What is the purpose of a secondary clarifier in the activated sludge process?

• A. To aerate the influent and oxidize organics.
• B. To settle mixed-liquor suspended solids and produce clarified effluent, returning sludge to the aeration basin.
• C. To remove dissolved metals via chemical precipitation.
• D. To store treated effluent before discharge.

Answer: (B)

The main idea here is that a secondary clarifier is the gravity-based separator that cleans the liquid after biological treatment. In the activated sludge process, air is used in the aeration basin to keep microorganisms active and to break down organics, producing a mixed liquor full of water, microbes, and suspended solids. When this mixture flows into the secondary clarifier, gravity settles the suspended solids, separating them from the clarified liquid. The clarified effluent leaves the clarifier as the treated water, while the settled solids become sludge. A portion of that settled sludge is returned to the aeration basin as return activated sludge to maintain the microbial population and treatment capacity, and excess sludge is wasted to control solids levels. So the clarifier’s purpose is to produce clarified effluent and to recycle a portion of the biomass back to the aeration basin to keep the process working efficiently. This is different from aeration (which happens in the aeration basin), chemical metals removal (which occurs elsewhere in treatment or pretreatment), or storage of treated water (which is not the clarifier’s function).