Detention time in a sedimentation basin is determined by flow rate and what other factor?

• A. Temperature
• B. Turbidity
• C. Volume
• D. pH

Answer: (C)

Detention time in a sedimentation basin depends on how long water stays in the basin, determined by the basin’s volume and the flow rate. The hydraulic detention time is t = V / Q, where V is the basin volume and Q is the influent flow rate. A larger volume means water remains longer, allowing more time for particles to settle, while a higher flow rate pushes water through faster, reducing the time available for settling. Temperature, turbidity, and pH influence treatment effectiveness, not the residence time itself. To achieve a target detention time, engineers size the basin so that V/Q matches the desired duration; real-world factors like dead zones can cause actual time to differ from the theoretical value.