After a main break, the water must be dosed with chlorine to a concentration of what?

• A. 100 mg/L for 60 minutes
• B. 25 mg/L for 2 hours
• C. 10 mg/L for 24 hours
• D. 300 mg/L for 15 minutes

Answer: (D)

After a main break, you need rapid, effective disinfection of the distribution system. This is done with shock chlorination: a very high chlorine concentration for a short period to ensure the disinfectant penetrates through the system and creates a strong enough concentration-time (CT) to inactivate microbes that may have entered during the break. Dosing to around 300 mg/L for about 15 minutes provides a large CT value, meaning a robust disinfecting effect in a short timeframe. This approach minimizes the chance of surviving contaminants and allows the system to be flushed and re-tested quickly. Lower doses or longer times (for example, 100 mg/L for 60 minutes or 25 mg/L for 2 hours) would not achieve the same rapid, comprehensive disinfection. After the shock treatment, the system is flushed and monitored to ensure residual chlorine returns to safe levels before normal operation resumes.