This work was developed within the Horizon 2020 D-NOSES project, with the purpose of validating a new methodology for odour pollution management based on citizen science. The work was carried out in the area of Castellanza, in the Region of Lombardy (Italy), and involved 4 plants that have been identified as potential causes for odour emissions. In this complex situation, it was proposed to combine the D-NOSES methodology with the “traditional” way involving olfactometric measurements and dispersion modelling, in order to identify the causes of the odour problem. The results of the olfactometric surveys allowed to identify the major odour emissions of the plants under investigation. Moreover, the preliminary analysis of the 358 odour observations collected in the period between May 14th and September 30th 2020 by a group of trained citizens, resulted in the disprove of the common belief that a particular product of the chemical plant caused the release of odorous chemical substances generating malodours at the municipal WWTP.