Why polyaluminium chloride can remove colloidal substances in water? Polyaluminum chlorides enable the removal of colloidal substances in water due to some physicochemical interactions during coagulation because: (1) Adsorption Polymeric aluminum chloride is a polymeric substance that acts as an adsorptive bridging agent in water , Leaving the water particles bonded to each other into large particles, and then use the precipitation method to remove colloidal substances. (2) Neutralization Since PAC produces a large amount of highly-charged positive ions in water, most of the colloidal substances in natural water are negatively charged, so that they attract each other and neutralize each other, thereby eliminating the problem of colloidal particles Of electrostatic repulsion, and can grow into large particles, weight loss by settlement. (3) the role of surface contact Flocculation process is the core of the particles as the core on the surface, leaving the particle surface contact, and bonded into large particles, by precipitation to remove. (4) the role of filtration flocculation Settlement in the water process, like a filter down, so wrapped with other particles to settle together. Colloidal substances are removed by these complex processes as described above.