Surface Overflow Rate (SOR)
Flow divided by clarifier surface area, a key settling performance check.
SOR = Flow (gpd) ÷ Clarifier Surface Area (sq ft), expressed in gallons per day per square foot (gpd/sq ft).
Typical secondary clarifiers are designed for 400–800 gpd/sq ft at average flow, with peak flow limits often around 1,000–1,200 gpd/sq ft.
Exceeding the design SOR, often during wet-weather peak flows, can cause solids to be carried over the weir before they've had time to settle.
If SOR is chronically near the limit, options include taking an additional clarifier online, reducing MLSS, or flow-equalizing peak wet weather flows.
Solids loading rate (SLR = MLSS × Flow ÷ Clarifier Area, in lbs/day/sq ft) is a second, equally important clarifier check — a clarifier can pass an SOR check yet still fail on SLR if MLSS is running unusually high.
Common mistake: evaluating SOR only at average flow — the real design constraint is almost always the peak instantaneous flow condition, which can be several times the average during a wet weather event.
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