Process Design for Decolorization and Desalination of Textile Dyeing Wastewater

I. Design Basis and Core Problem Analysis

1.1 Wastewater Sources and Characteristics

Textile dyeing wastewater is a typical high-concentration, refractory industrial wastewater. Its complexity primarily stems from the various dyes, auxiliaries, sizing agents, and chemicals used in processes like desizing, scouring, bleaching, dyeing, printing, and finishing. The core treatment challenges are centered on "color" and "salinity."

• High Color, Complex Composition: Contains large amounts of residual dyes (reactive, disperse, vat, sulfur, etc.), sizing agents, and surfactants, leading to extremely high color (thousands to tens of thousands of times). It also contains refractory aromatic and azo compounds.

• High Salinity: Dyeing and fixing processes heavily use inorganic salts (e.g., NaCl, Na₂SO₄), resulting in high wastewater TDS (Total Dissolved Solids), with conductivity potentially reaching 5000-15000 µS/cm or higher. This significantly impacts biological treatment efficiency and reuse potential.

• High COD, Poor Biodegradability: The BOD₅/COD ratio is typically low (0.2-0.3), containing many bio-inhibitory substances.

• Significant Fluctuations in Water Quality and Quantity: Closely related to production orders, colors, and fabric types, leading to strong shock loads.

• Variable Temperature and pH: Wastewater from some processes is high temperature, with a wide pH fluctuation range.

1.2 Design Objectives and Challenges

• Core Objectives:

  1. Deep Decolorization: Effluent color stably ≤ 20 times (meeting the indirect discharge limit in Table 2 of Emission Standard of Water Pollutants for Dyeing and Finishing of Textile Industry(GB 4287-2012) or stricter requirements).

  2. Efficient Desalination: Effluent TDS ≤ 1000 mg/L (suitable for reuse in some dyeing processes or strict sewer discharge), or achieving salt resource recovery/reduction through fractionation.

  3. Comprehensive Compliance: COD ≤ 60 mg/L, NH₃-N ≤ 8 mg/L, TN ≤ 12 mg/L.

• Core Challenges:

  • How to efficiently destroy chromophores while avoiding the introduction of new salts or interfering substances.

  • How to economically and efficiently remove high concentrations of inorganic salts and address the resulting high-salinity concentrate treatment problem.

  • How to construct a stable, shock-resistant treatment system adaptable to water quality fluctuations.

II. Core Process Route: "Advanced Oxidation for Color Breaking - Biological Enhancement - Membrane Desalination"

Addressing the dual challenges of "difficult color removal and salt removal," this scheme adopts an integrated process route: "Categorized Pretreatment - Advanced Oxidation for Color Breaking - Biological Mainstream Reduction - Membrane Graded Desalination - Concentrate Ultimate Disposal." The core lies in treating decolorization and desalination as two relatively independent yet synergistic objectives, achieving maximum resource recovery through graded reuse.

2.1 Full-Process Decolorization and Desalination Flow Diagram

III. Key Points for Technical Design of Critical Units

3.1 Advanced Oxidation Decolorization Unit (Core for Color Breaking)

• Function: Destroys chromophores in dye molecules (e.g., azo bonds, anthraquinone structures), oxidizes refractory macromolecules into smaller ones, improves biodegradability. This is key to achieving deep decolorization.

• Process Selection and Design:

  • Fenton Fluidized Bed Oxidation:

    • Advantages: Strong oxidation capability, high decolorization rate (>95%) for complex dyes (especially reactive, disperse dyes), relatively lower investment.

    • Design Points: Requires precise control of pH=3-4, H₂O₂/Fe²⁺ molar ratio, and reaction time. Using a fluidized bed carrier improves mass transfer efficiency and reduces iron sludge production. Effluent requires neutralization and sedimentation, producing chemical sludge.

  • Ozone Catalytic Oxidation:

    • Advantages: Fast reaction, no secondary pollution, high automation, no sludge production, suitable for connection to membrane systems.

    • Design Points: Uses supported metal oxide catalysts (e.g., MnO₂, CuO/Al₂O₃) to improve ozone utilization and ·OH yield. Ozone dosage 30-80 mg/L, contact time ≥30 minutes. Requires ozone generator and off-gas destructor.

3.2 Biological Enhancement Treatment Unit

• Function: Removes small organics and ammonia nitrogen after oxidation, ensuring stable COD, NH₃-N, and TN compliance, providing qualified feed water for subsequent desalination membrane systems.

• Process Design: "Hydrolysis Acidification + Two-Stage A/O-MBR".

  • Hydrolysis Acidification: Further improves biodegradability.

  • Two-Stage A/O-MBR: The MBR process replaces the secondary clarifier, offering very high sludge concentration, high nitrogen removal efficiency, and effluent with near-zero suspended solids. It effectively protects subsequent NF/RO membranes and is an ideal link between biological and membrane processes.

3.3 Membrane Desalination and Salt Fractionation Unit (Desalination Core)

• Function: Selectively or deeply removes salts based on reuse water quality requirements, achieving high-quality water reuse and salt concentration.

• Process Selection and Design:

  • Nanofiltration Desalination Route:

    • Applicable Scenario: Primarily used to remove divalent ions causing hardness and residual color (SO₄²⁻, Ca²⁺, Mg²⁺) and macromolecular organics. Has lower rejection (≈40-70%) for monovalent salts (NaCl).

    • Value: Product water salinity is moderately reduced, suitable for direct reuse in processes with lower salinity requirements like rinsing and dyeing. Operating pressure is lower, energy consumption is about 30-40% less than RO. NF concentrate is enriched with dye molecules and divalent salts, facilitating targeted subsequent treatment.

  • Reverse Osmosis Deep Desalination Route:

    • Applicable Scenario: Required when product water must be near-pure, for boiler feed, high-end dyeing, or approaching zero liquid discharge (ZLD).

    • Design Points: Use fouling-resistant brackish water membranes. Strict softening, silica removal, and antiscalant treatment are mandatory upstream to prevent membrane scaling. RO concentrate TDS can be concentrated to 20,000-50,000 mg/L, sent to the concentrate treatment unit.

  • Concentrate Treatment and Salt Fractionation:

    • Membrane Re-concentration: Further reduce RO concentrate volume using Disc Tube RO or Electrodialysis.

    • Thermal Fractional Crystallization: For high-concentration brine, use the "NF Salt Fractionation + MVR Evaporation Crystallization" process. NF separates sodium sulfate from sodium chloride; MVR crystallizes them separately to produce salt cake (Na₂SO₄) and industrial salt (NaCl). This achieves mixed salt resource recovery, a key to solving the salt sludge disposal problem and improving project economics.

IV. Main Design Parameters and Economic Analysis

4.1 Key Unit Design Parameters (Example: 2000 m³/d Treatment Scale)

4.2 Economic Analysis

• Total Capital Estimate:

  • Conventional "Advanced Oxidation + Biological + NF" reuse route: ≈ 8 - 12 million RMB.

  • "Advanced Oxidation + Biological + RO + Evaporation Crystallization" near-ZLD route: ≈ 20 - 35 million RMB. The main cost difference lies in the RO system, evaporation crystallization, and salt fractionation equipment.

• Operating Cost (Unit: RMB/ton of water):

  Process Route	Direct Operating Cost	Composition Notes
  Oxidation + Biological + NF Reuse	3.5 - 5.5	Electricity (1.0-1.5), Chemicals (1.5-2.5, mainly oxidants), Membrane Replacement (0.5-1.0), Maintenance & Labor (0.5)
  Oxidation + Biological + RO + Evaporation Crystallization	8 - 15	Electricity (3.0-5.0, MVR high consumption), Steam/Heat (1.0-2.0), Chemicals (2.0-3.0), Maintenance & Depreciation (2.0-5.0)

• Benefit Analysis:

  • Reuse Benefit: NF/RO permeate reuse can save >70% in freshwater costs and equivalent discharge fees. At a water price of 5 RMB/ton, annual savings ≈ 2.5 million RMB (2000 tons/day).

  • Resource Recovery Benefit: Sale of salt cake and industrial salt from fractional crystallization can partially offset the high operating cost of the evaporation crystallization unit.

  • Environmental & Social Benefit: Stable compliance mitigates environmental risk; enables water resource recycling, enhancing corporate green image.

V. Conclusion and Operational Recommendations

This integrated "Advanced Oxidation for Color Breaking - Membrane Desalination" scheme provides a systematic technical pathway for achieving deep decolorization, efficient desalination, and resource recovery in textile dyeing wastewater treatment. Fenton/Ozone oxidation is the powerful tool for color breaking, NF/RO membranes are the core for desalination and reuse, and NF salt fractionation coupled with MVR evaporation is the key to achieving salt resource recovery and overcoming the ZLD bottleneck.

Keys to Successful Operation:

1. Source Segregation and Reduction: Strive for separate collection and pretreatment of high-concentration dye liquors and printing pastes to reduce color and salinity load at the source.

2. Precise Control of Oxidation Processes: Dynamically optimize oxidant dosage and reaction conditions based on changes in influent dye types, balancing decolorization effectiveness with cost control.

3. Comprehensive Protection of Membrane Systems: Biological effluent must undergo precision pretreatment like UF to ensure SDI<3; RO systems require strict softening and antiscalant treatment—this is the lifeline for long-term stable membrane operation.

4. Explore Concentrate Disposal Pathways: Actively engage with industrial parks or regional facilities to explore possibilities for co-disposal of concentrate. If self-disposal is necessary, the fractional crystallization process route and product market must be planned in advance.

This scheme features a complete technological chain, combining technical advancement with engineering feasibility. It is a preferred technical solution for the textile dyeing industry to achieve green transformation and address increasingly stringent environmental and water resource challenges.