In the slug method for disinfecting a pipe, what minimum chlorine concentration and contact time are required?

• A. 100 mg/L, 1 hour
• B. 200 mg/L, 2 hours
• C. 300 mg/L, 3 hours
• D. 400 mg/L, 4 hours

Answer: (C)

Disinfection by the slug method relies on delivering a slug of highly chlorinated water that stays in contact with the pipe interior long enough to inactivate microbes, even where debris or pipe material potential chlorine demand exists. Because large mains and complex piping can create areas where disinfectant contact is limited, a relatively high concentration must be maintained for a substantial contact period to ensure all surfaces are adequately disinfected. The minimum practical combination used in this method is 300 mg/L of free chlorine held for 3 hours, which provides a strong enough CT value to overcome chlorine demand and achieve a thorough disinfection throughout the line. After the contact time, the line is flushed and samples are taken to verify that the distribution system has an acceptable chlorine residual before normal service resumes. Lower concentrations or shorter contact times would not reliably ensure complete disinfection in all areas of the pipe.