In an effort to treat N-rich streams in a more sustainable way, recent years have seen the development of new technologies, most of which are based on autotrophic denitrification via nitrite (anammox). In order to attain a suitable influent for that process, the wastewater must be treated by partially oxidising the ammonium to nitrite. In that aspect, this article presents the start-up and operation of a Partial Nitritation Sequencing Batch Reactor (PN-SBR) treating urban landfill leachates. Stable partial nitritation has been reached treating high ammonium loads (1-1.5 kg Nm(-3)d(-1)), demonstrating the feasibility of this technology as a previous step of anammox process. This study has also given away the importance of pH influence over ammonium oxidising bacteria (AOB) activity, thus it has been possible to determine the values of the half inhibition constants for free ammonia (k(I,FA)=605.48+/-87.18 mg N-NH L(-1)) and free nitrous acid (k(I,FNA)=0.49+/-0.09 mg N-HNO2 L(-1)), as well as the half-saturation constant for bicarbonate (k(HCO3-) = 0.01 +/- 0.16 mg CL(-1)).