## 5 Secondary Waste Management as the Last Element of a Circular Economy  
Two main secondary waste types are generated during incineration of municipal solid waste: hazardous fly ash and non-hazardous slag. The fly ash in Waste Catalogue  
Figure 3: Ideal circular economy.  
is classified under the code 19 01 07* (* means that this is hazardous waste.)--solid waste from gas treatment and slag is under the code 19 01 12--slags and bottom ash [30] (different than hazardous) [34; 35]. The fly ash is light gray and it has a very fine fraction (specific surface area 745 m\({}^{2}\)/kg). It is very hygroscopic and dusty with an acid smell. The slag is gray with visible glass particles. The slag is similar to aggregate, while the fly ash is similar to cement. Figure 4 shows slag (Figure 4a) and fly ash (Figure 4b).  
The basic technical properties of secondary waste under test are presented in Table 1. The moisture content in the secondary waste was specified in accordance with the PN-EN ISO 18134-3:2015-11 Standard [36], while the bulk density was carried out in accordance with the PN-EN 1097-3:2000 Standard [27].  
The total moisture in fly ash is 1.45% and 8.65% in slag. Slag prior to use in concrete should be dried to reach the moisture level below the minimum of 3% because of pozzolanic activity [27]. The bulk density for fly ash is 469.03%, while for slag is 1077.33%. This is an important parameter for carrying secondary waste, especially for fly ash requiring a special vehicle.  
The use of secondary waste in the building industry is not easy and obvious, although slag belongs to non-hazardous waste but fly ash (or APCr--Air Pollution Control residues) and bottom ash are hazardous wastes. Until now, this hazardous waste has been stored underground but there are a lot of research projects underway on how to manage it. One must be aware of the heterogeneous composition of the municipal waste and, consequently, the variable composition of the secondary waste. The tables below show the content of components in fly ash, determined in October. Table 2 presents the results of heavy metal concentration tests for fly ash and slag.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & **Symbol** & **Unit** & **Fly Ash** & **Slag** \\ \hline Moisture & M & \% & 1.45 & 8.65 \\ \hline Bulk density & \(\varrho\) & kg/m\({}^{3}\) & 469.03 & 1077.33 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic technical properties of fly ash and slag.  
Figure 4: Municipal solid waste incineration plant secondary waste: (**a**) Slag after valorization; (**b**) Fly ash.  
The determination of heavy metal content in secondary waste is carried out in two stages. First, the waste is mineralized according to the PN-EN 13657:2006 Standard, and then, heavy metals in the mineralized matter are determined using the method of atomic emission spectrometry with excitation in inductively coupled plasma (ICP-OES) according to the PN-EN ISO 11885:2009 Standard [37].  
Managing fly ash is more complicated than managing slag. Table 1 shows that the concentration of heavy metals in fly ash is much higher than in slag. The exception is copper because it is a non-magnetic heavy metal and it cannot be removed in magnetic separators during slag cleaning. The level of manganese, cobalt, and chrome is about 2 times higher, and 4 times higher for nickel and lead. The test results shown may significantly differ depending on the waste to be incinerated or the season.