Forward Osmosis, Reverse Electrodialysis and Membrane Distillation: New Integration Options in Pretreatment and Post-treatment Membrane Desalination Process

Lower fouling propensity and energy demand compared with pressure-driven membrane units make FO a suitable option to boost the efficiency of current seawater membrane desalination systems. Recent optimistic investigations indicate that FO can potentially decrease the energy input to RO seawater desalination down to 1.5 kWh/m3, not far from the thermodynamic threshold (B1 kWh/m3). However, preliminary studies indicate that the present commercial FO membranes do not guarantee a sustainable implementation of FO in the desalination industry due to the high capital investment cost (CAPEX). The inversion point is predicted in proximity of a targeted membrane cost of 25 h/m2 c.a. or a transmembrane flux higher than 15 L/m2 h. Integration of RED in membrane desalination systems is a promising approach for concurrent production of fresh water and electrical energy. In theory, such a concept might enable a technological solution to low energy desalination. Similarly, MD integration can increase water recovery factor above 90% [52]. Moreover, the adverse effects of brine discharge to the ecosystem and the pollution of the environment by the greenhouse gases released from power plants that supply energy to desalination plants could be minimized. Such advantage from the synergistic integration of RED and MD with other membranebased technologies is consistent with the process intensification strategy and zero liquid discharge paradigm. However, implementation of these integrated membrane systems at the industrial level requires a significant research effort. It is well recognized that the power output at SGP-RED stage is highly influenced by the presence of divalent ion and fouling phenomenon during operations with natural feeds; therefore, the development of new materials, particularly ion exchange membranes able to overcome the adverse effect of divalent ions as well as fouling, is necessary. Ion exchange membranes for RED should have low resistance ion-conductive membrane materials at a low cost (,2 h/m2) and with high permselectivity (.95%) for operations under real conditions. Addressing these issues will have a significant impact on the possibility of commercial implementation of RED technology.