Tertiary wastewater treatment is described as removing nutrients, toxins, and pathogens through chemical treatment. Which of the following best matches this description?

• A. Mechanical filtration
• B. Biological nitrification
• C. Chemical treatment removes nutrients, toxins, and pathogens
• D. Ultraviolet disinfection

Answer: (C)

Tertiary treatment aims to push water quality to a higher level by removing remaining nutrients, toxins, and pathogens, often using chemical methods. The description that this step involves chemical treatment to remove those contaminants directly matches how tertiary processes operate: adding chemicals to precipitate nutrients such as phosphorus, to neutralize or detoxify certain toxins, and to disinfect to kill or inactivate pathogens. Coagulation and chemical precipitation target nutrients and metals, while disinfectants like chlorine or ozone achieve pathogen reduction and can also address some toxins.

In contrast, mechanical filtration relies on physical barriers to remove solids; biological nitrification is a microbial process that converts ammonia to nitrogen species; ultraviolet disinfection uses light rather than chemicals. So the description describing chemical treatment removing nutrients, toxins, and pathogens best fits tertiary treatment.