Renewable energy powered membrane technology: Impact of solar irradiance fluctuation on direct osmotic backwash

Abstract

Direct osmotic backwash (OB) during solar energy fluctuations is an important observation in directly coupled, battery-less solar energy powered nanofiltration/reverse osmosis (NF/RO) membrane systems. OB occurs spontaneously and has the potential to induce membrane self-cleaning. The impact of controlled solar irradiance fluctuation conditions, feed water salinity (1–10 g/L) and membrane type (NF membrane NF270, RO membrane BW30) on OB were investigated by periodic step-response tests. The objective of this research was to investigate the OB mechanism under solar irradiance fluctuations. The results show that solar energy fluctuations lead to a change of filtration hydrodynamic conditions (feed flow rate, applied pressure), operating time, and permissible backwash time. This results in variations of OB driving force which is determined by the osmotic pressure difference during filtration. High feed water salinity and solar irradiance before fluctuation enhance the OB process, while relative high solar irradiance during fluctuation weakens the OB process. Furthermore, the BW30 membrane shows a higher OB flow rate and less accumulated volume than NF270. This was attributed to the higher salt retention of BW30 resulting in a larger backwash driving force and much lower flux of BW30 which in turn caused a thinner CP layer. These findings indicate that solar irradiance fluctuation conditions may potentially delay fouling and hence enhance the reliability and robustness of battery-less photovoltaic-membrane systems.