From electron supply to electron economy: a unified framework for N₂O control in membrane aerated biofilm reactors

Membrane aerated biofilm reactors (MABRs) create a unique counter-diffusion biofilm through bubble-free aeration, spatially decoupling oxygen penetration from bulk substrate diffusion and thereby reshaping electron competition among nitrogen transformation pathways. This reactor-specific redox stratification offers a distinctive platform for selective nitrous oxide (N₂O) control under low-carbon-to-nitrogen (C/N) conditions. Here, an Electron Economy Framework (EEF) is proposed to reinterpret nitrogen removal and N₂O dynamics in MABRs from the integrated perspectives of electron source expansion and pathway prioritization. Within this framework, existing strategies are unified into three synergistic dimensions: (i) inorganic electron source expansion using hydrogen, reduced inorganic sulfur compounds, and iron-based materials to alleviate electron scarcity; (ii) electron demand and pathway regulation through ammonia diversion by PN/A, algal assimilation for nitrification bypass, and bioelectrochemical electron channeling to reduce reductive burden or selectively suppress N₂O formation; and (iii) EEF based on the electron expansion and pathway regulation was proposed to quantify electron allocation toward target denitrification pathways, thereby improving electron utilization efficiency and mitigating N₂O emissions. This framework bridges reactor architecture with electron resource management, providing a theoretical basis for low-carbon, high-efficiency nitrogen removal with minimized N₂O emissions in next-generation biological treatment systems.