Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 247

affected. In the biodegradation process, the contaminant serves as an effective
energy source. The contaminant may take part as primary, secondary or as a co-
metabolite in the bacterial metabolism (Kolekar et al. 2008 ). The structure of
contaminant has a high impact on degradation process, followed by its purity and
solubility (Khan et al. 2013 ).

3.2 Oxygen Availability

The decolorization of various textile dyes by mixed aerobic and facultative anaer-
obic microbial consortia has been also reported earlier (Nigam et al. 1996 ; Moosvi
et al. 2005 ). Although many of these cultures could grow aerobically, decolorization
was achieved only under anaerobic or static anoxic conditions (Kolekar and Kodam
2012 ). Pure bacterial strains, such as Pseudomonas luteola, Aeromonas hydrophila,
Bacillus subtilis, Pseudomonas sp., Proteus mirabilis, Bacillus fusiformis and Ali-
shewanella sp. decolorized dyes under both anoxic and static anoxic conditions
(Chang et al. 2001 ; Kolekar et al. 2008 ; Kolekar and Kodam 2012 ). It was observed
that under static anoxic conditions, the dye decolorization of different dyes was more
than 75 % as compared to agitation, which caused less than 30 % decolorization
(Kolekar et al. 2008 ; Chaudhari et al. 2013 ). This indicates that azo dye degradation
primarily starts in anoxic conditions. The facultative anaerobes were able to grow
under both aerobic and anaerobic conditions and capable of decolorizing dyes under
anaerobic condition (Nigam et al. 1996 ). In the rst step of biodegradation, a

reductive cleavage of the azo bond gives rise to the production of colorless
metabolites, mainly aromatic amines which are further degraded in aerobic condi-
tions. The

rst step usually occurs in anaerobic/static anoxic conditions; however, it
was observed by several researchers that aerobic microbes were able to cleave the
azo group by synthesizing oxygen insensitive azoreductases in the presence of
molecular oxygen (Nachiyar and Rajkumar 2005 ). Bacterial strains, that decolorize
textile dyes aerobically, were reported for few years. Many of these cultures require
external carbon sources, because they cannot utilize dye as terminal electron
acceptor (Stolz 2001 ). Textile dye was decolorized by P. aeruginosa in the presence
of glucose under aerobic conditions (Nachiyar and Rajkumar 2003 ). Azo dye
decolorization by mixed, as well as pure cultures generally requires complex organic
sources, such as yeast extract, peptone, or a combination of complex organic source
and carbohydrate (Chen et al. 2003 ; Khehra et al. 2005 ).

3.3 Optimization of Biodegradation Conditions

Microbial degradation depends on various physico-chemical operational conditions
like, pH, temperature, salinity, heavy metals, radioactive molecules, chlorinated
compounds and other contaminants (Boopathy 2000 ). In this scenario, wastewater