Which stage of sewage treatment primarily involves the settling of solids by gravity?

• A. Secondary sewage treatment
• B. Tertiary sewage treatment
• C. Primary sewage treatment
• D. Sewage treatment

Answer: (C)

Gravitation-based settling is a physical separation step, and primary sewage treatment is the stage that uses this process. In primary treatment, wastewater flows into large clarifier tanks where the flow is slowed so heavier suspended solids can settle to the bottom as sludge, while lighter materials may rise and be skimmed off. This removes a large portion of the solids and some organic matter before biological treatment, making the next stages more efficient. Secondary treatment relies on microbes to biologically digest dissolved organics, and tertiary treatment focuses on advanced polishing and removal of remaining nutrients or pathogens, not gravity settling.