A laboratory scale membrane bioreactor (MBR) consisting of worms was operated for 214 days. The objective was to evaluate the treatment and operating performance of the MBR with and without the addition of Ametryn which is a toxic and persistent herbicide. Removal of Ametryn was doubled (up to 80%) in the MBR when the worms were present. Increased rate (2.5 kPa/day) of trans-membrane pressure (TMP) and low concentration of MLSS (5.5 g/L) were recorded when the worm population was high (80-100 worms per 70 mu L). Short-term critical flux values were increased from 7.5 to 15 and then to 30 L/m(2)/h when the worm numbers decreased from 90 to 35 and then to 18 per 70 mu L of mixed liquor respectively. Further, high levels of carbohydrate concentration of soluble microbial products (SMP) and smaller sludge floc-sizes were found when the worm numbers were high. (C) 2013 Elsevier Ltd. All rights reserved.