Environmental Impact of the Disposal of Solid By-Products from Municipal Solid Waste Incineration Processes

Incineration is often regarded as a very efficient technique for municipal solid waste (MSW) management. However, the environmental impacts of MSW incineration need to be carefully taken into account. The most relevant problem with MSW incineration is flue gas treatment. However, another of ten overlooked issue is the disposal of solid byproducts of the incineration process. MSW incinerators essentially produce two types of solid by-products, that is, slag, or bottom ash, and fly ash, often mixed with various other chemicals used for flue gas treatment. Bottom ash and–even more–fly ash are regarded as dangerous wastes mainly due to their potentially toxic elements (PTE) content and their tendency to leach such PTE to the environment. Different approaches have been proposed to mitigate the impact of fly ash disposal. The most commonly considered technology is stabilization in a cement matrix, with the aim of producing ash–concrete aggregates. Another, more radical, approach consists of using a plasma torch to heat fly ash to the point where it becomes a melt, which is then cooled down into a vitreous, nonleaching material. In light of its characteristics, plasma torch–based vitrification plants appear to constitute a promising answer for the disposal of solid by-products from MSW incineration processes. However, the high costs of plasma-based plants represent a serious drawback, particularly relevant in a context in which energy costs appear set to increase dramatically in the near future.