The general topic of this research project is the modelling of wastewater treatment processes for anaerobic digestion effluents, which are characterized by high nitrogen concentrations. In order to avoid eutrophication of lakes and rivers caused by nutrient enrichment as it happened in Switzerland a few decades ago, the implementation of efficient nitrogen removal processes for those effluents remains nowadays of importance. Moreover nitrogen removal will even become a pre-requisite for efficient micropollutants techniques to be implemented. The first objective of this master project is the modelling of the biological nitrogen removal process Anammox in the software gPROMS. This process converts ammonium and nitrate (close to 1:1 influent concentration ratio) into dinitrogen and nitrate by Anammox bacteria. In order to obtain an influent with the proper concentration ratio, this process can be coupled to a Sharon reactor operated for partial nitrification (two stages operation). In this study, the Anammox biological developed model has been integrated into a continuous stirred tank reactor (CSTR) combined with a particles separator. The two modeled reactors, Sharon and Anammox, have then been run in series and steady-state results have been obtained. Through a comparison with literature results (lab experience and pilot-plant), the steady-state simulation results in terms of nitrogen removal efficiency and effluent concentrations have been considered as reliable. However, a calibration of the model with real data still needs to be investigated. The second objective of the project is the study of key influencing parameters (pH and temperature) on the operation of the two stages Sharon-Anammox process. A sensitivity analysis showed that temperature and pH are influencing the nitrite to ammonium ratio Sharon effluents and consequently affecting the efficiency of the Anammox reactor; the growth and selection of bacteria in the Sharon reactor are determined by those two parameters (chemostat principle). The optimal range of operation for the Sharon reactor in order to maximize the efficiency of the Anammox reactor are found to be between 306K and 308K and between 7.8 and 8.0 for the pH according to the steady-state simulations; those results are coherent with literature findings and operation of full-scale Sharon reactors. The final objective of the project is the comparison of the Sharon-Anammox process with an activated sludge system in terms of energy consumption and nitrogen removal efficiency. By minimizing in both cases the mmonium effluent concentration and playing with the aeration needs, savings of 75% in benefit of the Sharon-Anammox system have been observed; the order of magnitude obtained by optimization is coherent with the stoichiometry implemented in the model. The biological Anammox model itself will be implemented in other types of reactors models (one stage technologies involving biofilms) and the models predictions will be compared with operational data (at pilot or full-scale). Modelling wastewater treatment processes is a help to better understand the processes themselves, their interactions, their optimal operation conditions both by taking into consideration the energy consumption and the water quality through optimization techniques. The challenge of modelling will however still remain the modelling of bacteria interaction and inhibition, as well as a deep understanding and quantification of various mass transfer processes involved. This will need a close collaboration with expert scientists in order to better understand these phenomena. Modelling is a good support for experimental studies, as it can help to plan efficient experimental campaign by minimizing the required experimentations run.