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

## TL;DR

This study examines how zinc oxide and titanium dioxide nanoparticles affect wastewater treatment by altering microbial activity and sludge structure.

## Contribution

The study provides a comparative analysis of ZnO-NPs and TiO2-NPs toxicity on activated sludge, highlighting distinct structural and biological impacts.

## Key findings

- ZnO-NPs showed stronger toxicity than TiO2-NPs, with significant inhibition of enzymatic activity at 0.3 g/L.
- Both nanoparticles caused floc fragmentation and increased ultrafine particles, affecting sludge settleability.
- ZnO-NPs induced more severe destabilization, while TiO2-NPs showed partial re-aggregation at higher concentrations.

## Abstract

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