The modified fouling index using ultrafiltration membranes (MFI-UF): characterisation, filtration mechanisms and proposed reference membrane

Abstract The existing modified fouling index (MFI0.45), based on cake filtration, uses a 0.45 μm microfiltration membrane to measure the particulate fouling potential of feedwater. To incorporate smaller colloidal particles into the MFI measurement, the application of 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 (MWCO) in tap water experiments. The stability of the MFI-UF value over time 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/l2) were significantly higher than the MFI0.45 expected for tap water, (1–5 s/l2), 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. Whereas, 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 range as most likely the cake itself acts as a second membrane, determining the size of particles retained and the resultant MFI-UF∗.