## 3 Results and Discussion
### Characterization
For sewage sludge, the C concentration effect is observable if the percentage values are considered on a dry ash free (daf) basis, less observable or not observable at all if the percentage values are considered on a dry basis. Thus, on a daf basis, the sludge mixture and the hydrochar obtained at 190 \({}^{\circ}\)C had a comparable carbon content (about 49%), and the carbon content rose for all the other hydrocarbons, both raw and KOH-modified, and it was in the range of 54%-67%. Therefore, for these types of substrate, the carbon enrichment takes place at the net of their ash content: the concentration effect of minerals masks the carbon concentration effect which is typical of HTC. Regarding proximate analysis data, the ash content shows a significant increase after HTC, as expected. Moreover, a further increase is observed for the alkali-modified samples: among all the samples, the highest ash content (about 52%) was measured for the 250 MHC. The volatile matter of the sludge mixture reduced from about 63% to 52%-55% with the carbonization treatment, justifying the corresponding increase in mineral ashes [36]. Results show that there is no a significant difference in the VM content of the three raw hydrochars. Furthermore, the KOH treatment reduced even more the VM content of the hydrochars, which resulted in a range from about 51% (190 MHC) to 45% (250 MHC). Accordingly, the ash content after the KOH treatment increased up to 52% (250 MHC). However, the decrease in VM was not accompanied by an increase in fixed carbon content. This indicates that VM was also converted into other products, such as CO\({}_{2}\) or liquids [37; 38]. Despite the reported data of FC content, which honestly appears quite controversial (actually, in the general case HTC increases FC, which is not the case here), the same thermogravimetric analysis testifies that both raw hydrochar and modified hydrochar samples show an increased thermal stability if compared to raw sludge samples, as expected. This is clear when considering the derivative thermogravimetric (DTG) curves in N\({}_{2}\): Figure A1 in Appendix A. The temperature at which the decomposition rate was maximal was around 280 \({}^{\circ}\)C for digestate, palatable and mixture sludge, while it was in the range 450-480 \({}^{\circ}\)C for raw and modified hydrochars. Considering H/C and O/C atomic ratios, general trends appear even if some data are out of trend, likely due to the heterogeneity of this kind of substrates. H/C and O/C atomic ratios decreased after HTC, and they decreased even more after KOH post-treatment. The higher the temperature of the HTC process, the lower are the H/C and O/C ratios. Thus, increasing the reaction severity resulted in a decrease of H/C and O/C ratios. The BET specific surface area values of hydrochars were 31.00, 8.82 and 11.85 m\({}^{2}\) g\({}^{-1}\) for 190 HC, 220 HC and 250 HC, respectively. Thus, sample 190 HC had the highest surface area value. However, after treatment with KOH, samples 190 MHC and 220 MHC showed very low BET surface areas in comparison with raw hydrochars, accounting for 0.29 and 2.74 m\({}^{2}\) g\({}^{-1}\) respectively. Similar results and trends were observed also for cold alkali modification of other hydrocarbons as reported in previous studies [10; 14; 28]. A low apparent BET surface area can be related to pore blockage due to organic compounds that were not transferred to the liquid phase during KOH treatment [10]. On the contrary, the BET surface area of sample 250 MHC slightly increased after the treatment with KOH reaching the value of 13.36 m\({}^{2}\) g\({}^{-1}\). Figure 1 shows adsorption/desorption branches for nitrogen isotherms at 77 K of raw (Figure 1a) and modified (Figure 1b) hydrochars. The desorption branch of sample 190 MHC is not reported because it is superimposed onto the adsorption one, due to the low BET surface area value.