Engineering Application of Sulfur-Containing Wastewater Treatment Process Design Scheme for the Dyeing and Printing Industry

I. Design Basis and Engineering Objectives

1.1 Source and Characteristics Analysis of Wastewater

Sulfur-containing wastewater in the dyeing and printing industry primarily originates from dyeing and reduction cleaning processes using sulfur dyes (e.g., sulfur black, sulfur blue), as well as some dye synthesis and size decomposition processes. The challenges for engineering treatment are prominent, with specific characteristics as follows:

• High Sulfide Concentration: Present as S²⁻, typically ranging from 200-2000 mg/L. It is highly toxic, corrosive, prone to generating foul odors (H₂S), and severely inhibits microbial activity. It is the primary target for pretreatment.

• High Chroma and High COD: Dyes and by-products cause extremely high color (up to thousands of times) and high Chemical Oxygen Demand (COD), containing components resistant to biodegradation.

• High Alkalinity and Temperature: Production processes often result in wastewater pH > 10 and elevated temperature.

• Contains Complex Organics: May contain organic sulfides like mercaptans, thioethers, and residual auxiliaries.

1.2 Design Influent and Effluent Water Quality (Example)

1.3 Core Engineering Application Objectives

1. Safe and Efficient Sulfur Removal: Ensure sulfides are completely and stably removed, eliminating toxicity, corrosiveness, and safety hazards, creating conditions for subsequent biological treatment.

2. Deep Degradation of Pollutants: Achieve effective removal of COD and color, ensuring stable discharge compliance.

3. Economical and Reliable Operation: Mature process, high automation, controllable sludge production, reasonable operating costs.

II. Core Process Route Selection and Engineering Flow Diagram

Following the core principle of "priority sulfide removal", this engineering scheme recommends the main route: "Enhanced Physicochemical Pretreatment for Sulfur Removal + Biological Treatment for Main Degradation + Advanced Oxidation for Deep Decolorization". This route is technologically mature and suitable for large-scale engineering applications.

The specific engineering process flow diagram is as follows:

III. Key Points for Engineering Design of Treatment Units

3.1 Enhanced Pretreatment Unit for Sulfur Removal (Engineering Core)

• Process Selection: Chemical precipitation is recommended as the mainstream engineering process, with Ferrous Sulfate (FeSO₄) as the preferred reagent. Its reaction is complete, operation is simple, operating costs are relatively low, and it is easiest to achieve stable control in engineering applications.

• Engineering Design and Control Points:

  • Precise Reaction pH Control: Use online pH meters and automatic acid dosing systems (e.g., H₂SO₄) to precisely control wastewater pH within the optimal range of 8.5-9.5. Under these conditions, the reaction between Fe²⁺ and S²⁻ to form Ferrous Sulfide (FeS) black precipitate is most complete.

  • Chemical Dosing and Mixing: Implement a two-stage mechanically stirred reaction tank system. The primary tank ensures rapid and uniform mixing of the FeSO₄ solution and wastewater; the secondary tank provides sufficient reaction time (≥20 minutes). The dosage must be determined by testing, typically 1.1-1.3 times the theoretical amount (Fe:S ≈ 1.1:1).

  • Safety and Redundancy Design: An emergency acid dosing point and exhaust gas collection piping can be reserved at the front end of the equalization or reaction tank to handle abnormal spikes in sulfide concentration. If necessary, a short-term "acidification stripping" emergency mode can be activated, with the generated trace H₂S directed to an exhaust gas scrubber (alkali spray) for treatment.

3.2 Main Biological Treatment Unit

• Hydrolysis Acidification Tank: After most sulfides are removed, the wastewater enters the hydrolysis acidification tank. Here, refractory macromolecules like dye molecules are broken down into smaller molecules, increasing the B/C ratio and creating favorable conditions for subsequent aerobic treatment. Dissolved oxygen should be controlled at <0.5 mg/L.

• A/O (Anoxic/Oxic) or A²/O Process:

  • Anoxic Zone (A): Performs denitrification for nitrogen removal while further degrading organics.

  • Oxic Zone (O): Utilizes activated sludge or biological contact oxidation. With blower aeration, microorganisms oxidize and decompose organic matter. This is the core unit for COD removal. Dissolved oxygen must be strictly controlled between 2-4 mg/L.

• Secondary Sedimentation Tank: Achieves solid-liquid separation. A portion of the sludge is returned to the front of the biological tank; excess sludge is discharged to the sludge handling system.

3.3 Advanced Treatment and Safeguard Unit

• Advanced Oxidation Processes (AOPs): The color and COD of the secondary clarifier effluent may still be near limits, requiring AOP "polishing" treatment to ensure absolute and stable compliance.

  • Fenton Oxidation: Widely applied in engineering, with relatively low investment. Involves dosing H₂O₂ and Fe²⁺ to generate highly oxidative hydroxyl radicals (·OH) that destroy chromophores. Requires a dedicated reaction tank, pH adjustment, and a final settling tank.

  • Ozone Catalytic Oxidation: Suitable for space-constrained sites or where low secondary sludge production is required. High equipment integration, no secondary sludge, but higher investment and power consumption.

3.4 Sludge and Exhaust Gas Treatment Unit

• Sludge Treatment: The sulfur-containing chemical sludge from the primary clarifier and the excess biological sludge are mixed, resulting in stable properties. After thickening, a high-pressure plate and frame filter press is used for dewatering, reducing the cake moisture content to below 60%. This sludge, due to containing heavy metal sulfides, typically needs to be managed as hazardous waste and disposed of by a licensed entity.

• Exhaust Gas Treatment: The pretreatment workshop and sludge dewatering room require a negative pressure collection system to capture potentially emitted trace sulfurous and dusty exhaust gases, which are then treated by an "alkali spray scrubber" before discharge via a stack.

IV. Main Process Parameters and Equipment Selection (Example: 1000 m³/d Scale)

V. Engineering Economic Analysis

• Capital Cost Estimate: Approximately 6 - 10 million RMB. Main cost components: Civil works, corrosion protection, special equipment (filter press, oxidation equipment), automation control system, exhaust gas treatment unit.

• Operating Cost: 8 - 15 RMB per ton of water, mainly including:

  • Chemical Costs: 3-7 RMB/ton (FeSO₄, acid, alkali, H₂O₂, PAM, etc., highest proportion).

  • Power Costs: 2-4 RMB/ton (blowers, pumps, mixers, etc.).

  • Sludge Disposal Costs: 2-3 RMB/ton (high cost for hazardous waste disposal).

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

• Benefits: Eliminates sulfide pollution, ensures stable operation of the biological system, avoids environmental risks, and achieves stable compliance for the entire plant's wastewater.

VI. Operation Management and Engineering Implementation Recommendations

1. Source Monitoring and Segregation: Strive to collect and pre-treat sulfur-containing wastewater separately at the workshop outlet to prevent shock loads on the combined wastewater treatment system.

2. Online Monitoring of Key Parameters: Online monitoring and automatic control of pH and ORP in the pretreatment unit, DO and MLSS in the biological unit, and ORP in the advanced treatment unit are essential; they are the "eyes" for stable system operation.

3. Pilot Testing: Continuous pilot testing on actual wastewater is mandatory before implementation, especially to verify the optimal FeSO₄ dosage, pH, and impact on the subsequent biological system under different sulfide concentrations.

4. Strict Corrosion Protection: All concrete tanks, metal equipment, pipes, bolts, etc., in contact with sulfur-containing wastewater (especially low-pH areas) must have appropriate corrosion protection based on the medium characteristics; this ensures the project's lifespan.

5. Safety Contingency Plan: Develop detailed hydrogen sulfide leak detection and emergency response plans. Equip with portable H₂S detectors, positive pressure air respirators, and provide specialized training for operators.

This engineering application scheme, centered on the mature and reliable "iron salt precipitation for sulfur removal" technology, combined with efficient biological and advanced treatment processes, forms a complete technical chain for treating sulfur-containing dyeing wastewater. It balances treatment effectiveness, operational stability, and economy, possessing value for widespread application in large-scale dyeing and printing enterprises.