Generally, raw water contains dissolved mineral impurities viz. Chlorides, Sulphates, Alkalinity, Silica, Sodium, Hardness due to Calcium, Magnesium, etc. These are dissolved impurties form the total dissolved solids. Total Dissolved Solids(TDS) can be measured online with reference to conductivity of water. For specific applications, the dissolved mineral impurities / conductivity levels have to be within the acceptable limits. The dissolved impurities in water can be reduced by use of Ion Exchange Process which is well known. The ion exchange system can be desinged to produce treated water with a TDS of < 5 ppm and in terms of conductivity < 30 ms/cm at 25 deg C.

The influent water is led to a cation unit containing ion exchange resins in Hydrogen (H+) form. When the influent water containing calcium, magnesium, sodium (Cations in water) comes in contact with the resins in Hydrogen form, Hyrdrogen ions are released in water in exchange of Calcium, Magnesium and Sodium. A very small quantity of Sodium does escape through. Depending on the feed water quality, the water is either led to a degasser tower or to an anion exchange unit. Typically, a degasser tower is an atmospheric vessel containing a set of plastic packing rings. The decationised water is sprayed in to the packing rings against an upflow of air. This leads to a scrubbing action and dissolved gas viz. CO2 gets released and is vented out. The degassed water is collected in a tank and then pumped to the Anion Exchange unit. The influent water Decationised / degassed water is led to the anion unit containing ion exchange resins in Hydroxy (OH) form. The influent water containing chloride, sulphate, alkalinity, silica (anions in water) comes in contact with the resins in Hydroxyl (OH) form, the OH ions gets released in water in exchange of chloride, sulphate, Alkalinity and silica. A small quantity of silica slips through the anion exchange unit. Ion Exchange reaction is a reversible one. On a predetermined time or output (when all the H+ in cation resin and OH- in anion resin is exhausted), the unit is regenerated with HCl and NaOH respectively.