<br />Dyestuffs can be classified according to their origin, chemical and/or physical properties, or characteristics related to the application process. Another categorization is based on the applications sector (e.g., inks, disperse dyes, pigments, or vat dyes). A systematic classification of dyes according to chemical structure is the color index, namely, nitroso, nitro, monoazo, disazo, trisazo, polyazo, azoic, stilbene, carotenoid, diphenylmethane, triarylmethane, xanthene, acridine, quinoline, methine, thiazole, indamine/indophenol, azine, oxazine, thiazine, sulfur, lactone, aminoketone, hydroxyketone, anthraquinone, indigoid, phthalocyanine, natural, oxidation base, and inorganic. Synthetic dyes are also classified according to their most predominant chemical structures, namely, polyene and polymethine, diarylmethine, triarylmethine, nitro and nitroso, anthraquinone, and diazo. Approximately 10,000 different dyes and pigments are manufactured worldwide with a total annual market of more than 7×105 tonnes per year. There are several structural varieties of dyes, such as acidic, reactive, basic, disperse, azo, diazo, anthraquinone-based, and metal-complex dyes. They all absorb light in the visible region. Untreated dye effluent is highly colored and hence reduces sunlight penetration, preventing photosynthesis. Many dyes are toxic to fish and mammalian life, inhibit growth of microorganisms, and affect flora and fauna. They are also carcinogenic in nature and hence can cause intestinal cancer and cerebral abnormalities in fetuses. The physical and chemical methods for the treatment of dye-containing effluent includes physicochemical flocculation combined with flotation, electroflotation, flocculation with Fe(II)/Ca(OH)2, membrane filtration, electrokinetic coagulation, electrochemical destruction, ion-exchange, irradiation,photochemical precipitation, oxidation, ozonation, adsorption with activated carbon, and the Katox treatment method, which involves the use of activated carbon and air mixtures. The chemical color removal process leads to 60 to 70% reduction in the color, while the decrease in biological oxygen demand (BOD) is only about 30 to 40%.<br /><br />Textile Dyes<br />Textile industries consume two thirds of the dyes manufactured. The requirement for reactive dyes is high since cotton fabric with brilliant colors has a high demand. The reactive dyes bind to the cotton fibers by addition or substitution mechanisms under alkaline conditions and high temperature. Also, a significant fraction of the dye is hydrolyzed and released. Colored wastewater is a consequence of batch processes both in the dye manufacturing and the dye-consuming industries. Two percent of dyes that are produced are discharged directly in the effluent, and a further 10% is lost during the textile coloration process. Generally, the wastewater contains dye concentrations around 10 to 200 mg/L, as well as other organic and inorganic chemicals used in the dyeing process. The wastewater discharged from a dyeing process in the textile industry is highly colored and has low BOD and high chemical oxygen demand (COD) (because of the presence of grease, dirt, and/or sizing agents, as well as nutrients from dye bath additives). Alkali or acids from the bleaching, desizing, scouring, and mercerizing steps also end up in the effluent, resulting in extreme pH and high salt content.<br />Conventional biological processes have also been resorted to for the treatment of textile wastewater. This includes adsorption of dyestuff on activated sludge , decolorization of reactive azo dyes by transformation using Pseudomanas luteola , and biosorption of cationic dyes by dead macro fungus Fomitopsis carnea . Activated sludge has also been used as biomass in the adsorption of dyestuff, achieving about 90% of BOD, 40 to 50% of COD reduction, and 10 to 30% of color removal <br />Aerobic biological treatment alone generally cannot effectively decolorize<br />wastewaters containing water-soluble dyes; hence a chemical treatment is a necessary primary stage. Effluent collected from a textile mill was chemically treated with sodium bisulfite and sodium borohydride as the catalyst and reduction agent, respectively, followed by aerobic biological oxidation leading to an 80% reduction in color, 98% reduction in BOD, 80% reduction in COD, and 95% reduction in TSS.<br /><br />Reactive azo dyes, which are used for dyeing cellulose, produce the colored wastewater (Fig. 10-2). These dyes make up ∼30% of the total dye market. Because of their stability and xenobiotic nature, reactive azo dyes are not totally degraded by conventional wastewater treatment processes that involve light, chemicals, or activated sludge. Azo dyes are not readily metabolized under aerobic conditions. Under anaerobic conditions, many bacteria reduce the electrophilic azo bond in the dye molecule to colorless amines. Although these amines are resistant to further anaerobic mineralization, they are good substrates for aerobic degradation through a hydroxylation pathway involving a ring-opening mechanism. Hence a combined anaerobic treatment followed by an aerobic one could be very effective. Microbial species, including bacteria, fungi, and algae, can remove the color of azo dye via biotransformation, biodegradation, or mineralization. Decolorization of azo dyes by bacteria is carried out by azoreductase-catalyzed reduction or by cleavage of azo bonds under anaerobic environment.<br />The efficiency of color removal depends on several factors, which include<br />level of aeration, temperature, pH, and redox potential. The composition of<br />textile wastewater is varied and can include in addition to the color, organics, nutrients, salts, sulfur compounds, and toxicants. The concentration of dye in the solution affects the rate of biodegradation; possible reasons include toxicity of the dye, toxicity of the metabolites formed during the degradation of the dye molecule, and ability of the enzyme to recognize the dye efficiently at very low concentrations.<br /><br />