Brewery spent grain valorization through fermentation: Targeting biohydrogen, carboxylic acids and methane production

This study investigated three different fermentation approaches to explore the potential for producing biohydrogen, carboxylic acids, and methane from hydrolysates of thermally dilute acid pretreated brewer's spent grains (BSG). Initially, the research focused on maximizing the volumetric hydrogen production rate (HPR) in the continuous dark fermentation (DF) of BSG hydrolysates by varying the hydraulic retention time (HRT). The highest HPR reported to date of 5.9 NL/L-d was achieved at 6 h HRT, with a Clostridium -dominated microbial community. The effect of the operational pH (4, 5, 6, and 7) on the continuous acidogenic fermentation was then investigated. A peak carboxylic acid concentration of 17.3 g CODequiv./L was recorded at pH 6, with an associated volumetric productivity of 900.5 ± 13.1 mg CODequiv./L-h and a degree of acidification of 68.3 %. Lactic acid bacteria such as Limosilactobacillus and Lactobacillus were dominant at pH 4–5, while Weissella , Enterococcus , and Lachnoclostridium appeared at pH 6 and 7. Finally, this study evaluated the biochemical methane potential of the DF broth and the unfermented hydrolysates and found high methane yields of 659 and 517 NmL CH4/g-VS added , respectively, both within one week. Overall, the results showed that pretreated BSG can be a low-cost feedstock for the production of bioenergy and valuable bio-based chemicals in a circular economy. • First study on continuous dark fermentation of pretreated BSG. • Maximum hydrogen productivity of 5.9 NL H2/L-d at 6 h HRT. • Methanization after dark fermentation: energy recovery of 6994.2 MJ/ton dry BSG. • Acidogenesis degree of 68.3 % during continuous acidogenesis at pH 6. • Lactic acid predominated at pH 4–6; pH 7 resulted in mixed-type fermentation.