Today's models of activated sludge systems are based on average composition of biomass (macroscopic models). With the introduction of cell internal storage compounds (structured biomass) in ASM2 and ASM3 cell composition influences the kinetic behavior of activated sludge. Since the kinetics of most processes is not linear, adding up the behavior of individual cells (microscopic models) does not result in the same model prediction as obtained when predictions are made with average cell composition. Based on first simple computations with microscopic models which consider up to 100,000 individual bacteria, differences in model prediction are identified. It becomes clear that kinetic parameters for lumped, macroscopic models are system specific: whereas biomass from SBR systems yields kinetic parameters suited for application in microscopic models – biomass from continuous flow systems can only be used for the calibration of macroscopic models. Application of SBR models to continuous flow systems requires the use of microscopic models.