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Why is a small chlorine residual kept in public water distribution systems?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

A small chlorine residual is maintained in public water distribution systems to ensure that the water remains safe for consumption by the time it reaches the points of use.1

While primary disinfection occurs at the treatment plant, maintaining a residual concentration provides ongoing protection as water travels through the pipes. This practice is essential for monitoring the continued effectiveness of the disinfection process throughout the entire distribution network. Unlike other methods such as ozone treatment, which leaves no residual and makes it difficult to track water quality after it leaves the plant, chlorine provides a lasting disinfecting effect.12

To further enhance this protection, some systems use chloramines—a combination of chlorine and ammonia—because their disinfecting effect lasts longer than that of chlorine alone, helping to preserve water quality more effectively throughout the distribution system.1