Valorization of spent coffee grounds into biofuels and value-added products: Pathway towards integrated bio-refinery

Coffee is the second largest traded commodity after petroleum and the second most popular beverage after water. This big industry is believed to generate huge amount of waste with spent coffee grounds (SCGs) represents one of the main by-products. Recycling of such waste to fuels and value-added products through biorefineries is a promising way to solve the problem of many countries that face daily challenges and heavy cost in waste disposal. This review aims to shadow the light on SCGs recycling potential in which over 230 published papers on SCGs recycling topic were gathered and discussed. Various opportunities to produce biofuels such as biodiesel, biogas, bioethanol, bio-oil and fuel pellets besides value-added products such as bioactive compounds, adsorbents, polymers, nanocomposites, and compost were discussed. Moreover, the potential of membrane technology related to various processes of biorefining, separation and purification in the proposed SCG-integrated biorefinery are presented. Based on the presented review, it is obvious that recycling of SCGs offers many worthwhile options to policymakers that can contribute towards huge financial saving on taxpayers of running and maintaining landfills besides saving the environment from harmful emissions. In conclusion, this review emphasizes that SCG-integrated biorefineries to produce different types of biofuels and value-added products are a very promising approach that shall be economically more scrutinized in the foreseen future.