High rate biomethanation of carbon monoxide rich gases via a thermophilic synthetic co-culture

Abstract

Carbon monoxide-fermenting microorganisms can be used for the production of a wide range of commodity chemicals and fuels from syngas (generated by gasification of e.g. wastes or biomass) or industrial off-gases (e.g. from steel industry). Microorganisms are normally more resistant to contaminants in the gas (e.g. hydrogen sulfide) than chemical catalysts, less expensive and self-regenerating. However, some carboxydotrophs are sensitive to high concentrations of CO, resulting in low growth rates and productivities. We hypothesize that cultivation of synthetic co-cultures can be used to improve overall rates of CO bioconversion. As case study a thermophilic microbial co-culture, consisting of Carboxydothermus hydrogenoformans and Methanothermobacter thermoautotrophicus was constructed to study the effect of co-cultivation on conversion of CO-rich gasses to methane. In contrast to the methanogenic monoculture, the co-culture was able to efficiently utilize CO or mixtures of H2/CO/CO2 to produce methane at high efficiency and high rates. In CSTR-bioreactors operated in continuous mode, the co-culture converted artificial syngas (66.6% H2: 33.3% CO) to an outflow gas with a methane content of 72%, approaching the 75% theoretical maximum. CO conversion efficiencies of 93% and volumetric production rates of 4 m3methane/m3liquid/day were achieved. This case shows that microbial co-cultivation can result in a significant improvement of gas-fermentation of CO-rich gasses.