Why Emulsified Oil Is the Biggest Headache in Oily Wastewater Treatment?
In industrial wastewater treatment, oily wastewater is always a key target for remediation, and emulsified oil among it is even a recognized tough nut to crack for environmental protection practitioners. Unlike free oil that floats on the water surface and can be removed with simple separation, emulsified oil is stably dispersed in water in the form of tiny oil droplets. Conventional treatment methods are ineffective, and even complex processes require overcoming multiple obstacles to achieve discharge standards. This article will break down the core reasons why emulsified oil is difficult to treat and explain the logic behind this environmental governance pain point.
First, Understand: What Is Emulsified Oil?
To understand the difficulty of treating emulsified oil, it is first necessary to distinguish it from ordinary oils. Oils in oily wastewater are mainly divided into four categories: free oil, dispersed oil, emulsified oil, and dissolved oil. Among them, emulsified oil has the smallest particle size, usually between 0.1 and 10 micrometers, and can even reach the nanometer level, much smaller than dispersed oil (10~150 micrometers) and free oil (>100 micrometers). More importantly, emulsified oil is not a simple suspension of oil droplets, but a stable mixed system formed by oil and water under the action of emulsifiers (mostly surfactants)—similar to the state where fat particles are uniformly dispersed in milk and difficult to layer, with a stability period of 1 to 6 months.
The formation of emulsified oil requires the dual action of external force and emulsifier: in industrial production processes such as machining, metal cutting, food processing, and oil extraction, oil is broken into tiny particles by external forces such as stirring and impact, and emulsifiers adsorb on the surface of oil droplets to form a protective film. This film isolates the contact between oil droplets and water, and also prevents the collision and fusion between oil droplets, allowing them to remain dispersed for a long time and unable to achieve oil-water separation through natural standing. According to the characteristics of emulsifiers, emulsified oil can also be divided into water-in-oil type (oil as the continuous phase and water as the dispersed phase) and oil-in-water type (water as the continuous phase and oil as the dispersed phase). Among them, the oil-in-water type commonly found in industrial wastewater is more difficult to treat.
Core Difficulty 1: Stable Structure Is Hard to Break, Demulsification Is the Primary Hurdle
The fundamental reason why emulsified oil is difficult to treat lies in its strong stability, which stems from two core supports. To treat emulsified oil, it is necessary to break this "protection" first, which is what the industry calls "demulsification"—the most critical and tricky step in the entire treatment process. The first stability guarantee is the "double electric layer structure". Most emulsifiers are surfactants, which are hydrophilic at one end and lipophilic at the other. After adsorbing on the surface of oil droplets, they make the oil droplets carry a uniform negative charge. Oil droplets with the same charge will produce repulsive force, just like two like magnets repelling each other, making it impossible to approach, collide, or aggregate into large oil droplets. The double electric layer formed by this charge repulsion firmly locks the dispersed state of oil droplets, and even physical methods such as centrifugation and precipitation are difficult to break this balance.
The second stability guarantee is the "emulsifier protective film". After emulsifiers adsorb on the surface of oil droplets, they form a dense film that not only isolates the contact between oil droplets and water but also resists external physical impact and chemical interference. This film is like putting a "protective suit" on the oil droplets, preventing the coalescence of oil droplets and making it difficult for conventional chemical agents to contact the oil droplets themselves and play a role. More importantly, the types of emulsifiers in industrial wastewater are complex, and the formulas of emulsions in different industries vary greatly, so a single demulsification method is often unable to adapt to all scenarios.
Core Difficulty 2: Conventional Treatment Methods Are Ineffective, Requiring Customized Complex Processes
For free oil and dispersed oil, conventional physical methods can achieve effective treatment: free oil can naturally float up in oil separators using the density difference between oil and water, and dispersed oil can be separated by air flotation where bubbles adsorb oil droplets and float up. However, these methods are almost ineffective against emulsified oil.
The limitations of physical methods are very obvious: gravity sedimentation relies on the weight of oil droplets to sink or float, but emulsified oil droplets are extremely small and light, and the sedimentation speed is negligible, making it difficult to layer even after standing for several months; the bubble size of ordinary air flotation is much larger than that of emulsified oil droplets, which cannot effectively adsorb tiny oil droplets and is difficult to achieve separation; filtration will cause rapid clogging of filter media due to the viscosity of emulsified oil, which not only drastically reduces treatment efficiency but also increases operation and maintenance costs.
Although chemical methods can achieve demulsification, they have many constraints. Currently commonly used chemical demulsification methods include acid-base demulsification and addition of coagulants/flocculants: acid-base demulsification neutralizes the charge on the surface of oil droplets and destroys the double electric layer by adjusting the pH value of wastewater, but the efficiency is low, and the oil content of the effluent is difficult to meet the standard, requiring combination with other processes; inorganic coagulants (such as aluminum salts and iron salts) can make oil droplets aggregate through adsorption and bridging, but they will produce a large amount of sludge, increasing the difficulty of subsequent treatment; organic flocculants have strong pertinence and good demulsification effect, but they are expensive and diverse, making actual selection difficult, and most are still in the laboratory stage. More tricky is that industrial wastewater often contains impurities such as heavy metals and organic substances, which will interfere with the demulsification reaction and reduce the treatment effect.
Biological treatment methods face "adaptation difficulties". The BOD/COD ratio of emulsified oil wastewater is relatively high, and theoretically, residual organic substances can be degraded by microorganisms. However, emulsified oil will coat the surface of microorganisms, inhibit their metabolic activity, and lead to the failure of the biological treatment system. Therefore, biological methods can only be used as advanced treatment methods, which can be used only after thorough demulsification and removal of most oils in the front end.
Core Difficulty 3: Easy Rebound After Treatment, High Threshold for Up-to-Standard Discharge
Even if demulsification and oil-water separation are completed, the treatment of emulsified oil wastewater is not over—the remaining emulsifiers and tiny oil droplets are likely to cause a rebound in treatment effect. Some uncompletely decomposed emulsifiers will re-adsorb oil droplets during the flow of wastewater, forming an emulsified system again and causing the oil content of the effluent to exceed the standard.
At the same time, the continuous tightening of environmental protection discharge standards has further increased the difficulty of treatment. Currently, China has extremely high requirements for petroleum indicators in industrial wastewater discharge, and the detection accuracy requirements for residual emulsified oil have also increased accordingly. Once demulsification is incomplete, the remaining trace emulsified oil will cause discharge exceeding the standard, and enterprises may face fines, production suspension and other penalties. In addition, the oily sludge generated during the treatment is a hazardous waste that needs harmless treatment, which not only increases the treatment cost but also puts forward higher requirements for the treatment process.
Conclusion: The Difficulty of Emulsified Oil Stems from Dual Plights of Stability and Adaptation
In summary, the reason why emulsified oil has become a difficult problem in oily wastewater treatment is that its stable double electric layer structure and emulsifier protective film are difficult to break, and conventional treatment methods are not adaptable; at the same time, the complexity of industrial wastewater, the easy rebound after treatment, and the strict discharge standards further amplify the difficulty of governance.
With the development of environmental protection technology, new technologies such as ultrasonic demulsification and electric sedimentation demulsification have emerged, and up-to-standard treatment can also be achieved through combined "physical + chemical + biological" processes. However, these technologies generally have the problems of high cost and complex operation and maintenance, and have not been widely promoted. In the future, the key to solving this environmental problem is to develop efficient and low-cost customized demulsification technologies.
