Zejnilovic, E. (2022). Optimization of process performance and mixotrophic cultivation of Clostridium carboxidivorans for the production of biofuel alcohols [Diploma Thesis, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2022.102605
In times of limited energy resources, and increasingly significant negative climate and environmental changes, the focus is on renewable energy sources. Biofuels have proven to be a very good alternative to currently used fossil fuels. Biowaste and residual materials can be sensibly reused.Industrial bioethanol production from CO2/CO-rich waste gases (e.g. steel mills industry) is an effective way for the production of valuable biofuels. Acetogenic clostridia have the ability to use organic and inorganic (gaseous) substrates simultaneously, which is referred to as anaerobic, non-photosynthetic mixotrophy in which alcohols are produced by syngas fermentation under optimized conditions. The main syngas components, carbon monoxide, carbon dioxide, and hydrogen, are converted into alcohols.Clostridium carboxidivorans was only investigated using autotrophic substrates, which led to low biomass formation and low productivity. In order to solve this problem, C. carboxidivorans was tested for the use of heterotrophic and mixotrophic substrates and their conversion into alcohols such as ethanol, butanol, but also hexanol.In the first experiment, C. carboxidivorans was cultivated in serum bottles with different sugar concentrations in order to additionally test the influence of the sugar concentration on the effect of the mixotrophic conversion. The study has shown that mixotrophy performed successfully and a lower sugar concentration with added carbon monoxide leads to higher alcohol production, achieving 0.53 g/L butanol, 2.07 g/L ethanol, and 0.12 g/L hexanol.In recent years, attempts have been made to achieve high productivity of the continuous process in order to achieve high butanol, ethanol and hexanol yields.In the second experiment C. carboxidivorans was cultivated in a continuous process with mixotrophic substrates (glucose and various gas mixtures). Heterotrophic and mixotrophic substrates were compared in the formation of products. The setpoint with added carbon monoxide (60.12 % H2, 9.54 % CO2, 10.57 % CO, 19.77 % N2) achieved the highest results. C. carboxidivorans can use carbon monoxide to form alcohols, which makes mixotrophy successful. Alcohol concentrations in steady-state conditions reached values of 0.37 g/L butanol, 4.89 g/L ethanol, and 0.75 g/L hexanol.The study also showed that lowering the pH in the chemostat was not possible, even from pH 6 to 5.5. Biomass or OD dropped sharply overtime at pH 5.5. Low pH led to a low growth rate.With this work, a first continuous fermentation with C. carboxidivorans using mixotrophic substrates was carried out with the first evidence of mixotrophy and the effect of different gas mixtures.