Coagulation is the process of destabilizing colloidal particles by neutralizing their surface charges. Flocculation is the process of agglomerating small particles into fragile networks of larger particles called flocs. The resulting flocs can subsequently be separated from the bulk liquid by either flotation, sedimentation, or filtration. In natural waters, most colloids are negatively charged due to the adsorption of a variety of negative functional groups on their surfaces.  Thus inorganic coagulants are almost exclusively metal salts having high, positive valences. These include alum, ferric sulfate, ferric chloride, aluminum chlorohydrate, and polyaluminum chlorides. Determining the optimal coagulant for a given application generally requires onsite field evaluation.  The coagulant dosage required for adequate performance often correlates better with the amount of electrically-charged colloidal material present in the untreated water, rather than on the untreated turbidity itself.

Alum, ferric sulfate, ferric chloride, aluminum chlorohydrate, and polyaluminum chlorides are very effective coagulation and flocculation agents because:

• They possess high positive valences for charge-neutralization.
• They form, via hydrolysis, metal hydroxide-based floc particles which adsorb onto and agglomerate with suspended and colloidal particles. The size of the resulting floc particles facilitates their removal by the combined action of sedimentation and filtration.