What is the recommended detention time for a sedimentation basin?

• A. 2 hours
• B. 90 minutes
• C. 20 minutes
• D. 6 hours

Answer: (A)

Detention time, or hydraulic retention time, is the amount of time water spends in the sedimentation basin to let suspended solids settle by gravity. For a typical sedimentation basin used as a primary clarifier, about two hours is the standard design detention time because it provides enough time for the majority of settleable solids to settle before the water exits, while keeping basin size practical for the plant’s throughput. If the time were too short, like twenty minutes, solids wouldn’t settle adequately, leading to poor clarification. If it were much longer, such as six hours, the basin would be oversized for the given flow and unnecessary in most designs. A time around ninety minutes sits in between but is not the common convention taught for this application.