Which two factors determine the detention time in a sedimentation basin?

• A. Temperature and pH
• B. Flow Rate and Volume
• C. Turbidity and Color
• D. Influent velocity and headloss

Answer: (B)

Detention time in a sedimentation basin is defined by how long water stays in the basin, which is set by the basin’s volume and the flow rate through it. The fundamental relationship is t = V / Q, where t is the hydraulic retention time, V is the basin volume, and Q is the inflow (or through) rate.

Thus, the two factors that determine detention time are the flow rate and the volume. If you increase the volume while keeping the same flow, water remains longer, increasing detention time. If you increase the flow while keeping the same volume, water spends less time in the basin, reducing detention time.

Other factors like temperature, pH, turbidity, or color describe water quality, not how long water sits in the basin. Influent velocity and headloss relate to flow conditions and energy losses, not the basic residence time calculation. For example, with a flow of 1000 m^3/h and a basin volume of 2000 m^3, the detention time is 2 hours. Doubling the volume or halving the flow would increase the time accordingly.