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Chlorine Residual
That portion of the total available chlorine residual remaining in water or wastewater at the end of a specified contact period. Chlorine residual will react chemically and biologically as hypochlorous acid (HOCl) or hypochlorite ion (OCl–). This does not include chlorine that has combined with ammonia, nitrogen, or other compounds.
1. At 30 minutes after the addition of sodium hypochlorite there should be no more than 2.0 mg/L of free chlorine residua l present (this ensures the water does not have an unpleasant taste or odor).
2. At 24 hours after the addition of sodium hypochlorite to containers that are used by families to store water there should be a minimum of 0.2 mg/L of free chlorine residual present (this ensures microbiologically clean water).
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