Synergistic pretreatment for biorefinery of spent coffee grounds: Enhancing hemicellulose and methane yield with black liquor-catalyzed composting

Among pretreatment options, alkali methods are promising for spent coffee ground (SCG) hemicellulose preservation and process scale-up, but efficiency, resource recyclability, and environmental impacts are the key research priorities. This study established a closed-loop, triple-integrated biorefinery for SCG combining alkali-urea pretreatment, anaerobic co-digestion, and black liquor (BL)-catalyzed composting. Comparative analysis of KOH:Urea (KU) and NaOH:Urea (NU) pretreatments under mild conditions revealed reagent-specific effects: KU excelled in hemicellulose preservation (423.3 mg/g xylose), while NU achieved superior delignification (88.5%). XPS confirmed acetyl group retention in KU-treated samples, while solid-state 13 C NMR provided definitive evidence of carbamate group formation (R–O–C(=O)–NH₂) via a distinct signal at 160.5 ppm exclusively in KU-pretreated SCG. Co-digestion with corn stover yielded 353 mL/gVS methane (80.25% of theoretical maximum). BL recycling catalyzed composting of lignin-rich digestate in 20 days, 60% faster than control, with Py-GC/MS revealing increased syringyl-to-guaiacyl ratio (0.84 to 1.16), confirming enhanced oxidative lignin depolymerization. The integrated system achieved 90% resource recovery, demonstrating a scalable solution for sustainable SCG valorization within a circular economy framework. • KU pretreatment preserved 423.3 mg/g xylose from SCG. • NU pretreatment removed 88.5% lignin for fractionation. • Co-digestion achieved 353 mL/gVS of CH 4 , 80.25% of theoretical yield. • Black liquor catalysis cut composting time by 60% to 20 days. • S/G ratio increased from 0.84 to 1.16, supporting lignin depolymerization.