Grease, oils, and fats that enter a treatment plant will ultimately end up as part of the ___ ___ in an anaerobic digester.

• A. Gas Cap
• B. Scum Layer
• C. Sludge Blanket
• D. Scouring Layer

Answer: (B)

Grease, oils, and fats are lighter than water, so they float and rise to the surface in an anaerobic digester. This floating material forms the scum layer at the top, sitting above the liquid and the sludge at the bottom. The gas produced in digestion collects above the liquid in the gas cap, not within the scum. Keeping the scum layer in check is important because it can hinder mixing and gas collection if it builds up. So the greases end up as part of the scum layer.