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

Mineral scaling is a key limiting factor for membrane desalination and results in increased operation and maintenance costs. In directly coupled battery-less photovoltaic-powered nanofiltration/reverse osmosis (PV–NF/RO) membrane systems, solar energy fluctuations cause intermittent operation and direct osmotic backwash (OB) spontaneously. This potentially induces a self-cleaning mechanism to mitigate scaling. In this study, the impact of different flux decline/scaling mechanisms (concentration polarization (CP), surface crystallization (SC) and bulk crystallization (BC)), membrane type (BW30 and NF270) and controlled solar irradiance fluctuations on OB cleaning efficiency were investigated to explore the feasibility of OB for scaling (CaSO4 and CaCO3) control. The results showed that OB could restore 30–96% flux and partially remove scaling crystals. OB cleaning was more effective for CP and SC than for BC. High-level solar irradiance before fluctuation and long operating time led to severe scaling, which reduced OB cleaning efficiency. Low-level solar irradiance during fluctuations did not affect the scaling formation, but improved OB cleaning efficiency by strong OB. Those results show that spontaneous OB can act as a natural self-cleaning process in directly coupled PV-membrane systems to control scaling during solar irradiance fluctuations. [Display omitted] •Osmotic backwash during fluctuations partially removes scalants and restores flux.•Osmotic backwash is more effective for surface blockage than cake layer formation.•NF270 membrane exhibited more severe scaling (especially CaSO4) than BW30 membrane.•Nature of solar irradiance fluctuations affect osmotic backwash cleaning efficiency.