Pre-coagulation of surface water for mitigating membrane fouling during submerged nanofiltration

Membrane fouling becomes a challenge in nanofiltration (NF) when pre-filtration, such as microfiltration or ultrafiltration, is not performed. This study aimed to identify the potential of pre-coagulation for minimizing membrane fouling during submerged NF membrane treatment. Pre-coagulation (i.e., rapid and slow mixing) before submerged NF treatment was performed by adding optimized doses of ferric chloride (FeCl 3 ) or polysilicate iron (PSI). The FeCl 3 -based pre-coagulation only slightly reduced membrane fouling compared to that without pre-coagulation, with a transmembrane pressure (TMP) increase of 9 kPa compared to 18 kPa over 12 days. In contrast, PSI-based pre-coagulation achieved a smaller TMP increase (7 kPa) compared to filtration without pre-coagulation (26 kPa) over 15 days. Interestingly, considerable deposition of foulants was observed on the surface of both NF membranes. Regardless of pre-coagulation, foulants on the membrane surface were removed using one-wipe sponge cleaning, fully restoring membrane permeance. Membrane characterization indicated that FeCl 3 or PSI pre-coagulation resulted in a fouling layer comprising iron, which had low hydraulic resistance. This study identified the potential of PSI-based pre-coagulation for considerably mitigating NF membrane fouling, which reduce the frequency of physical cleaning. • FeCl 3 pre-coagulation slightly mitigated fouling during submerged NF treatment. • In contrast, polysilicate iron (PSI) substantially mitigated membrane fouling. • Formation of a fouling layer with high iron content was observed. • The fouling layer caused low hydraulic resistance of the membrane. • PSI pre-coagulation can reduce chemical cleaning frequency.