Catalytic and co-hydrothermal treatment of sewage sludge: Hydrochar properties and fate of heavy metals and per and polyfluoroalkyl substances (PFAS)

Understanding the fate of heavy metals (HMs) and per and polyfluoroalkyl substances (PFAS) during hydrothermal treatment (HT) of sewage sludge (SS) is crucial for the development and adoption of the HT process for sludge valorisation. However, there are limited understanding on the conversion efficiency of SS, fate of contaminants, product distribution, and hydrochar properties during HT processes, including non-catalytic, catalytic, and co-hydrothermal treatments. Therefore, this study investigates the behaviour and fate of HMs and PFAS in hydrothermal products (hydrochar, bio-oil, and aqueous phase) under sole HT, catalytic HT with Ca(OH)₂, and co-HT with alum sludge at 270 °C for 30 min. Catalytic HT and co-HT enhanced bio-oil yields by 10 % and 15 %, respectively, while exhibiting contrasting effects on hydrochar yield. Co-HT increased hydrochar production to 51.3 %, while catalytic HT reduced hydrochar yield to 21 % compared to sole HT of PS. Both processes improved hydrochar porosity, enhancing its potential as an adsorbent. Heavy metals followed distinct pathways, including precipitation, adsorption, co-precipitation, and complexation. Catalytic HT concentrated HMs such as Cu (1151.4 mg/kg) and Zn (1569.5 mg/kg) in hydrochar under alkaline conditions, while co-HT increased mass percentages but diluted their concentrations with Cu and Zn reducing to 660.4 and 467.8 mg/kg. PFAS degradation varies by chemical structure. Approximately >99 % PFOA was degraded during HT, while PFOS degradation was limited, with a maximum of 41 % degradation observed. These findings highlight the need for further process optimisation to improve PFOS breakdown during catalytic HT and co-HT. Overall, this study provides valuable insights into the fate of HMs and PFAS during HT, offering strategies to optimise HT for sustainable sludge management. • Investigated the fate of HMs and PFAS during sole HT, catalytic HT and co-HT. • Product distribution varied across the three HT processes. • Lime and alum sludge additives influenced HMs retention in the hydrochar. • All HT processes led to 99 % removal of PFOA. • A reduced PFOS concentration was observed for co-HT of PS with AlS.