## 4 Conclusions  
Sewage sludge hydrochars were tested as adsorbents as such and after a KOH post-treatment step. Results indicate that both raw and alkali-modified hydrochars could be used as adsorbents for MB solutions and, more in general, for the treatment of wastewater containing dyes. Batch adsorption tests demonstrated good adsorption capacity of hydrochars with a higher MB removal efficiency for the alkali post-treated hydrochars. Among raw hydrochars, the sample produced at 190 \({}^{\circ}\)C achieved the best adsorption results and, correspondingly, was characterized by a higher surface area compared to the other hydrochars. After KOH post-treatment, the sample carbonized at 250 \({}^{\circ}\)C showed the highest MB removal: the alkali modification contributed to a homogenization of the hydrochar surface favoring the interaction between surface functional groups and MB molecules, thus enhancing the adsorption capacity of the sample. Comparing the adsorption results with all physicochemical data from various characterization techniques, a complex mechanism for MB removal can be suggested, where surface active sites, chemical interactions and acid-base or redox equilibria between adsorbents and adsorbates can play an important role. As a whole, this work demonstrates a relatively simple process, i.e., HTC possibly followed by a cold alkali post-treatment, that allows the conversion of sewage sludge to an adsorbent material usable for water remediation. Conceptualization, R.F., L.F. and G.A. ; methodology, R.F., R.C., L.F. and G.A. ; validation, R.F., R.C., L.F. and G.A. ; formal analysis, R.F. and V.M. ; investigation, R.F., V.M., R.C. ; resources, L.F., G.A., R.C. ; data curation, R.F., V.M., R.C., L.F. and G.A. ; writing--original draft preparation, R.F. and L.F.; writing--review and editing, R.F., L.F., R.C., G.A. ; supervision, L.F.; funding acquisition, L.F. and G.A. All authors have read and agreed to the published version of the manuscript. This research was partially funded by ECOOPERA SpA [[https://www.ecoopera.coop/it/](https://www.ecoopera.coop/it/)]([https://www.ecoopera.coop/it/](https://www.ecoopera.coop/it/)). Authors want to thank the help of some technicians of the University of Trento: [PERSON] for SEM acquisition, [PERSON] for support in the adsorption tests, [PERSON] for ultimate analysis data. Authors want to thank also prof. [PERSON] for TGA and [PERSON] for his contribution in the experimental activity. The authors declare no conflict of interest. Figure A1: Derivative thermogravimetric (DTG) curves in N\({}_{2}\) atmosphere of (**a**) digestate, palatable, mixture sludge and hydrochars and (**b**) mixture sludge and modified hydrochars. **Figure A2. ** Scanning electron microscopy (SEM) images of (**a**) 190 \({}^{\circ}\)C, (**c**) 220 \({}^{\circ}\)C, (**e**) 250 \({}^{\circ}\)C raw hydrocarbons and (**b**) 190 \({}^{\circ}\)C, (**d**) 220 \({}^{\circ}\)C and (**f**) 250 \({}^{\circ}\)C modified hydrocarbons. **Figure A3. ** pH values at different MB concentrations. DW-0 represents deionized water without any MB. Standard deviation for the pH is \(<\pm\)0.1. **Table A1. ** Content (mg kg\({}^{-1}\)) of heavy metals, alkali metals and nutrients in the various samples. Absolute errors as follows: N \(<\pm\)900, Al \(<\pm\)800, Cd \(<\pm\)0.1, Ca \(<\pm\)2000, Cr \(<\pm\)1, Fe \(<\pm\)5000, P \(<\pm\)1000, Mg \(<\pm\)2000, Hg \(<\pm\)0.1, Ni  
Figure 14: Linear Langmuir plots for the adsorption of MB on hydrocarbons (HC) (**a**) and modified hydrocarbons (MHC) (**b**); linear Freundlich isotherm plots for the adsorption of MB on HC (**c**) and MHC (**d**); linear [PERSON] isotherm plots for the adsorption of MB on HC (**e**) and MHC (**f**) obtained at different HTC temperatures (triangle 190 \({}^{\circ}\)C, circle 220 \({}^{\circ}\)C and square 250 \({}^{\circ}\)C-initial MB concentrations: 10–300 mg L\({}^{-1}\); contact time: 48 h).