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Hydrothermal carbonization as a promising approach towards the removal of polyethylene microplastics and trace organic contaminants from wastewater sludge
Summary
Scientists found that heating sewage sludge with water under pressure (a process called hydrothermal carbonization) removed about 71% of microplastics and completely eliminated drug residues like caffeine and carbamazepine. This matters because sludge is often used as fertilizer, so cutting down on these contaminants could reduce how much plastic and medication residue ends up in soil, water, and eventually our food supply.
Microplastics from sludge need to be eliminated since they can enter the environment through agronomic utilization and threaten the organisms as well as water quality. This study provides novel insights into the concurrent removal of polyethylene microplastics and trace organic contaminants (TrOCs) from two types of sludges (oxidation ditch sludge (ODS) and aerated lagoon sludge (ALS)) using hydrothermal carbonization (HTC), a low-energy process, by analyzing the impacts of temperature and time on removal efficiency. HTC at 220°C (± 3 °C) for 3 hours removed about 71% (± 7.52%) of microplastics from sludge. Temperature and contact time were observed to have a significant effect ( p < 0.05) on microplastic removal efficiency. Morphological characterizations of residual microplastics after HTC revealed that exposure to subcritical water under hydrothermal conditions caused surface deterioration and structural changes. Several TrOCs were detected in sludges, and caffeine exhibited the highest concentration (16.34 µg/g) in the solid phase of ODS, whereas carbamazepine showed the highest concentration (0.695 µg/L) in the aqueous phase. Furthermore, caffeine had the highest concentration (0.240 µg/L) in the aqueous phase of ALS, whereas carbamazepine was the most concentrated (19.96 µg/g) in the solid phase. The initially observed TrOCs were not detected in the aqueous phase nor the solid phase after the HTC, irrespective of varying temperatures and contact times, which signifies their complete removal. These findings demonstrated that HTC is a sustainable sludge management strategy since it can effectively eliminate microplastics and TrOCs from sludge and thus minimize their detrimental effects on the environment.