A systematic approach to determine the fouling index for a RO/NF membrane process

A membrane fouling index such as the silt density index (SDI) and modified fouling index (MFI) is an important parameter in design and operation of reverse osmosis (RO) and nanofiltration (NF) membrane processes for desalination. Nevertheless, SDI and MFI often fail to predict actual membrane fouling due to their limited capability to measure fouling potential by colloidal deposition and organic adsorption. In this study, a novel method to determine fouling index was attempted using a set of different membrane filters for better prediction of RO/NF fouling potential. Three MF/UF membranes were used for MFI tests in order to increase sensitivity to fouling potential of particles, hydrophobic organics, and sub-micron colloids. The fouling indices determined using different membrane filters were compared with RO filtration results. Experimental results indicated that fouling indices using different test membranes showed variable results for untreated and pretreated waters. Nevertheless, no single method was sufficient for accurate measurement of fouling potential. Accordingly, a new fouling index (CFI: combined fouling index) was developed by combining various fouling indices. The CFI exhibited good agreement with experimental data compared with other fouling indices such as SDI, MFI, and MFI–UF.