Natural organic matter (NOM) rejection by, and flux-decline of, nanofiltration (NF) and ultrafiltration (UF) membranes

Nanofiltration (NF) and ultrafiltration (UF) membranes performance was investigated in terms of natural organic matter (NOM) rejection and associated fouling. This study focused on (1) demonstrating the effects of both hydrophobic and electrostatic interactions between NOM and the membrane surface, (2) determining influential factors in NOM rejection and flux-decline, (3) determining major fouling materials, and (4) developing NOM-rejection prediction equations and flux-decline (NOM adsorption) models.; NOM characterization was performed in terms of MW distribution (for size), humic content (for hydrophobicity), and acidity measurement (for charge density). For some NOM characterization, XAD-8 and XAD-4 resins were used to isolate or fractionate hydrophobic/hydrophilic acids and neutrals/bases. Clean and NOM-fouled membranes were characterized in terms of MWCO (for pore size), contact angle (for hydrophobicity), and zeta potential (for surface charge). NOM and membrane characterizations were evaluated in terms of several mechanisms and interactions that may influence NOM rejection and flux-decline by membranes.; A cross-flow filtration and a dead-end unit were used for NOM rejection and fouling tests with a wide range of NF/UF membranes and various NOM-source waters. A simple equation with a flux-decline coefficient was formulated to describe flux-decline trends, and a series resistances model was used to represent different resistances for flux-decline. NOM rejection was predicted by an empirical equation with independent parameters including specific UVA and a f/k ratio (a hydrodynamic operating condition). NOM-fouled membranes were cleaned by a caustic cleaning agent to determine various resistances and were measured in terms of contact angle, zeta potential, and FTIR spectrum.; Results indicated that electrostatic repulsion between NOM acid components and the membrane surface was an influential factor in increasing NOM rejection and decreasing flux-decline. The hydrodynamic operating condition (f/k ratio) was also an influential factor for optimizing membrane performance. Hydrophilic NOM (particularly hydrophilic neutrals such as polysaccharides) was found to be major foulants for typical NOM-source waters, being proved by MW distribution comparisons, XAD resin fractionations, and ATR-FTIR.