Depends on purpose water purification or treatment systems are different. There are different natural sources of water; in most cases containing much impurity. Sea water is not amenable for water treatments, but obtained from the others natural sources are amenable for water conditioning (treatment). In the process of water purification impurities may be removed completely or partly depending on uses. Following are the main purposes for which treated water could be applied.
For purified water different soluble or insoluble impurities, bacteria, bad test and bad order are eliminated. Usually the municipal water is used for drinking purpose and other household purposes. Insoluble impurities, bad test, bad order and bacteria should not tolerate for drinking purpose. In this case some soluble impurities are useful for health.
For boiler purpose calcium carbonate, calcium sulphate, magnesium carbonate deposits are formed if hard water or without conditioning water are used. This deposit is harmful for boiler. H2SO4 and HCl can be formed in the presence of sulphate and chloride ion which cause corrosion. Zero hardness water is best for boiler purpose.