Bioethanol and biobutanol production from CO and CO2 using solventogenic acetogens

Abstract

This research aimed at enhancing ethanol and butanol production from H2/CO2 and CO by anaerobic sludge from wastewater treatment plants via investigating the environmental parameters in a semi-gas fed bioreactor and enrichment of CO-converting solventogenic Clostridium bacteria. In the first part of the study, a fermentation process that converts CO2 to ethanol using H2 as electron donor and anaerobic granular sludge as inoculum was studied at different temperatures. Besides, the effects of pH, carbon source (HCO3- and glucose supplementation) and trace metals on ethanol production were also investigated. Results from the experiments showed that fermentation at 25°C and initial pH of 6 achieved the highest ethanol production (17.1 mM). Ethanol production occurred when both the pH decreased to 4.7 and acetic acid accumulated to 15 mM at 25°C by granular sludge using H2/CO2 as the substrate. with Clostridium being the functional microorganism at genus level at both 25 and 37°C. However, methane was produced from H2/CO2 at 55°C. The use of glucose and CO2 as co-substrate enhanced butyric acid production (3.3 mM), while ethanol production occurred at a pH as low as 4. The presence of 10 μM W and 2 μM Mo enhanced the ethanol production by 7.0 and 5.4-fold, respectively. A continuous fermentation further showed the change of H2/CO2 to 100% H2 in the gaseous substrate enhanced butyric acid and hexanoic acid production at pH 4.5. The second part of this study investigated ethanol and butanol production using CO as the sole carbon source with anaerobic sludge as inoculum in an intermittent gas-fed bioreactor. CO and syngas metabolizing solventogenic acetogens were enriched from anaerobic sludge at pH 5.7-6.5 and produced up to 6.8 g/L butanol in an intermittent gas-fed bioreactor. Additional tests under controlled pH demonstrated that the lowest pH of 5.7 stimulated ethanol and butanol production in the enriched culture. The applied enrichment procedure efficiently selected for a range of Clostridium species. Thereafter, successive enrichment obtained a high (74%) amount of unidentified Clostridium species, which produced 11.7 g/L ethanol with low accumulation of acetic acid over a wide pH range of 6.45-4.95. Besides, the enriched Clostridium bacteria in the present study produced 2.7 g/L butanol from exogenous butyric acid with a 100% conversion efficiency using CO as reducing power. The highest butanol concentration was produced (3.4 g/L) with 14 g/L butyric acid addition using CO as gaseous substrate by the Clostridium bacteria. Butanol was significantly enhanced up to 2.4 fold when adding CO after endogenous butyric acid production from glucose compared to when adding CO initially with glucose (co-fermentation).