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. The highest concentrations of ethanol and butanol produced were respectively of 122 mg.L-1 and 97 mg.L-1 for the thermal pretreatment (after 710 hours), and of 87 mg.L-1 and 143 mg.L-1 for the acidic-thermal pretreatment (after 210 hours). Butyrate was consumed and acetate was produced in all assays. A 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.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Mass balances, some free Gibb's enery calculations and some chemical equations were wrong in first version. Now they are properly corrected.
7 List of Abbreviations Used
- STP
- Standard temperature and pressure (0° C and 1 atm)
- ppH2
- Partial H2 pressure (átm)
- R2
- Levenberg-Marquardt algorithm correlation coefficient
- TVS
- Total volatile solids (mg L-1)
- OTU
- Operational taxonomic unit
- HPLC
- High performance liquid chromatography
- PCR
- Polimerase chain reaction
- RDP
- Ribossomal data project
- VFA
- Volatile fatty acid
- HBut
- Butyrate
- HProp
- Propionate
- HAc
- Acetate
- But(OH)
- Butanol
- Prop(OH)
- Propanol
- Eth(OH)
- Ethanol
- Concentration in respect of time (mg L-1)
- Time (h)
- Initial time for exponential growth phase (h)
- Ending time for exponential growth phase (h)
- Time in which maximum production rate () is achieved (h)
- Maximum production rate (mg L-1 h-1)
- Maximum concentration (mg L-1)
- ΔGr
- Variation of Gibbs’ free energy at a given condition (kJ mol-1)
- ΔG°r
- Variation of Gibbs’ free energy at standard condition (kJ mol-1)
- ΔGFr
- Variation of Gibbs’ free energy at final experiment condition (kJ mol-1)
- ΔGIr
- Variation of Gibbs’ free energy at initial experiment condition (kJ mol-1)