Which statement best describes the primary clarifier's function?

• A. It primarily aerates the wastewater.
• B. It primarily removes suspended solids by gravity.
• C. It primarily chemically treats the water.
• D. It primarily transports sludge to digesters.

Answer: (B)

The main concept here is gravity-driven physical separation. A primary clarifier is designed so wastewater slows down enough for suspended solids to settle to the bottom as sludge, while lighter materials may rise to the top as scum. The clarified liquid then exits the tank for downstream treatment, and the accumulated sludge is removed and sent to sludge processing or digestion. This unit’s job is to reduce solids (and some associated organic load) through settling, not to introduce air or do chemical treatment. Aeration happens in the biological treatment stage, and chemical treatment (when used) occurs in other processes. While the clarifier does feed the sludge handling system, its primary function is the gravity separation of solids from the liquid.