The Structural and Biological Effects of Zinc and Titanium Oxide Nanoparticles on the Condition of Activated Sludge from a Municipal Wastewater Treatment Plant

The increasing environmental presence of metal oxide nanoparticles (NMOPs) raises concerns regarding their influence on biological wastewater treatment. This study comparatively evaluates the structural and biological effects of zinc oxide (ZnO-NPs) and titanium dioxide (TiO₂-NPs) nanoparticles on activated sludge from a wastewater treatment plant. Experimental exposure covered nanoparticle concentrations of 0.05-0.3 g/L and contact times up to 180 min, with analysis of enzymatic activity (dehydrogenase activity, TTC-SA method), sludge settleability, and particle size distribution. Inhibition of microbial metabolic activity was observed in a clear dose- and time-dependent manner, with ZnO-NPs showing stronger toxicity than TiO₂-NPs. At the highest dose (0.3 g/L), enzymatic activity nearly disappeared after 90 min (0.04 µg TPF/mg MLSS). Both nanoparticles caused floc fragmentation, decreased sludge volume index (SVI), and increased the proportion of ultrafine particles (<0.3 µm). ZnO-NPs induced more severe destabilization, while TiO₂-NPs showed partial re-aggregation of suspended particles at higher concentrations. Additionally, particle size distribution in the supernatant was analyzed, revealing distinct aggregation and fragmentation patterns for ZnO- and TiO₂-NPs. These structural and functional alterations suggest potential risks for treatment efficiency, including reduced nutrient removal and impaired sludge settleability. The study provides a comparative contribution to understanding toxicity mechanisms of ZnO- and TiO₂-NPs and emphasizes the need to monitor NMOPs in wastewater and to develop mitigation strategies to ensure stable plant operation.