Advances in biological techniques for sustainable lignocellulosic waste utilization in biogas production

Improper disposal of lignocellulosic wastes may produce a large quantity of greenhouse gases and pollute the environment. Through anaerobic digestion processes, lignocellulosic wastes can be recycled to produce clean and renewable biogas. However, the lignin in lignocellulose limits its potential as such a biomass resource, and the efficacy of biogas production is not satisfactory although recent research efforts have attempted to address this issue. In this review, the physicochemical characteristics of three lignocellulosic wastes, including municipal solid waste, forestry waste, and crop straw, are summarized. Then, the mechanism and influencing factors of biogas production from these wastes through anaerobic digestion are presented. Biological pretreatment techniques have been confirmed to increase lignocellulose hydrolysis and then enhance biogas production, among them, co-culture systems, metabolic engineering and anaerobic co-digestion are worthy of focus in future research. Furthermore, natural lignocellulose degrading systems, like xylophagous insects and ruminants, also have potential for improving the anaerobic digestion system. This review also considers the future perspective of anaerobic digestion of lignocellulosic wastes, including kinetics and model studies to optimize anaerobic digestion process, and policy to facilitate biogas production from lignocellulosic wastes. This article aims to comprehensively review challenges with anaerobic digestion of lignocellulosic wastes and summarize available pretreatment methods focusing mainly on biological techniques to find efficient and low-cost strategies for improving the anaerobic digestion process and biogas production.