The influence of natural organic matter and cations on fouled nanofiltration membrane effective molecular weight cut-off

Abstract

This study investigated the influence of water matrix characteristics, specifically natural organic matter (NOM) and divalent cations, on fouled nanofiltration (NF) membrane rejection by size exclusion. Membrane bioreactor effluent, Lake Ontario water and laboratory-prepared waters were filtered and the membrane effective molecular weight cut-off (MWCO) was measured. Analysis of the results for the laboratory-prepared waters showed that NOM caused a statistically significant reduction in effective MWCO (from 385 ± 13 Da to 343 ± 12 Da) whereas neither the influence of cations nor the interaction of NOM and cations was significant. The MWCO of the NF membrane was reduced from 385 ± 13 Da to 222 ± 46 Da and 348 ± 28 Da following filtration with the MBR effluent and Lake Ontario water, respectively. A previous study by the authors [A.M. Comerton, R.C. Andrews, D.M. Bagley, C. Hao, The rejection of endocrine disrupting and pharmaceutically active compounds by NF and RO membranes as a function of compound and water matrix properties, J. Membr. Sci., 313 (2008) 323] observed an increase in the rejection of endocrine disrupting compounds (EDCs) and pharmaceutically active compounds (PhACs) from MBR effluent and Lake Ontario water by the NF membrane when compared to laboratory-grade water. However, in this study the effective MWCO measured following filtration with Lake Ontario water was larger than the molecular weight of all 22 compounds (<300 Da) from the previous study. This implies that compound interaction and association with organic matter in the water matrix itself may have contributed to previously observed increases in EDC and PhAC rejections.