Abstract: In the analysis of China Electroplating wastewater Based on the treatment of the current situation, in combination with relevant national policies and regulations Electroplating wastewater treatment Various technological processes, and the future electroplating waste water treatment The future development of the electroplating industry will provide a reference for the realization of "clean production, energy conservation and emission reduction".
1. Current situation of electroplating wastewater
With the rapid development of the national economy, the discharge of industrial wastewater in China has increased significantly, and the investment in industrial wastewater treatment has also increased year by year. In 2006, the total discharge of industrial wastewater in China was close to 25 billion tons, accounting for about 45% of the total discharge of wastewater. At the same time, the total investment in industrial wastewater treatment in China in that year was nearly 15 billion yuan, close to 1/3 of the total investment in industrial pollution treatment in the country, which brought heavy economic burden and environmental pressure to enterprises and the country. Therefore, in response to the national call of "clean production, energy conservation and emission reduction", it is of great significance to develop more advanced and efficient industrial wastewater treatment technology to achieve sustainable development of the national economy.
According to statistics, there were about 15000 electroplating plants in China in 2006, with an average annual discharge of electroplating wastewater of 4 billion tons, accounting for 1/6 of the total industrial wastewater discharge, but about 50% of electroplating wastewater did not meet the national discharge standards. For a long time, China's electroplating enterprises have been characterized by extensive operation that consumes a lot of resources. Compared with foreign countries, China's electroplating industry has a significant gap. It is reported that the average water consumption of 1m2 of electroplating parts in foreign countries is only 0.08t, while the average water consumption in China is 0.82t, more than 10 times that in foreign countries, Every year, the treatment cost of electroplating wastewater containing heavy metals alone in China is more than 400 million yuan. The quality of electroplating wastewater is complex, involving various heavy metal ions and cyanide, and some also contain carcinogenic, teratogenic and mutagenic highly toxic substances, which are very harmful to human beings. If these electroplating wastewater does not reach the discharge standard and enters the environment directly, it will certainly cause extensive and serious harm to the ecological environment and human beings. In addition, the recovery of heavy metals in electroplating wastewater can fully utilize resources, which is of great economic value. Therefore, the treatment of electroplating wastewater is the most important issue of industrial wastewater treatment.
2. Relevant national policies
The state also supports and encourages electroplating wastewater treatment in terms of relevant policies and regulations. In the 11th Five Year Plan Outline for National Economic and Social Development of the People's Republic of China, the determination to strengthen the control of heavy metal pollution is put forward in the third section of Chapter 24, Chapter 6. Because electroplating wastewater contains a large amount of heavy metals, electroplating wastewater treatment has been listed as a key project of the 11th Five Year Plan. The Cleaner Production Promotion Law of the People's Republic of China came into force on January 1, 2003. Article 19 of the Law stipulates that "in the process of technological transformation, enterprises shall comprehensively utilize or recycle the wastes, waste water and waste heat generated in the production process; And adopt pollution prevention and control technologies that can meet the national or local pollutant discharge standards and total pollutant discharge control indicators. "It can be seen that the treatment and comprehensive utilization of electroplating wastewater is also a manifestation of cleaner production of enterprises. In addition, Article 35 of the Law of the People's Republic of China on the Promotion of Cleaner Production stipulates that "VAT shall be reduced or exempted by the tax authorities in accordance with relevant national regulations for those who use waste to produce products and recover raw materials from waste". It can be seen that the treatment of electroplating wastewater is a project encouraged and supported by the state. If a process for effectively treating electroplating wastewater can be proposed, It is bound to be strongly supported and widely promoted by the state.
In May 2005, the National Development and Reform Commission and the State Environmental Protection Administration prepared the Cleaner Production Evaluation Index System for the Electroplating Industry, which stipulates that enterprises use the index system to check and measure their own cleaner production level, so that departments engaged in relevant management activities can audit the cleaner production of enterprises according to the index system. At present, the main environmental problem of electroplating industry is water pollution, and wastewater treatment accounts for a large proportion in the indicator system. Therefore, it is a very necessary law enforcement activity to treat and comprehensively utilize electroplating wastewater. The Science and Technology Standards Department of the State Environmental Protection Administration began to prepare the pollutant discharge standards for the electroplating industry in July 2005, and stipulated that the national standard GB21900-2008 should be used to set the standard limits for wastewater discharge in the electroplating industry from 2008.
3. Electroplating wastewater treatment method
At present, there are four commonly used methods to treat electroplating wastewater in China: chemical method, biological method, physicochemical method and electrochemical method. Chemical methods include precipitation method, oxidation method, reduction method, neutralization method and air flotation method; Biological methods include: bioflocculation, biosorption and biochemistry; Physicochemical methods include ion exchange method, membrane separation method, adsorption method and evaporation concentration method; Electrochemical methods include galvanic cell method, electrodialysis method, electrocoagulation gas flotation method, electrochemical oxidation method and electrochemical reduction method. This paper summarizes these methods of electroplating wastewater treatment, and analyzes and compares them.
3.1 Chemical method
Chemical method is to decompose toxic and harmful substances into non-toxic and harmless substances by redox reaction or neutralization precipitation reaction, or directly remove heavy metals from wastewater by precipitation or flotation.
(1) Precipitation method. Add precipitator to electroplating wastewater to make it react directly with the pollutants to be removed in the wastewater to form insoluble solids for separation and removal. The disadvantage of general chemical precipitation method for wastewater treatment is that it cannot achieve the effect of advanced treatment, and other high molecular flocculants must be used together.
(2) Oxidation method. By adding oxidants, the toxic substances in electroplating wastewater are oxidized into non-toxic or low toxic substances, which are mainly used to treat CN -, S2 -, Fe2+, Mn2+and other low valent ions in wastewater, as well as various organics and pathogenic microorganisms that cause chromaticity, taste, smell, etc.
(3) Reduction method. By adding reducing agent, the toxic substances in electroplating wastewater are reduced to non-toxic or low toxic substances, which are mainly used to treat high valence heavy metal ions such as Cr6+and Hg2+in wastewater.
(4) Neutralization method. Through acid-base neutralization reaction, adjust the pH of electroplating wastewater to make it neutral or near neutral or within the pH range suitable for further treatment, which is mainly used to treat the pickling wastewater of electroplating plants. In addition, calcium carbide slag is used as neutralizer to treat acid wastewater with good effect, which can achieve the goal of "treating waste with waste".
(5) Air flotation method. The principle of the air flotation method is to use a large number of micro bubbles released by the sudden decompression of the working water of the pressure vessel to adhere to the agglomerate produced by the preliminary treatment of electroplating wastewater, so that its specific gravity is less than water and floats to the water surface to become scum for removal, so that the wastewater can be purified. The advantages of chemical method are: less investment, simple treatment device, low treatment cost, easy installation, maintenance and operation. However, its disadvantages are low treatment efficiency, poor effluent quality, shallow treatment depth, inability to recycle heavy metals and large amount of sludge.
3.2 Biological method
The research on the treatment of electroplating wastewater by microorganisms originated in the 1980s. The mechanism of biological treatment lies in the relationship of mutualism and symbiosis, and there are three levels of mutual cooperation mechanisms of chemistry, physics and genetics. Some microbial metabolites can change the valence of heavy metal ions in the wastewater. At the same time, the microbial flora itself has strong biological flocculation and electrostatic adsorption, which can absorb metal ions and make heavy metals pass through Solid-liquid separation After that, the sludge cake is fed to make the wastewater discharge or reuse up to the standard.
(1) Biological flocculation method. Flocculation and sedimentation are carried out by microorganisms or metabolites produced by microorganisms. At present, there are about a dozen varieties of biological flocculants that can flocculate heavy metals. The amino and hydroxyl groups in biological flocculants can form stable chelates with heavy metal ions such as Cu2+, Hg2+, Ag+, Au2+and precipitate.
(2) Biosorption. The method of adsorbing heavy metal ions dissolved in water by using the chemical structure and composition characteristics of organisms, and then removing metal ions through solid-liquid separation.
(3) Biochemical method. The wastewater containing heavy metals is treated by microorganisms, and the soluble ions are converted into insoluble compounds and removed through the direct chemical reaction between microorganisms and metal ions. The advantages of biological method are: strong selectivity, large adsorption capacity, no use of chemicals; The treatment technology is relatively simple, the operation cost is low, and the enrichment of metal ions by functional bacteria is high. In addition, the amount of sludge generated by biological method is small, the metal concentration in sludge is high, the secondary pollution is significantly reduced, and the heavy metals in sludge are easy to recover, with high recovery rate. However, its disadvantage is that the reaction efficiency of functional bacteria and metal ions in wastewater is not high, and the consumption of culture medium of culture bacteria is large, resulting in high treatment costs. In addition, due to the excessive addition of biological bacteria, the residual organisms in the water can reproduce, which may also cause secondary water pollution.
3.3 Physicochemical method
Physicochemical method is to remove impurities contained in electroplating wastewater by means of ion exchange or membrane separation or adsorbent. It is widely used in industry, usually in combination with other methods.
(1) Ion exchange method. The method of separating harmful substances in wastewater with ion exchanger is used. When the wastewater containing heavy metals passes through the ion exchanger, the ions on the exchanger exchange with the metal ions in the water to remove the metal ions in the water. Resin process is the most common method in electroplating industry, and the economic benefit of ion exchange resin in treating precious metal wastewater is the most significant.
(2) Membrane separation method. The technology of material separation by using the selectivity of polymers, including electrodialysis, reverse osmosis, membrane extraction, etc.
(3) Adsorption method. A method of removing heavy metal ions by using the unique structure of adsorbent. Traditional adsorbents include activated carbon, humic acid, glycan resin, algal soil, etc.
(4) Evaporation concentration method. It is a treatment method for evaporation of electroplating wastewater to concentrate heavy metal wastewater and recycle it. It is generally applicable to the treatment of electroplating wastewater containing chromium, copper, silver, nickel and other heavy metals. At present, it is generally used as an auxiliary means of other methods. Physicochemical method has the advantages of convenient management, easy automation, high separation efficiency, good deep treatment effect, and the effluent can be reused, which is widely used in wastewater treatment. However, the physicochemical method has many disadvantages in different degrees, such as complex process, high energy consumption, high cost, large floor area, high operation cost, secondary pollution, etc.
3.4 Electrochemical method
The application of electrochemical method in wastewater treatment started in the 1940s, but it was not until the 1960s that electrochemical method was really used in wastewater treatment process with the development of power industry. Electrochemical method is called an "environmentally friendly" process for electroplating wastewater treatment. With the development of electrode materials, reactors and the improvement of traditional electrochemical processes, electrochemical methods are more and more widely used in the field of electroplating wastewater treatment.
(1) Galvanic cell method. Granular carbon, coal cinder or other conductive inert substances are used as cathode, iron filings are used as anode, and the conductive electrolyte in the waste water plays a conductive role to form a primary battery. The purpose of treating waste water is achieved through the primary battery reaction.
(2) Electrodialysis. Electrodialysis technology is a kind of membrane separation technology. It is to alternately arrange anion and cation exchange membranes between positive and negative electrodes, and separate them with a special barrier. Under the action of electric field, with potential difference as the driving force, the electrolyte is separated from the solution by using the selective permeability of ion exchange membrane, so as to realize the concentration, desalination, refining and purification of electroplating wastewater.
(3) Electrocoagulation gas floatation method. Soluble anode (Fe, Al, etc.) materials are used to generate a large number of cations such as Fe2+, Fe3+, Al3+, etc., and precipitates such as Fe (OH) 2, Fe (OH) 3, Al (OH) 3 are generated through flocculation to remove pollutants in water. At the same time, a large number of H2 microbubbles are generated on the cathode, and a large number of O2 microbubbles are generated on the anode. These bubbles are used as air flotation carriers to float together with the flocculated sewage. A large number of flocs float up quickly with rich micro bubbles to purify water quality.
(4) Electrochemical oxidation method. It is an oxidation method to oxidize and degrade organic matters in electroplating wastewater by using the active free radicals with strong oxidizing ability generated by the reaction between the high potential of the anode and the catalytic active anode electrode.
(5) Electrochemical reduction is a method of removing pollutants through cathodic reduction reaction. It can be divided into two categories. One is direct reduction, that is, the pollutants get electrons directly on the cathode and are reduced; The other is indirect reduction, which uses the reduction products of cathode to degrade pollutants. The advantages of electrochemical method are: (1) no additional redox reagent is needed to avoid secondary pollution caused by additional reagent; (2) Fast reaction speed, less sludge production, stable effluent quality, easy to realize automatic operation; (3) Short processing time, small equipment volume and small floor area; (4) It can simultaneously recover valuable metals and remove organics;
(5) It has the functions of flotation, flocculation and sterilization. Disadvantages are: high power consumption, large electrode plate consumption and high processing cost; At low concentration, the current efficiency is low and the potential window is not wide enough; Corrosion, passivation and deactivation of electrodes are prone to concentration polarization and scaling; The composition of precipitated hydroxide is unstable and may be re dissolved, which may cause secondary pollution.
4. Outlook
In the face of increasingly serious environmental pollution, the electroplating industry, as one of the three major polluting industries in China today, only by implementing cleaner production, energy conservation and emission reduction, can enterprises cope with increasing environmental pressure and achieve sustainable development. At present, the main environmental problem of electroplating industry is the pollution of electroplating wastewater, so only by solving the problem of electroplating wastewater treatment first, electroplating enterprises can improve their market competitiveness, create more market profits, and assume the social responsibility that enterprises should do. The future development of electroplating wastewater treatment will focus on the following four directions:
(1) Integrated technology is the hot spot of electroplating wastewater treatment technology in the future. Due to the differences between enterprises and processes, the use of only one wastewater treatment method often has its limitations and cannot achieve ideal results. Only according to the composition of electroplating wastewater, the application of integrated technology of multiple treatment technologies can achieve ideal results. At the same time, according to the principles of system engineering, reasonably arrange the connection between each treatment technology.
(2) Zero discharge technology will be the development direction of electroplating wastewater treatment technology in the future. Electroplating wastewater is a misplaced resource. To implement circular economy, promote cleaner production, improve the conversion rate and recycling rate of electroplating materials and resources, realize green electroplating technology from the source, and at the same time, carry out the whole process control, combined with the comprehensive treatment of wastewater, so that electroplating wastewater can be recycled into electroplating process after treatment, Thus, electroplating wastewater will not be discharged to the surrounding environment, and finally zero discharge of electroplating wastewater will be realized.
(3) Centralization is also a trend in the development of electroplating wastewater treatment in the future. At present, there are a wide range of electroplating enterprises in China. There are less than 600 large enterprises with an annual electroplating amount of more than 1 million m2. The electroplating enterprises produce relatively small and dispersed concentrated electroplating wastewater. If every electroplating factory has established up to standard wastewater treatment equipment, it will certainly impose a heavy economic burden on small and medium-sized electroplating plants. From the perspective of economy and environmental protection, centralized recycling in one city or region can be considered to establish a regional electroplating wastewater treatment center (similar to Urban sewage treatment To reduce the unit cost of electroplating wastewater treatment.
(4) Electrochemical method is the main force of electroplating wastewater treatment technology in the future, and its final development direction is cheap, efficient and easy to operate. In the future, the development of electrochemical methods will focus on three aspects: ① developing cheap and efficient electrodes, such as using non rare noble metal electrode materials and using cylindrical electrodes or three-dimensional electrode brushes; ② Design a new type of electrolytic cell, such as fluidized bed or multi-stage series electrode device; ③ Promote the application of pulse power supply in electrochemical method and improve the pulse electrolysis technology, so as to break through the limitations of traditional electrochemical method in energy consumption, effectively reduce energy consumption and greatly improve the treatment efficiency of electroplating wastewater.
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