A study of the membrane bioreactor (MBR) equipped with anaerobic/anoxic switching tanks was conducted to investigate the effect of different internal recycling modes on nutrient removal. The system consisted of a pre-anoxic tank, a first anaerobic/anoxic switching tank, a second anaerobic/anoxic switching tank and an MBR tank. To study the effect of the change in the condition of anaerobic/anoxic switching tanks on nutrient removal, three different modes were tested. These modes were classified according to the varying operating conditions of the anaerobic/anoxic switching tanks. In mode 1, the internal recycle was introduced to the pre-anoxic tank and second anaerobic/anoxic switching tank. In mode 2, the internal recycle was introduced to the pre-anoxic tank and first anaerobic/anoxic switching tank. In mode 3, the internal recycle was introduced to the pre-anoxic tank. To investigate the effect of each mode, GPS-X was used. Subsequently, a pilot-scale experiment was conducted. At the site of this study, phosphorous loading of the influent wastewater was comparatively high: therefore after finishing the experiment, the pilot plant was operated under mode 3. The phosphorous removal efficiency increased up to 88.3% for mode 3 as the anaerobic phase increased. Also, nitrogen removal efficiency was the highest for mode 2. (C) 2010 Elsevier B.V. All rights reserved.