All the possible mechanisms and factors affecting flux decline, natural organic matter (NOM) rejection, and NOM transport were studied in this paper. Electrostatic exclusion (charge repulsion) was determined to be an influential factor in minimizing flux decline and membrane fouling, and in maximizing NOM rejection; the effective MWCO (membrane molecular weight cutoff) concept was used to support the notion of electrostatic interactions and to describe an apparent MWCO by a charged membrane. This allowed for a negatively-charged tight ultrafiltration (UF) membrane to be comparatively compared to a nanofiltration (NF) membrane in terms of NOM rejection. The f/k ratio, relating relative back-diffusional transport to convective transport, was identified as a major influential factor in terms of flux decline and NOM rejection. The effect of hydrophobicity on flux decline was not significant when the same f/k ratio was used for different NOM-membrane pairs. The convectional and diffusional transportation of NOM was also evaluated for NF and UF membranes using existing thermodynamic models.

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