The objective of this research was to obtain a more detailed insight in the kinetics and stoichiometry of poly-β-hydroxybutyrate (PHB) metabolism in activated sludge cultures. The process of storage and degradation of PHB under denitrifying conditions was studied. Dynamic conditions as occurring in activated sludge processes were simulated in a 2L sequencing batch reactor (SBR) by subjecting a mixed sludge population at a sludge retention time (SRT) of about 6 days to successive periods of external substrate availability (feast period) and no external substrate availability (famine period). Under these conditions intracellular storage and degradation of PHB occurs. It was shown that the process of storage and degradation of PHB is the same under anoxic and aerobic conditions. About 70% of the amount of acetate consumed in the feast period is used for synthesis of PHB, and the remainder is used for growth processes. The anoxic specific acetate uptake rate was 3-4 times lower than aerobically. The PHB metabolism was evaluated on the basis of a metabolic model. In a traditional macroscopic model, 10 parameters would be needed to describe the process. By relating these parameters to metabolic coefficients, only 2 parameters were needed in the description. Degradation of PHB could be described by a power law equation in PHB concentration. The order n in these experiments equaled 0.59, close to a value previously reported for PHB degradation by bio-P-cultures. However, PHB degradation by bio-P-cultures occurs at a much higher rate. Storage of PHB in the feast period can increase the required COD/N ratio for denitrification by 70% compared to a situation without storage. (C) 2000 John Wiley and Sons, Inc.