**4. Conclusions**

Biorefineries that are based on HTL and utilize feedstocks with high nutrient loads can add value to the production chain of liquid biofuels relatively easily through the addition of struvite-producing units. For high-protein and low-ash feedstock, such as *SPR*, a one-step approach to directly precipitate struvite from the HTL liquid phase recovers most of the phosphate. Still, ammonium is present in such substantial loads that additional treatment becomes mandatory. Since ammonia stripping is usually a disadvantage of struvite precipitation, such e ffect might be forced tothe extent that ammonia is recovered as, for example, ammonia sulfate during the production of struvite. The sewage sludge that is processed by HTL provides a solid residue that is suitable for recovering phosphate as struvite by means of acid leaching and the addition of released phosphate to the ammonium-rich HTL liquid phase. The larger problem in recovering phosphate is to recover the fine particulate solid residue that is dispersed in the bio-crude oil. Further development of possible in-line filtration during HTL might resolve this problem and enable, along with a future optimization study of precipitation and leaching step, a way to higher phosphate recovery rates for the HTL solid residue, as illustrated by the mass flow.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/13/2/379/s1, As supplementary materials are available: Figure S1. The particle-size distribution in HTL oil phase from PSS; Figure S2. XRD analysis of HTL solid phase from PSS and *SPR*; and Figure S3. XRD analysis of precipitate from PSS and *SPR.*

**Author Contributions:** Conceptualization: E.O., G.C.B.; analysis and investigation: E.O.; data analyses: E.O., G.C.B.; writing and preparation of manuscript: E.O., G.C.B., A.K.; funding acquisition: G.C.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the European Union's Horizon 2020 research and innovation program under gran<sup>t</sup> agreemen<sup>t</sup> No 764734 (HyFlexFuel-Hydrothermal liquefaction: Enhanced performance and feedstock flexibility for e fficient biofuel production).

**Acknowledgments:** We gratefully thank the Department of Engineering at Aarhus University (Patrick Biller and Konstantinos Anastasakis) for providing samples for investigation. We are grateful to Michael Zimmermann (Karlsruhe Institute of Technology (KIT)) for XRD and SEM analyses.

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