Recommended detention time for sedimentation basins is approximately

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

Answer: (A)

Detention time in a sedimentation basin is the average time water spends in the basin, giving suspended solids a chance to settle by gravity. It’s effectively the hydraulic retention time, calculated from the basin volume and the flow rate entering the basin. For typical municipal sedimentation basins, about two hours provides enough time for the common solids to settle so the water leaving the basin is clearer and with fewer solids carried downstream. If the time were shorter, like a few tens of minutes, more solids would remain suspended and reduce downstream treatment efficiency. If it were much longer, such as several hours, it would unnecessarily reduce plant throughput and lead to oversized basins. So, around two hours is the balanced, practical detention time.