**5. Conclusions**

This study defined the three P-containing phases in SSA which are completely recovered by HCl acid digestion. This implies that the recoverable P-content of SSA studied (sludge burned at 850 ◦C) lies not in a single mineral phase, but in multiple element associations. Any recovered phosphorus product from SSA will inevitably require costly purification. At the same time, acid digestion does not recover the full P-content. Legal obligations to recover the full content are likely to result in economic actors not pursuing P-recovery from SSA at all. Thus, several routes are probable: SSA not being used for P-recycling (too costly), legal requirements being adjusted or SSA being optimized—perhaps the P-content can be reorganized in easily recoverable phases by optimizing the combustion regime. Workers with SSA combusted at higher temperature report less elementary complex phases, and our work shows that higher P-phases are composed of fewer elements. Thus, the impurities in recovered phosphoric acid that need to be stripped off are reduced in complexity. Future work could address SSA from different wastewater treatment plants incinerated at the same furnace to shed light on P-deportment. This would enable the study of provenance effects and should help to differentiate between incineration- and provenance-based effects. Another possible route would be P-recovery prior to incineration, ye<sup>t</sup> this work focused on developing a new spectra naming/grouping strategy for SSA, which is recommended to use during future work for advancing this particular area of waste management.

P-recovery from SSA should, ideally, start with exploring a combustion regime that optimizes P-phase shift towards recoverable P-phases with fewer elements. This study showed that higher P-content in material phases is accompanied by fewer elements. Future work could focus on improving combustion for an efficient P-recovery from SSA.

**Author Contributions:** Following an idea of M.B., conceptualization by B.S. and M.B. was followed by V.-G.G. carrying out experiments. A.C.G. evaluated data and drew conclusions and proposed spectra labelling. Draft preparation and writing were done by A.C.G., M.B. and B.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the German federal ministry for economic affairs and energy, BMWi FKZ 03EFGSN097 and the German federal ministry of education and research, BMBF FKZ 033R099E. These were granted to M.B. and his institute.

**Acknowledgments:** The authors would like to thank Wirbelschichtfeuerungsanlage Elverlingsen GmbH for providing the samples. Furthermore, we are grateful for Sabine Gilbricht of the Geometallurgy Laboratory at the Department of Economic Geology and Petrology, TU Freiberg, for kindly supporting the MLA analyses. The authors would like to express gratitude towards the three anonymous reviewers whose comments improved this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
