Conventional treatment removes only a small fraction of nitrogen and phosphorus. Specialized biological processes can be implemented to achieve higher removal rates.

8.1. Nitrogen Removal: Nitrification and Denitrification

This is typically a multi-stage process:

1. Carbonaceous BOD Removal: An initial aerobic stage reduces the bulk of the organic matter.
2. Nitrification: In a second aerobic stage, ammonia (NH₄⁺) is oxidized first to nitrite (NO₂⁻) by Nitrosomonas bacteria and then to nitrate (NO₃⁻) by Nitrobacter bacteria.
3. Denitrification: In a final anaerobic stage, nitrate and nitrite are converted to nitrogen gas (N₂), which escapes to the atmosphere. This stage requires a carbon source, often supplied by adding methanol or bypassing a portion of the raw wastewater.

Nitrifying bacteria are sensitive to inhibition by free ammonia (0.1 to 1.0 mg/L) and free nitrous acid (0.22 to 2.8 mg/L).

8.2. Phosphorus Removal

While conventional biological treatment removes 20-40% of phosphorus via cell synthesis, higher removal can be achieved through:

• Enhanced Biological Phosphorus Removal (EBPR): Modifying processes to create conditions for “luxury phosphorus uptake” by certain bacteria. This requires a plug-flow reactor, adequate dissolved oxygen, and slightly alkaline pH.
• Chemical Addition: The most effective method involves adding alum or ferric salts to precipitate phosphates, which are then removed with the sludge.