A continuous stirred tank reactor with built-in bioelectrochemical system (CSTR-BES) was developed for azo dye Alizarin Yellow R (AYR) containing wastewater treatment. The decolorization efficiency (DE) of the CSTR-BES was 97.04 +/- 0.06% for 7 h with sludge concentration of 3000 mg/L and initial AYR concentration of 100 mg/L, which was superior to that of the sole CSTR mode (open circuit: 54.87 +/- 4.34%) and the sole BES mode (without sludge addition: 91.37 +/- 0.44%). The effects of sludge concentration and sodium acetate (NaAc) concentration on azo dye decolorization were investigated. The highest DE of CSTR-BES for 4 h was 87.66 +/- 2.93% with sludge concentration of 12,000 mg/L, NaAc concentration of 2000 mg/L and initial AYR concentration of 100 mg/L. The results in this study indicated that CSTR-BES could be a practical strategy for upgrading conventional anaerobic facilities against refractory wastewater treatment. (C) 2016 Elsevier Ltd. All rights reserved.