*Article* **Influence of Acids and Alkali as Additives on Hydrothermally Treating Sewage Sludge: Effect on Phosphorus Recovery, Yield, and Energy Value of Hydrochar**

**Vicky Shettigondahalli Ekanthalu 1,\*, Satyanarayana Narra 1,2, Jan Sprafke <sup>1</sup> and Michael Nelles 1,2**

<sup>1</sup> Department of Waste and Resource Management, Faculty of Agricultural and Environmental Sciences, University of Rostock, D-18059 Rostock, Germany; satyanarayana.narra@uni-rostock.de (S.N.); jan.sprafke@uni-rostock.de (J.S.); michael.nelles@uni-rostock.de (M.N.)

<sup>2</sup> Deutsches Biomasseforschungszentrum GmbH (DBFZ), D-04347 Leipzig, Germany

**\*** Correspondence: vicky.ekanthalu@uni-rostock.de

**Abstract:** The high moisture content present in sewage sludge hinders the use of sewage sludge in incineration or energy application. This limitation of moisture present in sewage sludge can be obviated by using the hydrothermal carbonization (HTC) process. In sewage sludge management, the HTC process requires less energy compared to other conventional thermo–chemical management processes. The HTC process produces energy-rich hydrochar products and simultaneously enables phosphorus recovery. This study investigates the influence of organic acids, inorganic acid, and alkali as additives on phosphorus transformation, yield, proximate analysis and the heating value of subsequently produced hydrochar. The analysis includes various process temperatures (200 ◦C, 220 ◦C, and 240 ◦C) in the presence of deionized water, acids (0.1 M and 0.25 M; H2SO4, HCOOH, CH3COOH), and alkali (0.1 M and 0.25 M; NaOH) solutions as feed water. The results show that phosphorus leaching into the process-water, hydrochar yield, proximate analysis, and the heating value of produced hydrochar is pH- and temperature-dependent, and particularly significant in the presence of H2SO4. In contrast, utilization of H2SO4 and NaOH as an additive has a negative influence on the heating value of produced hydrochar.

**Keywords:** hydrothermal carbonization; sewage sludge; phosphorus recovery; hydrochar; processwater; pH
