**Taina Lühmann and Benjamin Wirth \***

Biorefineries Department, Deutsches Biomasseforschungszentrum Gemeinnützige GmbH, Torgauer Straße 116, 04347 Leipzig, Germany; taina.luehmann@ru.nl

**\*** Correspondence: benjamin.wirth@dbfz.de; Tel.: +49-341-2434-449

Received: 25 July 2020; Accepted: 26 August 2020; Published: 26 August 2020

**Abstract:** As the use of sewage sludge as a fertilizer in agriculture is increasingly restricted in the European Union, other ways to utilize this waste stream need to be developed. Sewage sludge is an ideal input material for the process of hydrothermal carbonization, as it can convert wet biomass into a solid energy carrier with increased mechanical dewaterability. Digested sewage sludge was hydrothermally carbonized at 160–200 ◦C for 30–60 min with initial pH levels of 1.93–8.08 to determine optimal reaction conditions for enhanced dewaterability and phosphorus release into the liquid phase. Design of experiments was used to develop response surface models, which can be applied to optimize the process conditions. For optimal dewaterability and phosphorus release, low initial pH values (pH 1.93) and mild temperatures around 170 ◦C are favorable. Because holding time had no statistically relevant effect, a dependency of reaction time was investigated. Though it did not yield substantially different results, it could be included in investigations of short reaction times prospectively. Low reaction temperatures and short holding times are desirable considering economic reasons for scale-up, while the high acid consumption necessary to achieve these results is unfavorable.

**Keywords:** dry matter; moisture; dewatering; sewage sludge; hydrothermal treatment; phosphorus; RSM; DoE; optimization; regression model
