Biological nitrogen removal of high-strength ammonium wastewater was studied in his thesis. Applying specific operational conditions in an activated sludge system, partial nitrification or nitritation (oxidation of ammonium to nitrite) was achieved, avoiding the consequently oxidation of this nitrite to nitrate. This reduction in the nitrification process provides some advantages in comparison to the complete nitrification process, such as the reduction of the oxygen requirements during nitrification and the organic matter during denitrification, the increasing of the denitrification rates and the reduction of the biomass production. The thesis was presented as a compendium of publications and a brief summary of the papers were presented: In Paper I, a partial nitrification system consisting of a pilot plant with three continuous stirred tank reactors in series is presented. This pilot plant was inoculated with sludge from a municipal WWTP and it was operated with a control loop of the nitrogen loading rate which was applied modifying the inflow rate depending on the OUR values measured in the reactors. With this control loop and low sludge retention time the washout of the nitrite-oxidizing bacteria was performed. Simultaneously, almost a complete partial nitrification to nitrite with a high nitrogen loading rate was achieved during a long term operation. Subsequently, in Paper II, the heterotrophic denitrification of the nitrite obtained with the pilot plant developed in Paper I was presented. Different organic carbon sources such as ethanol, glycerol, landfill leachate, fermented primary sludge centrate and fermented secondary sludge centrate were used in the heterotrophic denitrification. This study was carried out in sequential batch reactors (SBR) and a complete denitritation of a high-strength nitrite wastewater was achieved using these organic carbon sources with the exception of fermented secondary sludge centrate. In Paper III the study of the effect of a long-term starvation of a partial nitrification system during 30 days was presented. Four ammonium-starved reactors under different conditions of aeration were used. During this period the ammonia-oxidizing bacteria activity was evaluated using respirometric tests and the FISH analysis. It was observed that is better to shut-down a partial nitrification system under anoxic conditions and, if it is possible, no more than two weeks. Finally, a fast recovery of the system was achieved using the OUR control loop used in the pilot plant of Paper I. Finally, in Paper IV a study of the inhibitory effect by free ammonia and free nitrous acid on the ammonia-oxidizing bacteria under total inorganic carbon limitations and without total inorganic carbon limitations was presented. It was observed that the inhibition by free ammonia can be described accurately using the Haldane model and the inhibition by free nitrous acid using a non-competitive inhibition model. The effect of these inhibitions increased under total inorganic carbon limitation, being much higher in the case of the free nitrous acid.