Improving MFI-UF constant flux to more accurately predict particulate fouling in RO systems: Quantifying the effect of membrane surface porosity

This study aimed to quantify the effect of membrane surface porosity on particulate fouling predicted by the MFI-UF method at constant flux. Firstly, the surface porosity of polyethersulfone UF membranes (5–100 kDa) was determined using ultra-high resolution SEM. Thereafter, the MFI-UF was measured using suspensions of polystyrene particles (75 nm), which were pre-washed to remove surfactant and particle fractions smaller than the pores of MFI-UF membranes, thus ensuring complete retention of particles during MFI-UF measurements. Consequently, the MFI-UF values of washed polystyrene particle suspensions were independent of the pore size and depended only on the surface porosity of MFI-UF membrane. The results showed that the membrane surface porosity decreased with MWCO from 10.5% (100 kDa) to 0.6% (5 kDa), and consequently the MFI-UF increased from 3700 to 8700 s/L2, respectively. This increase in MFI-UF was attributed to the non-uniform distribution of membrane pores, which is exacerbated as surface porosity decreases. Consequently, preliminary correction factors of 0.4–1.0 were proposed for MFI-UF measured with UF membranes in the range 5–100 kDa. Finally, the surface porosity correction was applied to predict particulate fouling in a full-scale RO plant. However, additional research is required to establish correction factors for different types of feed water.