Industrial wastewater treatment plants (WWTPs) have to provide 100% reliability and availability for the discharging facilities at an industrial site. Varying production schedules at these facilities and specific components occurring in the industrial wastewater considerably hinder the optimisation of industrial WWTPs. In this context it is shown in this paper that model-based optimisation is an efficient and cost-reducing way to ensure that an industrial WWTP functions well. The aim of the study presented was two-fold. The first step was to show the usefulness of a proposed procedure to build and calibrate a model for the industrial WWTP. The second objective was to use the model for optimisation of the WWTP. As an example, a large set of possible production schedules in the different discharging facilities was simulated. Based on these simulations it could be predicted which schedules allow the effluent standards to be met and which do not. The calibrated and validated model was also used to investigate different operating strategies such as the in-series operation of the two available aeration tanks. In fact, with the model it was shown that a 20% reduction of the degradable COD concentration in the effluent could be achieved by operating the tanks in series instead of in parallel. This case study shows how the approach presented can lead to fast and cost effective modelling and optimisation of an industrial WWTP. (C) 2004 Society of Chemical Industry.