Effects of fouling and scaling on the retention of trace organic contaminants by a nanofiltration membrane: The role of cake-enhanced concentration polarisation
The combined impact of humic acid fouling and CaCO3 scaling on the retention of trace organic contaminants by a commercially available nanofiltration membrane was investigated in this study. Due to the presence of humic acid in the feed solution, CaCO3 scaling behaviour differed substantially from that of a pure CaCO3 solution. A prolonged induction period was consistently observed prior to the onset of membrane scaling. In addition, membrane scaling following humic acid fouling did not result in a complete loss of permeate flux. This is consistent with the absence of any large CaCO3 crystals. In fact, the CaCO3 crystals on the membrane surface were quite small and similar in size, which would result in a relatively porous cake layer. At the onset of CaCO3 scaling the retention of all three trace organic contaminants started to decrease dramatically. The observed decrease in retention of the trace organic contaminants was much more severe than that reported previously with a single layer of either organic or colloidal fouling. Such severe decrease in retention can be attributed to the extended cake-enhanced concentration polarisation effect occurring as a result of the combination of membrane fouling and scaling. The porous CaCO3 scaling layer could lead to a substantial cake-enhanced concentration polarisation effect. In addition, the top CaCO3 scaling layer could reduce the wall shear rate within the underlying humic acid fouling layer, causing an additional concentration polarisation effect.