been shown that, of the 15 hazard-relevant properties, only the property HP14 \"ecotoxic\" could be applicable [12; 13; 14]. The entry HP14 \"ecotoxic\" is defined by the following in Council Regulation (EU) No 2017/997 [15]: \"Waste which presents or may present immediate or delayed risks for one or more sectors of the environment\". The further explanations in the regulation list the substances that lead to this classification. These are the substances to which the hazard statements H400, H410, H411, H412 and H413 are assigned. These substances are classified as acutely hazardous to water (H400) or chronically hazardous to water category 1 (H410), category 2 (H411), category 3 (H412), or category 4 (H413). Cut-off values of 1% (H400, H411, H412, H413) or 0.1% (H410) apply. If the concentration c is below the cut-off value, the substance is not counted. A waste is classified as hazardous according to HP14 if one of three sums results in a value greater than or equal to 25%:  
(1) \(\Sigma\) c(H400);  
(2) \(100\times\Sigma\) c(H410) + \(10\times\Sigma\) c(H411) + \(\Sigma\) c(H412);  
(3) \(\Sigma\) c(H410) + \(\Sigma\) c(H411) + \(\Sigma\) c(H412). A complete list of hazardous substances can be found in Annex VI of CLP Regulation (EC) No. 1272/2008 [10]. The current version of the list (18 th Adaption to Technical Progress, ATP18) contains 4372 entries. Of these, numerous substances have the hazard statements H400 and H410-413, but the vast majority of entries are, of course, irrelevant for the classification of bottom ash. The calculation rules for classification (1-3, see above) also indicate that the hazard statement H410 is particularly relevant for a substance. Even when the concentration of a substance with the hazard statement code H410 is just above the cut-off value of 0.1% the cumulative value of calculation rule (2) is already 10% because the concentrations of H410 substances are multiplied with the factor 100. As far as possible, H410 substances in IBA-only heavy metal compounds are to be considered. It is important to note that the concentrations c refer to the molar mass of the compounds rather than, e.g., leaching limits where the limit values are usually defined for an element (or ion). One exception is lead (Pb) where the concentrations apply to the element (\"Note 1\" in Annex VI of the CLP regulation). Therefore, only heavy metals with concentrations above approx. 500 mg/kg need to be considered for the HP14 classification (in the case of Pb: 1000 mg/kg). According to the literature data on the elemental composition of IBA, these are Cu, Zn, Pb, and Cr (at least when maximum values of the range are taken into account) [16; 17; 18; 19]. Data for two samples from the present study together with the data from [PERSON] et al. [16] are listed in the Supporting Information (SI, Table S1). Usually, it is not exactly known which chemical compound of the mentioned elements is present in IBA. However, this is needed for the HP14 classification. Otherwise, a \"worst case\" composition (related to the highest stochiometric factor) has to be assumed [14], i.e., for Cu and Zn, the sulfates CuSO\({}_{4}\) and ZnSO\({}_{4}\), with stochiometric factors of 2.51 and 2.47, respectively. Elemental concentrations of 500 mg/kg each for Cu and Zn would result in a sum value of 24.9%, only slightly below the limit value of 25%. If crystal water were considered (Cu: penta-hydrate, Zn: hepta-hydrate) the sum would be above the limit value. However, information on the composition of IBA can be obtained by an approach based on the grouping of substances [13]. For IBA, four substance groups can be defined: 1. elemental and alloyed metals, 2. readily water-soluble substances, 3. slightly and sparingly soluble substances, and 4. substances with strong chemical bonding. Elemental and alloyed Cu and Zn, at least the larger particles are, present in IBA and are usually recovered as the non-ferrous metals fraction [20]. Zn particles smaller than 1 mm are classified as H410, fine particles of Cu only as H411.