Settleable solids removal relies on gravity during which state of the sewage?

• A. Turbulent state
• B. Quiescent state
• C. Aerated state
• D. Fully mixed state

Answer: (B)

Settleable solids are removed by gravity when the wastewater is calm and still. In a quiescent state, there’s little to no turbulence or mixing, so heavier particles can slowly settle to the bottom while clearer liquid remains above. If the sewage is turbulent or aerated, energy keeps solids dislodged and suspended, which hinders gravity-driven settling. Fully mixed or aerated conditions are used for biological treatment, not for gravity settling, and they prevent the solids from settling out.