Applications of the MFI-UF

182 183 The MFI-UF was developed to include smaller colloidal particles not measured in the existing SDI and MFI 0.45 fouling indices. This research investigates its application to measure and predict the particulate fouling potential of reverse osmosis (RO) feedwater. MFI-UF measurements were carried out under constant pressure filtration at the IJssel Lake and River Rhine RO pilot plants of the influent feedwater and after pretreatment processes e.g. coagulation, sedimentation, conventional filtration, ultrafiltration etc. Using the MFI-UF results, the pretreatment efficiency was evaluated and a comparison made with the SDI, MFI 0.45 , and particle counts. The MFI-UF of the influent feedwater was circa 700-2400 higher than the corresponding MFI 0.45 and SDI, due to the retention of smaller particles. A pretreatment efficiency of >80%, was found by MFI-UF at both plants. For the larger particles the MFI 0.45 gave a 90-≈100% reduction. Minimum predicted run times for a 15% flux decline from MFI-UF measurements were shorter than that observed at the IJssel Lake plant. This was most likely due to almost negligible particle deposition in the RO systems and/or particle removal from the cake formed under cross flow. Moreover, it was shown that cake resistance increased with ionic strength in MFI-UF tap water experiments and therefore, a correction of the MFI-UF index is required for salinity effects in RO concentrate. Finally, it was suggested that the MFI-UF be carried out under constant flux (CF) filtration to more closely simulate fouling in RO systems. Preliminary experiments were promising, the MFI-UF could be determined under CF filtration within ≈2 hours for the low and high fouling feedwater examined. 184