*2.3. Analyses*

### 2.3.1. Solid and Oil Phase

The inorganic elemental compositions of the feedstocks, the HTL solid and organic phases and the solid phases that were obtained during nutrient recovery were determined by ICP-OES (Agilent 715) after acid microwave-assisted digestion. The feedstock, HTL solid phase, and solid phase after leaching and precipitation (Figure 1) were digested by means of the INVERSE AQUA REGIA method in accordance with the procedure that has been described in reference [43]. The Feed-X, HTL-X-SP, and HTL-X-LSP were not completely dissolved during digestion since recalcitrant minerals, such as silica, were not affected; nevertheless, they are not of interest in the current study. Hence, in spite of the recognition of the applied procedure, the total element concentration may differ from that which was obtained due to the limitations of the digestion method. For digestion of the oil phase, the first step was to add 10 mL of 65% HNO3 and 1 mL of 37% HCl to 100 mg of the organic phase. The solution was then digested in a microwave (CEM Discover) for 5 min at 220 ◦C. Subsequently, the completely dissolved samples were diluted to 25 mL with Milli-Q water. All digested solutions were correspondingly diluted with 1% HNO3 to avoid matrix effects and to adjust to the calibration range of ICP-OES. The nitrogen content of the HTL oil and solid-phases was identified by an elemental analyzer (HekaTech, Euro EA). The ash content of the HTL solid samples was determined according to DIN 51719. The analyses were performed twice.

To specify the minerals that are associated with HTL solid phases and precipitates, the samples were characterized by powder x-ray diffraction (XRD). The diffraction patterns were recorded on a PANanalytical X'Pert Pro X-Ray diffractometer with a monochromatic CuKα radiation source. The raw data were processed with Xpert-II software, and the mineral phases were identified with the Inorganic Crystal Structure Database (ICSD). The morphology of the precipitates was characterized by scanning electron microscopy (SEM; GeminiSEM 500 from Zeiss).
