Sludge blanket depth should not be higher than bottom ___ of the clarifier sidewall depth.

• A. 1/2
• B. 1/4
• C. 1/3
• D. 3/4

Answer: (B)

Sludge blanket depth in a clarifier should be kept shallow to prevent solids from being carried over with the effluent and to maintain stable clarification performance. The safe, commonly used limit is a depth no higher than one-quarter of the clarifier’s sidewall depth, measured from the bottom up. Keeping the blanket within this quarter-depth helps ensure clear effluent and reliable sludge removal by the clarifier mechanism.

For example, if the clarifier is 8 feet deep, the sludge blanket should not exceed about 2 feet from the bottom. Options that imply a deeper blanket (such as one-half, one-third, or three-quarters of the depth) are more likely to cause solids carryover and poorer effluent quality, so they are not used in practice.