Weir Loading Rate (Weir Overflow Rate)

Weir loading is flow divided by the length of the lip the water leaves over, and its unit is the one that confuses people: cubic metres per day per metre of weir, which reduces to square metres per day. Five thousand cubic metres a day over twenty-five metres of crest is 200 m³/(d·m); in North American units the same quantity is quoted as gallons per day per foot, and one m³/(d·m) is 80.5 gpd/ft. Ten States Standards caps a water-plant settling basin at 20,000 gpd/ft — about 250 m³/(d·m) — and Metcalf & Eddy puts a secondary clarifier lower, nearer 125 to 185 at average flow, because activated-sludge floc is far easier to drag over a lip than alum floc is.

Two things get this number wrong in practice, and neither is the arithmetic. The first is measuring the weir: an inboard launder collects on both sides, so its length counts twice, and a V-notch plate is measured along its crest line rather than around the notches. The second, and the serious one, is that weir loading is a proxy for something it does not measure. What actually pulls solids out of a clarifier is the upward velocity in the water converging on the weir, and that velocity is set as much by density currents and by whether the crest is level as by the average loading. A weir a few millimetres out of level sends nearly all the flow over its low end, multiplying the real local loading several times while this calculation goes on reporting the average serenely. Check level with a hose or a laser before adding weir length, because releveling costs a morning and a new launder costs a season.