Ametryn, which is a second generation herbicide, was introduced to a lab-scale MBR at a concentration of 1 mg/L and a 20-40% removal was observed at HRT ranging from 7.8 to 15.6 h for an average influent Ametryn concentration of 0.8 mg/L. Components of EPS (protein and carbohydrates) increased in the bioreactor and the observed biomass production reduced after the addition of Ametryn. In a batch study, GAC was added to MBR mixed liquor and removal of Ametryn via biodegradation and adsorption were measured. Five common bacterial colony types (Gram negative and positive bacilli and Gram negative cocci) were identified and three of these were resistant to Ametryn up to 5 mg/L. GAC was found to be a very effective Ametryn adsorption medium and in some occasions Ametryn may have acted as a nutrient source for bacteria. (C) 2011 Elsevier Ltd. All rights reserved.