Preface

Incineration technology has become predominant for pollution control and resource reuse of municipal solid wastes (also namely as refuse, garbage, municipal solid waste (MSW), domestic waste, etc.) in developed countries and many developing nations. Currently, China has over 500 incineration plants with a scale of 1000 to 9000 t/d each plant. As a by-product, around 25–30% bottom ash and 1.5–3% fly ash of those incinerated will be generated in these incineration plants. The huge quantity of bottom and fly ashes has become a great challenge for the development of incineration technology.

Bottom ash (BA, or MSWI BA) contains mainly inert components such as glass, synthetic ceramics, metals, silicates, phosphates, sulfates or carbonates, and some unburned organic matters. 3.3–5.9 Zn, 1.2–2.4 Cu, 1.8–2.4 Pb, 0.6–1.1 Cr, 0.1–3 Ni, 0.022–0.06 Cd, 0.00003 Hg (all in g/kg) may contain in the bottom ash. Over 60% of the bottom ash was >4 mm in size, with a typical composition of SiO₂ (35.3–42.3%), CaO (19–27.2%), Al₂O₃ (7.4–7.8%) and Fe₂O₃ (3.9–5.1%). The contents of CaO, P₂O₅, MgO, TiO₂ increase and SiO₂, Al₂O₃, Fe₂O₃, Na₂O, K₂O decrease, as the particle size of the bottom ash decrease. Moreover, the concentrations of heavy metals also increase as the particle size of bottom ashes decrease. It may be considered that the coarser particles may originate from the wastes incinerated and the smaller ones are formed during the incineration process. The contents of heavy metals in both the bottom ash and fly ash are found to be inter-related and depend on the volatilization of the heavy metals involved. The heavy metals with low volatilization mainly remained in the bottom ash, which present as carbonates and in residual forms with high stabilization. As the leachability of heavy metals in the bottom ash are lower than the leachability toxicity limit of wastes according to the China leaching standard and USA TCLP, the bottom ash is considered to be a non-hazardous waste in many countries and to use as building materials for brick making and cement additives etc. Actually, however, most bottom ash is being placed in landfills for it is in fact a lower quality waste, and the physical and chemical and even biological changes will occur in the landfill during its long-term weathering and stabilization processes.

Fly ash (FA, or MSWI FA) has been classified as a hazardous waste due to the presence of leachable heavy metals Zn, Cu, Hg, Cd, Cr, As, Ni and dioxins, which need to be disposed of carefully. Fly ash can be treated with different ways. The most frequent means for fly ash treatment is to stabilize the fly ash using organic and inorganic phosphorus and sulfur chemicals and then placed in a security landfill. Immobilization and vitrification at high temperature and hydrometallurgical selective separation of heavy metals by water washing, solvent extraction or by dissolution in acidic or alkaline medium, or used as the ceramic additives, have also been practiced, but limited due to their high costs. The fly ash used as cement additive after water-washing had been extensively studied and pilot-tested, but finally proved infeasible in engineering as the Cl present can not be removed to an acceptable level.

The placement of fly ash in a security landfill will result to an irreversible effect on the environment, as the heavy metals in the ash would never disappear in the landfill. Moreover, the cost for such a disposal method is quite unacceptably high in many cases, as the investment for the landfill is always high. The immobilization using cement and other solidification agents often show relatively low stabilization efficiency according to its long-term leachability tests. The capability of cement matrix to capture and stabilize the heavy metals is limited and may deteriorate after long-term storage in the landfill or recycling uses such as road pavement materials. Moreover, the volume of the solidified products usually increases considerably, leading to an increase of transportation cost. Modifications have been used to raise the long-term stabilization effects for the cement method. Three principally different cement solidification technologies exist: (1) solidification of unwashed (original) fly ash; (2) solidification after neutral/basic washing, and (3) solidification after acid washing. The first method ends up with a residue with high chlorine and high heavy metals contents. Consequently, a large amount of expensive high quality cement with good hydraulic properties is used. In the second technology, the soluble heavy metals chlorides are transformed to heavy metals hydroxides, which precipitate. After filtration and solidification with a low amount of cement, this process yields a residue with low chlorine content but high heavy metals. However, the heavy metals will continuously release, though slowly, in wet environment. Most heavy metals may be removed when the fly ash is washed in an acidic medium, which is actually a hydrometallurgical process as most the metals may dissolve in the acidic solutions. The post-solidification treatment for the resultant residue seems unnecessary.

The predominated fly ash disposal method currently practiced involves the combined solidification/stabilization using chemical agents and cement, and then the resultant products are placed at a security (dedicated) landfill or at a sanitary landfill used for MSW disposal. If co-disposed with MSW, the fly ash must be placed independently, without mixing with MSW. However, the MSW landfill can become stabilized and be recycled after closure for years, but the fly ash at landfill will be kept unchanged forever and never become detoxified. When the fly ash is placed together with MSW, this land might have to be closed forever without any recycling possibility.

In this book, the general incineration process of refuse, characterization, properties, stabilization and solidification, treatment and final disposal, recycling of bottom ash and fly ash, is fully described, though focused on and took China situation an example. The main contents include municipal solid waste incineration process and its generation of incineration ashes, characterization and recycling of bottom ashes, weathering process and biological dechlorination of bottom ash at landfill, characterization of heavy metals in fly ash, thermal characterization and use in ceramic tile making of fly ash, chemical stabilization of fly ashes, and solidification/stabilization process engineering design of fly ash. The readers include solid waste engineers, managers, technicians and maintenance staff, recycling coordinators and government officials, undergraduates and graduates, and researchers.