The SHARON process permits the partial nitrification of wastewaters with high ammonium content and coupled with the Anammox process represents a more sustainable alternative for N removal than a typical nitrification-denitrification. A mathematical model describing a continuously aerated SHARON reactor for the treatment of landfill leachate is presented. Haldane kinetics are used in both steps of the nitrification since ammonium-oxidizing and nitrite-oxidizing organisms are inhibited by their own substrates. Special attention is given to pH because it is a key factor, implementing an algorithm for its calculation. A preliminary calibration of the model using synthetic feed is presented.