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 R3 showed DB22 decolorization rates 73 and 123% higher than R1, respectively. At the same time, the corresponding values of estimated methane production were 35 and 37% lower. Thus, oxygen apparently changed the balance between methanogenesis and reductive decolorization, inhibiting the first process and enhancing the second. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.