purpose of this study was to evaluate the biodegradability of synthetic azo dyes in anaerobic treatment system. Azo dye biodegradation study was conducted in batch experiments in 1000 ml conical flask with 500 ml total working volume. The dye concentrations used in the study were in the range of 100 to 600 mg/l of synthetic azoic diazo5, Diazo Fast Red Salt (DFRS). Testes were performed at a temperature of 30oC for four days at 50 rpm in rotary shaker. Color removal and dye degradation efficiencies were monitored for absorbance, COD, BOD5, TN, NH4 +-N, S-2 and SO4 -2. The results from the absorbance indicated that the color removal and dye degradation efficiencies were 92.6, 83.8, 67.3, 41.2, 18.7 and 10.6 % for the dye concentrations of 100, 200, 300, 400, 500 and 600 mg/l, respectively. COD and BOD5 removal efficiencies of the batch bioreactors were also ranged from 85-20 % and 86-16 % for the dye concentrations of 100-600 mg/l, respectively. The TN hydrolysis of the azo dye removal efficiencies were also fount to vary from 64, 48, 23.2, 14.8 and 8.3 % with the corresponding production of ammonium-nitrogen ranging from 368-247.8 mg/l for 100, 200, 300, 400 and 500 mg/l, respectively. The reduction of TN and production of ammonium-nitrogen significantly correlated with absorbance values indicating the biodegradation of the azo dyes. Sulfide removal of the batch bioreactors were also 87.2, 72.5, 58.2, 36.7 and 18.6 % for 100, 200, 300, 400 and 500 mg/l, respectively, consistent with other parameters. The result from this study showed potential application of the system using tannery sludge biomass for the treatment of azo dye containing textile and other similar industries effluents. Key Words: Azo dye, biodegradation, decolorization, anaerobic batch bioreactor