The sludge blanket in a secondary clarifier should be kept at or below which fraction of the clarifier sidewall depth?

• A. 1/8 to 1/7
• B. 1/6 to 1/5
• C. 1/2 to 2/3
• D. 1/4 to 1/3

Answer: (D)

Keeping the sludge blanket depth within about one-quarter to one-third of the clarifier sidewall depth ensures a stable layer of settled solids while preserving a clear zone for the clarified water and preventing solids from being drawn into the effluent. If the blanket becomes too deep, the solids are more likely to be entrained during peak flows or by the overflow weirs, increasing effluent turbidity and risking sludge carryover. If the blanket is too shallow, there isn’t enough residence time for solids to settle efficiently, which also harms treatment performance and makes sludge withdrawal harder. This range therefore balances effective removal with protecting effluent quality and allowing reliable sludge collection.