The biological phosphorus removal process is a process which depends basically on three internal storage compounds. Poly-beta-hydroxybutyrate (PHB) produced during the anaerobic phase is used as substrate for biomass, polyphosphate, and glycogen formation. The reaction rates of the aerobic processes are primarily determined by the PHB content of the cells. This PHB content is highly dynamic due to the conversions during the anaerobic and aerobic phase of the cycle and the ratio between substrate addition and biomass present in the reactor. The amount of biomass present in the reactor is determined by the sludge retention time and growth rate. A metabolic model of the biological phosphorus removal process was developed and verified over a wide range of growth rates. The effect of different growth rates on the internal fractions of stored components was determined and described mathematically. One set of kinetic parameters was capable of describing the measured conversions of ail components observed in the reactor as a function of the sludge retention time.