What is meant by Desalination of brackish water? How it is carried out by Electro dialysis?

Desalination:The process of removing salts like sodium chloride from the water is known as desalination.

Commonly employed methods for desalination of brackish water are:

i) Electro dialysis ii) Reverse osmosis

i) Electrodialysis:

 It is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential.

Principle: 

 When direct current is passed through saline water using electrodes, salt ions present in saline water

migrates towards their respective electrodes through ion selective membrane, under the influence of applied emf.

Apparatus: The electro dialysis unit consists of a chamber, two electrodes a cathode and an anode. The chamber is divided into three compartments with the help of thin, ion selective membranes which are 

permeable to either cation or anion

Process:

 When direct electric current is passed through saline water, the Na+ ions moves towards cathode (-ve pole) and the chloride ions moves towards anode (+ve pole) through membrane. 

 The concentration of brine decreases in the central compartment,where as the concentration in outer compartments increases.

 Desalinated brine (pure water) is removed from time to time, while concentrated one is replaced by fresh brine.

 Ion selective membranes are employed for more efficient separation,which has permeability for one kind of ions.

 Cation selective membrane (which possesses functional groups such as RSO3- ) is permeable to cations only.

 Anion selective membrane (possess functional groups such as R4N+Cl- ) is permeable to anions only

Electrodialysis cell:

 It consists of a large number of paired sets of rigid plastic membrane.

 Saline water is passed at a pressure of 5-6 kg m-2

between membrane pairs.

 Electric field is applied to the direction of water flow

 Fixed +ve charges inside the membrane repel +ve charge ions ( Na+ ) and permits –vely charged ions.

 Fixed -ve charges inside the membrane repel -ve charge ions (Cl-) and permits +vely charged ions.

 Alternative streams of pure water and brine water are obtained.

Advantages:

 It is a compact unit, economical.

 Best suited if electricity is easily available.