Impurities in water and the treatment steps can make it either acid or alkali. When water has undergone coagulation with Alum (see COAGULATION) the pH is reduced.
Lime may also be added to set the pH to lower levels, which is the level where chlorine kills germs most effectively. Once the chlorine has done it’s work (see CHLORINATION) then the water is set back to neutral which is safe and ready to go to your house.