Process Design Plan for Oily Wastewater Treatment in the Dyeing and Printing Industry

I. Design Basis and Objectives

1.1 Source and Characteristics of Wastewater

Oily wastewater in the dyeing and printing industry primarily originates from processes such as desizing, scouring, dyeing, printing, and finishing. It has a complex composition with the following key characteristics:

• Complex Water Quality: Contains residual dyes, sizing agents (e.g., PVA), various auxiliaries (wetting agents, leveling agents, softeners, etc.), fiber impurities, and mineral/vegetable/animal oils.

• High Chroma: Dyes cause high color intensity, leading to severe visual pollution, with some dyes being resistant to biodegradation.

• High COD: High concentration of organic matter, with Chemical Oxygen Demand (COD) typically ranging from 800-3000 mg/L, or even higher.

• Oil Content: Fluctuating oil content (50-500 mg/L), often in an emulsified state, which can impede oxygen transfer and biological treatment efficiency.

• Variable Biodegradability: Typically has a low B/C ratio (BOD5/COD) of 0.2-0.4, indicating relatively poor biodegradability.

• Significant Fluctuations in Quality and Quantity: Varies with production orders and process changes.

1.2 Design Influent and Effluent Quality (Example)

1.3 Treatment Objectives

To ensure that after treatment, all wastewater indicators stably meet national and local discharge standards, possess potential for reuse, and achieve economical and reliable operation.

II. Selection of Treatment Process Route

Addressing the characteristics of "high chroma, high COD, oily, and generally moderate biodegradability," the overall technical route is determined as: "Pretreatment (Demulsification/Oil Removal & Conditioning) + Main Biological Treatment (Anaerobic + Aerobic) + Advanced Treatment (Decolorization/Degradation)."

The core principles are:

1. Enhanced Pretreatment: Efficient demulsification, removal of most suspended solids and oils, improvement of wastewater biodegradability, creating favorable conditions for biological units.

2. High-Efficiency Biological Treatment: Utilizing anaerobic processes to enhance biodegradability and degrade macromolecular organics, followed by aerobic processes for further COD and BOD removal.

3. Guaranteed Decolorization: Employing chemical or biological methods to ensure stable compliance with chroma standards.

III. Recommended Process Flow and Description

The recommended process flow diagram is as follows:

flowchart TD

3.1 Pretreatment Unit

• Bar Screen: Removes larger floating solids and fibers from the wastewater to protect downstream pumps and equipment.

• Equalization Tank: Equalizes water quality and quantity. Equipped with a cooling system (e.g., cooling tower) to control influent temperature below 35-40°C, meeting requirements for biological units. Aeration or mixing devices are installed to prevent sedimentation.

• Coagulation Air Flotation (CAF) System: Core pretreatment unit. Demulsifier (for emulsified oils), coagulant (PAC, Poly Aluminum Chloride), and flocculant (PAM, Polyacrylamide) are dosed. Air flotation generates numerous micro-bubbles that adhere to oil droplets and flocs, causing them to rapidly float and separate. This efficiently removes oils, colloids, and partially removes COD and color. This unit can remove over 60% of oils and 30-50% of COD.

3.2 Main Biological Treatment Unit

• Hydrolysis Acidification Tank: Subjects the CAF effluent to anaerobic treatment here. No aeration is provided. Hydrolytic and acidogenic bacteria decompose refractory macromolecular organics (e.g., dyes, PVA) in the wastewater into smaller, soluble molecules, significantly increasing the wastewater's B/C ratio and creating favorable conditions for subsequent aerobic treatment. It also provides some decolorization.

• Biological Contact Oxidation Tank: Core degradation unit. The tank is filled with combined packing media supporting a rich biofilm of microorganisms. Blower aeration supplies oxygen, enabling microbes to decompose organic matter into CO2 and H2O. This process offers strong resistance to shock loads, low sludge production, and high treatment efficiency, effectively removing most COD and BOD.

• Secondary Sedimentation Tank: Achieves solid-liquid separation. The clarified effluent proceeds to advanced treatment. A portion of the sludge is recycled to the biological contact oxidation tank to maintain biomass, while excess sludge is sent to the sludge handling system.

3.3 Advanced Treatment and Decolorization Unit

To ensure stable compliance with COD and chroma standards, the secondary sedimentation tank effluent requires advanced treatment.

• Recommended Process: Fenton Fluidized Bed Oxidation or Ozone Catalytic Oxidation.

  • Fenton Fluidized Bed: Under acidic conditions, utilizes the reaction between Fe²⁺ and H₂O₂ to produce highly oxidative hydroxyl radicals (·OH), effectively degrading recalcitrant COD and chromophores. Fluidized bed technology enhances reaction efficiency, reduces chemical dosage and sludge production. Effluent requires pH adjustment and coagulation sedimentation.

  • Ozone Catalytic Oxidation: Ozone, in the presence of a catalyst, generates more ·OH, offering higher oxidation efficiency and no secondary pollution, but with higher equipment investment and operating power consumption.

• Alternative Process: For better water quality or lower requirements, "Coagulation Sedimentation" can be used for advanced decolorization and COD removal.

3.4 Sludge Handling Unit

CAF scum, excess biological sludge, etc., are sent to a sludge thickener. After thickening, sludge is dewatered by a dewatering machine (e.g., belt filter press or plate & frame filter press) to form sludge cake with moisture content ≤60%, which is then transported off-site for disposal by a licensed entity.

IV. Main Structure and Equipment Design Parameters (Example: 1000 m³/d scale)

V. Economic Analysis

• Capital Cost Estimate: Approximately 3-5 million RMB (depends on water quality, standards, location, automation level).

• Operating Cost: Approximately 3.5 - 6.0 RMB/ton of water, mainly including:

  • Electricity: 1.5-2.5 RMB/ton (mainly blowers, pumps).

  • Chemicals: 1.2-2.5 RMB/ton (demulsifier, PAC, PAM, Fenton reagents, etc.).

  • Labor: 0.3-0.5 RMB/ton.

  • Sludge Disposal: 0.5-1.0 RMB/ton.

  • Maintenance & Depreciation: 0.5-1.0 RMB/ton.

VI. Operation Management and Recommendations

1. Strengthen Source Management: Implement cleaner production to reduce the use of hard-to-treat auxiliaries, and segregate high-concentration wastewater streams.

2. Precision Chemical Dosing: Optimize the dosage of demulsifiers, coagulants, and oxidants based on online influent quality monitoring data (e.g., pH, ORP) to reduce chemical consumption.

3. Biological System Maintenance: Regularly monitor biological phase, DO, SV30, etc., promptly adjust aeration rate and nutrient (N, P) dosing to maintain microbial activity.

4. Automation Control: Implementing a PLC-based automatic control system for monitoring and interlocking key parameters is recommended to improve stability and reduce labor costs.

5. Pilot Testing: For specific, hard-to-treat dyeing wastewater, conducting on-site pilot testing before finalizing the scheme is advised to optimize process parameters and ensure effectiveness and economy.

This plan systematically addresses the challenges of treating oily wastewater in the dyeing and printing industry through a combined process approach. While ensuring compliance, it strives for technical rationality, operational economy, and management convenience, providing technical support for the enterprise's environmental compliance and sustainable development.