Chemical characterisation and leaching properties of air pollution control residues (APCr) from municipal solid waste incineration (MSWI) sites in the UK

The thermal treatment of Municipal Solid Waste (MSW) in Energy from Waste (EfW) facilities is continuously increasing in the UK as a sustainable practice to reduce the amount of waste that is sent to landfill. However, the MSW incineration (MSWI) causes the volatilisation of hazardous compounds contained in the waste. These contaminants are removed from the gaseous emissions by air pollution control units. The solid waste generated during this cleaning process, known as air pollution control residues (APCr), are classified as hazardous because their chemical composition and leaching properties cause an environmental impact. The variable concentrations of metals and soluble salts make APCr treatment a difficult practice; hence, this waste must be carefully characterised before being treated either for disposal or for recycling. In this research, APCr from 22 EfW facilities in the UK, were analysed for heavy metals, mineralogy and leaching behaviour. The contents of heavy metals were determined by Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES), revealing that the elements of major concern, given their high contents (122 to 10,500 mg/kg), were Pb, Zn, Ba and Cr, of which the most concerning metal was Pb with leachable concentrations from 7 to 2,570 mg/kg. The mineralogy was analysed by X-Ray diffraction (XRD). From a total number of 45 mineral phases identified in APCr, the most common were Calcite, Quartz, Calcium hydroxychloride, Halite and Sylvite. Regarding soluble salts, chlorides exhibited the highest leaching rates, from 9,756 to 330,275 mg/kg; while the leaching of sulphates was between 53 and 27,566 mg/kg, and the leaching of fluorides varied from 23 to 44 mg/kg. The APCr characterisation provides valuable information for the implementation of a suitable treatment aimed to reduce or stabilise the contents of heavy metals and soluble salts, improving their leaching properties and enabling the APCr for further recycling.