**4. Conclusions**

The results of this study provide useful information for the design and optimization of HTC systems for sewage sludge treatment. It aimed at finding the optimal HTC reaction conditions for a maximized mechanical dewaterability and phosphorus release in the liquid phase. In the investigated design space (160–200 ◦C, 30–90 min, pH 1.93–8.09), low initial pH levels are desirable and holding time can be minimized as it had no statistically relevant effect. The influence of reaction temperature is not completely clear. While low temperatures are somewhat more advantageous for phosphorus release, medium temperatures are favorable for dewaterability.

It was investigated if using reaction time as an input parameter instead of holding time would cause different results. Though only minor changes were observed, some models were more refined. However, with regard to future experimental plans using batch reactors, aiming at identifying the impact of HTC process parameters on several product characteristics, it could be considered to exclude holding time from the parameter variation. Residence time is of course a main factor in the design of larger plants, but can only be addressed sufficiently in lab-scale using equipment allowing continuous operation to minimize actual heating and cooling times.

A numerical optimization to maximize mechanical dewaterability and phosphorus release yielded achievable dry matter contents of 48.6% and a phosphorus share of 70.3% in the liquid phase at 170 ◦C and an initial pH of 1.93. This would be accompanied by an HHV of 13.95 MJ kgdb<sup>−</sup>1.

**Author Contributions:** Conceptualization, T.L. and B.W.; Formal analysis, T.L.; Funding acquisition, B.W.; Investigation, T.L.; Methodology, T.L. and B.W.; Project administration, T.L.; Supervision, B.W.; Validation, T.L. and B.W.; Visualization, T.L.; Writing—original draft, T.L.; Writing—review and editing, B.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received funding by the Federal Ministry of Food and Agriculture of Germany (BMEL) and by the Sächsische Aufbaubank (SAB) financed by the European Regional Development Fund (ERDF) under grant no. 100283030.

**Acknowledgments:** The authors would like to thank all of the laboratory and technical staff at DBFZ for conducting the experiments and analyses. Furthermore, the authors would like to thank L. Prokot for her contributions to the experimental part of this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
