Pretreatment methods to enhance solubilization and anaerobic biodegradability of lignocellulosic biomass (wheat straw): Progress and challenges

Abstract

Agro-waste (wheat straw) is considered most effective (heating value 16 MJ/kg) for energy recovery through anaerobic digestion (AD). However, the complex lignocellulosic structure obstructs their biotransformation and is a rate-limiting step of the AD process. The pretreatment of agro-waste could remove the physical and chemical barriers and accelerate the downstream AD process. The physical pretreatments (mechanical, thermal, sonication) improve the biodegradation kinetics of wheat straw (>50% and can reach up to 92%) but require more energy. High external or internal heat sources may lead to the formation of recalcitrant compounds, i.e., furan derivatives from cellulose. Chemical pretreatment is effective for rapid reaction rate but is uneconomical due to high chemical cost. It may produce toxic intermediary products, and hinder microbial activities in AD. The ionic liquids are emerging chemical pretreatment and are renewable, recoverable, difficult to oxidize, and bio-based salts. The chemical pretreatment induces cellulose loss up to 59% and lignin up to 69%, while the bio-degradability is as high as 88.3%. In biological pretreatment, bacterial pretreatment is superior to fungal and algal pretreatments, and also helps in the substrate bio-degradation during AD. The FTIR, TGA-DTG, XRD, and SEM technologies can validate the efficiency of the pretreatment method, such as modifications in lignocellulosic functional groups, mass loss, crystal feature, surface topography, and morphology of lignocellulosic biomass. Different mathematical models, especially the modified Gompertz model, are employed to facilitate the insight into the AD research. Energy use and balance are necessary for the pretreatment process to figure out the real synergic effects rather than aiming to achieve enhanced methane production.