Syngas Fermentation by Mixed Microbial Consortia: Enrichment and Continuous Fermentation

Abstract

Over the last decades, the development of second generation biofuel technologies have been posed as a necessity due to the increasing energy demand in the transportation sector and the rising concerns about the climate change. Common approaches for the production of second generation biofuels involve the conversion of recalcitrant biomasses via either biochemical or thermochemical methods such as biomass pretreatment and fermentation of carbohydrates, and biomass gasification coupled to Fischer-Tropsch (FT) processes. Syngas fermentation has also been proposed as a promising alternative to the aforementioned technologies as it presents higher biomass conversion efficiency compared to biochemical methods, and lower operational costs when compared to FT processes.1 The fermentation of syngas into ethanol has been mostly studied using pure cultures. Nevertheless, several acetogenic species have been reported to be partially inhibited by the impurities of raw syngas, which would increase the raw syngas cleaning requirements.2 A possible alternative to circumvent the additional gas clean-up step is using open mixed microbial consortia (MMC) as their inherent microbial diversity confers them with a higher resilience towards toxic compounds, thus reducing partial inhibition phenomena.3 Additionally, the fact that sterile operation is not required in MMC-based processes contributes to reducing the operational costs by minimizing the utility consumption. On the other hand, the microbial interactions prevailing within microbial communities are often poorly understood, which leads to a decreased performance in terms of yield and product selectivity. In order to overcome these limitations, this work focused on studying the effects of operational conditions on the enrichment of MMC for enhancing the product selectivity towards ethanol, as well as on studying the effects of operational parameters on the ethanol yield and productivity of a MMC-based continuous process using a 2 L stirred tank reactor. The results showed that the enrichments at different pH conditions had a strong effect on the microbial activity and the final ethanol yield in batch experiments, obtaining a maximum ethanol yield of 17.7%, 33.3% and 59.8% of the theoretical maximum at pH 6, 5.5 and 5, respectively. Further experiments in continuous mode showed that pH, HRT and gas loading rate are essential operational parameters governing microbial interactions within the microbial consortium, and consequently the ethanol yield and product distribution.