# A Review of the Water Desalination Technologies
## 2 The State of the Art
### Reverse Osmosis (RO)  
RO is a desalination technology based on semipermeable membranes, which are specific layers allowing the passage only to selected molecules. In nature, if two solutions with different concentrations of solutes are separated by a semipermeable membrane, the solvent flows spontaneously from the more diluted solution to the more concentrated one, in order to balance the energy potential of both solutions, as shown in Figure 11 (see case a). This flow can be progressively reduced if an increasing external pressure gradient \(\Delta p\) is applied to the semipermeable membrane (see case b) [33]. Figure 10: A schema of a TVC and MED unit. Figure 9: A schema of a simple Thermal Vapor Compression (TVC) desalination unit. The exact value able to stop the solvent flow is defined as Osmotic Pressure \(\Delta p_{osm}\) (see case c). If the external pressure gradient is greater than the osmotic pressure, the solvent flow is inverted, so the solvent can be extracted from the concentrated solution (see case d) [33]. For each solution the absolute osmotic pressure \(p_{osm}\) can be defined according to [PERSON]'s equation [33; 34] (In chemistry, the amount t[c] is called \"normality\", indicating the number of equivalents in a unitary volume. The equivalent represents the amount of a specific substance able to react with (or supply) one mole of hydrogen (H\({}^{+}\)) in an acid-base reaction or react with (or supply) a mole of electrons in a redox reaction):  
\[p_{osm}=\iota[c]R\tau \tag{1}\]  
assuming the following notation:  
* \(\iota\) is the dimensionless van't Hoff index (also called the number of osmotically active particles), given by the relation \(\iota=1+e(\
u-1)\), where \(e\) is the degree of dissociation representing the ratio of how many original solute molecules are dissociated, and \(\
u\) is the number of ions formed by the molecule dissociation (the stoichiometric coefficient of dissociation reaction). As an example, in the case of sodium chloride (NaCl), \(\epsilon\approx 1\), \(\
u=2\), consequently \(\iota=2\). * \([c]\) is the molar concentration of the solute. * \(R\) is the ideal gas constant equal to \(8.31441\) J K\({}^{-1}\)mol\({}^{-1}\). * \(\tau\) is the absolute temperature of the solution. As the salt concentration is negligible in freshwater and consequently its osmotic pressure (the minimal pressure required to stop the solvent flow) is equal to the osmotic pressure of saline water. For seawater, the salt concentration ranges between \(0.51\) and \(0.68\) mol L\({}^{-1}\)[33]. Thus, considering an environmental temperature equal to \(25\)\({}^{\circ}\)C, the osmotic pressure according to [PERSON]n't [PERSON]'s equation ranges between \(25\) and  
Figure 11: A depiction of the phenomenon of osmosis, according to the external pressure applied to the two sides of the membrane. Case (**a**). Forward osmosis. Case (**b**) Retarded osmosis. Case (**c**) Zero flow. Case (**d**) Reverse osmosis. 33 bar. Greater values can be measured, as in the extreme case of Dead Sea, where the osmotic pressure is equal to 290 bar [33]. Figure 12 visualizes the phenomenon of osmosis: until the external pressure gradient is lower than the osmotic pressure, the solvent flows from the more diluted solution to the more concentrated one. In this working region, it is also possible to extract energy from the mixing of two solutions having different concentrations. This approach can be used in the estuary of the rivers. Indeed, the exploitation of the saline gradient energy source is currently under development [35]. The case without an external pressure gradient is identified as \"Forward Osmosis\", a condition which is used in the other osmosis desalination technique, analyzed in the following subsection. By applying an external pressure gradient greater than the osmotic pressure, freshwater is extracted from saltwater. For desalination purposes, an external pressure between 15 and 25 bar is normally applied for brackish water, and between 54 and 80 bar for seawater [23; 36].