Improving hydrogen and volatile fatty acids production through pretreatment of spent coffee grounds

Consumption of coffee produces large amounts of waste in the form of spent coffee grounds (SCG), a lignocellulosic material rich in carbohydrates, proteins, and polyphenols. This abundant feedstock is promising in terms of biofuels and value-added product generation. This study investigated the impact of pretreatments, such as alkaline (NaOH), ultrasound, and static magnetic field, on SCG bioconversion in terms of biomolecule release, H₂ potential and volatile fatty acids production. Following treatment, the slurry (solid and liquid fraction mixture) was utilised in anaerobic fermentation tests at varying volatile solid (VS) concentrations (23.3 and 46.7 g VS/L). The highest H₂ production range, 25 - 30 mL H₂/g VS, was obtained using the alkaline-pretreated SCG slurry at 23.3 g VS/L. Nevertheless, inhibition of H₂ production was observed when utilising the alkaline-pretreated slurry at 46.7 g VS/L owing to the excessive use of NaOH for pretreatment and chemicals to adjust the initial pH. In contrast, increasing the VS concentration had a positive impact on volatile fatty acids accumulation, with acetic (HAc) and caproic acid being dominant. Ultrasound-pretreated SCG achieved 3260.0 mg HAc_eq/L at a concentration of 46.7 g VS/L.