Sarayu. K and Sandhya. S*
National Environmental Engineering Research Institute,
CSIR-Complex, Chennai - 600113, India.
*e-mail: drssandhya@gmail.com

Industrialization is considered to be the key for the development in economic terms. However, it is also recognized to be the root cause for environmental perspective. The recognition that environmental pollution is a worldwide threat to public health has given rise to new initiatives for environmental restoration for both economic and ecological reasons. The industrial effluents contain toxic and hazardous pollutants. One particular class of synthetic chemicals which is of major concern is synthetic dyes and dye intermediates. The dyes are extensively used for textile, paper printing, and color photography, cosmetic, pharmaceutical and leather industries. In 1994 the world production of dyes was around 1 million tons, of which more than 50% were azo dyes (Stolz, 2001). India, Eastern European countries, including the Russia, China, South Korea, and Taiwan together consume approximately 600,000 tonnes of dyes per annum (Ishikata et al., 2000). Synthetic dyes are widely used in a variety of products, of which textiles are the primary. The worldwide annual textile production is more than 30 million tonnes with an expected growth of 3% per annum.

The textile dyeing and furnishing industries use wide variety of dyestuffs due to the rapid changes in the customer's demands. More than 100,000 commercially available dyes are known and the world annual production of the dyestuffs amounts to more than 7x10⁵ tonnes (Robinson et al, 2001). Azo dyes which are used as textile colorants designed to bond covalently with cellulosic fibers (i.e. cotton) are extensively used because of their wide variety of color shades, high wet fastness profiles, ease of application, brilliant colors and minimum energy consumption. It has been estimated that more than 10 - 15 % of the total dyestuff used in dye manufacturing and textile industry is released in to the environment during their synthesis and dying process. Almost 2, 80,000 tonnes of textile dyes are discharged every year worldwide (Mass and Chaudhari, 2005). Waste streams generated from textile industries are hazardous and difficult to biode-grade owing to the presence of recalcitrant dyes and pigments. India being a major producer and exporter of textiles, the problem of environmental pollution from textile industry has a serious dimension. An example for textile wastewater discharged into the environment is shown in Figure 1.

Fig. 1. Discharge of colored wastewater into a riverine system (Tiruppur)

Textile industry wastewater due to the presence of dyes is difficult to treat by traditional treatment technology. This colored industrial effluent when discharged cause considerable damage to the receiving water bodies by imparting color. The efficient removal of dyes from textile industry effluents is still a major environmental challenge. Typically textile wastewater consists of a variety of waste streams from different operations. Some dyestuffs are highly structured polymers and are very difficult to decompose.

The degradation products of textile dyes are often carcinogenic. Furthermore, the absorption of light due to textile dyes creates problem to photosynthetic aquatic plants and algae. The presence of even trace concentrations of dyes in effluent is highly visible and undesirable. The release of colored wastewater in the ecosystem is a remarkable source of esthetic pollution, eutrophication, and perturbations in aquatic life.

Textile manufacturing consumes a considerable amount of water in its manufacturing processes. Considering both the volume generated and the effluent composition, the textile industry wastewater is rated as the most polluting among all industrial sectors. It has been estimated that an average of 100 m³/ tonnes of product and discharge approximately 40 - 50 thousand tonnes/year of dyes in the environment. Wastewater from textile finishing industries is complex and highly colored. Coloration of the liquid effluent results from wastage and washing during dyeing and printing processes, with the degree of coloration being dependent on the color/shade dyed and type of dye used. Water insoluble dyes (disperse and vat dyes) generally exhibit good exhaustion properties (i.e. most of the dye bonds to the fiber) and can be removed from the effluent by physical means such as flocculation. However, due to synthetic origin and complex structures deriving from the use of different chromospheres groups dyes are extremely recalcitrant. Along with the recalcitrant nature of dye wastewater, the frequent daily variability of characteristics of such wastewater adds to the difficulty of treatment. When this effluent is discharged to a conventional sewage treatment plants, most of the color gets adsorbed to the biomass (Shore, 1995). Virtually all the known physicochemical and biological techniques have been explored for treatment of extremely recalcitrant textile wastewater, but none however has emerged as a panacea. A single universally applicable end-of-pipe solution appears to be unrealistic and combination of appropriate techniques is deemed imperative to devise technically and economically feasible options. Physico-chemical methods are applied for the treatment of textile wastewater, achieving high dye removal efficiency (Ishikata et al., 2000).

On the other hand, in recent years there is a tendency to use biological treatment systems to treat textile wastewater. It was considered that the recalcitrant nature of textile wastewater imparts toxicity to micro organisms making aerobic treatment difficult. The treatment under anaerobic condition generates aromatic amines which are more toxic to environment.