Toxicity and intermediates of C.I. Direct Red 28 dye through sequential anaerobic/aerobic treatment

Toxicity and intermediates of C.I Direct Red 28 (DR 28) azo dye was investigated through decolorization and mineralization of DR 28 azo dye in a sequential upflow anaerobic sludge blanket reactor (UASB)/continuous stirred tank reactor (CSTR) system. System performance was determined by monitoring the conventional parameters such as COD, inert COD, TOC, color, ammonia, nitrate, volatile fatty acid (VFA) and total aromatic amine (TAA) analyses. Under steady state conditions color was effectively removed under anaerobic conditions. Total aromatic amines could be mainly removed under aerobic conditions while COD was removed under both anaerobic and aerobic conditions. Samples from effluents of the system were analyzed by high performance liquid chromatography–diode array detection (HPLC–DAD) and gas chromatography–mass spectra (GC–MS) methods to detect the intermediates produced in the effluents of anaerobic and aerobic steps. Benzidine produced under anaerobic conditions was mineralized significantly under aerobic conditions. Simple qualitative analyses, NO−3–N, HPLC and GC analyses showed that Direct Red 28 could be chiefly mineralized by the anaerobic/aerobic sequential system. Reduction of toxicity was also observed in anaerobic toxicity assays (ATA) respiration/inhibition and Daphnia magna tests through the continuous operation of anaerobic/aerobic sequential treatment system.