The leachate treatment plant of the landfill in Mechernich including biological pretreatment, reverse osmosis and evaporation and drying of the concentrate has been in operation since the beginning of 1994. Balancing the nitrogen transformations in the biological contactor plant as the nitrification step in the biological pretreatment, a large amount of the ammonium can be eliminated under aerobic conditions by deammonification. It succeeded obviously under the special conditions to establish a non-negligible amount of denitrifying ammonia oxidants. Experiences concerning this effect were described several times in the past.