Explain biological nitrogen removal from wastewater.

Biological nitrogen removal involves nitrification and denitrification. Nitrification converts ammonia to nitrate under aerobic conditions by Nitrosomonas (NH4 to NO2) and Nitrobacter (NO2 to NO3). Denitrification reduces nitrate to nitrogen gas under anoxic conditions using heterotrophic bacteria with organic carbon as electron donor. Process configurations include MLE (Modified Ludzack-Ettinger) with pre-anoxic zone, Bardenpho process, and oxidation ditch with alternating zones. SRT > 10 days needed for nitrifiers.