What is the minimum free chlorine residual at the customer tap that is considered acceptable for ongoing disinfection?

• A. Trace
• B. 0.1 mg/L
• C. 0.2 mg/L
• D. 0.5 mg/L

Answer: (C)

Maintaining a free chlorine residual at the customer tap is essential to keep disinfection active as water travels through the distribution system. The commonly accepted minimum for ongoing disinfection is 0.2 mg/L. This level provides a reliable barrier against microbial regrowth and contamination, even as chlorine is consumed by reacting with organics and biofilms along the way. A residual described as trace or as low as 0.1 mg/L isn’t guaranteed to sustain disinfection throughout the system, and while a higher residual like 0.5 mg/L can be effective, it’s not the minimum requirement. Therefore, 0.2 mg/L is the baseline that utilities aim to meet to ensure ongoing protection.