**2. Desalination techniques**

Desalination of brackish/saline water can be done in a variety of ways. Among the distillation methods, multistage flash desalination (MSFD), multi-effect distillation (MED), and reverse osmosis (RO) are considered the most commercial and economic viable technologies for distillation. These techniques are the most effective, and they will continue in the future [17]. Desalination can be done in several ways, as listed below.

### **2.1 Multistage flash distillation (MSFD)**

MSFD is based on flashing of evaporation. Evaporation of saltwater occurs as a result of a decrease in pressure rather than a rise in temperature. Broadly speaking, regenerative heating is used to achieve a maximal production and best MSFD's economics. The seawater flashing in the flashing chamber regenerates and transfers its thermal energy to the saltwater passing *via* the flashing action. Because this is a regenerative heating system, it must be completed in stages. Then, it would be better if we increased the incoming saline water temperature [18]. The condensation heat (released when condensating the water vapor) is responsible for the progressive raise in saline water temperature. The heat input, heat recovery, and heat rejection are the most important factors for MSFD unit [19].

Multistage evaporators, with roughly 19–28 stages, are utilized in recent MSFD plants [20]. The MSFD operating temperatures are between 90 and 120°C. The more the operating temperature of unit, the higher the efficiency of the system. Also, the pressure should be controlled under the water saturation temperature.

In the unit (see **Figure 3**), various devices or accessories such as demisters, decarbonators, and vacuum deaerators are employed for various reasons. Demisters are used to prevent the carryover of saline water into condensed distillate. Removing the dissolved gases in brine is the function of decarbonator and vacuum deaerator [22].

#### **2.2 Multiple-effect distillation (MED)**

MED method is the earliest of all distillation techniques. Thermodynamically, it is the most constitutional technique among all other distillation methods. The term "effect" of the MED name refers to how many evaporators' series. Then, the main fundamental of MED is the decrease in ambient pressure through various effects. In this process, no additional heat is required after the first effect. This is due to that

**Figure 3.** *Illustration of processes of MSFD [21].*

#### *Distillation Processes - From Solar and Membrane Distillation to Reactive Distillation…*

MED automatically makes the saline water to be boiled many times through the corresponding pressure.

In this process, the saline water temperature is increased to its boiling value at the first effect through the heat exchanger tubes. The entering saline water is sprayed over the evaporator surface for better vapor generation. The created vapor is condensed on the other side of tubes. On the second hand, the condensation process is utilized on the same time to heat up the saline feedwater. **Figure 4** illustrates the desalination with solar energy.

MED economy relies on its effects number regarding its chained processes [8]. So, the first effect has some generated vapor from the seawater entering the effect. After that, the remaining water is fed to the next effect. The amid tubes are warmed up using the first-effect created vapor. Also, that vapor is condensated to produce potable water. Besides, the generated vapor heats up the remaining saline water of the next effect. This action is repeated from one effect to another under low pressure and temperature till the end of process with around 4–21 effects [23].

### **2.3 Vapor compression distillation (VCD)**

As it is known, raising the pressure of steam vapor leads to increase its temperature, and hence, the heat released from this vapor is increased. This concept is utilized in the VCD method, where the vapor-increased heat is utilized to evaporate the incoming

**Figure 4.** *Desalination by solar energy [2].*

#### *Thermal Desalination Systems: From Traditionality to Modernity and Development DOI: http://dx.doi.org/10.5772/intechopen.101128*

saline water. So, the main key of VCD is reducing the medium pressure, which leads to decrease the boiling temperature of water [24]. This process can be achieved by either steam jet technique (thermo-compressor technique) or mechanical compressor device (electrically driven technique), and these techniques are utilized to condense vapor content into distillate and use the corresponding latent heat as a heat source for evaporating the incoming saline water.

In the technique of thermo-compressor, a venturi orifice is used in the stream of steam jet to create low pressure. After that, the vapor content is compressed *via* the steam jet with the help of the venturi orifice device. Therefore, the generated vapor content is condensated over the tubes' surfaces with releasing heat to vaporize the incoming salt water.

In addition, there is another kind of VCD, which depends on lowering the temperature inside the VCD, and hence, this kind needs only power to run. This technique of VCD is useful for small desalination units due to its simple construction, better efficiency, and process reliability [25]. As a result, VCD is applicable in resorts, industries, and drilling sites.

#### **2.4 Reverse osmosis (RO)**

It is known that the water flows naturally from the freshwater direction to saltwater direction. RO depends mainly on a critical parameter called osmotic pressure, which should force the saltwater to flow in the opposite direction of natural flow. As a result, we need an external source of pressure to overcome the osmotic pressure. That external pressure must be more than the osmotic pressure. Hence, the name of this method (reverse osmosis) refers to the process meaning reversing the direction of normal water flow through the membrane. By applying this process, the salts in the saline water are left behind and not allowed to cross the membranes [26]. This process produces potable water (permeate) and brine water (concentrate) as illustrated in **Figure 5**. Also, **Figure 5** illustrates the main components of RO unit such as the pretreatment, membranes, high-pressure pumps, and post-treatment.

The pretreatment process is important for eliminating the undesirable materials that damage the membrane [27]. It relies on the membrane kind and configuration, properties of feedwater, recovery ratio, and required permeate quantity and quality. According to those parameters, the pretreatment process has different techniques to be applied such as chlorination, coagulation, acid addition, micron cartridge filtration, multimedia filtration, and dechlorination. Moreover, to overcome the osmotic pressure, high-pressure pumping system such as the centrifugal pump is utilized. Additionally, the different membrane configurations such as the spiral wound and hollow fine fiber (HFF) strongly affect the performance of the RO unit [28]. Besides, adjusting the pH and adding H2S and CO2 are performed in the post-treatment process [29].

#### **2.5 Freezing**

Another type of desalination processes is nominated freezing, which is simple to conduct and operate. This process depends mainly on the fact that the dissolved salts are removed while forming the ice crystals from the saline water. First, we clean and wash the saline water mixture to leave the salt in the left water behind before freezing the whole water. Then, we can get the freshwater *via* melting the ice crystals. As a result, freezing has the processes of saline water cooling, partial creation of ice, separation of ice from saline water, ice melting (obtaining freshwater), and finally refrigeration and heat rejection [30].

*Distillation Processes - From Solar and Membrane Distillation to Reactive Distillation…*

#### **Figure 5.** *Schematic of RO desalination technique [16].*

Freezing units have the merits of low power consumed, few corrosions, and eliminated scaling factors. But it has the demerits of handling water and ice mixtures because it is mechanically hard to treat. Unfortunately, this method still needs numerous improvements to be applied on the commercial level. Limited freezing stations exist all over the globe such as that was built in Saudi Arabia [31].

#### **2.6 Solar desalination**

Here, the sun energy is the driving force for such solar desalination systems, which actually are alike the natural hydrological cycle. The natural hydrological cycle takes every day in the form of heating the seawater using the solar radiation to produce vapor, and this vapor content is condensated due to the low temperature in the heights. An application on the solar distillation systems is the greenhouse solar distiller [25, 32].

#### **2.7 Potabilization**

This process is almost linked to MSFD systems. This is because MSFD produces distillate with small impurities amounts, and then, the potabilization is utilized to eliminate these impurities [33]. The potabilization process can be conducted *via* two different methods: injecting CO2 and hydrated lime [34] and carbonated water is passed through limestone bed filters [35]. Potabilization has four main processes: carbonation, liming, chlorination, and aeration. The main functions of liming and carbonation are increasing the hardness, alkalinity, mineral content, and pH of the targeted water. Also, chlorination (performed *via* using chlorine gas or calcium hypochlorite) aims at avoiding the infected water [36], while aeration aims at replacing the oxygen inside the water to enhance its taste.

#### **3. Desalination economics**

The economics of any system determine its success. In desalination stations, the parameters of fixed cost, running costs, station location, maintenance cost, and

#### *Thermal Desalination Systems: From Traditionality to Modernity and Development DOI: http://dx.doi.org/10.5772/intechopen.101128*

energy consumption are the controlling factors of the station economics. There are two opposite directions for determining the economics of the desalination systems. First is the improvements in desalination systems, which leads to reduce the cost of the system as a whole. Second is the pollution and contamination of water, which raises the cost of the desalination system.

The economics and technical properties of the desalination method with the targeted quantity of freshwater are the factors based on which we select the distillation technique. The technical properties include the energy-driving source, consumed energy, freshwater specifications, land space of unit, station reliability, operational aspects, plant size, and the maintenance of spare parts, while the economic parameters include the fixed costs of the station, operating costs, interest rate, life cycle of station, and maintenance costs …etc. [37]. The cost of distilled freshwater is described by \$/m3 . The cost of desalinated water is determined using the following equation.

 *All spent and estimated costs through station lifetime Cost of desalinated water Total produced water quantity* <sup>=</sup> (1)

### **4. Solar distillers**

As explained before, solar distillation is one of the desalination techniques. Solar distiller is one of the solar distillation devices' family. It is a simple in construction, cheap, and easy to maintain device [38]. But it has the demerit of few freshwater production. As a result, the scientists do their best to improve the yield of solar distillers [39–45]. The solar distiller has the parts of glazing cover, basin, collecting trough, and some instruments as shown in **Figures 6** and **7**. The basin is fed by saline water to be heated and vaporized inside the distiller. Then, the vapor is condensed on the inner glazing surface. After that, the condensated droplets are taken out using a hose into the calibrated flasks. The surfaces of the solar distiller are painted with black for maximizing the absorbed solar radiations. Also, the device body is insulated by saw dust or fiberglass for avoiding the thermal losses. Moreover, the measuring instruments are utilized to be able to evaluate the solar distiller performance.

**Figure 6.** *Single-basin distiller [46].*

**Figure 7.** *Solar still with simple basin [40].*

#### **5. Methods of improving the solar distiller performance**

One of the biggest problems of the twenty-first century is the global freshwater scarcity, which has numerous side effects on the mankind [47, 48]. The widespread of the water problem on a global level has made more impacts on the lives of people who live far from urbanization and remote areas, and more and more on the lives of the poor who do not have the costs of using high technology to produce water. As a result, the science developed various high and low water desalination technologies to be used at the level of industrial and commercial production and at the level of individuals and families [49]. The water desalination technologies can be categorized by membranes or thermal processes. Nevertheless, the high technologies demand building complex and large central installations, which causes them to be infeasible for developing regions such as distributed, poor, and remote areas. In addition, the rural, arid, and remote areas need desalination methods with no or minimum maintenance, supervising, and operating requirements [41, 45]. Consequently, the solar-powered desalination units such as solar distillers meet all these conditions, which make them as an efficient candidate to provide drinkable water in these regions [42, 43].

Nevertheless, the solar distillers have low output productivity (1.5–2.5 L/m2 day) and low thermal efficiency (≈ 30%), which are the main bottlenecks of this distillation technique [50–52]. As a result, numerous investigations focused on improving the performance of solar distillers. Consequently, the solar stills such as stepped type [53–55] (the absorber of the basin takes the steps shape), disk type [56] (the main effective absorber is a rotating disk), vertical type [57] (the distiller horizontal width is very small compared to its vertical height), tubular type [58–60] (the outer shape of the distiller is tubular/cylindrical), drum type [61, 62] (the absorber is a rotating drum inside the basin still), PV/T active type [63, 64] (distiller powered by PV panels), finned type [65] (the absorber of the distiller is a collection of fins), trays type [66–69] (the main effective absorber has trays to enlarge the surface area), inclined type [70] (the absorber is inclined), wick type [10, 71, 72] (the wick material is spread over the absorber of distiller), corrugated type [73, 74] (the absorber of the distiller is a collection of corrugated shapes), spherical type [75] (the distiller takes the shape of sphere), double-effect type [76–78] (the distiller has two stages of water basins), multistage type [79] (the distiller has multistages of water basins), inverted absorber type [80] (the distiller has inverted absorber inside it), hemispherical type [81] (the distiller takes the shape of sphere), convex type [82, 83] (the absorber has a convex shape), and pyramid type [84–86] (the distiller

*Thermal Desalination Systems: From Traditionality to Modernity and Development DOI: http://dx.doi.org/10.5772/intechopen.101128*

takes the form of pyramid) are found in the literature. In addition, numerous modifications were performed to improve the distiller performance. These modifications included the use of condensation unit [87–89], nanofluids [90, 91], heat exchanger [92], floating aluminum sheet [93], desiccant [94], solar ponds [95], glass cooling [44], volcanic rocks [96], wick materials [71], rotating parts [97–99], coated absorbers [100, 101], phase change materials (PCM) [102–104], fins [105], half barrel and corrugated absorbers [106], solar collectors [107], sun-tracking systems [108], multiple-effect basins [109], reflectors [110], vapor extraction [111–113], and reusing latent heat of evaporation [114].

### **6. Solar distiller types**

The solar distillers can be classified into single-effect and multi-effect distillers (according to the number of effects of distiller) with a subcategory of active and passive distillers (according to the vaporization heat source) inside every classification [2]. In the passive distillers, the vaporization occurs directly without external sources of heat, while using external heat sources such as collectors and concentrators are used in the active solar distillers.

#### **6.1 Single-effect distillers**

This is the traditional solar distiller (or conventional distiller) without any modifications [115]. Also, it is used as a reference for the other modified distillers' performances. Here, in this type of distillers, there is just one glass cover over the basin water. Also, the thermal losses in this type of distiller are large due to the single glass cover, which hence decreases its performance. Therefore, the efficacy of this distiller is low around 30–40% [2]. As a result, numerous experimental and empirical investigations were performed to augment the distiller performance. This type of distillers (single-effect solar distillers) is categorized into active and passive distillers.

#### *6.1.1 Active solar distiller*

The word "active" means that the solar distiller is integrated with somewhat external source of heat such as the solar concentrators and collectors [2]. Then, the active distillers include the following items:


**Figure 8** shows the active distiller integrated with evacuated collector.

*Distillation Processes - From Solar and Membrane Distillation to Reactive Distillation…*

**Figure 8.** *Active distiller integrated with evacuated collector [116].*

## *6.1.2 Passive solar distiller*

Here, the heat source for basin water vaporization is only from inside of distiller [2]. Then, the passive solar distillers include the following kinds.


For example, **Figure 9** obtains a passive distiller incorporated with condenser, and **Figure 10** reveals a passive distiller incorporated with internal and external mirrors.

#### **6.2 Multi-effect distillers**

Multi-effect solar distillers are different in design from the single-basin distillers. Also, the modified design of multi-effect distillers helped enhancing *Thermal Desalination Systems: From Traditionality to Modernity and Development DOI: http://dx.doi.org/10.5772/intechopen.101128*

#### **Figure 9.**

*Passive distiller integrated to condenser [117].*

**Figure 10.**

the performance of the distillers. Here, in these types of distillers, the condensation latent heat is utilized as a recovery source of heat to obtain more vaporization through the effect of the distillers, and hence, the freshwater production is augmented [119]. Now, multi-effect solar distillers are categorized into two main sections: active and passive distillers as the following.

#### *6.2.1 Active distiller*

It is the same concept of the single-effect distiller. The word "active" means that the solar distiller is integrated with somewhat external source of heat such as the solar concentrators and collectors. Various distiller kinds can be found in the literature under the category of active distiller-based multi-effect distillers.


A multi-effect active distiller with collector is illustrated in **Figure 11**. Also, **Figure 12** shows a condensation-evaporation active distiller.

**Figure 11.**

*Double-effect single-slope active still: a coupled with solar collector in thermosiphon mode and b coupled with solar collector in forced circulation mode [120].*

**Figure 12.**

*Condensation–evaporation active distiller [121].*

**Figure 13.** *Wick-passive distiller [122].*

*Thermal Desalination Systems: From Traditionality to Modernity and Development DOI: http://dx.doi.org/10.5772/intechopen.101128*

**Figure 14.** *Double-effect double-basin passive distiller [46].*
