## 2 New Membrane Operations Aligned with PIS
### Energy Production in Desalination Sector
#### 2.2.1 Reverse Electrodialysis (RED)  
In hybrid RED-membrane desalination systems, highly concentrated reject brine (from both thermal and membrane desalination plants) is used for energy recovery. The generated electricity can be used to fulfill (partly or entirely) energy requirements of desalination system. Various integrations of RED with RO are possible [18]. RED can serve as a pretreatment, post treatment, or both steps for RO. Depending upon the configuration applied, RED can serve as energy reducer to the net energy generator in an RO desalination plant [18]. Similarly, integration of RED with MD can give synergetic effects. Application of MD at RO retenate can increase recovery factor while RED connected at retenate of MD can produce electricity while simultaneously decreasing its concentration.  
Recently, [PERSON] et al. experimentally evaluated the integrated application of RED and MD for clean water and energy generation [22]. The conceptual illustration of the investigated hybrid system  
Figure 3: Specific water cost of different integrated configurations without considering sale of salt (patterned bars) and after considering sale of salt (solid lines). ER: Mechanical energy recovery; TE: Thermal energy recovery; MF, Microfiltration; NF, Nanofiltration; MD, membrane distillation; 1 = Configuration 1: RO alone; 2 = Configuration 2: NF-RO; 3 = Configuration 3: MF-NF-RO; 4 = Configuration 4: MF-NF(-MCr)-RO; 5 = Configuration 5: MF-NF-RO(-MCr); 6 = Configuration 6: MF-NF(-MCr)-RO(-MD); 7 = Configuration 7: MF-NF(-MCr)-RO(-MCr).  
is presented in Figure 4. In particular, MD was operated with seawater RO retentate (1 M NaCl). Brine with concentrations of up to 5.4 M of NaCl (near supersaturation) was obtained from direct contact membrane distillation (DCMD) operated on RO brine (1 M NaCl) at a feed recirculation time of 20 h and a temperature gradient of 30 \({}^{\circ}\)C [15]. The DCMD brine was then used for energy recovery by RED system. The level of output power of RED depends on feed (MD brine) concentration and operating parameters like temperature and flow rate. For example, the maximum power density of a RED system was observed to increase from 0.9 W/m\({}^{2}\) to 2.4 W/m\({}^{2}\) when the MD brine concentration was increased from 4 M NaCl to 5.4 M NaCl, respectively [15]. The output power from the RED was also observed to increase with temperature (0.027 W/\({}^{\circ}\)C on average) and flow velocity, which can be optimized to set appropriate working conditions for improved system performance. In general, energy recovery from DCMD brine by RED enables the supply of extra energy required for desalination. This represents a promising strategy towards low energy desalination and Near-Zero Liquid Discharge.  
The feasibility of an integrated MD-RED depends on several factors affecting the performance of individual system units. Different strategies can be followed to improve the performance of the system in terms of water production capability, output power, and energy efficiency. The energy consumption in RO or MD can be reduced by controlling the operating conditions, using appropriate pre-treatment techniques, and designing optimal membranes and modules [12; 21]. The potential and hence the output power from a RED unit can be improved by the development and use of highly permeable and low resistance IEMs (ion exchange membranes) [11]. Besides the benefit obtained at higher temperatures for both systems (MD and RED), the coupled system enables the possibility to use and convert waste heat to electricity.