Acute and chronic responses of activated sludge viability and performance to silica nanoparticles

Recently, the potential health and environmental risks of silica nanoparticles (SiO₂ NPs) are attracting great interest. However, little is known about their possible impacts on wastewater biological nitrogen and phosphorus removal. In this study, the acute and chronic effects of SiO ₂ NPs on activated sludge viability and biological nutrient removal performance were investigated. It was found that the presence of environmentally relevant concentration (1 mg/L) of SiO₂ NPs caused no adverse acute and chronic effects on sludge viability and wastewater nitrogen and phosphorus removal. However, chronic exposure to 50 mg/L SiO₂ NPs induced the increase of effluent nitrate concentration, and thus depressed the total nitrogen (TN) removal efficiency from 79.6% to 51.6% after 70 days of exposure, which was due to the declined activities of denitrifying enzymes, nitrate reductase and nitrite reductase. Wastewater phosphorus removal was insensitive to 1 and 50 mg/L SiO₂ NPs after either the acute or chronic exposure, because the critical factors closely related to biological phosphorus removal were not significantly changed, such as the activities of exopolyphosphatase and polyphosphate kinase and the intracellular transformations of polyhydroxyalkanoates and glycogen. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that the bacterial community structure was changed after long-term exposure to 50 mg/L SiO₂ NPs, and the quantitative PCR assays indicated that the abundance of denitrifying bacteria was decreased, which was consistent with the declined wastewater nitrogen removal.