Azo dyes are a widespread class of poorly biodegradable industrial pollutants. In anaerobic environments, azo bonds are reductively cleaved yielding carcinogenic aromatic amines, many of which are assumed to resist further metabolism by anaerobes. Here we report for the first time that an azo dye compound is completely biodegradable in the absence of oxygen. A pharmaceutical azo dye, azodisalicylate, constructed from two 5-aminosalicylic acid (5ASA) molecules was mineralized in an adapted methanogenic consortium to CH4 and NH3 with transient accumulation of 5ASA as a degradation intermediate in both batch assays and continuous bioreactors. The anaerobic metabolism of 5ASA was shown to provide the electrons required for the initial reductive cleavage of the azo group. Our results suggest that it is possible to design azo dyes that are fully mineralized in the environment; thereby, avoiding ac cumulation of notorious toxic intermediates.

Seven anaerobic sludges were screened in order to obtain the most suitable methanogenic inoculum for the anaerobic treatment of wastewaters containing long-chain fatty acids (LCFA). The selection was made on the basis of the toxicity of a model compound, oleate, to acetoclastic methanogens in different sludges. The effects of three biological factors, sludge origin, specific acetoclastic methanogenic activity and sludge adaptation to lipids, and a physical factor, specific surface area of sludge, on the degree of toxicity were investigated and compared. Values of the fifty percent inhibition concentration (IC50) of oleate obtained from 40°C batch toxicity tests ranged from 0.26 to 3.34 mM for the various sludges examined. It was found that the toxicity of oleate to anaerobic sludges was not dependent upon the three biological factors. Instead, it was closely correlated to the specific surface area of sludge. Suspended and flocculent sludges, which have higher specific surface area, suffered much greater inhibition than did granular sludges. This paper suggests the use of granular sludges as appropriate inocula for reactors treating lipids (fats, oils and greases) wastewaters, to decrease the toxic impact from their hydrolysis products-LCFA.

Biological treatment of wastewaters discharged by the textile industry could potentially be problematic due to the high toxicity and recalcitrance of the commonly-used azo dye compounds. In the present report, the fate of two azo dyes under methanogenic conditions was studied. Mordant Orange 1 (MO1) and Azodisalicylate (ADS) were completely reduced and decolorised in continuous UASB reactors in the presence of cosubstrates. In the MO1 reactor, both 5-aminosalicylic acid (5-ASA) and 1,4-phenylenediamine were identified as products of azo cleavage. After long adaptation periods, 5-ASA was detected at trace levels, indicating further mineralization. ADS, a pharmaceutical azo dye constructed from two 5-ASA units, was completely mineralized even in the absence of cosubstrate, indicating that the metabolism of 5-ASA could provide the reducing equivalents needed for the azo reduction. Batch experiments confirmed the ADS mineralization. These results demonstrate that some azo dyes could serve as a carbon, energy, and nitrogen source for anaerobic bacteria.

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