What does chlorine residual refer to in water treatment?

Chlorine residual refers specifically to the amount of chlorine that remains in the water after the disinfection process has taken place. This residual is crucial for ensuring that the water continues to be protected from potential pathogens as it moves through the distribution system. It is an indicator of the effectiveness of chlorination in killing bacteria and viruses and also serves to maintain water quality during storage and transportation.

This leftover chlorine can exist in two forms: free chlorine, which is the active form that disinfects, and combined chlorine, which has reacted with contaminants. Monitoring chlorine residual helps water treatment facilities maintain a safety threshold to ensure that treated water remains safe for consumption while preventing the regrowth of microorganisms in the distribution system.

The other options do not accurately describe chlorine residual. The total amount of chlorine applied refers to the initial quantity used for treatment, while the initial concentration relates to the concentration before any reactions occur. The rate of chlorine depletion concerns how quickly chlorine is consumed and does not indicate the amount that remains after disinfection.