**4. Conclusion**

*3.2.3.1. Advantages and disadvantages of MD*

a limitation for this process [11].

• Freezing and ion exchange processes

*3.4.1. Advantages and disadvantages*

**3.5. Ion exchange: Solvent process**

present in solution. For example, Na<sup>+</sup>

exchanger and Cl−

**3.3. Alternative processes**

cesses [23]

344 Desalination and Water Treatment

**3.4. Freezing**

• Membrane distillation is simple and operates at low temperature. Hence it utilizes less amount of heat. Coupling MD units with solar energy sources makes it more attractive.

• Main disadvantage is that MD requires more space compared to other membrane pro-

• In this process the feed water should not have any organic pollutants. This fact turns to be

Though a number of other methods have been used in water desalination process, but none of them turned to be commercially as successful as MSF, RO, and ED technologies. Some

The principle involved in freezing desalination is that, in the process of freezing, the dissolved salts present in the feed water are separated during the formation of ice crystals. Seawater can be desalinated by cooling the water to form crystals under controlled conditions. Before the total amount of feed water has been frozen, the mixture is washed and rinsed to remove the salts present in the remaining water or that is sticking to the ice crystals. The ice is then melted to produce fresh water. Since the main heat transfer processes involved are freezing and melting which are regenerative, this method is said to have very high energy efficiency [24].

• This process uses theoretically lower energy and has less chance for corrosion and scaling.

• The disadvantage in this process is handling ice and water mixtures which are mechani-

Ion exchangers are generally organic or inorganic solids which are capable of exchanging one type of cation (or anion) immobilized on the solid surface for another type of cation (or anion)

the complete 'demineralization' of a NaCl solution. This process can be reversed by regenerat-

ions can be similarly replaced with OH<sup>−</sup>

ing the cation exchanger with an acid, and the anion exchanger with a base.

ions in solution can be replaced with H+

by a cation

by an anion exchanger, resulting in

• It operates at a lower pressure than other pressure-driven membrane processes

alternative methods are also used for desalination process. They are

• It produces pure drinking water and also water for irrigation

cally complicated to move and to be processed [25].

The desalination of brackish water and seawater proves to be a reliable source of fresh water and is proves to be a solution for the world's water shortage problem. Desalination processes are normally used to produce drinking water in areas where only seawater or brackish water is the source of water. A number of technologies have been developed and many more methods are under R&D for desalination. They can be used for small scale, that is, supplying water for small communities (e.g. solar distillation) and large scale to supply water to cities, that is, huge plants (e.g. reverse osmosis).

Though desalination costs seem to be progressively decreasing, but they are still costlier than conventional drinking water processes. Coming to environmental aspects each desalination plant has to take proper measures in case of intake of water, pre-treatment of water as well as disposing concentrate reject water that is produced in the process because environmental aspects are equally important as commercial aspects.
