Hybrid forward osmosis-reverse osmosis for wastewater reuse and seawater desalination: Understanding the optimal feed solution to minimise fouling

Abstract To enhance the seawater desalination energy efficiency forward osmosis − reverse osmosis (FO-RO) hybrid system has recently been developed. In this process, the FO “pre-treatment” step is designed to dilute the seawater (SW) with reclaimed wastewater (WW) before the desalination step, thereby reducing the energy demand for the SWRO process. However, membrane fouling is a major issue that needs to be addressed. Proper selection of suitable WWs is necessary before proceeding with large-scale FO-RO desalination plants. In this study, long-term experiments were carried out, using state-of-the-art FO membrane with real WW solutions. A combination of water flux modelling and membrane characterisation were used to fouling to process performance. Initial water fluxes as high as 22.5 L.m −2 .h −1 were observed when using biologically treated secondary effluent. It was also found that secondary effluents cause negligible flux decline. On the other hand, biologically treated wastewater and raw wastewater caused mild and severe flux decline respectively (25% and 50% of flux decline after 80 hrs, compared to no-fouling conditions). Ammonia leakage to the diluted seawater was also measured, concluding that, if biologically treated wastewater is used as feed, the final NH 4 + concentration in the draw is likely to be negligible.