The biological treatment of industrial wastewater containing high nitrogen concentrations (>1000 mg N L-1) and low biodegradable organic matter (bCOD) such as landfill leachate is nowadays challenging. Conventional nitrification-denitrification of such wastewater implies high operational costs associated with aeration requirements and external BCOD supply. Partial nitritation (PN) combined with anaerobic ammonium oxidation (anammox) has become a more sustainable alternative treatment of this kind of industrial wastewaters. PN is the previous step to a subsequent anammox reactor which objective is to obtain a suitable effluent to feed the later anammox reactor. The research presented in this thesis involves changes on operational parameters such as aeration phases length and operational temperature decrease in a PN sequencing batch reactor (PN-SBR) treating mature landfill leachate to reduce energy requirements. The PN-SBR startup and operation was assessed at fully aerobic conditions at 25 and 35^oC as well as the implementation of anoxic feedings to promote heterotrophic denitrification via NO₂-at 35^oC. Special attention was focused on the effluent quality, microbial selection using molecular techniques and N2O production under anoxic and aerobic conditions. The PN-SBR effluent was suitable in all cases despite operational temperature (25 and 35^o C) and the inclusion of anoxic conditions during feeding in terms of both NO_² -:NH₄+molar ratio and BCOD content. With regards to N2O production from the PN-SBR, it was demonstrated the production and later emission of 3.6% of the influent N as N2O. The microbial selection achieved in the PN-SBR was also assessed by several molecular techniques. In all the experiments one ammonium oxidizing bacteria phylotype was enriched at all cases well adapted to stringent conditions present in the PN-SBR. Heterotrophic community was low diverse but well adapted to PN-SBR
conditions. Successful demonstration of the PN process robustness was achieved despite changing operational temperature or the inclusion of anoxic feedings. The application of the studied operational changes in full scale reactors should be analyzed in terms of the requirements for the subsequent anammox reactor and environmental law (in the case of N2O emissions). The present thesis results also demonstrated bacterial acclimation to stringent and variable conditions.