additives from 0.1 M to 0.25 M obviously decreased the pH of the resulting feedstock slurry, but it did not greatly influence TP leaching into process-water.

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The initial pH of feedstock slurry produced using CH3COOH and HCOOH additive was ~6.3 and ~5.6, and ~6.2 and ~5.3, respectively, at 0.1 and 0.25 M concentration. The TP in the process-water was observed to be in the range of 65.5–126 mg/L and 66–105 mg/L when produced using CH3COOH additive at 0.1 and 0.25 M concentration. Similarly, the

197

TP in process-water was in a similar range with 59.3–66.9 mg/L and 62–86.3 mg/L when produced using HCOOH additive at 0.1 and 0.25 M concentration.

Factors influencing the TP immobilization during the HTC process include treatment conditions (temperature, reaction time, and additive properties), and the feedstock itself [28]. The formation of phosphorus salts (calcium phosphate, magnesium ammonium phosphate, and magnesium phosphate) are known to immobilize phosphorus into the hydrochar and this immobilization is influenced by the presence of higher inorganic content of the feedstock (such as the level of Ca, Mg, and others), pH, temperature and additives during HTC.

The element composition of the feedstock, particularly the presence of phosphate precipitating metals (viz., Fe, Al, and Ca) has a higher potential in deciding the phosphate retention in the hydrochar product [34]. During HTC of sewage sludge, the presence of a higher concentration of multivalent metal ions such as Al3+, Ca2+, Fe3+, and Mg2+ are responsible for forming phosphate with low solubility and in turn enabling the phosphate to be retained in subsequently produced hydrochar. However, the previous studies indicated that the treatment using H2SO4 as an additive tends to reduce the level of Ca, Fe, and Mg in hydrochar [33]. Analyzing the conductivity aids in understanding the metal ion concentration in the process-water, and the experimental analysis indicated higher conductivity in the process-water following the use of H2SO4 additives in comparison with other organic acids as additives (see Figure 4). The presence of increasing metal ion concentration can explain the higher level of P immobilization into the process-water, particularly with H2SO4 additives. Nevertheless, despite having relatively higher conductivity following the use of NaOH as an additive, TP concentration in the process-water was comparatively less. One explanation for increased conductivity following the use of alkali additive can be simultaneously induced ionic salts with NaOH additive utilization. The investigated results suggest that HTC of sewage sludge significantly immobilizes phosphorus into hydrochar in all but mineral acid additives. Results are consistent with another study carried out by Ekpo et al. (2016) demonstrating lower TP leaching into the process-water during HTC of swine manure in the presence of CH3COOH, HCOOH, and NaOH as additives.

### **4. Conclusions**

The influence of organic acids, an inorganic acid and alkali as additives on phosphorus mobilization, energy value, yield, and dewaterability by hydrothermally carbonizing sewage sludge was analyzed. Phosphorus extraction into the process-water is pHdependent and particularly significant in the presence of inorganic acid (H2SO4). The use of H2SO4 and NaOH as additives has decreased the FC content of produced hydrochar, which negatively influences the heating value of the consequently produced hydrochar. A relatively higher reduction in the binding force of the sludge particles was observed during HTC using H2SO4 in the reaction medium; this, in turn, improved the hydrochar dewatering performance in comparison with other additives. In conclusion, if the HTC of sewage sludge is designated to leach the phosphorus into the process water, the use of inorganic acid at a higher concentration is favorable; however, compromises will be made in the fuel characteristic of the hydrochar.

**Author Contributions:** Conceptualization, V.S.E. and S.N.; investigation, V.S.E.; writing—original draft preparation, V.S.E.; writing—review and editing, V.S.E., S.N., J.S., and M.N.; supervision, S.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This publication was funded by the German Research Foundation (DFG) and the Open Access Publication Fund of the University of Rostock.

**Data Availability Statement:** The authors confirm that the data supporting the findings of this study are available within the article.

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