Development of novel downstream fermentation processes for the recovery and valorisation of Volatile Fatty Acids (VFAs)

This PhD research investigates the development and optimisation of integrated systems for the recovery of bio-based volatile fatty acids (VFAs) from organic waste streams, with the aim of bridging the gap between waste management and high-value bio-based production. Addressing critical challenges such as product inhibition, low VFA concentrations and the complex composition of fermentation effluents, the research focuses on the use of innovative recovery technologies to improve process efficiency and sustainability. The research consisted of three interconnected studies. The first study demonstrated the benefits of in-line pertraction using biodiesel as a bio-based extraction solvent in a semi-continuous fermentation system. This approach led to increase VFA productivity, achieving a yield of 52 gCOD/gTVS (up to 65% increase) at an optimum hydraulic retention time (HRT) of 3 days, but also reduced operating costs by minimising the need for pH correction. Despite its effectiveness, challenges related to biodiesel viscosity and selectivity highlighted the need for alternative recovery methods. The second study introduced a closed-loop liquid-liquid membrane contactor (MC) system to recover bio-VFAs from the effluent of a UASB reactor treating winery wastewater. The system achieved a VFA recovery efficiency of 27% and demonstrated compatibility with downstream bioprocesses for the production of high-value bio-based products and biomaterials (e.g. polyhydroxyalkanoates (PHAs)). The integration of an electrochemical acidification cell further enhances the sustainability of the process, providing precise pH control with minimal energy input, which accounts for less than 1% of the total power consumption of the UASB-MC system. The third study investigated the production of SCP using purple phototrophic bacteria (PPB) in an anaerobic photobioreactor (AnPBR). VFAs were transferred as a gaseous carbon source via a membrane contactor (MC), allowing high quality SCP production without direct contact with the fermentation stream. The system achieved a biomass production rate of 0.36 ± 0.01 kgXPBm-3d-1, with proteins representing over 60% of the biomass. The process also enabled the co-production of valuable compounds such as bacteriochlorophylls, carotenoids and PHAs, further enhancing the nutritional and economic value and demonstrating the versatility and scalability of this approach. The results demonstrate the potential of integrating advanced VFA recovery systems into bio-based production chains, transforming waste streams into valuable resources while aligning with the principles of the circular economy and EU waste directives. Future research will focus on further refining membrane technologies to optimize selectivity, enhance scalability, and explore broader applications for sustainable bioprocess integration.