What chlorine concentration is used initially for new water main disinfection?

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Disinfecting a new water main relies on creating and maintaining a strong free chlorine residual inside the pipe so microorganisms are inactivated during the disinfection period. The minimum concentration used for this initial disinfection is no less than 25 mg/L of free chlorine, kept in contact with the pipe interior for the required dwell time (often around a full day) before flushing and testing. This level is high enough to ensure effective disinfection of the interior surfaces, including areas that might harbor microbes, while avoiding unnecessary excessive tastes, odors, or corrosion.

A concentration of 10 mg/L would not reliably disinfect the pipe interior, while higher values like 50 mg/L or 100 mg/L may be used in some protocols or special situations but are not the stated minimum for this standard.