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

Abstract

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.

Mathematical Modelling as a Tool for Optimized PHA Production

“…The mathematical modeling of similar processes was also performed by Dias and coworkers [86][87][88] . Primarily, these authors have developed a simple two-compartmental metabolic model for PHA production by mixed microbial culture 86 .…”

“…Primarily, these authors have developed a simple two-compartmental metabolic model for PHA production by mixed microbial culture 86 . After that, they decomposed the metabolic network into separate EFMs to develop a dynamic/hybrid/semi-parametric model 87 that was able to simulate the application of propionic acid as C-source.…”

Composition of mixed culture PHA biopolymers and implications for downstream processing

“…To facilitate a deeper understanding of polymer development, knowledge of the metabolic pathways involved in the synthesis of PHA is very important for anticipating the copolymer composition as it is evolving. For this reason, the mathematical/metabolic model described by Dias et al (2005Dias et al ( , 2008 evaluated the influence of carbon substrate composition (acetic and propionic acids) on enrichment and accumulation stages of PHA production by mixed cultures using an ADF regime. This model considered acetyl-CoA and propionyl-CoA both as biomass and PHA precursors.…”

“…Existing metabolic models assume that the use of acetyl-CoA and propionyl-CoA for active biomass synthesis is proportional to the acetic and propionic acid uptake ). These models are based on the metabolic approach described by van Aalst-van Leeuwen et al (1997) and are a set of empiric equations to describe volatile fatty acid uptake and polymer production (Dias et al 2005, Johnson et al 2009), and monomer composition ).…”

“…Adopting this approach, Dias et al (2005) extended the model to include a set of empiric equations to describe acetic acid uptake, residual biomass growth and PHB production. As the production of the co-polymer PHBV is more advantageous given its mechanical properties , the model was extended to include the case of mixed substrates (acetic and propionic acid) for the production of PHBV (Dias et al 2008).…”