In slug method disinfection, the required chlorine concentration and contact time are?

• A. 150 mg/L, 2 hours
• B. 300 mg/L, 3 hours
• C. 350 mg/L, 4 hours
• D. 400 mg/L, 5 hours

Answer: (B)

Slug method disinfection relies on delivering a short burst of high free chlorine to a defined portion of the distribution system and keeping it in contact long enough to inactivate pathogens in that slug. The standard target for this approach is about 300 mg/L of free chlorine maintained for roughly 3 hours. This combination provides sufficient microbial inactivation (a practical CT value) while staying operationally feasible and avoiding excessive taste/odor or byproduct concerns downstream. In practice, you would ensure thorough mixing, isolate the slug, monitor chlorine levels as it passes, and then flush and restore normal residuals once disinfection is confirmed. Lower doses or shorter contact times would not achieve adequate disinfection, while higher doses or longer times are unnecessarily aggressive for this method.