Nitritier-enriched activated sludge was used in batch degradation of bisphenol A (BPA) and nonylphenol (NP) to verify if ammonium-oxidizing activity within nitrifiers is responsible for the biodegradation of those compounds. Decreases in both BPA and NP concentrations occurred simultaneously with ammonium (NH4+) oxidation into nitrate (NO3-)by nitrifying sludge. However, when ammonium was replaced by nitrite (NO2-) in the medium, an acclimation period was required prior onset of significant degradation of BPA and NR In the presence of inhibitors such as allylthiourea or Hg2SO4, BPA and/or NP reduction decreased significantly, implying that removal of BPA and NP was mostly mediated by biological activity rather than by physicochemical adsorption onto sludge flocs. (C) 2007 Elsevier Ltd. All rights reserved.