Polyaluminium chlorides are synthetic polymers dissolved in water. They react to form insoluble aluminium poly-hydroxides which precipitate in big volumetric flocs.The flocs absorb suspended pollutants in the water which are precipitated with the PAC and can together be easily removed.

Chemical formula:[AL₂(OH)_nCL_6-n]m

Polyaluminium chloride (PAC) is increasingly used for water treatment. Against the conventional use of aluminium sulphate (alum) it is showing distinct advantages.PAC can be used as a flocculant for all types of water treatment treatment, drinking water, industrial waste water, urban waste water and in the paper industry.