Wastewater math
Clarifier surface overflow rate in gpd/ft², from the flow in MGD and a circular clarifier's diameter in feet.
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The formula
Surface Overflow Rate, gpd/ft² = Flow, gpd ÷ Area, ft²WPI Formula/Conversion Table (2023), Printed page 6 of 10, Surface Loading Rate or Surface Overflow Rate
Area = (0.785)(Diameter²)WPI Formula/Conversion Table (2023), Printed page 1 of 10, Area of Circle
Worked example
A 50 ft diameter clarifier receives 1.2 MGD. What is the surface overflow rate?
Find the surface area: 0.785 × 50 ft × 50 ft = 1,962.5 ft².
Put the flow in gallons per day: 1.2 MGD × 1,000,000 gal/MG = 1,200,000 gpd.
Divide the flow by the area: 1,200,000 gpd ÷ 1,962.5 ft² = 611 gpd/ft².
Answer: 611 gpd/ft²
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