What chlorine residual must remain after 24 hours for a new water main?

• A. At least 1 mg/L
• B. At least 20 mg/L
• C. At least 10 mg/L
• D. 0 mg/L

Answer: (C)

Chlorine residual is the amount of disinfectant that remains in the water to guard against microbial growth as water moves through the system. For a new water main, you want to confirm the line still has a measurable free chlorine after a period of time, which shows the disinfection is persisting through the pipe and flushing process. Requiring a residual of 10 mg/L after 24 hours provides a robust margin to overcome the initial chlorine demand of the fresh pipe and any contaminants that could be released during commissioning. A lower residual, such as 0 mg/L or 1 mg/L, wouldn’t guarantee continued disinfection, while a much higher value like 20 mg/L would be unnecessarily excessive and could cause taste or odor issues. Thus, 10 mg/L is chosen to ensure the line remains adequately disinfected during the critical commissioning period.