Balancing yield and quality: Effects of aqueous phase recirculation in sewage sludge hydrothermal liquefaction

This study involved a series of six recirculation cycles of aqueous phase (AP) into the hydrothermal liquefaction (HTL) process of sewage sludge. Most of the nitrogen from the feedstock was distributed into AP and N-heterocyclics (41.6 %-53.3 %) were the most abundant compounds which accumulated continuously. AP recirculation improved the bio-crude yield from 32.1 wt% to 40.2 wt%, within a simultaneous deterioration in carbon content from 76.48 wt% to 64.81 wt%, and a significant increase in oxygen content from 7.97 wt% to 20.07 wt%. The increasing oxygen content affected heating values (HHV) of the obtained bio-crudes which systematically decreased from 38.1 to 31.02 MJ/kg. However, the more problematic heteroatom nitrogen, remained at a stable level varying between 3.66 wt% and 4.13 wt%. The study indicates that AP recirculation, enables to achieve a positive quality-to-quantity ratio and could be a suitable solution for the industrial advancement of the HTL technology of sewage sludge.