Figure 10 Sources for colored wastes from textile dyeing operations from

On the basis of Eqs (5) and (6), the part of dyestuff released as hydrolyzed dye can be estimated using Eq. (7).

nffCpLR

cdLR

When a color depth of 5% (50 g dyestuff per 1 kg of goods) is used as basis for a calculation and a dyestuff fixation of 80% is observed at a liquor ratio of 1:10 (10 L of dyebath for 1 kg of goods) then a mass of 40 g dyestuff is fixed on the textile while 10 g remain in the dyebath as hydrolyzed dye. The dyestuff concentration cD in the used bath is then 1 g/L (^0.05, LR=10, cD=1 g/L).

While at LR 1:10 a fixation of 80% is observed, a reduction of LR to 1:5 lowers the losses of dyestuff to approximately 11% and a degree of fixation of 89% is expected. These results clearly indicate the importance of a low liquor ratio to optimize the degree of dyestuff fixation.

Another source of highly colored dyebaths is found in continuous dyeing processes where the last filling of the padder required to complete the process at well-defined conditions has to be withdrawn at the end of the padding process. Dyestuff concentrations of 50 g L-3 technical dyestuff are quite usual for such dye liquors.

For a dyestuff fixation of 70-80% and a color depth of 5% a concentration of 1.5-1 g/L hydrolyzed dye is expected in the wastewater, when 10 L of washing water is applied per 1 kg of goods. The emission of colored wastewater here can be divided into two different sources, the wastewater from the washing of the dyed material and the residual filling of the padder.

Depending on the length of the dyed piece (800-5000 m) the contribution of the filling of the padder to the total dyestuff concentration in the wasted water is estimated between 50 and 20%.

In general there are two different qualities of colored wasted water:

• The fillings of the padder. High dyestuff concentrations of approximately 50 g/L, high concentration of alkali;

• Spent dyebaths and washing baths. Low concentration of dyestuff, approximately 1 g/L, low concentration of alkali.

Besides an optimization of the dyestuff and the dyeing processes with regard to improved dyebath, exhaustion, the problem of colored wastewater released from dyehouses, has led to numerous technical developments proposed to overcome it.

A large number of techniques have been described in the literature, for example, dyestuff adsorption, oxidative and reductive treatments, electrochemical oxidation or reduction methods, electrochemical treatment with flocculation, membrane separation processes, and biological methods [37-55]. Each of these techniques offers special advantages, but they can also be understood as a source of coupled problems, for example, consumption of chemicals, increased COD, AOX, increased chemical load in the wastewater, and formation of sludge that has to be disposed.

The techniques for decolorization of dye-containing solutions can be applied at different stages:

• Treatment of concentrated dyestuff solutions (e.g., filling of padder), which is an efficient way to handle such concentrates, but as shown in Figure 10 usually only part of the released dyestuff is decolorized by treatment of such baths.

• Treatment of separately collected and reconcentrated baths that initially contain dyestuff concentrations of approximately 1 g/L and are reconcentrated to approximately 10-20 g/L dyestuff by membrane filtration. Such techniques yield considerable amounts of recyclable water, but care has to be taken to avoid any disturbing effect during reuse caused by salt and alkali content in the regenerate. The concentrated dyestuff solution can be treated with similar methods as concentrated dye solutions from fillings of padder.

• Treatment of the total wastewater: this technique will be discussed in Section 8.3, "End-

of-pipe Technologies." The general scheme of such treatments is shown in Figure 11.

Vat Dyes. Vat dyes are normally present in their insoluble oxidized form. During their application in the dyeing process the dyestuffs are reduced in alkaline solution by addition of reducing agents, for example, dithionite, hydroxyacetone, formaldehydsulfoxylates. Vat dyes normally exhibit an excellent degree of fixation; thus, the problem of colored wastewater is of minor relevance. In addition, vat dyes are readily reoxidized in the wastewater into the insoluble oxidized form that precipitates and thus shows lower absorbance. The main problem in the wastewater released form reducing agents which cause certain load in the effluents (XX1). In the case of dithionite, sulfate is formed that can cause corrosion of concrete tubes, and in the case of hydroxyacetone, the COD is increased considerably. A substitution of the nonregenerable reducing agents by electrochemical reduction has been proposed in the literature [56].

Sulfur Dyes. Similar to the vat dyes, sulfur dyes are applied in reduced form. Owing to the lower redox potential of the dyes, reducing agents such as sulfide, polysulfide, glucose, hydroxyacetone, or mixtures of glucose with dithionite are in use. Sulfides should be replaced by other organic reducing agents mentioned above; in such cases the COD is increased but the products are easily biodegradable. In comparison to the vat dyes the degree of fixation is lower with sulfur dyes. As such, dyes are mainly used for dark shades and colored effluents have to be treated with methods similar to the processes mentioned with reactive dyes.

Indigo. Dyeing with indigo for the Denim market (jeans) is unique. Here a nonuniform dyeing through the cross-section of the yarn is the desired type of quality. There is only one dye in use, indigo. For this type of textile the warp is dyed before the weaving process and special techniques are applied on unique dyeing machines specialized to produce indigo-dyed warp yarn [57]. Figure 12 presents a scheme of the dyeing process. After the warp yarn has been wetted and squeezed, it is immersed into the dyebath, which contains the reduced indigo dye (from 1 to 5 g/L) for a few seconds. After mangling to 80-90% expression, the reduced dyestuff on the material is oxidized completely during an air passage that lasts for 60-120 s. The immersion/squeezing/

CancrDlritol d) fSlil IT solution DJIutcd dye.it uff solution e.g. Filling Of padtW C.&. wash in£ Ijallis

Dirttt treolmnl uy

Pretipiulion UnxlcgnidltLtin Reduction. procures Oxidation processes F.lcctrocbcnucaJ procissci

CancrDlritol d) fSlil IT solution DJIutcd dye.it uff solution e.g. Filling Of padtW C.&. wash in£ Ijallis

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Pretipiulion UnxlcgnidltLtin Reduction. procures Oxidation processes F.lcctrocbcnucaJ procissci

Figure 11 Treatment scheme for colored wasted water (from Ref. 54).
Figure 12 Flow scheme for indigo recovery in continuous yarn dyeing for denim (from Ref. 57).

oxidation cycle is repeated several times and the dyestuff is applied layer by layer. After the last oxidation passage the dyed material is washed and dried. Table 9 presents the typical data describing the production scale of such a dyeing unit. Two main difficulties exist at present:

• the indigo content in the wasted water, which causes colored wasted water;

• the sulfate or COD content in the washing water due to the use of dithionite or hydroxyacetone as reducing agents.

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