The effect of supplementary carbon addition for the treatment of high-technology industrial wastewater in a membrane bioreactor (MBR) was investigated. The MBR was operated for 302 days under different C/N (BODL/NH4+-N) ratios, i.e. 0.9-1 to 20 days, 1.6-21 to 42 days, 2.9-43 to 82 days. 3.6-83 to 141 days, 4.8-165 to 233 days and 9.3-240 to 302 days. Irrespective of the C/N ratios investigated, SS and BOD5 removal efficiencies were above 95% and above 80% COD removal efficiency was observed. In addition, complete nitrification was observed throughout the investigation. However, denitrification and total nitrogen removal efficiencies reached their maximum values at the highest C/N ratio (9.3) investigated. Real-time PCR analysis revealed 10 times higher ammonia oxidizing bacteria to total bacteria ratio under the highest C/N ratio condition (9.3) compared to the low C/N ratio condition (1.6). (C) 2012 Elsevier Ltd. All rights reserved.