# A Review of the Water Desalination Technologies
## 2 The State of the Art
### Reverse Osmosis (RO)
Thus, according to the values reported above, the RO essentially requires electrical (or mechanical) energy to run the pumps to significantly increase the seawater pressure before the semipermeable membrane. A simple diagram of a RO desalination unit is depicted in Figure 13 [37]. Seawater, after a pre-treatment to remove solid particles, is pressurized by a High Pressure Pump (HPP) in order to supply the RO desalination unit [38]. In desalination applications, the Recovery Ratio (RR) is defined as the ratio between the freshwater flow and the saline feedwater flow [12]. \[RR=\frac{Q_{f}}{Q_{s}} \tag{2}\]  
Considering the working conditions, RR assumes values between 35% and 50%, so practically only half (or less) of the seawater flow becomes freshwater and the remaining  
Figure 12: Solvent flow as a function of the external pressure gradient. Figure 13: A diagram of a simple Reverse Osmosis (RO) desalination unit. part is expelled as brine [39]. To increase the freshwater extraction, the pressure before the semipermeable membrane should be increased, but there are several technical constraints, essentially related to the mechanical resistance of the membrane [40]. As semipermeable membranes are not perfect, a limited amount of salts can be found in the freshwater output [23]. The brine flow has a high energy potential, as its pressure is practically the same as the saline input water. In fact, the pressure drop inside the brine circuit is about 2-3 bar [37]. To reduce the total energy consumption for desalination, many studies have been realized since the 1970s. In addition to the improvement of membrane properties, the main goal was energy recovery from the brine flow by the introduction of an Energy Recovering Device (ERD). The solutions can be classified as [41]:  
* Centrifugal device
* Isobaric device  
In the first case, two technologies have been proposed. The first, introduced in the 1980s, is the installation of a hydro turbine (Pelton) to recover the energy of the brine flow and transfer it to the main HPP. To complete the energy demand of the pump, an electrical motor is used. The pressure increase is realized by the main HPP to the entire saline feedwater flow. This solution is depicted in Figure 14 [37]. Pelton turbines are normally used in hydropower to exploit high heads, between 300 m and 1000 m. Of course, only a part of the available energy of the brine flow (about 70%) is transferred to the saline feedwater due to the double energy conversion (from the fluid to the mechanical shaft and then to the fluid) [42]. At the end of the 1980s, another centrifugal ERD solution was introduced, represented by the turbocharger ERD. The working principles of this system are shown in Figure 15.  
In detail, the saline water is pressurized in two steps: the first one is entrusted to an HPP, driven by an electrical motor, and the latter one is realized by a turbocharger device, which is composed of a hydro turbine and a pump, directly coupled. This solution shows a greater energy efficiency in comparison to the previous one, as the rotary speed of the turbocharger can be modulated independently by the HPP [43]. Figure 14: A schema of a RO unit with a Pelton Turbine. Figure 15: A schema of a RO unit with a Turbocharger. To improve the energy efficiency even in part-load, limiting the adoption of passive regulating systems, a Dual Turbine System has been proposed. Considering the turbocharger system, shown in Figure 15, a part of the brine flow is spilled to run a Pelton turbine and reduce the power load of the electrical motor in the HPP. A schema of this solution is reported in Figure 16[37]. The last centrifugal solution is represented by the HEMI (Hydraulic Energy Management Integration) proposed by FEDCO (Fluid Equipment Development Company) [44]. In detail, the HEMI solution consists of the adoption of an electrical motor in the turbocharger system, modulating the final pressure of the saline feedwater before the semipermeable membrane. In Figure 17, the HEMI solution is depicted.