The invention is directed to systems and methods of biological and chemical treatment of wastewater comprising organic nitrogen compounds. Systems may include: an aerobic reactor, a first separation module for separating liquid and solid components of the wastewater; an oxidation module for removing organic materials from the wastewater; and a post-anoxic reactor for denitrifying at least a portion of the wastewater. Systems may include a second separation module and various feedback recycle lines between the components. Methods may include: degrading by the aerobic reactor more than 95% of organic compounds to ammonia, oxidizing by the oxidation module at least a portion of the ammonia to nitrates, and degrading by the post-anoxic reactor at least a portion of the nitrates to nitrogen gas and water. Systems and methods may reduce total organic carbon of the wastewater by more than 90%, and total nitrogen of the wastewater by more than 90%.