Integrated Hydrothermal Carbonization and Anaerobic Digestion of Agro-industrial Wastes: Volatile Fatty Acid Production and Analytical Benchmarking

Volatile fatty acids (VFAs) are key intermediates for circular biorefineries because they connect organic waste treatment with bioenergy, bio-based chemicals and downstream upgrading routes. This work evaluates a combined hydrothermal carbonization (HTC) and anaerobic digestion (AD) strategy for agro-industrial residues from Lesvos, Greece, with emphasis on VFA formation, biogas production and analytical reliability. Anaerobic sludge and cheese whey were co-digested with 100 mg of char additive, using cocoa biochar or hydrochars produced from wine sludge and anise residues. HTC tests were conducted at 200-250 °C, 14-55 bar and 2 h residence time. The highest biogas yield was obtained with cocoa biochar, reaching 666.9 mL g-1 VS, while anise hydrochar promoted the highest total VFA load by GC-BID, 705.1 mg L-1. The GC-BID system resolved a broader acid spectrum than GC-FID, including formic, hexanoic and heptanoic acids. The results show that char-assisted AD can improve resource recovery from wet agro-industrial streams and that detector selection is critical for reliable VFA monitoring.