The kinetics of production and degradation of poly-β-hydroxybutyrate (PHB) by a mixed activated sludge culture growing on acetate was studied in a sequencing batch reactor (SBR). Occasionally a very high amount of acetate was added to the steady state system in order to obtain high PHB concentrations in the cells (f_PHB). This made it possible to follow PHB production and degradation over a wide range of f_PHB-data (between 0 and 0.8Cmol/Cmol). The results were compared with data available in literature and with equations derived by metabolic modeling. This led to some remarkable observations. For the feast period, the ratio q_PHB^feast/-q_Ac^feast (specific PHB production rate over specific acetate uptake rate) was used to indicate which fraction of the substrate is stored. Experimentally and theoretically it was shown that this ratio has a constant value for dynamically fed systems operated at a sludge retention time (SRT) >2d. This value is 0.6Cmol/Cmol under aerobic conditions and 0.4-0.5Cmol/Cmol under anoxic conditions, irrespective of the specific growth rate of the biomass and the specific acetate uptake rate in the feast period. Degradation of internal stored PHB could be described with a first order degradation rate with respect to the PHB content of the cells. Degradation of PHB appeared to be independent of the type of electron acceptor present in the system and independent of the SRT of the system. The kinetic descriptions can be used to predict PHB production and consumption in general in dynamic fed wastewater treatment systems, and they provide some trends for modeling purposes.

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.

This paper discusses the poly-β-hydroxybutyrate (PHB) metabolism in aerobic, slow growing, activated sludge cultures, based on experimental data and on a metabolic model. The dynamic conditions which occur in activated sludge processes were simulated in a 2-L sequencing batch reactor (SBR) by subjecting a mixed microbial population to successive periods of external substrate availability (feast period) and no external substrate availability (famine period). Under these conditions intracellular storage and consumption of PHB was observed. It appeared that in the feast period, 66% to almost 100% of the substrate consumed is used for storage of PHB, the remainder is used for growth and maintenance processes. Furthermore, it appeared that at high sludge retention time (SRT) the growth rate in the feast and famine periods was the same. With decreasing SRT the growth rate in the feast period increased relative to the growth rate in the famine period. Acetate consumption and PHB production in the feast period both proceeded with a zero-order rate in acetate and PHB concentration respectively. PHB consumption in the famine period could best be described kinetically with a nth-order degradation equation in PHB concentration. The obtained results are discussed in the context of the general activated sludge models.