Coupling of forward osmosis with desalination technologies: System-scale analysis at the water-energy nexus

Couplings of forward osmosis (FO) with reverse osmosis (RO) or membrane distillation (MD) are investigated at the water-energy nexus. The treatment of low and hypersaline feed solutions was assessed, followed by discussion of the most effective hybrid scheme for different conditions. Two FO configurations are presented, suggesting the potential applicability of a versatile multi-stage approach for treating low-saline wastewater sources under co-current membrane module design. Subsequently, energy and exergy consumption of the post-treatment RO / MD were evaluated. Finally, the coupling of FO and RO or MD units is investigated, highlighting the dependence of the two hybrid systems upon the operating parameters in FO. While FO-RO coupling is the most efficient solution in terms of power and exergy consumption, it is narrowed by the choice of the salinity gradient in the draw solution. A 2 order of magnitude higher power consumption is required by the MD to drive back the draw solution in FO while treating low saline wastewater. When dealing with hypersaline solutions instead, the FO-MD becomes more competitive, mostly from the exergy standpoint, highlighting the ability to use low-grade heat. Overall, FO-MD is more versatile, showing a broader application range while potentially approaching zero liquid discharge.