In an activated sludge plant, the clarifier provides detention time for solids to compress into a sludge blanket that will have solids concentration approximately 3 to 10 times as much as in the aeration tank.

• A. Sludge mat
• B. Sludge layer
• C. Sludge bed
• D. Sludge blanket

Answer: (D)

In an activated sludge plant, the solid matter that settles in the clarifier forms a dense layer known as the sludge blanket. The clarifier’s detention time lets these settled solids pack tighter as water drains away, so the solids concentration in that blanket becomes much higher than in the aeration tank—roughly 3 to 10 times as high. This dense, suspended layer travels slowly through the clarifier, separating from the clear effluent as it’s carried toward the bottom and then removed or recycled as needed.

This term is the standard way to describe the concentrated zone of settled solids in the clarifier. A sludge mat, by contrast, implies a surface or bottom accumulation that isn’t the typical settled- solids blanket within the clarifier; a sludge bed refers to a fixed, bottom layer in other types of systems; and a sludge layer is a more general label that doesn’t specify the distinct settled zone in the clarifier.