The aim of this study was to assess p-nonylphenol removal mechanisms under nitrifying conditions in a bioreactor system. The removal of a technical mixture of p-nonylphenol isomers was evaluated in a membrane sequencing batch reactor without the addition of an organic carbon source in enriched nitrifying sludge. The p-nonylphenol concentration was quantified in the liquid and solid phases to accurately differentiate between biodegradation and sorption mechanisms. The predominant removal mechanism was p-nonylphenol biodegradation (62%), followed by sorption, which resulted in a total removal of 96%. In addition, ammonium-oxidizing microorganisms controlled the biodegradation of p-nonylphenol, and 1,2-benzenedicarboxylic acid, mono(2-ethylhexyl)ester was identified as the main intermediary metabolite. These results suggested that co-metabolic biodegradation by heterotrophic microorganisms is necessary to completely mineralize p-NP isomers under nitrifying conditions. This study provides an important contribution to existing knowledge of nonylphenol biodegradation. (C) 2015 Elsevier B.V. All rights reserved.