Electroactive carriers promote simultaneous nitrification and denitrification (SND) coupled with enhanced biological phosphorus removal (EBPR) via synergistic microbial enrichment, metabolic intensification, and electron transfer facilitation

Integrated fixed-film activated sludge (IFAS) systems incorporating Fe-C and FeMn-C-based electroactive sponge carriers achieved removal efficiencies 95.6% for COD, 82.8% for total nitrogen, and 99.9% for total phosphorus. The porous structure and negatively charged surfaces of electroactive materials facilitated localized NH₄+ enrichment at the carrier-biofilm interface (16%–20% effect), thereby enhancing nitrification efficiency. FeMn-C further promoted electron transfer, strengthened nitrification–denitrification coupling, and increased electron transport system activity. Batch denitrification experiments, together with Mn valence transformation revealed by X-ray photoelectron spectroscopy, suggested that Mn-associated redox processes may supplement mixotrophic denitrification as an auxiliary pathway. Concurrently, Fe/Mn precipitates served as microbial scaffolds, and together with improved hydrophilicity, promoting biofilm development and biomass retention. Accordingly, carriers in electroactive systems exhibited 1.6- and 3.5-fold higher EPS secretion and enhanced NH₄+-N oxidation and NO₃−-N reduction rates, with the latter reaching 11.8- and 16.3-fold higher levels on Fe-C and FeMn-C carriers, respectively, compared to normal carriers. These enhancements reshaped the microbial community, promoting the co-enrichment of key functional groups involved in SND and enhanced biological phosphorus removal (EBPR) (e.g., Nitrospira, Nitrosomonas, Thauera, Azonexus), along with EET-related taxa. Functional gene analysis showed increased abundance of nitrification (amoA/B/C, hao), denitrification (narG/I, nirK, norC), phosphorus metabolism (ppk1, pstA, adk), and EET-related genes (nuo, mtrA/C, pilA, cyc2, mtoB, omcS). However, electroactive carriers also increased nxrA/B abundance, indicating non-selective stimulation of nitrite oxidation and a potential risk of nitrate accumulation. These findings provide mechanistic insights into electrochemically enhanced C-N-P removal via SND-EBPR coupling in IFAS systems.