Anaerobic digestion of coffee pulp and husk: constraints, pretreatment, process enhancement, and scale-up

Coffee pulp (CP) and coffee husk (CH) are abundant agro-industrial residues with potential for biomethane production through anaerobic digestion (AD), but their contrasting characteristics impose different conversion constraints. This review critically examines the AD of CP and CH, focusing on feedstock characteristics, inhibition, pretreatment, process enhancement, and scale-up. CP is characterised by high moisture, a relatively accessible biodegradable fraction, acidity, soluble phenolics and caffeine, and susceptibility to rapid acidification, whereas CH is drier and more structurally recalcitrant, with hydrolysis and lignocellulosic accessibility as major limitations. Phenolic, caffeine, tannin, furanic, and pretreatment-derived inhibitors are examined in relation to their concentration-dependent and combined effects on anaerobic microbial activity. Physical, thermal, hydrothermal, chemical, biological, and hybrid pretreatments are compared, with emphasis on trade-offs among substrate accessibility, methane recovery, inhibitor formation, and energy and chemical requirements. Process-enhancement strategies, including adsorbents and conductive materials, trace-element supplementation, co-digestion, and semi-continuous, continuous, and two-stage digestion, are evaluated using quantitative evidence where available. Energy balance, techno-economic and life-cycle performance, feedstock logistics, regional applicability, system configuration, and scale-up are also considered. CP is primarily suited to strategies targeting inhibitor control, nutrient and buffering management, and process stability, whereas CH requires greater emphasis on structural deconstruction and hydrolysis. Future research should prioritise residue-specific optimisation, long-term reactor validation, integrated techno-economic and life-cycle assessment, and pilot-scale demonstration. A constraint-driven pathway linking feedstock characterisation, constraint identification, targeted intervention, long-term validation, and scale-up is proposed for sustainable coffee-residue biomethane production.