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In the context of water purification, ion-exchange is a rapid and reversible process in which impurity ions present in the water are replaced by ions released by an ion-exchange resin. The impurity ions are taken up by the resin, which must be periodically regenerated to restore it to the original ionic form. (An ion is an atom or group of atoms with an electric charge. Positively-charged ions are called cations and are usually metals; negatively-charged ions are called anions and are usually non-metals).

Sand Filter
Sand Filtration Sand filtration can be either rapid or slow. The difference between the two is not a simple matter of the speed of filtration, but in the underlying concept of the treatment process. Slow sand filtration is essentially a biological process whereas rapid sand filtration is a physical treatment process. Slow sand filters have an advantage over rapid sand filters in that they produce microbiologically "clean" water which should not require disinfection to inactivate any bacteria, although the addition of a disinfectant to provide a residual for the distribution system is still advisable. However, because of their slow flow rate, slow sand filters require large tracts of land if they are to supply large populations and can be relatively labour intensive to operate and maintain. As the reestablishment of the schumtzdecke takes several days, the plant has to have sufficient capacity to supply the water demand when one or more filters are out of action. Rapid sand filtration is now commonly used worldwide and is far more popular than slow sand filtration. The principal factor in this decision has been the smaller land requirement for rapid sand filters and lower labour costs. However, rapid sand filters do not produce water of the same quality as slow sand filters and a far greater reliance is placed on disinfection to inactivate bacteria. It is also worth noting that rapid sand filters are not effective in removing viruses.

Carbon filter
Granular activated carbon (GAC) is commonly used for removing organic constituents and residual disinfectants in water supplies. This not only improves taste and minimizes health hazards; it protects other water treatment units such as reverse osmosis membranes and ion exchange resins from possible damage due to oxidation or organic fouling. Activated carbon is a favored water treatment technique because of its multifunctional nature and the fact that it adds nothing detrimental to the treated water. Most activated carbons are made from raw materials such as nutshells, wood, coal and petroleum. Typical surface area for activated carbon is approximately 1,000 square meters per gram (m2/gm). However, different raw materials produce different types of activated carbon varying in hardness, density, pore and particle sizes, surface areas, extractable, ash and pH. These differences in properties make certain carbons preferable over others in different applications. The two principal mechanisms by which activated carbon removes contaminants from water are adsorption and catalytic reduction. Organics are removed by adsorption and residual disinfectants are removed by catalytic reduction.

Softwater Treatment Plant
Softening is the ion exchange process, it is done through resin, the most common and probably the easiest method of removing hardness (that is calcium and magnesium) from water and suitable for utility purpose. As the name implies ion exchange is a process in which undesirable ions are exchanged for more desirable ions. The softening process consists of passing raw water containing hardness through a bed of Cation resin in sodium form. The hardness ions Ca & Mg are taken up by resin and in exchange, the sodium ions are relinquished from the resin. This is called the service cycle, where the hard water is being softened.
The following ions are widely found in raw waters :

Cations Anions
Calcium (Ca2+) Chloride ( Cl-)
Magnesium (Mg2+) Bicarbonate (HCO3-)
Sodium (Na+) Nitrate (NO3-)
Potassium (K+) Carbonate (CO32-)

Ion Exchange Resins
There are two basic types of resin - Cation-exchange and anion-exchange resins. Cation exchange resins will release Hydrogen (H+) ions or other positively charged ions in exchange for impurity cations present in the water. Anion exchange resins will release hydroxyl (OH-) ions or other negatively charged ions in exchange for impurity anions present in the water.
The application of ion-exchange to water treatment and purification. There are three ways in which ion-exchange technology can be used in water treatment and purification : first, Cation-exchange resins alone can be employed to soften water by base exchange; secondly, anion-exchange resins alone can be used for organic scavenging or nitrate removal; and thirdly, combinations of Cation-exchange and anion-exchange resins can be used to remove virtually all the ionic impurities present in the feedwater, a process known as deionization. Water deionizers purification process results in water of exceptionally high quality.

For many laboratory and industrial applications, high-purity water which is essentially free from ionic contaminants is required. Water of this quality can be produced by deionization. The two most common types of deionization are :
Mixed-bed deionization
In mixed-bed deionizers the Cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The thorough mixture of Cation-exchangers and anion-exchangers in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants. As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant.
Although more efficient in purifying the incoming feedwater, mixed-bed plants are more sensitive to impurities in the water supply and involve a more complicated regeneration process. Mixed-bed deionizers are normally used to ‘polish' the water to higher levels of purity after it has been initially treated by either a two-bed deionizer or a reverse osmosis unit.