In an anaerobic digester, the scum layer forms at which part of the tank?

• A. Bottom Surface
• B. Middle Layer
• C. Side Walls
• D. Top Surface

Answer: (D)

The main concept here is buoyancy and how layers form in a digester. Scum is composed of lighter materials—fats, oils, greases, and other floating solids—that rise to the surface of the liquid. As heavier solids settle toward the bottom, forming sludge, the lighter materials float and accumulate at the very top. This top surface layer is the scum layer, and it’s typically removed with a skimmer to keep the digestion process efficient. The middle remains mostly liquid with suspended particles, and the bottom holds the settled sludge. So, the scum layer forms at the top surface.