Application of multiple modified fouling index (MFI) measurements at full-scale SWRO plant

Recently, a modified fouling index (MFI) is published on ASTM, and thus, is expected to become a standard method for evaluating fouling potential, as a supplement to silt density index (SDI). Employing membranes with smaller pore size in MFI measurements has been continuously suggested in many previous studies. However, there was a lack of practical studies verifying the usefulness of MFI. In this study, the applicability of multiple MFIs (i.e., MFI_0.45, MFI-UF_{100 kDa} and MFI-UF_{10 kDa}) were evaluated at a full-scale UF/RO seawater desalination plant located in the Middle East. For a year, MFIs were measured with RO feed water and the results were correlated with increments in differential pressure (DP) of RO process. Regression analysis was conducted in order to find out whether the fouling index could predict RO performance deterioration. The observed MFIs clearly showed correlation with DP variation, while SDI and water quality parameters (e.g. turbidity and UV₂₅₄) exhibited relatively low accordance. Particularly, MFI-UF_{100 kDa} was the most sensitive, indicating that colloidal fouling is the main cause of fouling in SWRO desalination. The regression results were reconfirmed through specific events where feed water quality was severely changed due to red tide or ship movement at intake. These events provided unconventional feed water characteristics, but similar trend was observed that MFI-UF_{100 kDa} was most sensitive. The application of MFI in an actual plant provided valuable contribution for the further advancement of MFI measurements.