What is the diameter of the primary clarifier used in the example?

• A. 60 ft
• B. 40 ft
• C. 70 ft
• D. 50 ft

Answer: (A)

The diameter of a primary clarifier is chosen to provide enough surface area for solids to settle at the plant’s expected flow, using the surface overflow rate (SOR) as the design guide. In the example, the given flow and target SOR are matched by a 60 ft diameter, which gives a surface area of π × (60/2)^2 = 900π ft^2 ≈ 2,827 ft^2. That area yields the required SOR for the influent flow, so this diameter fits the design goals. If a smaller diameter were used, the surface area would be less and the SOR would be higher than intended, potentially reducing settling efficiency. A larger diameter would lower the SOR too much and might not meet the desired detention time for solids. Hence, 60 ft is the diameter that satisfies the example’s design conditions.