Mathematical modelling of a mixed culture cultivation process for the production of polyhydroxybutyrate.

Mixed cultures submitted to acetate "feast" and "famine" cycles are able to store intracellularly high quantities of polyhydroxybutyrate (PHB). It was demonstrated in a previous study that the intracellular PHB content can be increased up to 78.5% (g HB/gVSS) of cell dry weight in a sequencing batch reactor (SBR) with optimised operating conditions. The specific PHB formation rate was also shown to be higher for mixed cultures than for pure cultures. Such high intracellular PHB contents and specific productivity open new perspectives for the industrial production of polyhydroxyalkanoates (PHA) using mixed cultures instead of pure cultures. The main goal in this work was to develop a mathematical model of mixed cultures envisaging the optimisation of PHB production. A relatively simple two-compartments cell model was developed based on experimental observations and other models proposed in the literature. A convenient experimental planing allowed to identify the kinetic parameters and yield coefficients. Experiments were performed with and without ammonia limitation enabling the analysis of PHB formation independently of the cell growth process. The experimental true yields partially confirm the theoretical values proposed in the literature. The final model exhibited high accuracy in describing the process state of most experiments performed, thus opening good perspectives for future model-based optimisation studies.