**4. Conclusions**

The present study probed the impact of hydrothermal processing conditions and sewage sludge withdrawal point on resulting hydrochars. The hydrothermal carbonization (HTC) of sludge proceeds similarly to many other wet biomasses; as harshness (time and temperature) of the process increases, the solid yield decreases, the ash (inorganic) content increases, total COD decreases but soluble COD increases, and solid pH decreases. However, there are distinct differences between the hydrochars produced from primary (thickened) versus secondary (digested and dewatered) sludge. Thickened

sludge carbonized at moderate conditions (220 ◦C, 30 min) produced the most viable solid fuel with the highest HHV, moderate ash content, and high volatile matter content. However, with O:C and H:C ratios higher than typical bituminous coals of similar heating content, the thermal reactivity of the hydrochar was significantly higher than coals typically combusted. This suggests that sludge hydrochars could be co-fired with coal but are not ideal solid fuels. On the other hand, hydrochars produced from secondary sludge are more viable as potential soil amendments. The carbonized digested sludges show relatively neutral pH, low COD, and enhanced phosphorous, along with enhanced Ca, Mg, and Al concentrations to help mobilize P. Their heavy metal composition is well below International Biochar Initiative standards, though the elemental oxygen content and lower volatile matter content warrant future inquiry into this pathway.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/13/11/2890/s1, Figure S1: Product distribution among solid, gas and liquid (by difference) phases following hydrothermal carbonization of three sludge samples, Figure S2: Impact of hydrothermal carbonization on total COD (top) and soluble COD (bottom) of three sludge samples, Figure S3: pH of raw and hydrothermally carbonized sludge samples, Figure S4: Proximate analysis of hydrothermally carbonized sludge samples with black circles indicating fixed carbon, red squares are volatile matter, and blue plus symbols are ash content (a: Thickened sludge, 30 min HTC; b: Thickened sludge, 60 min HTC; c: Digested sludge, 30 min HTC; d: Digested sludge, 60 min HTC; e: Dewatered sludge, 30 min HTC; f: Dewatered sludge, 60 min HTC), Figure S5: Ultimate analysis of hydrothermally carbonized sludge samples, Figure S6: Inorganic element distribution in raw sludge samples with: a high concentrations and b: low concentrations; error bars indicate 95% confidence interval, FigureS7: Inorganic elements as a function of sludge sample and carbonization conditions, Figure S8: Visual evidence of hydrochar dewaterability, Table S1: ICP-OES analysis of inorganics present in three sludge feedstocks, Table S2: ICP-OES analysis of inorganics present in hydrothermally carbonized sludge samples, Table S3: Summary of observations of sludge hydrochars.

**Author Contributions:** Conceptualization, J.G., G.A. and L.F.; methodology, F.M., J.G., G.A. and L.F.; validation, F.M., M.V. and T.M.; formal analysis, F.M., J.G., T.M. and M.V.; investigation, F.M.; resources, J.G., G.A. and L.F.; data curation, F.M., J.G. and M.V.; writing—original draft preparation, F.M. and J.G.; writing—review and editing, J.G., M.V. and L.F.; visualization, F.M. and J.G.; supervision, G.A. and L.F.; project administration, G.A. and L.F.; funding acquisition, G.A. and L.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partially supported by Atzwanger Spa http://www.atzwanger.net/.

**Acknowledgments:** The authors appreciate the help of Lihui Gao and Giulia Ischia in running the thermogravimetric analysis and of Fabio Valentinuzzi and Stefano Cesco for inductively coupled plasma-optical emission spectroscopy. J. Goldfarb acknowledges support of the U.S.-Italy Fulbright Commission.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
