Desalination: The process of removing salts like sodium chloride from the water is known as desalination. Desalination or desalting of water consists of a water treatment process by which sea or brackish water is converted into potable water for supplying communities that have the most difficulty accessing freshwater.
Brackish water refers to a water source that is somewhat salty (more so than freshwater) but not as salty as seawater. The exact amount of salinity will vary depending on environmental factors and can not be precisely defined. The salinity is usually measured in a range rather than an exact amount.
There are two primary distillation methods used to desalinate brackish water: multistage flash distillation and solar distillation. Multistage flash distillation is used in large-scale operations, while solar distillation is used in small-scale operations and in small communities.
Two distillation technologies are used primarily around the world for desalination: thermal distillation and membrane distillation. Thermal distillation technologies are widely used in the Middle East, primarily because the region's petroleum reserves keep energy costs low.
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 a semi-permeable membrane, under the influence of an electric potential. Electro Dialysis (ED) is a membrane process, during which ions are transported through a semi-permeable membrane, under the influence of an electric potential.
The membranes are cation- or anion-selective, which basically means that either positive ions or negative ions will flow through.
Principle:
When a direct current is passed through saline water using electrodes, salt ions present in saline water migrate toward their respective electrodes through the 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
Reverse osmosis. (R.O)
Reverse osmosis is one of the membrane filtration processes. The process is used to remove salts and organic micro pollutants from water.
Principle:
Osmosis: When two solutions of different concentrations are separated by a semi-permeable membrane, solvent flows from the region of low concentration to the region of high concentration. This is called Osmosis.
The Pressure developed on the membrane is called osmotic pressure. If the pressure higher than the osmotic pressure is applied on the concentrated side, then the solvent flows in the reverse direction i.e from the higher concentrated region to the lower concentrated region. This is called reverse osmosis.
In this process, pure solvent (water) is separated from its contaminants, rather than removing contaminants from water. Sometimes it is also called super/hyperfiltration.
Method:
- The reverse osmosis cell consists of a chamber fitted with a semi-permeable membrane above which, seawater or impure water is taken.
- Pressure (of the order 15-40 kg cm-2) is applied to the seawater/impure water.
- Pure water is forced through the semi-permeable membrane.
- The membrane consists of very thin films of cellulose acetate/ superior membrane made of polymethacrylate and polyamide polymers, affixed to either side of the perforated tube.
Advantages:
- Removes ionic and non-ionic colloidal particles and high molecular weight organic matter.
- It removes colloidal silica
- The lifetime of a semi-permeable membrane is high.
- The membrane can be replaced within a few minutes.
- Low capital and operating cost & high reliability