It is generally believed that water hardness below 50 ppm has no adverse effect, and water with a hardness of 20 ppm is considered of high quality.

Such water quality is also acceptable for cone yarn dyeing, but attention should be paid to the content of copper and iron ions in the water.

Typically, a hardness below 50 ppm is optimal for reactive dyeing, with the key factor being the metal ion content—especially iron ions, which can cause shade variation. It is recommended to test the water’s conductivity.

A primary cause of dye streaking when using discharge-standard reclaimed wastewater for bleaching and dyeing processes is excessively high water conductivity. Therefore, conductivity must be closely monitored when reusing bleaching and dyeing wastewater. Conventional wastewater treatment methods cannot effectively remove the metal ions that elevate conductivity; reverse osmosis membrane treatment is required for their removal.

Excessively high water hardness leads to scale formation on boiler walls, and calcium and magnesium ions in the water can cause soap precipitation. This results in uneven dyeing, poor fabric hand feel, and fabric yellowing. Additionally, it reduces the activity of desizing enzymes, decreases the solubility of sizing materials, and causes incompatibility with chemicals in finishing formulations.

As for treatment methods, a dosage of 2g/L of EDTA or sodium tripolyphosphate can be tried, but using a specialized metal ion sequestering agent yields better results. If unsoftened water is used for dyeing, or if heavy fabrics contain inherent metal ions, uneven dyeing defects often occur. Thus, chelating dispersants should be added during dyeing to mitigate metal ion interference and enhance dye removal efficiency.

However, chelating dispersants such as EDTA (Ethylenediaminetetraacetic Acid), DTPA, NTA, phosphates, and gluconic acid may chelate metal ions from the dye molecules, leading to reduced light fastness and color fading. For this reason, some industry professionals recommend using polyphosphate or polyhydroxy acid-based chelating dispersants.

A factory currently uses untreated groundwater for cotton dyeing, without any softening or filtration processes. Previously, a noticeable layer of sludge was visible on the dyed cone yarns. After adding a small amount of bleaching powder to the water, some improvement was observed, but the dyed cone yarns still exhibited a harsh hand feel on the inner layers, along with dull and lighter shades. Despite adjustments to machine rotation directions and the addition of water softeners, the results remained unsatisfactory. Further investigation revealed that during the pre-treatment scouring stage, stratification issues had already emerged, with the water hardness measuring approximately 130 ppm.

Excessively hard groundwater containing high levels of impurities caused contamination on the inner layers of the dyed cone yarns. The optimal solution is to pump groundwater to the surface for sedimentation and softening, followed by filtration. This approach will completely resolve the issue, though it requires a relatively high initial investment.

Cationic softener working solutions are unstable in river water, as they flocculate with microorganisms present in the water, forming yellow flocs that adhere to fabrics. After drying and heat setting, these flocs develop into yellow stains on the fabric surface. This explains why quality issues related to yellow staining are particularly prevalent in May and June, when temperatures are ideal for algal growth, while such problems rarely occur in autumn and winter.

Water quality significantly impacts the stability of softener working solutions and the whiteness of fabrics after soft finishing. Therefore, printing and dyeing factories should prioritize using treated water in production.

Some manufacturers still encounter similar issues even when using activated carbon-filtered water, because microorganisms are too small to be fully adsorbed and filtered by activated carbon. To enhance treatment effectiveness, cationic surfactants can be added to the sedimentation tank prior to filtration, causing microorganisms to flocculate; subsequent activated carbon filtration will then remove these flocs more efficiently. Additionally, activated carbon must be regularly cleaned or replaced.

If the aforementioned methods are unfeasible or fail to eliminate the problem, it is recommended that manufacturers switch to non-ionic or anionic softeners, or use a blend of non-ionic softeners and silicone oil for hand feel finishing during periods of heavy algal growth.

Manufacturers experiencing frequent yellow staining issues have reported that using amino silicone oil for finishing effectively prevents such problems.

Winter is also a challenging season for printing and dyeing factories due to generally poorer water quality during this period.

For reactive dyeing, maintaining a pH range of 7–9 results in normal color consistency, indicating that this pH range has minimal impact on shade variation. The presence of trace alkalinity does not alter or damage the chromophores of reactive dye molecules.

When the pH value reaches 8.5, the fabric’s soap fastness and wet rubbing fastness only reach Grade 3, failing to meet customer requirements. As the pH value increases further, color fastness deteriorates significantly. This demonstrates that elevated alkalinity in water negatively impacts the color fastness of dyed products.

For wastewater reuse systems, the influent water typically requires a pH < 9 to ensure that microorganisms in the treatment system are not harmed by excessive alkalinity and can function properly. Meanwhile, the quality standard for reclaimed water specifies a pH value of 6.5–8.5, with a neutral pH (near 7) being optimal.

The primary salt present in wastewater reuse systems is sodium sulfate, which originates from two sources:

The maximum tolerable salinity in reuse systems is 924 mg/L, which does not negatively affect fabric dyeing quality.

During pre-treatment, iron ions accelerate the decomposition of hydrogen peroxide, leading to the formation of holes on the fabric surface.

Iron ions also cause dullness and specks on dyed fabrics. Regarding removal methods, first test the iron ion content—levels below 15 ppm are generally harmless. If concentrations are slightly higher, chelating agents can be added to address the issue.

Chloride ions alone have no adverse effect on reactive dyeing, as common salt (sodium chloride) is actually used as a dyeing accelerator in the process. However, chloride ions can affect dyeing machine cylinders, depending on whether the steel used in the cylinders is chloride-resistant.

In the case of tap water, which contains residual chlorine for disinfection purposes, concentrations exceeding specified limits will negatively impact dye molecules.