In this work, food waste was processed through HTL at different temperatures (240-295 °C) and residence times (0-60 min) to evaluate the effect of operating conditions on aqueous phase yield and composition. Aqueous phase organic constituents, which covered a wide range of molecular weight and degree of aromaticity, were characterized using a high-resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS). Total organic carbon (TOC) and total nitrogen (TN) were used to determine the C and N recovery in the HTL process. Inorganic compounds, like Na, K, P, Mg, Ca, and Zn, were quantified by inductively coupled plasma-optical emission spectrometry (ICP-OES).
Higher yields of aqueous phase were obtained at low temperatures and short residence times, up to 28.7 wt.% (from 16.7 wt.% at higher temperature and longer residence time). The resulting aqueous phase TOC and TN were higher for food waste (21.2 g/L and 0.8 g/L, respectively) that what is typically expected from lignocellulosic biomass. FT-ICR MS results indicated an increased complexity of the aqueous phase compared to the bio-crude oil. The bio-crude oil composition is dominated by fatty acids, and di- and tri-glycerides, which are not present at high relative abundances in the aqueous phase. The predominant inorganic species in the aqueous phases were Ca, P, Mg, with a substantial level of Na and K, which can cause difficulties for catalytic upgrading process.