| Summary: | Air pollution control (APC) residues are by-products of the flue gas cleaning process in energy-from-waste (EfW) plants treating municipal solid waste. They are classified as a hazardous waste in the EU Waste Catalogue and are a priority hazardous waste stream in the UK due to high alkalinity, concentrations of volatile heavy metals and soluble salts. Plans currently exist to increase the number of EfW plants in the UK, with the potential to increase future arisings of APC residues. Stabilisation/solidification (S/S) is an inexpensive treatment technology, involving mixing of the waste with cementitious binders. The main objective of this research is to assess the effectiveness of CEM I and ground granulated blast furnace slag (GGBS) as S/S binders for the treatment of APC residues. The ultimate goal is to expand existing knowledge on S/S systems and assist development of more sustainable treatment methods for APC residues. S/S APC residue specimens were prepared varying the waste-to-binder and water-to-solids ratios and subsequently tested for physical properties and contaminant leaching according to international standards. Geochemical modelling was used to assess contaminant release-controlling processes and contribute to more efficient mix and treatment design. Results from this study indicate that mechanical properties of 50 wt.% CEM I and GGBS mixes exceed UK landfill disposal criteria (1.0 MPa), achieving unconfined compressive strength (UCS) values of up to 21 MPa. CEM I mixes with 10 and 20 wt.% binder addition also met the criterion of 1.0 MPa, achieving UCS values of up to 10 MPa. In contrast, 10 and 20 wt.% GGBS mixes exhibited inferior mechanical properties (UCS < 1.0 MPa). S/S is hampered predominantly by high concentrations of chloride in APC residues. All monolithic S/S samples exceeded relevant UK waste acceptance criteria (monWAC) for chloride (20,000 mg/m2) within the first two days of the 64-day monolithic leaching test. Altough partial immobilisation occurs through the formation of chloro-complexes, S/S of APC residues would require binder additions greater than 50 wt.% to meet UK requirements for landfill disposal. Leaching of Pb also becomes problematic for mixes with 10 and 20 wt.% binder addition, exceeding UK monWAC (20 mg/m2). Nevetheless, the amphoteric nature of heavy metals and the high solubility of chloride salts could favour extraction of potentially valuable elements through washing procedures. Modelling results indicate that a simple washing step may be able to extract 650 mg of Pb and 120 mg of Zn per kg of APC residues treated, while removing approximately 90% of available chloride.
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