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Abstract: Based on the activated sludge model no.3 (ASM3), a new kinetic expression describing substrate removal and biomass growth mechanism occurring in activated sludge system under aerobic condition was established. The new model proposed that under feast condition, one part of substrate was utilized directly for biomass growth and the other part was stored as internal storage products and simultaneously the storage products were used for biomass growth. The model was successfully calibrated on oxygen uptake rate (OUR) data and off-line soluble chemical oxygen demand (COD) dynamic variations obtained from batch experiments with biomass from a full-scale wastewater treatment plant (WWTP). OUR predictions with the calibrated model could reasonably describe the OUR profile after pulse addition of acetate, i.e., the OUR transiently reached a very high level, and then increased gradually to a maximum level until the initial substrate was taken up for storage and biomass growth. This new model also, for the first time, highlighted the significant effect of the biomass storage products concentration before pulse addition of acetate on OUR profile. [Copyright &y& Elsevier]