Settleable solids are removed by gravity when the sewage is brought to a ___ state.

• A. Turbulent
• B. Aerated
• C. Compressed
• D. Quiescent

Answer: (D)

Gravity settling relies on still water for heavier particles to fall out of the wastewater. When the sewage is brought to a quiescent state, the flow is calm with minimal mixing, allowing settleable solids to descend to the bottom while clearer liquid remains above. Turbulence or aeration introduces agitation that keeps particles suspended, preventing gravity from acting effectively. A compressed state isn’t conducive to settling because it doesn’t create the calm conditions needed for particles to settle out. So, a calm, quiescent condition is what enables gravity to remove the settleable solids.