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How does flocculation help purify 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.

In the context of public health and water treatment, flocculation is a critical process used to remove suspended particles that are too small or light to settle out of water on their own.1

Following the addition of chemicals during coagulation, which destabilizes particles so they can clump together, flocculation involves a period of gentle agitation. In a dedicated basin, large paddles slowly mix the water to encourage these particles to collide and aggregate into larger, heavier masses known as "floc".21

By increasing the size and weight of these particles, flocculation allows them to settle more efficiently at the bottom of a sedimentation basin. This clarification step is essential because it significantly reduces the load of suspended solids, microorganisms, and other impurities before the water moves on to filtration and final disinfection, ensuring the water is safe for public consumption.2