Colour is usually the first contaminant to be recognized in a wastewater because a very small amount of synthetic dyes in water ( < 1 ppm) are highly visible, affecting the aesthetic merit, transparency and gas solubility of water bodies. They adsorb and reflect the sunlight entering water, thereby interfering with the aquatic species growth and hindering photosynthesis. Additionally, they can have acute and or chronic effects on organisms depending on their concentration and length of exposure. Removal of colour from dye-containing wastewater is the frrst and major concern, but the point of degrading dyes is not only to remove colour, but to eliminate, or substantially decrease, the toxicity (i.e. detoxification).
Government legislation regarding the removal of dyes from industrial effluents is becoming increasingly stringent, especially in the more developed and developing countries (Robinson et al. 200l). Enforcement of the law will continue to ensure that textile and other dye-utilizing industries treat their dye-containing effluent to the required standards. In India, colour limits in industrial waters have also been set and have been made more stringent in the last few years. Table 4.3 presents the colour concentrations, their limits and the quantity of water generated from textile and other industries in United States and India (Anjaneyulu et al. 2005). European Community (EC) regulations are also becoming more stringent (O'Neill et al. 1999). A large variety of dyes can be found in real effluents. It has been estimated that – 9% (or 40 000 tons) of the total amount ( 450 000 tons) of dyestuffs produced in the world are discharged in textile wastewaters (O'Neill et al. 1999). Desirable criteria when producing those dyes are their fixation degree to fibre and fastness (i.e. high stability in light and washing) and resistant to microbial attack. Indeed, dyes are design to resist to very harsh conditions, difficulting colour removal from textile wastewaters by the conventional wastewater treatments. The degree of fixation of an individual dye varies with the type of fibre, shade and dyeing parameters.
Dye fixation rate values are useful in giving an idea of the amount released, but can only be approximated. These losses are
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