Phosphorus removal from wastewater is achieved during which stage?

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

Answer: (C)

Phosphorus removal is achieved during tertiary treatment because this stage is the polishing step focused on nutrient management after the main biological and physical treatments. In tertiary treatment, phosphorus is removed mainly through chemical precipitation: adding coagulants like ferric chloride or alum binds phosphate to form insoluble compounds that can be settled out or filtered, greatly lowering the phosphorus in the final effluent. Primary treatment mainly removes solids, not dissolved nutrients, and secondary treatment targets organic matter and general nutrient uptake by microbes (which may remove some phosphorus in certain designs but isn’t relied upon for consistent removal). Disinfection is about killing pathogens and does not remove phosphorus. So the reliable stage for phosphorus removal is tertiary treatment.