This study investigated the fate of nano-CeO2 during municipal wastewater treatment using a laboratory-scale activated sludge (A/S) system fed with primarily-treated municipal wastewater and nano-CeO2 (55.0 mg Ce/L). Nano-CeO2 was highly removed during A/S treatment (96.6% total Ce). Extensive removal of CeO2 <200 nm was also attained and the concentration escaping treatment was only 0.11 mg Ce/L. Elimination occurred mainly by aggregation and settling of CeO2 particles, promoted by circumneutral pH values and by nanoparticle interactions with organic and/or inorganic wastewater constituents. Biosorption also contributed to CeO2 removal as shown by sludge analysis and batch adsorption studies. Batch bioassays demonstrated that nano-CeO2 only exerted inhibition of O-2 uptake by A/S at concentrations exceeding those in the bioreactor feed (50% inhibition at 950 mg CeO2/L). These findings indicate that A/5 treatment is expected to provide extensive removal of nano-CeO2 in municipal wastewaters. (C) 2011 Elsevier Ltd. All rights reserved.