**1. Introduction**

Sewage sludge is widely used in agriculture because high contents of phosphorus, nitrogen, and potassium, as well as organic carbon make it a favorable organic fertilizer [1,2]. Because of concerning substances, such as heavy metals, organic residues, microplastics, and various pathogens, stronger regulations regarding the use as fertilizer have been passed. This has been combined with a requirement to recover phosphorus from sewage sludge in countries such as Germany [3,4], so that new ways for valorization are sought after. After landfilling became restricted in the European Union (EU) in 1999, incineration has become the preferred solution in the EU-15 countries [5]. During incineration, sewage sludge can be mono-combusted in specifically designed furnaces, co-combusted in existing waste incineration plants or added in small quantities in cement kilns [6]. Thermal dewatering, an energy intensive process, is the major drawback of incineration [6].

Hydrothermal carbonization (HTC) is a process that can convert sewage sludge into a more easily mechanically dewaterable substance by breaking up the cell structure, thereby releasing bound water [7–10]. Essentially, it is a process that converts biomass in hot pressurized water yielding an upgraded solid biofuel [11,12]. Typical reaction conditions reported in literature are 180 to 250 ◦C for 1 to 12 h holding time [11] at an elevated pressure due to water saturation pressure and additional pressure by gaseous compounds formed during the reaction. To measure the dewaterability of sewage sludge, different approaches such as filtration and mechanical dewatering were examined. Danso-Boateng et al. [8] used a filtration cell and calculated the specific cake resistance as a measure for dewaterability. They found that the reaction temperature was the defining factor for dewaterability, especially when compared to holding time. When Escala et al. [9] mechanically dewatered hydrochar (HC) from stabilized sewage sludge that was acidified, they were able to reach dry matter contents of 52 ± 5.5%, compared to the dry matter content of sewage sludge of 30%. Besides improved dewaterability, another advantage of applying HTC to sewage sludge is the opportunity for phosphorus recovery, the most valuable component of wastewater. Distribution of phosphorus in liquid and solid phase as well as chemical composition has been increasingly studied [13–16]. Shi et al. [14] investigated the effect of initial pH and reaction temperature on the distribution of phosphorus. They found that higher temperatures caused phosphorus to be more present in the solid phase while adding large amounts of acid could shift phosphorus into the liquid phase.

The aim of this study is to investigate the dewaterability and phosphorus release of hydrothermally treated sewage sludge as minimum required reaction conditions are yet to be determined. A method to investigate the optimal reaction conditions in a defined reaction space is facilitated by a Design of Experiments (DoE)/Response Surface Model (RSM) approach. For HTC, this has been already utilized frequently [13,17–20] and for sewage sludge dewatering after HTC by Danso-Boateng et al. [8]. While they tested the filterability as an indicator for dewatering, the objective of this study was to take a more practical approach by investigating mechanical dewatering using a screw press at lab-scale. This experimental set-up was implemented to reflect a possible industrial application. Sewage sludge was used as received (without any pre-treatment) from a wastewater treatment plant (WWTP) to identify if mild HTC conditions are suitable for valorization.

Although the process parameters reaction temperature, holding time and initial pH all play a role on phosphorus release and dewaterability, they have only been investigated individually so far. To also consider interaction effects, a DoE approach was taken. This allowed for the development of regression models to predict the above-mentioned parameters as well as typical HTC characteristics. Reaction conditions that are close to industrial applicability, for which low reaction temperatures and short holding times are of interest, were investigated.
