Fermentation of organic wastes and CO2 + H2 off-gas by microbiotas provides short-chain fatty acids and ethanol for n-caproate production

Abstract This study describes a two-stage process for the valorization of organic wastes and CO2/H2 off-gas for the production of short-chain fatty acids (SCFAs) and ethanol, as precursors for n-caproate formation. Three complex organic wastes with previously identified acetogenic members were used to inoculate batch reactors: 1) fibers and 2) leachates from a bagasse fermentation process, and 3) an anaerobic granular sludge from a reactor treating brewery effluents. In the first stage, the reactors loaded with a CO2/H2 [25:75, v/v] gas mixture produced SCFAs at similar concentrations between 4,274 and 5,381 mg/L. In this stage, fiber reactors had the highest H2 consumption. In the second stage, reactors consumed all available H2 and accumulated solvents as well as low concentrations of n-caproate. In particular, sludge reactors produced 3,528 mg/L of solvents with more than 50% ethanol content, and leachate reactors produced 617 mg/L of n-caproate. By the end of the second stage, in a CO2-enriched atmosphere, leachate reactors produced 6,157 mg/L of n-caproate at the expenses of the SCFAs and solvents. Fiber and leachate microbiotas shared autotrophic members and SCFA-elongating bacteria while sludge microbiota differed in composition. This study demonstrated that the use of CO2/H2 off-gas shifted the composition of SCFAs to a mixture with ethanol which promoted its condensation into n-caproate. A CO2-enriched atmosphere improved significantly the final production of n-caproate. The results of this study can be used to integrate the n-caproate production, and possibly other medium-chain fatty acids, with biorefinery schemes in which SCFAs and CO2/H2 off-gases are produced.