High strength industrial wastewater was treated using a pilot scale submerged membrane bioreactor(MBR). The MBR was maintained at higher sludge concentration of 14,500 mg/l at prolonged sludge retention time (SRT). It was found that the prolonged SRT MBR could achieve both COD and TOC overall removal efficiencies at more than 99%. The MBR was operated for 300 days without sludge discharge and no deterioration of TOC/COD removal efficiencies being observed. A low specific oxygen utilization rate (SOUR) was found in the prolonged SRT MBR system. This showed that smaller amount energy was required by the microorganisms during the prolonged SRT MBR condition, where maintenance metabolism played the dominant role. Sludge production in the prolonged SRT MBR was consistently low at 0.0016 kgVSS.d(-1) with low F/M ratio of 0.11.