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

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 use seawater (SW) as draw solution to filter the wastewater (WW) while reducing the seawater osmotic pressure. Thereby reducing the operating pressure of the RO to desalinate the diluted SW. 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, using real WW and SW solutions. A combination of water flux modelling and membrane characterisation were used to assess the degree of membrane fouling and the impact on the process performance. Initial water flux as high as 22.5 Lm⁻² h⁻¹ was observed when using secondary effluent. It was also found that secondary effluent causes negligible flux decline. On the other hand, biologically treated wastewater and primary effluent caused mild and severe flux decline respectively (25% and 50% of flux decline after 80 hours, 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.