Spruce is the most resistant type of lignocellulosic material to anaerobic digestion. Different parts of spruce tree, i.e., leaves, wood, branches, bark, fruit, and mixtures of these parts, were pretreated using the leading pretreatments, i.e., concentrated phosphoric acid, dilute sulfuric acid, and ethanolic organosolv pretreatment, to improve biogas production. The pretreatment effects on methane yield from both solid and liquid fractions of the pretreatments were evaluated. Different parts of the tree showed different behaviors. The maximum methane yields from the pretreated solids were obtained after phosphoric acid pretreatment, which yielded methane yields of 211.7 and 225.5 mL per g volatile solids (VS) of pretreated spruce leaves and mixture, respectively. The improvements were related to opened-up structure, crystallinity reduction, and delignification. Sulfuric acid pretreatment improved biomethane yield from liquids, but not solids. Moreover, organosolv pretreatment improved the yield from solids, but not wood, bark, and the mixture. The maximum methane production yield was 245.3 NmL CH4/gVS which was obtained from the organosolv liquor of spruce leaves. The highest overall methane production (252.9 L per Kg) was gained from the whole solid and liquid fractions from organosolv pretreatment of leaves. This methane yield corresponded to 2.4-fold improvement compared to that of the untreated leaves (107 NmL CH4/gVS). Thus, leaves and branches are less recalcitrant parts and do not need severe pretreatment, while bark and wood are the most recalcitrant parts and required severe pretreatment like concentrated phosphoric acid. Moreover, the mixture of all parts also required severe pretreatment for efficient biogas production. © 2022 Elsevier Ltd
