### Ash Main Elements and Heavy Metal Content  
Table 8 shows the main elemental analysis of the ash fractions of the reference run. It can be seen that the elemental composition for all ash fractions was similar. The major components of the ash were calcium, silica, phosphorous, aluminium and iron. These elements originated from the mineral content of waste water and additives for phosphorous precipitation. Due to the high phosphorous content, the ash can be used as fertilizer if it fulfils the respective regulations or as raw material for industrial fertilizer production. Table 9 shows the heavy metal concentration in dry sewage sludge and the ash fractions from the gasification experiments. Also the German legal limits for fertilizers [66] in general and specifically for the deployment of sewage sludge as fertilizer [67] are given. It can be seen that the used dry sewage sludge already fulfils the legal limits for all components except Thallium. Depending on the species, the concentration of the heavy metals in the ashes are different to those of the sewage sludge. For more volatile elements such as Cd, Pb and Tl, the concentration in the bed ash is reduced compared to the sewage sludge, but these elements are found again in the fly ashes with increased concentrations. It is gratifying, that the bed ash is therefore, compared to sewage sludge, depleted of Cd, Hg, Tl and has reduced Pb concentrations. The bed ash fulfils all legal limits except of Nickel, however the high Ni concentrations of the bed ash are assumed to come from the high temperature steel pipe of the reactor and are not attributed to the Ni present in the sewage sludge. Since in industrial plants are refractory lined, the industrial ash is expected to meet the Ni limits. The bed ash is only slightly below the Cu limit, though. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**run** & \(\theta\) & \(n_{\rm SC}\) & \(n_{\rm O2}\) & \(n_{\rm WHSV}\) & \(\mathbf{Y_{\rm ash,fly}}\) & \(\mathbf{Y_{\rm ash,bed,bal}}\) & \(\mathbf{Y_{\rm C,ash,fly}}\) & \(\mathbf{Y_{\rm C,ash,bed}}\) \\ \hline \(\theta\)1 & 659 & 1.03 & 0.259 & 0.54 & 0.072 & 0.404 & 0.110 & 0.045 \\ \(\theta\)2 & 778 & 0.97 & 0.256 & 0.53 & 0.063 & 0.413 & 0.092 & 0.031 \\ \(\mathbf{ref}\) & 842 & 1.05 & 0.250 & 0.47 & 0.062 & 0.414 & 0.085 & 0.017 \\ \(\theta\)4 & 894 & 1.03 & 0.254 & 0.51 & 0.059 & 0.417 & 0.047 & 0.006 \\ \(\mathbf{W1}\) & 848 & 1.21 & 0.252 & 0.27 & 0.065 & 0.411 & 0.066 & 0.016 \\ \(\mathbf{SC4}\) & 849 & 1.98 & 0.256 & 0.54 & 0.070 & 0.406 & 0.055 & 0.014 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Fly ash and bed ash yields and carbon content. It has to be noted, that in this study only the total value of chrome was measured, but not CrVI for which strict limits are imposed [66]. However, from a commercial air-blown sewage sludge fluidized bed gasifier the heavy metal concentrations of the bed ash were reported, and the CrVI concentration was below 0.5 mg kg\({}^{-1}\) and with that below the legal limit [62]. From this commercial facility also very low concentrations of Cd, Hg and Tl were reported, each below 0.2 mg kg\({}^{-1}\), 0.05 mg kg\({}^{-1}\) and 0.5 mg kg\({}^{-1}\), respectively, and Pb was reported with 20 mg kg\({}^{-1}\). These values correspond well to the findings of this study. The Ni concentration was reported to be much lower in the commercial gasifier, underlining the hypothesis that, in this study, the Ni loading of the ash came from the high temperature steel of the reactor pipe. Arsenic, at 3 mg kg\({}^{-1}\), was reported to be much lower in the commercial gasifier compared to this study, however there could also be differences in the sewage sludge composition, which is not considered in [62].