Rainwater does not contain any metal ions, which is natural soft water. But when rainwater seeps out from the surface, it can absorb metal ions from the soil and rocks which flows through. If rainwater flows through soft rocks such as chalk or limestone, it can dissolve these minerals. When more alkaline earth metals such as calcium and magnesium are dissolved, the hardness of water will become some harmful elements in the ice, which will affect the production process. Among them, the existence of calcium, magnesium, iron and manganese will cause serious problems.

About 50% of the defects in dyeing and finishing products are caused by poor water quality. Alkaline earth metals and heavy metal ions in water can react with other substances and cause defects. Because desizing, scouring, bleaching and dyeing, printing and finishing are all carried out in water before treatment, the root cause of the formation of defective fabrics needs to be removed from water quality treatment, while sequestering agent plays an important and irreplaceable role.

Reactive dyes, direct dyes and acid dyes containing sulfonic acid and carboxylic acid groups (water-soluble groups) may synthesize insoluble salts by complexing calcium and magnesium ions with two or more water-soluble groups. On the one hand, the solubility of dyes in water is reduced, forming the macromolecules to lead to the decrease of diffusibility of dyes to fibers, resulting in uneven dyeing, which also makes the color of dyes dark. Therefore, hard water is the main reason for dyeing irregularity and color fading of water-soluble dyes. The solution is to add sequestering agent in dyeing solution. However, if metal complexing dyes are encountered, it is not appropriate to select sequestering agent with high stability constant, so as to avoid the capture of heavy metals from metal complexing dyes, resulting in the change of color phase and the decrease of fastness of disperse dyes to dye at high temperature, and some groups (-NHCOCH3, -CH2CH20COCH3, -CH2CH2COOCH3, -CN and azo groups)  are prone to happen hydrolysis and reduction, resulting in light and color change. Cu2+, Fe2+ in dyeing bath promote the above reaction. Ca2+, Mg2+ in water can precipitate the anionic dispersant (lignosulfonate, naphthalene sulfonic acid formaldehyde condensate) coated on the outer layer of disperse dye particles and destroy their dispersion, resulting in condensation of dye particles, affecting dyeing, and reducing chrominance. The addition of sequestering dispersant agent is helpful to leveling dyeing, but the addition of sequestering agent with high temperature resistant in the dyeing bath of other dyes can prevent some anthraquinone disperse dyes, reactive dyes and acid dyes from discoloring and fading due to complex alloy ions, prevent the oxidation of vat dyes and eliminate the metal ions from their cryptochromes. The influence of metal ions on the color and light of sulfur dyes can also be prevented; the diazo salt of insoluble azo dyes can be destroyed by trace Cu2+ and Fe2+. The addition of sequestering agent can prevent the reaction of these metal ions; coating dyeing can prevent agglomeration between coatings particles due to the destruction of dispersants, resulting in instability of dyeing bath and reduction of levelness. The reactive dyeing, the removal of float are important steps to improve the fastness of dyes. The purpose of adding sequestering agent in soaping solution is to avoid the formation of insoluble deposits of float dyes with calcium and magnesium ions, which makes it difficult to remove float dyes, and make the float dyes re-stain on fabrics.