Surface Overflow Rate (SOR) is how we measure the hydraulic loading for a clarifier.  It’s calculated by dividing effluent flow of a clarifier by the clarifier surface area.  Typical values are 800 gal/ft²·d for average flow and 1200 gal/ft²·d for peak conditions. Higher flow rates result in higher SORs and stress the clarifiers more.  Tracking SOR can help in troubleshooting clarifier performance.