However, the contents of cyanide, COD and ammonia nitrogen in the external drainage of coking wastewater treated by the above process still cannot meet the standard. Aiming at the problem that coking wastewater is complicated in composition, difficult to treat and can not meet the discharge standard after being treated by traditional methods, a large number of research results on coking wastewater treatment at home and abroad in recent years are synthesized, the advantages and disadvantages of three methods used in advanced treatment of coking wastewater are systematically introduced, several examples and shortcomings of coking wastewater reuse are listed, and the future development direction of coking wastewater treatment technology is pointed out.
Coking wastewater mainly refers to the wastewater generated in the process of coal coking, gas purification, chemical product recovery and chemical product refining. Due to the influence of raw coal properties, product recovery, production technology and other factors, the composition of wastewater is extremely complex. The organic matter in coking wastewater is mainly phenolic compounds, accounting for more than half of the total organic matter, and the remaining organic matter is mainly heterocyclic organic matter and polycyclic aromatic organic matter containing sulfur, oxygen and nitrogen.
Coking wastewater has a large amount of discharge, complex composition and difficult treatment, so it is difficult to reuse or meet the discharge standards. Therefore, it is an urgent problem to reduce the concentration of pollutants in coking wastewater and improve the reuse rate of wastewater.
With the strengthening of people's awareness of environmental protection and the state's attention to environmental protection issues, the Ministry of Environmental Protection of China issued "Pollutant Discharge Standard for Coking Industry" (GB16171-2012) in June 2006, which not only gave stricter discharge standards for the main pollutants in wastewater, but also exceeded the original standard. At present, it is urgent to develop and study new, efficient and low-cost wastewater treatment technology, optimize and improve the existing technology, improve the wastewater treatment effect and make it reach the discharge standard.
Over the years, although predecessors have done a lot of basic research work in coking wastewater treatment, there is still no breakthrough research progress in coking wastewater treatment due to the large discharge of coking wastewater, many kinds of pollutants in water, and some pollutants are difficult to biodegrade. Therefore, it is the focus of coking wastewater treatment research in the future to develop an efficient, low-cost and effective wastewater treatment technology and optimize and improve the existing technology.
In this paper, the physicochemical method, oxidation method and membrane treatment method used in advanced wastewater treatment are analyzed and compared, and several examples of coking wastewater reuse and their shortcomings are listed, and the development direction of coking wastewater treatment technology in the future is pointed out.
1 advanced treatment technology of coking wastewater
1. 1 physical and chemical methods
1. 1. 1 coagulation and precipitation method
Coagulation and precipitation method is to treat coking wastewater by the principle of electric neutralization. The specific treatment process is as follows: under certain conditions, coagulant is quantitatively added into coking wastewater, and the charged substances in wastewater are electrically neutralized by coagulant to form large particle micelles, and then coking wastewater is further precipitated and purified.
Lu, et al. developed a special coagulant M 180 for treating effluent from A/O biochemical pool of Baosteel Coking Plant. The results show that when the pH value is 6.0 ~ 6.5 and the dosage of coagulant is 300 mg/L, the special coagulant has good treatment effect on the indexes such as COD, chromaticity and CN of coking wastewater.
A new composite flocculant PFASSB was developed by He Li and compared with PFASSB, PFASSB and PFASSB. The treatment effects of PFASSB, PAC, PFAC and new flocculant PFASSB on COD and turbidity of coking wastewater were investigated.
The experimental results show that under the same conditions, the treatment effect of the new composite flocculant on coking wastewater is obviously better than PAC, PFS and PFAC, and the dosage of the new flocculant is obviously lower than other flocculants. When the PH value of wastewater is 8 and the dosage of new flocculant is 10 mg/L, the effluent after flocculation treatment is SS.
Zheng Yi and Zhang Zhuo studied and compared the treatment effects of aluminum sulfate, polymeric ferric sulfate and polyacrylamide on the effluent from biochemical pool of coking plant, and combined them to investigate their combined treatment capacity for coking wastewater. Through experiments, it is found that the treatment effect of polymeric ferric sulfate combined with polyacrylamide is obviously better than that of using various coagulants alone. When the pH is 5 and the dosage is 40 mg/L polyferric sulfate and 6 mg/L polyacrylamide, the composite coagulant has the best treatment effect on coking wastewater. At this time, the chromaticity of effluent is 70 times, COD is 68 mg/L, and the removal rates are 73.08% and 62.22% respectively.
Through the above analysis, it is found that coagulation and precipitation method has a good effect on the removal of chromaticity, COD and other indicators of coking wastewater, and the treated coking wastewater can meet the discharge standards. However, in the process of advanced treatment of coking wastewater by coagulation and sedimentation, a large number of solid sediments will be produced, which is difficult to be ignored and causes new pollution to the environment. In addition, the treatment of coking wastewater by coagulation and sedimentation requires adjusting the pH value of the inlet and outlet water of the sedimentation tank, and the coagulant needs to be added manually, so the operation is complicated, and the treated wastewater can only be discharged outside and cannot be reused.
1. 1.2 adsorption method
The adsorption treatment of coking wastewater mainly uses adsorbent as a porous substance with large specific surface area, which has good adsorption performance for pollutants such as macromolecular organic substances, oil substances and some solid suspended solids. The adsorbent is separated from coking wastewater after adsorption treatment.
And Li used powdered coal ash as adsorbent to deeply treat ammonia nitrogen in biochemical effluent of coking wastewater. Three main influencing factors, such as dosage, pH value and adsorption time, were investigated by experiments. The experimental results show that when the wastewater pH is 5, the dosage of fly ash is 150 g/L, the dosage of quicklime is 2.5 g/L, and the adsorption time is 1 h, the ammonia nitrogen content in coking wastewater is reduced from 77.67 mg/L to below 25 mg/L, and the ammonia nitrogen removal rate is over 70%.
In Wang Hongmei and Zheng Zhenhui, modified bentonite was used for advanced treatment of biochemical effluent from coking wastewater. The experimental results show that when the pH value of coking wastewater is 8.0 ~10.0 and the dosage of modified bentonite is1200 ~1500mg/L, the decolorization rate of coking wastewater is over 65%, and the removal rates of cyanide and CODcr are 3 1% and 200% respectively.
Sun Hema carried out technical transformation on two coking wastewater treatment stations in Nangang, adding activated carbon filtration devices on the basis of the original treatment stations and improving the original operation methods. Through the improvement of activated carbon filtration device, the effluent of coking wastewater treatment station in Nangang was upgraded from the original national second-class standard to the national first-class discharge standard, and the operating cost of wastewater treatment station was reduced and the service life of activated carbon was prolonged by improving the operation method.
Li Mao and Han Yongzhong used the combined process of resin adsorption and Fenton oxidation to treat high concentration coking wastewater. The experimental results show that when the adsorption resin and Fenton reagent are in the best working state, the removal rate of phenolic organics in coking wastewater can reach 100%, and the removal rate of COD can reach 74.82%, and the biodegradability of resin adsorption -Fenton oxidation combined process for treating high concentration coking wastewater has also been greatly improved.
Zhang Changming et al. used fly ash as adsorbent to deeply treat the biochemical effluent from coking plant of Shanxi Coking Group Co., Ltd. When the dosage of fly ash was 17.47 g/L, the treatment effect of coking wastewater was good, and the contents of pollutants such as COD, chromaticity, oil, sulfide, cyanide and volatile phenol in the wastewater reached the national discharge standard except ammonia nitrogen. The adsorbed fly ash can be reused by burning bricks or building roads. The adsorption treatment of coking wastewater by fly ash embodies the environmental protection concept of treating waste with waste.
The advanced treatment of coking wastewater with activated carbon as adsorbent has good effect, and the treated wastewater can meet the discharge standard. However, due to the high price of activated carbon, the difficulty of regeneration and the high cost of wastewater treatment, most enterprises have abandoned it at present. The advanced treatment of coking wastewater with fly ash as adsorbent has good treatment effect, and the adsorbed fly ash can still be reused by burning bricks and building roads without affecting its quality. Moreover, using fly ash as adsorbent to reuse coking waste conforms to the current national green chemical recycling policy.
1. 1.3 chemical precipitation method
Chemical precipitation method not only makes the ammonia nitrogen content in wastewater meet the national discharge standard, but also indirectly improves the biodegradability of wastewater. However, at present, there are few studies on the treatment of coking wastewater by chemical precipitation, and the technology is not mature enough to realize industrialization.
Apply.
1.2 oxidation method
1.2. 1 Fenton oxidation method
Fenton reagent can reduce the COD value and chromaticity of coking wastewater by oxidizing and decomposing refractory macromolecular organic matter into small molecular organic matter, and at the same time improve the biodegradability of coking wastewater to a certain extent, so that coking wastewater can be better treated.
1.2.2 ozone oxidation method
Oxygen atoms in ozone molecules have strong electrophilic or proton affinity and strong oxidation activity, and ozone can oxidize and decompose macromolecular organic substances in coking wastewater. Ozone oxidation technology has the characteristics of strong oxidation ability, fast reaction speed, high treatment efficiency, no temperature influence and no sludge.
2 conclusion
In recent years, with the country's increasing attention to environmental protection issues and the continuous improvement of national environmental awareness, wastewater discharge standards have become more and more stringent. Scholars from all over the world have developed some new coking wastewater treatment technologies through continuous exploration, such as electrochemical oxidation technology, photocatalytic oxidation technology, membrane technology and so on.
These technologies thoroughly treat pollutants in coking wastewater and will not produce secondary pollution. However, these technologies have high investment and operation costs, and many of them are still in the stage of theoretical research and laboratory research, so it is difficult to realize large-scale industrial application. Therefore, while deeply studying the advanced treatment technology of coking wastewater, we should also fully tap the advantages of the existing technology, optimize and improve the existing technology and improve its treatment efficiency.
Through the above analysis, it can be found that the adsorption effect of fly ash is good, which conforms to the national environmental protection and energy-saving policy of treating waste with waste. Membrane technology has also been applied in some factories and achieved good results. Pretreatment of coking wastewater by fly ash adsorption can remove most of the organic matter in wastewater, reduce the burden of membrane filtration, improve its service life and reduce the treatment cost. The synergistic effect of fly ash adsorption technology and membrane technology should be the focus of future research.
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