Membrane filtration of natural organic matter: factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane

We studied natural organic matter (NOM) rejection and the membrane’s flux decline during natural water filtration using a charged ultrafiltration membrane based on thin-film-composite technology. NOM rejection mechanisms such as steric exclusion and aromatic/hydrophobic and charge interactions were considered. Water composition factors affecting NOM rejection and flux decline were investigated, including ionic strength, pH, and calcium ion concentration. The membrane’s effective relative molecular mass cutoff for the NOM in our study was between 1500 and 2300 (significantly lower than the manufacturer’s nominal value of 8000) and depended on the NOM characteristics in the source water. In particular the ratio of UV absorbance at 254 nm to dissolved organic carbon (related to the humic content) correlated with the rejection. Comparison of relative molecular mass distributions between fractionated NOM and recovered membrane foulants indicates that the foulants are the larger-sized neutral and/or basic NOM components, and not the humic substances that were efficiently rejected by this membrane.