Surface Overflow Rate

Divide flow by plan area and you get a velocity: the speed at which water rises through a settling tank. That is the whole of Hazen's 1904 ideal-basin theory: any particle whose settling velocity exceeds the overflow rate reaches the floor, any slower particle is carried over the weir, and the tank's depth does not enter into it. A clarifier taking 8640 m³/d over 216 m² of surface runs at 40 m/d, which is the same thing as 40 m³ of water per square metre per day and, in North American units, about 980 gallons per day per square foot.

Design values are worth memorising: 30–50 m/d for a conventional water-treatment sedimentation basin, 30–50 m/d for a primary clarifier at average flow, and only 16–28 m/d for a secondary clarifier following activated sludge, because the light, flocculent biological floc settles far more slowly than grit or alum floc. The counter-intuitive consequence of Hazen's result is that making a shallow tank deeper does not improve capture at all, only adding area does, which is exactly why tube and plate settlers were invented, stacking many shallow settling surfaces inside one tank footprint. Depth still matters in practice, for sludge storage and for keeping currents from scouring the floor.