Environmental hazards of incinerator ash

With approximately 25% of the municipal solid waste input volume, bottom ash is by far the largest volume of solid residue arising from municipal solid waste incinerators (MSWI). Bottom ash has the following average composition:

Potentially the main environmental hazards related to the handling, utilization and disposal of bottom ash can be summarized as follows:

• leachate generation
• dust emissions
• gas emissions
• temperature increase

All of these hazards can be minimized by direct treatment of bottom ash by INASHCO technologies, because they optimally separate metals and minerals into fractions ready for use in the metals industry and the building and construction sector.

Together with the Technical University of Delft, INASHCO has set up a strong research program to optimize the characteristics of the mineral fractions produced by the INASHCO concentrator. INASHCO expects to be able to offer the market a 100% recycling rate of bottom ash into saleable products towards the end of 2009.

Leachate generation

The potential environmental impact of leaching includes contamination of soil, groundwater and surface water bodies. Leaching of contaminants from incinerator ash (also known as incinerator slag or bottom ash) may occur during the temporary storage, treatment or reuse as well as during the final disposal of the material.

The following elements must be considered as hazardous contaminants potentially leachable from bottom ash: As, Al, B, Ba, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Zn, Br^-, Cl^-, CN-,F-, NH4⁺, NO₃-, NO₂-, SO4 ^2-. The distinction between the short- and the long-term leaching behavior is a key factor. Whilst information is available concerning the short-term behavior of bottom ash, long-term behavior is uncertain.

Dust emissions

Incineration ash contains a fine fraction of particulate matter passing the 74 μm mesh sieve that accounts for 1–10% of the bottom ash volume. Friability of bottom ash may result in an increased percentage of finer particles after processing operations. The fine bottom ash particles typically contain chloride and sulfate salts as well as heavy metals like Pb, Cu and Zn. The easy airborne nature of fine particles leads to the dispersion of pollutants, which in turn can give rise to health risks for exposed, unprotected workers and the public, as well as soil contamination. To prevent or minimize dust emissions, bottom ash is normally kept wet (5–15% humidity) and transported by covered and watertight trucks.

Gas emissions

Gas emissions from MSWI bottom ash are generated as a result of metallic aluminum hydration. Hydrogen production from unquenched bottom ash is higher when compared to quenched bottom ash. Hydrogen generation can proceed even over a long period of time. A case of deflagration (explosive combustion) occurred after sealing off a monofill that contained up to 20-year-old residues.

Temperature development

Bottom ash storage sites are affected by heat generation as a result of different exothermic reactions, such as hydration of alkaline and alkaline earth oxides, corrosion of metals and carbonation of portlandite. Temperature development can last over long periods (decades or longer) due to the low rate of heat transfer through the residue bulk.