Biodegradation of azo dyes in cocultures of anaerobic granular sludge with aerobic aromatic amine degrading enrichment cultures

A prerequisite for the mineralization (complete biodegradation) of many azo dyes is a combination of reductive and oxidative steps. In this study, the biodegradation of two azo dyes, 4-phenylazophenol (4-PAP) and Mordant Yellow 10 (4-sulfophenylazo-salicylic acid; MY10), was evaluated in batch experiments where anaerobic and aerobic conditions were integrated by exposing anaerobic granular sludge to oxygen. Under these conditions, the azo dyes were reduced, resulting in a temporal accumulation of aromatic amines. 4-Aminophenol (4-AP) and aniline were detected from the reduction of 4-PAP. 5-Aminosalicylic acid (5-ASA) and sulfanilic acid (SA) were detected from the reduction of MY10. Subsequently, aniline was degraded further in the presence of oxygen by the facultative aerobic bacteria present in the anaerobic granular sludge. 5-ASA and SA were also degraded, if inocula from aerobic enrichment cultures were added to the batch experiments. Due to rapid autoxidation of 4-AP, no enrichment culture could be established for this compound. The results of this study indicate that aerobic enrichment cultures developed on aromatic amines combined with oxygen-tolerant anaerobic granular sludge can potentially be used to completely biodegrade azo dyes under integrated anaerobic/aerobic conditions.