Textile Dyes Degradation: A Microbial Approach for Biodegradation of Pollutants - Microbial Degradation of Synthetic Dyes in Wastewaters - page 185

pollutants are eliminated or converted into less toxic (or even useful) substances.
Natural biodegradation is often largely catalyzed by indigenous microbial or plant
populations in soil or aquatic ecosystems. Biodegradation has at least three de

ni-
tions (i) a minor change in an organic molecule leaving the main structure still intact,
(ii) fragmentation of a complex organic molecule in such a way that the fragments
could be reassembled to yield the original structure, and (iii) complete mineraliza-
tion. Mineralization is the transformation of organic molecules to mineral forms,
including carbon dioxide or methane, plus inorganic forms of other elements that
might have been contained in the original structures. In recent years, several studies
have focused on the use of microorganisms that are capable or potent to biodegrade
and/or bio-accumulate toxic compounds (Aksu 2005 ). The bioremediation tech-
nology offers several advantages; it can be performed on site; generally has lower
cost and minimum inconvenience in the process; eliminates the waste permanently;
can be used in conjunction with methods of physical and chemical treatments; has
minimal environmental impact and, therefore, has wide public acceptance and also
encouraged by regulatory authorities (Boopathy 2000 ;Dias 2000 ).

2 Textile Ef uents and Their Degradation

The wastewater treatment system is mainly based on physical and chemical pro-
cedures such as absorption, coagulation- ltration, and

electrochemical. Although these methods are effective, but there are also some
disadvantages and limitations, such as high cost, formation of hazardous by-
products, operational problems and intensive energy requirements. Therefore,
biological process is getting more and more attention, since it is cost-effective,
environmental friendly and does not produce a large quantity of sludge (Seong et al.
1995 ). Microbial decolorization of dyes is currently in wide use in textile industry
and also safe for environment due to non formation of toxic by-products during and
after degradation. Several attempts have been made for dye degradation in the past,
but yet complete dye degradation or decolorization is an important task to the
researchers.
A wide variety of microorganisms (fungi, bacteria and actinomycetes) growing
in biological treatment systems are the biological agents for biodegradation.
Microbial degradation is usually only means for complete mineralization of organic
molecules. Besides, microbes can concentrate, accumulate and absorb heavy metals
inside cell or cell walls. Various microorganisms including, yeasts Proteus sp.,
Enterococcus sp., Streptococcus sp., Bacillus subtillis and Streptococcus sp. have
been previously isolated to degrade azo compounds (Brown 1981 ). Immobilized
microorganisms are also being used for water puri

occulation, oxidation,

cation e.g. immobilized
mycelium of Coriolus versicolour is being used for removing colors/pigments from
Kraft mill wastes. In order to develop suitable technology to decolorize or degrade
dyes discharged in the ef uent and to convert them into bene cial products

simultaneously, a well-planned scienti cally acceptable technology is needed.