Why do we add alum to purify water?

Chemical formula of alum is KAl(SO₄)₂ known as Potassium aluminium sulphate.In raw water, bicarbonate salts are present as impurities. Alum, when added react with alkaline bicarbonates and forms a gelatinous precipitate. This also attracts other fine and suspended particles and settled down at the bottom of the container. In this way we can get purified water.