*2.1. Digested Sewage Sludge*

Digested sewage sludge (DSS) was collected from a municipal WWTP in Leipzig, Germany, where the wastewater undergoes mechanical (screening, grit removal, primary sedimentation), chemical (phosphate precipitation) and biological treatment (aeration reactor, secondary sedimentation). Sludge stabilization includes sludge thickening, digestion, and mechanical dewatering, reaching a dry matter content of 24.3%. The composition of the sample used in this study can be found in Table 1. A sample of approximately 30 L was homogenized and separated into 27 subsamples, which were stored at 4 ◦C. Before being used in the experiments, they were left to warm up at room temperature.

**Table 1.** Properties of the tested samples with standard deviations given in parentheses.


‡ = triplicates; † = duplicates; HHV = higher heating value; meas = measured; calc = calculated; db = dry basis.

The calculated higher heating value (HHV) is slightly lower than the actual measured one as high ash contents of a sample can severely impact the results of various calculation formulas [21]. However, the correlation of ultimate analysis data and HHV used here is reported to fit measured HHVs of HCs very well and the value for DSS was added as a comparison.

#### *2.2. Experimental Method*

The DoE included the variation of reaction temperature, holding time, and initial pH. A face-centered central composite design (FCCD) was chosen. Corner points were carried out in duplicate and the center point in triplicate, resulting in 27 experiments. The reaction conditions were (i) reaction temperature: 160, 180, 200 ◦C, (ii) holding time (starting when the reaction temperature was reached): 30, 60, 90 min, and (iii) initial pH: 2, 4.5, 7. Though holding time only accounts for part of the timespan during which reactions take place, it is the common measure for comparison of HTC studies. For sewage sludge, Escala et al. [9] used the timespan above 180 ◦C for each experiment. Wang et al. [22] concluded that the conversion of sewage sludge starts at around 130–150 ◦C, observed by a rapid rise of the organic load found in process water. However, they did not observe decomposition of these organic compounds below 180 ◦C. In this study, it was investigated if using the timespan above 140 ◦C instead of holding time for analysis would yield substantially different regression models, in accordance to the values provided by Wang et al. [7].

A graphical illustration of the design space can be found in Figure 1 and the randomized experimental design in Table A1 in Appendix A.

**Figure 1.** Design space of the face-centered central composite design (FCCD), red dots indicate triplicates, green dots duplicates, and blue dots single repetitions.

In preparation for the pH adjustment, the amount of acid needed for the targeted pH levels was determined according to DIN EN 15933 [23]. After homogenizing a mixture of 10 g of sludge and 100 mL of deionized water on a vibrating plate for 10 min at 130 rpm, concentrated sulfuric acid (18 M) was added whilst stirring until the desired pH levels were reached and held constant for 30 min. The amount of acid needed for the actual amount of sludge could then be extrapolated from these results.

For each experiment, around 350 g of DSS were weighed out and the calculated amount of sulfuric acid (18 M) was added. Of that mixture, 290 g were used for the HTC experiment and 10 g for pH measurement of the acidified sample. The measured pH was later used for regression modeling instead of the targeted pH. Except for the addition of acid, the DSS was not further diluted with water but utilized as received from the WWTP. The experiments were conducted in a 500 mL high-pressure stirred-tank reactor made of stainless steel (BR-500, Berghof Products + Instruments GmbH, Eningen unter Achalm, Germany). The stirrer was set to 100 rpm and the reactor was heated up electrically with a heating rate of 2 K min<sup>−</sup>1. After reaching the desired temperature, the reactor temperature was kept constant over the holding time. Then, the heater of the reactor was switched off and the reactor was allowed to cool down to room temperature by the environment.

After the process, the HC-water-slurry was separated with a screw press with an integrated scale. The screw press was tightened until a pressure of 300 kg (which corresponds to 2.2 bar) was applied and retightened when the weight dropped by 10 kg. Pressing was stopped when there was no liquid flow for more than one minute. After the pressing process was completed, the HC was divided into two homogeneous samples according to DIN EN ISO 14780 [24] by coning and quartering. The HC was dried and the process water was stored at 4 ◦C until further use.
