When the aeration is turned off, the clear water that forms on top of an aerobic digester is called?

• A. Backwash
• B. Supernatant
• C. Centrate
• D. Filtrate Nitrogen

Answer: (B)

When the aeration stops, the mixture in the digester starts to separate: solids settle toward the bottom while clearer liquid remains on top. That clear liquid layer is called the supernatant. It represents the liquid portion that has separated from the settled solids and can be drawn off or decanted without disturbing the sludge below. This term is used to distinguish the liquid above settled sludge from the heavier, suspended solids beneath.

Backwash refers to the process of cleaning filters by reversing flow, so it isn’t the liquid atop a digester. Centrate comes from dewatering or centrifugation of sludge, not from a resting digester. Filtrate is the liquid that has passed through a filter, and “filtrate nitrogen” isn’t the standard term for the layer in a digester.