Ageing of membranes for water treatment: Linking changes to performance

•Five practical performance factors contribute to membrane ageing.•Changing performance results from many chemical and physical changes in membranes.•Both operational and classic analytical methods illuminate changing characteristics.•Research gaps include foulant effects on ageing, changing cleaning rate and others.•To optimize membrane life, all performance factors should be considered together.

Fouling and cleaning of membranes are well understood, but long-term changes (ageing) are not as established. A large body of literature has emerged on membrane ageing in the past 15 years. An array of changes in physical and chemical characteristics, as well as in membrane performance, has been attributed to ageing. The present review and analysis of published literature emphasize that research into membrane ageing is complex; it can only be comprehensively addressed by considering performance factors, chemical/physical membrane characteristics, and analytical methods in parallel, as well as the links between these. Of the performance factors, membrane resistance and breach frequency are widely discussed, but there is still debate over the membrane characteristics that cause them to change. Changing membrane fouling rate has been attributed to fewer causes, whereas membrane cleaning rate has not been linked to membrane characteristics. Chemical and physical changes in PVDF membranes can be understood using classic analytical techniques as well as methods that mimic membrane operation. Several research avenues emerged from the literature review. These include heightened attention to foulant effects, identifying the cause of changes to cleaning rate, and considering all performance factors simultaneously in economic assessment of membrane ageing.