What is the usual detention time range in a sedimentation basin?

• A. 1-2 hours
• B. 2-4 hours
• C. 4-6 hours
• D. 6-8 hours

Answer: (B)

Detention time is the average time water spends in a sedimentation basin, determined by the basin volume divided by the flow rate. In these basins, enough time must pass for solids to settle by gravity, but not so long that the basin becomes unnecessarily large and costly. For typical drinking-water sedimentation basins, the usual target is about two to four hours. This range provides a practical balance: it allows effective removal of suspended solids given common particle sizes and water conditions, while keeping basin size and operating costs reasonable. Shorter times, around one to two hours, often don’t allow sufficient settling, whereas longer times, such as six to eight hours, would require much larger basins with diminishing returns for most plants.