Enhanced Anaerobic Digestion of Spent Coffee Grounds: A Review of Pretreatment Strategies for Sustainable Valorization

Spent coffee grounds (SCG) constitute a significant organic waste stream with considerable potential for bioenergy recovery. This review critically examines the viability of anaerobic digestion (AD) as a sustainable valorization pathway for SCG, addressing both technical and environmental challenges. Due to their elevated lignin levels, lipid content, and inhibitory substances, SCG exhibit strong recalcitrance that limits their direct digestibility in anaerobic systems. Therefore, a range of pretreatment methods, including oil extraction, alkaline hydrolysis, thermo-alkaline processes, oxidative treatments, and hydrothermal techniques, are evaluated for their effectiveness in enhancing biodegradability and methane yields. Co-digestion with nutrient-rich substrates is explored as a strategy to improve process stability, mitigate inhibitory effects, and optimize nutrient balance. Furthermore, techno-economic and life cycle assessments underscore the feasibility of SCG-based AD compared to conventional waste management practices. The integration of SCG digestion into biorefinery models offers a promising approach to energy recovery, resource efficiency, and waste minimization within a circular bioeconomy framework. This review highlights the need for continued optimization and scale-up to fully harness the potential of SCG in renewable energy systems.