Nitrogen is removed from sewage by the following... Organic-nitrogen is converted to ammonia-nitrogen (NH4) by a mostly anaerobic process called Ammonification... Secondly, Ammonia-nitrogen (NH4) is converted to nitrate-nitrogen (NO3) by an aerobic biological process called nitrification... Last but not least Nitrate-nitrogen (NO3) is converted to nitrogen gas biologically in a low-oxygen (anoxic) environment. During denitrification, nitrogen gas bubbles harmlessly out of wastewater into the atmosphere.