Effect of membrane property and feed water organic matter quality on long-term performance of the gravity-driven membrane filtration process

This study investigated the influence of membrane property and feed water organic matter quality on the permeate flux and water quality during gravity-driven membrane (GDM) filtration. GDM filtration was continuously carried out over 500 days at hydrostatic pressure of 65 mbar in dead-end mode without any back-flushing or membrane cleaning. Three ultrafiltration (UF) membranes (PES-100 kDa, PVDF-120 kDa, and PVDF-100 kDa) and one microfiltration (MF) membrane (PTFE-0.3 μm) were tested for treating lake water with varied organic matter qualities due to algal growth. The fluxes of the four membranes rapidly decreased to ~8 L/(m² × h) within a week of GDM filtration. The flux variations were quite similar for the four membranes during the entire GDM filtration, indicating that membrane property has a little effect on the flux. The flux strongly depends on the feed water organic matter quality. The average flux in treating low organics containing water (7–60 days) was ~5 L/(m² × h) and decreased to ~2 L/(m² × h) in treating high organics containing water (60–300 days). The accumulation of algal-derived biopolymers was mainly responsible for the flux decline by forming biofilms with high permeation resistance. The average flux in 300–500 days increased to ~3.5 L/(m² × h) when the feed water contained lower levels of biopolymers and higher levels of easily biodegradable organics, which created open and heterogeneous biofilms with lower permeation resistance. Removal efficiency for Escherichia coli was more than 5 log, while the removal efficiency for total bacterial cells was 1 log–2 log for the four membranes, indicating some bacterial regrowth after the filtration. Removal efficiency for the MS2 phage was 2.4 log and 1.5 log for the fouled PES-UF and PTFE-MF membranes.