Which wastewater treatment stage primarily removes settleable solids?

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

Answer: (B)

Settling by gravity is the main idea here. In the first stage of a wastewater plant, water flows slowly into large clarifiers where heavier solids fall to the bottom as sludge and lighter materials rise as scum. This physical separation removes most settleable solids before the water moves on to biological treatment, reducing the load on microbes and helping downstream processes work more efficiently. The clarified water then proceeds to the biological stage, where microbes tackle dissolved and suspended organic matter. Other stages focus on different goals: secondary treatment mainly biodegrades organics, tertiary treatment polishes the water by removing remaining nutrients or contaminants, and disinfection kills remaining pathogens. So the stage that primarily removes settleable solids is the primary treatment.