Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 61

under aerobic conditions, these intermediates were further metabolized, resulting in
complete mineralization. Not only, anaerobic-aerobic systems are able to mineralize
azo dyes, but the improved degradation of Golden Yellow using a consortium was
found when aerobic conditions was followed by microaerophilic conditions
(Waghmode et al. 2011 ). Many times, aromatic amines, resulting from anaerobic
azo dye decolorization, undergo partial degradation in aerobic environments,
because the metabolites of azo dye are auto-oxidised upon exposure to oxygen and
the products are unable to degrade further. The individual cell densities of
consortium also vary signi

cantly with the change in oxygen amount (Solis et al.
2012 ). Chan et al. ( 2011 ) have observed that during microaerophilic conditions,
Enterococcus casseli avus is predominant, while Enterobacter cloacae and Cit-

robacter freundii are predominant under aerobic conditions.

2.3 Bacterial Degradation of Azo Dyes by Immobilized Cells

Currently, immobilized bacterial cells have been used for degradation study of azo
dyes. Immobilization of cells is done by two ways, i.e., attachment and entrapment.
In attachment, cells adhere to surfaces of inert materials or other organisms. While
in case of entrapment, bacterial cells get entrapped in the interstices of brous or
porous materials. During last few years, different reactor designs have been pro-
posed for an effective continuous anaerobic/aerobic treatment of azo dyes. Immo-
bilized bacterial cells have higher level of stability to environmental perturbations,
such as change in pH, or exposure to high concentrations of dyes, than their free
cell counterparts. Immobilization of cells for degradation of dyes is a promising
method, as it is easy to perform under aseptic condition in situ, prevents cell
washouts, and allows a high cell density to be maintained in a continuous reactor
and also easy to scale up. Catalytic stability and substrate uptake are often improved
by immobilization because of increased availability of nutrients. Numerous carriers,
such as sintered glass, nylon web, polyurethane foam, activated carbon, pine wood
and porous polystyrene, are used for immobilization of bacterial cells. Barragan
et al. ( 2007 ) studied degradation of several azo dyes by immobilizing cultures of
Enterobacter sp., Pseudomonas sp. and Morganella sp., on Kaolin, bentonite and
powdered activated carbon (PAC) to degrade several textile dyes. Similarly, a
reactor, using polyvinyl alcohol (PVA) and co-immobilized cells of A. hydrophila,
Comamonas testosterone and Acinetobacter baumannii, was used for decoloriza-
tion of azo dye Red RBN and it was found that decolorization was effective even at
higher concentration of dye (Chen et al. 2003 ). They have optimized various
operating conditions, such as bed expansion, cell bead number, density and initial
dye concentration, hydraulic retention time and diameters of beads for decolor-
ization of dye (Puvaneswari et al. 2006 ). Moreover, bacterial entrapment within
natural or synthetic materials has an application for azo dye degradation on a large
scale (reactor scale), because it creates anaerobic environment favourable to dye