# A Review of the Water Desalination Technologies
## 2 The State of the Art
### Multi-Stages Flash (MSF)  
The first MSF plant was realized in the 1950s in Scotland and after few years it became the most used desalination technology [24]. MSF is also commonly used on ships and along the coastline in several parts of the world, like USA, Middle East and Korea, to produce freshwater from seawater [30]. The MSF process shows some similitudes with MED, previously described. In fact, an initial heat supply is also required, using steam spilled from a power plant, and the decreasing pressure is used to force the vapor production [5]. Electricity is required to run the several pumps distributed along the desalination plant [31]. Figure 6 shows the diagram of a once-through MSF unit [23; 26; 27]. The plant can be conceptually divided in two sections: the first is the brine heater section, where the feedwater receives heat from an external supply, and the latter one is the heat recovery section, where the thermal energy is recovered to preheat the feedwater [6]. Figure 5: A schema of a Multi-Effect Distillation (MED) unit. Focusing the attention on the picture reported above, the saline feedwater is firstly used as cooling water for the condenser and then as a raw source to produce freshwater. The saline water increases its temperature progressively, flowing inside the pipes and forming the heat exchangers inside the flash stages. To start the process, the saline water is heated inside the brine heater by using steam usually spilled from a power plant. This steam condenses inside the brine heater (outside the tube bundle), so it can be reused in the steam power plant [24]. As the saline feedwater flows inside pipes in the brine heater and in the flash stages, the maintenance operations to remove the scaling are simpler than in MED [23]. For this reason, MSF is the most widespread thermally driven desalination technology, representing 17.6% of the total installed desalination capacity in the world [12]. After the initial heating, saline water is laminated inside the first flash stage. The vapor produced inside the chamber condenses thanks to the heat removal by the saline feedwater inside the heat exchanger. The brine collected in the lower part of the chamber is laminated in the subsequent chambers, where the internal pressure is reduced linearly from the first stage to the terminal one [30]. The vacuum is obtained by the utilization of steam ejectors supplied by high pressure steam (as shown in Figure 6), or by using vacuum pumps [6]. Thanks to the pressure drop, the introduction of heated saline water produces the \"flashing effect\", for which the saline water boils rapidly inside the chamber, producing vapor [24]. To maximize the energy efficiency of the system, MSF units are typically composed of several flash stages, with the total number ranging from 15 to 25 stages (greater values are related to bigger MSF plants). This technology is able to satisfy a freshwater demand of about 4000 to 57,000 m\({}^{3}\)/day, requiring heating at 90 \({}^{\circ}\)C-110 \({}^{\circ}\)C [23]. In more recent plants, a few changes have been introduced. Instead of the condenser, a heat rejection section is inserted, which is composed of two or three flash stages (see Figure 7). In detail, seawater is used as cooling fluid in this section. After this step, a part of seawater is rejected and the other part is mixed with a part of brine extracted by the last flash stage. This salt solution is used in the main section of the desalination unit, in the same way as shown in the previous diagram. This technique is applied to increase the energy efficiency of big desalination plants, composed of 19-40 flash stages and 2-3 heat rejection stages [6; 31]. In the last two decades, the reliability of the system has been improved thanks to scaling control (adding substances to limit the phenomenon), the introduction of automation and control systems and the choice of better materials for the realization of the desalination units [23].