Brewer’s Spent Grain (BSG) is roughly 85% of the total by-product generated from beer brewing. It is usually used as animal feed. BSG contains proteins and carbohydrates, including residual starch and (hemi)celluloses from the hull. It is therefore usable as carbon and/or nutrient source in biotechnological processes, including in mixed culture fermentation processes. Our aim is to identify the best acidogenic fermentation conditions in order to produce a mixture of Medium Chain Carboxylic Acids (MCCAs). The targeted MCCAs, n-caproate and n-caprylate, are valuablechemicals, which can be used as intermediate molecules for green chemistry. In our preliminary approach, the mixed culture fermentation is performed with BSG as raw substrate, wastewater sludge as inoculum, H2 as electron donor and exogenous addition of ethanol. The influence of the Organic Loading Rate (ORL), Hydraulic Retention Time (HRT), H2 and ethanol exogenous addition on the carboxylates production rates will be presented.