A comprehensive review on the pretreatment of lignocellulosic wastes for improved biogas production by anaerobic digestion

Biogas production from agricultural residues represents an effective and sustainable option for handling vast quantities of lignocellulosic waste for meeting the global energy demand. However, the recalcitrance of lignocellulosic biomass due to the presence of cellulose and hemicellulose in a structurally complex lignocellulosic matrix, and the crystallinity of cellulose create a hindrance in biogas production by restricting the availability of fermentable sugars to microbial action. This paper assesses the different pretreatment strategies adopted for making the lignocellulosic biomass amenable to anaerobic digestion by altering the structure of lignocellulose and eliminating lignin, thus increasing the accessibility of microbes to the easily degradable components. The review highlights the advantages and limitations of each technology—physical for size reduction (chipping, milling, extrusion, cavitation), thermal for breaking down of hydrogen bonds (conventional heating, steam explosion, microwave irradiation, hydrothermal), chemical for decreasing the crystallinity and polymerization degree of cellulose (alkalis, acids, gases, oxidizing agents, various solvents), biological for enhancing the digestibility (microbial, enzymatic), and their effect on improving the degradation efficiency and biogas yield. Further, the review discusses the role of some emerging technologies and other less explored options which may require optimization at higher scale so that the confidence in the translation of this knowledge can be increased and this abundantly available raw material can be effectively utilized. With the goal of improving the applicability of pretreatment technologies, some recommendations are put forth so that the efficiency of anaerobic digestion can be increased and the development of pretreatment technologies can be promoted on a large scale.