“…Nevertheless, it still contains a variety of valuable organic compounds like organic acids, alcohols, aldehydes, ketones and phenolics (Additional file 1 : Table S1) that can potentially serve as substrates for microbial fermentations. Current research approaches dealing with a microbial valorization of aqueous pyrolysis fractions range from the production of value-added compounds like lipids [ 10 ] and solvents [ 11 – 13 ] to their conversion into biogas via anaerobic digestion [ 14 – 16 ] and the utilization as substrate for H 2 production in microbial electrolysis cells [ 17 , 18 ]. However, the broad spectrum of carbons might also be detrimental for a potential microbial utilization of the condensate as it includes several compounds that could act as inhibitors for growth and production of various organisms [ 19 , 20 ].…”
Background
The pyrolytic aqueous condensate (PAC) formed during the fast pyrolysis of wheat straw contains a variety of organic carbons and might therefore potentially serve as an inexpensive substrate for microbial growth. One of its main components is acetic acid, which was recently shown to be a suitable carbon source for the filamentous fungus Aspergillus oryzae. However, the condensate also contains numerous toxic compounds that inhibit fungal growth and result in a tolerance of only about 1%. Therefore, to enable the use of the PAC as sole substrate for A. oryzae cultivations, a pretreatment seems to be necessary.
Results
Various conditions for treatments with activated carbon, overliming, rotary evaporation and laccase were evaluated regarding fungal growth and the content of inhibitory model substances. Whereas the first three methods considerably increased the fungal tolerance to up to 1.625%, 12.5% and 30%, respectively, the enzymatic treatment did not result in any improvement. The optimum carbon load for the treatment with activated carbon was identified to be 10% (w/v) and overliming should ideally be performed at 100 °C and an initial pH of 12. The best detoxification results were achieved with rotary evaporation at 200 mbar as a complete removal of guaiacol and a strong reduction in the concentration of acetol, furfural, 2-cyclopenten-1-one and phenol by 84.9%, 95.4%, 97.7% and 86.2%, respectively, were observed. Subsequently, all possible combinations of the effective single methods were performed and rotary evaporation followed by overliming and activated carbon treatment proved to be most efficient as it enabled growth in 100% PAC shake-flask cultures and resulted in a maximum cell dry weight of 5.21 ± 0.46 g/L.
Conclusion
This study provides a comprehensive insight into the detoxification efficiency of a variety of treatment methods at multiple conditions. It was revealed that with a suitable combination of these methods, PAC toxicity can be reduced to such an extent that growth on pure condensate is possible. This can be considered as a first important step towards a microbial valorization of the pyrolytic side-stream with A. oryzae.
“…Nevertheless, it still contains a variety of valuable organic compounds like organic acids, alcohols, aldehydes, ketones and phenolics (Additional file 1 : Table S1) that can potentially serve as substrates for microbial fermentations. Current research approaches dealing with a microbial valorization of aqueous pyrolysis fractions range from the production of value-added compounds like lipids [ 10 ] and solvents [ 11 – 13 ] to their conversion into biogas via anaerobic digestion [ 14 – 16 ] and the utilization as substrate for H 2 production in microbial electrolysis cells [ 17 , 18 ]. However, the broad spectrum of carbons might also be detrimental for a potential microbial utilization of the condensate as it includes several compounds that could act as inhibitors for growth and production of various organisms [ 19 , 20 ].…”
Background
The pyrolytic aqueous condensate (PAC) formed during the fast pyrolysis of wheat straw contains a variety of organic carbons and might therefore potentially serve as an inexpensive substrate for microbial growth. One of its main components is acetic acid, which was recently shown to be a suitable carbon source for the filamentous fungus Aspergillus oryzae. However, the condensate also contains numerous toxic compounds that inhibit fungal growth and result in a tolerance of only about 1%. Therefore, to enable the use of the PAC as sole substrate for A. oryzae cultivations, a pretreatment seems to be necessary.
Results
Various conditions for treatments with activated carbon, overliming, rotary evaporation and laccase were evaluated regarding fungal growth and the content of inhibitory model substances. Whereas the first three methods considerably increased the fungal tolerance to up to 1.625%, 12.5% and 30%, respectively, the enzymatic treatment did not result in any improvement. The optimum carbon load for the treatment with activated carbon was identified to be 10% (w/v) and overliming should ideally be performed at 100 °C and an initial pH of 12. The best detoxification results were achieved with rotary evaporation at 200 mbar as a complete removal of guaiacol and a strong reduction in the concentration of acetol, furfural, 2-cyclopenten-1-one and phenol by 84.9%, 95.4%, 97.7% and 86.2%, respectively, were observed. Subsequently, all possible combinations of the effective single methods were performed and rotary evaporation followed by overliming and activated carbon treatment proved to be most efficient as it enabled growth in 100% PAC shake-flask cultures and resulted in a maximum cell dry weight of 5.21 ± 0.46 g/L.
Conclusion
This study provides a comprehensive insight into the detoxification efficiency of a variety of treatment methods at multiple conditions. It was revealed that with a suitable combination of these methods, PAC toxicity can be reduced to such an extent that growth on pure condensate is possible. This can be considered as a first important step towards a microbial valorization of the pyrolytic side-stream with A. oryzae.
“…Current density and hydrogen productivity were observed to be linearly correlated with the organic loading rate. Prior studies have shown similar correlations between organic loading and hydrogen productivity up to an organic loading rate of 10 g/L-day using biomass pyrolysis aqueous phase [24,33,34], and up to 30 g/L-day using corn stover fermentation product [22].…”
Section: Electrochemical Performance Of Mecs Under Closed Circuit Conmentioning
Background Microbial electrolysis is a promising technology for converting aqueous wastes into hydrogen. Substrate adaptability is an important feature, seldom documented in Microbial Electrolysis Cells (MECs). The correlation between substrate composition and community structure has not been well established. This study used a MEC capable of producing over 10 L/L-day of hydrogen from a switchgrass-derived bio-oil aqueous phase and investigated four additional substrates. The additional substrates included a red oak-derived bio-oil aqueous phase, a corn stover fermentation product, a mixture of phenol and acetate, and acetate alone. Results The MEC fed with the corn stover fermentation product resulted in the highest performance among the complex feedstocks, producing an average current density of 7.3 A/m2, although the acetate fed MEC outperformed complex substrates, producing 12 ± A/m2. 16S rRNA gene sequencing showed that community structure and community diversity were not predictive of performance, and replicate community structures diverged despite identical inoculum and enrichment procedure. The trends in each replicate, however, were indicative of the influence of the substrates. Geobacter was the most dominant genus across most of the samples tested, but its abundance did not correlate strongly to current density. High-performance liquid chromatography (HPLC) showed that acetate accumulated during open-circuit conditions when MECs were fed with complex feedstocks and was quickly degraded once closed-circuit conditions were applied. The largest net acetic acid removal rate occurred when MECs were fed with red oak bio-oil aqueous phase, consuming 2.93 ± 0.00 g/L-day. Principal component analysis found that MEC performance metrics such as current density, hydrogen productivity, and COD removal were closely correlated. Net acetic acid removal was also found to correlate with performance. However, no bacterial genus correlated to performance metrics, and the analysis suggested that less than 70% of the variance was accounted for by the two components. Conclusions This study demonstrates the robustness of microbial communities to adapt to a range of feedstocks and conditions without relying on specific species, delivering high hydrogen productivities, thus indicating functional adaptation vs. compositional requirement. MECs may,, play a central role in the 21st-century bioeconomy as factories producing a zero-emission fuel.
“…Current density and hydrogen productivity were observed to be linearly correlated with the organic loading rate. Prior studies have shown similar correlations between organic loading, current density, and hydrogen productivity up to an organic loading rate of 10 g/L-day using biomass pyrolysis aqueous products [ 24 , 33 , 34 ], and up to 30 g/L-day using corn stover fermentation product [ 22 ]. Fig.…”
Section: Resultsmentioning
confidence: 88%
“…Prior studies have shown similar Table 1 Substrate COD before addition to reactors and compound concentration as a percent of the COD of the substrates 4-hydrozybenzaldehyde (4HB) was not detected in any of the substrates. The concentrations of acetic acid and phenol for the phenol/acetate blend (PHE/ ACE) and acetate (ACE) were determined by weight measurement during their preparation, while the complex substrates' composition was determined by HPLC correlations between organic loading, current density, and hydrogen productivity up to an organic loading rate of 10 g/L-day using biomass pyrolysis aqueous products [24,33,34], and up to 30 g/L-day using corn stover fermentation product [22]. Anode Coulombic efficiency varied more considerably from substrate to substrate, which was lowest when phe/ ace was fed to MECs, reaching 44.7 ± 1.56%.…”
Section: Electrochemical Performance Of Mecs Under Closed Circuit Conmentioning
Background
Microbial electrolysis is a promising technology for converting aqueous wastes into hydrogen. However, substrate adaptability is an important feature, seldom documented in microbial electrolysis cells (MECs). In addition, the correlation between substrate composition and community structure has not been well established. This study used an MEC capable of producing over 10 L/L-day of hydrogen from a switchgrass-derived bio-oil aqueous phase and investigated four additional substrates, tested in sequence on a mature biofilm. The additional substrates included a red oak-derived bio-oil aqueous phase, a corn stover fermentation product, a mixture of phenol and acetate, and acetate alone.
Results
The MECs fed with the corn stover fermentation product resulted in the highest performance among the complex feedstocks, producing an average current density of 7.3 ± 0.51 A/m2, although the acetate fed MECs outperformed complex substrates, producing 12.3 ± 0.01 A/m2. 16S rRNA gene sequencing showed that community structure and community diversity were not predictive of performance, and replicate community structures diverged despite identical inoculum and enrichment procedure. The trends in each replicate, however, were indicative of the influence of the substrates. Geobacter was the most dominant genus across most of the samples tested, but its abundance did not correlate strongly to current density. High-performance liquid chromatography (HPLC) showed that acetic acid accumulated during open circuit conditions when MECs were fed with complex feedstocks and was quickly degraded once closed circuit conditions were applied. The largest net acetic acid removal rate occurred when MECs were fed with red oak bio-oil aqueous phase, consuming 2.93 ± 0.00 g/L-day. Principal component analysis found that MEC performance metrics such as current density, hydrogen productivity, and chemical oxygen demand removal were closely correlated. Net acetic acid removal was also found to correlate with performance. However, no bacterial genus appeared to correlated to these performance metrics strongly, and the analysis suggested that less than 70% of the variance was accounted for by the two components.
Conclusions
This study demonstrates the robustness of microbial communities to adapt to a range of feedstocks and conditions without relying on specific species, delivering high hydrogen productivities despite differences in community structure. The results indicate that functional adaptation may play a larger role in performance than community composition. Further investigation of the roles each microbe plays in these communities will help MECs to become integral in the 21st-century bioeconomy to produce zero-emission fuels.
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