Table 6 Processing of Cotton Process Steps and Selected Parameters

Process step Critical parameter Desizing COD + BOD Scouring COD + BOD

Complexing agents

_Component_

Starch, modified starch, PVA, polyacrylates Organic load released from cotton and added auxiliaries

EDTA, phosphonates

pH

NaOH

Bleach

Hypochlorite AOX

Chlorinated compounds

Peroxide

Complexing agents

EDTA, phosphonates

Mercerization

pH

NaOH

Dyeing

Direct

Salt

NaCl, Na2SO4

Reactive

Color

Hydrolyzed dyes

Salt

NaCl, Na2SO4

pH

NaOH

Vat

PH

NaOH

Sulfate

Na2SO4, Na2SO3

Indigo

Color

Indigo

Salt

Na2SO4

Printing

Printing pastes

Concentrated chemical load

Washwater (COD, BOD,

Thickener, dyestuff

color)

Finishing

Filling of padder

Concentrated chemical load

Table 7 COD and BOD per Mass of Size Released

Type of size

COD C (mg/g) BOD B (mg/g)

Starch

900-1000

500-600

CMC

800-1000

50-90

PVA

1700

30-80

Polyacrylate

1350-1650

<50

Galactomannane

1000-1150

400

PES-dispersion

1600-1700

<50

Protein

1200

700-800

Source: Ref. 3.

Desizing of m=1000 kg of goods, which contain 5% of weight starch size (p=0.05) cause a load zcod=50 kg and zBOD=30 kg. Using 10 L of water for desizing of 1 kg of fabric, a total volume of 10,000 L will be required and the load Z,COD =50 kg will be diluted in this volume. As a result, a COD value of 5000 mg/L can be calculated for the effluent.

Two different paths can be followed to describe the behavior of sizes released in effluents:

• Biodegradation, which refers to the complete biodegradation of sizes like starch. Here high values of COD are coupled to high BOD.

• Bioelimination is detected by BOD, which is rather low BOD, compared to the COD. In such cases the polymer is removed from the waste stream in the WWT/CWWT by flocculation, adsorption, hydrolysis, and, to a certain degree, by biodegradation. Representatives are PVA, CMC, and acrylate sizes [16,17].

The strategies to handle size-containing wastes are dependent on the type of size and particularly on the technique of desizing (Fig. 5). In the case of starch, the desizing step is usually performed by enzymatic degradation, and in some cases oxidative degradation is used. However, the starch is degraded and a reuse is not possible in such cases. The disadvantage of a high COD caused by the released partially degraded starch is accompanied by easy biodegradation, thus the effluents can be treated in a WWT/CWWT with sufficient capacity for biodegradation with no further problems.

Figure 5 Desizing and treatment of size-containing wastes (from Refs 1824).

Water-soluble sizes permit a recycling of the polymer for further weaving processes. Various techniques have been proposed to regenerate sizes released from the fabric. General requirements that have to be considered as fundamentals for possible reuse of sizes are summarized as the following:

• easy and short distance transportation of recovered size to sizing/weaving plant;

• known composition of sizes;

• development of standardized recipes;

• stable composition of recovered size/no degradation.

In practice, a recycling of sizes is hindered for a number of reasons. In many cases various qualities of fabric containing different sizes are treated in a dyehouse and the type of size is often not known. The selection of sizes with regard to easy biodegradation/bioelimination is necessary. When a regeneration is intended a direct interaction between the selection of size, desizing procedure, recycling processes, and the sizing/weaving process have to be considered.

Two general technological strategies have been developed and proposed:

• removal of water soluble sizes by washing;

• reconcentration in the washing machine or by UF/evaporation. Figure 6 gives an overview of these two techniques.

Washing techniques have been proposed for PVA and acrylate sizes [18]. When applying washing techniques the volume of concentrated washwater for each size is limited by the volume actually spent in the following up sizing process (e.g., 900 L in Fig. 6) [19-21]. The use of higher amounts of water would increase the mass of recovered size, but the dilution of the regenerate is too much and hinders a reuse without reconcentration. A typical balance for a full process for acrylate sizes is shown in Figure 7 [22].

The advantage of UF techniques is the higher rate of size recovery, because a reduction of volume is possible. In some cases an evaporation step is used as final concentration step because the viscosity of the sizes increases and the permeate flow is reduced substantially. Problems can result from a change in the composition of the size due to changes in the molecular weight distribution as a result of the cutoff of the UF membrane. Attention has to be paid to avoid biodegradation of the recovered sizes, which changes the properties of the polymer and causes intensive odor of the regenerates.

Washing technique*

Walci soluble size

L000 kg <500ki

Squeezing pick-up v

Fabric 6WL

UF-mbniquri

Waier soluMe siw Washing

I Diluted solution 12 I SgfL

EtHoaoarintfaM S0-150 g/L UF. cvipnmliaii

Pcrmcale Water

Ritcniatc Recycling

Figure 6 Recycling of sizes (from Refs 18-24).

Figure 6 Recycling of sizes (from Refs 18-24).

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