Microbe-Mediated Degradation of Synthetic Dyes in Wastewater - Microbial Degradation of Synthetic Dyes in Wastewaters - page 202

Azo dyes are the most widely used dyes and represent over 60 % of the total
dyes (Fu and Viraraghavan 2001 ). They are complex aromatic compounds with
signi

cant structural diversity. Their properties provide a high degree of chemical,
biological and photocatalytic stability and therefore, resist breakdown over the time,
exposure to sunlight, microorganisms, water and soap, in other words. Thus, they
are very resistant to degradation process (Savin and Butnaru 2008 ). Over the years,
a number of biological and physicochemical methods have been developed for the
removal of industrial azo dyes (Forgacs et al. 2004 ; Saratale et al. 2011a ). As the
laws are becoming more and more stringent due to increasing environmental
concern a large number of investigations have been recently carried out to nd more

ef cient methodologies for the treatment of wastewater. Suitable microorganisms

have been selected for the textile wastewater treatments. This review summarizes
the recent achievements in the microbial technologies developed for the removal of
azo dyes using simulated ef

uents. The principal
factors that affect dye removal were analyzed, and the microbial decoloration
systems based on the use of algae, bacteria,

uents and real textile industry ef

lamentous fungi and yeast, genetically

modi ed strains, microbial consortia, and microbial processes in combination with

AOPs and MFCs are discussed in details with particular reference to the analysis of
the toxicity of the metabolic products of azo dye decoloration. Concomitant with
the in-house multi-dimensional pollution minimization efforts, a number of
emerging material recovery/ reuse and end-of-pipe decolorization technologies are
being proposed and tested at different stages of commercialization. However, due to
their synthetic origin and complex structure deriving from the use of different
chromophoric groups, dyes are extremely recalcitrant (Robinson et al. 2001 ). Along
with the recalcitrant nature of dye wastewater, the frequent daily variability of
characteristics of such wastewater adds to more dif

rses
et al. 2002 ). Despite the fact that virtually all the known non-biological and bio-
logical techniques have been explored for decolorization, (Hao et al. 2000 ), none
has emerged as a panacea. While cost-competitive biological options are not very
effective non-biological processes are restricted in scale of operation and pollution
pro

culty to its treatment (G ü

uent. A list of advantages and disadvantages with different tech-
niques has been re

le of the ef

ected in Table 1 . It appears that a single, universally applicable
end-of-pipe solution is unrealistic, and only a combination of different techniques is
required to devise a technically and economically feasible option for dyes waste-
water. Besides a wide range of hybrid decolorization techniques have been also
tried as depicted in Fig. 1 .

2 Decolorization Process

In the following discussion, the biological and non-biological processes for dye
decolorization have been discussed with focus on azo dyes, as they represent the
largest class of dyes extensively and widely used in industries.