Long-term effect of metal oxide nanoparticles on activated sludge

This study evaluated the influence of metal oxide nanoparticles (NPs) (Ag₂O, TiO₂) and their mixture on activated sludge for 180 days. When tested, a mixture of NPs at 1 and 10 mg/L had greater impact than individual NPs, in which maximum reduction in chemical oxygen demand (COD) elimination (76.3%) was observed after 150 days for 1 mg/L (500 mg/L COD concentration) and after 180 days (70.2%) for 10 mg/L (250 mg/L COD concentration). TiO₂ had higher inhibition on COD reduction than Ag₂O NPs at 1 and 10 mg/L. An antagonistic effect was observed in which the combination of individual NPs had a greater effect than a mixture of NPs. Reduction in COD elimination was found to be dependent on NP type and concentration (p < 0.05). Further, metal ion concentration was higher in sludge than supernatant irrespective of NPs, while microscopic analysis showed the presence of NPs inside activated sludge. Among NPs tested, the concentration of Ti⁴⁺ ion was greater in sludge than in the Ag⁺ ion, thus indicating that TiO₂ has a greater affinity than Ag₂O NPs. All three factors (NP type, NP concentration, organic concentration) play a significant role in imparting COD removal (p < 0.05). Future studies are required to quantify NP concentration to minimize NP effect on plant performance.