Until now the majority of sewer networks in Italy have been built and managed considering small as well as local wastewater treatment plants (WWTPs). However, pushed by Italian and EU regulations, centralised and large WWTPs are becoming more common in Italy. One major technical aspect caused by the changing from local to centralised WWTPs is the development of significant in-sewer processes. These processes are also emphasized by the building of long interceptor sewers characterized by high hydraulic retention times and absence of tributary sewers. In this paper a model derived from the well known WATS model (Wastewater Aerobic/anaerobic Transformation in Sewers) has been set up and applied to an Italian case study located in Palermo. To assure a good degree of reliability the model needs a number of parameters that can be measured or calibrated by the combined use of field data and laboratory analyses. To support the planning of the experimental activity a sensitivity analysis to assess the influence of each model parameter has been carried out. Indeed, the sensitivity analysis allowed identifying the most significant model parameters. As a matter of the fact, by means of sensitivity analysis it was possible not only to pin down the most important model parameters but also to address the field survey towards the most sensitive model parameters in order to optimize the model application. The results are interesting and enabled to reduce the number of parameters to be calibrated from 24 to 14. More specifically, COD fractions, Y(Hw) and mu(H) showed a strong influence on the model results addressing the field campaign to an accurate evaluation of their value.