Comparison of two mechanical pre-treatment systems for impurities reduction of source-separated biowaste

The treatment of source-separated biowaste is still a challenge due to its high proportion of impurities. Biowaste bins are intended exclusively for the collection of biodegradable matter, such as food, kitchen and garden waste. However, plastics, metals, glass and textiles are also found in biowaste bins. If not properly removed, these impurities cause problems to the treatment facility and depreciate the quality of the final product, when the biowaste is converted to compost. There is ongoing discussion whether the existing treatment systems are able to remove impurities, especially plastics, from biowaste thoroughly enough to ensure that the produced compost complies with state regulations. In this work, two wet mechanical pre-treatment systems were tested for their efficiency to remove impurities. The first system consisted of a screw mill, a star screen, and a food unpacking machine (process I). The second system consisted of a shredder, followed by a piston press with 12 mm pore size (process II). Both processes produced a dry output, which contained the concentrated impurities, and a wet output, which could be used as substrate for anaerobic digestion. Results showed that, although 99% of the incoming plastics were efficiently removed in process I, the impurities concentration was still too high to meet the legal standards of plastics concentration in the final product, according to the German Federal Compost Quality Association (Bundesgütegemeinschaft Kompost e.V.). The removal efficiency of glass particles was low for both processes: at least 80% of the incoming particles were transferred to the wet output.