Coagulation is the process of adding chemicals to surface waters to collect particulate matter and colloids into clusters that can be removed from the solution by subsequent sedimentation and filtration through granular media.

Coagulants:- The most widely used coagulants for water and wastewater treatment are aluminum and ion salts. The common salt is aluminum sulfate, which is a good coagulant for water-containing organic matter. Iron coagulants are effective over a wider pH range and are generally more effective in removing color from water, but they usually cost more.

Aluminum sulfate (filter alum) — Aluminum sulfate is the standard coagulant used in water treatment.  A formula used for filter alum is Al2(SO4)3. 18 H20 with a molecular weight of 600.