The leachate of landfill sites, partial flow from sludge treatment or effluents of anaerobic digestors and composting plants are characterized by unwanted high amounts of nitrogen and low carbon concentrations. The common elimination of nitrogen is mostly realized by nitrification and denitrification. To maintain the denitrification process most of the plants use acetic acid or methanol as the main carbon source. Due to the aeration during the nitrification process, the nitrogen removal becomes a highly energy consuming process. Additionally, the carbon source increases the surplus sludge and can cause temperature problems due to the waste heat of the process. The paper describes the plant optimization for the treatment of landfill leachate. By the stepwise reduction of added methanol and lowered aeration, high concentrations of Nitrite and Ammonia were detected in the activated sludge and the downstream activated carbon. During the changeover we measured the nitrogen species, COD and the hydraulic data of the plant. Additionally, the Planctomycetes responsible for the anaerobic ammonium oxidation (ANAMMOX) were monitored by Fluorescence in situ Hybridisation (FISH). We verified the ANAMMOX related bacteria in the activated sludge and the downstream activated carbon. Last but not least, ANAMMOX reduces the costs for methanol, energy and surplus sludge as well as the CO2 emissions of the plant.