Assessment of long-term pH developments in leachate from waste incineration residues

Environmental assessment of residue disposal needs to account for long-term changes in leaching conditions. Leaching of heavy metals from incineration residues are highly affected by the leachate pH; the overall environmental consequences of disposing of these residues are therefore greatly influenced by changes in pH over time.

The paper presents an approach for assessing pH changes in leachate from municipal solid waste incineration (MSWI) air-pollution-control (APC) residues. Residue samples were subjected to a stepwise batch extraction method in order to obtain residue samples at a range of pH Values (similar to common pH-dependence tests), and then on these samples to determine leaching of alkalinity as well as remaining solid phase alkalinity.

On a range of APC residues covering various pretreatment and disposal options, this procedure was used to determine leachable and residual alkalinity as a function of pH. Mass balance calculations for typical disposal scenarios were used to provide data on pH as a function of the liquid-to-solid (L/S) ratio in the leaching system.

Regardless of residue type and pretreatment, pH was found to stay above 7 for L/S ratios up to about 2000 L kg(-1) corresponding to about 100 000 years in typical landfill scenarios. It was found that pH changes were mainly governed by alkalinity decreases from leaching processes rather than neutralization reactions.

The results suggest that leaching testing for assessment purposes should be carried out in the alkaline range, for example, at pH 9. The paper offers a thorough basis for further modelling of incineration residue leaching and for modelling the environmental consequences of landfilling and utilization of these residues.