Product responses to aqueous phase recirculation during hydrothermal carbonization of sewage sludge

Ten-cycle aqueous-phase recirculation induced stage-specific product responses. • Secondary-char yield increased from 12.70% to 15.93% after recirculation. • Oil-phase yield rose from 28.52% to 60.40% with compositional restructuring. • Recirculation increased hydrochar peak combustion temperature to about 393 °C. • Process-water reuse created trade-offs between fuel and adsorption performance. Hydrothermal carbonization generates an organic-rich aqueous phase (AP) whose recirculation can modify the reaction medium and subsequent product formation. However, how aqueous phase recirculation (APR) coordinates product formation remains unclear. This study conducted ten-cycle APR during HTC to evaluate product yields, carbon partitioning, hydrochar characteristics and oil phase composition. Hydrochar yield increased from 57.86% to 60.54% after the first recirculation, whereas secondary char yield increased from 12.70% to 15.93%. The oil phase yield rose from 28.52% to 60.40%, accompanied by marked compositional redistribution. The relative abundances of fatty acids and hydrocarbons decreased from 18.20% to 0.35% and from 11.99% to 0.90–1.86%, respectively, while nitrogen-containing heterocycles and higher-molecular-weight compounds became enriched. Despite limited changes in bulk elemental composition, hydrochar underwent surface deoxygenation and carbon-skeleton reconstruction, increasing its peak combustion temperature from approximately 314 °C to 393 °C. Its adsorption applicability nevertheless declined with further recirculation, and the hydrochar obtained after the first recirculation provided the best balance between fuel and adsorption performance. The oil phase shifted from acids and hydrocarbons towards nitrogen-containing heterocycles and higher molecular weight compounds. Multivariate analyses further associated oil-phase compositional complexity with hydrochar structural stabilization and functional differentiation. These responses were stage specific rather than uniformly cumulative, indicating that evolving AP composition selectively regulated intermediate transformation and cross-phase carbon partitioning during each HTC run. APR therefore acts as an in-process reaction regulator that can couple process water reuse with product oriented resource recovery, although additional cycles did not consistently improve hydrochar performance. Download: Download high-res image (196KB) Download: Download full-size image