Successful performance of a sedimentation basin depends greatly upon the

• A. pH of influent
• B. Turbidity of effluent
• C. Chlorine residual
• D. Surface settling rate

Answer: (D)

Gravity-driven settling is the main mechanism in a sedimentation basin, so the rate at which solids can settle before leaving the basin is the key factor. This is captured by the surface settling rate, which describes how much water can pass through the basin per unit area while allowing particles to settle. If the surface settling rate is too high, there isn’t enough time for solids to settle, and more particles escape with the effluent, increasing turbidity. A lower surface settling rate provides longer residence time for particles to settle, improving removal.

Other factors shown have different roles. The influent pH can influence how well flocs form and settle, but it doesn’t directly set the settling rate. Chlorine residual is for disinfection, not the physical removal of solids. Turbidity of the effluent reflects performance, but it’s an outcome, not the parameter that governs how quickly solids settle.