Thesis title: Efficient Production of Polyhydroxyalkanoates with Mixed Microbial Cultures: Combining Process Engineering, Modeling tools and Downstream Strategies
This PhD research addresses one of the major challenges in the transition toward a circular economy: developing sustainable and economically viable alternatives to conventional plastics. The work focuses on the production of polyhydroxyalkanoates (PHAs) using mixed microbial cultures (MMCs), with the main goal of overcoming the economic and technological barriers that currently limit their large-scale adoption. Adopting a multi-approach strategy, the research integrates process engineering approaches, experimental investigation, and modeling tools to converge toward the improvement of process efficiency, stability, and scalability. This combined framework allowed the development of an innovative continuous process configuration, the implementation of a sustainable downstream recovery route, and the implementation of a kinetic model capable of describing and interpreting system dynamics.
The thesis establishes, for the first time, the feasibility of a fully continuous process for PHA production using MMCs, operating from microbial selection to polymer accumulation under steady-state conditions. This represents a conceptual and technological advancement compared to conventional batch systems, paving the way for more stable, productive, and scalable bioprocesses. Moreover, the influence of feedstock composition on process performance and PHA-products characteristics was explored, demonstrating the potential for tailoring copolymer properties through substrate modulation. A green alkaline-oxidative recovery strategy was integrated into the continuous process, providing a simpler, safer, and environmentally sound alternative to solvent-based extraction while maintaining competitive productivity. Finally, a kinetic model was developed to complement the experimental findings, serving as a descriptive and predictive tool that connects process parameters, microbial behavior, and polymer characteristics. Overall, from data collected at laboratory scale during the experimentation it was possible to estimate a continuous production of purified PHA (with purity degree of 92% wt/wt) accounting for 4.5 g/d, corresponding to a volumetric productivity of 0.6 gPHA/Ld.
Together, these elements bridge key research gaps and provide both practical and conceptual advances toward cost-effective and sustainable PHA production systems ready for scale-up within a circular economy framework.