[Enhanced Nutrient Removal and Microbial Community Structure in a Step-feed A²/O Process Treating Low-C/N Municipal...

To resolve the issue of insufficient influent carbon sources in existing municipal wastewater treatment plants (WWTPs) in China, a pilot-scale step-feed A²/O process was used to treat low-C/N (C/N<5) municipal sewage with five different inflow distribution ratios. In this study, the effects of influent flow distribution on the removal efficiencies of chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N), total nitrogen (TN), and total phosphorus (TP) were investigated. The results showed that optimal removal efficiencies of 89.41% for COD, 95.30% for NH₄⁺-N, 83.00% for TN, and 90.09% for TP were obtained at an inflow Q_PA:Q_AN:Q_A distribution ratio of 0.1:0.2:0.3. The activated sludge exhibited excellent settleability characteristics, showing a sludge volume index (SVI)<120 mL·g^-1 with an average volatile suspended solids (VSS) total suspended solids (TSS) ratio of 0.84 (Phase 5), and no filamentous bacteria bulking occurred during the 120-day experiment. Moreover, the microbial community structure in the oxic zone was detected by high-throughput sequencing. The results demonstrated that excellent nutrient removal and sludge settling performance of the system were closely related to the enrichment of six types of heterotrophic bacteria, four types of denitrifying bacteria, five types of polyphosphate-accumulating organisms (PAOs), and two types of zoogloea and the elimination of three types of filamentous bacteria.