Recycling rainwater by submerged gravity-driven membrane (GDM) reactors: Effect of hydraulic retention time and periodic backwash.

Rainwater recycling has been considered as an alternative cost-effective decentralized water supply. The low cost and effective gravity-driven membrane (GDM) filtration technology has been introduced to treat the rainwater prior use. In this study, we investigated the effects of hydraulic retention time (HRT; 27 h, 51 h, and 156 h) and periodic backwash durations (2 min, 5 min, 10 min, and 30 min per 2-3 days' filtration) on the permeate quality, flux and fouling mechanism in lab-scale submerged GDM reactors. Compared to the performance at HRT of 51 h (40% of DOC removal and ~2.9 L/m h), better permeate quality and higher membrane flux were achieved at HRT of 27 h (51% of DOC removal and ~4.2 L/m h) and 156 h (48% of DOC removal and ~5.0 L/m h). Although the hydraulically reversible resistance was predominant (up to 90% of the total fouling resistance), the permeate flux could not be fully recovered by periodic backwash, regardless of the backwash durations. After several filtration-backwash cycles, the stabilized flux of GDM reactor with backwash was even worse than those without backwash. However, no correlation can be established between the stabilized flux (i.e., cake layer resistance) and the soluble organics and microbial cells in the cake layer of the GDM system during rainwater treatment.