Innovative char-sparger for improving volatile fatty acids (VFA) production in homoacetogenic fermentation of H2/CO2 with microbial mixed cultures (MMC)

The homoacetogenic fermentation of CO2 and H2 allows the production of renewable volatile fatty acids (VFA) using microbial mixed cultures (MMC). The main challenges of such approach, which would open a reliable power-to-X pathway, are the low product concentration and low volumetric productivity. To address such key features, which are due to product inhibition and low solubility of H2, biomass-derived biochar and polystyrene foam were used to manufacture a porous monolith (sparger) that allows the delivery of H2/CO2 to bacterially colonisable surfaces. Such device was used within a novel char-based biofilm sparger reactor (CBSR), which was tested for acetogenic fermentation of H2/CO2 with variable dilution rates. Noticeably, a low dilution rate (0.1 d-1) achieved the highest VFA concentration ever obtained (58 gCOD L-1 and 52 g/L acetic acid) with MMC, whereas a higher dilution rate (0.8 d-1) provided 15 gCOD L-1 VFA with a volumetric productivity of 18.5 gCOD L-1 d-1. According to the 16S rRNA sequencing of the biofilm microbiota, Acetobacterium was the dominant genus with a 90% relative abundance ratio. Such performances, which are close to that obtainable with optimised axenic fermentation, suggest that the innovative char-sparger could enhance mass transfer rate, shield from product inhibition and/or improve the growth of biofilm.