Case study on sampling, processing and characterization of landfilled municipal solid waste in the view of landfill mining

Landfilling has been the major method for municipal solid waste (MSW) disposal during recent decades. Recently utilization of deposited materials, commonly referred to as landfill mining, has been increasingly considered due to increasing raw material costs and environmental reasons. When mining municipal solid waste (MSW) landfills, finding suitable treatment and utilization routes for different types of materials is essential because of the economic aspects and to minimize the re-landfilled fraction of the waste. This paper describes a case study to sample, characterize and process wastes of a potential landfill mining site. The study combines manual sorting with full-scale mechanical treatment to better assess the treatability of the landfilled wastes. The approach also aimed at complying with the challenges of the heterogenous nature of the landfilled MSW. An approximately 30 m high and 10-year-old Finnish landfill was sampled in connection with the drilling of three gas collections wells, producing samples from two layers of waste of slightly different ages (ca. 5–10 years). Manual sieving and sorting into seven waste fractions as well as full-scale mechanical processing was performed. Further characterization included the fuel properties of the calorific fractions and the leaching properties of the fine materials. Manual sorting of the materials yielded 40–45% (w/w) of the possible fuel fraction in the landfilled waste with a net calorific value of approximately 20 MJ/kg dry matter. The metal fractions recovered in the manual sorting amounted to 3–4% (w/w). The landfilled waste was also processable by full-scale mechanical processes, including shredding, magnetic separation, screens and a wind sieve, despite the moisture and impurities in the landfilled waste. Results from the mechanical process showed that approximately 30% (w/w) of the material could be recovered as solid recovered fuel with similar calorific values to the fuel fraction from the manual sorting. Approximately 1% (w/w) could be recovered as magnetic metals in the mechanical process. The yields of fuel and metal fractions are site-specific but could likely be improved by optimizing the mechanical process for landfill mining purposes, as is indicated by the results from the manual sorting of the materials. The amount of fine materials (<20 mm in the manual sorting and <30 mm in the mechanical process) was found to be ca. 50% (w/w) which supports previous reports of the amount of the fines. The fine materials require attention to minimize the waste remaining from landfill mining. The Fe and Al contents of the fine fraction, at approximately 5% (w/w) both in the manually sorted and mechanically treated waste, are interesting for recovery purposes. The findings from this study highlight the importance of proper exploration stage in a landfill mining project in order to plan the best applicable full-scale processes for material recovery.