## 3 Results and Discussion  
### Characterization  
Mass yields, elemental and proximate compositions and specific surface area of the various samples are reported in Table 1. Results reveal that the mass yield and the C content of the raw hydrochars are consistent with literature studies [35]. There was a significant decrease in mass yield at increasing HTC reaction temperature: when the reaction temperature rose from 190 to 250 \({}^{\circ}\)C, the hydrochar yield dropped down from about 83% to 63% and the C percentage increased from 26.9% to 36.3%. The KOH treatment reduced the mass of the hydrochar with a clear trend: the higher the temperature at which the hydrochar was produced, the higher the KOH treatment yield. This behavior is in agreement with previous results obtained coupling HTC with classical KOH chemical activation [11] and testifies the higher stability of hydrochars obtained at higher temperatures. Considering the elemental composition (H, C, N, O), the O percentage is maximal for the three raw sludges and decreases with the HTC reaction temperature, as expected. Interestingly, the KOH treatment significantly reduced the O content in the samples obtained at 190 and 220 \({}^{\circ}\)C. Once more, \begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Mass Yield** & \multicolumn{3}{c}{**Elemental Composition (wt\%, db)**} & **Ash** & **VM** & **FC** & **Atomic Ratio** & **Surface Area** \\  & [\%] & **C[\%]** & **H[\%]** & **N[\%]** & **O[\%]** & **[\%]** & **[\% db]** & **[\% db]** & **H/C** & **O/C** & **[m\({}^{2}\) g\({}^{-1}\)] \\ \hline Digestate & - & 25.6 & 4.0 & 3.6 & 21.9 & 45.0 & 50.2 & 4.8 & 1.88 & 0.64 & - \\ Palatable & - & 35.9 & 5.4 & 5.8 & 24.4 & 28.4 & 65.6 & 5.9 & 1.81 & 0.51 & - \\ Mixture & - & 34.4\({}^{\circ}\) & 5.2\({}^{\circ}\) & 5.5\({}^{\circ}\) & 24.1 & 30.8 & 63.4 & 5.7 & 1.81 & 0.53 & - \\
190 Fk & 83.3 & 26.9 & 5.1 & 3.0 & 20.5 & 44.5 & 55.2 & 0.3 & 2.28 & 0.57 & 31.00 \\
220 HC & 76.3 & 28.2 & 4.0 & 1.8 & 18.5 & 47.5 & 52.4 & 0.1 & 1.70 & 0.49 & 8.82 \\
250 HC & 62.9 & 36.3 & 5.0 & 5.0 & 8.7 & 45.0 & 54.9 & 0.1 & 1.65 & 0.17 & 11.85 \\
190 MHC & 51.5 & 29.4 & 4.1 & 1.9 & 15.8 & 48.8 & 51.1 & 0.1 & 1.67 & 0.40 & 0.29 \\
220 MHC & 71.2 & 33.6 & 4.4 & 3.0 & 9.5 & 49.5 & 50.0 & 0.5 & 1.57 & 0.20 & 2.74 \\
250 MHC & 84.7 & 30.6 & 4.0 & 2.1 & 11.2 & 52.1 & 45.3 & 2.6 & 1.57 & 0.27 & 13.36 \\ \hline \hline \end{tabular}  
* oxygen content is estimated as follows: O = 100 \(-\) (C+H+N+ash). 2 calculated as the average of the values of digestate and palatable (weighted average considering the relevant dry matter content) the absolute errors for mass yield are \(\leq\)\(\pm\)2.4, the absolute errors for C, H, N are, respectively, \(\leq\)\(\pm\)1.0, \(\pm\)0.2, \(\pm\)0.3.  
\end{table} 
Table 1: Mass yields, main physicochemical characteristics and elemental and proximate compositions of all the samples. this testifies the higher stability of hydrochars obtained at higher temperatures. The reduction in O can be due to the loss of volatile matters comprising some surface functional groups containing O [35]. Conversely, results related to the content of N and H do not show a clear trend in both raw and modified hydrochars. The carbon content data are particularly interesting and testify to the peculiarity of this type of substrate, which behaves differently than the more standard biomass (e.g., agro-industrial biowasate, ligno-cellulosic biomass, organic fraction of municipal solid waste). If for standard biomasses the HTC leads to an increase in the C percentage in the solid, here, on the contrary, the C percentage in some cases even decreased, passing for example from the value of 34.4% in the sludge mixture to the value of 26.9% in the hydrochar obtained at 190 \({}^{\circ}\)C. This atypical trend is explained by the remarkable ash content that characterizes the sewage sludge. HTC actually concentrates the ashes which, for the two samples mentioned above, passed from the value of 30.8% to the value of 44.5%.