Where does sludge go after concentration to 4-8%?

• A. Aeration basin
• B. Disinfection chamber
• C. Digester
• D. Primary clarifier

Answer: (C)

Concentrating sludge to about 4–8% solids prepares it for stabilization and volume reduction. The next step is digestion, usually an anaerobic digester, where the concentrated sludge is kept in a sealed, oxygen-free environment so microorganisms break down the organic matter. This process reduces the sludge mass, stabilizes it to lessen odors and pathogens, and produces biogas that can be captured for energy. After digestion, the sludge is more stable and easier to dewater and dispose of.

The other options aren’t the correct path for sludge after thickening: an aeration basin treats the wastewater itself with air for biological oxidation, a primary clarifier collects settleable solids from influent but isn’t where thickened sludge is stabilized, and a disinfection chamber treats the effluent, not the sludge.