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.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.