Continuous bioconversion of CO/CO2 to ethanol through bacterial fermentation in a gas–liquid separation biofilm reactor

Ethanol production by bacterial fermentation using carbon-containing gases is an effective way to achieve the dual carbon goal and green energy production. However, low gas mass transfer efficiency in liquid and insufficient carbon source lead to low ethanol production efficiency. To solve the problem of gas transmission limitation, polyvinylidene fluoride (PVDF), one kind of hydrophobic breathable membrane that can realize gas–liquid separation was used as the attached carrier for the bacterial biofilm in this study. By doing this, carbon-containing gases and liquid culture flow into the bio-reactor respectively from the side of PVDF membrane and biofilm to build. And most of CO/CO₂ mixed gas that passed through the PVDF membrane could be directly utilized by the attached biofilm. The mass transfer coefficient K_La (O₂) reached up to 0.035 s⁻¹, which was nearly 10 times higher than that in the bubbling bio-reactor. The utilization rate of CO gas was improved as high as 50 %. Furthermore, to improve ethanol production, the culture pH was regulated based on the requirements of the mixed bacteria at different growth stages. If the pH in the acidogenic stage was controlled at 6.0 and the later solventogenic stage at 4.5, the maximum ethanol concentration (12.9 g L^-1) was obtained. And the ethanol production rate could stabilize at a high value of 1.08 g L^-1 d^-1 during the continuous fermentation mode. Therefore, pH regulation in the gas–liquid separation biofilm reactor is one of the effective methods to adjust the bacterial metabolic activity and realize stable ethanol production. Moreover, the gas–liquid separation biofilm reactor presents an effective approach to enhance gas–liquid mass transfer and augment ethanol production within the domain of synthesis gas fermentation. Simultaneously, the scalable gas–liquid separation membrane bioreactor also establishes the foundation for the industrialization of synthesis gas fermentation.