Process Integration and Application of Reverse Osmosis Membrane Technology in Treating Chemical Oily Wastewater

Abstract

Chemical oily wastewater is characterized by its complex composition, diverse forms of oil (including free oil, dispersed oil, emulsified oil, and dissolved oil), high concentration of organic matter, and poor biodegradability, making it one of the challenges in industrial wastewater treatment. Reverse osmosis (RO) membrane technology can efficiently remove dissolved salts, small-molecule organic matter, and trace oil fractions, serving as a core process for the advanced treatment and reuse of this type of wastewater. However, oily substances readily cause irreversible organic fouling and wetting of the membranes, posing an extremely high risk for direct treatment. Therefore, constructing an integrated process system centered on "enhanced pretreatment for oil removal, synergistic prevention and control of membrane fouling, and safe concentrate disposal" is key to the successful application of RO technology in treating chemical oily wastewater. This article systematically elaborates on the technical route, key integrated processes, technological innovation directions, and engineering application examples for the RO treatment of chemical oily wastewater, aiming to provide a systematic solution for the engineering promotion of this technology.

1. Characteristics of Chemical Oily Wastewater and Treatment Challenges

1.1 Wastewater Sources and Oil Occurrence Forms

• Main Sources: Oil-water separation water, wash water, condensate, accidental drainage, etc., from processes such as petroleum refining, petrochemicals, coal chemical, fine chemicals, lubricant production, and mechanical processing.

• Oil Forms:

  • Free Oil and Dispersed Oil: Particle size >100 µm, removable by physical methods.

  • Emulsified Oil: Particle size 0.1-100 µm, stabilized by surfactants or fine solid particles; a key focus for pretreatment removal.

  • Dissolved Oil: Dispersed in molecular state, typically <0.1 µm, difficult to remove by conventional physicochemical methods; one of the primary targets for RO membrane treatment.

• Composite Water Quality Characteristics: Besides oil, often accompanied by high salinity, high COD, high suspended solids, characteristic organics (e.g., benzene series, phenols), and toxic substances.

1.2 Core Challenges for RO Membrane Treatment

• Membrane Fouling: Oily substances (especially emulsified and dissolved oils) readily adsorb and deposit on the membrane surface and within pores, forming a hydrophobic fouling layer. This leads to a sharp flux decline, difficult cleaning, and can even cause permanent wetting (loss of membrane salt rejection).

• Extremely High Pretreatment Requirements: Must reduce the feed water oil content below the safe threshold for RO membranes (typically required <0.5-1.0 mg/L).

• System Operation Risk: Fluctuations in water quality can lead to pretreatment failure, allowing oil breakthrough and causing impact fouling to the membrane system.

  

2. Integrated Process System Centered on Reverse Osmosis

Addressing the above challenges necessitates adopting an integrated process strategy of "separate treatment by quality and grade, synergistic protection." A typical flow is: "Raw Water → Oil Separation/Equalization → Demulsification/Oil Removal → Advanced Physicochemical Treatment → Precision Filtration/Ultrafiltration → Reverse Osmosis → Concentrate Disposal".

2.1 Enhanced Pretreatment Unit: The Core Lies in Efficient Oil Removal and Water Quality Assurance

• Primary Oil Removal: Employ technologies like API separators, plate separators, and flotation to remove most free and dispersed oil. Advanced flotation (e.g., cavitation air flotation, dissolved air flotation) can effectively remove some emulsified oil.

• Demulsification and Advanced Oil Removal:

  • Chemical Demulsification-Coagulation: Dose demulsifiers (e.g., electrolytes, reverse demulsifiers) and coagulants to destabilize the double layer of emulsified oil droplets, causing them to coalesce, followed by removal via coagulation-sedimentation or flotation. This step is critical and requires optimization of chemicals through testing.

  • Advanced Oxidation Pretreatment: For wastewater containing refractory organics and stable emulsified oils, use technologies like ozone catalytic oxidation or Fenton oxidation. These degrade COD while altering oil-water interface properties, aiding demulsification and reducing subsequent membrane fouling potential.

• Advanced Purification and Guard Filtration:

  • Multi-Media Filtration: Further removes residual suspended solids.

  • Barrier Role of Ultrafiltration: Serves as the core protective barrier for RO. Employing fouling-resistant, organic solvent-tolerant externally pressurized or tubular Ultrafiltration (UF) can retain nearly 100% of colloids, macromolecular organics, bacteria, and residual fine oil droplets/oil micelles after pretreatment. This ensures effluent with SDI<3 and oil content <0.5 mg/L, significantly reducing the risk of oil fouling on RO membranes.

2.2 Optimized Design of the RO Membrane System

• Selection of Oil-Fouling-Resistant Membranes: Prioritize RO membrane elements that are hydrophilically modified, have low surface energy, and are of a fouling-resistant type. Hydrophilic surfaces can reduce the adsorption tendency of hydrophobic oils.

• Process Design Optimization:

  • Conservative Recovery Rate Design: Initial design employs a moderately conservative recovery rate (e.g., 50-70%) to lower the pollutant concentration gradient at the membrane surface, delaying fouling.

  • High Cross-flow Velocity Operation: Maintain a relatively high velocity at the membrane surface to enhance shear force and reduce pollutant deposition.

  • Inter-stage Layout Optimization: Rationally distribute flux across stages to avoid overly rapid fouling in the front-end membranes.

• Intelligent Chemical Dosing:

  • Specialized Antiscalant/Dispersant: Use compounded antiscalants with oil-fouling dispersing functions to prevent the synergistic deposition of oils and inorganic scale.

  • Precise Reducing Agent and Biocide: Strictly control oxidizing agents; dose non-oxidizing biocides to inhibit microorganisms (which can form composite fouling with oils).

2.3 Membrane Fouling Monitoring and Efficient Cleaning

• Online Monitoring and Early Warning: Real-time monitoring of changes in normalized flux, inter-stage differential pressure, and salt rejection. Establish fouling early warning models based on trend analysis.

• Specialized Cleaning Strategies:

  • Cleaning Agent Formulations: For oil fouling, use composite formulations containing strong alkaline cleaners (NaOH, pH>12) combined with surfactants and organic solvents (e.g., ethanol, isopropanol; membrane compatibility requires careful evaluation). Proprietary cleaners recommended by membrane manufacturers can be selected.

  • Cleaning Procedure Optimization: Employ steps like "alkaline cleaning → acidic cleaning" (if descaling is needed). Explore techniques like hot water cleaning and chemically enhanced cleaning-in-place (CIP) to improve effectiveness.

2.4 RO Concentrate Treatment and Resource Recovery

• Concentrate Characteristics: Enriched with salts, refractory organics, and trace oils, making it difficult to treat.

• Disposal Pathways:

  • Further Concentration and Volume Reduction: Use technologies like High-Pressure RO, Electrodialysis, or Membrane Distillation for further volume reduction.

  • Advanced Oxidation Treatment: Employ Wet Air Oxidation or Fenton-like oxidation to degrade residual organics and reduce toxicity.

  • Ultimate Disposal: Feed into an evaporation crystallization system or consign as hazardous waste. Explore the potential for recovering valuable solvents from the concentrate.

3. Key Technological Breakthroughs and Innovation Directions

3.1 High-Performance Oil-Fouling-Resistant Membrane Materials

• Develop biomimetic, fouling-resistant membranes with surfaces grafted with zwitterionic polymers or superhydrophilic coatings to resist oil adsorption at the source.

• Research ceramic-based or specialty polymer RO membranes resistant to solvents and oils.

3.2 Deep Coupling Technology of Pretreatment and Membrane Separation

• Membrane Coagulation Reactor: Integrates the coagulation process with UF membrane separation to enhance emulsified oil removal.

• In-situ Coupling of Advanced Oxidation and Membrane Filtration: Introduces mild catalytic oxidation processes on the membrane surface or in the feed water to achieve immediate degradation of pollutants, alleviating membrane fouling.

3.3 Intelligent Operation and Maintenance Systems

• Integrate online oil analyzers and membrane fouling sensors to achieve early warning of oil fouling.

• Utilize artificial intelligence algorithms to optimize the dosing strategy for demulsifiers/cleaners and cleaning cycles.

4. Typical Application Case Analysis

Case: Oily Chemical Wastewater Reuse Project at a Petrochemical Enterprise

• Feed Water: Oil content 50-200 mg/L (primarily emulsified oil), COD 800-2000 mg/L, TDS ~3000 mg/L.

• Process Flow: "Flow/Quality Equalization → High-Efficiency Cavitation Air Flotation → Catalytic Ozonation for Demulsification → Coagulation-Sedimentation → Multi-Media Filtration → UF → Primary RO → Partial Concentrate via High-Pressure RO".

• Key Technical Points:

  1. Catalytic ozonation effectively demulsifies and improves wastewater biodegradability.

  2. Stable operation of UF ensures RO feed oil content <0.3 mg/L.

  3. Use of fouling-resistant RO membranes,配合 (coupled with) specialized antiscalant/dispersants.

• Operational Performance: System water recovery >75%. RO permeate meets make-up water standards for circulating cooling systems. Membrane cleaning cycles extended to 4-6 months, with stable operation.

5. Economic Analysis and Application Outlook

5.1 Economic Analysis

• Investment and Costs: The integrated system involves relatively high investment, concentrated mainly in the pretreatment unit and membrane system. Operating costs primarily consist of chemicals, energy, membrane replacement, and concentrate disposal fees.

• Benefits:

  • Water Resource Benefit: Achieves wastewater reuse, saving on freshwater and discharge fees.

  • Environmental and Compliance Benefit: Meets stringent discharge standards, reducing environmental risk.

  • Long-term Economics: With extended membrane life and operational optimization, the cost per unit of treated water becomes competitive.

5.2 Outlook

RO membrane technology in the field of chemical oily wastewater treatment is developing towards higher fouling resistance, lower energy consumption, and stronger intelligence. In the future, through new material R&D, deep innovation in process integration, and the construction of smart water management platforms, RO technology will play an even more critical and reliable role in achieving "near-zero discharge" and resource recovery of high-difficulty chemical wastewater. It will provide important technical support for the green, low-carbon, and circular development of the chemical industry.