Combination between Composting and Vermicomposting of OFMSW: A Sicilian Case Study

In nature, earthworms process enormous quantities of plant debris, ingesting and converting them into vermicast, the final product of their digestion process. Vermicast is widely recognized as an organic fertilizer suitable for plants, usually obtained from the transformation of animal manure. Indeed, controlled vermicomposting of the organic fraction from municipal solid waste (OFMSW) has yielded contradictory results, limiting the extension of laboratory results to larger-scale initiatives. This study aims to analyze a combined composting–vermicomposting process using Eisenia fetida (also known as Californian red worm) for the treatment of OFMSW (containing different impurities, such as heavy metals), mixed with a suitable substrate for earthworms, consisting of a blend of animal bedding and pre-selected food scraps (SOM—pre-selected organic material). Different fractions from a municipal composting plant were tested for the biological process: raw OFMSW, pre-composted (PC, organic matrix that has completed the thermophilic biodegradation phase), and compost (C). Laboratory tests involved five different applications with varying mixing proportions, partly aided by the addition of OM. The physical–chemical parameters (e.g., pH, temperature, moisture) and worm growth rate in the different treatments were measured and compared. The results showed that the process improved the quality of the final product, especially for the selected matrices, and revealed a significant reduction in the carbon-to-nitrogen ratio (lower than 14 in all tests) when vermicomposting was applied to the mixed matrices and SOM. Worms increased during the process in net weight and growth rate, even if there was an accumulation of heavy metals in the “worm tea”. Worm mass reached a little over double the initial value in all tests (from 200 g to more than 500 g in the SOM test), except for an increase of only 87% in the OFMSW test, while heavy metal content in the solid matrix was reduced in all tests compared to the starting content.