This thesis presents the results of a sampling and analysis campaign conducted at the Viikinmäki wastewater treatment plant (Helsinki, Finland). The collected data were made available to the DIGICARBA project team, which used them to calibrate the plant digital twin currently under development. For this reason, the sampling strategy, laboratory analyses, and data processing were designed and carried out in accordance with guidelines, standards, and methods specifically developed for wastewater treatment plant modelling. During six sampling days, a 24-hour sampling programme was implemented, consisting of eight 3-hour composite samples per day. These samples were analysed to assess the diurnal variation of organic matter and suspended solids loads at three strategic points within the plant.