purpose of this study was to evaluate the biodegradability of synthetic azo dyes inrnanaerobic treatment system. Azo dye biodegradation study was conducted in batchrnexperiments in 1000 ml conical flask with 500 ml total working volume. The dyernconcentrations used in the study were in the range of 100 to 600 mg/l of synthetic azoicrndiazo5, Diazo Fast Red Salt (DFRS). Testes were performed at a temperature of 30oC forrnfour days at 50 rpm in rotary shaker. Color removal and dye degradation efficienciesrnwere monitored for absorbance, COD, BOD5, TN, NH4rn+-N, S-2 and SO4rn-2.rnThe results from the absorbance indicated that the color removal and dye degradationrnefficiencies were 92.6, 83.8, 67.3, 41.2, 18.7 and 10.6 % for the dye concentrations ofrn100, 200, 300, 400, 500 and 600 mg/l, respectively. COD and BOD5 removal efficienciesrnof the batch bioreactors were also ranged from 85-20 % and 86-16 % for the dyernconcentrations of 100-600 mg/l, respectively.rnThe TN hydrolysis of the azo dye removal efficiencies were also fount to vary from 64,rn48, 23.2, 14.8 and 8.3 % with the corresponding production of ammonium-nitrogenrnranging from 368-247.8 mg/l for 100, 200, 300, 400 and 500 mg/l, respectively. Thernreduction of TN and production of ammonium-nitrogen significantly correlated withrnabsorbance values indicating the biodegradation of the azo dyes. Sulfide removal of thernbatch bioreactors were also 87.2, 72.5, 58.2, 36.7 and 18.6 % for 100, 200, 300, 400 andrn500 mg/l, respectively, consistent with other parameters. The result from this studyrnshowed potential application of the system using tannery sludge biomass for therntreatment of azo dye containing textile and other similar industries effluents.rnKey Words: Azo dye, biodegradation, decolorization, anaerobic batch bioreactor