Free chlorine residual values are based on a contact time of how many minutes?

Free chlorine residual values are typically assessed based on a contact time of 30 minutes. This is due to the need for sufficient time to allow chlorine to effectively disinfect water by inactivating pathogens. The 30-minute contact time is a standard used in many water treatment protocols to ensure that the chlorine has adequate time to react with microbial contaminants, ensuring higher disinfection efficacy.

A shorter contact time would not provide enough opportunity for chlorine to achieve the desired disinfection, potentially leading to an inadequate removal or inactivation of harmful microorganisms. This makes the 30-minute standard particularly important in ensuring safe drinking water quality. The values derived from this duration are crucial for determining adequate chlorine dosing needed to maintain effective residual levels in the distribution system after treatment.