Compositing of coffee silverskin with carbon rich materials leads to high quality soil amendments

Abstract Purpose: Coffee silverskin (CS) is the integument covering the raw coffee bean, representing the primary waste product of the coffee-roasting industry. Despite the growing attention in seeking potential reuse of this material, the majority of CS is commonly used as a firelighter or discharged to landfills. The study aimed to test co-composting as a low-cost solution that meets the circular economy paradigms proposed by the European Union.Methods: Four composting mixtures were prepared mixing CS with pruning waste and biochar at different ratios, aiming to maximize the amount of compostable CS per batch and monitored for 60 days.Results: The contents of macro-, micro- and trace elements of the final composts matched the strictest requirements of the Spanish national regulation on compost quality (Class A amendments), proving that CS composts area high-value amendment rich in N and K. Despite the highly phytotoxic effect of CS raw material, the seed germination tests showed that all the mature composts exhibited phytostimulant properties allowing their harmless application to the soil.The four composts had a high water holding capacity (237 - 351 % dw) and they are likely to promote the persistence of plant-available water in the soil.Conclusion: The present study showed that composting the whole CS produced in Europe would lead to a recovery of 2,420 to 3,481 tons of nitrogen and 1,873 tons of potassium, reducing the dependency on mineral fertilizers, thus meeting the growing demand for sustainable and low-cost amendments.Statement of noveltyCoffee silverskin is the protective layer of the coffee beans, the main by-product of coffee roasting process that largely accumulates in consuming countries. Despite its high content in nitrogen and potassium, and its extremely high water retention capacity, the presence of phytotoxic compounds limited the use of coffee silverskin as soil amendment.In this study we demonstrate that co-composting with carbon rich materials, such as gardening prune and biochar, is a valid alternative to remove the phytotoxicity effect of raw coffee silverskin maintaining its beneficious agronomical properties.Composting coffee silverskin appears to be an easy process and a low-cost opportunity: up to 3,481 tons of N and 1,873 tons of K could be recovered in Europe from this by-product.