What is the typical detention time for a primary clarifier, and what is its main function?

• A. About 5 to 10 hours; sedimentation for final removal.
• B. About 0.5 hours; neutralization.
• C. About 24 hours; dewatering.
• D. About 1 to 3 hours; settle primary solids and remove portion of BOD/TSS.

Answer: (D)

Solids must settle by gravity in a primary clarifier, so the detention time is kept short enough for efficient settling without excessive sludge buildup. About 1 to 3 hours is typical. The unit’s main function is to settle primary solids and remove a portion of the organic load, specifically BOD and suspended solids, from the incoming wastewater. The settled solids form sludge that’s removed periodically, while lighter materials rise to the surface as scum and are skimmed off. This step reduces the load on downstream treatment and improves effluent quality for secondary treatment.

Shorter times, like half an hour, wouldn’t give enough opportunity for particles to settle; much longer times, around 5–10 hours or more, aren’t practical for a primary clarifier and would lead to other process dynamics. A 24-hour detention time would resemble digestion rather than primary clarification, and neutralization or dewatering are different treatment objectives not addressed by a primary clarifier.