Which of the following are tertiary treatment methods? Choose the option that lists all true methods.

• A. Ozone
• B. Granular filtration
• C. Chemical precipitation
• D. Granular filtration, carbon adsorption, and chemical precipitation

Answer: (D)

Tertiary treatment focuses on removing remaining contaminants after secondary treatment using advanced physical, chemical, or adsorption steps to meet stricter standards. Granular filtration physically removes fine suspended solids and reduces turbidity, providing clearer water for polishing downstream. Carbon adsorption targets dissolved organics, taste-odor compounds, and trace contaminants that slip past earlier steps. Chemical precipitation removes nutrients like phosphorus (and can help with certain metals), addressing issues that secondary treatment doesn’t fully resolve. Ozone is a powerful oxidant used for disinfection and oxidation of organics, but in many wastewater treatment frameworks it’s classified as a disinfection/advanced oxidation step rather than a traditional tertiary treatment method. Therefore, the combination of granular filtration, carbon adsorption, and chemical precipitation represents the true tertiary treatment methods.