Conversion of spent coffee grounds into bioenergy and biomass using Clostridium species: A sustainable pathway for renewable energy

Coffee, one of the most widely consumed beverages worldwide, generates significant lignocellulosic waste. This study investigates the use of spent coffee grounds (SCGs) as a substrate for biomass and biohydrogen (H₂) production in pure cultures of obligate anaerobic Clostridium species (C. pasteurianum, C. beijerinckii, C. acetobutylicum, and C. intestinale), providing an environmentally friendly bioenergy solution. Bacteria were grown on the physicochemically pre-treated SCGs (4 % w/v) hydrolysate at pH of 5.5–7.5. Kinetics of H₂ production, oxidation-reduction potential (ORP) and pH were evaluated over 168 h, with standard media serving as a control. Quantitative analyses of total carbohydrates (TCs) and volatile solids (VSs) showed a strong correlation between nutrient consumption and microbial activity. Results revealed that, compared with controls and pH 5.5, all four strains demonstrated enhanced growth, H₂ yield and rate on SCG hydrolysate, at an initial pH 7.5. C. beijerinckii and C. pasteurianum exhibited the best growth (OD₆₀₀ of 1.5). With C. pasteurianum, the peak of H₂ yield was 1621 ± 10 ml L⁻¹ g⁻¹ TCs when utilizing SCGs. As bacterial growth progressed and pH decreased, ORP also decreased, reaching a value of −460 ± 10 mV. C․ beijerinckii exhibited the highest carbohydrate-removal efficiency of 73 % at 168 h, whereas C. acetobutylicum demonstrated the lowest at 44 % under the same conditions. The results highlight the potential of SCGs as a cost-effective and renewable feedstock for biomass and bio-hydrogen production, underscoring the ability of Clostridium species to utilize lignocellulose coffee waste, paving the way for sustainable bioenergy applications.