Opportunities to reach economic sustainability in forward osmosis–reverse osmosis hybrids for seawater desalination

• A flux threshold of 30 L·m− 2·h− 1 was defined for FO economic viability.
• Current membrane developments do not match the fixed minimum flux threshold.
• Membranes with higher A (> 5 L·m− 2·h− 1·bar− 1) and lower S (< 100 μm) are required.
• Pressure assisted osmosis could be an economically favourable alternative to FO.

Despite being energetically attractive, the economic sustainability of forward osmosis (FO) - reverse osmosis (RO) hybrid process for seawater desalination has not yet been fully demonstrated. This study is the first to carry out an extensive economic evaluation of FO-RO hybrid, benchmarked against stand-alone RO system. This assessment clearly highlights that FO-RO hybrid can be beneficial, comparatively to RO, only for high energy costs and/or substantial operational costs savings. It is also demonstrated that improvement in water permeation flux, typically above 30 L·m− 2·h− 1 for classical water recoveries, is an absolute prerequisite to lower investment costs down to an economically acceptable level. Such fluxes are not achieved with current membrane developments when realistic feed and draw solutions are considered. In a second step, a sensitivity analysis was carried out to assess the effects of process parameters on flux in FO, compared to the above-mentioned benchmark. Results indicate that for novel FO membranes, typically a coupling of higher water permeability (A > 5 L·m− 2·h− 1·bar− 1) and lower structural parameter (S < 100 μm) is required. This study also shows that the concept of pressure assisted osmosis (PAO) can help to reach higher fluxes with current commercial membranes, thereby demonstrating more favourable economics compared to state-of-the-art FO membranes.

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