Design for Decolorization and Compliance in Color Printing Wastewater Treatment Process Scheme

I. Design Basis and Core Problem Identification

1.1 Wastewater Characteristics and Treatment Challenges

Color printing wastewater primarily originates from processes such as press cleaning, fountain solution replacement, ink preparation, plate washing, and workshop cleaning. Its water composition is complex, making it a typical refractory industrial wastewater. The core treatment challenges focus on the deep removal of "color" and "composite pollutants":

• High Chroma, High COD: Contains large amounts of residual dyes, pigments, binders, organic solvents, etc., resulting in extremely high color (up to thousands of times) and high Chemical Oxygen Demand (COD), including refractory macromolecular organics.

• Poor Biodegradability: The BOD₅/COD ratio is typically low (0.2-0.4), limiting the efficiency of direct biological treatment.

• Significant Fluctuations in Water Quality and Quantity: Closely related to print orders, color types, and cleaning frequency, resulting in strong shock loads.

• Presence of Heavy Metals and Additives: Some inks may contain heavy metal ions like lead, chromium, cadmium, and various surfactants, defoamers, etc.

• High Compliance Pressure: Must stably meet the Class III standard of the Integrated Wastewater Discharge Standard(GB 8978-1996) or the stricter indirect discharge limits of the Emission Standard of Water Pollutants for Dyeing and Finishing of Textile Industry(GB 4287-2012), where color (≤50 times) and COD (≤500 mg/L) are key restrictive indicators.

1.2 Design Objectives and Principles

• Core Objectives:

  1. Efficient Decolorization: Ensure effluent color is stably ≤ 40 times (meeting or bettering the 50 times standard, with margin).

  2. Deep COD Reduction: Ensure effluent COD is stably ≤ 80 mg/L (meeting indirect discharge or stricter requirements).

  3. Comprehensive Compliance: Simultaneously control other indicators like pH, suspended solids, ammonia nitrogen, and total phosphorus to meet standards.

• Design Principles:

  • Enhanced Pretreatment: Focus on efficient physicochemical decolorization to break chromophores and improve wastewater biodegradability.

  • Biological Core: Employ efficient, shock-resistant biological processes for main organic degradation.

  • Depth Safeguard: Incorporate "polishing" units like advanced oxidation to handle fluctuations and ensure absolute compliance.

  • Economical and Reliable: Select mature, cost-controllable process combinations for stable operation.

II. Core Process Route: "Three-Stage Decolorization, Two-Stage Biological, One-Stage Safeguard"

Addressing the pain points of "difficult color removal and COD reduction" in color printing wastewater, this scheme establishes the main technical route of "Physicochemical Pretreatment (Color Breaking/Enhancement) — Biological Treatment (COD Reduction/Denitrification) — Advanced Oxidation (Compliance Safeguard)". The core lies in achieving staged, efficient removal of color and COD through multi-level, complementary technical means.

2.1 Full-Process Core Process Design Diagram

The diagram below clearly illustrates the complete technical path from wastewater collection to compliant discharge, highlighting the core role of each unit in decolorization and compliance.

III. Key Design Points for Core Unit Decolorization and Compliance Technology

3.1 Pretreatment Unit (Core of Physicochemical Decolorization)

• Function: Serves as the "first barrier" of the entire system, primarily aiming to rapidly and efficiently remove most color, suspended solids, colloids, and partial COD, significantly reducing the subsequent biological load and improving wastewater biodegradability.

• Core Technology: High-Efficiency Coagulation Flotation.

  • Process Principle: Dosing Demulsifier, Coagulant, and Flocculant. Hydrophobic dye particles, ink particles, colloids, etc., are rapidly separated as scum via adhesion to micro-bubbles.

  • Design Key Points:

    1. Chemical Screening and Optimization: Determine the most effective Composite Decolorant and PAC/PAM combination and dosage for the target wastewater through testing. pH is typically adjusted to 7-9.

    2. High-Efficiency Flotation Equipment: Use dissolved air flotation or cavitation air flotation to ensure fine bubbles and good separation. This unit can remove 60-80% of color and 30-50% of COD.

3.2 Biological Treatment Unit (Main COD Reduction and Biological Decolorization)

• Function: Utilizes microorganisms to degrade soluble organics, further reducing COD and color, and achieving nitrogen and phosphorus removal.

• Core Technology Combination:

  • Hydrolysis Acidification Tank: Breaks down refractory macromolecules like dyes and additives into smaller organics, increasing the wastewater B/C ratio, creating favorable conditions for subsequent aerobic treatment, while providing some biological decolorization.

  • A/O (Anoxic/Oxic) Process:

    • Anoxic Zone: Performs denitrification for nitrogen removal and also has a reductive decolorization effect on some azo dyes, etc.

    • Oxic Zone: Uses activated sludge or biological contact oxidation. With sufficient aeration, aerobic microorganisms thoroughly oxidize and decompose organic matter. This unit is the core for COD removal and further reduces color.

3.3 Advanced Treatment Unit (Ultimate Safeguard for Compliance)

• Function: Serves as the "polishing" and "safeguard" unit after biological treatment, completely destroying residual refractory chromophores and COD, ensuring effluent stably meets standards under any conditions.

• Core Process Selection:

  • Ozone Catalytic Oxidation (Preferred Recommendation):

    • Advantages: Ozone, under the action of a catalyst, produces highly oxidative hydroxyl radicals, which can non-selectively oxidize and decompose organics, achieving excellent decolorization and COD reduction, with no secondary pollution and high automation.

    • Design Points: Control ozone dosage (20-50 mg/L), contact time (≥30 minutes), catalyst selection (supported metal oxides).

  • Fenton Fluidized Bed Oxidation (Alternative):

    • Advantages: Strong oxidation capability, relatively lower investment, suitable for situations with large water quality fluctuations.

    • Design Points: Requires precise control of pH (3-4), H₂O₂ to Fe²⁺ ratio, and supporting neutralization-sedimentation facilities, generating chemical sludge.

3.4 Sludge Treatment Unit

• Design Key Points: Physicochemical sludge from pretreatment and advanced treatment is mixed with excess biological sludge, thickened, and dewatered using a High-Pressure Plate and Frame Filter Press to achieve cake moisture content ≤60%. This sludge has complex composition and must be managed as hazardous waste, disposed of by a licensed entity.

IV. Key Design Parameters and Operational Control

4.1 Core Unit Design Parameters (Example Scale: 100 m³/d)

4.2 Intelligent Control and Operational Optimization

• Online Monitoring: Install online pH, ORP, Colorimeter, COD Analyzer at flotation effluent, biological effluent, and final discharge point for real-time feedback.

• Automatic Dosing: Automatically adjust dosages of coagulant, oxidant, acid/alkali based on influent flow and online water quality signals for precise control and chemical savings.

• Warning and Emergency: Set alarm thresholds for key parameters; automatically trigger alarms and initiate emergency dosing or recirculation procedures upon abnormality.

V. Techno-Economic Analysis

• Capital Cost Estimate: Core treatment system (excluding civil works) investment is approximately 0.8 - 1.5 million RMB. Main cost components: flotation equipment, biological system, ozone generator, filter press, control system.

• Operating Cost: 8-15 RMB per ton of water.

  • Power Cost: 2-4 RMB/ton (flotation, blowers, pumps, ozone generator).

  • Chemical Cost: 4-8 RMB/ton (coagulant, oxidant, acid/alkali, nutrients).

  • Sludge Disposal Cost: 1-2 RMB/ton.

  • Equipment Depreciation & Maintenance: 1-2 RMB/ton.

• Benefits: Ensures corporate environmental compliance, avoids production suspension risks and high fines; partial reuse of treated water for initial rinsing saves freshwater; enhances corporate green image.

VI. Conclusion

The combined process of "High-Efficiency Coagulation Flotation Decolorization + A/O Biological Degradation + Ozone Catalytic Oxidation Safeguard" proposed in this design scheme is an effective and reliable solution for the challenges of decolorization and compliance in color printing wastewater. Enhanced pretreatment is the foundation, efficient biological treatment is the core, and advanced oxidation is the safeguard, with the three steps interlinked. Through refined design, optimization of key parameters, and intelligent control, effluent indicators such as color and COD can be ensured to stably meet the strictest discharge standards in the long term, achieving a balance of environmental and economic benefits. Systematic water quality testing and pilot verification are strongly recommended before implementation to determine the optimal chemical formula and process parameters.