Which component is typically the final settling unit in an activated sludge plant?

• A. Aeration basin
• B. Aeration tank
• C. Sludge digester
• D. Final clarifier

Answer: (D)

In an activated sludge plant, the last stage that separates the treated liquid from the biomass is the final clarifier. After the mixed liquor is aerated and biologically treated, it flows into this settling basin where solids settle by gravity. The clarified liquid then proceeds to disinfection or out to the effluent, while the settled solids are routed to sludge handling (return and waste activated sludge). The aeration basin provides the oxygen and mixing for biological treatment, not settling, and a sludge digester handles stabilization and storage of sludge, not the final separation of solids from the treated water.