimization of reactor operating parameters is critical for the engineering application of aerobic granular sludge (AGS) technology. Volume exchange ratio (VER) identified as a key parameter but the comprehensive effects of VER on AGS systems remain underexplored. In this study, two sequencing batch reactors were operated at VERs of 33 % and 50 % to systematically evaluate the influence of VER on pollutant removal, extracellular polymeric substances (EPS), microbial community and key metabolic activities. Results showed that the reactor operated with a 33 % VER exhibited higher ammonia (92.1 % ± 11.0 %) and total nitrogen (73.8 % ± 8.64 %) removal efficiencies, along with denser granule structures and better settling performance. Microbial analysis and functional gene prediction demonstrated that low VER facilitated the enrichment of ammonia-oxidizing bacteria and nitrification-related genes, thereby strengthening nitrification pathways. Although the high VER promoted tricarboxylic acid (TCA) cycle and EPS secretion, the reduced hydrophobic amino acid metabolisms weakened EPS hydrophobicity, negatively impacting sludge stability. This study reveals that VER affects the nitrogen removal and sludge characteristics of AGS system by regulating the microbial community and EPS, providing a theoretical basis and practical reference for the optimization of the AGS process and its engineering application.