Microorganisms Characterization in Anaerobic Solventogenic Processing of Volatile Fatty Acids

1 Abstract Background Production of alcohols from wastes through biological processes is environmentally and economically interesting, since they can be valorized as drop-in liquid fuels, which have a high market value. Using microbial mixed cultures in such processes is of great interest since it confers more stability, a higher resistance to both toxicity and contamination, and an increased substrate flexibility. However, there is still a lack of fundamental knowledge on such microbial populations used as inoculum in solventogenic processes. This work evaluates the effect of four different physicochemical pretreatments (acidic, thermal, acidic-thermal and thermal-acidic) on an anaerobic inoculum used for alcohols production from volatile fatty acids. Results All experiments were conducted in single batches using acetate and butyrate as substrates, at 30°C and with a pressurized headspace of pure H2 at 2.15 atm (218.2 MPa). Higher productions of both ethanol and butanol were achieved with both thermal and acidic-thermal pretreatments of the inoculum. Mathematical modelling shows that the highest attainable concentrations of ethanol and butanol produced were respectively of 122 mg.L-1 and 97 mg.L-1 for the thermal pretreatment (after 17.5 days), and of 87 mg.L-1 and 143 mg.L-1 for the acidic-thermal pretreatment (after 18.9 days). Acetate was produced in all assays. Mass balance study indicated that the inoculum provided part of the substrate. Thermodynamic data indicated that a high H2 partial pressure favored solventogenic metabolic pathways. Finally, sequencing data showed that both thermal and acidic-thermal pretreatments selected mainly the bacterial genera Pseudomonas, Brevundimonas and Clostridium. Conclusion The acidic-thermal pretreatment selected a bacterial community more adapted to the conversion of acetate and butyrate into ethanol and butanol, respectively. A higher production of ethanol was achieved with the thermal pretreatment, but at a slower rate. The thermal-acidic pretreatment was unstable, showing a huge variability between replicates. The acidic pretreatment showed the lowest alcohol production, almost negligible as compared to the control assay.