Electroactive FeMn-C carriers enhance C-N-P removal in low-temperature IFAS sewage treatment system via microbial regulation and metabolic reprogramming

Low-temperature impairs biological nutrient removal by suppressing microbial activity and destabilizing sludge structure. To address this challenge, Fe-C and FeMn-C electroactive carriers were incorporated into IFAS systems to enhance simultaneous C-N-P removal at 15 °C. Low-temperature-induced nitrification inhibition caused DO accumulation, which was mitigated by higher-frequency intermittent aeration, achieving total nitrogen removal efficiencies of 70.0%, 73.6%, and 77.0% in the normal hydrophilic polyurethane sponge, Fe-C, and FeMn-C systems, respectively, while COD removal (>96%) and effluent phosphorus (<0.1 mg/L) remained stable. Structurally, low-temperature triggered excessive EPS secretion in flocs, leading to reduced VSS concentration and biomass loss. Electroactive carriers redirected EPS production toward carrier-biofilms, enhancing sludge structural stability and biomass retention, and increasing nitrogen transformation potential. IFAS systems exhibited a spatial functional differentiation, with denitrifiers and PAOs enriched in flocs and nitrifiers preferred carriers, while cold stress reinforced this partitioning by inducing Nitrosomonas migration to carriers. Metagenome revealed FeMn-C carriers-induced metabolic reprogramming under cold stress. Carbon metabolism shifted from TCA cycle toward the glyoxylate shunt, accompanied by enhanced gluconeogenesis and PHA accumulation, supporting energy conservation. Nitrogen metabolism transitioned from oxygen-sensitive nar-type to oxygen-tolerant nap-type nitrate reduction pathways, while phosphorus metabolism was enhanced via increased abundances of phosphate regulation and transport. Moreover, electroactive carriers promoted intracellular electron generation, flavin-mediated indirect electron transfer, and direct extracellular electron transfer via transmembrane electron conduits, together with the enhancement of quorum sensing modules indicated enhanced redox coupling and coordinated microbial cooperation. Overall, this study revealed FeMn-C electroactive carriers-driven metabolic and spatial adaptations at low temperature.