The environmental impact of the use of engineered nanoparticles (NPs) is undeniable based on many studies. Considering that NPs end up in wastewater treatment systems, the potential impact of selected NPs (TiO2, SiO2 or ZrO2) on the activated sludge, especially on Microthrix parvicella or Nocardia bacteria cells, was investigated using laboratory-scale batch reactors through short-term, 7-day exposure to 100, 200, 300 mg/L of NPs. Changes in the oxygen uptake rate (through respirometric measurements), viability of cell (fluorescence microscopy using a live/dead analysis), morphology of flocks or filaments (based on Gram staining and image analysis) and fluorescence in situ hybridization (FISH) of the activated sludge compared with the controls were investigated. The shock loads of NPs caused no lethal effects on activated sludge. The presence of NPs in the activated sludge (depending on the concentration) supported the presence of Nocardia cells in the case of TiO2 (oxygen consumption was maintained); however, SiO2 and ZrO2 NPs caused a restriction in the activity of the sludge (oxygen consumption and Microthrix parvicella cell presence); viability was not significantly affected for any of the NPs or concentrations.Keywords: nanoparticle toxicity; activated sludge; microbial respiration; Microthrix parvicella; Nocardia.
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