Research Report on PCB wastewater treatment
In recent years, with the rapid development of PCB industry, China has become the most important PCB production base in the world. At present, China's PCB output value has accounted for more than 25% of the world's total output value, surpassing Japan as the world's first PCB manufacturing place. With the continuous development of PCB industry, the environmental protection of PCB wastewater has become increasingly prominent. Based on the analysis of the current situation of PCB wastewater, this research discusses the main treatment technologies and existing problems of PCB wastewater, in order to benefit the subsequent treatment of PCB wastewater.
1. PCB wastewater status analysis
1.1 PCB wastewater discharge
The water consumption of PCB production mainly depends on the number of layers of the circuit board. The more layers, the more inner layer circuit manufacturing processes, and the corresponding increase in water demand. According to the investigation of water consumption and drainage of several domestic PCB enterprises, the water consumption of different types of PCB production is shown in the table.
Water consumption in PCB production
The water loss of PCB production line is mainly due to the evaporation of cylinder fluid, drying of wet products, recovery of waste cylinder fluid, etc. Considering that a certain amount of liquid medicine is added to the production line, the drainage can be calculated as 95% of the water consumption when the wastewater is not recycled after treatment. It can be seen that PCB wastewater discharge is large. Statistics show that the total waste water volume of PCB industry in China increased from 278 million tons to 336 million tons in 2007 to about 600 million tons in 2010.
1.2 PCB wastewater quality
Source and classification of PCB wastewater
It can be seen that the wastewater from different production processes contains different pollutants, including a large number of heavy metal compounds such as Cu, Ni, Ag, Au, Sn and Pb, as well as synthetic polymer organics and a variety of organic additives. If it is not treated and directly discharged into the nature, it will cause great harm to the environment and human beings. Because the content of metal ions and organic matter in PCB wastewater changes greatly, the concentration is high, the composition is complex and the form is different, it brings great difficulty to PCB wastewater treatment technology. Therefore, PCB wastewater must be collected and treated separately.
2. Current situation and problems of PCB wastewater treatment technology
Different types of wastewater have different water quality and treatment difficulty. Brush grinding wastewater and low concentration heavy metal wastewater can easily reach the reclaimed water reuse standard after being treated by ion exchange method and biochemical method; comprehensive wastewater can be used as general cleaning water after being treated; cyanide containing nickel wastewater can be removed by traditional methods such as sedimentation method and electrolysis method after being treated by cyanide removal. The complex copper wastewater and printing ink wastewater contain a large number of complex copper and high concentration of refractory organics respectively. The traditional treatment method has limited treatment effect, which will affect the subsequent biochemical treatment effect, which is the technical difficulty of PCB wastewater classification treatment.
2.1 main treatment technologies and existing problems of copper complex wastewater
At present, the treatment methods of complex copper in wastewater mainly include Fenton oxidation, ion exchange, chemical precipitation and chelating precipitation.
Fenton reagent is a kind of strong oxidant which is obtained by mixing H2O2 and Fe2 +. It can produce strong · OH radicals in the reaction, thus destroying the complex structure. Fenton oxidation can effectively break the stable complex structure of complex copper. After treatment, the concentration of copper in the wastewater can be reduced to below the national emission standard. Moreover, the oxidation can transform the organic compounds in the complex into CO2 and water, and remove part of COD. The disadvantage of this method is that the dosage of ferrous ion is large and it is easy to produce secondary pollution.
The ion exchange method mainly uses the exchange ion in the ion exchange resin to exchange with the heavy metal ion in the waste water to remove it. Since 1981, China began to use ion exchange process to treat copper complex wastewater, it has gained some experience in design, operation and management. Ma Xiaoou and others optimized the regeneration process conditions of the ion exchange column in the copper wastewater treatment unit of a circuit board plant. The results show that when the flow rate of regeneration liquid is 2000L / h and the mass fraction of regenerant is 8.5%, the total amount of copper exchanged each time is 26.1kg-29.0kg; when the secondary utilization process of regeneration liquid is adopted, the total amount of copper exchanged each time is 18.5kg-21.3kg, which can save the amount of regenerant and is beneficial to the recovery of copper. S2 + precipitation method can remove copper ions thoroughly, with simple equipment and low cost. However, the cus particles generated by the reaction are small and need to be added with flocculant to form larger alum flowers so as to make it precipitate quickly. It is difficult to remove copper by single S2 + precipitation, which needs to be combined with other methods to make the effluent copper concentration reach the discharge standard. Gong Bentao and others treat the copper plating waste liquid produced in the process of circuit board production in the electronic factory, first add Na2S for precipitation, then add PAM for coagulation reaction, finally add PAC (polyaluminium chloride) to make it form larger alum flower precipitation, and the effluent copper concentration can reach the discharge standard. At present, the process of copper removal by sodium sulfide is still under study, and sodium sulfide is only used as an auxiliary agent. Moreover, it is difficult to treat the sludge from the treatment of wastewater containing copper, which will cause secondary pollution to the environment.
The chelating agents used in chelating precipitation are TMT and DTCR. They are respectively for inorganic and organic copper complexes. The chelating precipitation method is simple, has good removal effect on copper, low requirements on equipment, good stability of the precipitate, and will not cause secondary pollution to the environment. These advantages make chelating precipitation method have broad application prospects and promotion value in industrial heavy metal wastewater treatment. However, the high price of chelating agent has become an important factor hindering its wide application.
2.2 main treatment technologies and existing problems of oil membrane wastewater
Ink wastewater contains a large number of biorefractory organics, which are usually treated by acidizing first. Most organics can be separated out in acid condition, and the removal rate of COD is up to 40% ~ 50%. However, there are still a lot of refractory organics in water. Scholars at home and abroad have carried out some research work on this kind of wastewater treatment and applied it in engineering practice.
Iron carbon micro electrolysis is widely used. The carbon containing iron filings are immersed in the electrolyte solution to form countless tiny Fe-C primary batteries, the anode generates Fe2 +, the cathode generates · oh and new ecology [H], which has high chemical activity and has the functions of oxidation, reduction, adsorption and flocculation with pollutants; the polymer formed by Fe2 + and water can realize flocculation, decolorization and purification. According to the characteristics of high pollution concentration and small water volume of waste ink, Wang Wen et al. Used electrolysis as the main process to treat the waste ink of a factory. The engineering practice showed that the removal rate of COD and BOD of the waste water was 47% and 60% respectively through the pretreatment of electrolysis and the comprehensive functions of oxidation, reduction, agglomeration and air flotation. The treatment of ink wastewater by electrolysis has the advantages of small equipment, less land occupation, simple operation and management, high efficiency, short time and good chroma removal effect. However, the electrode surface is easily polluted, which makes the electrode inactive. Therefore, the highly efficient catalytic electrode and the activation of the electrode are the key to application.
Coagulation is a kind of physical treatment method that adding flocculant to waste water to coagulate and flocculate the colloidal particles, such as pollutants, to form precipitates and be removed. It is a commonly used technology for treating waste ink. The key of coagulation is coagulant. The common flocculants are inorganic flocculant, organic flocculant, compound flocculant and biological flocculant. In the engineering practice of ink waste liquid treatment, inorganic flocculants such as aluminum salt, iron salt, polyaluminium and polyiron are commonly used in combination with organic flocculants. The disadvantage of coagulation method is that it will produce a lot of sludge and it is difficult to dewater. If it is not properly disposed, it will cause secondary pollution.
The biological method is to remove the organic matter in the ink wastewater through the microbial action under the aerobic and anaerobic conditions. Cai Yanxing et al. Mixed the supernatant of the ink wastewater after coagulation pretreatment with domestic sewage by 1 / 10. Under the conditions of contact oxidation process: temperature (30-32) ℃, dissolved oxygen concentration (4-5) mg / L, inflow flow (14L / h), contact time (15.7h), organic load cod (0.7-0.9) kg / m3 · D, after continuous operation, the COD removal rate can be maintained above 80%, and the effluent COD can be maintained below 100mg / L 。 Yang Pei et al. Pretreated ink wastewater by air flotation, and then treated by SBR, the effluent COD can reach below 150mg / L. In a word, the treatment of ink wastewater by biological method has the advantages of low operation cost and stable treatment effect, but it has the disadvantages of low removal effect of chroma and COD, especially the removal effect of refractory substances such as ammonia nitrogen, azo pigments and aromatic organic solvents in ink wastewater is not obvious, and the reaction time is long, covering a large area. In addition, a large number of excess sludge is produced during the operation of biological process accompanied by wastewater purification. How to treat these excess sludge and make it resource-based and harmless is a key link of ink wastewater treatment.
Advanced oxidation technology is a new chemical oxidation technology which can effectively treat refractory organic wastewater. At present, the reaction mechanism is generally believed to be free radical oxidation mechanism, that is, the use of composite oxidants, light irradiation, electricity or catalyst and other functions can induce the generation of various forms of strong oxidation active substances, especially the hydrogen oxygen free radicals can completely mineralize or partially decompose the vast majority of organic pollutants, such as Fenton oxidation, ultrasonic irradiation oxidation, photocatalytic oxidation and so on. Wen Haifeng, Guo Xinchao, et al. Studied the treatment effect of UV Fenton Method for PCB high concentration cod film removal wastewater. Wen Haifeng determined the best reaction conditions of UV Fenton Method for the treatment of Demembrane waste liquid: pH = 3; [H2O2] = 0.85mol/l; [Fe2 +] = 0.04mol/l; reaction time is 12min. Guo Xinchao studied the influence of various factors on the treatment effect, in the order of H2O2 / COD ratio > FeSO4 / H2O2 ratio > light time. Advanced oxidation technology is a new type of water treatment technology. Although it can degrade the refractory substances into small molecular substances and improve the biodegradability of ink wastewater, most of them are only used in the research of single substance at present, and only stay in the experimental stage, rarely involving the actual wastewater field.
2.3 Application of MBR technology in PCB wastewater treatment
Wushiwen et al. Used MBR process to treat the wastewater from developing and de filming and degreasing in a PCB circuit board factory in Wuhan. The operation results showed that under the conditions of influent cod 1500mg / L, Cu2 + 1.5mg/l, MLSS 6000-8000mg / L, do controlled at 2-4mg / L, system temperature at 20-40 ℃, residence time at 10h and aeration volume at 40-43m3min, the effluent water quality of MBR system was good and stable, COD was stable The average removal rate is more than 87%, and the average removal rate of Cu2 + is about 70%. At the same time, the system has strong resistance to load shock.
Inlet and outlet water quality of MBR system
Some studies have shown that A2O + MBR can treat high concentration organic nitrogen wastewater, and the front-end anaerobic intercepts high concentration and difficult to treat organic pollutants, so as to effectively complete the transformation of organic nitrogen and ammonia nitrogen to nitrogen by anoxic aerobic process. After treatment, no organic nitrogen is detected in the effluent, and ammonia nitrogen and total nitrogen meet the local discharge standards.
Application example: Comprehensive Wastewater of a circuit board plant, water volume 1500m3 · D-1, ammonia nitrogen = 100mg · L-1, total nitrogen = 180mg · L-1.
After treatment, the effluent ammonia nitrogen is less than 0.5mg · L-1 and total nitrogen is less than 7Mg · L-1, which meets the discharge standard of the enterprise.