Membrane filtration of natural organic matter: initial comparison of rejection and flux decline characteristics with ultrafiltration and nanofiltration membranes

Two source waters containing natural organic matter (NOM) with different physical and chemical characteristics were crossflow-filtered using four types of membranes having different material and geometric properties. Transport measurements of NOM rejection and flux decline were made. A resistances-in-series model was used to represent and quantitatively compare membrane flux decline and recovery. As anticipated, the resistance due to specific adsorption depended on the concentration at the membrane interface. For the two membranes showing evidence of NOM adsorption, reducing the initial flux (which we infer to also reduce the interfacial NOM concentration) also lowered the measured resistance assigned to adsorption in our protocol. Relative molecular mass (RMM) distribution measurements (by size exclusion chromatography) were used to calculate the average RMM of the NOM and persuasively illustrated that the nominal relative molecular mass cut-off (MWCO) of a membrane is not the unique predictor of rejection characteristics for NOM compounds. Size exclusion, electrostatic repulsion, and NOM aromaticity all influenced the NOM rejection. For a given water composition (including pH and ionic strength), membrane characteristics (such as the surface charge, hydrophobicity and nominal MWCO) can be combined with the NOM properties (such as total dissolved organic carbon, specific UV absorbance at 254 nm and humic content) to provide a consistent qualitative rationale for the transport results.