The effect of organic membrane fouling on the properties and rejection characteristics of nanofiltration membranes

The occurrence of trace organic contaminants in drinking water supplies, wastewater effluents and the environment is amplifying interest in membrane treatment methods due to the removal of a wide variety of organic solutes. During operation, organic matter present in source water accumulates at a membrane's surface. This fouling layer could potentially influence the degree of removal of dissolved constituents including organic contaminants of concern. The purpose of this study was to investigate the effect of organic matter fouling on membrane characteristics and the rejection of non-ionic organic solutes. Three nanofiltration membranes (NF-90, TFC-S, and NF-270) were fouled with microfiltered secondarytreated wastewater effluent at bench-scale under constant-flux filtration. The membranes were characterized prior-to and after fouling in terms of surface charge, hydrophobicity, effective pore size distribution, and phenomenological model coefficients. Rejection experiments were subsequently conducted with eight non-ionic trace organic contaminants of varying size to determine how membrane fouling affects the rejection of environmentally relevant organic contaminants. The effect of membrane fouling was found to be dependent upon the membrane evaluated. Membrane fouling resulted in an increase in hydrophobicity and a decrease in surface charge for the TFC-S and NF-270 membranes, while the hydrophobicity decreased and surface charge increased for the NF- 90 membrane. The effective pore size of the NF-90 membrane became slightly smaller with a larger deviation of pore sizes indicating that fouling affected the sieving mechanism of the membrane. However, the calculated effective pore size of the NF-270 membrane increased as characterization compound rejection was generally lower for the fouled membrane, which may be explained by cake-enhanced concentration polarization. The rejection of the non-ionic organic contaminants investigated by the NF-90 membrane was greater than 80% and was relatively unaffected by organic fouling. The NF-270 membrane provided variable rejection of the non-ionic organic contaminants and had markedly lower rejection for acetaminophen, bisphenol-A, and phenacetin after membrane fouling with effluent organic matter. Examination of rejection using the phenomenological model had advantages over examining rejection at a particular permeate flux, particularly, that rejection over a wide range of permeate flux (where rejection is variable) could be characterized by two model parameters.