Two-step pH regulating ethanol production through continuous CO/CO2 gas fermentation by mixed bacteria from rabbit faeces

Mixed bacterial syngas fermentation is a promising carbon-neutral method for producing acetic acid and ethanol. However, the bacterial community and metabolic pathways were different in the growth and ethanol production stages, whereas the activity of the bacterial community greatly depended on the pH of the culture. In this study, the effect of pH on syngas fermentation performance and microbial community composition under continuous gas supply was investigated. The mixed bacteria grew fastest at a pH of 7, while a constant pH of 6 led to the highest ethanol concentration of 5.15 g L −1 , which was 160 % higher than that at pH 7. Based on this, a two-step pH regulation method was proposed in this study to improve the ethanol production. Initially, the culture pH was maintained at 7 to create optimal growth conditions. Once bacterial density stabilized, it was then changed to a pH 6 to promote ethanol production. By doing this, compared with the control group, the maximum ethanol concentration (5.85 g L −1 ) increased by 148.9 %, the ethanol production rate (2.41 g L −1  d −1 ) increased by more than 8 times. This two-step pH regulation method ensured a high bacterial density and effectively stimulated ethanol production during syngas fermentation.