Advanced Treatment Processes for Coking Wastewater in the Iron and Steel Industry

Abstract

Coking wastewater is a typical, high-concentration, and highly toxic organic effluent generated during production processes in integrated iron and steel plants. It is characterized by its complex composition, poor biodegradability, and contains a large number of refractory pollutants, making it a recognized challenge in the field of water treatment. With increasingly stringent environmental regulations and the requirement for "near-zero discharge," tailwater from traditional biological treatment struggles to meet discharge standards stably, let alone reuse requirements. Therefore, developing efficient and stable advanced treatment processes to achieve the detoxification and resource recovery of coking wastewater is crucial for the green transformation and sustainable development of the iron and steel industry. This article systematically analyzes the water quality characteristics of coking wastewater, with a focus on elaborating integrated advanced treatment processes centered on the sequence of "enhanced pretreatment - core degradation - advanced desalination - resource recovery," and discusses their technical keys and development directions.

1. Sources, Water Quality Characteristics, and Treatment Challenges of Coking Wastewater

1.1 Main Sources

Coking wastewater is primarily generated during coking, gas purification, and chemical by-product recovery processes, including:

• Excess Ammonia Water: Generated during the coking process, accounting for a relatively large volume with a high pollution load.

• Gas Final Cooling Wastewater, Crude Benzene Separation Wastewater: Contain high concentrations of phenols, cyanides, ammonia nitrogen, and polycyclic aromatic hydrocarbons (PAHs).

• Gas Pipeline Condensate, Equipment Cleaning Water, etc.

1.2 Typical Water Quality Characteristics and Core Challenges

• High Pollutant Concentrations: COD typically ranges from 2000-5000 mg/L, ammonia nitrogen 200-500 mg/L, phenols 200-1000 mg/L, cyanides 10-50 mg/L, and also contains sulfides, oils, PAHs, etc.

• Complex Composition, High Toxicity: Contains a large number of biologically inhibitory, refractory aromatic compounds (e.g., benzene, naphthalene, anthracene, quinoline).

• Poor Biodegradability: BOD5/COD ratio is typically below 0.3, limiting the efficiency of traditional biological treatment.

• High Salinity: Contains high concentrations of ions like Cl⁻, SCN⁻, SO₄²⁻, posing challenges to microbial activity and subsequent membrane treatment.

• Significant Fluctuations in Water Quality and Quantity: Greatly influenced by coke oven operating conditions and chemical recovery operations.

2. Advanced Treatment Process Technology Roadmap

Advanced treatment of coking wastewater requires the construction of an integrated system with multi-barrier, synergistic effects, aiming for compliant discharge or high-quality reuse. The core process route can be summarized as: "Enhanced Pretreatment → Core Biological/Physicochemical Degradation → Advanced Purification and Desalination → Concentrate Resource Recovery/Ultimate Disposal".

2.1 Enhanced Pretreatment Stage

Aims to remove oils, suspended solids, and some toxic substances, condition the water quality, and improve wastewater biodegradability, creating suitable conditions for the core treatment units.

• Oil and Suspended Solids Removal: Uses a combination of "oil separation + flotation (e.g., DAF, CAF)" to efficiently remove free oils, emulsified oils, and some suspended solids.

• Preliminary Removal of Phenols, Cyanides, Ammonia, and Water Conditioning:

  • Steam Stripping or Air Stripping can be used to remove most ammonia nitrogen for potential recovery.

  • Chemical Precipitation (e.g., using iron salts) to remove some sulfides and cyanides.

  • Advanced Oxidation Pretreatment: Such as Fenton oxidation, ozone catalytic oxidation, or wet air oxidation, to partially oxidize refractory organics, destroy their biotoxicity, and increase the BOD/COD ratio.

2.2 Core Biological Degradation Enhancement Stage

Based on pretreatment, employs high-efficiency biological technologies to deeply remove COD, ammonia nitrogen, and some characteristic pollutants.

• Anaerobic Biological Treatment: Such as the hydrolytic acidification process, converting large organic molecules into smaller ones to further improve biodegradability.

• Upgraded Aerobic Biological Treatment:

  • A/O, A2/O and their variants: Enhance nitrogen removal and carbon oxidation.

  • Bioaugmentation Technology: Adding highly efficient degrading bacterial strains to specifically treat characteristic pollutants like phenols, cyanides, and PAHs.

  • Moving Bed Biofilm Reactor (MBBR), Membrane Bioreactor (MBR): Utilize biofilms to enrich specific microbial communities, improving system shock load resistance and sludge concentration, resulting in better effluent quality. MBR can replace secondary clarifiers, providing effluent with low suspended solids.

2.3 Advanced Purification and Desalination Stage

Biologically treated effluent still contains residual COD, color, dissolved salts, and trace pollutants, requiring further treatment for reuse or strict compliance.

• Advanced Oxidation for Advanced Treatment:

  • Ozone-Biological Activated Carbon (O3-BAC): Ozone oxidizes residual refractory organics, followed by adsorption and degradation on biological activated carbon, effectively reducing COD and color.

  • Electrochemical Oxidation, Fenton-like Oxidation: For the ultimate oxidation of extremely refractory pollutants.

• Membrane Separation Technology:

  • Nanofiltration/Reverse Osmosis Dual-Membrane Process: This is the core of advanced treatment and reuse. NF removes most divalent ions, organics, and color; RO provides deep desalination, producing water that can meet standards for circulating cooling water or even higher applications. The NF/RO concentrate requires specialized treatment.

  • Key Application Points: Must have comprehensive pretreatment (e.g., UF) to prevent membrane fouling; select fouling-resistant membrane elements considering the high salinity and organic content.

• Adsorption and Ion Exchange: Activated Carbon Adsorption and Resin Adsorption are used for the deep removal of trace organics (e.g., benzene series) or specific ions.

2.4 Concentrate Treatment and Resource Recovery/Ultimate Disposal

The concentrate produced by membrane systems (15-30% of feed volume) concentrates all difficult-to-treat pollutants and salts, and is the key to the success of "near-zero discharge."

• Concentrate Volume Reduction: Use technologies like High-Pressure RO, Electrodialysis (ED), or Forward Osmosis (FO) for further concentration, reducing the volume for evaporation.

• Detoxification via Advanced Oxidation: Treat high-concentration organic concentrate with processes like Wet Air Oxidation (WAO) or Supercritical Water Oxidation (SCWO) to completely degrade organics.

• Evaporation Crystallization/Drying and Solidification:

  • Multiple Effect Evaporation (MEE), Mechanical Vapor Recompression (MVR) Evaporation: Evaporate the concentrate to supersaturation, crystallizing out mixed salts. This is the mainstream end-of-pipe process for achieving zero liquid discharge.

  • Spray Drying/Rotary Kiln Incineration: Dry or incinerate high-organic concentrate for complete harmless disposal.

3. Examples of Typical Integrated Process Flows

Example 1: Advanced Treatment Route Targeting "Reuse"

"Raw Water → Equalization → Oil Separation/Flotation → Fenton Pretreatment → Hydrolytic Acidification → A2/O-MBR → Ultrafiltration → NF/RO Dual-Membrane Process"

• Product Water: RO permeate is reused in the circulating cooling water system.

• Concentrate: NF/RO concentrate enters a system of "Advanced Oxidation + High-Pressure RO + MVR Evaporative Crystallization" to achieve salt separation and zero liquid discharge.

Example 2: Route Targeting "Compliant Discharge + Resource Recovery"

"Raw Water → Equalization → Ammonia Stripping (recovering ammonia) → Flotation → Enhanced Biological Treatment (A/O + Bioaugmentation) → Ozone-Biological Activated Carbon → Discharge/Partial Reuse"

• For scenarios with less stringent discharge standards or lower reuse demand, focusing on resource recovery (ammonia) and stable compliance.

4. Techno-Economic Analysis and Development Trends

4.1 Techno-Economic Aspects

• Investment and Operating Costs: Advanced treatment systems, especially those including membrane processes and evaporative crystallization, have significantly higher investment and operating costs than traditional biological treatment. Main operating cost components are chemicals, energy (electricity, steam), membrane replacement, and maintenance.

• Benefits: Achieves water savings and fees from wastewater reuse; reduces discharge fees and environmental risks; partial resource recovery (ammonia, salts) can offset some costs. In regions with strict environmental requirements and high water prices, it demonstrates economic feasibility.

4.2 Technical Challenges and Development Trends

• Challenges: Inhibition and fouling of biological and membrane systems by high salinity, toxicity, and organics; high energy consumption and cost of concentrate treatment; difficulties in resource recovery and impurity separation of crystallized salts.

• Development Trends:

  • High-Efficiency, Low-Consumption Pretreatment Technologies: Develop new catalysts and reactors to reduce the cost of advanced oxidation.

  • Specialized Microorganisms and Bioaugmentation: Screen and cultivate highly efficient degrading bacteria to enhance biological system performance.

  • Fouling-Resistant, High-Selectivity Membrane Materials: Research and develop specialized separation membranes suitable for coking wastewater characteristics.

  • Concentrate Resource Recovery Technologies: Develop fractional crystallization technology to separate and recover valuable salts like NaCl, Na₂SO₄; explore low-energy concentration processes via deep thermal-membrane coupling.

  • Smart Water Management: Apply big data and artificial intelligence for intelligent full-process monitoring and optimized operation.

Conclusion

The advanced treatment of coking wastewater in the iron and steel industry is a complex systems engineering challenge; no single technology can solve all problems. Based on water quality characteristics, treatment objectives, and site conditions, it is essential to scientifically select and optimally combine various technologies—"physical-chemical-biological-membrane"—to construct an integrated process chain with graded treatment and progressive refinement. The technical path of "enhanced pretreatment to ensure biological system stability, high-efficiency biological technology as the core to reduce load, advanced oxidation and membrane separation as the key for advanced purification, and evaporation crystallization as the final guarantee for zero discharge" is an effective direction for achieving efficient, stable treatment and resource recovery of coking wastewater. With technological advancements and cost optimization, advanced coking wastewater treatment processes will provide solid technical support for realizing the green, low-carbon development and water resource recycling of the iron and steel industry.