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

• A. 8 ft
• B. 10 ft
• C. 12 ft
• D. 14 ft

Answer: (C)

The depth of a secondary clarifier is chosen to give enough settling distance and stable sludge behavior while keeping construction and operation practical. A depth of about 12 feet provides a solid balance: it gives the suspended solids a long enough path to settle before reaching the effluent, supports a reasonable sludge blanket, and keeps the surface area requirements and costs in check. If the clarifier were only 8 feet deep, solids might have insufficient residence time to settle, raising effluent turbidity; if it were much deeper, like 14 feet, the extra depth adds cost without proportionally improving settling under the same flow and surface area. So, describing the clarifier as 12 feet deep aligns with typical design practice and supports effective clarification at a reasonable cost.