Environmental Engineering Reference
In-Depth Information
chemical technologies toward organic synthesis and processing. The dye
industry corresponds to a relatively small part of the overall chemical
industry. Dyes and pigments are highly visible materials and so even the
minimum amount released into the environment may cause the appear-
ance of color in open waters [3,40].
Colored dye wastewater is created as a direct result of the production
of the dye and also as a consequence of its use in the textile and related
industries. There are more than 100,000 commercially available dyes with
over 700,000 tons produced annually. It is estimated that 2% of the dyes
are discharged as effluent from manufacturing operations, while 10%
is discharged from textile and associated industries. Among industries,
textile factories consume large volumes of water and chemicals for pro-
cessing of textiles. Wastewater stream from the textile dyeing operation
contains unutilized dyes (about 8-20% of the total pollution load due to
incomplete exhaustion of the dye) and auxiliary chemicals along with
a large amount of water. The rate of loss is approximated to be 1-10%
for pigments, paper and leather dyes. Effluent treatment processes for
dyes are currently able to eliminate only half of the dyes lost in waste-
water streams. Therefore, hundreds of tons daily find their way into the
environment, primarily dissolved or suspended in water [41,42]. Dyes
are synthetic aromatic compounds which are embodied with various
functional groups. Some dyes are reported to cause allergy, dermatitis,
skin irritation, cancer, and mutations in humans [43]. Beyond aesthetic
considerations, the most important environmental problems related to
dyes is their absorption and reflection of sunlight entering the water,
which interferes with the growth of bacteria, limiting it to levels insuffi-
cient to biologically degrade impurities in the water. It is evident that the
decolorization of aqueous effluents is of environmental, technical, and
commercial importance worldwide in terms of meeting environmental
requirements and water reuse [44]. Textile wastewaters exhibit a con-
siderable resistance to biodegradation, due to the presence of the dyes,
which have a complex chemical structure and are resistant to light, heat
and oxidation agents. Hence the removal of dyes in an economic and
effectual manner by the textile industry appears to be a most imperative
problem [45,46].
The dyestuffs in wastewaters cannot be efficiently decolorized by con-
ventional methods. There also are the high cost and disposal problems for
treating dye wastewater on a large scale in the textile and paper indus-
tries [47]. The technologies for color removal can be divided into three
categories as chemical, biological and physical [48].
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