Decolorizing Agents for the Textile and Dyeing Industry — Effective & Cost-Effective

Date: · Author: Hóa chất Lộc Thiên

Introduction

The textile and dyeing industry is the number one “culprit” when it comes to color pollution in industrial wastewater. A medium-sized textile and dyeing plant can discharge 2,000–5,000 m³ of wastewater per day with a color intensity of up to 3,000–5,000 Pt-Co, COD ranging from 800–3,000 mg/L, and high alkalinity due to the use of caustic soda and soda ash in the dyeing process.

Treating color in textile and dyeing wastewater is not as simple as “just adding PAC and calling it a day.” In this article, Loc Thien Chemicals will provide an in-depth analysis of the unique characteristics of textile dyeing wastewater and specialized decolorization solutions—helping you meet discharge standards while optimizing chemical costs.

Characteristics of Textile Dyeing Wastewater — Why Are PAC and FeCl₃ Insufficient?

Textile dyeing wastewater has four characteristics that make traditional inorganic coagulants (PAC, aluminum sulfate, FeCl₃) less effective:

1. Dyes exist in a true solution state

Unlike total suspended solids (TSS), dye molecules are completely dissolved in water at the ionic/molecular level, with extremely small sizes (1–5 nm). PAC and FeCl₃ primarily function through the “sweep floc” mechanism—effective for colloidal particles >100 nm but virtually “powerless” against truly soluble dye molecules.

2. Strong and Stable Negative Surface Charge

Reactive dyes, acid dyes, and direct dyes—the three groups accounting for >70% of textile dye production—all carry a sulfonate group (-SO₃⁻) with a strong negative charge. These groups create strong electrostatic repulsion between molecules, preventing them from naturally aggregating. PAC (with a +3 charge on Al³⁺) is not strong enough to completely neutralize the “charge cloud” surrounding each dye molecule.

3. Stable aromatic ring structures

Dye molecules have azo (-N=N-), anthraquinone, or phthalocyanine structures—resonant aromatic ring structures that are chemically very stable, resistant to biodegradation, and resistant to even some common oxidizing agents.

4. High pH and Temperature

Textile dyeing wastewater typically has a pH of 9–12 (from the alkaline dyeing and caustic washing stages) and a temperature of 40–60°C. PAC loses significant effectiveness at pH >8 (Al³⁺ converts to soluble aluminate Al(OH)₄⁻). FeCl₃ forms Fe(OH)₃ precipitates at high pH, but the resulting flocs are small, difficult to settle, and leave a yellow-brown color in the effluent.

Results: Many factories add PAC at doses of 500–1,000 mg/L, yet the color value in the effluent remains >200 Pt-Co, exceeding the permissible limit. This is precisely when specialized decolorizing agents for the textile dyeing industry are needed.

Specialized Decolorizing Agents for Textile Dyeing

Type 1: Strong Cationic Decolorizing Polymers (PolyDADMAC / Modified Polyamine)

This is the “main weapon” for textile dyeing wastewater:

• Composition: Polydiallyldimethylammonium chloride (PolyDADMAC) or modified polyamine with a very high positive charge density
• Mechanism: Each polymer unit carries a single positive charge (+), forming a long “cationic chain”—5–10 times more effective at neutralization than PAC per unit mass
• Advantages:
  • Low dosage (50–200 mg/L compared to 500–1,000 mg/L for PAC)
  • Generates 40–60% less sludge than PAC
  • Performs well over a wide pH range (4–9)
  • Decolorization efficiency of 85–95% after flocculation and sedimentation
• Limitations: Higher cost per kg than PAC; sensitive to certain competing anions (high concentrations of SO₄²⁻)

Type 2: Modified Melamine-Formaldehyde-Based Decolorizing Agent

A specialized product line for reactive dyes and reduction dyes:

• Mechanism: The melamine group forms hydrogen bonds and π-π interactions with the aromatic ring of the dye
• Efficiency: 90–98% decolorization for reactive dyes (Reactive Black 5, Reactive Red 120, etc.)
• Suitability: Extremely dark wastewater (>5,000 Pt-Co)

Type 3: Modified Tannin-Based Decolorizing Agents (Bio-based)

New trend — decolorizing agents derived from cationically modified plant tannins:

• Advantages: Biodegradable, non-toxic, environmentally friendly
• Performance: Better than PAC but lower than synthetic polymers (70–85%)
• Suitable for: Green-certified facilities (OEKO-TEX, bluesign), exports to the EU

Typical Case Study: Textile Dyeing Plant with a Capacity of 3,000 m³/day

Note: Data is based on a typical operating model and does not represent measurements from a specific plant. Actual performance depends on wastewater composition and operating conditions, and requires a jar test for verification.

Cost-Benefit Analysis:

1. 32% reduction in monthly chemical costs: Although the unit price per kg of polymer is ~3–5 times higher than that of PAC, the dosage is 4–7 times lower, resulting in a reduction in the amount of PAC required.
2. 43% reduction in sludge volume: The cost of treating hazardous sludge ranges from 2 to 5 million per metric ton—resulting in additional savings of tens of millions per month.
3. Stable output: Color <50 Pt-Co meets QCVN 40:2011 Column A (Kq=0.9; Kf=1.0) — no risk of fines.

Optimized Decolorization Process for Textile Dyeing

Inlet wastewater
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[1] Equalization tank ─── maintain pH 7–8 (use H₂SO₄ or CO₂ if pH is too high)
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[2] Rapid Coagulation Tank ─── Cationic polymer 100–200 mg/L, agitation at 200 rpm for 2–3 minutes
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[3] Flocculation tank ─── Agitation at 30–40 rpm for 10–15 minutes (+ PAC at 150–300 mg/L if additional flocculation is needed)
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[4] Settling tank ─── Retention time 2–4 h, surface load 0.8–1.2 m³/m².h
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[5] Fine treatment ─── Sand filtration / Activated carbon / Ozone (if color <20 Pt-Co is required)
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[6] Biological reactor ─── Treatment of remaining dissolved COD
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Effluent meets standards

FAQ — Quick Questions, Concise Answers

Q: My plant primarily uses reactive dyes—which decolorizing agent should I choose? A: Reactive dyes carry a very strong negative charge—you should prioritize strong cationic polymers such as PolyDADMAC or modified melamine-based decolorizing agents. Lộc Thiên Chemicals can provide sample kits so you can conduct a jar test directly on your factory’s wastewater.

Q: Do polymer decolorizing agents affect the microorganisms in the downstream biological treatment tank? A: When used at appropriate dosages, cationic polymers are not toxic to microorganisms. However, if there is an excess of polymer (overdose), the excess cations may cause inhibition. Always control the dosage through jar-testing and monitor the MLSS and SVI parameters in the biological reactor.

Q: Is it necessary to adjust the pH before using the polymer decolorizer? A: Cationic polymers work well at a pH of 4–9, but for optimal flocculation and to protect the downstream biological reactor, the pH should be adjusted to 7–8. If the pH is >10, use H₂SO₄ or CO₂ to lower it.

Q: Can I use a polymeric decolorizing agent without PAC? A: In many cases—yes. For textile wastewater with moderate color intensity (<2,000 Pt-Co), cationic polymer alone can achieve an efficiency of 85–90%. However, for very intense colors, combining polymer and PAC yields higher and more stable results.

Conclusion

Treating color in textile dyeing wastewater is not a matter of “expensive or cheap”—it’s a matter of “right or wrong.” Using the wrong decolorizing agent means you’ll waste money on chemicals, incur additional costs for sludge treatment, and face legal risks. Use the right one—and you’ll reduce costs, minimize sludge, and ensure stable output.

Call 0979 891 929 to have Loc Thien Chemicals send you free samples of decolorizing agents and provide consultation on the optimal solution for your textile dyeing plant.