Anaerobic textile wastewater treatment: toxicity to methanogenic organisms

Anaerobic toxicity tests were performed using aromatic azo dyes (tetra-azo Direct Black 22 - DB22 and diazo Direct Orange 4 - DO4) and an anthraquinone dye (Reactive Blue 19 - RB19), aiming to verify the influence of the different chemical groups of these dyes on the inhibition of methanogenic activity. For DB22 dye, two tests were performed with dye concentrations of 0.03; 0.06; 0.09; 0.12; and 024 mmol.L-1 (TTABD22) and 0.3; 0.6; 0.9;1.2; and 2.4 mmol.L-1 (TTADB22Adap, with adapted sludge). For DO4 (TTADO4) and RB19 (TTARB19) dyes, the concentrations used were 0.5; 1.0; 2.0; 3.0; and 4.0 mmol.L-1. The results indicated that both azo and anthraquinone chemical groups may be inhibitors for methanogenesis. Each dye resulted in a different toxic potential, depending on the nature of the dye, the concentrations used, and the by-products generated. Anthraquinone dye RB19 showed an inhibition rate of methanogenesis twice as high as azo dye DO4, indicating greater tolerance of methanogenic microorganisms to azo dye than to anthraquinone dye. Among the azo dyes, the highest inhibition rates were verified for DB22 (48%), which has a greater number of sulfonic groups in its structure than DO4. In general, higher percentages of dye removal were obtained for lower dye concentrations.