Water softening and demineralization are two important water treatment methods used for many water applications.
Water softening is replacement of hardness causing ions (calcium and magnesium) with equivalent quantity of sodium ions. The total dissolved solids content of the water remain unchanged. Softening process consists of passing raw water containing hardness through a bed of cation resin in sodium form. The hardness ions are replaced with sodium ion from resin. Raw water will continue to get softened till resin gets exhausted. Regeneration with salt solution will bring back the resin in to original form.
Demineralization or deionization is the removal of complete ions from water. The process of demineralization comprises of
- conversion of salts to their corresponding acids by hydrogen cation exchanger
- removal of acids by anion exchanger
The two exchangers are normally in series. Cation exchanger of the hydrogen type are strongly acidic are weekly acidic. Strongly acidic cations contain sulfonic acidic functional group SO3H. Weekly acidic cations contain carboxylic acidic group COOH. Similarly strongly base anion exchangers with ammonium functional group and week base anion exchangers with amine group.
Selection of particular system depends on quality of raw water and desired output quality. A typical DM plant consists of cation, degasser and anion exchangers. Addition of weak acid cation exchanger and weak base anion exchanger increases the plant efficiency. Also a having a mixed for final polishing improves the DM water quality. Degasser removes carbon dioxide generated by dissociation of carbonic acid at cation outlet.