The flow through a sedimentation basin should not exceed...

• A. 0.5 ft/min
• B. 0.8 ft/min
• C. 1.0 ft/min
• D. 1.5 ft/min

Answer: (A)

In a sedimentation basin, gravity does the work of pulling suspended solids down, so the water must move slowly enough for particles to settle before they reach the outlet. If the flow is too fast, particles won’t have time to settle and will be carried out with the effluent, reducing removal efficiency. That’s why a conservative maximum flow is used—about 0.5 ft per minute—so there’s enough detention time for typical solids to settle. Velocities higher than this shorten the time for settling and increase solids in the effluent, which is why the other options are not acceptable. Keep in mind that the exact rate can vary with particle size and basin depth, but 0.5 ft/min is the standard guideline.