Textile Industries are consuming large quantity of water. Major portion of water (60-70%) is used for wet processing of textiles. Although it is difficult to state definitive water demand for various processes, the raw material used in the greatest quantity in virtually every stage of textile wet processing is water. The quality of textiles produced by any manufacturing operation which employs wet processes, such as preparation, dyeing and finishing, is profoundly affected by the water quality.

Various textile processes are influenced in different ways by the presence of impurities in the water supply and there are several major water use categories to be considered including water for processing, potable purposes, utilities, and laboratory use. Each requires different water-quality parameters. Process water (for preparation, dyeing, and finishing) is to be mainly used for making concentrated bulk chemical stock solutions, substrate treatment solutions, and washing. Potable water is for drinking and food preparation. Utility use includes non-contact uses such as boiler use, equipment cleaning etc.

Water from almost all supply sources contains impurities to some extent. The type and amount of impurities depend upon the type of water source. The most common impurities that may contain in water are as follows :

• Calcium and magnesium (hardness)



• Heavy metals, such as iron, copper, and manganese



• Aluminium



• Chlorine



• Miscellaneous anions (sulphide, fluoride, etc.)



• Sediments, clay, suspended matter



• Acidity, alkalinity, and buffers



• Oil and grease



• Dissolved solids

There are many quick qualitative tests for detection of trace quantities of ions and elements in water. There are also quantitative tests for determining the exact concentration of cations such as calcium, magnesium, iron, copper, and manganese in water. Water contaminants, especially metals, can have a substantial effect on many textile wet processes. The effects are not always adverse but even when a process is enhanced by water impurities, it is not desirable to have variance in processes and product quality due to water quality changes. Such variations in the quality of water make process and machinery optimisation and control difficult.

Problems in the Textile Laboratory due to poor water quality :

It is a common practice in some mills to use potable water for the laboratory supply while using non-potable water for production processing. Since potable water is usually chlorinated, it can alter the shade of dyeings and contributes to poor lab-to-bulk reproducibility. Moreover, most work in analytical laboratories is done with distilled and/or deionized water. However, many situations arising in textile wet processing laboratories will require the use of process water in order to correlate well with production. The laboratory technician must be able to realize when to use process water and when to use distilled or deionized water.

Problems in Preparation Processes due to poor water quality:

Metallic ions in water can have a dramatic effect by either enhancing or inhibiting the action of many preparation processes. All of the wet preparation processes are affected in some way by metallic ion contaminants in water. In enzymatic desizing, the metallic ions may cause inactivation of the enzymes, resulting in poor size removal. These ions will precipitate soaps, forming a sticky insoluble substance which deposits on the substrate. Such deposits impair the fabric handle, cause resist in dyeing, attract soil to the material and cause inconsistent absorbency in subsequent processes.

Although most synthetic detergents used in scouring today do not precipitate in the presence of calcium and magnesium ions, the fatty acid hydrolysis products formed by the saponification of natural waxes, fats, and oils in the fibres will precipitate.The formation of complexes with alkaline and alkaline earth salts drastically reduces the solubility and the rate of dissolution of surfactants, thus impairing the wash removal ability of the surfactants. It is, therefore, imperative to use soft water in the scouring process.

Bleaching with hydrogen peroxide is greatly affected, even by trace quantities of metal ions in the water. The transition metal ions such as iron, copper, manganese, zinc, nickel, cobalt and chromium catalyze decomposition of hydrogen peroxide. The decomposition is so rapid that it frequently occurs before any significant bleaching can occur. In addition, the decomposition products attack cotton fibres leading to their degradation. Bleaching baths containing these ions will therefore lead to reduction in whiteness and high loss in fibre strength, as well as an increase in fluidity. The alkaline earth metal (magnesium), on the other hand, produces beneficial effects when present in peroxide bleaching solutions. These ions increase the stability of hydrogen peroxide under alkaline bleaching conditions, and as a result increased whiteness and less fibre degradation is obtained. Electrolytes of other metals may have a harmful effect.

Problems in Dyeing Processes due to poor water quality:

The most commonly observed dyeing problems caused by poor water quality include inconsistent shade, blotchy dyeing, filtering, spots, resists, poor washing off, and poor fastness. Inconsistent shade can be caused by chlorine contamination of the process water or iron, copper and other metals. The action of copper on the dyestuff can be prevented by a suitable complexing agent but not the action of iron. For iron, purification of water prior to dyeing is recommended. Chelating agents are frequently used in an attempt to eliminate the undesirable effect of these metals in process water, but in many cases, the chelate itself may cause unpredictable effects such as shade changes. The best strategy is to remove the metal from water before using it in processing.

The presence of calcium and magnesium ions in the process water can cause inconsistent and uneven washing-off of unfixed dyes, leading to blotches, and/or inconsistent shade. Hexametaphosphates are effective sequestering agents for removing these ions and are generally safe in the sense that they do not cause other undesirable effects such as shade variations.

Blotchy dyeing can result from acidity or alkalinity in the water, depending upon the application class of dyes. Even when the pH is neutral, water (and substrate) may contain substantial alkalinity. This can have effects on exhaustion, levelling and fixation of dyes. Similar types of defects can result from the residual chemicals, especially alum (aluminium) in water.

Filtering in package dyeing, resists and spots can result from sediments, alum or other residual flocking agents left over from water treatment, from organic contaminants, from metal hydroxides (copper and iron), or from fatty acid/hardness metal complexes. Generally, the stiffness of textile material dried after rinsing is greater, the higher the solids content of the rinsing water.

In order to avoid the problems outlined above, water for textile processing has to meet the following demands :

• Freedom from suspended solids and from substances that can give staining in processing



• No great excess of acid or alkali



• Neutral pH



• Freedom from substances affecting the textile processes, such as iron, manganese, Calcium or magnesium salts, and heavy metals



• Non-corrosiveness to tanks and pipelines, and



• Freedom from substances that give rise to foaming or unpleasant odour

Various measures and treatments may be employed in order to remove impurities from water and to avoid problems in textile processing, such as follows:

• Sedimentation and filtration treatments



• Softening treatments



• Reverse osmosis.



• Use of sequestering agents.

Source: Various references