Development of the Modified Fouling Index using Ultrafiltration Membranes (MFI-UF)

116 117 The existing Modified Fouling Index (MFI 0.45 ), based on cake filtration, uses a 0.45μπι microfilter to measure the particulate fouling potential of a feedwater. However, it is not sensitive to small colloidal particles. To incorporate these particles the MFI using ultrafiltration (UF) membranes was proposed. In this study a suitable reference membrane for the MFI-UF test was investigated using polysulphone (PS) and polyacrylonitrile (PAN) UF membranes of 1-100 kDa molecular-weight-cut-off in tap water experiments. The stability of the MFI-UF value over time for the PAN and PS membranes and the influence of MWCO on the MFI-UF obtained were examined. Field emission scanning electron microscopy (FESEM) of the membrane surfaces and an analysis using the constant pressure blocking and cake filtration models were carried out. A stable MFI-UF* was found for PAN membranes. Whereas, the MFI-UF continuously decreased over time for PS membranes. The measured MFI-UF* (2000-13 300 s/1 2 ) were significantly higher than the MFI 0.45 expected for tap water, (1-5 s/1 2 ), indicating smaller particles were retained. FESEM showed PAN membranes were homogeneously permeable, resulting in a stable MFI-UF* value as cake filtration was proven to be the dominant mechanism. FESEM showed the 1,2, and 5 kDa PS membranes were heterogeneously permeable and sparsely porous resulting in (i) artificially high MFI-UF*and (ii) limited cake filtration. Hence the MFI-UF never stabilised. The MFI-UF* appeared MWCO independent within the 3-100 kDa MWCO range as most likely the cake itself acts as a second membrane, determining the size of particles retained and the resultant MFI-UF*. 118