A novel biological process to upgrade biogas was developed and optimised during the current study. In this process, CO in the biogas and externally provided H were fermented under mesophilic conditions to volatile fatty acids (VFAs), which are building blocks of higher-value biofuels. Meanwhile, the biogas was upgraded to biomethane (CH >95%), which can be used as a vehicle fuel or injected into the natural gas grid. To establish an efficient fermentative microbial platform, a thermal (at two different temperatures of 70 °C and 90 °C) and a chemical pretreatment method using 2-bromoethanesulfonate were investigated initially to inhibit methanogenesis and enrich the acetogenic bacterial inoculum. Subsequently, the effect of different H:CO ratios on the efficiency of biogas upgrading and production of VFAs were further explored. The composition of the microbial community under different treatment methods and gas ratios has also been unravelled using 16S rRNA analysis. The chemical treatment of the inoculum had successfully blocked the activity of methanogens and enhanced the VFAs production, especially acetate. The chemical treatment led to a significantly better acetate production (291 mg HAc/L) compared to the thermal treatment. Based upon 16S rRNA gene sequencing, it was found that H-utilizing methanogens were the dominant species in the thermally treated inoculum, while a significantly lower abundance of methanogens was observed in the chemically treated inoculum. The highest biogas content (96% (v/v)) and acetate production were achieved for 2H:1CO ratio (v/v), with Acetoanaerobium noterae, as the dominant homoacetogenic hydrogen scavenger. Results from the present study can pave the way towards more development with respect to microorganisms and conditions for high efficient VFAs production and biogas upgrading.

The present study proposes a novel alternative method of the current biogas upgrading techniques by converting CO 2 (in the biogas) into valuable chemicals (e.g., volatile fatty acids) using H 2 as energy source and acetogenic mixed culture as biocatalyst. The influence of thermal treatment (90°C) on the inhibition of the methanogenic archaea and enriching the acetogenic bacteria in different inocula (mesophilic and thermophilic) was initially tested.The most efficient inoculum that achieved the highest performance through the fermentation process was further used to define the optimum H 2 /CO 2 gas ratio that secures maximum production yield of chemicals and maximum biogas upgrading efficiency. In addition, 16S rRNA analysis of the microbial community was conducted at the end of the experimental period to target functional microbes. The maximum biogas content (77% (v/v)) and acetate yield (72%) were achieved for 2H 2 :1CO 2 ratio (v/v), with Moorella sp. 4 as the most dominant thermophilic acetogenic bacterium.

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