Behavior of stabilized fly ashes in solid waste landfills

Abstract

In Japan, the fly ash generated by incineration of municipal solid waste has come to be handled as general waste under special management in recent years. At the same time, direct landfilling or ocean dumping of fly ash is legally prohibited. To avoid the possibility of causing damages affecting human health or life environment, fly ash must be subjected to intermediate treatment, and must stabilized, encapsulated and made hygienic by removing pollutants. Four methods can be given for fly ash intermediate treatment; (1) cement solidification, (2) treatment by chemicals, (3) acid and other solvents and (4) melting and solidification. On the other hand, various phenomena affecting fly ash occur on the landfilled waste layer, such as generation of organic acid and carbon dioxide gas due to decomposition of solid waste. Fly ash solidified by cement is deteriorated by water-soluble sulfate and organic acid and carbon dioxide gas produced by decomposition of organic substances in the municipal solid waste. This may result in a reduced strength of hazardous substances and re-melting of fly ash solidified by cement. Furthermore, encapsulated hazardous heavy metals may be eluted from fly ash treated by chemicals due to the change in chemical environments (pH, ORP, etc.) in the landfilled solid waste layer according to stabilization of the landfilled solid waste. However, not much is known about the physical and chemical properties of the fly ash after it has been disposed of with the general solid waste at the landfill site. Thus, in this present study, the large-sized landfill model lysimeter is filled with cement-solidified fly ash or chemically treated fly ash treated together with municipal solid waste (bottom ash and shredded solid waste) to obtain correct information on the changes in the physical and chemical properties of the cement-solidified fly ash, and temporal concentration of hazardous substances (Pb, Cd, Cr 6+) contained in seepage leachate or leachate in the landfilled solid waste layer, thereby clarifying the long-term stability of the disposed fly ash. This study has revealed that the pH value of the cement-solidified fly ash tends to be neutralized with the process of time, although the compression strength of the cement-solidified fly ash left to stay in the landfilled solid waste for three years is not reduced much below that at the time of experiment start-up. This trend of neutralization has been found to be conspicuous in the vicinity of the leachate collection pipe at the bottom landfill layer. Furthermore, the pH values of seepage leachate and leachate have been observed to be reduced by the disposal of the fly ash chemically treated with municipal solid waste. This indicates that the concentration of the heavy metal such as Pb and Cd is severely affected by pH values. Namely, the pH value of the leachate from the large-sized landfill model lysimeter filled with chemically treated fly ash is close to neutral value, and concentrations of the Pb and Cd tend to be greater than those of the leachate from the large-sized landfill model lysimeter not filled with chemically treated fly ash. This suggests that the chemical properties of the treated fly ash and the solid waste disposed of with it determines the pH value of the seepage leachate and leachate and, at the same time, greatly affects the concentration of the heavy metal.