What is the minimum detention time (DT) required for chlorine disinfection?

• A. 5 minutes
• B. 10 minutes
• C. 20 minutes
• D. 30 minutes

Answer: (D)

Detention time in the chlorine contact basin is the period water must stay in contact with chlorine to allow the disinfectant to work effectively. For chlorine disinfection, a minimum of 30 minutes of contact time is required to help ensure the CT value (concentration times time) meets the target for pathogen inactivation under typical operating conditions. This longer residence time accounts for variations in temperature, pH, and residual chlorine, giving a safety margin so disinfection remains reliable even if conditions aren’t perfect. Shorter times may not provide enough CT, risking inadequate disinfection, so 30 minutes is used as the minimum.