April 25, 2020
With the increasing need for clean water in the major cities in the world, the desalination of marine waters is becoming a crucial option to address the challenge. This is however hindered with the exorbitant technological and infrastructural costs involved in the process. With the world’s major water body (about 98%) being marine or brackish, desalination becomes an option that can be sustainable than any other known source of water in the world. Currently, the most economical means of desalination used commonly is evaporation by use of solar energy; this is despite the fact that this method needs quite a large surface area for effective operation (Humplike et al).
Several methods of sea water desalination exists, with differing advantages in relation to the areas they are applied in. All of them are however generally classified into three categories depending on their mode of extracting salt from the marine or brackish water. They are;
Mechanisms involving phase change:
This method has sea water undergo a phase change into ice or vapor. In this category, brine can be evaporated on the surface of highly heated vapor or flashed into low-pressure compartment to enhance evaporation. Methods utilizing this mechanism include; solar distillation using solar energy (as it is applied in the Solar Evaporation and Pumping System and solar stills), humidification or membrane distillation (John &Stephen, 1). This method requires large surface area to work efficiently. Freezing of sea water has also been used to desalinate since salt remains in the liquid form while the water solidifies. Freezing is regarded more economical in comparison with vaporization since the latent heat of fusion is theoretically lower than the heat of vaporization.
Mechanisms involving short-range interactions with a selective material:
Grouping of steric, dispersion and electrostatic interactions during grouping of water with other materials is used in desalination. This can be accomplished by use of chelating agents to adsorb metals or by use of selective media transport as widely used in Reverse Osmosis (Humplike et al, 3).
Mechanisms involving long-range electrostatic interactions:
This applies the use of long-range electrostatic fields to extract charged ions from water. By electrokinetics, ions can be adsorbed or transported from the water to desalinate it. Since water is not ionized, it is not affected by the electrostatic fields. This is as applied in electrocialysis and capacitive deionization (Humplike et al, 3).
The use of solar stills is a common practice in desalination of sea water. The stills utilize the evaporation-condensation mechanism to separate ions from water. They are made of transparent cover enclosing a pan of saline water, this condition causes the heating and evaporation occurs, leaving behind salt and other organic components. Water produced from them is of high quality and readily used by human (Kabeel & El-Agouz, 2).
With the rising concern to provide portable water to the rapidly increasing human population, the need to have better ways to improve desalination technology is also increasing. The chemistry and physical actions involved provide adequate grounds to understand and improve on the existing systems of desalination. Use of solar energy is of common application in desalination, but other mechanisms exists which need to be exploited to enable their improvement and better utilization to ensure adequacy in water provision.
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