Effects of operating conditions on the removal of trace organic contaminants by a loose nanofiltration membrane

In recent years, there has been considerable research effort focusing on the removal of specific individual contaminants at trace level concentrations instead of the traditional surrogate, and often ill-defined, water quality indicators. The need for this more precise water quality characterisation has been driven by stricter environmental regulations and legislation, and the increasing need to utilise non-traditional water resources including reclaimed municipal wastewaters. Amongst several advanced technologies used for high quality water treatment, removal of trace contaminants by nanofiltration membranes has been investigated to a considerable extent. However, it is known that such membrane filtration processes tend not to operate continuously under steady-state conditions. Accordingly, it is surprising that there has been very little published information regarding the impact of operating conditions on the separation performance, particularly on the removal of trace contaminants. This study examined the effects of the operating conditions including feed water chemistry and membrane fouling on the retention of trace organics by a loose nanofiltration membrane. Emerging trace organics with functional groups having pKa values in the environmental pH range and outside the environmental pH range were selected for investigation. Results reported here indicate that the solution pH and ionic strength can markedly influence the removal of ionisable trace organic compounds. These observations were explained by electrostatic interactions between the solutes and the membrane surface and by the speciation of the ionisable compounds. In addition, membrane fouling has also been shown to exert some considerable impact on the retention of trace organics. The underlying mechanisms however remain unclear and are subject to on-going investigation.