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Four selected azo dyes (acid orange 6, acid orange 7, methyl orange and methyl red) were completely decolourised in the presence of anaerobic granular sludge, while only methyl red was degraded in aerobic conditions using a conventional activated sludge. Additional experiments with culture broth devoid of cells showed that anaerobic decolourisation of azo dyes was performed by extracellular reducing agents produced by anaerobic bacteria. This was further confirmed by abiotic experiments with sulphide and NADH. The presence of redox mediators such as riboflavin led to dramatic acceleration of the anaerobic biodecolourisation process. The azo dye reduction products were found to be sulphanilic acid and 4-aminoresorcinol for acid orange 6; sulphanilic acid and 1-amino-2-naphthol for acid orange 7; N,N-dimethyl-1,4-phenylenediamine and sulphanilic acid for methyl orange; and N,N-dimethyl-1,4-phenylenediamine and anthranilic acid for methyl red. Anaerobic toxicity assays showed that the azo dyes were more toxic than their breakdown products (aromatic amines), except 1-amino-2-naphthol. In the presence of activated sludge, only anthranilic acid was completely mineralised while sulphanilic acid was persistent. 4-aminoresorcinol, 1-amino-2-naphthol and N,N-dimethyl-1,4-phenylenediamine underwent autooxidation in aerobic conditions yielding coloured polymeric products. On the contrary, in the presence of granular methanogenic sludge, 4-aminoresorcinol, 1-amino-2-naphthol and anthranilic acid were quantitatively methanised, sulphanilic acid was partially (70%) mineralised while N,N-dimethyl-1,4-phenylenediamine was only demethylated producing 1,4-phenylenediamine as an end product.

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Kinetics of anaerobic biodecolourisation (methanogenic environment) of four azo dyes (Acid Orange 6, Acid Orange 7, Methyl Orange and Methyl Red) was investigated with regard to their electrochemical properties as well as under variation of dye and sludge concentrations, pH and temperature. Cyclic voltammetry revealed a correlation between the potential of irreversible reduction peak of the dye and its first-order decolorisation constant. For each dye tested, this decolourisation constant was adversely proportional to dye concentration (0.086 -1.7 mM) and had a saturation (hyperbolic) dependency on sludge concentration (0.04 -1.1 g VSS/l), a bell-shape dependency on pH (4.0-9.0) and Arrhenius dependency on temperature (24 -40 8C). Transfer from methanogenic to sulphate reducing environment led to an increase of decolorisation constant for all the dyes investigated due to the abundant presence of sulphide as a reducing agent in the reaction medium. Similar transfer to a denitrifying environment resulted in an almost complete decease of decolourisation because nitrate easily outcompetes azo dyes as an electron acceptor.