## 2 Materials and Methods
### Materials Characterization  
Elemental composition (carbon, C; hydrogen, H; and nitrogen, N) of the samples was assessed by using a LECO 628 analyzer (LECO, Moenchengladbach, Germany). Each analysis was performed in duplicate using about 0.1 g of sample per trial. Considering the ash value resulting from proximate analysis, the oxygen (O) content was calculated based on the mass difference [23].  
In order to evaluate moisture (M), volatile matter (VM) and ash (A) contents of the samples, thermogravimetric analysis (TGA) was carried out by means of a Mettler Q5000 V3 (Columbus, OH, USA) thermobalance [24] and using about 30 mg of dried sample per trial. The thermal program, chosen from literature [25], can be considered as a modification of the ASTM reference methods E871, E872 and E830. To release residual moisture content, the sample was heated under air to 105 \({}^{\circ}\)C, temperature which was held for 30 min before heating at 16 \({}^{\circ}\)C/min from 105 to 900 \({}^{\circ}\)C (hold time: 7 min) in nitrogen to determine the volatile matter content. Ash was determined by switching to air while holding the temperature of 900 \({}^{\circ}\)C for an additional 30 min.  
The surface morphological structure of the hydrochars was examined using a scanning electron microscopy (SEM) system (JEOL JSM-7001F Field Emission SEM, JEOL, Tokyo, Japan). The functional groups of the samples were examined by using Fourier transform infrared spectroscopy (FT-IR, Avatar 330, Nicolet, Waltham, MA, USA). Wavenumbers between 4000 and 400 cm\({}^{-1}\) were covered using 64 scans in the investigation range with a resolution of 4 cm\({}^{-1}\). Moreover, the functional groups were examined using also FTIR-ATR (attenuated total reflectance) spectroscopy using a Spectrum One (Perkin Elmer, Boston, MA, USA) by averaging 16 scans with a resolution of 4 cm\({}^{-1}\) in the wavenumber range between 4000 and 650 cm\({}^{-1}\), employing a ZnSe crystal. The load applied to squeeze the powdered samples towards the diamond was \(130\pm 1\) N.  
The surface morphology and pore structure characteristics of the samples were determined by N\({}_{2}\) physisorption measurements performed at 77 K using a surface area porosimeter (ASAP 2010,Micromerities, Norcross, GA, USA). All the samples were degassed below 1.3 Pa at 25 \({}^{\circ}\)C prior to the measurement.  
The specific surface area was calculated following multipoint N\({}_{2}\)-Brunauer-Emmett-Teller (BET) adsorption method, in the interval 0.05 \(\leq\) (P/P\({}_{\rm o}\)) \(\leq\) 0.33. Pore size distribution curves were determined using the Brunauer-Joyner-Halenda (BJH) method applied both on the adsorption and the desorption branches of the isotherms.  
The chemical speciation analysis of alkali and heavy metals and other elements undetectable by ultimate analysis was carried out in duplicate on raw sludges, hydrocarbons and modified hydrocarbons. Heavy metals concentrations were determined by using an optical ICP system (Perkin Elmer 7300 DV) and following UNI EN 13,657 and UNI EN ISO 11885:2009 methods. Mercury concentrations were determined by using a flow injection mercury system (FIMS) (Perkin Elmer FIMS 100). Total nitrogen concentrations were determined following CNR-IRSA Method [26].