Characterization and biological stabilization of fine fraction from landfill mining

Landfilling has been the major method to dispose waste for the decades, thus there are thousands of landfills around the world. Landfills contain large amount of resources, which could be used as material or energy. There is an increasing interest for landfill mining which means excavation and processing of waste materials mined from landfills. While previous landfill composition studies have focused especially on metal recovery and combustible materials, they have shown that landfills contain significant amounts of soil type material with small particle size, referred as fine fraction (FF). As redisposal of FF after landfill mining is expensive and causes emissions for decades, FF should be treated to increase value for reuse. The aim of this thesis was to assess in details the characteristics of the FF and to evaluate the effects of different biological treatment methods on stability and characteristics of FF. In this study, FF was sampled from two landfills representing different eras of material consumption and waste management practices: Kuopio, landfilled 2001–2011, and Lohja, landfilled 1967–1989. The Kuopio landfill was found to contain 38–54 % of FF (< 20 mm) and the Lohja landfill 40–74%. FF contains in various amounts of organic matter (VS 6–27% of TS), nutrients (1.4–8 kg N/t TS, 1–1.5 kg P/t TS) and soluble organic compounds (e.g. 0.5–4.6 kg COD/t TS). The organic matter content, biomethane potential (0.4–27 L CH4/kg TS) and respiration activity (1.4–2.4 g O2/kg TS) were detected to be higher in top layer of new landfill (1–5 years old) while bottom layer of new landfill (6–10 years old) was similar to old landfill (24–46 years old). Biological activity may limit the utilization of FF after landfill mining, thus FF needs to be stabilized to reduce biological activity. Furthermore, FF may also contain hazardous compounds, which needs to be assessed when evaluating the use of FF. To reduce biological activity of FF, the anaerobic and aerobic stabilization of FF were studied in two laboratory experiments employing simultaneous four leach bed reactors operated for 173–180 days. In anaerobic stabilization, methane production was found to range from 9 to 18 m3 CH4/t VS for FFs from both landfills. Irrigation of FF was necessary for efficient methane production while sludge addition providing both moisture and inoculum deteriorated the characteristics of FF. Aerobic stabilization reduced more efficiently organic matter content and biological activity from FF compared with anaerobic treatment. Ammonium nitrogen in the leachate was removed rapidly in aerobic treatment due to nitrification. Organic matter and soluble compounds were efficiently removed with continuous water adding, regardless of anaerobic and aerobic conditions, while leachate recirculation introduced those back to the reactor. The scaling up of the anaerobic and aerobic stabilization methods of FF showed that applied technology, for example aeration or irrigation method, and size of treatment area have major effects on the costs of FF treatment. However, anaerobic stabilization and aerobic stabilization with passive aeration without continuous irrigation would have similar costs in similar sites. In conclusion, FF may need stabilization due to organic matter content and biological activity before utilization. Both anaerobic and aerobic stabilization improved the quality of FF by reducing organic matter content and biological activity. Both treatment methods can be used in full scale stabilization of FF. The treatment of FF has potential to increase the value and usability of FF. Treatment concept and technology should be further optimized in pilot and full scales.