Membrane filtration of natural organic matter: comparison of flux decline, NOM rejection, and foulants during filtration with three UF membranes

Three ultrafiltration membranes [thin-film composite, polyethersulfone (PES), and sulfonated PES] which have different chemistries and nominal relative molecular mass cut-offs (MWCO), were compared in terms of their characteristics of flux decline, rejection of natural organic matter (NOM), and the adsorbed foulants, with two very different (relatively hydrophilic and hydrophobic) NOM-containing source waters. To facilitate comparison, the ratio (Jo/k) of the initial pure water flux Jo to the estimated boundary layer mass transfer coefficient k was used to define similar initial hydrodynamic operating conditions for the three different membranes. The membranes showed differences in NOM rejection when filtering hydrophilic NOM-source water, and also exhibited differences in flux decline with the hydrophobic NOM-source water. Flux decline and NOM rejection were quantified using a resistances-in-series model and effective molecular mass cut-off, respectively. Non-charged NOM fractions (hydrophilic and hydrophobic neutrals/bases) were found to be significant foulants for these negatively charged membranes.