**2. Desalination Technology**

The main desalination technologies are divided into evaporation (distillation) and membranes. To evaporate, heat and electricity are necessary, while membranes only need electrical energy and have a considerably low consumption.

Torres [9] explained that in the distillation processes there are several systems depending on the use of the condensation heat of steam.

The most relevant are:


A multiple-effect evaporator is an apparatus for efficiently using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the previous one. MSF is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages that are essentially countercurrent heat exchangers.

The MED is similar to the previous process but operating at a lower pressure. In the case of multiple-effect evaporation plants, the exhaust vapors from the product are used to heat the downstream-arranged evaporation effect so that the steam consumption is reduced accordingly.

MVC refers to a distillation process where the evaporation of sea or saline water is obtained by the application of heat delivered by compressed vapor. This system is the most thermodynamically efficient process of single-purpose thermal desalination plants.

Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to remove ions, unwanted molecules, and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure. A part of the inlet water is desalinated, producing a certain amount of water with a high concentration of salt called brine.

Voutchkov [13] estimated a cost share, that approximately 35% goes to energy. Table 2 shows the energy consumption of the desalination technologies described.


**Table 2.** Desalination technologies' power requirements. Own elaboration.

Desalination by nanofiltration has a similar principle to the one used for reverse osmosis. The main difference with the latter is the characteristics of the semipermeable membranes used in this technique, which offer a higher percentage of rejection of some ions from salts, which can operate at lower pressures (Parlar et al. [19]). Nanofiltration is a process that has been used in recent years but basically limited to some stages of the drinking water purification, such as softening, discoloration, and elimination of micro contaminants.

Desalination by electrodialysis consists of the passage of ions under the effect of a continuous electric current through a series of selective cationic and anionic permeable membranes, which allows the electrochemical separation of ions. The membranes, separated from each other by a few millimeters, are placed between two electrodes so that the incoming water circulates. The membranes let the ions in, by being transferred through them from a low concentrate to a higher concentrate (Lee and Kang, [20]).

Electrodialysis has proved very viable, especially in brackish water desalination, in effluent treatment, and in industrial processes. It is suitable for connecting directly to photovoltaic panels, taking advantage of the use of solar energy, and it is particularly recommended in areas with isolated saline aquifers where the connection to the electrical network is difficult and expensive.

Asociación Española de Desalación y Reutilización, AEDYR [21] states that nowadays the most globally used technique to desalinate water is reverse osmosis, which reaches almost 70% of the total available technologies; followed by MSF (18%), MED (7%), nanofiltration (3%), and finally electrodialysis (2%).

The question is whether there is room for improvement in desalination technology, although advances in this field will undoubtedly continue to occur. Inside the reverse osmosis system, the key component is the membranes. The ones that are used at present are the result of more than 50 years of research in polymers. In the USA, MIT researchers are experimenting with graphene membranes, which require less pressure and therefore, less energy. Other researchers have studied the use of carbon nanotube membranes. Unfortunately, these technologies have not yet been developed for industrial use.

Jeff Urban [22] from Berkeley Laboratory described the line of open investigation to develop desalination by direct osmosis through a highly concentrated extraction solution to extract water from sea water. In a Berkeley Laboratory, they are developing extraction solutions, in gel form, which would extract water effectively and would then separate spontaneously from this water thanks to the application of low amounts of heat. This line is still in the research stage and the first steps would be taken by giving direct osmosis a complementary role in the brackish water treatment.

Another open line of investigation is desalination by solar energy. Many areas with water scarcity usually have a decent insolation level that can be used as solar energy. De Luis López and Gómez Benítez [23] mentioned small installations (up to 15 m3/day) of solar stills to provide drinking water in Greece. The Freeport Plant, in the Gulf of Mexico, is more important, with a multiple stage system (LTV, Long Tube Vertical Multiple Effect Distillation), which guarantees a relatively good output through a progressive evaporation process at a constant decreasing pressure, producing 4000 m3/day of desalinated water. It is a small installation if we compare it with the big desalination plants that have been built in recent years. Some current investigations also revealed a model which has a manifold, an evaporation tower, and a condensation tower, but with no conclusive results yet.

Subramani and Jacangelo [24] made a critical review of new emerging desalination technologies, considering membranes that incorporate nanoparticles, carbon, or graphene nanotubes; they also analyzed alternative technologies like the ones based on the deionization and on microbial desalination cells. From all these options only nanocomposite membranes have been commercialized.

Estevan and García [25] stated that in Spain, desalination has evolved very positively since the first facilities were launched in the early 1970s, that were designed by thermal type processes (MSF, MED and MVC). These facilities were large energy users with specific consumption which could exceed 30–40 kilowatts/hour per cubic meter of desalinated water. In the 1980s the first reverse osmosis installations were introduced and coexisted with the evaporation technologies, mainly MVC, and with important energy reduction consumption: 15 kWh/m<sup>3</sup> for vapor compression plants and 8–10 kWh/m<sup>3</sup> for those of reverse osmosis. The evolution of specific consumption in the field of desalination by reverse osmosis, through successive technological innovations in energy recovery systems, reduced to 3 kWh/m3, contributing very significantly to the huge increase of production capacity. The graph of the evolution of the installed capacity/specific consumption ratio in Spain done by Centro de Estudios y Experimentación de Obras Públicas (CEDEX) is attached below in Figure 1.

**Figure 1.** Evolution of energy consumption to desalinate in Spain 1970–2010. Source: CEDEX.

Today, reverse osmosis processes have been achieved under 2.7 kWh/m3, by slowly reducing the consumption through energy recovery systems and by achieving a higher efficiency in the membranes, which are the key elements in the process.

Jia et al [26] analyzed the energy consumption, greenhouse gas (GHG) emissions, and cost of seawater desalination in China. The energy consumption and GHG increased from 81 MWh to 1561 MWh from 2006 to 2016. The unit product cost (UPC) of seawater desalination is shown in the Table 3. They concluded that there was potential for energy consumption, GHG emission, and cost reduction with the application of energy recovery units, the integration of desalination plants and renewable energies or low potential heat, as well as the development of new technologies.


**Table 3.** Unit product cost of seawater desalination technologies.

However, the energy cost is still the most significant in large industrial desalination plants, as well as the consequences regarding the sustainability of generating the necessary energy.

Improvement is still possible, mainly in three aspects:


The next significant step would be to lower the working pressure, which would reduce energy consumption [20]. Nevertheless, this cost should be put in the context of what it really involves, by comparing it to other activities or development, which are either not considered or are even positive measures, without analyzing them altogether. This is like promoting the electric car without considering the origin of the energy necessary to charge those vehicles.

The Water Corporation [27], regarding energy consumption, estimated that desalination uses more energy than water supply by using traditional methods, such as the gravity feeding of water from a dam. However, the energy used to provide enough desalinated water daily for a family of four is the same quantity as to operate an air conditioner for just an hour.

AEDYR [21] explained the consumption equivalence to desalinate with the following reasoning: if we bear in mind that the energy consumption of an average home in Spain is 13,141 kWh/year, and the daily average consumption per person is 150 liters/day, taking as a reference that the average energy consumption to produce 1 m<sup>3</sup> of desalinated water is 3 kWh/m3, with the energy consumption of an average home, 80 people can be supplied with desalinated water for a whole year.

For years, there has been progress in renewable energy production plants, mainly solar and wind, associated with big desalination plants. Kalogirou [28] studied seawater desalination using renewable energy sources. Charcosset [29] provided a state-of-the art review on membrane processes associated with renewable energies for seawater and brackish water desalination. Eltawil [30] provided a review of renewable energy technologies integrated with desalination systems.

Petersen et al. [31] and Lindermann [32] studied wind and solar powered desalination plants for the Mediterranean, Middle East, and Gulf countries. Ghermandi and Messalem [33] provided a state-of-the art on renewable powered seawater desalination plants. Palenzuela et al. [34] valued the use of solar power and desalination plants in arid regions.

Initially, renewable energies were not efficient enough to meet the energy demand of large desalination plants. The technological development produced in recent years allows a wind or solar plant to guarantee the demanded electricity supply. For this reason, it is now common to award construction contracts for seawater desalination plants associated with photovoltaic or wind solar plants. In 2019, ACWA Power [35] awarded the Taweelah desalination plant (reverse osmosis) in the United Arab Emirates with a capacity of 909,000 m3/day, including the construction of a 40 MW solar photovoltaic plant. In 2020, ACWA Power [36] awarded the construction of the 600,000 m3/day Jubail 3A osmosis plant in Saudi Arabia, associated with a solar plant.

As another example, Southern Seawater Desalination Plant, SSDP, (Figure 2) located in Binningup, Australia, produces up to 100 billion liters of fresh drinking water a year, around 30% of Perth's water supply. It started production in 2011. The plant is owned by Water Corporation, a public company dependent on the Western Australian Government, which is the main provider of drinking water supply and wastewater treatment services to more than two million people throughout more than 2.6 million square kilometers in Western Australia. Water Corporation [37] says that since production commenced in 2011, WC has purchased energy from a wind farm and a solar farm near Geraldton: Mumbida Wind Farm [38] (55 MW) and Greenough Solar Farm [39] (80 Has, 10 MW). Both the wind and solar farms were developed on the back of a long-term energy purchase agreement associated with the Southern Seawater Desalination Plant.

**Figure 2.** SSDP, general view. Source: Water Corporation.

Stover [10], a member of the Board of Directors of the International Desalination Association, claims that reverse osmosis is still the dominant technology for desalination. Innovation is promoted to increase freshwater performance, to reduce residual brine, and to deal with harder water sources, because innovation stimulates the growth of desalination in industrial and inland brackish water applications.
