Can coffee silverskin be a suitable substrate for biogas production and odour impact mitigation of digestate?

Coffee silverskin (CS), the main by-product generated during coffee roasting in consuming countries, remains largely underutilized despite its high organic content. This study evaluates its suitability as a substrate for anaerobic digestion by determining its biochemical methane potential (BMP) and assessing its influence on digestate odour emissions. Laboratory-scale BMP tests were conducted under mesophilic conditions (40 degrees C) using an inoculum from a full-scale biogas plant. Results showed a cumulative methane yield of 112.33 +/- 61.33 L CH4 & sdot;kg-1 VS and a biogas production of 254.07 +/- 135.87 L kg-1 VS, with an average methane content of 45.4%. These values indicate a moderate methanogenic potential, limited by the high lignin fraction and low C/N ratio of CS, suggesting its preferential use in co-digestion with carbon-rich substrates. Odour impact was assessed using both an electronic nose and dynamic olfactometry. The addition of CS temporarily altered volatile compound fingerprints during digestion but did not lead to substantial improvements in perceived odour quality. Although odour concentration increased in treated samples, hedonic tone differences were minimal. Overall, the results highlight that while CS is not an optimal standalone substrate, its integration into co-digestion strategies could support renewable energy production and enhance the circularity of the coffee supply chain.