Correct Answer:
C. Primary water Treatment
The correct answer is Primary water Treatment. Sedimentation is a fundamental physical process used in the first major stage of municipal wastewater treatment to remove large, settleable solids from raw sewage.
The Role of Sedimentation in Primary Treatment
- Gravity Separation: In primary treatment, wastewater flows into large settling tanks or clarifiers, where the velocity is reduced. Heavier particles like sand, grit, and organic sludge sink to the bottom as primary sludge, while lighter materials like grease and oils float to the surface and are skimmed off.
- Removal Efficiency: Primary sedimentation typically removes 50–70% of suspended solids and about 25–40% of biochemical oxygen demand (BOD). This drastically reduces the organic load before biological (secondary) treatment.
- Distinction from Secondary and Tertiary: Secondary treatment uses microorganisms to break down dissolved and colloidal organic matter, not sedimentation as the primary mechanism. Tertiary treatment involves advanced filtration, nutrient removal, or disinfection. While sedimentation can occur in secondary clarifiers to settle biological sludge, its principal, defining role is in primary treatment.
- Disinfection: Disinfection (e.g., chlorination, UV) is a final step to kill pathogens and does not involve sedimentation.
Hence, sedimentation is an integral part of primary water treatment.