Wastewater New York State Practice Exam

What is the weir overflow rate of a primary clarifier that has a diameter of 60 ft., depth of 15 ft., and an influent flow of 3.0 mgd?

1. 10,000 gpd/ft of weir

2. 12,000 gpd/ft of weir

3. 15,924 gpd/ft of weir

4. 20,000 gpd/ft of weir

The correct answer is: 15,924 gpd/ft of weir

To calculate the weir overflow rate, it is essential to first determine the surface area of the clarifier and then use the influent flow rate to find the overflow rate per foot of weir. The primary clarifier is circular, so its surface area can be calculated using the formula for the area of a circle, which is A = πr². Given the diameter of the clarifier is 60 feet, the radius (r) would be half of the diameter, or 30 feet. Therefore, A = π(30 ft)² = π(900 ft²) ≈ 2827 ft² (using 3.14 for π). Next, given that the influent flow is 3.0 million gallons per day (mgd), we need to convert this into a more usable unit for our calculations. 1 mgd = 1,000,000 gallons per day, thus: 3.0 mgd = 3,000,000 gallons per day. Now, to find the weir overflow rate, we divide the influent flow by the total surface area of the clarifier. So, the weir overflow rate = Total flow / Surface area: = 3,000,000