A novel MABR-integrated system for sustained iron-mediated autotrophic nitrogen removal in municipal wastewater treatment

Achieving efficient autotrophic nitrogen removal from low C/N ratio wastewater remains a critical challenge. While the coupled Fe(III)-mediated ammonium oxidation (Feammox) and nitrate-dependent Fe(II) oxidation (NDFO) process emerges as a promising energy-efficient alternative, its engineering feasibility is fundamentally constrained by the absence of nitrate in actual wastewater. In this study, a Membrane-Aerated Biofilm Reactor (MABR) was introduced to achieve precise oxygen mass transfer, serving as an in-situ nitrate supply engine. Long-term operation (influent ammonia 28-100 mg/L, C/N < 2.0) maintained effluent ammonia and total nitrogen (TN) below 3 mg/L and 10 mg/L, respectively, achieving a maximum nitrogen removal rate exceeding 80.00 g N/m³/d. Batch experiments elucidated a spatial decoupling of metabolic pathways: nitrification primarily occurred within the MABR biofilm, whereas the coupled Feammox-NDFO process was confined to the strictly anoxic suspended sludge. Nitrogen mass balance based on uncertainty analysis indicates that autotrophic pathways contribute to at least more than half of TN removal. Furthermore, XPS and XRD analyses confirmed the dynamic coexistence of Fe²⁺ and Fe³⁺ species in the suspended sludge, providing robust physicochemical evidence for an active iron redox cycle. Microbial community analysis revealed a distinct spatial niche differentiation: nitrifying bacteria (Nitrosomonas, 5.41%; Nitrospira, 6.28%) were significantly enriched in the biofilm, whereas functional genera associated with Feammox (Ferruginibacter, 11.63%) and NDFO (Thauera, 12.22%, Denitratisoma, 8.90%) dominated the suspended sludge. Overall, these findings demonstrate that the MABR-driven spatial uncoupling strategy effectively sustains the Feammox-NDFO synergy, offering a solid theoretical foundation for its future large-scale application.