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Permanent

  • 27/02/2005

Permanent What do industry and consumers look for in a dye? Permanence: the colour is unaffected when exposed to light, washing, chlorine or ozone. This is why dye chemistry produces dyes that last forever. The result of their success: outstanding permanence, but resistance to treatment or removal in wastewater treatment systems.

Most experts agree it is extremely difficult to remove colour from wastewater. There is no universally applicable technique for all conditions. Research and development, therefore, focuses on sector-specific methods and technologies to remove colour and similar contaminants from different kinds of waste streams. Such technologies use three kinds of processes:

Physical treatment
adsorption: Adsorption, the attachment of the molecules of a liquid or gaseous substance to the surface of a solid, is commonly used to remove colour. Most wastewater treatment systems use activated carbon, a crude form of graphite commonly made from wood, coal, lignite and coconut shell, as an adsorbent. Activated carbon is highly porous; this imperfection differentiates it from tight graphite, but also provides it a very large surface area: 5 grammes of activated carbon is equivalent to the surface area of a football field. So, it can adsorb a wide range of components. Its physical adsorption forces are the strongest. Its adsorbing porosity is the best known to humankind.

filtration: In simple terms, a crossflow filtration system separates an influent stream into two effluent streams: the permeate and the concentrate. The former is what has passed through the semi-permeable membrane. The concentrate stream contains constituents the membrane rejects.

Nanofiltration is a proven method for colour removal because it can operate at much lower pressures.

Chemical treatment
electrolytic treatment: Research shows this can remove colour from wastewater, achieved by passing polluted water between two or more electrodes; colour-imparting material is absorbed, producing decolorisation.

ozonation: Due to its strong oxidative nature, ozone (combined with other physical, chemical or biological processes) can treat complex industrial wastes: colour molecules break down quickly. Industry and municipal bodies use this process extensively to remove colour from wastewater.

coagulation and flocculation: Both are crucial to water and wastewater treatment, and are commonly used to remove suspended matter or colour. The commonly used coagulants are ferric chloride, ferric sulphate and alum.

Biological treatment
Typical processes include biologically activated sludge oxidation and post coagulation, followed by chemical oxidation of the final effluent by sodium hypochlorite (naocl).

But the most important strategy is to minimise pollution. Simple steps exist: maximising dye use in the dyebath (thereby reducing what goes out as effluent); maximising dye fixation (reducing usage of dyes) and minimising washoff. These reduce dye consumption by as much as 10 to 20 per cent.

Dyes without metals should be used wherever possible. If a shade cannot be matched with a metal-free colour (say, with bright green, royal blue direct and fibre-reactive colours), reducing metal-bearing dye content is often possible by substituting part of the dye.

In sum, a question remains: how many factories actually use them? More than 70 per cent of dye manufacturing units are in the small-scale industries (ssi) sector, producing more than half of the total dye and dyestuff in the country. The ssi sector pays no heed to environmental regulations: effluents are discharged sans treatment. The same occurs in the unregulated textile dyeing sector. Pollution problems in towns like Panipat, Pali, Bichchri, Jetpur, Tirupur and Ludhiana testify to this fact.

The colour problem will only increase in future. Polluting industries in the West, facing stiffer environmental regulations, continue to shut shop and shift to countries like India, helped by weak enforcement and monitoring. Competition from countries like China will drive manufacturers to look for cheaper options. In this, the casualties will be health and the environment: sub-standard and cheaper inputs mean highly polluting, conventional materials and technologies will remain in use with no incentives for switching to better, less polluting technologies.

Regulatory authorities will have to look for novel solutions. The situation is unique; the solution, too, must be so. To start with, a major improvement in enforcement and monitoring is the need of the day. Authorities will have to encourage research and development to look for solutions that work in our conditions. Consumers will have to be better informed about the dangers the products they use, can cause. As it has been in the past, a demand for change

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