| Disposal of MSWI fly ash into ordinary landfill sites poses serious environmental problems, principally because this material contains high concentrations of leachable heavy metals. Thus, as a by-product, MSWI fly ash colleted by the baghouse or electrostatic precipitator is considered hazardous and should be properly treated. Traditionally, fly ash is sent to a hazardous landfill, after preliminarily solidified by cement or stabilized by chemical agents. Consequently, the cost for the treatment of MSWI fly ash is quite high. On the other hand, the life of heavy metals in the solidified products of fly ash may exist for a long time, but the life of hazardous landfill may be much shorter. Hence, it is necessary to seek for alternatives for fly ash treatment other than landfill or high temperature vitrification. The objective of this research was to investigate the feasibility of a combined washing-immobilisation process as a means of optimising the disposal of fly ash resulting from municipal solid waste incineration (MSWI) in cementitious matrices.The type of MSWI fly ash used in this study was collected from the baghouse of Shanghai Yuqiao incineration plants. Characterization involved chemical and physical analyses, observation under Environmental Scanning Electron Microscope (ESEM), X-ray Diffraction (XRD), Atomic Absorption Spectrometry (AAS) and Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) analyses. Toxicity Characteristic Leaching Procedure (TCLP) tests were performed on this research to assess the likehood of heavy metals being released into the environment. The benefits of water-immobilisation treatment of MSWI fly ash are twofold: namely, the cost savings stemming from reduction in the cement used, and the ecological advantage of ensuring a final destination for the ash.The experiment results indicate: The leaching toxicity for Shanghai Yu Nong MSWI fly ash is tested and found that Pb content in the leaching solution (15.6mg/L) exceed the toxicity standard set by the China regulations (3mg/L); The main pollution of fly ash comes from Pb, Cd, Zn and Cr. The leaching toxicity for fly ash samples after water-immobilisation treatment can meet the Chinese Identification standard for hazardous wastes. A two-stage washing pre-treatment with water (ambient temperature, water/solid (w/s) weight ratio=25, mixing time=15min, for each stage of treatment) is enough to maximize the incorporation of fly ash into cementitious matrices. Furthermore, an ash+sludge/cement ratio of 4.0 on a solid basis is enough to immobilize the fly ash. Besides, a wastewater produced by washing pre-treatment of flyash meets the requirements of China wastewater discharge regulations under pH condition between 6.5 and 7.5.Setting and leaching tests on cementitious mixes prove that the hazardous sludge produced from wastewater treatment can be completely mixed with washed fly ash and this mixture can be incorporated into cementitious matrices to a great extent without the risks of an unacceptable delay of cement setting and excessive heavy metals leachability from solidified products.The better performance of combined washing-immobilisation process as compared with immobilisation process of unwashed fly ash may be ascribed primarily to the ability of the washing step in promoting the formation of hydrate phases that incorporate and/or convert heavy metal compounds into less reactive forms and, secondarily, to its ability of removing significant amounts of alkali chlorides and sulphates from fly ash. As a result, MSWI fly ash is transformed into a material that adversely affects cement hydration to a much lower extent than unwashed fly ash.LUO Chunhui (Environmental Engineering) Supervised by: Associate Prof. LIU Zhenghong...
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