Roxarsone, widely used as feed additive for chicken and swine in China, significantly inhibits ammoniation and nitrification in soil. Understanding the fate of roxarsone in biological nitrogen removal processes and its potential risks is key for proper and efficient disposal of roxarsone-contaminated animal wastewater, yet remains largely unclear. We report an experimental study of the biotransformation process of roxarsone and its fate in biological nitrogen removal systems for roxarsone-contaminated wastewater treatment. Results showed negligible roxarsone removal during biological nitrogen removal processes, associating with emergent of small amount of 3-amino-4-hydroxyphenylarsonic acid (HAPA) and inorganic arsenic. Biological inhibition analysis revealed that roxarsone exerted inhibitive influence on biological nitrogen removal, especially on denitrification processes. Both trivalent and pentavalent arsenics were detected in liquid and extracellular polymeric substances (EPS), whereas cytoplasm contained pentavalent arsenic only. The results provide insight on the biological nitrogen removal process in roxarsone-containing wastewater treatment systems, which is important for guiding appropriate operation of animal wastewater treatment. (C) 2014 Elsevier B.V. All rights reserved.