What characterizes tertiary treatment of sewage?

• A. Removal of remaining pollutants
• B. Removal of remaining pollutants and disinfection
• C. Primary sedimentation
• D. Biological oxidation

Answer: (B)

Tertiary treatment is the polishing step after the primary and secondary stages, focused on finishing the cleanup by removing pollutants that the earlier stages miss and by inactivating microbes. After solids settle in primary and organic matter is biologically degraded in secondary treatment, tertiary processes target dissolved substances and nutrients, and they often include filtration, nutrient removal (like phosphorus and nitrogen), and disinfection to kill remaining pathogens. This combination—extra pollutant removal plus disinfection—best captures what tertiary treatment does. Primary sedimentation is part of the initial stage, not the polishing stage, and biological oxidation is the core mechanism of secondary treatment, not tertiary.