What is Desalination

Desalination (also called as Desalting) refers to the methods that remove the dissolved minerals, and salt from the brackish water (having salt concentrations from 5 ppt to 20 ppt; while ppt abbreviates for parts per thousand) or sea water ( > 20 ppt), to produce clean and pure water, which can be easily used for domestic or industrial purposes.

How Does Desalination Work

There are many desalination methods used today. We can classify them as:

1) Thermal Desalination

Thermal desalination methods are the widely used desalination techniques all over the world. In thermal desalination, the water brings to boil by the applied heat or the high pressure, and then the water vaporizes and condenses as clean water. The method used for the thermal desalination is called as Distillation.

The distillation of salt water refers to the use of heat energy. The water is heated at a constant temperature, until its vapor pressure becomes equal to the atmospheric pressure (its boiling point attained) and the water begins to vaporize. The salt and other sediments mostly have the greater boiling point then that of the water, so the sediments kept at the bottom, while pure water vaporizes, condensed and used as the pure water, while the sediments left behind are treated separately.

The distillation methods used for industrial desalination of salt water are:

• Multi-Stage Flash Distillation (MSF)

In MSF, the water to be treated is heated in a container called “Brine heater”. Then the heated water enters to the second low pressure container called as “Stage”. Some amount of water starts boiling quickly, because of this lower pressure. The remaining water then flows to the next stage, where the pressure is even lower then that of the second stage, causing more vaporization of water. In the same way, MSF have 15-25 stages, each having lower pressure then the previous.

• Multiple Effect Distillation

It is similar to MSF, because it also works with the series of “effects” (containers), each having lower pressure then the previous one. The difference is that unlike stage, effect is made up of a container and a heat exchanger. So, the water boils, condenses and give rise to the water temperature of next effect.

• Vapor-Compression Distillation

It is generally used for small sized plants. During distillation, a steam jet or mechanical compressor generates heat. The feed water enters, partially vaporizes, and condenses into clean water.

2) Membrane desalination

Membrane desalination can also be divided into two more categories:

2.1) Method uses electric current to attract salt molecules through membrane

• Electro-Dialysis (ED)

In electrodialysis desalination, the electric current is passed through the membranes. The salt and other mineral particles, being ionic attracts towards the membranes leaving desalinated water behind. It has higher water recovery then RO, but generally not usable for sea water, because of higher concentration of salt.

• Electro-Dialysis Reversal (EDR)

This is same as ED, but in reverse.

2.2) Method uses high pressure to force water through membrane

• reverse osmosis

It is the reverse phenomena of Osmosis. It can remove suspended solids, all minerals, organic elements like algae and bacteria and salt from the water. A semi-permeable membrane separates solutions with different concentrations of suspended minerals. An applied pressure forces the dissolved minerals from the higher concentration solution to move towards the lower concentration solution.

• Nano-filtration

It is some what similar to the RO, the membrane used in nano-filtration has larger pore size then that of the RO, and so the lesser pressure is needed to be exerted to pass the water through the membrane.

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