How does a higher surface overflow rate in a sedimentation basin specifically impact the removal of smaller, less dense particles?

A higher surface overflow rate in a sedimentation basin reduces the effective settling time for particles, particularly affecting the removal of smaller, less dense particles. The surface overflow rate is the flow rate of water divided by the surface area of the sedimentation basin; it essentially represents the upward velocity of the water in the basin. Sedimentation basins rely on gravity to settle suspended particles. Particles settle downwards at a velocity determined by their size, density, and shape, as described by Stokes' Law. Smaller, less dense particles have a lower settling velocity than larger, denser particles. In a sedimentation basin, a particle must settle to the bottom before the water reaches the outlet. If the upward velocity (surface overflow rate) is too high, particles with a slow settling velocity will be carried out of the basin before they can reach the bottom. A higher surface overflow rate means there's less time for these slow-settling particles to descend to the bottom before the water exits. As a result, the effluent water will contain a higher concentration of these smaller, less dense particles, reducing the overall effectiveness of the sedimentation process. For example, if the surface overflow rate is increased beyond the design capacity of the basin, very fine clay particles or light organic matter will be carried over, increasing the turbidity of the treated water.