Biogas production through combined thermochemical and biochemical processing of grape pomace

Grape pomace, the solid residue from winemaking, is produced in large seasonal quantities and represents a challenge for sustainable waste management due to its elevated lignin concentration (≈50 wt%, markedly higher than in other agricultural residues such as corn stover or wheat straw, ≈20 wt%), which severely limits its biodegradability in anaerobic digestion. Its potential as a feedstock for bioenergy production remains underexploited, largely because of this recalcitrant fraction. This study investigates the use of mild hydrothermal pretreatment under pressurized conditions as a strategy to restructure the lignin matrix of grape pomace and thereby enhance its subsequent anaerobic digestion and biogas production. A combined thermochemical and biochemical process was conducted at laboratory scale with grape pomace as biomass. The effects of temperature and solid yield on lignin content and bioconversion efficiency were analyzed. Results show that thermochemical pretreatment increases the overall lignin content while inducing morphological changes that improve its fermentability. As a result, the pretreated samples produced higher biomethane volumes up to 29 % with respect to the as-received biomass and referred to the lignin fraction. By comparison with literature on different biomasses, a correlation of biomethane yield with treatment temperature was obtained, showing a maximum increment of 20 % in the biomethane yield compared to the untreated biomass at around 130 °C. The findings confirm the viability of combining mild thermochemical treatment with anaerobic digestion as a strategy to valorize grape pomace, aligning with circular economy principles.