# A Review of the Water Desalination Technologies
## 1 Introduction  
To overcome the ever increasing freshwater demand due to population growth and welfare concerns, the first desalination plants were installed in the late 1950s [1; 2; 3]. The first technologies were thermally driven due to the low cost of fossil fuels (less than $3 for one barrel of oil). As the cost of energy has progressively increased, two lines of research have been pursed in order to minimize the total cost of water treatment [1; 4]:  
* Increase the energy efficiency of commercial technologies
* Investigation and proposal of new solutions  
For example, in the first case the number of stages in Multi-Stage Flash (MSF) distillation has progressively increased from 8-12 to 20 [5; 6]. As regards the proposal of innovative solutions, the introduction of semipermeable membranes represents a radical change in the desalination sector. In fact, nowadays Reverse Osmosis (RO) is the most used technology for this purpose [7]. The term \"desalination\" refers to the technological process used to extract freshwater from brackish or saltwater. Seawater is often the raw water source used to supply this process. Historically, the idea behind the desalination process was introduced by the Royal Navy (the United Kingdom's naval warfare force) at the end of the 18 th century with the purpose of increasing navigational autonomy without storing more water on the ships [8]. Since in that period ships were equipped with steam engines, the first desalination technology was single flash distillation, which was improved in the following years into the more efficient MSF. The first type of desalination unit was realized by the [PERSON] in 1885 in Glasgow (Scotland) [9]. This company, which later became known as Weir Westgarth, practically had a monopoly as a desalination unit builder until World War II. In the following years, desalination plants had been installed around the world for civil purposes. In 1907, a Dutch company installed the first desalination plant in the Arabian Gulf countries in the city of Jeddah [10]. By the order of King [PERSON], the same plant was replaced in 1928, with two units produced by Weir Westgarth being installed (with a total installed capacity of 135 m\({}^{3}\)/day) [10]. In 1953 other desalination plants were installed in Qatar and Kuwait. In detail, 5 units were installed in Qatar, with a total capacity equal to 682 m\({}^{3}\)/day, and 10 units in Kuwait, with a total capacity of 4545.5 m\({}^{3}\)/day. In 1955 another 10 units were installed in Shuwaikh (Kuwait), which were the same size [10]. From this moment on, desalination plants expanded around the world, with the birth of many companies such as Krupp in Germany, Westinghouse in the USA, and SIR (Societa Italiana Resine) in Italy [8]. The other mainstream technique is RO, based on semipermeable membranes. Historically, the phenomenon of osmosis was observed for the first time in 1748 by [PERSON], without any application for about two centuries [11]. In the USA the first studies were started by the researchers [PERSON] and [PERSON] in 1956 at the University of California and the University of Florida, respectively. The first membrane was realized in 1959 whereas the first pilot plant was installed in 1965, with a capacity of 19 m\({}^{3}\)/day [8]. An improvement of this technique was obtained by the development of asymmetric membranes that show a different porosity moving from a face to the other one, allowing a greater water flux through them [11]. The slow diffusion of RO was initially due to the high electricity consumption required to produce freshwater in comparison with other techniques, and the limited life of semipermeable membranes [8]. The first applications were related to brackish water, due to its lower osmotic pressure in comparison with seawater. The first desalination plant, based on RO, for a municipality was realized in 1977 in the USA, with an installed capacity of 11,350 m\({}^{3}\)/day.