Technical Pathway for Advanced Treatment of Coal Chemical Wastewater Using Membrane Processes and Coordinated Salt Control

Coal chemical wastewater is characterized by complex composition, high toxicity, poor biodegradability, and high salinity. Concentrations of COD, ammonia nitrogen, total dissolved solids (TDS), and characteristic pollutants (such as phenolic compounds, cyanides, and polycyclic aromatic hydrocarbons) are typically elevated. Employing integrated membrane processes for advanced treatment and resource recovery represents a critical technological pathway to achieve high-standard water reuse, approach zero liquid discharge, and enable systematic pollutant reduction. The following outlines a comprehensive control strategy for pollutants in coal chemical wastewater based on integrated membrane technologies.

I. Treatment Objectives and Technical Rationale

Core Objectives:

1. Achieve deep removal of conventional pollutants such as COD, ammonia nitrogen, and suspended solids, producing effluent that meets stringent reuse standards (e.g., make-up water for circulating cooling systems).

2. Effectively remove and separately recover inorganic salts (e.g., NaCl, Na₂SO₄) to enable resource recovery from concentrate streams, ultimately achieving zero liquid discharge from the system.

Technical Rationale:

Adhere to the overall treatment train of: "Pretreatment for load reduction → Biological degradation → Advanced membrane-based desalination and separation → Evaporation/crystallization for solidification." Membrane technologies primarily assume two core roles within this framework: advanced purification and graded salt separation. Their successful implementation necessitates seamless integration with pretreatment and final disposal units.

II. Analysis of Core Membrane Process Technologies

The membrane-based treatment of coal chemical wastewater is not a single-technology application but a multi-stage integrated membrane system.

1. Ultrafiltration Unit: Precision Pretreatment and Guard Filtration

• Functional Role: Positioned upstream of reverse osmosis systems as a critical pretreatment step.

• Mechanism of Action: Utilizes physical sieving through micropores (approximately 0.01-0.1 μm) to effectively retain colloids, suspended solids, macromolecular organics, and microorganisms present in biologically treated or advanced oxidation process effluent.

• Key Performance Indicators: Ensures effluent with a stable Silt Density Index (SDI) below 3 and turbidity below 0.1 NTU, providing qualified feed water for subsequent RO membranes and preventing colloidal and particulate fouling.

• Process Requirements: Often employs externally pressurized or submerged UF membranes with strong anti-fouling properties, coupled with regular air-water backwashing and chemical cleaning to maintain flux.

2. Reverse Osmosis Unit: Core Desalination and Advanced Purification

• Functional Role: Achieves high-efficiency removal of dissolved salts, low molecular weight organics, and ions; the core guarantee for producing reusable water.

• Mechanism of Action: Applies external pressure exceeding the osmotic pressure across a semi-permeable membrane, driving water molecules through while retaining the vast majority of dissolved salts (rejection >98%), ions, organics with molecular weight >100 Da, and polyvalent ions.

• Treatment Efficacy: Single-stage RO permeate typically achieves TDS <100 mg/L, COD <30 mg/L, and ammonia nitrogen <5 mg/L. This water can be directly reused in circulating cooling water systems or, after further polishing, as boiler feedwater.

• System Challenges: Faces challenges from high osmotic pressure due to salinity, and risks of membrane fouling and scaling from organics and scaling ions (Ca²⁺, Mg²⁺, SO₄²⁻, SiO₂). Reliance on comprehensive pretreatment and scientific antiscalant dosing programs is essential.

3. Nanofiltration Unit: Key to Salt Separation and Resource Recovery

• Functional Role: Used for separating salts in RO concentrate or for pre-desalination of biologically treated effluent. It is a core component for enabling salt resource recovery and reducing end-stage evaporation/crystallization costs.

• Mechanism of Action: Based on size exclusion and the Donnan effect, NF exhibits high rejection (>95%) for divalent and multivalent ions (e.g., SO₄²⁻, Ca²⁺, Mg²⁺) and lower rejection (20%-80%) for monovalent ions (e.g., Cl⁻, Na⁺).

• Application Modes:

  • Mode A (NF as RO Pretreatment): Performs salt separation on biologically treated effluent. NF permeate (predominantly NaCl) feeds the RO system, improving RO recovery and reducing scaling tendency. NF concentrate (rich in Na₂SO₄ and some COD) is treated separately.

  • Mode B (NF for RO Concentrate Treatment): Further separates RO concentrate. Yields a low-hardness permeate stream (primarily NaCl, suitable for further concentration to produce industrial salt) and a concentrate stream rich in Na₂SO₄ (suitable for evaporation/crystallization to produce anhydrous sodium sulfate).

• Value Proposition: Salt separation prevents the generation of mixed complex salts at the end-stage, enhances the quality and resource recovery value of crystallized salts, and significantly reduces hazardous waste disposal costs.

4. High-Pressure Membrane Unit: Ultimate Concentration for Volume Reduction

• Functional Role: Further dehydrates and concentrates NF or RO concentrate streams to minimize the volume of waste liquid sent to evaporation/crystallization units.

• Technology Options:

  • High-Pressure Reverse Osmosis: Operating pressures up to 12-18 MPa, capable of concentrating feedwater with TDS of 40,000-70,000 mg/L to over 100,000 mg/L.

  • Disc Tube RO: Features an open-channel design with strong anti-fouling capability, particularly suitable for further concentrating high-suspended-solid, high-fouling-potential brine streams.

• Core Function: Reduces brine volume by 60%-80%, directly and significantly decreasing the scale, capital cost, and steam energy consumption of subsequent evaporation/crystallization equipment.

III. Typical Integrated Process Train

For typical coal chemical wastewater, the following integrated membrane process chain is recommended:

"Pretreatment → Biological Treatment → Advanced Oxidation → Ultrafiltration → Nanofiltration → Reverse Osmosis → High-Pressure RO/DTRO → Evaporation/Crystallization"

1. Biological and Advanced Oxidation: Removes most biodegradable organics and ammonia nitrogen, and transforms refractory COD into more treatable forms.

2. Ultrafiltration: Serves as the protective barrier for membrane processes, providing stable, low-SDI feed water.

3. Nanofiltration: Enables initial salt separation, creating favorable conditions for downstream units.

4. Reverse Osmosis: The main permeate stream is reused; the concentrate proceeds to further treatment.

5. High-Pressure RO/DTRO: Performs ultimate concentration on the separated brine streams for maximum volume reduction.

6. Evaporation/Crystallization: Evaporates water from the final concentrate and crystallizes salts, producing industrial-grade sodium chloride and sodium sulfate separately, achieving zero liquid discharge.

IV. Technical Advantages, Challenges, and Countermeasures

Technical Advantages:

1. Excellent and Stable Effluent Quality: Membrane separation is a physical process with strong resistance to feedwater fluctuations, producing effluent that meets high standards for reuse.

2. High Water Recovery Rate: Overall system water recovery can reach 75%-95%, significantly conserving fresh water consumption.

3. Clear Resource Recovery Potential: Coupling NF salt separation with evaporation/crystallization enables the classified recovery of inorganic salts, turning waste into valuable products.

4. High Degree of Automation: Modular design facilitates automatic control and remote monitoring.

Key Challenges and Mitigation Strategies:

1. Membrane Fouling and Scaling:

   • Challenge: Organics, colloids, microorganisms, and inorganic ions in wastewater readily cause membrane fouling and chemical scaling.

   • Countermeasures: Strengthen pretreatment (coagulation, advanced oxidation) and UF guard filtration; optimize antiscalant and biocide dosing; establish online/offline chemical cleaning protocols based on water quality analysis; select fouling-resistant membrane elements.

2. High Cost of Concentrate Management:

   • Challenge: Energy consumption and capital investment for evaporation/crystallization units constitute a major portion of the total system cost.

   • Countermeasures: Maximizing volume reduction to the evaporator through NF salt separation and high-pressure membrane concentration is key to reducing overall process costs.

3. System Capital and Operating Costs:

   • Challenge: Significant investment for membrane systems and evaporators/crystallizers, with operating costs involving electricity, chemicals, and membrane replacement.

   • Countermeasures: Improve recovery rate and energy efficiency through detailed design; conduct life-cycle cost analysis balancing expenses against revenue from water reuse, reduced discharge fees, and by-product sales.

Conclusion

The adoption of an integrated membrane process centered on "nanofiltration for salt separation" and "reverse osmosis for deep desalination" represents an effective and essential technological pathway for achieving advanced pollutant removal and salt resource recovery from coal chemical wastewater. Its successful application relies on accurate analysis of water quality characteristics, reliable pretreatment safeguards, scientific combination and design of membrane processes, and a coordinated strategy for concentrate management. This technological approach aligns with the strategic requirements for green development and water resource recycling within the coal chemical industry, serving as a critical engineering choice for unifying environmental benefits and economic viability.