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 effectwas 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.