Reliable conversion of real syngas to volatile fatty acids (VFA) using mixed microbial cultures on Char-Based Sparger Reactor (CBSR).

The biological upgrading of gasification‑derived syngas represents a promising route for the valorisation of residual biomass and wastes. Still, its industrial implementation remains limited by process instability and sensitivity to variable gas composition. This work addressed these issues by using mixed microbial cultures (MMC) on char to ferment untreated syngas over 335 days. Single treatment with BES enabled the selection of a stable consortium dominated by Acetobacterium, Ercella, and Alkalibaculum that converted a H2:CO mixture into volatile fatty acids (VFAs) with high (>80%) selectivity for acetate. Both CO:CO2 and H2:CO:CO2 were consistently fermented to an acetate concentration of 30 g COD L-1, with volumetric productivity exceeding 5 g COD L-1 d-1 throughout the study. The key parameter was the concentration of fermentable gases (CO:H2). H2:CO:CO2 was converted with volumetric productivity up to 28 g COD L-1d-1, while providing the highest VFA (30 g COD L-1) concentration ever obtained in syngas fermentation. This work provides a laboratory-scale proof of concept and establishes a basis for future engineering and pilot-scale investigations.