## 4 Discussion and Conclusions  
Incineration bottom ash consists of roughly 10% metals (Fe and NFe such as Cu, Cu alloys, and Al) and 90% mineral material. The recovery of the elemental metals has been improved over the years, so that the mineral fraction contains only traces of metal particles [8; 15]. In Switzerland, there is a legal requirement to remove non-ferrous metals from IBA to less than 1% by weight [62]. The recovery of elemental metals is favorable economically due to achieved revenues and environmentally due to lower heavy metal content in the mineral fraction. In other European countries, including Germany, the recovery of NFe is common practice without being legally prescribed. The remaining mineral material should be at least suitable for landfilling. In Germany, a first legal definition of requirentions for landfilling of waste that cannot be otherwise used was the Technical Instruction Municipal Waste (TA Si), with two classes of landfills (\"Deponieklasse I and II\") [63]. Later, most of these limit values (defined for leaching tests with an L/S = 10 L/kg) were adopted into the German Ordinance on Landfills, with five classes (DK 0 for inert waste and DK I-IV for other waste) [64]. The requirements are mainly related to the leaching properties of IBA and are displayed for DK 0-II in Table 2. In the 1990s, it was unclear for operators of waste incination plants whether IBA fulfills the requirements for Deponieklasse I, mainly due to the lack of appropriate studies of its leaching behavior. The limit value for Pb in TA Si was 0.2 mg/L. At that time, the storage of IBA to allow for the completion of the aging reactions described above before landfilling or use was not commonly established. Leaching tests with fresh bottom ash frequently yielded Pb concentrations around 1 mg/L, i.e., 10 mg/kg (L/S = 10 L/kg), as also observed in the present study (see Figure 3). However, it was rapidly recognized that the heavy metal concentrations decrease with the pH of the leachate as a result of the aging reactions [22]. This finding is confirmed with the results from the present study. The storage of IBA prior to landfilling or use, allowing alkaline earth oxides and hydroxides to react with CO\({}_{2}\), is a necessity to immobilize contaminants. Figure 7: Comparison of batch test results (leached amount, \(y\)-axis) and column test results (cumulative release, \(x\)-axis) at L/S = 2 L/kg (blue symbols) and 10 L/kg (orange symbols). (**Left**): fresh IBA; (**Right**): IBA after 84 days of indoor storage. To save scarce landfill volume, today's aim in thermal waste treatment is to also use the mineral fraction of IBA. The most important use pathway of IBA is as secondary building material [47]. The requirements for the use of IBA in Europe were reviewed by [PERSON] et al. [65]. Some countries have requirements on total content for certain parameters (e.g., TOC, loss on ignition, organic compounds), but usually the leaching behavior is decisive. Applied test procedures are mainly batch tests with L/S ratios of 2 and 10 L/kg and to a lesser extent percolation tests with L/S ratios from 0.1 to 10 L/kg. The limit values from the German Ordinance for Secondary Building Materials (defined for L/S = 2 L/kg) were implemented later (2021) and are listed in Table 2 for the material classes HMVA-1 and HMVA-2, with HMVA-1 enabling more permitted installation methods. For comparison, the limit values from the German Ordinance on Landfills are also listed (all values are converted to leached amount in ug/kg). It is commonly accepted that the results from batch tests at an L/S = 2 L/kg are closer to field conditions than those at an L/S = 10 L/kg [66]. Nevertheless, it was shown that the results are at least in the same order of magnitude. It can be seen that the limit values for use according to HMVA-2 are less strict than the limit values for the inert landfill DK 0, although IBA is actually not classified as inert waste for DK 0. This is the case for most European countries except for Denmark and Sweden [14].