Integrated acid-thermal and enzymatic hydrolysis of hardwood residues with anaerobic digestion: Effects on carbohydrate and biomethane recovery

Abstract

Canada has one of the world's largest forestry industries, which generates substantial amounts of woody residues. The hardwood residues are often underutilized or disposed of at low value, creating both environmental and management challenges. This study investigates the enhancement of carbohydrate solubilization and biomethane recovery from two Canadian hardwood residues, poplar and maple, using combined acid-thermal pretreatment and enzymatic hydrolysis followed by anaerobic digestion (AD). The integrated pretreatment approach significantly improved carbohydrate solubilization and subsequent methane yields. Maximum carbohydrate recovery after acid-thermal pretreatment reached 73 % for poplar and 63 % for maple. With the addition of enzymatic hydrolysis at the acid-thermal pretreatment scenario of 170 °C, carbohydrate recovery further increased to 80 % and 70 %, respectively. As a result, biomethane yields increased up to 335 mL CH₄/g COD_added, with a maximum yield of 313 mL CH₄/g initial wood. Kinetic analysis revealed that the modified Gompertz model provided the best description of the biomethane production rate. Optimal conditions generally included lower solid/liquid ratios (5 %) and lower acid concentrations (0.5 %) at moderate temperatures (170 °C), which favored fermentable sugar production. However, higher temperatures (200 °C), while enhancing carbohydrate release, reduced subsequent AD performance, likely due to the generation of inhibitory or recalcitrant compounds. These findings demonstrate that combining pretreatment improves hardwood residue carbohydrate recovery and subsequent biomethane production. Additionally, this study provides a sustainable approach to managing abundant forestry waste streams, contributing to renewable energy generation and greenhouse gas reduction in Canada.