# A Review of the Water Desalination Technologies
## 2 The State of the Art  
Today, desalination can be realized using several technologies. In general, a desalination plant includes different processes to obtain freshwater, among which the desalination unit is the most energy expensive component. A desalination plant normally includes [6]:  
* Intake, composed by pumps and pipes to take water from the source (sea or brackish water)
* Pre-treatment, consisting of the filtration of raw water to remove solid components and the addition of chemical substances to reduce the salt's precipitation and the corrosion inside the desalination unit
* Desalination, where freshwater is extracted from saltwater
* Post-treatment, to correct pH by adding selected salts to meet the requirements of the final uses. As introduced before, the desalination process represents the most energy consuming water treatment. For this reason, this topic is quite widely assessed in literature. Before analyzing the specific solutions, a classification is required. [PERSON] suggested three main categories [13]: Evaporation and Condensation, Filtration and Crystallization. The following Figure 1 shows an upgrade of the classification proposed by [PERSON], integrating the new technologies currently under investigation. Evaporation and Condensation technologies are the first desalination techniques to be historically introduced and used for civil freshwater production. The idea is to supply thermal energy to seawater, producing a vapor, and then condensate it. This energy can be generated by using the heat from a thermal process (for example, waste heat or  
Figure 1: The classification of desalination technologies by working principle. fuel combustion), or through a mechanical process. In the first case, the most common technologies are MED, MSF, Thermal Vapor Compression (TVC) and Membrane Distillation (MD). Currently other approaches are under investigation, and among these we can find few new solutions supplied by solar radiation: Solar Still Distillation (SSD), Solar Chimney (SC) and Humidification-Dehumidification (HDH) desalination. Regarding the mechanical processes used to produce freshwater through the evaporation and condensation of seawater, the main technique is Mechanical Vapor Compression (MVC) [14; 15]. In case of filtration technologies, all solutions are essentially based on a semipermeable membrane, i.e., a layer that shows a different mode of crossing behavior according to the sizes or nature of molecules. The only exception is Ion-exchange resins (IXR), where natural or artificial materials are used to capture the dissolved ions in a chemical way [16]. In this context RO is the most used technology for desalination. The Electrodialysis (ED) and Ion Exchange Resin (IXR) are used to produce water with a very limited concentration of salts. Other techniques, as Forward Osmosis (FO), Nano Filtration (NF) and Capacitive Deionization (CDI) are in the development stage [17]. Finally, the Crystallization category comprises techniques that extract freshwater producing ice as intermediate product. As example, the main techniques are Secondary Refrigerant Freezing (SRF), Hydration (HY) and Vacuum Freezing (VF) desalination. All these approaches are under investigation [18]. A timeline showing the evolution of desalination is shown below in Figure 2.  
Another useful classification can be realized by considering the kind of energy mainly required to run the process, as shown in Figure 3. This aspect is important for select renewable energy sources to supply the desalination process [19]. In detail, four kinds of energy are considered here:  
* Thermal energy
* Mechanical energy
* Electrical energy
* Chemical energy  
The first category could be supplied by solar thermal or geothermal energy sources. It comprises the following technologies: MSF, MED, TVC, MD, SC, HDH and SSD. In particular, the last three approaches are designed to directly exploit solar radiation [20]. The group of technologies requiring a Mechanical Energy as input comprises MVC, RO, NF, SRF, HY.