Boosting biogas production from recalcitrant lignin-based feedstock by adding lignin-derived carbonaceous materials within the anaerobic digestion process

Lignocellulosic biomass can add to the worldwide resource of biogas; however, the aromatic structure of lignin is recalcitrant which impairs biodegradation. Direct interspecies electron transfer (DIET) may overcome limitations in the biodegradation of lignin derivatives. Within a circular bioeconomy system, lignin-derived biochar and activated carbon were assessed for their ability to enhance the digestion of a typical lignin monomer – syringaldehyde. Biochar at 5–10 g/L significantly reduced the lag-phase time by 33–42% possibly due to the enhancement of syntrophic hydrogenotrophic methanogenesis. In comparison, activated carbon at 1–10 g/L reduced the lag-phase time by 46–85% and significantly accelerated the degradation of volatile fatty acids, due to a combinational effect of enhanced syntrophic oxidation and DIET. When activated carbon was added at a higher dosage of 20 g/L, the highest biomethane yield (426.6 ml/g) was achieved; an increase of 33% compared to the digestion of syringaldehyde alone. The enhancement was ascribed to the metabolic shift from the hydrogenotrophic to the DIET pathway, which could be implied from the microbial community dominated by Methanosaeta. The superior function of activated carbon over biochar was speculated to be associated with its larger surface area and higher abundance of the CO group.