What is the diameter of the secondary clarifier described in the case?

• A. 30 ft
• B. 45 ft
• C. 60 ft
• D. 75 ft

Answer: (B)

In a secondary clarifier, the diameter is chosen to provide enough surface area for the design flow to settle efficiently. The surface area of a circular clarifier is A = πD^2/4, and the surface overflow rate is SOR = Q/A. The case describes a flow that requires about 1,600 ft^2 of surface area to meet the target SOR and allow proper settling. A 45-foot diameter yields about 1,590 ft^2 of surface area (A ≈ π × 45^2 / 4 ≈ 1,590 ft^2), which matches that requirement. A smaller diameter would reduce surface area and raise the SOR beyond the design limit, risking poor settling, while a larger diameter would be more than needed. So the diameter used in the case aligns with the required surface area, making 45 feet the correct choice.