The effect of selected homogeneous and heterogeneous catalysts and feedstock properties on the formation of water soluble components during hydrothermal liquefaction (HTL) of sewage sludge

This study investigates the composition of aqueous phase (AP) from 24 HTL trials of two different municipal sewage sludge (MSS) samples, using homogeneous (Na₂CO₃, Li₂CO₃, K₂CO₃, Ba(OH)₂) and heterogeneous (Fe₂O₃, CeO₂, NiO/MoO₃, MoS₂, Ni/NiO, SnO₂, FeS) catalysts. Principal Component Analysis (PCA) was applied to assess the influence of feedstock and catalyst on AP composition i.e. formation of water soluble components. MSS1-derived AP showed a higher proportion of oxygenated aliphatics (13.9–33.7 %), while MSS2 had elevated N-heterocyclic aromatics (19.6–43.3 %). Homogeneous catalysts increased concentration of phenols (up to 26.3 %) and carboxylic acids, with K₂CO₃ almost doubling the carboxylic acid derivatives. Heterogeneous catalysts affected nitrogen and total organic carbon contents. Whereas Fe₂O₃ increases the aliphatic N-heterocycles from 20.6 % to 30.2 % (MSS1) and from 12.7 % to 21.0 % (MSS2), FeS strongly decreases the aromatic hydrocarbons from 9.5 % to 1.1 % (MSS1). PCA analysis confirmed distinct clustering patterns based on the interactions of the feedstock and catalyst, highlighting their synergistic effects. Phenol and cresol were present in the highest concentrations for both sludge, ranged up to 15.6 % and 15.4 % for MSS1 and 12.6 % and 12.9 % for MSS2, respectively. Among the oxygenated aliphatics the most abundant were cyklopenten-1-one, ethanone and their derivatives. N-heterocyclics were represented by a broad mix of pyrazine, pyridine, pyridinole, pyrrolidine, piperidine and their derivatives. The study demonstrates that feedstock properties significantly affect the AP composition, additionally it highlights the role of catalysts applied. These findings provide key insights into optimizing HTL conditions for industrial-scale applications and supporting effective AP by-product management strategies.