From Demulsification to Separation: Breaking Through Emulsified Oil Dilemma
The discharge of oily wastewater is inevitable in industrial production processes such as mechanical processing, petrochemicals, and metal coating. Among them, emulsified oil, as a kind of "stubborn molecule", is not only difficult to treat but also poses a serious threat to aquatic ecosystems and wastewater treatment systems. Why is it hard to remove? What targeted treatment schemes are available currently? This article will break it down for you one by one.
I. Emulsified Oil: Why Is It a Tough Nut to Crack in Wastewater Treatment?
Emulsified oil is not a simple oil-water layered mixture, but a stable system formed by oil droplets wrapped in surfactants and uniformly dispersed in water. Its particle size is usually between 0.1~10μm, and can even reach the nanometer level, which is the core reason for its difficulty in treatment. Specifically, its stability mainly comes from two major "protective barriers":
1. The "Wrapping Effect" of Surfactants
Most surfactants in industrial wastewater are amphiphilic molecules. One end is a lipophilic group, which can firmly adsorb on the surface of oil droplets; the other end is a hydrophilic group, which extends into water to form a "protective film". This film breaks the natural property of oil and water being immiscible, preventing oil droplets from naturally aggregating and floating up, leaving them suspended in water as tiny particles. More importantly, some surfactants ionize in water, making the surface of oil droplets negatively charged, which further enhances stability.
2. The "Repulsive Barrier" of the Electric Double Layer
The surface of negatively charged oil droplets attracts positive ions in water, forming an electric double layer structure consisting of a "compact layer + diffusion layer", which generates a certain potential value on the surface of oil droplets. When two oil droplets approach each other, the electrostatic repulsion caused by the same charge prevents them from merging, just like installing a "repulsive shield" on each oil droplet. This repulsion not only hinders the aggregation of oil droplets but also weakens the adhesion between oil droplets and bubbles, making traditional flotation methods difficult to work.
In addition, industrial wastewater has a complex composition, often accompanied by impurities such as heavy metals and organic substances. These substances interact with surfactants to further strengthen the emulsified system, making treatment even more difficult. In contrast, free oil (particle size >150μm) can naturally float up by standing, and dispersed oil (particle size 10~150μm) can be aggregated and separated through simple treatment. However, the stable characteristics of emulsified oil make it a key and difficult point in oily wastewater treatment.
II. Overcoming Emulsified Oil: Technical Paths from "Demulsification" to "Separation"
The core of treating emulsified oil is "demulsification" — destroying its stable system to allow tiny oil droplets to aggregate into large ones, which are then removed by separation technology. Current mainstream technologies can be divided into three categories: physical methods, chemical methods, and advanced treatment methods. In practical applications, combined processes are often used to improve efficiency.
1. Physical Methods: Mild Demulsification and Efficient Separation
Physical methods destroy the emulsification balance through physical effects without introducing chemical agents, avoiding secondary pollution. They are suitable as pretreatment or core treatment units.
• Flotation Method: This is the most commonly used physical technology for treating emulsified oil. A large number of fine bubbles are generated through a dissolved air system. The bubbles adhere to destabilized oil droplets to form a "bubble-oil droplet" complex, which floats to the surface due to its lower density than water and is scraped off. Among them, Dissolved Air Flotation (DAF) has the highest efficiency. When used with coagulants, the removal rate of emulsified oil can reach more than 90%, which is widely used in mechanical processing, food processing and other industries. Studies have found that the cationic surfactant CTAB can optimize bubble size (average 71μm) and improve flotation separation efficiency.
• Membrane Separation Method: As an advanced physical technology, it separates oil droplets by the interception effect of special membrane materials. In recent years, flexible lignin-based carbon nanofiber membranes have become a research hotspot. They have both superhydrophilicity and underwater oleophobicity, enabling efficient separation driven by gravity. The separation efficiency of emulsified oil reaches 97%, and the flux recovery rate is still over 96.5% after 5 cycles of use. It is both environmentally friendly and energy-saving, providing a new direction for advanced treatment of emulsified oil.
2. Chemical Methods: Precise Demulsification with Strong Targeting
Chemical methods destroy the emulsified system by adding agents, featuring fast demulsification speed and stable effect. They are the core means for treating high-concentration emulsified oil wastewater, with the key lying in selecting suitable demulsifiers.
Demulsifiers can be divided into three categories according to their chemical structure: cationic types target negatively charged emulsified oil droplets and destabilize them through charge neutralization and bridging; anionic types are suitable for emulsions containing positively charged particles, with both charge neutralization and chemical reaction functions; non-ionic types adapt to different systems by adjusting the ratio of hydrophilic and lipophilic groups, featuring a wide application range and good stability. In practical applications, compound demulsifiers (mixing two or more types) use synergistic effects to cope with industrial wastewater with complex compositions and solve the problem of poor effect of single demulsifiers.
Chemical demulsification is often combined with coagulation precipitation: first, demulsifiers are added to destroy the electric double layer, then coagulants such as Poly Aluminum Chloride (PAC) and Polyacrylamide (PAM) are added to aggregate oil droplets into flocs, which are completely separated by precipitation or flotation. A chemical enterprise successfully reduced the oil content of high-concentration emulsified oil wastewater to meet discharge standards by adopting the process of "demulsifier + pH adjustment + flocculation".
3. Combined Processes: Graded Treatment for Compliance Assurance
A single technology cannot cope with complex industrial wastewater, so the combined process of "graded treatment and progressive refinement" has become the mainstream. The classic process is: Regulation Tank (homogenization of quantity and quality) → Oil Separator (removal of free oil) → Coagulation Flotation (demulsification and separation of emulsified oil) → Biochemical/Filtration (advanced treatment) → Effluent Reaching Standards/Reuse.
Among them, the oil separator serves as pretreatment to remove free oil and part of the dispersed oil, reducing the load on subsequent processes; coagulation flotation is responsible for demulsifying and removing emulsified oil, which is the core link; the terminal biochemical treatment (such as activated sludge process) degrades residual organic matter, and the filtration method (quartz sand, activated carbon) intercepts tiny oil droplets to ensure that indicators such as oil content and Chemical Oxygen Demand (COD) in the effluent meet standards. For scenarios with high reuse requirements, a reverse osmosis membrane can be added to further improve water quality.
III. Future Trends: Green and High Efficiency as the Core Direction
Traditional treatment technologies have problems such as high energy consumption, secondary pollution, and reliance on chemicals. In recent years, green and high-efficiency technologies have become the focus of research and development. In addition to lignin-based membrane materials, biological demulsifiers (such as plant extracts and microbial metabolites) are environmentally friendly and degradable, making them suitable for scenarios with high environmental requirements; advanced oxidation technologies (such as ozone oxidation) can directly destroy the structure of surfactants, fundamentally disrupting the emulsified system, and can greatly improve treatment efficiency when combined with traditional processes.
Overcoming the challenge of emulsified oil treatment requires not only technological innovation but also process optimization based on the source of wastewater. In the future, with the integration of materials science and environmental protection technology, more economical, efficient, and environmentally friendly treatment solutions will surely emerge, providing strong support for the resource utilization of industrial wastewater.
