When treating wastewater, what treatment removes phosphorus from the wastewater?

• A. Primary treatment
• B. Secondary treatment
• C. Tertiary treatment
• D. Disinfection

Answer: (C)

Phosphorus removal requires advanced treatment beyond the basic cleanup steps. In primary treatment, only solids are settled out, so dissolved nutrients like phosphorus stay in the water. Secondary treatment targets organic matter and relies on biological processes, but removing phosphorus reliably isn’t the main goal of this stage. Tertiary treatment is specifically designed to remove nutrients, including phosphorus, using methods like chemical precipitation (adding ferric chloride, alum, or lime to bind phosphate and let it settle) or enhanced biological phosphorus removal, where bacteria uptake phosphorus and are then removed with the sludge. Disinfection aims to kill or inactivate pathogens and does not remove phosphorus. So the step that actually removes phosphorus is tertiary treatment.