Identification of key parameters and microbial community modulation mechanisms in magnetite-enhanced activated sludge process

Municipal wastewater treatment plants (WWTPs) are facing mounting pressure from rising treatment demand and increasingly stringent discharge limits. The magnetite-enhanced activated sludge (MEAS) process offers a promising pathway for in situ WWTP upgrading, yet its key operating parameters and microbial mechanisms remain unclear. This study systematically evaluated MEAS performance under various operating conditions using continuous-flow anaerobic–anoxic–oxic (AAO) reactors. The results identified sludge-to-magnetite ratio (S/M) as a critical parameter: at S/M = 1:1, TN removal efficiencies reached 90.5 % (C/N = 10) and 82.8 % (C/N = 5), 1.8- and 1.7-fold higher than the control; meanwhile, NH 4 + -N removal remained ∼99 % under all conditions, which was 1.6 times higher than the control. In addition, magnetite addition also demonstrated rapid start-up and strong resilience to shock loads and influent fluctuations. The process was further verified using real wastewater and achieved 72.4 % TN and 98.6 % NH 4 + -N removal, alongside stable COD (95.2 %) and TP (96.0 %) removal performance, all meeting Class A discharge standards in Jiangsu Province. Metagenomic sequencing revealed that magnetite addition restructured the microbial community by selectively enriching key functional taxa while inhibiting potentially problematic groups. The relative abundance of core nitrogen-transforming phylum Proteobacteria increased by 4–5 % across the three functional units in the AAO reactor; in contrast, filamentous Chloroflexi declined by 3–6 %. These shifts indicate that magnetite fosters a more functionally integrated microbial network linking nitrification, denitrification, and internal carbon cycling, thereby reinforcing both treatment efficiency and process stability. These findings establish both the operational basis and microbial mechanisms of MEAS process, providing a technically feasible strategy for sustainable WWTP upgrading.