Activated sludge under nanoparticle stress: Fate and mechanistic effects of wastewater-borne Al₂O₃ nanoparticles

Wastewater-borne metal oxide nanoparticles may affect biological treatment, but their fate and mechanisms in activated sludge remain unclear. This study evaluated the effects of wastewater-borne Al₂O₃ nanoparticles in sequencing batch reactors operated under long-term controlled exposure conditions. Al-containing particles remained detectable in activated sludge throughout reactor operation and showed substantial temporal variability. During the 63-day reactor operation, SBR_NPs showed the most pronounced temporal changes in biological parameters, including decreases in dehydrogenase activity, microbial respiratory activity, β-glucosidase activity, and overall hydrolytic activity, together with increases in oxidative stress indicators (ROS, SOD, and GST) and LDH release. Changes in microbial community structure were also observed in SBR_NPs. PICRUSt2-based functional prediction further indicated shifts in community functional potential related to nitrogen transformation, phosphorus acquisition, and oxidative stress response. Strong negative correlations between oxidative stress and enzymatic activity parameters indicate that oxidative stress was closely associated with microbial inhibition. Integrated analyses showed that the biological responses observed in SBR_NPs coincided
with temporal changes in the total Al-containing particle pool. These findings highlight wastewater treatment systems as sites of Al-containing particle retention and transformation, with implications for treatment performance.
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