# A Review of the Water Desalination Technologies
## 2 The State of the Art
### Multi-Effect Distillation (MED)  
The first MED plant was realized in Kuwait in the 1950s, using a triple-effect submerged tube evaporator. Despite being the first technology for desalination to be introduced, MED did not spread because it is particularly affected by the scaling problem on the pipes in comparison with other thermally supplied desalination technologies [22]. However, from the 1980s several research projects have been realized about MED, investigating lower temperatures to reduce the scaling issues and the corrosion of the pipes [23]. MED is also currently used in food industries to extract juice from sugarcane, and to produce salts from seawater [24].  
MED units can be arranged in several configurations, taking into consideration the shape of the heat exchangers or the brine flow direction regarding the vapor direction. The effects can be assembled in one line or in two parallel lines, working at different pressures in order to optimize the heat recovery [22,25]. According to the Top Brine Temperature (TBT), MED can be classified as Low Temperature (below 90 \({}^{\circ}\)C) or High Temperature (over 90 \({}^{\circ}\)C).  
To reduce the energy costs, steam is usually spilled from a steam turbine inside a power plant or recovered from a waste energy source in industrial processes [23]. As the primary steam is not in direct contact with saline water, the condensate inside the evaporator is normally recycled to the boiler for reuse [23].  
The maximal temperature of brine is limited to 120 \({}^{\circ}\)C by the calcium sulphate scaling, while the minimal is related to the available temperature in the wave. Figure 5 shows the diagram of a MED plant with horizontal tubes [23,26,27].  
In general, this plant is composed of a steam supply, several effects, heat recovery exchangers, a condenser and a venting system [22].  
In detail, the saline water can be split into two lines, in order to recover the thermal energy of freshwater and brine produced by the system, as depicted in Figure 5. After this step, saline water is used as cooling fluid for the condenser, then preheated by using the heat recovery exchangers that are supplied by the steam produced in each effect chamber [5].  
Figure 4: Possible coupling between desalination technologies and renewable energy sources (RES).  
The preheated water is sprayed in the first chamber on the evaporator surface, producing a thin film to promote rapid boiling and evaporation thanks to the low pressure inside the chamber and the external thermal energy supply [23].  
The vapor produced inside this chamber is transferred by pipes to the following chamber. As the pressure inside the second chamber is lower than the first one, the boiling temperature is also lower. In this way, it is possible to condensate the vapor produced in the first chamber inside the pipes and at the same time produce other vapor inside the second chamber [28].  
This process is repeated in the subsequent chambers in the same way, using the steam generated in the previous flash chamber to produce more vapor at lower pressure. In the last chamber, the vapor is finally condensed inside the condenser, cooled by the saline feedwater.  
The brine produced in the previous chambers is usually transferred inside the subsequent chambers, in order to force the extraction of more freshwater, thanks to the lower pressure inside them [23].  
The pressure inside the chambers is kept below the atmospheric conditions by using a dedicated vacuum system. The energy efficiency of MED units depends on the number of effects, normally ranging between 4 and 21 [24].  
MED units are used to produce freshwater with a flowrate ranging from 2000 to 20,000 m\({}^{3}\)/day. To improve the energy efficiency, MED can be coupled with a Thermal or Mechanical Vapor Compression unit [28]. The biggest desalination plants are concentrated in China and Middle East [29].