**4. Conclusions**

Two methods for the nutrient recovery and fuel production were evaluated. For the treatments PS and PL the acid consumption per kg P recovered were, at around 14.2 kg (±0.3 kg) H2SO4, equal, whereas for PL an advantage was the reduction of acidified PW and doubling the P-concentration in the extract. The additional treatment step with sedimentation and decantation and the reduced P-recovery efficiency compared to PS are the only drawbacks of this treatment. In comparison, PS produced a char with a slightly higher HHV, which could be favorable as a fuel substitute for industrial-scale plants. Both treatments exceed the regulatory requirements for P recovery and additionally transfer around 50% of N into the liquid phase, while leaving the main part of heavy metals in the hydrochar. A reduction in char-N content improves as well the combustion emissions regarding NOx. Regarding the high char-S content, only industrial plants with appropriate exhaust gas treatments are applicable. Therefore, sewage sludge will be still available as a climate-neutral fuel for the cement industry and coal power plants.

HTC can compete with current costs for sewage sludge disposal and offers an advantageous alternative to mono-incineration as a sludge treatment, regarding fuel and nutrient recovery. For an implementation in current WWTP infrastructures plants with AD are favored because of their sludge property and the possibility of PW fermentation for higher methane recovery. To promote the P recovery from waste streams like sewage sludge, further investigations in a sustainable and economically feasible fertilizer production are needed.

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

**Funding:** This research was funded by cemsuisse, the Association of the Swiss Cement Industry. The article processing charges for the open access publication was funded by ZHAW Zurich University of Applied Sciences.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank for the financial support received from cemsuisse, the Association of the Swiss Cement Industry and ZHAW Zurich University of Applied Sciences for funding the open access publication. The authors would also like to thank Samuel Solin and Simon Kaiser, Institute of Bioenergy and Resource Efficiency at the FHNW University of Applied Sciences and Arts Northwestern Switzerland, for the help with the elemental analysis.

**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. The project funders agreed to publish the results.

#### **References**

