*2.3. Analytics*

In all solid samples (DSS and HC), dry matter content, elemental composition (C, H, N, S) and ash content was analyzed. Additionally, HHV of DSS was measured and the concentrations of phosphorus analyzed in DSS as well as process water.

Dry matter content was determined by placing the samples in a drying oven at 105 ◦C overnight referring to DIN EN ISO 18134-3 [25]. Elemental carbon, hydrogen, nitrogen, and sulfur (C, H, N, S) were measured according to DIN EN 15104 [26] with an elemental analyzer (vario MACRO CUBE, Elementar Analysensysteme GmbH, Langenselbold, Germany). Ash content was determined by incineration at 550 ◦C according to DIN EN 14775 [27] and the oxygen content was calculated by difference. HHV was determined by combustion with oxygen in a bomb calorimeter (Parr Instrument GmbH, Moline, IL, USA), according to DIN EN ISO 18125 [28]. Phosphorus was determined according to DIN EN ISO 11885 [29] with a spectrometer (ICP-OES Icap 6300, Thermo Fisher Scientific, Waltham, MA, USA). Every sample was analyzed three times.
