SHARON-ANAMMOX: A novel process for the energy-efficient removal of nitrogen from wastewater containing high ammonia concentrations (1-2 gram/liter), such as sludge-digestor- effluent. Ammonia/ammonium is first partially oxidized to nitrite in the presence of air. In the second reactor the Anammox bacteria oxidize the remaining ammonium to dinitrogen gas with nitrite as the electron acceptor.