What is the function of a clarifier in a conventional activated sludge system?

• A. To remove settled solids from the mixed liquor after biological treatment.
• B. To aerate the mixed liquor.
• C. To digest solids biologically.
• D. To chemically oxidize contaminants.

Answer: (C)

A clarifier uses gravity to separate solids from the liquid after biological treatment. The mixed liquor from the aeration zone flows into the clarifier, where the activated sludge and other solids settle to the bottom, forming sludge that can be recycled or wasted. The clarified liquid exits from the top as effluent for further treatment or discharge. Digestion of the solids happens in separate digesters, not in the clarifier, and aeration happens in the aeration basin, not in the clarifier. Chemical oxidation is also not the role of the clarifier. So the primary function is to remove settled solids from the mixed liquor after biological treatment.