Challenges and strategies for solid-state anaerobic digestion of lignocellulosic biomass

Solid-state anaerobic digestion (SS-AD) has gained increasing attention in recent years, especially for digesting lignocellulosic biomass. Compared to liquid anaerobic digestion (L-AD), SS-AD handles feedstocks with higher total solids content, and therefore, performs more effectively at higher organic loading rates and has higher volumetric biogas productivity. Challenges facing SS-AD of lignocellulosic biomass are primarily related to its relatively low methane yield, potential instability, and low value end-products. These challenges are either due to the inherent limits of SS-AD (e.g. retarded mass transfer caused by high solid content) or can be attributed to the nature of lignocellulosic biomass (e.g. components recalcitrant to biodegradation). To address these challenges, a variety of methods, including pretreatment of feedstock, improvement of inoculation efficiency, co-digestion of multiple feedstocks, and upgrading biogas to higher-value transportation fuels, have been examined to enhance the performance of SS-AD and increase the value of the end products. This review summarizes these challenges in SS-AD of lignocellulosic biomass and discusses the mechanisms and feasibility of potential strategies for resolving them.