**5. Conclusions**

This study shows the wide range of chemical and mineralogical differences of the FA in Switzerland and the properties influencing acid leaching. Consequently, the effort to recover heavy metals from FA varies widely is mainly dependent on metal and Al0 content and on the ANC. It is especially unfavorable for FA with low Zn content but high ANC to recover a fixed rate of heavy metals. This fact was considered in the implementation of the guidelines of the ADWO, which prescribes that the recovery rate of Zn and Pb to achieve is based on the initial concentration in FA [20]. After complete implementation, the FLUWA treatment of all Swiss FA will produce a significant quantity of recovered heavy metals. These quantities are otherwise disposed as pollutants in landfills and removed from the raw metal cycle. However, the remaining filter cake that is deposited still contains a significant load of contaminants, such as Sb and Sn. Further development is therefore needed to increase the recovery of heavy metals and to extend it to less easily recoverable elements.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2227-9717/8/12/1668/s1.

**Author Contributions:** Conceptualization, W.Z. and U.E.; methodology, W.Z., G.W. and U.E.; software, W.Z.; writing—original draft preparation, W.Z.; writing—review and editing, W.Z., U.E., M.W. and G.W.; visualization, W.Z.; supervision, G.W. and U.E.; project administration, U.E.; funding acquisition, U.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Swiss Federal Office for the Environment (FOEN).

**Acknowledgments:** We thank MSWI plant operators for providing sample material, Kaarina Schenk (FOEN) and Stefan Schlumberger (ZAR) for feedback and support. Analytical support by Christine Lemp, Martin Fisch (both University of Bern) and Ivo Budde is highly acknowledged.

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