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Secondary Clarifier Sizing & Overflow Rate Calculator environmental
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Secondary Clarifier Sizing & Overflow Rate Calculator

Determine Surface Overflow Rate (SOR), Solids Loading Rate (SLR), weir overflow rate, and hydraulic detention time per Ten States Standards for wastewater clarifiers.

Plant Flow & Sludge Return

Clarifier Basin Geometry

Clarifier Hydraulic & Solids Sizing

Surface Overflow Rate (Peak SOR)
-- GPD/ft²
-- GPD/ft² at average flow
Peak Solids Loading (SLR)
-- lb/ft²·d
-- lb/ft²·d average
Weir Overflow Rate (WOR)
-- GPD/lin ft
Max 20,000 GPD/ft
Total Settling Surface Area: -- sq ft
Hydraulic Detention Time: -- hrs (Avg) / -- hrs (Peak)
Ten States Standards Status: --
Ten States Standards limit secondary clarifier peak overflow rates to 1,000 - 1,200 GPD/ft² and peak solids loading rates to 35 lb/ft²·day.

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Frequently Asked Questions

What is Surface Overflow Rate (SOR) and why does RAS not affect it?

Surface Overflow Rate (SOR) is the upward liquid velocity expressed as gallons per day per square foot of clarifier surface area (GPD/ft²). Return Activated Sludge (RAS) is withdrawn from the bottom hopper of the tank, meaning only the forward effluent flow rises to the surface weirs. Thus, SOR is calculated using forward plant flow only.

What is Solids Loading Rate (SLR) and why must RAS be included?

Solids Loading Rate (SLR) measures the total mass of biological solids fed onto each square foot of clarifier floor per day (lbs TSS / ft²·day). Because both influent mixed liquor and recycled RAS flow together into the center inlet well, both flows must be summed to calculate the true solids loading entering the clarifier.

What are the Ten States Standards limits for secondary clarifiers?

Recommended Standards for Wastewater Facilities (Ten States Standards) specify: 1) Surface overflow rate shall not exceed 700 to 800 GPD/ft² at design average flow and 1,000 to 1,200 GPD/ft² at peak hourly flow, 2) Peak solids loading rate shall not exceed 35 to 40 lbs/ft²·day, and 3) Minimum side water depth should be 12 to 14 feet.