The statement 'Sludge from the primary clarifier is removed only one time a day' is true.

• A. False
• B. True
• C. Not sure
• D. Always true

Answer: (A)

The frequency of sludge removal from the primary clarifier isn’t fixed; it depends on how much solids are being produced and how the plant is operated. The clarifier must be kept free of excessive settled solids to maintain hydraulic performance and prevent resuspension, so sludge is drawn off when the sludge depth reaches a set limit or when process conditions require it. That means you might remove sludge several times a day in some plants, or less frequently in others, depending on influent characteristics, flow, and design. Saying it happens only once a day is not universally true, so the statement is false.