Landfill leachate can present extremely elevated concentrations of ammonium (up to 6,000 mg N‐NH4+ L−1) and a low biodegradable organic matter fraction. As an alternative to conventional systems, this wastewater can be treated on a more sustainable way by a fully autotrophic partial nitritation–anammox system. The operation of the first step of this system, the partial nitritation, is critical since the elevated concentrations of ammonium and nitrite in the reactor can severely inhibit ammonium oxidizing bacteria (AOB) activity. In this way, the inclusion of anoxic phases during the feeding events to promote the denitrification via nitrite can be a good option for upgrading the process performance and increasing the stability of the system. This paper deals with the evaluation of an anoxic–aerobic step-feed strategy for the operation of a partial nitritation SBR. Results of this study have revealed a decrease on the total nitrogen inside the reactor of more than 200 mg N L−1 without prejudice on the partial nitritation process. Furthermore, this study has also allowed detecting an AOB activity reduction at the end of aerobic phases due to bicarbonate limitation and/or free nitrous acid inhibition.

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