Raw Water Requirements in Textile Printing and Dyeing & Coating Industries

The textile printing and dyeing and coating industries have extremely high requirements for raw water quality — if raw water contains impurities such as suspended solids, hardness ions, heavy metals, and organic matter, it will directly affect product quality (e.g., color differences in dyed fabrics, bubbling and peeling of coating films) and may even damage production equipment (e.g., pipe scaling, nozzle clogging). Therefore, the core goal of raw water treatment is to remove harmful impurities and ensure the water quality meets production process standards (e.g., dyeing water for textile printing and dyeing, spray/electrophoresis water for coating).

Core Processes for Raw Water Treatment in Textile Printing and Dyeing & Coating Industries

Raw water treatment requires adopting a stepwise process of "pretreatment + advanced treatment + precision treatment" based on water quality indicators (e.g., hardness, SS [Suspended Solids], COD [Chemical Oxygen Demand]) and production process requirements (e.g., "soft water zone" for textile printing and dyeing vs. "high-purity water zone" for coating). The specific processes and their functions are as follows:

1. Pretreatment: Remove Large-Particle Impurities to Protect Subsequent Equipment

Pretreatment serves as the "first line of defense" in raw water treatment. It mainly removes suspended solids, colloids, and part of organic matter to prevent clogging or contamination of subsequent precision equipment (e.g., reverse osmosis [RO] membranes, ion exchange resins).

• Core Process 1: Sedimentation/Coagulation
• Core Process 2: FiltrationCommon filtration units: Quartz sand filters / Activated carbon filters / Precision filters (security filters)

2. Advanced Treatment: Remove Hardness, Heavy Metals, and Dissolved Salts

Advanced treatment is critical for meeting the water quality requirements of core processes in textile printing and dyeing/coating. It mainly addresses issues related to "hardness" and "dissolved salts," with mainstream technologies divided into two categories: softening treatment (hardness removal) and desalination treatment (total salt removal).

(1) Softening Treatment: For Raw Water with "Low Salt Content and High Hardness"

• Process 1:Ion Exchange Method (Sodium-Type Softening)

  Raw water passes through the sodium ion exchange resin, where Na⁺ in the resin exchanges with Ca²⁺ and Mg²⁺ in the water, removing the hardness ions. The effluent hardness can be reduced to below 0.03 mmol/L (nearly no hardness).

• Process 2: Membrane Softening (Nanofiltration, NF)Utilizing the "charge repulsion effect" of NF membranes, it retains divalent ions (Ca²⁺, Mg²⁺) while having a low rejection rate for monovalent ions (Na⁺, Cl⁻). This not only removes hardness but also retains some beneficial ions, with effluent hardness ≤ 5 mg/L.

(2) Desalination Treatment: For Raw Water with "High Salt Content and High Hardness"

• Process 1: Reverse Osmosis (RO)
• Process 2: Ion Exchange Desalination (Dual Bed + Mixed Bed)

3. Precision Treatment: Ensure Water Quality Stability (Configured as Needed)

For water quality-sensitive processes (e.g., digital printing in textile printing and dyeing, high-end electrophoresis in coating), a precision treatment stage must be added after advanced treatment:

• Ultraviolet (UV) Disinfection: Kills bacteria and algae (e.g., E. coli [Escherichia coli]) in raw water to prevent microbial contamination of dyes/coatings, avoiding fabric mildew or microbial growth on coatings.
• Degassing Devices: Remove dissolved oxygen (O₂) or carbon dioxide (CO₂) from raw water to prevent oxidative corrosion of coatings in the coating process, or the formation of calcium carbonate precipitation caused by CO₂ combining with hardness ions in the textile printing and dyeing process.