An up-flow packed-bed bioreactor was constructed to investigate the simultaneous removal of chromate and nitrate using biodegradable meal box as carbon source and biofilm carriers. The bioreactor was operated for 164 days with varying influent Cr(VI) concentrations (2.0-50.0 mg/L) and hydraulic retention times (HRT, 10-24 h). It was shown that complete denitrification and Cr(VI) reduction could be achieved when influent Cr(VI) concentrations were lower than 20 mg/L with a HRT of 17 h. Shortening the HRT could significantly reduce the effluent CODcr. It was also observed that Cr(III) was mainly immobilized on the biofilm. Further investigation on Cr distribution in the biofilm compartments indicated that Cr (VI) reduction occurred in all compartments and the intercellular Cr was dominant. High-throughput sequencing analysis showed that Proteobacteria, Bacteroidetes and Firmicutes were the dominant phyla in the biofilm and Cr(VI) stress had a negative effect on the abundance of most bacteria. (C) 2016 Elsevier Ltd. All rights reserved.