Evaluation of pre-treatment methods for landfill disposal of residues from municipal solid waste incineration

This study presents results concerning leaching of Pb, Cd, Ca, and Cl with reference to one individual batch of bottom ash and fly ash (5:1) resulting from a municipal solid waste incinerator (MSWI) in Japan. This ash mixture was stabilised by the following pre-treatments: water washing, carbonation, and phosphate stabilisation. Once the optimum processing condition for each pre-treatment was determined, the performances were evaluated using both pH-stat leaching (pH 6, 9, 12) and availability tests. These performance tests were carried out with only fly ash without considering the mixture of MSWI residues, in order to accurately determine leaching differences among the pre-treatments. Water washing effectively removed the major elements from MSWI residues and also reduced the leachability of trace metals, such as Pb and Cd. A washing time of 15 minutes with a liquid/solid ratio of 5 was reasonably effective. Carbonation had a significant effect on leachability in alkaline ranges (pH 9 and 12), when the reaction occurred only on the surface of MSWI residues, moreover a moisture content of 10-16.7% was proved suitable for carbonation. On the other hand, phosphate stabilisation, even with small amounts of phosphate (0.16 mol-PO4 3- kg-1), was very effective in reducing the leach-ability of heavy metals.