This study aims at investigating the simultaneous nitrate and chromate reduction by combining the advantages of sulfur-based autotrophic denitrification, heterotrophic denitrification and membrane bioreactor (MBR) technologies. A laboratory-scale MBR equipped with hydrophilic flat sheet polyethersulfone (PES) membranes (0.45 mu m) was used to evaluate the performance of mixotrophic denitrification at varying nitrate and Cr(VI) concentrations. Methanol was supplied at C/N (mg methanol/mg NO3--N) ratio of 1.33. In the absence of Cr(VI), almost complete denitrification of 50 mg/L NO3--N was obtained and the methanol requirement (3.60 +/- 0.9 mg COD/(mg NOT-N)) for heterotrophic denitrifiers, was quite close to the theoretical value (3.7 mg COD/(mg NO3--N)). Around 54% of the influent nitrate was denitrified by heterotrophs and the rest (56%) was denitrified by autotrophic sulfur oxidizers. The effluent sulfate averaged around 200 mg/L, which was below than the theoretical sulfate concentration if autotrophic denitrification process was used alone. Autotrophic denitrification activity completely ceased at 5 mg/L Cr(VI), but heterotrophic denitrification did not show any inhibition. Almost complete chromate and nitrate reduction was observed at I mg/L Cr(VI). MBR was operated for around 200 days and a weekly physical membrane cleaning was enough at a flux of 15 LMH. (C) 2016 Elsevier B.V. All rights reserved.