Production and recovery of valuable bioproducts through anaerobic fermentation of organic waste

Resumen

This thesis explored controlled anaerobic fermentation processes to gain insights into the feasibility of treating different sources of organic waste to obtain valuable fermentation bioproducts, such as volatile fatty acid (VFA) and additional bioproducts. Anaerobic fermentation will constitute an innovative approach to recover value from organic waste in future biorefineries and contribute to the Circular Economy. The VFA production was assessed by performing laboratory-scale experiments where the acidogenic potential of different agro-industrial and urban waste was evaluated. These tests provided a valuable methodology to understand the process chemistry, where the impact of process variables, i.e. temperature and pH, was evaluated. Besides, these tests provided valuable information for subsequent up-scaling. Mixed culture VFA production tests were up-scaled in a pilot-scale platform, where the results of laboratory-scale tests could be reproduced. Herein, co-fermentation of substrates of complementary nature showed potential benefits to enhance the acidogenic fermentation process. The VFA production process was evaluated in a continuous mode of operation at pilot-scale. Herein, different process variables were optimised, i.e. the pH and the hydraulic retention time. Organic Fraction of Municipal Solid Waste (OFMSW) showed potential benefits as co-substrate, which enabled to boost the VFA productivity and enrich the fermentation composition. Continuous acidogenic fermentation was a feasible alternative, where stable VFA processes could be obtained. Additionally, ultrafiltration technology was adequate to recover a VFA rich permeate. During the last phase of the thesis, the downstream of fermentation bioproducts was evaluated by assessing the performance of a novel membrane-based technology, namely forward osmosis (FO). During the downstream process, other value-added fermentation bioproducts beyond VFA were evaluated, such as succinic acid, lactic acid and ethanol. These consist of valuable bioproducts that can be obtained from controlled anaerobic fermentation processes, for example using pure bacterial strains and different sources of organic waste, such as third generation biomass. The performance of different types of FO membranes, i.e. cellulose triacetate flat sheet membranes and thin film composite hollow fibre membranes showed the potential of FO technology as a downstream technology to up-concentrate the fermentation bioproducts, enabling high rates of water removal and high solute concentration factors. The application of FO with synthetic binary mixtures of target chemicals enabled to obtain low volume and high concentrated solutions, which might constitute an essential step for the final downstream step and bioproduct recovery. The application of FO membranes was validated with real fermentation mixtures obtained from pure culture fermentation tests, which were performed with the OFMSW and macroalgae substrates. The application of FO with the real fermentation broths, confirmed the potential of the technology as a novel application to upgrade target chemicals. This may raise the interest of the private sector by becoming a core technology of future downstream processes in the biorefinery context and by opening new market opportunities. Overall, this thesis covered the whole picture of fermentation processes that could be performed in future biorefineries, from laboratory scale experiments to pilot-scale, by treating different sources of organic waste and exploring a potential downstream technology. Key words Anaerobic fermentation, biorefinery, VFA platform, membrane separation, forward osmosis