## 2 New Membrane Operations Aligned with PIS
### Energy Production in Desalination Sector
#### 2.2.2 Pressure Retarded Osmosis (PRO)  
PRO is one of the most interesting membrane processes to harness clean energy from a salinity gradient. In PRO, a semipermeable membrane is applied to separate high and low salinity solutions. The osmotic pressure extracts the fresh water from dilute to the concentrated solution. The pressure generated on the high salinity solution side can be used to run a hydro turbine [27; 28]. For this process, several combinations of feed and draw solutions have been tested including river and seawater [29], seawater brine and wastewater retentate, freshwater and synthetic NaCl solution [30], and seawater and municipal wastewater [31]. Synergetic effects can be achieved by introducing PRO into desalination systems in terms of reduction of waste footprint of feed solution, and the dilution of the draw solution, which can reduce the energy demand of desalination system and can minimize the environmental consequences of brine disposal. [PERSON] et al. [31] have reported experimental data on a RO unit getting a benefit from pressure generated by a PRO unit. The authors have also reported the power density data of PRO-RO system ranging between 1.1 W/m\({}^{2}\) to 2.3 W/m\({}^{2}\). The authors have concluded that the proposed system can bring the desalination energy demand down to 1 kWh/m\({}^{3}\). In order to increase the power density of the PRO process, [PERSON] et al. [30] have proposed the integration of PRO with a closed loop  
Figure 4: Integrated application of MD with RO and RED (reverse electrodialysis) for water and energy production [26]. HCC and LCC represent High Concentration Compartment and Low Concentration Compartment of the RED cell, respectively. MD process. MD maintains a high concentration of draw solution that extracts freshwater from the feed solution. The authors have claimed that the proposed system can achieve a high recovery factor, huge production of power, and minimum membrane fouling and environmental impacts. The concept of integrating PRO with other membrane operations is also gaining attention at industrial scales. Currently, Applied Biomimetics and partners have setup two pilot plants aiming to generate electricity from geothermal wastewater by using PRO technology (energyforskining.dk/node/8345). The proposed plan is expected to produce emission free electricity while at the same time bringing the salinity level of the geothermal brine below the permissible limits. The dilution of geothermal brine via PRO will also reduce the corrosion and scaling potential of a geothermal stream. A similar concept has been used in a Mega-ton project where a pilot plant has been constructed in Fukuoka (Japan). The plant uses 460 m\({}^{3}\)/day of RO brine which is mixed with 420 m\({}^{3}\)/day of wastewater. The plant has been able to achieve power density as high as 13 W/m\({}^{2}\) at 30 bar hydraulic pressure by using commercial hollow fibers from TOYOBO (Osaka, Japan). Another pilot-scale PRO-hybrid research project is being conducted under the name \"Global MVP (Membrane distillation, Valuable resource recovery, Pressure retarded osmosis) Project\" in Korea (Figure 5). The objective of this project was to evaluate the feasibility of the RO-MD-PRO hybrid process in terms of reducing the discharged water concentration and the energy consumption. In the hybrid process, the concentrated RO brine enters the MD feed side, and the further concentrated MD brine is then utilized as a PRO draw solution while the waste water effluent is used as the feed solution. Consequently, an improvement in the total plant efficiency compared to a stand-alone RO plant is expected due to the additional water production by MD and the reduction of net energy consumption resulting from the PRO energy generation. Specifically, the following pilot plant will be built: a RO system capable of 1000 m\({}^{3}\)/day water production, a MD system with a water production capacity of 400 m\({}^{3}\)/day, and a PRO system having a 5 W/m\({}^{2}\) power density [32].