Data scarcity in modelling and simulation of a large-scale WWTP: Stop sign or a challenge

•A methodology is proposed to address the impact of data quality and quantity problems on modelling and calibration of WWTPs•The plant operational modes, weather condition and sensor performance were the main sources of the data scarcity.•Monte Carlo analysis is proposed to quantify the uncertainty of the modelling results to avoid bad modelling practice.•The role of a well-designed data collection is highlighted for performance investigation and troubleshooting of treatment units.