Renewable energy powered membrane technology: Safe operating window of a brackish water desalination system

The safe operating window (SOW) of a renewable energy (RE) powered membrane filtration system for brackish water desalination is determined. The SOW is constrained by several factors: (i) operating limits of pump motor (pressure and flowrate), (ii) maximum recommended recovery, and (iii) the osmotic pressure of the feedwater. The membranes (and brackish feedwater salinities) used were BW30 (5500 and 10,000mg/L), aged BW30 (5500mg/L) and NF90 (5500 and 2750mg/L). At lower salinities (2750-5500mg/L) the main constraint was maximum recovery (30%), while at higher concentrations (10,000mg/L) osmotic pressure played a more limiting role. The optimum operating strategy is 'constant recovery'. This produces the highest flux at a given power consumption and thus the lowest specific energy consumption (SEC) while maintaining good retention. However, this operating strategy can be difficult to implement. Therefore, 'constant set-point' mode is recommended for this system in order to provide a robust and effective solution, despite a minor reduction in performance. This approach is attractive for being powered by a wind turbine or solar energy (photovoltaics) given the low SEC (~ 3kWh/m³) that enables operation over a very wide power range (70-280W) in order to achieve the desired pressure range (5-11.5bar). Overall, the SOW methodology can be used in the performance evaluation of a wide range of membrane filtration systems.