Conversion of Pyrolysis Products into Volatile Fatty Acids with a Biochar-Packed Anaerobic Bioreactor

The coupling of pyrolysis and acidogenic fermentation was here proposed as a new hybrid thermochemical-biological method to circumvent the hydrolysis bottleneck within lignocellulose valorization schemes. Pyrolysis products of fir sawdust, that is, the water-soluble (WS) fraction together with CO-rich syngas, were tested as feedstock for volatile fatty acid (VFA) production. WS/syngas conversion to VFA was particularly challenging due to the combined effect of the substrate (WS/syngas) and product (VFA) inhibition. To solve such an issue, a new type of bioreactor, based on packed biochar and a new acclimatization/bioaugmentation procedure consisting of co-feeding WS/syngas and glucose were developed and tested. The gradual switch from glucose to WS was monitored through various analytical techniques, observing the transition toward a “pyrotrophic” microbial mixed culture able to convert WS/syngas into VFA. Even without selective inhibition of methanogens, the main fermentation products were VFA (mainly acetic, butyric, and caproic acid), whose profile was a function of the WS/glucose ratio. Although the achieved volumetric productivity was lower (<0.6 gCOD L–1 d–1) than that observed in sugar fermentation, bioaugmented pyrotrophs could convert headspace CO, most of GC–MS detectable compounds (e.g., anhydrosugars), and a significant portion of non-GC–MS detectable compounds of WS (e.g., oligomers with MW < 1.45 kDa).