Policastro, Grazia (2022) Dark and photo fermentation for mixed culture biorefinery: Energy and chemicals production from waste. [Tesi di dottorato]

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Over the last decades, the interest of sustainable alternative technologies for energy and chemicals production is dramatically increasing. Contextually, the waste production is driving governments to promote more efficient processes to reduce the amount of residues and favour a more sustainable bioeconomy. In this context, biorefining systems, based on waste utilisation, are promising alternative to fossil fuels-based processes for biofuel and/or chemicals production. The use of waste biomass requires bioconversions to be performed by mixed cultures rather than pure ones. Mixed cultures with the desired metabolic capacity have the advantage to not require sterilization of the inflow, to confer higher robustness and lower costs to processes and can prevent inhibition phenomena. However, little is known about mixed cultures applications, based on the adoption of a selecting environment rather than a pure culture. In this thesis, different mixed culture biotechnologies have been studied and applied to improve by-products valorisation in a biorefinery concept. The thesis consists of three different parts related to: i) Valorisation of agri-food feedstocks by mixed-culture fermentation into lactic acid; ii) Photo fermentation of agri-food feedstocks and dark fermentation effluents for hydrogen and PHAs production; iii) Other biorefinery processes. In detail, different applications of the dark fermentation process have been tested, including lactic acid production and hydrogen production. Moreover, the generation of dark fermentation effluents rich in ethanol has been studied as a strategy to enhance the hydrogen production in the photo fermentative step of two stages dark and photo fermentation processes. Also, the photo fermentation process has been studied as a single stage process, under both experimental and modelling approaches. The photo fermentative concomitant generation of hydrogen and PHAs has been investigated. Moreover, strategies to produce PHBV have been extensively studied, as this specific co-polymer can represent one of the most interesting future applications to valorise dark fermentation effluents. Finally, lipids induction in continuous microalgal cultivation systems has been tested. Results show how to optimize operational conditions, to create selective environments in which mixed cultures displaying a desired functionality are enriched. Moreover, the feasibility and the advantages of mixed cultures applications over pure ones has been demonstrated. The main findings of this thesis represent a step forward in the field of mixed cultures biotechnologies and interesting starting points for many possible future research directions aimed at the scale up of the studied processes.

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