Aluminium sulphate is a chemical compound with the formula Al2(SO4)3. It is soluble in water and is mainly used as a coagulating agent in the purification of drinking water and waste water treatment plants, and also in paper manufacturing and as a mordant in dying and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the clarifier or more easily filtered.