Many fermentation processes have been used to convert biomass to various fuels. Currently, the fermentation of syngas, which is a mixture of CO, CO2, and H2, is being studied for the production of ethanol. In our studies, an anaerobic microbial catalyst Clostridium P11 was used and the key performance parameters, including growth rate and ethanol/acetic acid production rates, were compared. Experimental conditions, such as pH, ionic strength, and redox potential are important issues in anaerobic fermentation not only because they affect the activities of enzymes involved in the metabolic pathway, but also because they affect the concentrations of nucleotides (ATP, ADP, NADH, and NAD+) in the bacteria. Moreover, nucleotide concentrations in turn affect the synthesis of intermediates and product. Therefore, it is critical to distinguish how pH, ionic strength and redox potential affect cell growth and product formation, as well as possible methods to improve ethanol formation.
Studies were carried out in both 100 ml bottles with daily replacement of headspace gas and in 1-liter bioreactors with continuous gas flow. Experimental results showed that pH and redox potential of the media changed due to the bacteria metabolism. In order to assess the effects of experimental conditions on syngas fermentation, experiments with controlled pH and redox potential were designed and the results indicated that pH and redox potential play key roles on cell growth, product formation and product distribution. Furthermore, experimental conditions have different effects on fermentation during different phases. For example, cell growth is much better at pH=5.8 than pH=4.5. However, the ethanol production rate at pH=4.5 is better than pH=5.8. Based on the experimental results, a strategy involving controlling the pH and redox potential at different phases was applied to improve ethanol production. This work provides significant insights on how varying experimental conditions can affect the syngas fermentation process