Weir Overflow Rate Calculator
Calculate Hydraulic Loading for Clarifiers & Settling Tanks
Understanding Weir Overflow Rate
The Weir Overflow Rate (WOR), also known as Weir Loading Rate, is a critical hydraulic parameter in the design and operation of clarifiers and sedimentation basins in water and wastewater treatment plants. It measures the volume of water passing over the weir per unit length of the weir per day.
Proper calculation of the overflow rate ensures that the velocity of the water leaving the tank is low enough to prevent the carryover of suspended solids (floc) into the effluent launders. If the rate is too high, turbulence increases, and solids may be dragged out with the clean water, reducing treatment efficiency.
The Formula
The standard formula for calculating Weir Overflow Rate is:
Where:
- WOR = Weir Overflow Rate (typically in gpd/ft)
- Q = Total Flow Rate (in Gallons per Day, GPD)
- L = Total Length of the Weir (in Feet)
Typical Design Criteria
Design standards vary by region (e.g., Ten State Standards) and the type of treatment process. However, typical ranges include:
- Primary Clarifiers: 10,000 to 15,000 gpd/ft
- Secondary Clarifiers (Activated Sludge): 10,000 to 20,000 gpd/ft (peak flow can go higher)
- Small Plants: Generally require lower rates to provide a safety factor against hydraulic surges.
Calculation Example
Suppose a wastewater treatment plant has a peak daily flow of 2.5 MGD (Million Gallons per Day). The circular clarifier has a total weir length of 150 feet.
- First, convert MGD to GPD: 2.5 × 1,000,000 = 2,500,000 GPD.
- Apply the formula: 2,500,000 / 150.
- Result: 16,666.67 gpd/ft.
This result would be compared against the facility's permit limits or design specifications to ensure compliance.