**2. Materials and Methods**

### *2.1. Hydrothermal Liquefaction and its Products*

The HTL products that were used in this study were produced at Aarhus University in Denmark by a continuous pilot-scale HTL reactor with a feed capacity of up to 100 L·h−1, and a total volume of the system of ~20 L. The detailed HTL reactor system description and the procedure can be found in reference [38]. In brief, it includes a feed introduction system, heat exchanger, trim heater, reactor, an oscillation system, a take-off system, and a product collection zone. Primary sewage sludge was collected in February 2018 from the primary treatment of wastewater at a plant in Viborg, while the *SPR* was purchased from Inner Mongolia Rejuve Biotech Co. Ltd. The initial dry matter content of feedstock slurries amounted to 4 wt% and 16.4 wt% for the PSS and *SPR*, respectively. Hydrothermal liquefaction was performed under sub-critical conditions at 220 bars and 350 ◦C. The flow rate of the slurries was 60 L·h−1. The total duration of runs was 5 h and 6 h for the PSS and *SPR*, respectively. The HTL gaseous stream was separated from other HTL product streams in hydro-cyclone. The HTL oil phase that was produced was gravimetrically separated from the HTL liquid and HTL solid byproducts, which were subsequently separated by filtration [38]. The collected HTL oil, liquid, and solid phases were distributed to the HyFlexFuel partners for further investigation. The pre-dried feedstock and HTL solid phase that were obtained from Aarhus were dried at 105 ◦C and stored at room temperature for future handling. The HTL liquid phase that was obtained was immediately analyzed for elemental concentration and subsequently stored in a freezer at −24 ◦C prior to processing. The HTL oil phase that was obtained was stored at 4 ◦C. Figure 1 presents the simplified sample flow diagram and sample ID.

**Figure 1.** Simplified sample flow diagram and sample ID. SP solid phase; LP liquid phase; OP oil phase. X corresponds to primary sewage sludge (PSS) or *Spirulina* (*SPR*) and d to direct.
