Effects of biological activity and feed water characteristics on the flux stabilization of gravity-driven membrane: Perspectives from biocake structure and mass balance

Gravity-driven membrane (GDM) filtration could be operated without any cleanings, has captured increasing attentions in decentralized drinking water treatment. However, the mechanism of flux stabilization during GDM long-term filtration was not comprehensively understood. This study aimed at elucidating the mechanism of flux stabilization in GDM process from the perspective of biocake layer structure and organic mass balance. The results revealed that bacteria played the fundamental role in flux stabilization and level of GDM by both engineering a rough and heterogeneous structure of biocake layer, and also hydrolyzing the organic matters on the membrane surface to maintain the dynamic balance of total organics in biocake layer. Furthermore, protozoa & metazoa would not influence the flux stabilization of GDM, but benefited to improving the stable flux level (from 4.51 L m⁻²h⁻¹ to 8.68 L m⁻²h⁻¹) by engineering a more porous and permeable biocake layer through the bio-predation. Regarding non-biodegradable organics (e.g., humic substance), it could be hydrolyzed into low-molecular organics to penetrate the membrane to reduce organics accumulation in the biocake layer, and achieved the mass balance, resulting in flux stabilization (7.54 L m⁻²h⁻¹). However, easily biodegradable organic (e.g., sodium acetate) would promote excessive microbial growth and EPS secretion, leading to a relatively lower stable flux of GDM system (5.75 L m⁻²h⁻¹). Therefore, the flux stabilization of GDM process was determined by the biological action-based dynamic balance of both the structural properties and organic mass in the biocake layer. Overall, these findings will be beneficial to promote GDM extensive application in the complex decentralized water supply.