Which wastewater treatment process primarily removes settleable solids via physical sedimentation?

• A. Primary treatment
• B. Secondary treatment
• C. Tertiary filtration
• D. Disinfection

Answer: (A)

The main concept here is that the first stage of wastewater treatment relies on gravity to remove solids that readily settle out of the wastewater. In primary treatment, wastewater flows into large settling tanks called clarifiers. There, heavier solids settle to the bottom as sludge and lighter materials like grease rise to the top and are skimmed off. This physical sedimentation process significantly reduces the amount of settleable solids before the water moves on to downstream biological or polishing steps, making it the stage that primarily performs this task.

Secondary treatment centers on breaking down dissolved and suspended organic matter through microbial processes, with any remaining solids often removed later by additional clarification. Tertiary filtration provides a final polishing step to remove remaining fine solids and other contaminants, while disinfection focuses on killing remaining microorganisms rather than removing solids.