Lignocellulosic biomasses are considered promising feedstocks for the next
generation of biofuels and chemicals; however, the recalcitrance of lignocellulose
remains a barrier to its utilisation over conventional sources. Pyrolysis is the heating
of biomass to several hundred degrees Celsius in the absence of oxygen, which can
thermally depolymerise lignocellulose. Products of pyrolysis are a solid biochar,
liquid bio-oil and syngas. Biochar has roles in both carbon sequestration and soil
amendment however bio-oil has no defined use, despite a high concentration of
fermentable sugars. Bio-oil is a complex organic microemulsion with a host of
biocatalyst inhibitors that makes its microbial degradation a challenge.
In this work, the use of aerobic cultures using microbial communities isolated from
natural environments saw limited potential; however, the use of anaerobic digestion
(AD) successfully generated a higher volume of biogas from reactors with bio-oil
than controls. Biogas yield test reactors were set up with anaerobic digestate from a
wastewater treatment plant as the substrate for degradation and conversion of bio-oils.
Next-generation 16S rRNA gene sequencing was utilised to characterise the
communities in the reactors while the ultrahigh resolution mass spectrometry
technique of Fourier transform ion cyclotron resonance (FT-ICR) was used for
characterisation of the chemical changes occurring during AD. Both sets of high-resolution
data were additionally combined for multivariate analysis and modelling
of the microbial genera that correlated best with the changes in digestate chemistry.
This represents a novel analysis method for the microbial degradation of complex
organic products.
Bio-oil from common lignocellulosic feedstock was the most easily degradable by
the AD communities, with significant inhibition observed when bio-oils from
anaerobic digestate and macroalgae were used. Additionally it was found that the
inclusion of biochars that were pre-incubated in anaerobic digestate prior to use in
AD were capable of significantly reducing the lag time observed for biogas
production in bio-oil-supplemented reactors. The addition of biochars that were not
pre-incubated had no effect on biogas production. Specific inhibition of
methanogenesis was also capable of causing the digestates to accumulate volatile
fatty acids (VFAs) as a product of greater value than biogas. Scale-up experiments
will be required to confirm the precise practicalities of the addition of bio-oil to AD
as well as to establish the potential for isolation and purification of VFAs