Stability and microbial characteristics of simultaneous nitrification denitrification and phosphorus removal (SNDPR) granular sludge for treating real domestic wastewater

This study demonstrates the long-term stability and low-energy operation of a continuous-flow granular sludge system for simultaneous nitrification, denitrification and phosphorus removal (SNDPR) from real domestic wastewater. The impacts of distinct hydraulic retention time (HRT) and sludge retention time (SRT) combinations (9 h and 33 days, 6 h and 22 days, 4.5 h and 17 days) on system performance, granular sludge characteristics and microbial community dynamics (specifically archaeal and bacterial populations) were systematically evaluated. The results showed that the performance decreased with shortening of HRT. At an HRT of 4.5 h, the settling performance and stability of SNDPR granules deteriorated, while loosely bound proteins (LB-PN) and the protein-to-polysaccharide ratio (PN/PS) significantly increased. This suggests that exogenous proteins in real domestic wastewater, which were difficult to fully hydrolyse under short HRT, likely accumulated on the outer layer of the granules. High-throughput sequencing analysis of bacterial and archaeal communities revealed that under real domestic wastewater conditions, the abundance of denitrifying and phosphorus-removing functional microorganisms significantly decreased, while the rapid proliferation of filamentous microorganisms was identified as the primary factor contributing to the deterioration of granular sludge structure. Finally, the phylogenetic classification of functional genera indicates that archaeal and bacterial communities played significant roles in denitrification, phosphorus removal and maintaining the stability of SNDPR granules.