Chapter 14: Biogas Production from Lignin via Anaerobic Digestion

Anaerobic digestion (AD) is a well-established technology widely used for waste management and production of biogas. Biogas, mainly composed of methane and carbon dioxide, is the final product generated by anaerobic microbial degradation of organic materials. The general view is that it is mainly the carbohydrate, protein, and lipid fractions of organic materials that is degraded during AD, while the lignin fraction is difficult to degrade. However, AD of lignin has been observed in various natural environments and here we highlight the mechanisms of depolymerization and solubilization into small molecular weight aromatic compounds and their subsequent conversion into biogas. This degradation requires close cooperation between several groups of microorganisms, where the key step is the enzymatic depolymerization of lignin. The industrial applicability of AD for lignin valorization may be limited due to the slow rate of lignin depolymerization. However, various pretreatment and bioaugmentation methods are identified that may enhance lignin depolymerization. Furthermore, co-digestion and microbial acclimation are suggested for reducing the possible inhibitory effects of aromatic compounds on the microbial community.