Introduction of Electroplating Wastewater Treatment
Electroplating industry is widely and important in modern industry, and plays an indispensable role in China's economic and social development. At the same time, electroplating is also a highly polluting industry. In China, about 4 billion cubic meters of waste water, 500 million tons of solid waste and 30 million cubic meters of acid gas can be discharged annually, of which more than 50% fail to meet the national pollutant discharge standards. In the electroplating process, due to the large consumption of raw and auxiliary materials and fresh water, the electroplating wastewater is accompanied by a large number of cyanide, zinc, chromium, acid and alkali pollutants. Once these untreated wastewater enters the natural environment, it will bring serious harm to the local ecology and human health.
Due to the scattered distribution of plating enterprises in China, pollution sources are also relatively scattered, and there are a series of problems such as low level of clean production technology, low degree of automation specialization, low level of pollution prevention and control and low effective control rate. At the same time, in the process of production, due to the variety of plating species, the specific process links involved are also different, which will inevitably lead to multiple pollutant emissions, such as the coexistence of different concentrations of organic cyanide pollutants and heavy metal ions. In view of the pollution characteristics of electroplating wastewater, the Technical Specification for Electroplating Wastewater Treatment (HJ 2002-2010) emphasizes that electroplating wastewater should be collected and treated separately. During the 12th Five-Year Plan period, the clean and orderly development of the electroplating industry has made outstanding contributions to the overfulfillment of the total target of heavy metal emission reduction in China. In the 13th Five-Year Plan, it was clearly pointed out that we should focus on steel, cement, petrochemical, non-ferrous metals, glass, coal-fired boilers, paper-making, printing and dyeing, chemical industry, coking, nitrogen fertilizer, agricultural and sideline food processing, raw material drug manufacturing, tannery, pesticides, electroplating and other industries to promote the transformation of discharge standards. Under the new situation, in order to further respond to the call of the state, this article systematically identifies the causes of compound pollution in the electroplating industry and proposes a more effective overall solution.
1. Sources and characteristics of electroplating wastewater
Because the electroplating industry needs to consume a lot of water, most processes use water as solvent. The sources of wastewater can be roughly classified as follows: (1) Plating rinsing wastewater is one of the main sources of electroplating wastewater, accounting for more than 80% of the wastewater discharged from the workshop, during which a variety of heavy metal ions can be produced. Inorganic organic organic pollutants such as organic activator and stabilizer. (2) In the process of plating solution filtration, the waste water containing high concentration pollutants from residual plating solution, invalid or deteriorated plating solution and cleaning plating bath, although the amount of this part of wastewater is small, but the concentration of various pollutants is high, which generally needs to be treated separately. (3) Laboratory water mainly includes electroplating process analysis and wastewater, waste gas detection and other experimental analysis water. Its water quantity is not large, but its composition is complex. Generally, it is discharged into the electroplating mixed wastewater system after unified treatment and discharge. (4) Wastewater generated by other miscellaneous water, such as washing machinery and equipment, workshop floor, etc.
Although the electroplating industry has a variety of complex processes and the quality of electroplating wastewater varies greatly among different enterprises, the common feature is that it contains a large number of heavy metal ions, acids, alkalis and other pollutants. Common heavy metal ion pollutants include chromium, copper, nickel, zinc, gold, silver and lead. Common acid and alkali pollutants include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, sodium hydroxide, sodium carbonate and so on. In addition, wastewater contains a certain amount of organic matter, ammonia nitrogen and so on. Therefore, for different types of characteristic pollutants, plating wastewater is generally further divided into: (1) wastewater with high COD concentration; (2) cyanide wastewater; (3) wastewater with heavy metal ions; (4) wastewater with mixed pollutants. Considering the diversity of pollutants in electroplating wastewater, it is particularly important to treat the wastewater by different qualities. This is due to the interaction of various organic substances or ammonia nitrogen, which may occur with heavy metal ions in the process of electroplating wastewater generation, resulting in the difficulty of mixed treatment of wastewater. For example, nickel plating wastewater must be treated separately from cyanide wastewater because nickel and cyanide will form nickel cyanide, which is a relatively stable and difficult compound to treat. In copper plating process, the wastewater produced by pyrophosphate copper plating process generally contains ammonia and needs to be treated separately from other wastewater containing metal ions, otherwise gold will be generated. It belongs to complex, which makes it difficult to remove metal ions.
2. Multicomponent Pollution Control Technology of Electroplating Wastewater
2.1 Main treatment methods of electroplating wastewater
For a long time, the treatment of wastewater in electroplating industry is mainly based on Harmless Control of heavy metals and degradation of organic matter. It can be roughly divided into chemical method, physical chemical method and biochemical method. Among them, chemical precipitation is the most widely used method, mainly because it has a series of advantages such as less investment and relatively simple process. However, this method is affected by many factors, such as the fluctuation of wastewater quality, sedimentation time, stirring conditions and management level, which will lead to the unstable quality of effluent, and the separation of sediments and the secondary pollution of sludge can not be ignored. Physical principle is to separate and remove pollutants from the system according to the different physical properties without changing the chemical properties of substances. It is commonly used for the separation, removal or concentration of suspended pollutants or heavy metal ions in water, mainly adsorption, evaporation and concentration, and membrane separation. Biochemical treatment has the advantages of low cost, good environmental benefits, less sludge and no secondary pollution, and has broad application prospects.
Generally speaking, the applicability of different wastewater treatment methods is different, and it is difficult to discharge electroplating wastewater after treatment by relying too much on a single treatment method; therefore, how to screen out the best treatment effect, low cost, simple operation, no secondary pollution, and easy to realize industrialization in many treatment processes Treatment technology is the trend of electroplating wastewater treatment technology in the future. To achieve this goal, effective combination of two or several superior processes is the main content and direction of current research on electroplating wastewater treatment technology, such as ion exchange-electrodeposition combined method, chemical method-membrane separation technology, biofilm-electrolysis method, biological method-membrane separation technology and so on.
2.2 Composite Control Method Based on Membrane Separation Technology
Membrane separation technology is a means of separating substances by selective permeability of membranes, including microfiltration, nanofiltration, ultrafiltration, reverse osmosis, electrodialysis and diffusion dialysis. Membrane separation method is used to treat electroplating wastewater, especially copper, zinc and nickel plating wastewater. It has high efficiency, simple operation, easy recovery of metals and no secondary pollution. The technology also has shortcomings, such as short service life of membrane due to blockage, and high investment cost of membrane separation alone. Therefore, it is usually necessary to combine the corresponding processes before the membrane dialysis process to maximize the purification effect and prolong the service life of the membrane. The photocatalytic oxidation of titanium dioxide was introduced to treat wastewater in an electroplating workshop of a factory in Yunnan. The COD of pretreatment wastewater was reduced by more than 75%. The operating load of ultrafiltration and reverse osmosis was reduced, the cleaning cycle and service life of membrane were prolonged, and the water reuse rate was increased to 85%. Bai Xinping  embedded the nanofiltration system in the chemical precipitation tank to realize the interaction between membrane dialysis and chemical precipitation, which can remove all kinds of cations and heavy metal ions in the solution more effectively.
2.3 Composite Control Method Based on Ion Exchange
Ion exchange method is a reversible reaction of ion exchange between ion exchanger based on macromolecule resin and substance in solution . Ion exchange process is suitable for many electroplating wastewater with low concentration of pollutants. It has the advantages of low energy consumption, less use of chemical reagents, no sludge production and good treatment effect. However, it has high operating costs, equipment needs maintenance, and resin is easily polluted. By combining ion exchange with other technologies, the level of cleaner production can be further improved and the quality of effluent water can be improved. The waste electroplating solution containing copper ion was adsorbed by R32 type ion resin. After treatment, the desorption solution with high concentration entered the electrolytic cell to recover copper by electrolysis. The purity of copper was up to 99.7%. The advantage of the combined electrolysis method is to reduce the economic loss of equipment operation in the process of treatment, reduce the consumption of water resources and the impact on the environment. In addition, ion exchange and membrane dialysis can also be combined to give full play to their advantages in removing ionic pollutants. By coupling ion exchange and nanofiltration technology, Cr (VI) can be successfully separated from Cl-and concentrated to 3200 mg/L, which greatly improves the recovery efficiency of chromium.
2.4 Combination Control Method Based on Biofilm Reaction
Biofilm treatment is one of the biological methods. At present, the toxicity of heavy metals in electroplating wastewater is one of the most critical problems to limit the biological treatment process. How to reduce the toxicity of heavy metals on microbial life activities has attracted increasing attention. Membrane Bio-Reactor (MBR) is a new water treatment technology which combines biological treatment technology with membrane separation technology. The removal of pollutants in wastewater is mainly through the metabolism of microorganisms, and the efficiency of wastewater treatment is enhanced by the combination of efficient separation of membrane modules. At present, the main factors restricting its wide application are the fast fouling rate of membrane, the high cost of replacement and cleaning. By adding biofilm hydrolysis reactor to strengthen MBR treatment process, COD removal rate can be improved in the case of high concentration of heavy metals. In MBR reactor, suspended biological carriers were added to form a composite MBR process. Compared with conventional MBR process, suspended biological carriers changed the abundance ratio of Biophase in the reactor and provided more diversified living environment for microorganisms, thus effectively enriching heavy metals such as copper, nickel and chromium, COD and ammonia. The average removal efficiency of nitrogen, total nitrogen and other pollutants also reached 94.4%, 74.8%, 51.0%.
For different concentration and different kinds of multi-component pollutants in electroplating wastewater, the most suitable and efficient control technology should be combined from the point of view of waste recycling according to the interaction between pollutants. In this way, it not only realizes the advantages and avoids the disadvantages of each process, but also reduces the difficulty of maintenance of equipment, thus promoting the cleaner production of the whole electroplating industry.
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