Application of membrane separation technology in zero discharge of electroplating wastewater

In electroplating wastewater, there are a large number of heavy metal ions, cyanides, and other toxic substances, some of which are carcinogenic and mutagenic, and can cause serious harm to humans. When wastewater is discharged into water, it can also pollute the water body, poisoning it and posing an extremely serious threat to fish and other organisms living in it. Although electroplating wastewater poses serious harm to the environment and humans, there are some valuable metals in electroplating wastewater that can be recycled and reused. There are currently several technologies for treating electroplating wastewater, such as chemical precipitation, oxidation-reduction, ion exchange, electrolysis, etc. With the continuous deepening of research in this industry, some other methods are gradually being applied in the treatment process of electroplating wastewater, such as ferrite method, membrane separation technology, adsorption method, biological method and other new technologies. Applying these new technologies in the treatment process of electroplating wastewater can improve the recovery efficiency of wastewater and reduce the pollution caused by direct discharge of wastewater. Among them, membrane separation technology is an efficient recycling technology with high practicality, and is currently the most promising new technology for development.

Overview of Membrane Technology

1.1 Introduction to Membrane Technology

There are various membrane separation technologies currently available, including ultrafiltration, electrodialysis, dialysis, nanofiltration, liquid membrane, etc. It has been applied in various fields and has broad application prospects. However, it cannot be ignored that there are still some factors that hinder the widespread development of membrane separation methods. For example, high inlet water quality requirements, regular cleaning, high operating costs, and continuous research are needed.

1.2 Introduction to membrane technology principles

The basic principle of membrane technology for treating wastewater is mainly to utilize the penetrability of water molecules in aqueous solutions (raw water), so that the separation membrane can maintain the phase change of the substances passing through, and under the action of external forces, the aqueous solution (raw water) can be separated from solutes or other impurities, ultimately obtaining relatively pure water, achieving the goal of treating wastewater and improving water quality. This technology essentially belongs to the category of physical separation, where substances do not undergo phase changes when passing through membranes, resulting in a relatively high energy conversion rate and good separation efficiency. It also has advantages such as energy saving, ease of operation, and the ability to achieve automation. Therefore, in future research, this is a promising new water treatment technology worth exploring.

1.3 Characteristics of membrane separation technology

Membrane separation technology refers to the technique of filtering and classifying various substances through membranes, which can selectively permeate different substances and complete the separation, purification, and concentration of substances. Membrane separation technology is a physical technology that has no phase change, low energy consumption, high efficiency, and can achieve energy-saving goals. Its treatment process is easy to control and will not cause pollution to the outside world. According to the size of the substance to be separated, membrane separation technology can be divided into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Microfiltration is more suitable for pre filtration of electroplating solutions, while ultrafiltration is mainly used in the recovery process of electrophoretic paints. Nanofiltration and reverse osmosis are widely used in the post-treatment of chemical treatment and the preparation of process pure water. By using membrane technology to separate and concentrate heavy metal rinse water during electroplating production, metal ions can be recovered, and water resources can be recycled to form a closed-loop cycle between the metal and water in the rinse water. Currently, membrane separation technology is widely used in the treatment of electroplating water, and countries are actively strengthening research and development on this technology.

Application of membrane separation technology in electroplating water treatment

Scholars have adopted membrane separation technology in the treatment process of nickel electroplating rinse water. Through concentration and reuse of rinse water, it has been found that this technology is feasible for application in electroplating solutions. Membrane separation technology can effectively intercept nickel ions, with a retention rate greater than 99%. The nickel ion concentration selected in the experiment is 145mg/L. After membrane separation technology concentration, the nickel ion concentration in the concentrated solution reaches 50g/L, while the remaining permeate can be recycled after treatment. This process can utilize both the metal and water in the molten metal, but due to the use of two-stage reverse osmosis, it consumes more energy and requires a larger investment.

In membrane separation technology, the membrane is a crucial substance, and its filtration effect has a direct impact on the treatment effect of the metal liquid. Some scholars use isopropanol as the main raw material, and through acid hydrolysis, alcohol removal, drying, and sintering processes, produce ceramic ultrafiltration membranes. Under an external pressure of about 0.2 MPa, ultrafiltration separation is carried out, and after precipitation treatment, clear liquid on electroplating wastewater is finally obtained. The experimental results show that the membrane throughput will continuously decrease, and the decrease rate is relatively fast. For example, after about 10 minutes of filtration, the membrane throughput is about 2.61 cubic meters, and after 70 minutes, the membrane permeability becomes 0.5 cubic meters. After precipitation, metal ions in electroplating water mainly exist in the form of complexes and coordination compounds, which can be intercepted by ceramic ultrafiltration membranes with relatively small pore sizes. The removal rate of copper metal reaches 70%, and the removal rate of chromium metal is about 10%. In the permeate, the concentrations of copper, chromium, and nickel are 0.0663mg/L, 0.0051mg/L, and 0.0763mg/L, respectively.

Development of membrane separation technology in China

Since the 1970s, there have been factories in China applying membrane separation technology to treat nickel electroplating wastewater and recover nickel. After more than 20 years of development, membrane separation technology began to be widely used in large scale at the beginning of this century. Some companies use membrane separation technology to recover nickel and water from electroplating foam nickel wastewater. Subsequently, more and more companies began to adopt membrane separation technology for electroplating wastewater treatment, such as Ningbo Koningda Industrial Co., Ltd., Ningbo Guanghua Battery Co., Ltd., Changsha Liyuan New Materials Co., Ltd., etc., all of which have applied membrane separation technology extensively. For example, Changsha Liyuan New Material Co., Ltd. is a well-known continuous ribbon foam nickel manufacturer in China, whose output is in a leading position in the world, but during the electroplating production process, a lot of nickel containing wastewater has been produced. Changsha Liyuan membrane separation project is a project to separate and treat the nickel ions in the company's electroplating wastewater. At the beginning, the company used chemical treatment to treat the nickel ions, which produced poor results. Moreover, the concentration of nickel ions in the discharged wastewater is still relatively high, and more nickel containing sludge has also been produced, which has caused more and more serious environmental pollution. With the application of membrane separation technology, the content of metal ions in its wastewater discharge is reduced, and the treated water resources can also be used. After treatment, the total dissolved solids in the recovered water are less than 10mg/L, making it a high-quality electroplating process water. During operation, it basically meets industry production standards and environmental emission requirements.

3.1 Textile wastewater

The components contained in the wastewater generated by the textile industry are relatively complex and diverse, and various substances may undergo rapid changes in this type of wastewater. At present, some textile enterprises in China have introduced and started using membrane separation technology to treat their wastewater. For example, Ningbo Shenying Knitting Industry and Trade Co., Ltd. effectively utilizes membrane separation technology to treat and reuse printing and dyeing wastewater in the enterprise. The Ningbo Branch of the Weapon Science Research Institute has developed a new wastewater treatment technology based on the application of nanofiltration membranes and reverse osmosis membranes. The principle of the new technology is that under a certain pressure, 99.5% of sodium ions in water cannot pass through the reverse osmosis membrane, and particles larger than sodium ions are even more difficult to pass through the "sieve holes". Only relatively pure water can pass through. After membrane technology treatment, the water has low hardness and can be recycled. The effective utilization of reverse osmosis membranes is of great significance for printing and dyeing enterprises. Not only can it effectively recycle and utilize printing and dyeing raw materials, greatly reducing the cost of enterprises, but it also hopes to achieve the goal of zero or micro discharge of wastewater for printing and dyeing enterprises, saving the use of fresh water such as tap water, which is a great contribution to environmental protection, economic benefits, etc.

3.2 Papermaking wastewater

In recent years, the main wastewater treatment methods for small and medium-sized paper-making enterprises in China have been acidification and ultrafiltration. The principle is to separate lignin from black liquor while reducing COD and BOD. However, with the popularization of membrane separation technology and its advantages of low cost and small footprint, Du Ming et al. used microfiltration coagulation precipitation method to treat papermaking wastewater. The principle is to use microfiltration to recover pulp and coagulation precipitation to remove the main pollutants in papermaking wastewater. In papermaking enterprises, the use of nanofiltration for wastewater treatment and the recovery of useful substances from wastewater can further effectively control the pollution caused by wastewater discharge. Pan Luting et al. used TOA (tri-n-octylamine) emulsion liquid membrane method to effectively treat black liquor. This method can achieve a COD removal rate of over 98%, and the extracted lignin can also be used as raw materials and products for other chemical industries in future production, achieving a dual effect of pollution control and comprehensive utilization.

3.3 Pharmaceutical wastewater

Most of the wastewater generated in pharmaceutical companies is mixed wastewater formed by the discharge of multiple drug production processes. Previously, both domestically and internationally, the anaerobic aerobic biochemical combined treatment method was mainly used for wastewater treatment, which has achieved certain results. However, this method has its limitations. For example, the anaerobic treatment process has relatively high requirements for factors such as temperature and pH value. At the same time, the operating range of this method is very limited, and the residence time of the structure is relatively long. On the other hand, using anaerobic treatment methods to treat biogas yields low yields and has relatively low economic value. Moreover, direct discharge after treatment can lead to secondary pollution of the environment, which may cause other safety hazards. In addition, the drawbacks of a single conventional aerobic biochemical treatment method are also quite obvious. The emergence of membrane technology can to some extent reduce the difficulty. PW membrane bioreactor technology is mainly composed of membrane components and bioreactors, which can maintain high MLSS concentration and long SRT time for a certain period of time. Moreover, due to the isolation function of the membrane, slow growing nitrifying bacteria can accumulate in the reactor for time. As the MLSS gradually increases, the number of nitrifying bacteria also continues to increase, enhancing the nitrification capacity of the reactor. As a result, after passing through the membrane, most of the water produced does not contain harmful substances such as bacteria, viruses, parasite eggs, etc., with relatively low turbidity, fully meeting the national wastewater discharge standards.

3.4 Heavy metal wastewater

Heavy metal wastewater pollution is one of the factors causing environmental pollution. Especially for enterprises such as electroplating, metallurgy, and mining, they are high-yield areas for heavy metal wastewater. During the production process, a large amount of wastewater containing metal ions such as chromium, nickel, copper, and cadmium will be generated. At present, the main methods for treating heavy metal wastewater both domestically and internationally are liquid membrane method and ion exchange method. And the main recovered material is ZN2 in acidic water, but it is difficult to effectively recover NA2SO4 and H2SO4. Today's nanofiltration membrane technology can ensure that 90% of heavy metal containing wastewater is effectively recovered or purified, and the concentration of heavy metal ions gradually increases throughout the separation process, even reaching the standard for recycling. If conditions permit, it is even possible to separate and recover other ions, such as Ni2 and CD2.

4 Conclusion

With the promotion of membrane technology and its advantages in wastewater treatment, this has created opportunities for the development and application of membrane technology in the future. The development of membrane technology has promoted the development of wastewater treatment and provided people with more convenience in their daily lives and production. However, it is a relatively young new technology developed in recent years, which is still not mature enough. There are few examples of large-scale industrial membrane separation, and more separation technologies are still in the process of exploration and research. There are many problems that need to be solved and improved, which is also a challenge for the future development of membrane technology.

The electroplating industry is an important industry in industrial production. During the electroplating production process, a lot of electroplating wastewater is generated. Direct discharge of wastewater can cause serious pollution to the environment and pose a threat to people's health. Therefore, wastewater should be treated before discharge. The traditional treatment method is chemical precipitation treatment, which can generate other precipitation pollutants and has poor filtration effect. Membrane separation technology can separate various heavy metal ions from metal solutions, separate heavy metal ions from aqueous solutions, and achieve efficient recovery of many valuable metals in electroplating water. Therefore, membrane separation technology can promote the sustainable development of the electroplating industry and achieve clean production.