Takayuki Shimaoka

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, Cr6+) 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.

Mechanisms for the Aging-Induced Reduction of Lead Solubility in Scrubber Residues from Municipal Solid Waste Combustion

This manuscript elucidates the mechanisms responsible for aging-induced reduction in lead leaching from scrubber residues. Leaching tests (JLT13) were conducted on 48 types of scrubber residues and lead solubility was found to be significantly reduced independent of incinerator type or type of gas treatment method. Reaction kinetics that result in lead solubility reduction were shown to be proportional to carbon dioxide partial pressure and in many cases were limited by mass transfer to the residue. With forced gas convection through the residue and a CO2 partial pressure of 0.3%, the concentration of lead in leachate was reduced from 84 mg L-1 to < 0.5 mg L-1 in two days. Ettringite analogs (Ca6Al2(SO4)3(OH)2•26H2O) were identified by X-Ray Diffraction in the wetted scrubber residues early in the aging process; these appear to have been converted into gypsum (CaSO4•2H2O) and vaterite (CaCO3) in the aged material. All of these solid phases are believed to substitute lead into their crystal structure and evidence for this attenuation mechanism is presented.

Pollutant transformations in landfill layers

Abstract The behaviour of pollutants, in particular organic carbon and nitrogen, in solid waste landfill layers and factors affecting their decomposition have been studied using four large scale simulated lysimeters in four landfill thicknesses, and operated under semi-aerobic conditions. This paper presents about 800 days of data through November 1991. Data in this study indicate that the thicker solid waste layer has lower production rates of Total Organic Carbon (TOC) and Total Nitrogen (T-N) than does the thinner layer. If the layer, however, becomes too thick, no significant increase in the removal rate of T-N is observed, as nitrogen is leached out in the form of NO 3 − -N without being demitrified at the bottom of the landfill layer. It is also assumed that temperature and pH in the landfill layer play important roles not only in microbial degradation, but also in dissolution of organic carbon and nitrogen.

The mechanism of lead (Pb) leaching from incinerator fly ash in monodisposal landfill

This study considers the leaching behaviour of heavy metals from Municipal Solid Waste Incinerator (MSWI) fly ash contained in a landfill dedicated solely to fly ash disposal. Large scale lysimeters were used to study four materials: untreated fly ash; two types of chemically treated fly ash; and a molten fly ash slag. The study also considers the effect of highly saline solutions to accelerate the leaching of lead and concludes that the concentration of calcium ions influences the leaching process.

Heavy metal elution characteristics from municipal solid waste scrubber residue by a centrifugation method

The objective of this study is to clarify the elution characteristics of heavy metals into scrubber residue solution such as bound water and pore water. The L/S ratio of wet scrubber residue is smaller than that the L/S ratios used in elution tests such as: the one based on Notification No. 13 of the Japanese Environment Agency (Japanese official method, JLT13), the Toxicity Characteristics Leaching Procedure (U.S.A. official method) and the Availability test (Dutch official method). A centrifugation method employed to sample soil solution was adopted for use to sample two types of scrubber residue solutions, and to compare the concentrations of heavy metals in the leachates with that obtained from the JLT13 and the Availability test as well as the pH dependency test.