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How does coagulation remove particles from drinking water?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

Coagulation is a critical chemical process in water purification used to remove suspended particles that are too small or light to settle on their own.1

The process typically involves two stages:

Coagulation: Chemicals known as coagulants—most commonly aluminum sulfate (alum)—are added to the water during a rapid mixing phase. This ensures the chemicals are dispersed evenly to react with suspended particles.1
Flocculation: Following rapid mixing, the water undergoes a period of gentle agitation. This slow mixing encourages particle collisions, causing the coagulated particles to clump together into larger, heavier masses called "floc".12

Once these larger aggregates are formed, they become heavy enough to settle out of the water in a sedimentation basin. This step is essential for public health, as it significantly reduces the load of suspended solids before the water proceeds to filtration and final disinfection to eliminate harmful microorganisms.12