Combined anaerobic–aerobic treatment of azo dyes—A short review of bioreactor studies

The most logical concept for the removal of azo dyes in biological wastewater treatment systems is based on anaerobic treatment, for the reductive cleavage of the dyes’ azo linkages, in combination with aerobic treatment, for the degradation of the products from azo dye cleavage, aromatic amines. Since the 1990s, several research papers have been published on combined, sequential or integrated, anaerobic–aerobic bioreactor treatment of azo dye-containing wastewater. The extent of azo dye reduction in the anaerobic phase of those bioreactor systems was generally high, albeit the process often required long reaction times, a limitation that can easily be remedied by making use of the property of redox mediators to speed up the process. The consequent removal of aromatic amines under aerobic conditions was less unequivocal. Although analytical data indicate that many of the aromatic amines were removed from the wastewater, and although the limited amount of available toxicity data all show far-reaching detoxification during aerobic treatment, it is clear that not all aromatic amines can be completely mineralized.