Inoculum dependence of methane formation from lignocellulosic biowastes

Anaerobic digestion (AD) is a well-established process to produce methane from recalcitrant waste materials, such as lignocellulosic substrates (LSs). This study investigated the interaction between three LSs, i.e., hazelnut skin (HS), spent coffee grounds (SCG), and almond shells (AS), and different inocula, i.e., crushed granular sludge (CrGS), digestate from buffalo manure (DBM), and digestate from sewage sludge (DSS). The CrGS was the less suitable inoculum, resulting in 219.1 (± 11.8) and 21.2 (± 4.3) mL CH4/g VS from SCG and AS, respectively, whereas no methane was produced from HS with CrGS. A methanol-organosolv pretreatment improved the methane potential of SCG by 21% and unlocked AD of HS with CrGS producing 289.6 (± 9.9) mL CH4/g VS, but no significant effect was observed on AS. DSS was the best-performing inoculum to digest the investigated LSs, achieving similar results to DBM for HS and SCG, but increasing the methane potential of AS up to 90.7 (± 4.3) mL CH4/g VS. DSS was particularly rich in microorganisms of the Synergistota (13.0%) and Chloroflexi (25.0%) phyla, which are capable of degrading complex biomolecules such as proteins and lipids present in HS, SCG, and AS.