Electroplating is a process that uses electrochemical methods to decorate, protect, and obtain certain new properties on metal and non-metal surfaces. To ensure the quality of electroplated products, ensure that the metal coating has a smooth and smooth appearance, and firmly bond with the substrate, it is necessary to thoroughly clean the dirt (oil, rust, oxide skin, etc.) on the surface of the plated parts before plating, and clean the adhesion solution on the surface of the plated parts after plating. Therefore, a large amount of electroplating wastewater is inevitably discharged during the general electroplating production process, and electroplating wastewater treatment and heavy metal wastewater treatment must be carried out.

Electroplating wastewater treatment is a complex and difficult to thoroughly treat mixed heavy metal wastewater. Electroplating wastewater treatment must be based on factors such as the quality and quantity of electroplating wastewater, the process conditions of electroplating production, production load, operation management, and water usage. The water quality of electroplating heavy metal wastewater treatment is complex and the composition is difficult to control. It contains heavy metal pollutants such as cyanide, acid, alkali, hexavalent chromium, copper, zinc, cadmium, nickel, gold, silver, etc., which are highly toxic. Some of them are highly toxic substances that cause cancer, deformity, and mutation, posing great harm to humans.

Electroplating wastewater treatment mainly targets electroplating rinsing wastewater, passivation wastewater, plating acid pickling wastewater, wastewater from brushing floors and plates, as well as wastewater generated by "running, emitting, dripping, and leaking" due to poor operation or management. In addition, there is also the discharge of self used water during the wastewater treatment process and laboratory drainage. There are two main ways for electroplating wastewater to pollute the environment. One is the discharge of electroplating wastewater with low volume and high concentration, and the other is the discharge of electroplating wastewater with large volume and relatively low concentration (mainly cleaning wastewater).

The purpose of electroplating wastewater treatment is to separate and treat or recycle harmful and toxic substances in wastewater, or to modify toxic substances into non-toxic substances.

Chemical precipitation method is a method of transforming dissolved heavy metals in wastewater into insoluble heavy metal compounds, including neutralization precipitation method and sulfide precipitation method.

Add alkali to wastewater containing heavy metals for neutralization reaction, allowing heavy metals to form insoluble hydroxide precipitates for separation. The neutralization precipitation method is a commonly used method for treating wastewater due to its simple operation. Practice has shown that the following points need to be noted in operation: (1) After neutralization and sedimentation, if the pH value in the wastewater is high, it needs to be neutralized before discharge; (2) Multiple heavy metals often coexist in wastewater. When wastewater contains zwitterionic metals such as Zn, Pb, Sn, Al, etc., the pH value is relatively high and there may be a tendency for re dissolution. Therefore, pH value should be strictly controlled and segmented precipitation should be implemented; (3) Some anions in wastewater, such as halogens, cyanide ions, humic substances, etc., may form complexes with heavy metals, so pre-treatment is required before neutralization; (4) Some particles are small and difficult to precipitate, so flocculants need to be added to assist in precipitation generation.

The method of adding sulfide precipitants to generate sulfide precipitation and remove heavy metal ions from wastewater. Compared with neutralization precipitation method, the advantage of sulfide precipitation method is that the solubility of heavy metal sulfides is lower than that of their hydroxides, and the pH value of the reaction is between 7-9. The treated wastewater generally does not need to be neutralized. The disadvantages of sulfide precipitation method are [2]: the sulfide precipitate particles are small and easy to form colloids; Sulfide precipitants themselves remain in water and generate hydrogen sulfide gas when exposed to acid, resulting in secondary pollution. In order to prevent secondary pollution, British scholars have developed an improved sulfide precipitation method, which selectively adds sulfide ions and another heavy metal ion to the wastewater to be treated (the equilibrium concentration of sulfide ions in this heavy metal is higher than that of sulfides in the heavy metal pollutant to be removed). Due to the fact that sulfides of added heavy metals are more easily dissolved than sulfides of heavy metals in wastewater, the original heavy metal ions in wastewater are separated first compared to the added heavy metal ions, while preventing the generation of harmful gases such as hydrogen sulfide and residual sulfide ions.

Cr in electroplating wastewater mainly exists in the form of Cr6+ions. Therefore, reducing agents are added to the wastewater to reduce Cr6+to slightly toxic Cr3+, and lime or NaOH is added to produce Cr (OH) 3 for precipitation, separation, and removal. Chemical reduction method is one of the earliest applied treatment technologies for electroplating wastewater, which is widely used in China. Its treatment principle is simple, easy to operate, and can withstand the impact of large amounts and high concentrations of wastewater. According to the different addition of reducing agents, chemical reduction methods can be applied to treat Cr containing wastewater, such as FeSO4 method, NaHSO3 method, iron filings method, SO2 method, etc. Lime is generally used for alkalization, but there is a lot of waste residue; By using NaOH, there is less sludge, but the cost of chemicals and treatment is high, which is a disadvantage of chemical reduction method.

Ferrite technology has been developed based on the principle of producing ferrite. Add excess FeSO4 to wastewater containing Cr, reducing Cr6+to Cr3+and oxidizing Fe2+to Fe3+. Adjust the pH value to around 8 to produce hydroxide precipitates for Fe ions and Cr ions. Stir with air and add hydroxide to continuously react, forming chromium ferrite. Its typical processes include intermittent and continuous. The sludge formed by the ferrite method has high chemical stability and is easy to separate and dehydrate from solid and liquid. The ferrite method is particularly suitable for electroplating mixed wastewater containing heavy metal ions, in addition to being able to treat wastewater containing Cr. The application of ferrite method in China has a history of several decades, and the treated wastewater can meet the discharge standards. It is widely used in the domestic electroplating industry.

The ferrite method has the advantages of simple equipment, low investment, easy operation, and no secondary pollution. However, heating (about 70 degrees Celsius) is required during the formation of ferrite, which results in high energy consumption, high salinity after treatment, and the disadvantage of not being able to treat wastewater containing Hg and complexes.

The electrolysis method for treating wastewater containing Cr has a history of more than 20 years in China, which has advantages such as high removal rate, no secondary pollution, and the recyclability of precipitated heavy metals. There are approximately 30 types of metal ions in wastewater solutions that can be electrodeposited. Electrolysis is a relatively mature treatment technology that can reduce the generation of sludge and recover metals such as Cu, Ag, and Cd. It has been applied in wastewater treatment. However, the cost of electrolysis is relatively high, and generally, electrolysis after concentration has better economic benefits.

Solvent extraction method [4] is a commonly used method for separating and purifying substances. Due to liquid-liquid contact, continuous operation is possible and the separation effect is good. When using this method, it is necessary to choose extractants with high selectivity. Heavy metals in wastewater generally exist in the form of cations or anions. For example, under acidic conditions, they undergo complexation reactions with extractants, are extracted from the aqueous phase to the organic phase, and then re extracted to the aqueous phase under alkaline conditions to regenerate the solvent for recycling. This requires careful selection of aqueous acidity during extraction operations. Although the extraction method has significant advantages, the loss of solvents during the extraction process and the high energy consumption during the regeneration process limit the application of this method.

Adsorption is an effective method for removing heavy metal ions by utilizing the unique structure of adsorbents. The adsorbents used for treating electroplating heavy metal wastewater by adsorption method include activated carbon, humic acid, sepiolite, and polysaccharide resin. Activated carbon equipment is simple and widely used in wastewater treatment, but the regeneration efficiency of activated carbon is low, and it is difficult to meet the reuse requirements for water treatment. It is generally used for pre-treatment of electroplating wastewater. Humic acid substances are relatively inexpensive adsorbents, and humic acid resin has been successfully used to treat wastewater containing Cr and Ni. Related studies have shown that chitosan and its derivatives are good adsorbents for heavy metal ions. After crosslinking with chitosan resin, it can be reused 10 times without a significant decrease in adsorption capacity [5]. The use of modified sepiolite to treat heavy metal wastewater has good adsorption capacity for Pb2+, Hg2+, and Cd2+. The heavy metal content in the treated wastewater is significantly lower than the comprehensive sewage discharge standard. There are also literature reports that montmorillonite is also a high-performance clay mineral adsorbent. Aluminum zirconium pillared montmorillonite has a removal rate of 99% for Cr6+under acidic conditions, and the content of Cr6+in the effluent is lower than the national emission standard, which has practical application prospects

Membrane separation method is a technology that utilizes the selectivity of polymers for material separation, including electrodialysis, reverse osmosis, membrane extraction, ultrafiltration, etc. The electrodialysis method is used to treat electroplating industry wastewater, and the composition of the treated wastewater remains unchanged, which is beneficial for reuse in the tank. Wastewater containing Cu2+, Ni2+, Zn2+, Cr6+and other metal ions is suitable for electrodialysis treatment, and a complete set of equipment has been established. The reverse osmosis method has been widely used for the treatment of Zn, Ni, Cr rinse water and mixed heavy metal wastewater. The reverse osmosis method is used to treat electroplating wastewater, and the treated water can be reused to achieve closed-loop circulation. There are many research reports on the treatment of electroplating wastewater by liquid film method. In some fields, liquid film method has entered the initial industrial application stage from basic theoretical research. For example, China and Austria both use emulsion liquid film technology to treat wastewater containing Zn, and it is also applied in the treatment of Au plating waste liquid [7]. Membrane extraction technology is an efficient and non secondary pollution separation technology, which has made great progress in metal extraction.

Ion exchange treatment is a method that uses ion exchanger to separate harmful substances from wastewater. The applied ion exchanger includes ion exchange resin, zeolite, etc. The ion exchange resin includes gel type and macroporous type. The former has selectivity, while the latter has complex manufacturing, high cost, and high consumption of rejuvenators, which greatly limits its application. Ion exchange is achieved through ion exchange between the freely moving ions carried by the exchange agent itself and the ions in the treated solution. The driving force behind ion exchange is the concentration difference between ions and the affinity of functional groups on the exchange agent for ions. In most cases, ions are first adsorbed and then exchanged, and ion exchange agents have a dual effect of adsorption and exchange. The application of this material is increasing, such as bentonite [11], which is a clay mainly composed of montmorillonite. It has good water absorption and expansion, large specific surface area, strong adsorption capacity and ion exchange capacity. If improved, its adsorption and ion exchange capacity is stronger. However, it is difficult to regenerate, and natural zeolite has greater advantages than bentonite in the treatment of heavy metal wastewater: zeolite [9] is an aluminosilicate mineral with a grid structure, which is porous inside, has a large specific surface area, and has unique adsorption and ion exchange capabilities. Research has shown [10] that the mechanism by which zeolite removes heavy metal ions from wastewater is mostly a dual effect of adsorption and ion exchange. As the flow rate increases, ion exchange will replace adsorption as the main factor. If NaCl is used for pretreatment of natural zeolite, the adsorption and ion exchange capacity can be improved. Through adsorption and ion exchange regeneration processes, the concentration of heavy metal ions in wastewater can be increased by 30 times. Zeolite can remove copper, with a removal rate of over 97% in the NaCl regeneration process. It can be adsorbed and exchanged multiple times for regeneration cycles, and the removal rate of copper does not decrease.

Due to the drawbacks of high cost, complex operation, and difficulty in treating harmful pollutants with high flow rate and low concentration in traditional governance methods, biological governance technology has been increasingly valued after years of exploration and research. With the progress of research on heavy metal toxic microorganisms, the use of biotechnology to treat electroplating heavy metal wastewater has shown a vigorous development trend. According to the different mechanisms of biological removal of heavy metal ions, it can be divided into biological flocculation method, biological adsorption method, biochemical method, and plant remediation method.

With the implementation of global sustainable development strategies, circular economy and clean production technologies are receiving increasing attention. The treatment of electroplating heavy metal wastewater has developed from terminal treatment to comprehensive prevention and control stages such as clean production processes, material recycling, and wastewater reuse. In the future, the treatment of electroplating heavy metal wastewater will highlight the following aspects:

(1) Implement circular economy and attach importance to the development and application of clean production technology; Improve the conversion rate and recycling rate of electroplating materials and resources; Reduce the production of heavy metal pollutants from the source, adopt full process control, combine with comprehensive wastewater treatment, and ultimately achieve zero discharge of wastewater.

(2) There are many treatment technologies for electroplating heavy metal wastewater, among which biotechnology is a technology with great development potential, with advantages such as low cost, high efficiency, and no secondary pollution. With the development and application of genetic engineering, molecular biology and other technologies, efficient and toxic strains have been continuously cultivated, providing favorable conditions for the widespread application of biotechnology. For already polluted and extensive external environments, plant remediation technology can be used for treatment. While treating pollution, it not only beautifies the environment but also achieves certain economic benefits.

(3) Integrated technology is a hot topic in the treatment of electroplating wastewater in the future. There are various types of electroplating wastewater, and there are significant differences in various electroplating processes. Using only one wastewater treatment method often has its limitations and cannot achieve ideal results. Therefore, an integrated technology that integrates the characteristics of various governance technologies has emerged.

Electroplating wastewater has significant differences in water quality, so the selected processes vary greatly. Each process has specific control parameters and a range of parameters for normal operation, which need to be regularly recorded by dedicated management personnel.

The variation of parameters directly determines whether the equipment is operating normally, and is also the basis for determining the optimal cleaning time and cleaning effect of each equipment. Record these parameters on time every day and file them for record. If abnormal data is found, the cause should be promptly identified and a simple fault analysis should be conducted, or our company's after-sales service department should be contacted in a timely manner. Record the causes of these malfunctions.

1. The staff should conduct daily inspections of each workshop in the electroplating plant to ensure the normal collection of wastewater. Waste water such as coking copper, plating solution, chemical nickel, organic dyes, etc. cannot be discharged to the wastewater station and need to be collected and sent separately for external treatment. High concentration wastewater such as cleaning solution cannot be discharged at once and needs to be discharged in small quantities multiple times.

2. Staff should carefully inspect the equipment before starting work every day and promptly correct any obvious and easily detectable issues. For example, whether the water tank is leaking, etc.

3. Staff should pay attention to the order in which the equipment is turned on. The operation of the equipment is semi automated. The startup sequence of manually operated devices is particularly important, as improper operation can cause damage to the equipment. For example, whether the valves of each stage before equipment operation are opened normally, and whether the pump cannot idle without water.

4. After the equipment is running normally, the staff should record the operating parameters of the equipment in different time periods. And supervise the operation status of the equipment. If there are any abnormal situations, emergency measures should be taken. If it cannot be handled, it should be reported to the person in charge.

5. The staff should do a good job in daily equipment cleaning, and have a comprehensive understanding of the cleaning timing, cleaning methods, water volume, etc. for each processing structure to ensure the long-term stable operation of the equipment. Especially for ultrafiltration membranes and reverse osmosis membranes, their cleaning effect directly determines the wastewater recovery rate, effluent quality, and membrane service life.

6. The staff should be careful and meticulous in every operation to ensure the normal and stable operation of each system. In chemical precipitation systems, sufficient time is required for cyanide reduction and chromium reduction; In the reclaimed water reuse system, the pre-treatment system should be cleaned at least once a day.

7. The staff should turn off the wastewater treatment equipment at the end of each day. For equipment that operates all day, the wastewater treatment station should be equipped with sufficient personnel to operate and monitor it all day to ensure the normal and stable operation of the equipment.

8. The wastewater treatment workshop should do a good job in hygiene to ensure a clean and hygienic environment.

9. Staff should regularly (1 or 2 months) perform maintenance and upkeep on the equipment during shutdown to ensure uninterrupted operation of the processing equipment.