Which unit relies primarily on gravity to settle solids out of wastewater?

• A. Aeration basin
• B. Trickling filter
• C. Sedimentation tank
• D. Maturation pond

Answer: (C)

Gravity-driven settling is the key mechanism here. A sedimentation tank, also called a clarifier, is designed so wastewater flows very slowly. That slow flow lets suspended solids settle to the bottom under the influence of gravity, forming sludge, while clearer water leaves from the top. This step removes the settleable solids early in the treatment process.

In contrast, an aeration basin relies on air transfer and mixing to supply oxygen for microorganisms, keeping solids oscillating and suspended rather than settling. A trickling filter treats wastewater by passing it over a bed of media with a biofilm, focusing on biological degradation rather than gravity settling. A maturation pond uses natural processes like sunlight, algae, and microbial activity for polishing and stabilization, with settling being only a secondary, ancillary effect.