Surface Overflow Rate (SOR) Calculator
Calculate the Hydraulic Loading for Clarifiers and Settling Tanks
Understanding Surface Overflow Rate (SOR)
In wastewater treatment and water purification, the Surface Overflow Rate (SOR)—also known as surface loading rate—is a critical design parameter for sedimentation basins and clarifiers. It measures the volume of water applied per unit of surface area of the tank per unit of time.
The SOR represents the upward velocity of the water. For a solid particle to settle effectively, its downward settling velocity must be greater than the upward velocity of the water (the SOR). If the flow rate is too high relative to the surface area, solids will be carried over the effluent weirs rather than settling to the bottom.
The SOR Formula
The calculation for SOR is straightforward:
Where:
- Q is typically measured in Gallons per Day (GPD) or cubic meters per day.
- A is the surface area of the water in the tank (Length × Width for rectangular tanks, or π × r² for circular tanks).
Standard Design Ranges
Depending on the type of clarifier, typical SOR values in wastewater treatment include:
- Primary Clarifiers: 600 – 1,200 GPD/sq.ft
- Secondary Clarifiers (Activated Sludge): 400 – 800 GPD/sq.ft
- Secondary Clarifiers (Trickling Filter): 400 – 600 GPD/sq.ft
Practical Example
Suppose you have a rectangular primary clarifier that is 80 feet long and 25 feet wide. The plant is currently processing 1.5 million gallons per day (MGD).
Step 1: Calculate Surface Area
Area = 80 ft × 25 ft = 2,000 sq.ft
Step 2: Calculate SOR
SOR = 1,500,000 GPD / 2,000 sq.ft = 750 GPD/sq.ft
Interpretation: This value falls within the standard range for primary clarification (600-1200), suggesting the tank is sized appropriately for the flow.
Why Does SOR Matter?
Maintaining the correct overflow rate ensures that the hydraulic detention time is sufficient for gravity to pull solids to the tank floor. If the SOR exceeds design limits (due to storm surges or plant overloading), the "rising" water velocity overcomes the "settling" velocity of the sludge, leading to high effluent turbidity and potential regulatory violations.