What is the primary purpose of tertiary treatment?

• A. Final polishing and disinfection to achieve final water quality.
• B. Primary solids removal.
• C. Biodegradation of organics.
• D. Sedimentation.

Answer: (A)

Tertiary treatment is the final polishing stage of wastewater treatment, aimed at bringing the effluent up to stringent water-quality standards for discharge or reuse. It focuses on removing what remains after primary and secondary processes—often targeting residual suspended solids, nutrients, and microbes—and typically includes filtration, nutrient removal steps, and disinfection to achieve the final water quality. The other options describe processes that occur earlier in the treatment train: primary solids removal and sedimentation happen during primary treatment, while biodegradation of organics occurs during secondary biological treatment. Hence, the primary purpose of tertiary treatment is final polishing and disinfection to achieve final water quality.