In terms of color removal, both the flocculent and the granular biomass rapidly adapted to the presence of the azo dye. In fact,high and stable color removal yields of 75–80% were observed inFSBR1 and GSBR1 after 10 days of operation (Fig. 2) with an initialdye concentration of 20 mg L−1at the start of each reaction phase.These results suggest that the color removal yield was independent of the biomass concentration levels attained in the flocculent and granular SBR systems. Color removal through azo dye reduction with aromatic amine formation was confirmed, in both SBR systems, by HPLC analyses of mixed liquor clarified samples. Examples of chromatograms obtained at the onset of the reaction phase(WW0), at the end of the anaerobic phase (WWanaer) and at the end of the aerobic phase (WWfinal) of selected cycles are presented in Fig. 3A for FSBR1 and in Fig. 3B for GSBR1. The intensity of peak b,corresponding to Acid Red 14, was significantly reduced during the anaerobic reaction phase (WWanaer) with the concomitant increase in the intensity of peak a, identified as one of the aromatic aminesresulting from azo dye reduction (4-amino-naphthalene-1-sulfonicacid). The second aromatic amine (1-naphthol-2-amino-4-sulfonicacid) has an unstable character and was not identified in the chromatograms.