What are nitrification and denitrification in wastewater treatment?

Nitrification and denitrification are biological processes for nitrogen removal from wastewater: Nitrification is a two-step aerobic process: ammonia (NH4+) is oxidized to nitrite (NO2-) by Nitrosomonas bacteria, then nitrite is oxidized to nitrate (NO3-) by Nitrobacter. Requires dissolved oxygen >2 mg/L and is sensitive to pH (optimal 7.5-8.5) and temperature. Denitrification is an anaerobic (anoxic) process where heterotrophic bacteria (Pseudomonas, Bacillus) use nitrate as electron acceptor instead of oxygen, reducing it to nitrogen gas (N2). Requires carbon source (BOD or added methanol). The sequence is: NO3- -> NO2- -> NO -> N2O -> N2. Treatment trains often use alternating aerobic and anoxic zones (A2O process) or modified configurations to achieve nitrogen removal while removing organic matter.