The use of air pollution control residues in landfill covers and for soil stabilization
Abstract: About 66% of all heat energy generated in Sweden originates from the incineration of municipal solid waste and biofuels such as wood, cardboard and peat. In addition to heat energy, incinerators generate about 1.7 Mt of ashes each year, of which 500 000 t are air pollution control (APC) residues. APC residues from municipal solid waste incineration are usually landfilled whereas those from biofuel incinerators are used in roads, as soil fertilizers, and as amendments for stabilizing APC residues classified as hazardous waste. The physico-chemical properties of compacted APC residues are such that they could be viable alternatives to clay in landfill cover liners, and could also replace cement in cases where a solidified structure is desired. However, APC residues are classified as waste and their use may rise concern due to the release of harmful substances into the environment. This thesis examines the environmental impact of APC residues when used together with other waste materials (e.g. sewage sludge, bottom ash, compost) in a landfill cover. In addition, the suitability of APC residues as amendments for stabilizing contaminated soil and their effect on the stability of treated soil under landfill conditions was investigated. The environmental impacts of alternative landfill cover were evaluated by field and laboratory tests that examined two water streams: the landfill cover’s drainage water and its leachate. The drainage water that seeps above the landfill cover liner and is drained away from a landfill cover was contaminated with Cl-, N, TOC and the trace elements As, Cu, Mo, Ni, Se, Pb and Zn. This water stream will need treatment for at least three to four decades. The amounts of landfill cover leachate that percolated through the liner in the field test ranged from 3 to 30 l (m2 y)-1, which was below the legal limit on the leachate flow in non-hazardous waste landfills (<50 l (m2 y)-1). Due to its quality and the small amount produced, the landfill cover leachate is expected to have low impact on the quality of the leachate from an entire landfill. Screening tests using three different APC residues and soils with low and moderate concentrations of organic matter demonstrated that treatment of contaminated soils with APC residues reduced the release of both As from the soil and Pb from the APC residues. Arsenic was immobilized due to the precipitation of Ca-As minerals, the substitution of SO42- by AsO43- in ettringite, and adsorption by portlandite and calcite. The treatment also caused some mobilization of Cr, Cu, Zn and dissolved organic carbon, but concentrations of these species were quite low compared to the leaching limit values for disposal in hazardous waste landfills. Unconfined compressive strength measurements showed that mixing 30% of APC residues with 70% sandy soils yielded a blend with acceptable geotechnical stability for landfilling whereas organic soils needed more than 50% of APC residues to achieve the same solidification. The gradual decline in the pH of leachates from strongly alkaline to neutral was identified as the main factor that may affect the release of contaminants from the landfill cover and treated soil. However, the acid neutralization capacity of the studied APC residues was high, indicating that the trace elements in the landfill cover liner and treated soil can be expected to remain stable over the long term. Geochemical modelling indicated that clay minerals may precipitate within the APC residues over time, which could enhance the long term stability of immobilized elements while reducing the hydraulic conductivity of the liner and stabilized waste.
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