4 Material und Umwelt
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- Bottom ash (1)
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Organisationseinheit der BAM
The production of secondary materials from waste materials requires, in most cases, significantly lower energy amounts than the primary material production of raw materials. Along with lower energy demand, the greenhouse gas emissions produced are also lower. The duty of a modern waste management system should therefore be to collect and sort the waste materials in a way that the highest amounts of single material fractions with the highest qualities can be generated. In this contribution, the greenhouse gas balances of the theoretical treatment of the household waste, if collected as mixed waste in sanitary landfills, in waste incineration plants, or in mechanical-biological treatment plants, are compared to the existing separate waste collection and treatment in Germany in 2014. The results show that the treatment of the mixed collected household waste in sanitary landfills would lead to a significant release of greenhouse gases. The treatment in MBTs with the recovery of valuables and the further disposal of the biologically stabilized fraction on landfills, as well as the treatment of the high calorific fraction (also called refuse derived fuel – RDF) in RDF plants, coal-fired power plants, or cement kilns, would lead to small amounts of avoided greenhouse gas emissions. The thermal treatment in waste incineration plants would lead to moderate amounts of avoided greenhouse gases. Only with the actually practiced separate collection and treatment of household waste were significant amounts of greenhouse gas emissions avoided. In total, this is approximately 5.5 million tons of carbon dioxide equivalents for approximately 45.5 million tons of separate collected and treated household waste in Germany in 2014.
Wastewater contains a diverse array of organic and inorganic compounds and its complex composition strongly depends on the location and the connected dischargers. However, municipal wastewater as a carrier of feces and urine generally contains considerable amounts of the main nutrients nitrogen and phosphorus. The latter is in the focus of the discussions about the recovery potential of wastewater due to the relatively high mass flows of phosphorus in wastewater and the finite nature and decreasing quality of phosphate rock reserves. But due to the presence of the whole periodic table of elements, wastewater might contain further valuable components of interest for recovery including those defined as critical raw materials by the European Commission. Phosphorus and most of the other critical raw materials are fixed in the sewage sludge and after incineration in the sewage sludge ash (SSA). This is accompanied by high concentration factors from wastewater via sludge to ash. However, the mass fractions of the majority of elements in sewage sludge are comparable to those of the earth crust, indicating no relative enrichment. Nevertheless, enrichment factors of 100 or higher are given for phosphorus, copper, zinc, cadmium, silver, tin, lead and the platinum group elements indicating an anthropogenic input. An economic value of sewage sludge was estimated to $460,-/t calculated on the basis of the respective market prices for high purity elements – a theoretical value. A German survey of sewage sludge ashes showed that the mass fractions and the mass flows of most of the elements present in SSA are probably too low for an economic recovery. In most cases the mass flows are rather small compared to the imports and the chemical forms are not suitable for recovery. An exception is phosphorus that is present in high mass fractions up to 13% and that bears a high substitution potential. If the application of P-recovery technologies lead to a further concentration of valuable elements e.g. as by-products in side streams of the process, it would probably make also the recovery of other elements of economic interest.
Waste-to-energy (WtE) is one of the leading technologies for municipal solid waste (MSW) treatment in Europe. According to Eurostat data, in 2015, 27 % of MSW was utilized in WtE plants, which represents more than 80 million tons per year. Therefore, the European annual production of incineration bottom ash (IBA) is about 20 million tons, as it is about 25 wt% of input MSW. In the European List of Waste, IBA is listed as mirror entry (i.e. waste materials which should be classified as either non-hazardous or hazardous, depending on its hazardous properties and/or content of hazardous substances) under codes 19 01 11 and 19 01 12.
Recent trends indicate that WtE allows, apart from utilization of the energy content of waste, also the recovery of various valuable components. Hence, WtE can be included in the key technologies that can put the circular economy concept into practice. Secondary raw materials in the case of WtE are solid residues, especially IBA, as it is a secondary source, particularly of ferrous metals (Fe) and non-ferrous metals (NF) and glass. Moreover, the residual mineral fraction can be used for various applications in the construction industry, i.e. as aggregates substitute for bound or unbound applications, in cement manufacturing or, as indicated by recent research, also in more sophisticated applications, e.g. for ceramics production. Recovery of these metals can also cause huge greenhouse gas savings. Alone in Europe, metal recovery from IBA reduces greenhouse gas emissions by approximately 3.2 million tonnes of CO2 equivalent.