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The Activated Sludge Model No. 3 (ASM3) and Dutch calibration guidelines (STOWA) were evaluated in the modelling of an activated sludge system treating effluents from a large oil refinery. The plant was designed to remove suspended solids, organic matter and nitrogen from wastewater at an average water temperature of 34°C. The plant consists of three tanks in series; the first two tanks operate in on-off aeration mode with pure oxygen for N-removal, whilst extra methanol is added for the denitrification, and the third tank is maintained as constantly aerobic. Calibration was performed based on a simplified influent characterisation and extra batch experiments (nitrification and denitrification). With the adjustment of only four parameters the model proved capable of describing the performance of the plant concerning both the liquid phase and the biomass. The model was further used to analyse possible modifications in the plant layout and optimize operational conditions in order to reduce operating costs. Modelling results indicated reduction in methanol dosage by implementing an idle time between aerobic and anoxic phases. In this way, surplus methanol was prevented from entering during the aerobic period. Moreover, simulations showed that the most cost-effective option regarding the denitrification process was a combined pre-post-denitrification scheme, without the need for enlarging existing basins. It can be concluded that although ASM3 and STOWA guidelines were originally developed for domestic wastewater application at a temperature range of 10 to 20°C, they proved well capable of describing the performance of an oil refinery wastewater treatment plant operating at 34°C. Moreover, the plant model proved useful for optimization of the plant performance regarding operational costs. [ABSTRACT FROM AUTHOR]