Which term describes the solids that settle out and form sludge in primary treatment?

• A. Sludge
• B. Supernatant
• C. Effluent
• D. Filtrate

Answer: (A)

In primary treatment, the solids that settle to the bottom during gravity settling form a distinct layer called sludge. This material is the accumulated, settled solids itself. The liquid that remains above that layer is the supernatant, and the water that eventually leaves the plant as treated effluent is, well, the effluent. Filtrate refers to liquid that has passed through a filter, which isn’t the situation here. So the term that best describes the solids that settle out and form sludge is sludge, because it specifically denotes the settled solids layer produced in the clarifier during primary treatment.