Lignin degradation under anaerobic digestion: Influence of lignin modifications -A review

Large-scale implementation of Biorefineries for the substitution of oil-based fuels with biofuels and bio-products will result in the production of large amounts of lignin. Furthermore, the paper and pulp industry produces lignin residues such as Kraft-lignin, which has limited use today. Lignin needs to be degraded before this high molecular weight molecule can be converted into valuable bio-products. Biodegradation of native lignin has been described for fungal strains such as white and brown rot fungi and for a variety of different bacterial genera. Anaerobic biodegradation of lignin is, however, far less understood today and most studies has been done using small molecular weight lignin degradation compounds, while little work has been done on the conversion of native lignin. In this paper we will discuss the influence of lignin modifications pertaining to its degradation during anaerobic digestion. Wet oxidation reactions using oxygen at a temperature of ca. 175 degrees C for 25 min was found to increase the degree of methoxylation of lignin with simultaneous increment in conversion during anaerobic conditions. Several studies have further found that de-methoxylation is the first step during anaerobic degradation of lignin followed by ring cleavage and fermentation into methane and carbon dioxide. As lignin is an important component of the materials that are fed to biogas plant, we have futher discussed the potential of these findings in this paper.