New trends in physicochemical characterization of solid lignocellulosic waste in anaerobic digestion

•TGA, FTIR and XRD applied in lignocellulosic solid material in biomethanization.•Artichoke, asparagus and cellulose chosen as representative lignocellulosic materials.•New information sources complemented with traditional monitoring of biomethanization.•Remaining acetate in solid state identified by FTIR can be further solubilized.•Crystallinity rate and degradation temperature peaks decreased by biomethanization. In this paper, the use of techniques such as TGA, FTIR-ATR and XRD has provided valuable information about the lignocellulosic wastes in anaerobic digestion (AD) processes. A deep study of their break-down in solid state could improve the management of these recalcitrant substrates. Representative wastes (artichoke –ART-, asparagus –ASP-, the co-digestion of both –AcoD- and pure cellulose –CELL-) were assessed in batch. The main obtained results were related to biochemical parameters. As the biomethanization progressed, hemicellulose, cellulose and lignin were weakened as shown by TGA, and cellulose crystallinity decreased as demonstrated by XRD. Additionally, results from FTIR-ATR allowed hypothesizing that the formation of acetate in solid state did not always lead to its solubilisation, thus hindering biogas formation. AcoD and CELL were the most successful in releasing the newly formed acetate from the solid fraction to the aqueous medium, increasing its bioavailability, which was supported by the higher biogas production.