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

Abstract

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).

Two-stage conversion of syngas and pyrolysis aqueous condensate into L-malate

“…Examples include also the enrichment of Clostridium sensu stricto (up to 17.5%) during the degradation of tars from rice husk gasification, where biochar facilitated syntrophic relations with Methanosaeta [ 26 ]. Another study documented the enrichment of Clostridium sensu stricto 1 and 12 up to 5% abundance during the co-fermentation of syngas and PAC in a packed biochar reactor [ 18 ]. Similarly, during the anaerobic digestion of phenol-rich coal gasification wastewater with addition of graphene, about 10% of the rector microbiota was composed of Clostridium sensu stricto 5 and Clostridium sensu stricto 1 [ 77 ].…”

“…Similarly, Caproiciproducens , a genus commonly found in chain-elongating microbial communities, produces butyrate and caproate [ 86 – 88 ]. The production of longer-chain carboxylic acids, such as valerate and caproate, has been previously documented in the co-fermentation of syngas and PAC at 30 °C [ 18 ]. Anaerococcus , enriched concurrently with Caproiciproducens , can ferment a wide range of carbohydrates, peptone or amino acids to produce carboxylic acids [ 89 ] and it has been previously correlated with iso -butyrate production in syngas reactors [ 90 ].…”

“…Other studies reported methanogenesis inhibition during the co-fermentation of PAC and syngas. At 30 °C and pH 6, methanogenesis was severely inhibited, and acetate, butyrate, and other carboxylic acids up to caproate accumulated in the fermentation broth [ 18 ]. Although that system was not optimized for gas fermentation, 46% of the CO fed into the system was metabolized throughout the whole experimental period [ 18 ].…”

“…At 30 °C and pH 6, methanogenesis was severely inhibited, and acetate, butyrate, and other carboxylic acids up to caproate accumulated in the fermentation broth [ 18 ]. Although that system was not optimized for gas fermentation, 46% of the CO fed into the system was metabolized throughout the whole experimental period [ 18 ]. In another work, syngas and PAC were co-fermented in shaking flasks under mesophilic and thermophilic conditions.…”

“…Others have utilized the diversity and functional redundancy of the microbial network in the anaerobic digestion process to convert PAC components into biogas. However, methanogenesis was severely inhibited by the toxicity of PAC, leading to the accumulation of short-chain carboxylates in the medium [17,18]. To improve methane production, recent efforts have successfully employed community enrichment or biochar amendments [5,6,17,[19][20][21][22][23][24][25][26][27][28].…”