Biodegradation of a monoazo dye-Acid Orange 7 (AO7) was investigated by using an internal circulation baffled biofilm reactor. For accelerating AO7 biodegradation, endogenous electron donors produced from AO7 by UV photolysis were added into the reactor. The result shows that AO7 removal rate can be accelerated by using its endogenous electron donors, such as sulfanilic and aniline. When initial AO7 concentration was 13.6 mg/L, electron donors generated by 8 h UV photolysis were added into the same system. The biodegradation rate 0.4 mg(0.05) h(-1) was enhanced 60% than that without adding electron donor. Furthermore, sulfanilic and aniline were found to be the main endogenous electron carriers, which could accelerate the steps of the azo dye biodegradation. (C) 2016 Elsevier B.V. All rights reserved.