When using a 300 mg/L chlorine dose for a water main break disinfection, what is the required contact time?

• A. At least 30 minutes
• B. At least 60 minutes
• C. 5 minutes
• D. At least 15 minutes

Answer: (D)

Disinfection effectiveness depends on the CT value—the product of chlorine concentration and contact time. A higher chlorine dose increases the CT, so you can achieve the needed inactivation in a shorter period. For a water main break disinfection using a 300 mg/L dose, the minimum contact time specified is 15 minutes. This duration ensures enough exposure for the disinfectant to act while avoiding unnecessary longer contact. A shorter time like 5 minutes would typically not provide enough exposure at that dose, while longer times (30 or 60 minutes) aren’t required as the high dose already achieves the needed CT in 15 minutes. After the 15-minute contact period, you’d verify chlorine residual and continue with flushing and sampling per protocol.