BACKGROUND A biofilm-electrode reactor (BER) was developed on the basis of hydrogenotrophic denitrification. The BER can be strengthened by adding organic matter, and the nitrate removal will be enhanced by a combination of heterotrophic and autotrophic denitrification. Denitrification in a biological system can be affected by operating parameters, and the microbial community and functional genes can be changed at the same time. In this study, the effect of hydraulic retention time (HRT) and current on the denitrification of a low chemical oxygen demand to nitrogen ratio (COD/N), saline wastewater in a continuous-flow BER was evaluated and compared to a conventional biofilm reactor (BR). RESULTS The highest extent of nitrate removal was obtained at the optimum HRT (8 h) and current (10 mA). Low levels of ammonia (0.5-4 mg L-1), produced through dissimilatory nitrate reduction to ammonia (DNRA), were observed. Quantitative real-time polymerase chain reaction analysis revealed that the proportion of nirS-type denitrifiers (3.75-8.30%) exceeded nirK-type denitrifiers (0.24-0.90%) in both BR and BER. The genera of T78, Paracoccus and Azoarcus were identified as the dominant denitrifying bacteria in the BER. The cooperative activity of nitrate, nitrite and nitrous oxide reductase is necessary to reduce nitrate to dinitrogen in the denitrification process. The highest ratio of nosZ/nir genes in the bacterial community was 0.25 when the applied current was 10 mA. CONCLUSION Compared to other denitrification reactors, the BER is more effective to treat low-COD/N, saline wastewater with short HRT and low current. (c) 2018 Society of Chemical Industry