Emulsions are widely used in mechanical processing, automotive engine processing, cooling and lubrication of rolling rollers and steel plates. Emulsion is subject to the influence of metal dust and surrounding environmental media during recycling, aging and deterioration, and must be replaced regularly. The chemical properties of the replaced emulsion wastewater are extremely stable, making it very difficult to treat. The author reviewed the treatment technology of emulsion wastewater in order to provide some reference for emulsion wastewater treatment.
Characteristics of Emulsion Wastewater
1.1 Formation of Emulsion
A large amount of surfactant is added to the emulsion, which reduces the surface free energy of the system, and the surfactant molecules are directionally adsorbed at the oil-water interface and form an interfacial facial mask, which prevents the collision between oil drops from becoming larger, so that oil drops can exist in water stably for a long time. Therefore, when treating emulsion wastewater, it is necessary to destroy its stability and try to eliminate or weaken the ability of surfactants to stabilize the emulsion in order to achieve oil-water separation.
1.2 Characteristics of Emulsion Wastewater
Emulsion wastewater is a kind of industrial wastewater that is difficult to treat, and its chemical stability and pollution load are extremely high. According to relevant data, the concentration of oil in emulsion wastewater is as high as 15000~20000 mg/L, COD reaches 18000~35000 mg/L, and BOD reaches 5000~10000 mg/L. In order to improve the performance of the emulsion, a large number of additives are added, such as oily additives, extreme pressure additives, antirust additives, anti mold additives, anti foam additives, etc., which make the composition of the emulsion extremely complex and increase the difficulty of treatment.
2. Treatment Technology for Emulsion Wastewater
At present, chemical coagulation, co coagulation gas floatation, electrocoagulation, advanced oxidation, ultrafiltration and biochemical combination process are mainly used to treat emulsion wastewater. Among them, co coagulation gas floatation and electrocoagulation are developed on the basis of chemical coagulation, while advanced oxidation and ultrafiltration respectively use Advanced oxidation process and membrane technology in water treatment. The biochemical combination process is developed on the basis of the above methods in combination with biochemical treatment, The current application status of them in the treatment of emulsion wastewater is introduced separately.
2.1 Chemical coagulation method
Chemical coagulation method is a traditional method for treating emulsion wastewater, which involves adding chemical coagulants to the emulsion wastewater. On the one hand, hydrolysis reactions occur to generate colloidal adsorbed oil droplets, and on the other hand, polymerization occurs to form different degrees of macromolecular polymers. Through adsorption flocculation and bridging, oil droplets are removed to achieve demulsification and achieve oil-water separation.
In the early research on the treatment of emulsion wastewater by chemical coagulation, inorganic coagulants such as iron sulfate and aluminum sulfate were commonly used. However, due to the unsatisfactory effect of traditional inorganic coagulants, many applications and studies of inorganic polymer coagulants have emerged in recent years. Wu Keming et al. used water glass and sulfuric acid to produce a polysilicate aluminum sulfate composite coagulant for the treatment of high concentration emulsion wastewater with turbidity of 10 910 NTU, oil of 3 446 mg/L, and COD of 21 006 mg/L, with corresponding removal rates of 99.9%, 99.7%, and 99.5%, respectively. Zhang Jianpeng et al. used a composite polyaluminum iron coagulant to treat emulsion wastewater, which not only achieved good demulsification effect, but also achieved average removal rates of CODCr and oil of over 90% and 99%, respectively. The coagulated effluent also had high biochemical properties. Lin Yongzeng et al. applied polyaluminum chloride (PAC) prepared from pickling wastewater as raw material to the treatment of emulsified wastewater from secondary cold rolling, with a COD removal rate of over 95%, achieving the goal of treating waste with waste.
In addition, organic coagulants also have certain applications in the treatment of emulsion wastewater. Li Zhengyao et al. used organic demulsifier SYS and polyaluminum chloride to jointly treat the cold rolling emulsion wastewater of a certain steel company with an oil concentration of 6200 mg/L and a COD of 34000 mg/L. After the second stage demulsification, the oil removal rate reached 99.58% and the COD removal rate was 97.79%, achieving very ideal results.
2.2 Coagulation gas flotation method
The co coagulation air flotation method is a method that combines chemical coagulation with air flotation technology. Due to the large particle size oil droplets and flocculent substances generated after chemical coagulation can collide and adhere to the micro bubbles generated by the air flotation machine, forming larger particle size aerated flocs. Therefore, its removal effect is more significant than that of coagulation precipitation method, with stronger adaptability to pH, water temperature, and pollutant load, less dosage, and shorter reaction time.
At present, research on co coagulation air flotation treatment of emulsion wastewater has been conducted in detail abroad. A. I. Zouboulis et al. used the co coagulation gas flotation method to treat simulated emulsion wastewater containing n-octane. The research results indicate that the main influencing factors of this method include the dosage of flocculants, initial pH, concentration of chemical additives (such as demulsifiers), concentration of flotation collectors, and cycle ratio. Under the optimal experimental conditions, 95% of the emulsified oil was separated from the simulated emulsion wastewater with an initial oil concentration of 500 mg/L. K. Bensadok et al. found that when the conventional demulsification method had low COD removal rate, the combined use of dissolved air flotation method resulted in a 29% reduction in COD and a 71% reduction in turbidity compared to the original process.
Good results have also been achieved in the research on co coagulation air flotation treatment of emulsion wastewater in China. Cao Fu et al. used polymeric aluminum iron chloride (PAFC) for co coagulation air flotation treatment of emulsion wastewater. When PAFC was 1 g/L, the turbidity removal rate reached over 98%. Xu Zhi et al. used the co coagulation air flotation demulsification adsorption method to treat emulsion wastewater. On the basis of adding PAC and PAM, the remaining sludge from the sewage treatment plant with certain adsorption capacity was added to the emulsion wastewater. It was found that when the sludge dosage was 15 g/L, the best treatment effect was achieved for COD. The COD of the wastewater could be reduced from 5000 to 2800 mg/L before treatment to 75 mg/L after treatment, and the treatment effect reached the national first level comprehensive sewage discharge standard.
2.3 Electrocoagulation method
The electrocoagulation method uses soluble metals as electrodes, and under the action of an electric field, the metals lose electrons and are oxidized to form hydroxide colloids. It utilizes adsorption and condensation, as well as the redox reactions that occur during the electrolysis process, to achieve the removal of oil stains. Due to its ability to greatly reduce the use of coagulants and its good treatment effect, this method has great application prospects. Usually, the electrocoagulation mechanism varies depending on the electrode material. When using metal as the anode and inert material as the cathode, the electrolysis process will produce metal colloids. The role of electrode reactions is manifested in reduction decolorization, electrochemical action, coagulation, adsorption, etc. The research materials are mainly iron filings and coke. Chen Yilan et al. used the rotating electrocoagulation demulsification technology to treat metal processing emulsions, and the removal rates of oil and COD were above 59.9% and 28.5%, and the raw water B/C could be increased from 0.21 to 0.32.
When using metals as negative and positive electrodes, NaCl is usually added. The electrode reaction produces metal colloids, strong oxidants chlorine gas, and hypochlorite, which can play a role in coagulation, adsorption, air flotation, and oxidation and reduction. P. Ca 觡 izares et al. use aluminum as the electrode, with a spacing of 9 mm between the plates and a current density of 1.01 × Under the condition of 10-2 A/cm2, the emulsion was treated by electrocoagulation and compared with the chemical coagulation method with the addition of AlCl3 or Al2 (SO4) 3. The experimental results show that the effectiveness of the two methods is not related to the dosage, but to the concentration and pH of aluminum ions in the water. At the optimal pH of 5-9, the COD removal rate is higher. Wu Keming et al. used aluminum plates as electrodes to prevent passivation. They used timed polarity reversal and added NaCl to treat emulsion wastewater. The organic matter in the emulsion wastewater was oxidized by chlorine gas and hypochlorite generated from the reaction, and the aluminum complex ions and aluminum hydroxide generated during the electrolysis process were used to remove the organic matter and suspended matter. The results show that the method has high removal rates for turbidity, oil, and COD, reaching 99.1%, 98.6%, and 99.3%, respectively.
Researchers have discussed the design parameters of the electrocoagulation method. For external power supply forms, research has shown that AC has better coagulation effect than DC, and frequency control at 60 Hz has higher economic suitability. Zhou Liancheng et al. pointed out that excessive spacing between electrode plates, high current density, and long electrolysis time are the reasons for the failure of electrolysis demulsification, and proposed the optimal operating conditions for electrode plate spacing of 8-15 mm, current density of 0.004-0.006 A/cm2, and electrolysis time of 40-50 minutes. Cao Fu et al. used aluminum plates as electrodes and added NaCl to treat steel rolling emulsion wastewater. In the experiment, when pH=6, current density was 0.004A/cm2, time was 40 minutes, NaCl was 1.25 g/L, and electrode spacing was 1 cm, the COD removal rate reached 99.5%, achieving good treatment results.
2.4 Advanced oxidation method
The use of advanced oxidation method to treat emulsion wastewater is based on the strong oxidation of · OH, and Fenton oxidation is the main research focus in this regard. A. C. S. C. Teixeira et al. used Fenton and photoassisted Fenton methods to treat emulsion wastewater containing different concentrations of PDMAS (an amino organosilicon polymer). Through analysis of COD, nitrate, iron, and ferrous ions, it was found that PDMAS was removed during the oxidation process, mainly due to the degradation of surfactants in the emulsion, allowing for further aggregation of PAMAS and the action of · OH. M. The research results of A. Tony et al. also indicate that the photoassisted Fenton method has a good treatment effect on emulsified oily wastewater. It can not only effectively remove COD and oil, but also significantly improve the water quality of emulsified wastewater. To reduce the use of ferrous in Fenton oxidation, Tang Wenwei et al. used a wet hydrogen peroxide oxidation process that replaced some or all of the air with H2O2 to treat emulsion wastewater, significantly reducing the amount of ferrous added. At 150 ℃ and influent COD of 50 540 mg/L, the removal rate reached 82.4%. Li Chuncheng combined micro electrolysis and Fenton method to treat emulsion wastewater, and under the optimal operating conditions, the COD removal rate can reach 97.16%.
2.5 Ultrafiltration method
The ultrafiltration method for treating emulsion wastewater mainly utilizes significant differences in the size of oil-water molecules and adopts cross flow filtration to filter oil-water. Water molecules are smaller than pores and pass through the ultrafiltration membrane, while oil molecules are larger than pores and cannot pass through the ultrafiltration membrane, thus achieving oil-water separation. For the treatment of emulsion wastewater, organic membrane was used in the early stage of ultrafiltration. However, because the cost of organic membrane was too high, and it was not resistant to high temperature, low mechanical strength, easy hydrolysis, etc., inorganic membrane represented by Ceramic membrane quickly occupied the market. When treating emulsion wastewater, the stability of ultrafiltration system operation, adaptability to emulsion changes, operation management, and treatment cost are all superior to oxidation method. Therefore, it has been widely used in the field of emulsion wastewater treatment. To address the issues of rapid decrease in membrane flux and easy fouling of ultrafiltration membranes, Zhao Wei et al. studied the impact of operating parameters on ultrafiltration systems. By controlling the operating temperature to be (60 ± 5) ℃, pH to be 9-11, and performing alkaline washing and acid washing operations every 3 operating cycles, ideal results were achieved. P. Janknecht et al. compared the treatment effects of 14 different pore size ultrafiltration membranes and microfiltration membranes on industrial cutting emulsion wastewater, and finally determined the suitable filter membrane for treating cutting emulsion wastewater through experiments.
At present, research on ultrafiltration is mostly focused on the use of combined processes. Shu Li et al. applied ultrasonic technology to the ultrafiltration process for treating emulsion wastewater, which not only greatly improved the pollutant removal rate, but also increased membrane flux and reduced membrane pollution. I. S. Chang et al. combined ultrafiltration with advanced oxidation to treat emulsion wastewater from automotive parts factories. During the ultrafiltration process, the non permeated oil can be reused, and the permeated liquid can be treated with ozone oxidation as reused water.
2.6 Biochemical combination process
The demulsification operation can disrupt the stabilizing effect of surfactants in the emulsion and achieve oil-water separation, but the COD of the treated emulsion remains at a high level and further treatment is needed to meet the discharge or reuse standards. Due to the removal of oil substances, the emulsified wastewater after demulsification has a certain degree of biodegradability, making biochemical treatment possible. Cheng et al. treated the effluent after demulsification with calcium chloride and alum, PAC and PAM coagulation treatment. The use of hydrolysis aerobic activated carbon adsorption can achieve COD of 50-70 mg/L, SS of 75 mg/L, petroleum of 5.4 mg/L, and chromaticity of 5 times. Lin Ming et al. used demulsification, membrane filtration, Fenton oxidation, and biochemical processes to treat high concentration emulsified oil wastewater, with COD ranging from 3 ×  104~2 × The treatment effect is good when the concentration drops from 106 mg/L to below 50 mg/L. Zhu Jing et al. used the coagulation air flotation SBR filtration process to treat emulsion wastewater. The COD, BOD, and oil levels decreased from 22400, 2680, and 1420 mg/L to 137, 25, and 0.8 mg/L, with removal rates of 99.38%, 99.06%, and 99.94%, respectively. For details, please refer to http://www.dowater.com More relevant technical documentation.
3 Outlook
(1) Chemical coagulation, co coagulation gas flotation, electrocoagulation, advanced oxidation, and ultrafiltration can all be used as effective demulsification processes for the treatment of oily emulsion wastewater. The co coagulation gas flotation method and electrocoagulation method have better treatment effects than traditional coagulation processes, and should be a key research object for emulsion treatment, just like advanced oxidation method and ultrafiltration method. (2) At present, the electrocoagulation method is widely used in practical engineering, as it can greatly reduce the use of chemical coagulants. Therefore, the focus of research on this method should be on further optimization of electrode operating parameter conditions. (3) The oil-water separation effect of advanced oxidation method is not as good as that of coagulation method or ultrafiltration method, so it is advisable to use it as a physicochemical treatment method after demulsification to further reduce COD and improve biodegradability. Due to the direct impact of the pore size of ultrafiltration membranes on the molecular permeability, the research focus should be on the selection of pore size for ultrafiltration membranes when processing different types of emulsions. (4) Two or more processes can be considered to be organically combined for the treatment of emulsion wastewater to achieve mutual promotion. If ultrasonic technology is used for ultrafiltration treatment of emulsion wastewater, it can not only improve pollutant removal rate, but also increase membrane flux and reduce membrane pollution; The use of advanced oxidation to treat ultrafiltration effluent can further improve the quality of the effluent; If the biochemical process is used as a post-treatment, it will have higher economic efficiency. Therefore, combined processes, especially those that use biochemical processes as post-treatment, will become a hot research topic in the treatment of emulsion wastewater.