Where should the smallest particles be expected to settle within the average detention time in a sedimentation tank?

• A. Far end
• B. Near end
• C. Middle
• D. Top

Answer: (A)

In a sedimentation tank, how fast a particle settles depends on its size and density—its settling velocity. With a fixed average detention time, the larger, heavier particles have enough time to reach the bottom early in the flow path, while the smallest, slowest-settling particles need more time. As water moves toward the outlet, these tiny particles continue to settle only slowly, so within the given detention time they are most likely to settle toward the far end of the tank (the outlet side). In other words, the smallest particles are expected to be found settling at the far end because they require the extra time to overcome their low settling velocity.