Waste minimization at source

Research has focused on both the salt requironents of these dyes and the dyeing process itself Low-salt reactive dyes with enhanced application characteristics (Sewekow, 1993 Shukla, 1999) have been developed and are commercially available. Modifications of the reactive dyeing process have also been suggested. Scheyer et al. (2000) proposed a two-bath process with the first bath being neutral and containing dye and salt, and the second bath providing the alkaline conditions essential for the chemical reaction to take place. Such a process sequence allows for the first bath to be re-used and dye hydrolysis to be drastically diminished. 

Solvent dyeing is another possibility to where reactive dyes may be applied with little loss due to hydrolysis. Lim et al. (2001) used methylene chloride with a small amount of water as the dyeing medium. The dye uptake was higher than in water and the organic solvent could be recycled and reapplied. Alberghina et al. (1990) described a method to solvent-dye ceUulosic fibers. 

Vat dyes are usually reduced with sodium dithionite and solubilized under alkaline conditions, applied to the textile material and reoxidized (Rivlin, 1992). Unfixed vat dye enters the waste stream in the form of water-insoluble pigments and can be recovered with ultrafdtration (Elliot, 1996). The reduction step, however, can also be achieved by electrochemical means (Roessler and Rys, 2003). Electrochemical reduction with electrocatalytic hydrogenation or a redox mediator reduces the need for chemicals and thus pollution of effluents. 

Alberghina G, Fisichella S, Occhipinti S (1990), The use of solvents in dyeing of cellulosic fibers , Colourage Annual, 3-6. 

Allegre C, Maisseu M, CharbitF, Moulin P (2004), Coagualtion-flocculation-decantation of dye house effluents concentrated effluents , J. Hazardous Mater., article in press. 

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No matter how much effort is put into cleaner produetion and waste minimization at source, it is rarely practical to convert 100% of raw materials into product or to design a process with a 100% recoverable and... 

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