**2. Materials and Methods**

### *2.1. Sampling and Sample Processing*

The sampling campaign was launched in December 2016 at all 29 MSWI plants in Switzerland. For three weeks, 100–200 g of FA was sampled daily to obtain a representative sample of several kilograms of FA. This timespan was chosen to avoid bias from daily fluctuation [9]. All but three MSWI plants were sampled as requested in January 2017. The others performed the sampling in the following months. One MSWI plant was not included in the study because it was shut down in 2020. The samples were split, stored in sealed bags and aliquots for XRD and XRF, and were dried at 35 and 105 ◦C, respectively. All FA samples were anonymized.

### *2.2. Chemical Analysis*

Elemental composition was determined by energy dispersive X-ray fluorescence analysis (ED-XRF) using a Xepos spectrometer (SPECTRO, Kleve, Germany); with matrix adjusted calibration. The measurement was performed on pressed powder pellets (32 mm diameter) using 4.0 g of ground sample material and 0.9 g of Hoechst wax C from Merck as binder. Trace elements and rare metals were analyzed by Actlabs (Canada) by the Ultratrace5 program. Actlabs uses INAA and ICP-MS to determine the respective concentrations in the lower ppm and ppb range.
