During this study, a mathematical model simulating piggery wastewater treatment was developed, with the objective of process optimisation. To achieve this, the effect of temperature and free ammonia concentration on the nitrification rate were experimentally studied using respirometry. The maximum growth rates obtained were higher for ammonium-oxidising biomass than for nitrite-oxidising biomass for the temperatures above 20 °C; values at 35 °C were equal to 1.9 and 1.35 day−1, respectively. No inhibition of nitrification was observed for free ammonia concentrations up to 50 mgN/L. Using these data with others experimental data obtained from a pilot-scale reactor to treat piggery wastewater, a model based on a modified version of the ASM1 was developed and calibrated. In order to model the nitrite accumulation observed, the ASM1 model was extended with a two-step nitrification and denitrification including nitrite as intermediate. Finally, the produced model called PiWaT1 demonstrated a good fit with the experimental data. In addition to the temperature, oxygen concentration was identified as an important factor influencing the nitrite accumulation during nitrification. Even if some improvements of the model are still necessary, this model can already be used for process improvement.