Renewable energy powered membrane technology: System resilience under solar irradiance fluctuations during the treatment of fluoride-rich natural waters by different nanofiltration/reverse osmosis membranes

Directly-powered photovoltaic membrane filtration (PV-membrane) systems possess no energy storage components, hence operate under energy fluctuations that result in variations in the membrane performance. Here, the resilience of a PV-membrane system under such conditions as a function of membrane type and feedwater quality is investigated. Five Tanzanian natural waters characterised by high fluoride (F) concentrations were treated using four nanofiltration/reverse osmosis (NF/RO) membranes. The key performance indicators—including flux, permeate electro-conductivity (EC) and specific energy consumption (SEC)—were instantaneously monitored throughout a solar day that exhibited 1-h of extreme fluctuations. The resilience indicators were resilience factor (RF) and recovery time. The PV-membrane system demonstrated resilience—defined in terms of both permeate quantity and quality—that was more dependent on membrane type. Tighter NF/RO membranes showed high resilience to variations in permeate quality, but poorer resilience to SEC variations, due to lower flux compared to the looser NF membrane. System resilience exhibited a recovery time range of 50–350 s which was determined by the duration of the fluctuation. Monitoring the permeate quality in terms of F concentration by using EC as a substitute for F through a linear correlation, showed that EC is a good indicator for F in these waters.