Abstract
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•Research on recycling spent coffee grounds (SCG) into biogas is relatively new.•Micronutrient supply and pH are key inputs for the successful anaerobic digestion.•The biogas yield from SCG was 0.500-0.598 m3/kg with CH4 concentrations of 55–61%.•The methane yield form the SCG was 310 ± 2 CH4 mL/g VS.•The methane yield from the defatted spent coffee grounds (DSCG) was 336 ± 7 mL CH4/g VS.•Co-digestion with other waste streams provided similar yields as obtained from SCG.•Research on the techno-economics of biogas production from SCG is recommended.
As a by-product of coffee consumption, millions of tons of spent coffee grounds (SCG) are produced annually. SCG can be recycled in a biorefinery-based process to produce biofuels and value-added products, avoiding the environmental issues, hazardous emissions and costs associated with its disposal. The recycling potential of SCG into biogas by the anaerobic digestion (AD) and co-digestion was reviewed in this work. Such types of review papers are novel and not much work was done on this aspect. The AD of SCG produces 0.500–0.598 m3/kg dry organic matter of biogas with a methane (CH4) concentration of 55–61 %. Nevertheless, the long-term mono-digestion of SCG has been linked with instability and volatile fatty acid (VFA) accumulation. Co-digestion with other waste streams, such as waste activated sludge, food waste and spent tea waste, resulted in the same or larger amounts of CH4 emission. Additionally, the defatted spent coffee grounds (DSCG) have been reported to have advantage over the SCG as the average CH4 yield was 336 ± 7 CH4 mL/g vS compared to 310 ± 2 CH4 mL/g vS from SCG. Additionally, the positive impact of the pretreatment process in increasing the CH4 yield during the AD of the SCG has been observed. Based on the presented work, it is clear that recycling SCG and DSCG is a worthwhile option that can help taxpayers save money on landfill operations and maintenance besides protecting the environment from numerous hazardous emissions. Valorization of biogas products and approaches to enhance the economics of the SCG/DSCG-based biorefinery are indicated in this review as areas where future work in biogas generation from SCG and DSCG can be done. In conclusion, this paper suggests further investigation of the techno-economic analyses and life cycle assessment (LCA) of a biorefinery based on the AD of the SCG and DSCG with other organic waste streams.