Assessing the post-collection recycling potential of mixed municipal solid waste and associated treatment residues

Abstract

Austria's mixed municipal solid waste (MSW) contains several hundred thousand tons of waste that should be collected separately as a recyclable fraction. This recyclable waste is lost during residual waste treatment, as most of it is incinerated. Only metals are partially recovered during pre-treatment or from incineration residues.Simultaneously, the EU defined recycling targets for various waste materials. Austria is approaching or has already achieved most of these targets. However, significant increases are necessary for some materials – especially plastic packaging. This study investigates how recyclable materials can be recovered more efficiently from mixed MSW and incineration residues to further increase recycling rates, approach closing material cycles, and reduce the need for landfill materials.Hence, two areas of mixed MSW treatment are being investigated: the recovery of recyclable materials through automated sorting and the utilization options for bottom ash from waste incineration.The first part is addressed by analyzing mechanical treatment plants. These investigations include regular sampling and characterization of selected material flows as well as processing tests of mixed MSW with a focus on recovering recyclable materials. These investigations are carried out on an industrial scale so that scaling factors are largely minimized, and robust results are produced. The theoretical recovery potential of packaging plastics from the plants investigated averages 6,500 tons per year. Extrapolated to Austria, this generates a potential of around 83,000 tons per year. Considering losses during further treatment, such as sorting and recycling, around 30,000 tons of recyclate per year could be returned to plastic production. This result would correspond to an increase in the recycling rate of packaging plastics from 25% to 35%.The topic of waste incineration residues includes investigating and comparing bottom ash from grate incineration and fluidized bed combustion. The analyses are carried out at incineration plants, which are almost exclusively fed with residual waste and located in the same city. The analyses of the ashes focus on their recyclable material content, chemical composition, and leaching behavior in wet environments. The content of recyclable material is of particular interest not only for metals but also for the content of potentially recyclable glass. The chemical analyses mainly concern the mineral components of bottom ash. These minerals could be recycled outside landfill sites if their chemical and environmental properties are below limit values. It is shown that most of the recyclable materials from bottom ash from fluidized bed combustion are of better quality than the ash from grate incineration. Metals in fluidized bed ash are less corroded, glass contains fewer impurities, minerals contain fewer heavy metals, and their leaching behavior is more favorable. In addition, metals and glass hardly melt in the fluidized bed combustion and, therefore, do not conglomerate with other materials compared to grate incineration. This makes it much more feasible to recover metals and glass. Concerning the input of the incineration plant, more aluminum and significantly more glass can be recovered from the fluidized bed ash. The disadvantage is that fluidized bed combustion produces around five times more fly ash per unit of incinerated waste, which has to be disposed of in landfill sites.The utilization of the mineral content in incinerator bottom ash outside landfills is also considered in a European context. Legally authorized fields of application and their requirements in the entire EU, including associated partner countries, are identified. The work shows many parameters to be analyzed for the total and eluate content. In addition, some countries have adopted landfill limit values for bottom ash utilization. Such an approach is not considered appropriate as the environmental impact of the material when disposed of in a landfill is different from that of non-landfill recovery. The overall utilization rate of incinerator bottom ash outside landfills in the countries studied is around 54% by weight. This rate does not necessarily depend on how good the legal regulation is. Instead, it seems to result from the political commitment to utilization and the economic conditions.These findings are supplemented by secondary literature and combined into an Austrian mixed MSW treatment model. Various scenarios are analyzed using material flow analysis software. Compared to the base scenario, increases in the recovery and associated recycling rates are possible for all material flows analyzed. This is not unexpected as packaging plastics, glass, mineral materials, and paper has not been recovered from mixed MSW for material recycling. Similarly, the existing potential for non-ferrous metals cannot yet be utilized, and there are still significant losses. Ferrous metals are already being recovered to a large extent, meaning there are only minimal rates of increase. Although the recycling rates for plastic packaging are not fully achieved through automated waste sorting, its integration into waste treatment strategies can contribute to achieving the recycling targets. By recovering recyclable materials before incineration, improving bottom ash processing, and utilizing the mineral content of bottom ash, up to 200,000 tons per year of input to landfill sites can be avoided. These savings correspond to one-third of the annual input from mixed MSW treatment and can, therefore, contribute to extending the use-phase of landfill sites.