Effect of hexavalent chromium on performance of membrane bioreactor in wastewater treatment.

The presence of toxic hexavalent chromium poses a great challenge in biological wastewater treatment. In this study, the performance of a membrane bioreactor (MBR) for the treatment of synthetic domestic wastewater in the presence of chromium was investigated. The carbonaceous pollutant removal is not affected by Cr(VI) with concentration ranging from 0.4 to 10 mg/L; it becomes slightly lower when the Cr(VI) is 50 mg/L. The nitrification efficiency of above 99% can be achieved when the waste stream is free of the metal or contains 0.4 mg/L chromium. When its concentration is 10 mg/L, nitrification efficiency above 50% is found; however, it becomes deteriorated in the presence of 50 mg/L chromium. The positive biomass growth, though lower than conventional activated sludge process, can be achieved at Cr(VI) concentration less than 10 mg/L; a decline in the cell growth occurs when the metal concentration is increased to 50 mg/L. Significant accumulation for the metal is observed when its concentration is 0.4 mg/L; however, almost no metal removal is observed when the concentration is above 10 mg/L. During eight-month continuous operation, the presence of Cr(VI) has an insignificant effect on the flux. The nitrifiers in the MBR are more sensitive to the presence of Cr(VI) than heterotrophs.