Enhancing performance in gravity-driven membrane systems through pre-coating with aluminum-based flocs: Mechanism and energy saving analysis

The gravity-driven membrane (GDM) system is an energy-efficient and environmentally sustainable water purification process; however, after prolonged operation, its membrane flux remains relatively low, making it necessary to adopt effective strategies for improving system performance. In this study, the effects of hydrostatic pressure (60, 100, 200 mbar) and pre-coating with aluminum-based flocs (ABF) on GDM flux and organic matter removal were investigated, and the regulatory mechanisms of the bio-cake layer were explored through interactions between morphological structure, composition and microbes. The results showed that the stable flux of the GDM-ABF system at a hydrostatic pressure of 60 mbar was almost equal to that at 100 mbar, and it outperformed higher hydrostatic pressure in organic matter removal, resulting in a more porous bio-cake layer structure. GDM-ABF system at 60 mbar achieved 38.51% energy saving compared to that at 100 mbar. Increased hydrostatic pressure led to a denser biofouling layer and higher EPS concentrations, whereas pre-coating reduced the EPS concentration and resulted in a looser biofouling layer. Hydrostatic stress and pre-coating determined membrane fouling by regulating microbial communities and key metabolites. Increasing hydrostatic pressure down-regulated arginine and proline metabolism and aggravated membrane fouling, while pre-coating ABF up-regulated arginine and proline metabolism, down-regulated galactose metabolism, and alleviated the membrane fouling. Hydrostatic stress and pre-coating altered the abundance of keystone species involved in extracellular polymeric substances (EPS) formation within the bio-cake layer. Pre-coating with ABF at low hydrostatic pressure can achieve stable flux and effective water purification in GDM systems, similar to high hydrostatic pressure conditions, with the added benefits of being more environmentally friendly and low-carbon. This study proposes a strategy to balance flux and energy consumption in GDM systems, providing theoretical and technical support for the efficient application of GDM technology in membrane water treatment processes.