Influence of operation modes on gravity-driven membrane process in treating the secondary effluent: Flux improvement...

A low flux level of the gravity-driven membrane (GDM) process constrained its extensive application in treating the secondary effluent. In this study, different operation modes were introduced to the GDM process without aeration, backwashing, and chemical cleanings, hoping to develop simple and economic flux regulating strategies, and their influences on the filtration performances and biocake layer characteristics were systematically investigated. The results indicated that the stable fluxs in the intermittent GDM systems elevated by 40%-100% relative to the continuous GDM case, attributing to the synergetic effects of forming more permeable, mushroom-like structures and reducing the concentrations of EPS and SMP within biocake layers. The quantitative analysis of biocake layer properties suggested that the structural parameters of porosity and absolute roughness were closely related to the flux variation compared to the thickness and relative roughness. Besides, the intermittent GDM system generated an apparent detachment of the biocake layer from the membrane surface along with a persistent flux increase than in the continuous GDM case during long-term filtration, achieving its self-sustained operation in a higher flux level without any interferences. The periodical flux recovery and decline occurred daily in each intermittent GDM system since the biocake layer attached to the membrane surface was mainly reversible. Although there were no significant differences in removing dissolved organic pollutants under different operation modes, the manganese removals decreased by 0%-25% in the intermittent GDM filtrations compared to the continuous GDM scenario. The optimized daily operation mode was 16 h on / 8 h off (operation of 16 h, interruption of 8 h), considering the trade-off effects between membrane flux level and water production. These findings provide a new simply-feasible optimized GDM process operation strategy and benefit promoting the application of the GDM system in the reclamation of wastewater.