## 3 Results  
### Morphological Characterization of Sludge Derived Biochar  
Raman analysis (see Figure 4) on non-thermal annealed biochar shows a high degree of fluorescence. Raman spectra appear not quite uniform at different points of the sample. This could be due to a nonuniform carbonization process at low temperatures (650 \({}^{\circ}\)C for 20 min). The thermally annealed biochar (1000 \({}^{\circ}\)C for 30 min) shows sharp Raman spectra with narrow D and G peaks. Spectra collected at different points show similar shapes in the thermal annealed sample, which supports a uniform carbonization process. Sharp D and G peaks from high thermal temperature annealing are due to the crystallization process of the carbon structure.  
Figure 3: Rectangular waveguide connected to a Vector Network Analyzer (VNA).  
Figure 2: Master mold, silicone mold and spacer with sample.  
Figure 5 shows the FESEM characterization of non-thermal annealed sludge biochar and thermal annealed sludge biochar.  
Non-thermal annealed sludge biochar has a slightly rough surface on both a large and reduced scale. Using a high magnification, grains with an average dimension of around 1 \(\upmu\)m can be observed. Thermal annealed sludge biochar has a very rough surface with similar grains. In this case, the grains are smaller and more defined with respect to the carbonaceous matrix. This could be explained by the results of the EDX analysis shown in Figure 6 and in Table 1. This analysis maps an area of around \(60\times 60\)\(\upmu\)m (mapping images).  
Figure 4: Raman spectra of thermally treated and non-thermally treated biochar.  
Figure 5: FESEM captures of (**a,b**) non-thermal annealed and (**c,d**) thermal annealed sludge derived biochar at different magnifications.  
Thermal annealing considerably reduced the carbon content of the sludge derived biochar from 55.20 wt.% down to 39.70 wt.%. This was unusual because, generally, with the increase of pyrolytic temperatures, the carbon content increases [38]. In this particular case, we suppose that the high starting inorganic content promoted a drastically radical degradation of organic matter with a concentration of inorganic species [39]. This explains the detection of elements such as Ti, Fe, Zn and the increase of all  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Element**} & \multicolumn{2}{c}{**Composition [wt\%]**} \\ \cline{2-3}  & **Non-Thermal Annealed Sludge** & **Thermal Annealed Sludge** \\ \hline C & 55.20 & 39.70 \\ O & 39.70 & 28.67 \\ F & 2.07 & Not detected \\ Mg & 0.32 & 1.39 \\ Al & 0.52 & 3.44 \\ Si & 0.62 & 5.54 \\ P & 0.64 & 4.17 \\ S & 0.38 & 2.18 \\ Cl & 0.57 & 0.29 \\ K & Not detected & 0.40 \\ Ca & Traces & 6.57 \\ Ti & Not detected & 0.60 \\ Fe & Not detected & 5.75 \\ Zn & Not detected & 1.29 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Elemental composition of carbonized sludge and annealed sludge.  
Figure 6: Energy Dispersive X-ray (EDX) analysis of (**a**) non-thermal annealed and (**b**) thermal annealed sludge derived biochar at different magnifications.  
the others with the exception of halides, which could form volatile compounds dragged away by the inert carrier gas flux [40].  
FESEM analysis was performed on the composites with 10 wt.% and 20 wt.% of biochar in order to study the biochar dispersion. The results are shown in Figure 7. A higher content of biochar can be seen in the composite with 20 wt.% biochar as compared to the composite with 10 wt.%. In both cases, a good dispersion of the filler was observed in the different sample portions analyzed. Furthermore, a rise in inorganic content led to a more granular structure in the composites.