Designing a flexible anaerobic biorefinery for the valorization of red grape marcs into biofuels and biochemicals

Global grape production reaches 70 million tons annually, with Europe leading 61 % of global wine production, mainly from Italy, France, and Spain. This study examined red grape marc(RGM), a winemaking byproduct, in a laboratory-scale biorefinery to produce fatty acids, hydrogen, and methane. The effects of hydraulic retention time (HRT) and organic loading rate (OLR) were investigated. Short HRTs (2–6 days) inhibited production, yielding minimal fatty acids and biogas. Optimal conditions for short-chain fatty acids, caproic acid, and hydrogen occurred at 10 days HRT and 5 gCOD/L⋅d OLR, with yields of 30 %, 5 % w/w, and 25 LH₂/kgVS, respectively. Trace heptanoic and octanoic acids indicated chain elongation onset. A downstream purification process using centrifugation and ultrafiltration produced a fatty acid stream exceeding 90 % purity, to be used for single cell oils in future studies. Methane production peaked at 20 days HRT and 2.5 gCOD/L⋅d OLR, yielding 160–170 LCH₄/kgVS. The study also evaluated polyphenols’ impact, finding their removal reduced methane production by 30 %, attributed to the co-extraction of fermentable sugars essential for methane generation.