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The effects of discontinuous micro-aeration on the biotransformation of the tetra-azo dye Direct Black 22 (DB22) and of its anaerobic by-products (aromatic amines) were tested using sequencing batch reactors. The experimental set-up consisted of three independent reactors (R1, R2, and R3) working with distinct cycles: R1 was completely anaerobic, while both R2 and R3 included a micro-aerated stage after an anaerobic stage (R2 continuously and R3 intermittently micro-aerated). Average DB22 removal efficiencies in R1, R2, and R3 were 81.4, 74.5 and 76.8%, respectively. R1 produced an effluent with high acute ecotoxicity (1.13% as effective concentration causing 50% of bioluminescence inhibition in Vibrio fischeri) due to aromatic amines accumulation. In contrast, no acute ecotoxicity was detected in the effluent of R2 and R3, reflecting aromatic amines removal. At the anaerobic stage of the cycle, R2 and …