A process of partial nitrification and denitrification in a sequencing batch reactor (SBR) treating leachate was simulated by applying a modified version of activated sludge model no. 3 (ASM3), named ASM3_2step. This model modifies the ASM3 model by separating nitrification and denitrification into two steps with nitrite as an intermediate substrate. Three periods, including long term period, steady state and cycle evolution, were used for calibration. Three main processes were observed, including biomass production, nitrification (focusing on nitrite accumulation) and denitrification. The kinetic and stoichiometric parameters (\documentclass[12pt]{minimal}
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\begin{document}$$ \mu_{{{\text{A\_NH}}}} $$\end{document}, \documentclass[12pt]{minimal}
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\begin{document}$$ Y_{{{\text{A\_NH}}}} $$\end{document}, \documentclass[12pt]{minimal}
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\begin{document}$$ Y_{{{\text{A}}\_{\text{NO}}_{2} }} $$\end{document}, \documentclass[12pt]{minimal}
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\begin{document}$$ Y_{{{\text{H}}\_{\text{O}}_{2} }} $$\end{document}, bA, \documentclass[12pt]{minimal}
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\begin{document}$$ K_{{{\text{A\_NH}}}} $$\end{document}), were determined from biokinetic and respirometry tests. Some of the default values of kinetic and stoichiometric parameters available in the ASM3 model and in the literature were kept constant, while some others were adjusted step by step until observed state variables fit with experimental data. The maximum specific growth rate of nitrite oxidizing bacteria (0.108 day−1) (simulated by the model) and that of ammonium oxidizing bacteria (0.61 day−1) (from biokinetic tests) are the parameters which have the highest influence on the nitrite accumulation, even more than oxygen supply intensity or Kla value. Other important parameters were \documentclass[12pt]{minimal}
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\begin{document}$$ K_{{{\text{AO\_NH}}}} $$\end{document} and \documentclass[12pt]{minimal}
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\begin{document}$$ K_{{{\text{AO}}\_{\text{NO}}_{2} }} $$\end{document}, calibrated at the values of 1.37 and 1.59 mg O2/L, respectively. The modified model and values of the kinetic and stoichiometric parameters obtained from the modeling process will be used for optimization of the partial nitrification in the next study.
