Activated sludge models (ASMs), extended with an N2O emission module, are powerful tools to describe the operation of full-scale wastewater treatment plants (WWTPs). Specifically, such models can investigate the most contributive N2O production pathways and guide towards N2O and carbon footprint (CF) mitigation measures. A common practice is to develop and validate models using data from a single WWTP. In this study, a successfully validated model in one plant (Slupsk/Poland) was extrapolated to another full-scale WWTP (Viikinmäki/Finland). For this purpose, the previously developed ASM No. 2d with the N2O module was used. Moreover, the results of calibration and validation of that model were compared with those obtained on the basis of the ASM No. 3 with an N2O module. A novel, rigorous calibration protocol, based on the system engineering approach, was implemented to minimize the number of adjusted parameters without compromising the accuracy of model predictions. The validated model accurately predicted the behavior of the system in terms of the liquid N2O production in the bioreactor and gaseous N2O emissions. Model-based identification of N2O production pathways revealed the key role of heterotrophs duo to their high abundance in the microbial community. The N2O emission factor (EF) at the studied plant was found between 0.9 and 0.94% of the influent TN-load for the validation and calibration period, respectively. Based on the model predictions, it was estimated that the aerobic zones contributed to over 93% of the N2O emitted to the atmosphere, while the remaining portion (7%) resulted from the N2O liquid-gas transfer in the non-aerated zones. The difference between the predicted N2O EF and the empirical EF calculation would lead to almost 1000 tonnes of CO2 equivalent reduction of the annual CF of the plant, which highlights the importance of model applications in CF studies.