Optimizing procedures for the treatment of dye-containing wastewater is an enormous task, made extremely complex by the hundreds of dyestuffs commercially available in the market. Dyes among various classes e.g., azo, anthraquinone (reactive/vat/disperse) exhibit different chemical characteristics & hence respond differently to treatments aimed at their removal. The complexity of textile dye wastewater prohibits a universal effluent treatment system. Detailed wastewater characterization is an integral part of determining the wastewater methods to be applied. The quantification values of impurities and hazardous ingredients are governed & part of individual geographic area’s legislation with frequent modifications and so not depicted.
Dyehouse effluents are generally discharged into local authority sewer lines/rivers or any other line (e.g., centrally coordinated effluent system), but in all these modes of discharges, there are certain permissible limits of impurities and toxicity. Dyehouse effluent undergoes great variations in content and ph. It is therefore necessary to discharge into a primary storage tank to obtain comparative uniformity by randomization. It also helps in the sedimentation of some solids. Excess acidity or alkalinity is corrected by a dosing mechanism, which is controlled by a pH meter. Excess of caustic alkalinity can be removed by using the carbon dioxide in the flue gas from the boiler. By this means alkalinity can be reduced to ph 9 at virtually no cost.
One of the most common effluent treatment methods is that of precipitation. This involves precipitation of insoluble salts, coagulation of colloidal material, and flocculation. The degree of contamination or impurities are examined by some important parameters like,
The highest standards of effluent treatment require combinations of different types of treatment, despite the expense; in today’s time, it is more relevant due to regulatory controls to be maintained day in day out.
The reactive dye effluent is associated with the most serious ecological problems arising from a high salt load. So, the fundamental approach to the objective of minimizing contamination quantity in effluents by salts/unfixed dye is,
The most initial stage involves the treatment of oxidizing and reducing agents targeted at removal of color and ready for the next stage. Chemical coagulation is also used to treat organic dyes and pigments. The textile plant often uses coagulation and flocculation compounds such as lime, alum, ferric salts, or polyelectrolytes followed by sedimentation. The disadvantage of using coagulation and flocculation is the generation of sludge in big amounts that require disposal. So coagulant is not the final answer for color removal too.
Wastewater treatment by bisulphite catalyzed sodium borohydride reductive technology is commercialized for the removal of metal cations and use of cationic polymer along with also removing color. Although the actual treatment procedure depends on the specific spices in the wastewater. The general guideline is as under.
The life expectancy of commercial dyes
(Courtesy Environment Chemistry of Dyes & Pigments by A. Reife & H. S. Freeman-John Wiley & Sons Inc.)
Dye class life expectancy-years
Acid 15
Basic 14.5
Disperse 13.4
Direct 19.5
Reactive 15.4
Activated carbon adsorption is a very helpful method in the reduction of color effluent with various classes of dyes, but it is not proven efficient and is also not economical in practice. To make it efficient and economical it is used in tandem with polymer flocculation, chemical reduction, and biodegradable treatment. Particularly azo dyestuffs are soluble due to sulphonic groups and induced polarity; they are not readily absorbed on tot the no-polar carbon surface. The dyes must be degraded, and their degraded products are easily absorbed.
The technologies that involve reverse osmosis and membrane filtration, adsorption on active carbon, or generation of coagulants by electrochemical techniques are costly propositions.