(723e) Evaluating the Impacts of Fouling Index Values on Reverse Osmosis Membrane Fouling

In reverse osmosis (RO) applications, silt density index (SDI) and modified fouling index (MFI) have been most widely applied for assessing a fouling potential and evaluating the requirement of pre-treatment. However, their limitations have been reported by many researchers so far.

In the present study, the impacts of membrane fouling indices on reverse osmosis membrane fouling were investigated systematically. Bench-scale fouling experiments with thin-film composite (TFC) RO membranes were performed at various combinations of particle size, particle concentration, and membrane type. Mono-dispersed silica suspensions with mean diameters ranging from 0.1 to 20 µm were used as model colloids.

Existing fouling indices such as SDI and MFI showed the impact of various particle and membrane characteristics on their measurements. Specifically, fouling indices significantly increased as feed particle size decreased under identical particle loading, however, the effect of membrane materials on MFI were not markedly observed when using particles larger than 3.0 µm although MFI value for the MCE membranes was higher than PVDF membrane due to increased membrane resistance.

Prediction of MFI by using Happel cell model based on the hydraulic resistance of the particle cake layer was performed. Modeling of experimental data revealed no noticeable changes in cake layer structure when the RO membrane was fouled by inorganic particles larger than 1.0 µm particles. However, the model predicted MFI values did not exhibit good agreement with the experimental results for particles smaller than 1.0 µm.

Overall, based on the experimental results, when the inorganic particles were the primary cause of RO membrane fouling, both the SDI and the MFI values were not closely related to the true fouling potential of feed water. Therefore, fouling index values of SDI and MFI could not predict RO membrane fouling accurately and were not absolute numbers for the design of RO membrane processes.