Which process results in solids settling out as sludge in a settling tank?

• A. Primary treatment
• B. Secondary treatment
• C. Tertiary treatment
• D. Quaternary treatment

Answer: (A)

Gravity-driven separation of solids in a settling tank is the hallmark of primary treatment. In this stage, incoming wastewater sits in a primary sedimentation tank where heavier suspended solids settle to the bottom and form sludge, which is periodically pumped out for sludge processing. Lighter materials may float to the top as scum and are skimmed off. This step mainly removes settleable solids and a large portion of organic matter before the wastewater moves to the biological treatment that follows. Secondary treatment would handle remaining dissolved organics biologically, tertiary treatment polishes the effluent, and higher stages add advanced purification. So, the formation of sludge by solids settling in a tank is characteristic of the primary treatment stage.