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What happens at a water-treatment plant before water reaches homes?

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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.

At a water-treatment plant, the process of turning raw water into potable water is essential for protecting public health. While specific methods depend on the source quality, the process generally follows these steps before reaching homes:12

Pretreatment and Clarification

Intake: Surface water first passes through intake screens to remove debris and fish.1
Coagulation and Flocculation: Chemicals are added to cause small suspended particles to clump together. Large paddles then mix the water to form larger clumps known as "floc".3
Sedimentation: The water enters a basin where the heavy floc settles to the floor as sludge, which is then removed.3
Filtration: The water is percolated through porous materials, such as sand or anthracite coal, to remove remaining suspended particles, microorganisms, and algae.34

Final Treatment

Disinfection: This final, critical step destroys pathogenic microbes like bacteria, viruses, and protozoa. It is typically achieved using chlorine, chloramines, ozone, or ultraviolet radiation.35
Additional Processes: Depending on the source, plants may also use fluoridation to prevent tooth decay, as well as softening, aeration, or carbon adsorption to improve taste, odor, and water quality.31