Ferric chloride (FeCl3, also known as iron chloride hexahydrate, ferric trichloride hexahydrate, ferric trichloride and iron chloride) is extremely aggressive, readily attacking most metals; however it is an excellent flocculent. When added to water, it hydrolyses into electropositive ferric hydroxide, attracting negatively charged colloidal substances and thereby forming flocs that clump together and precipitate out.
Through this mechanism, ferric chloride reduces biological oxygen demand and removes heavy metals, larvae eggs, pathogenic content, suspended solids and colloidal content. It precipitates phosphates, which is a major advantage in the fight to avoid eutrophication of rivers and lakes. Ferric chloride is not the cheapest flocculating agent, but it is widely viewed as the most cost-effective because the required dosage is lower.