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The increased consumption of water, and associated water pollution, has deteriorated the European surface waters. Hence, wastewater treatment has become obligatory. Modeling has proved to be a useful tool to enhance the knowledge on the treatment processes. The existing models that describe the biological wastewater treatment processes, are generally of high complexity and require a time consuming fractionation step to convert the measurements into the state variables of the model. Hence, the application to real wastewater treatment plants is still limited. This master thesis introduces a modified model structure in which the available measurements can be introduced without a preliminary fractionation procedure. The ASM2d model is selected as a starting point since it takes into account biological phosphorus removal, which has gained importance over the last decades. The new model is designed with the aid of the wastewater treatment set-up of Geel and, finally, the model is validated by application to the wastewater treatment plant of Mol. The required measurements for the new model are the biochemical oxygen demand, the chemical oxygen demand (on a raw and a filtered sample) and the total suspended solids for the organic carbon components, complemented with measurements of the total nitrogen, total phosphorus (both on a raw and a filtered sample), phosphate, nitrate, nitrite and ammonium content for the nutrients. The proposed modifications in this thesis eliminate the fractionation of these measurements in the following way. First, the fractionation of the biochemical oxygen demand between fermentable substrate and fermentation products, is eliminated by focusing on one organic biodegradable substrate based on the ultimate biodegradable oxygen demand. Furthermore, all particulate organic material is considered to be biodegradable. Moreover, since examination of the modeled wastewater treatment plant of Geel revealed a systematic underestimation of the inflow of organic nitrogen and phosphorus by the ASM2d model, the latter, in soluble and particulate form, are introduced as new state variables. The associated hydrolysis, ammonification and phosphatization processes are added. Hence, the measured total organic nitrogen and phosphorus can be exploited to represent the real influent more closely. The proposed model structure is capable of reproducing the output of the original model properly for both examined wastewater treatment plants and takes into account a more realistic inflow of organic nutrients. In addition, coupling to preceding sewer models and subsequent river models is feasible since these models exploit the same type of measurements, as such, enabling integrated modeling of the water cycle.