Single-step pretreatment enables direct fermentation and accelerated methanogenesis from spent coffee grounds

The rising global energy demand and depletion of fossil fuel reserves highlight the need for sustainable waste-to-energy solutions. Spent coffee grounds (SCG), a lignocellulosic residue of the coffee industry, represents an abundant but underutilized feedstock. This study proposes an integrated cascade biorefinery approach enabling the production of ethanol and methane from SCG via a single-step dilute-acid pretreatment, which eliminates the need for subsequent enzymatic saccharification or detoxification steps. The hydrolysate, containing 33 g L −1 total sugars and some inhibitory compounds, was directly fermented, without detoxification, by an evolved thermotolerant Kluyveromyces marxianus yeast strain achieving 8.54 ± 0.24 g L −1 ethanol, a yield of 0.47 gC-ethanol g −1 C-sugars, 70.5% conversion efficiency, and a production rate of 1.22 g L −1 h −1 . The residual hydrolysed solid fraction, enriched in cellulose (+10%), lignin (+27%), and fats (+15%) but depleted in hemicellulose (−82%), was anaerobically digested, reaching a methane yield of 301.14 ± 7.64 mL CH 4 g −1 VS and a production rate of 61.62 ± 2.60 mL CH 4 g −1 VS d −1 , representing increases of 29% (equivalent to ∼9% when normalised to the original dry SCG mass) and 55% (∼32% on the same basis), respectively, compared to the process using untreated SCG. Microbial analysis revealed enrichment of syntrophic acetogens and acetoclastic methanogens when SCG was anaerobically digested. This dual valorisation strategy demonstrates an efficient and simplified route for the conversion of SCG into ethanol and methane, offering a compelling model for waste-to-energy biorefineries.