Deciphering the Role of Low Superficial Gas Velocity (SGV) in Integrated Fixed-Film Activated Sludge (IFAS) System under Aniline Stress: Effects on Microbial Assembly and Electron Transfer Process

Integrated fixed-film activated sludge (IFAS) systems provide an energy-efficient method for nitrogen removal in aniline wastewater treatment. However, the effects of low SGV on microbial community dynamics and electron transfer under aniline stress in continuous-flow IFAS systems remain insufficiently understood. Herein, we systematically evaluated IFAS performance under varying SGVs (0.15, 0.10, and 0.04 cm/s) in treating 400 mg/L aniline wastewater. Aniline removal remained consistently high (>99%) across all conditions, while the total nitrogen removal efficiency declined from 82.65% at 0.15 cm/s to 46.58% at 0.04 cm/s. Reduced SGV, a key determinant of dissolved oxygen (DO), induced metabolic stress on microbial consortia and suppressed nitrification by reducing ammonia-oxidizing bacteria (AOB) abundance and downregulating amoA and hao. Community assembly analyses revealed a shift from deterministic selection at higher SGVs to stochastic processes (ecological drift and dispersal limitation) at lower SGVs. Microbial compositional shifts were observed, with Actinobacteria (aniline degraders) enrichment at reduced SGVs. Across all conditions, biofilms demonstrated a dominant role in nitrogen removal over suspended sludge. Electron transfer adaptations exhibited a strategic microbial response, characterized by the recovery of related functional gene abundance under lower SGVs.