# Reuse of Spent Coffee Grounds: Alternative Applications, Challenges, and Prospects--A Review
## 4 Lipids from SCGs as an Additive for Biodiesel Production  
SCGs contain extractable lipids in their composition. The study by [PERSON] et al. (2014) used organic solvents for the lipid extraction process. These included a mixture (50:50 vol/vol) of pure hexane and isopropanol, a mixture (50:50 vol/vol) of commercial hexane and isopropanol, a mixture of recovered hexane and isopropanol, pure hexane, and commercial hexane. The best results, i.e., lipid content equal to 6 wt%, were obtained using pure hexane and a 50:50 (vol/vol) mixture of pure hexane and isopropanol. The lipid content is higher when using dry SCGs compared to wet SCGs. The extracted lipids were tested for their acidic properties, density at 15 \({}^{\circ}\)C, viscosity at 40 \({}^{\circ}\)C, iodine number, and calorific value, which are negatively affected by water and solvents used for lipid extraction. For lipids with high free fatty acid content, the best procedure for conversion to biodiesel was a two-step process of acid esterification followed by alkali transesterification, instead of one step of direct acid transesterification. The iodine number was about 70 g per 100 g of biodiesel, about 1.8 mg KOH per 1 g of biodiesel, and 86% ester content. The parameters tested did not meet the requirements of NP EN 14214:2009 [27]. Nonetheless, biodiesel from SCGs shows potential when blended with higher-quality oils prior to transesterification or with fossil diesel, making it easier to meet the standard's requirements [28]. The use of SCGs for biodiesel production has potential, but there are significant limitations before it can be applied on an industrial scale. The lipid extraction process requires the use of organic solvents, which involve costs and serious environmental concerns. Biodiesel production from SCGs leads to secondary contamination caused by processing byproducts. Mixing SCG biodiesel with higher-quality oils increases costs and reduces profitability. Future research should focus on optimizing the lipid extraction process, improving the quality and stability of biodiesel, and balancing the environmental impact. Direct conversion of SCGs to biodiesel and bioethanol proved to be an unfavorable option due to the slow rate of the process. It is desirable to extract lipids from SCGs and convert them to biodiesel by a non-catalytic transesterification reaction. The bioethanol and biodiesel production yields were 0.46 g/g and 97.5 \(\pm\) 0.5%, confirming that SCGs can be a good material for biodiesel production. The disadvantage of producing fuel from coffee waste is the complicated and complex processing. The process requires advanced technology and precise control, which increase costs and production time [29]. Although lipid extraction and their conversion into biodiesel can yield promising results, the complexity of processing and the necessity for advanced technology are associated with high production costs, making it inefficient on an industrial scale. It also requires increased energy and resource consumption. Research and innovation are necessary to simplify the process and reduce costs. The researchers used extraction of lipids from SCGs using hexane and isopropanol in a 50:50 mass ratio. The investment in the project, purchase of equipment, and relevant reagents was USD 123 million. Profits generated from the sale of biodiesel turned out to be low. The project was not economically viable, which points to another major drawback of producing biodiesel from SCGs. High initial costs, associated with the purchase of specialized equipment and reagents, may discourage investment in this type of project. Future research should include better adjustment of biodiesel production from SCGs lipids in relation to its price [30]. High costs amounting to USD 123 million and low profits from sales indicate that the developed technology is not competitive. Expensive extraction methods and technological requirements discourage such investments. There is a need to find cheaper and more efficient solutions.