Anaerobic–aerobic biotreatment of simulated textile effluent containing varied ratios of starch and azo dye

Combined anaerobic–aerobic treatment was used to treat a simulated textile industry wastewater (overall HRT 1.8 days). The azo dye (PROCION Red H-E7B) and starch concentrations were varied in a series of 1-week experiments to determine the effect of starch:dye ratio on COD, BOD and colour removal. The treatment efficiency of the system at 1.9 g l−1 starch and 0.15 g l−1 dye remained constant over 130 days despite seven intervening 1-week periods of operation at other starch:dye ratios. Most colour removal occurred in the UASB reactor (1 d HRT), and the BOD:COD of the UASB reactor effluent increased by up to 47%. The maximum overall COD removal was 88% and the BOD removal was up to 99%. A maximum of 77% colour removal overall was achieved with starch and dye concentrations of 3.8 and 0.15 g l−1, giving a final true colour of 0.21 TCU. At both 0.15 and 0.75 g l−1 dye a starch concentration of 3.8 g l−1 rather than 1.9 g l−1 gave a significant improvement in colour removal. However, at 3.8 g l−1 starch volatile fatty acid levels in the UASB reactor rose, while at 2.9 g l−1 starch they did not. It is recommended that if colour removal efficiency decreases, carbohydrate should be added to the anaerobic reactor at a maximum sludge loading rate between 0.11 and 0.15 kg COD kg−1 TVS d−1.