What term describes the layer of settled solids that forms in the clarifier?

• A. Sludge blanket
• B. Biofilm
• C. Scum
• D. Sludge digest

Answer: (A)

In a clarifier, solids that settle out of the wastewater collect together to form a layer called the sludge blanket. As the mixed liquor slows down, suspended solids drop out of suspension and accumulate near the bottom, creating this blanket. It’s distinct from other surface or surface-attached features: scum floats on the top, consisting of oils and fats; biofilm is a microbial layer on surfaces; sludge digest is a separate tank where sludge is stabilized. The sludge blanket is the identifiable layer of settled solids within the clarifier itself, often continuously removed as waste sludge to keep the process effective.