SOR is flow divided by clarifier surface area. It controls clarification.
Clarifier Surface Overflow Rate
Formula
Area = 0.785 x Diameter² x Number of clarifiers SOR, gpd/ft² = Flow, gpd / Area, ft²
Inputs used
- Flow (influent, not including RAS) (MGD)
- Clarifier diameter (ft)
- Clarifiers in service (units)
- Or total area (overrides diameter) (ft²)
Worked example (White River WRRF, fictional)
- Area = 0.785 x 90² x 4 = 25,447 ft²
- SOR = 8,000,000 / 25,447 = 314 gpd/ft²
Tips
- Use influent flow only (RAS leaves through the floor).
- Check both average and peak hour.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.