Aluminum hydrolysis species as metabolic drivers: Novel insights into biofilm function regulation in gravity-driven membrane (GDM) systems.

Aluminum (Al) salt coagulation is frequently employed as a pretreatment strategy in gravity-driven membrane (GDM) filtration to alleviate fouling, however, its actual efficacy remains highly variable and occasionally counterproductive, with the underlying mechanisms largely unexplored. This study systematically revealed how Al hydrolytic species dictated biofilm functionality and overall GDM performance, shifting the conventional perspective from viewing Al as a simple foulant precursor to recognizing it as a decisive metabolic regulator. The formation of ε‑Al13 under high coagulant doses induced cytotoxicity, oxidative stress, and enzymatic inhibition, thereby suppressing energy metabolism and nitrogen transformation pathways. In contrast, optimized dosing promoted the generation of Al6 species, which established a low-mass-transfer-resistance, biocompatible microenvironment and acted as sustained-release nutrient carriers selectively enhancing nitrification, denitrification, and anammox activities, while curtailing extracellular polymeric substances (EPS) accumulation and inhibiting biofilm-to-biofouling transition. Based on these mechanistic insights, a micro-flocculation-GDM (MGDM) system was developed, achieving a flux approximately 2.3‑fold higher than conventional GDM alongside significantly improved pollutant removal (87.17% for NH4+‑N and 60.61% for UV254). These findings not only provided critical theoretical guidance for optimizing GDM operation but also established a refined species‑oriented biofilm regulation framework, offering a practical technological pathway for scalable engineering applications of GDM systems in surface water treatment.