Biological nitrogen removal has traditionally been performed by the conventional nitrification-denitrification process. Leachates generated in urban solid waste landfills are characterized by high ammonia concentration and low biodegradable organic matter content, as well as high salinity. In order to reduce the economic costs associated with the conventional process currently used for this kind of effluents, it is necessary to develop alternative treatment procedures. One of these alternative biological systems is the anammox (anaerobic ammonium oxidation) process, previous partial nitritation from ammonia to nitrite. It is an autotrophic process that converts ammonia and nitrite to dinitrogen gas under anaerobic conditions. When compared to conventional treatments, the combined partial nitritation and anammox processes have lower dissolved oxygen consumption for partial nitritation and organic matter is no longer needed for autotrophic denitrification.