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Magnetic properties driving nitrogen removal improvement in magnetite-enhanced activated sludge: Mechanistic insights
Jun Shi1, Luo-Kai Li1, Li-Han Lin1
1State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Urban Water Supply, Water Saving and Water Environment Governance in the Yangtze River Delta of Ministry of Water Resources, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, PR China; Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, PR China.
Abstract:
The magnetite-enhanced activated sludge (MEAS) process offers a promising in situ strategy for upgrading wastewater treatment plants (WWTPs) to meet increasing treatment demands and stricter discharge regulations. Unlike conventional materials, magnetite possesses intrinsic magnetic properties, yet their influence on biological treatment efficiency and microbial ecology remains underexplored. This study systematically evaluated three types of magnetite particles with varying properties, focusing on their roles in denitrification, sludge settling, and microbial responses. Batch experiments under low carbon-to-nitrogen conditions (C/N = 4.4) demonstrated that magnetite with high saturation magnetization (65.9 emu/g) achieved 79.3 ± 10.2 % nitrate removal, 3.3 times higher than the control. It reduced the sludge volume index (SVI) from 84.7 to 28.4 mL/g by promoting compact floc formation through extracellular polymeric substance (EPS) protein conformational changes and enhanced microbe-particle interactions. It also increased bio-capacitance of the sludge and achieved a 77.0 % increase in electron transport system activity (ETSA). Surface analysis confirmed that magnetite served as a passive electron mediator rather than actively participating in redox cycling. Metagenomic sequencing further demonstrated the selective enrichment of denitrifying and magnetotactic bacteria and enrichment of key nitrogen metabolism genes (narG, nirK, narK, narH). Validation in an anaerobic-anoxic-aerobic (AAO) reactor treating real municipal wastewater achieved NH4+-N and total nitrogen removal efficiencies of 98.7 % and 73.6 %, respectively, meeting stringent discharge limits. These results identify saturation magnetization as a critical parameter for selecting or engineering magnetite materials and provide mechanistic insights and engineering guidance for deploying MEAS as an efficient, retrofit-friendly technology for WWTP upgrading.
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