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Technical difficulties and prospects of near-zero discharge of modern coal chemical wastewater

来源:工业水处理 作者: 韩洪军,等 阅读:2003 网友评论0 Updated: 2019-09-17 11:02 Source: industrial water treatment Author: Han Hongjun, etc. Read: 2003 2010 0 comments

The near-zero discharge technology of modern coal chemical wastewater is the key to coordinating the contradiction between ecological environment and energy demand. At present, the biochemical treatment technology has changed from the emphasis on unit technology development to the key technology integration that considers process connection, processing system capacity, and source governance. Membrane separation + quality and salt treatment technology can improve the utilization rate of water resources while recovering salt resources, so it is the most reliable coal chemical concentrated brine treatment technology at present. The technology and application status are analyzed, and combined with the technical characteristics, it provides a reference for solving the difficulties of near-zero discharge treatment of modern coal chemical wastewater and the future development direction.

The coal chemical energy industry has developed rapidly due to the primary role of coal in China's energy reserves. The modern coal chemical industry uses coal as a raw material and converts it into gas, liquid and solid fuels and chemicals through chemical processing. In recent years, the rapid development of the national economy has increased the demand for energy. At the same time, the energy characteristics of "poor oil and low gas" have highlighted the contradiction between supply and demand. Therefore, the development of modern coal chemical industry is the guarantee of China's energy supply and sustainable economic development.

The production and construction of modern coal chemical projects are mainly based on the coordinated development of ecological environment and energy. Coal chemical industry is a high water consumption industry. The average water consumption per unit product of coal-to-oil, coal-to-olefin and coal-to-natural gas amounts to about 10, 27, and 6 tons. However, coal-to-chemical projects are usually distributed in areas with abundant coal resources and water scarcity. In addition, coal chemical industry will produce a type of wastewater containing high concentrations of phenols, high concentrations of ammonia nitrogen and a large number of toxic and harmful substances in the production process. And because the environmental capacity of wastewater in the area where the coal chemical project is located is limited, the requirements for the treatment of coal chemical wastewater in terms of environmental protection are very strict. In order to coordinate the contradiction between the ecological and environmental problems brought by coal chemical industry and the energy demand, and to solve the bottleneck of the development of modern coal chemical industry, many researchers apply biochemical treatment technology, physical and chemical treatment technology and biochemical-physical-chemical coupling technology to treat coal chemical wastewater. There are still some difficulties in the treatment of coal chemical wastewater, and the near-zero emission technology needs to be developed and optimized.

1 Status of near-zero discharge treatment technology for modern coal chemical wastewater

Coal chemical water treatment system includes water purification treatment, circulating water treatment, biochemical treatment, reclaimed water treatment, concentrated brine treatment and evaporative crystallization treatment (see Figure 1). Therefore, the realization of nearly zero discharge of modern coal chemical wastewater needs to address many issues. The technical difficulties of wastewater treatment and utilization can achieve the goal of high water resource utilization and no wastewater discharge. At present, the modern coal chemical wastewater treatment has established a technical process of pretreatment + biochemical treatment + reused water treatment + concentrated brine treatment and salt and mass fractionation. Among them, biochemical treatment and concentrated brine treatment are the key links for nearly zero discharge of coal chemical wastewater.

Biochemical treatment technology can remove more than 90% of pollutants in coal chemical wastewater, especially polyphenols and nitrogen-containing heterocycles, which are difficult to degrade organic matter, which require biochemical treatment to remove. At present, modern coal chemical wastewater treatment technology mainly consists of pretreatment, biological treatment and advanced treatment, including physical and chemical + biochemical, anaerobic + aerobic and their optimized treatment processes. Analysis and statistics of the wastewater biochemical treatment process of 18 coal chemical enterprises and 2 park wastewater treatment plants in Inner Mongolia, Shaanxi, Shanxi and other places, showing that the application of core biochemical processes for coal chemical wastewater treatment is related to wastewater quality (see Figure 2) , Including SBR, CAST, A / O, A2O, MBR and contact oxidation.

The core technologies of coal water slurry gasification and pulverized coal gasification are more selective, and there are differences in the biochemical treatment process between the crushed coal pressurized gasification wastewater treatment projects already under construction and under construction, such as the successful operation of the China Coal Tuke Coal Chemical Project The zero-emission biochemical treatment of wastewater uses Harbin Institute of Technology's EBA technology. The effluent COD is less than 60 mg / L, the total phenol is 10 mg / L, the ammonia nitrogen is 2 to 3 mg / L, and the average COD removal rate is 98%. The average ammonia nitrogen removal is The rate reached 99%. As the stable operation of the crushed coal pressurized gasification project is recognized by the industry, the wastewater from the crushed coal pressurized gasification projects currently under construction shall be treated with multi-stage A / O process or EBA process.

Concentrated brine treatment is the last step to achieve near-zero discharge of coal chemical wastewater treatment. Coal chemical brine is the reverse osmosis concentrated water discharged from the coal chemical reuse water system. The COD can reach more than 1 000 mg / L, and the total dissolved solids can reach 30 000 to 100 000 mg / L. Species of salt and heavy metals. At present, the treatment of concentrated brine in coal chemical industry is mainly based on two technologies: "membrane concentration + evaporation crystallization to prepare mixed salt" and "membrane separation + evaporation crystallization and salt separation". The mixed salt prepared by evaporation and crystallization needs to be disposed of in accordance with hazardous wastes because it contains traces of toxic substances and heavy metals. Due to the secondary environmental pollution risks of mixed salt treatment technology, it cannot meet the requirements of ecological environment protection. Therefore, modern coal chemical concentrated brine should be separated by membrane technology, and then industrial salt should be prepared by evaporation and crystallization technology to achieve resource utilization.

2 Technical difficulties in near-zero discharge treatment of modern coal chemical wastewater

The modern coal chemical wastewater has a complex water quality, contains a large amount of organic matter, and has a COD of 10 000 to 20 000 mg / L. It contains a large number of phenols, alkanes, esters, pyridine, quinoline, and heterocyclic substances that have inhibitory and toxic effects on biological metabolism. At present, there are still many technical difficulties in modern coal chemical wastewater treatment technology.

01 Characteristic hard-to-degrade organic matter pretreatment technology

Phenol oil is a characteristic pollutant to be removed in the pretreatment of modern coal chemical wastewater. The phenolic pollutants in the wastewater can reach 2 900 ~ 3 900 mg / L, and the ammonia nitrogen is 3,000 ~ 9 000 mg / L. The phenolic pollutants have very strong biological toxicity to the biochemical system, and when the pretreatment system is not stable The COD of the biochemical inlet water can reach 3 times or even 10 times during operation, which directly affects the stable operation of the biochemical treatment system. At present, the focus of phenol ammonia recovery is to study different extractants and extraction sequences. Chen et al. Studied the treatment effect of the extraction solvents methyl isobutyl ketone (MIBK) and diisopropyl ether (DIPE) on wastewater. The results showed that the MIBK partition coefficient was higher and the total phenol removal rate was increased from 50% to 67%. In the single column deacidification and deamination technology, MIBK is used as the extraction agent, and the removal rates of COD, ammonia nitrogen and total phenol reach 98%, 99%, and 100%, respectively.

The mass concentration of oil in the wastewater after phenol ammonia recovery treatment is 100 ~ 200 mg / L. These oil substances will hinder microbial metabolism and generate a large amount of foam in the aerobic section, causing sludge loss, which will seriously affect the operation of subsequent biochemical systems. Modern coal chemical projects use air flotation technology to bring oil and some pollutants out of wastewater, mainly including oil barrier sedimentation + air flotation, multi-stage air flotation and nitrogen gas barrier flotation. After applying the two-stage pretreatment process of oil and gas separation, the oil in coal chemical wastewater was reduced from 90 mg / L to 20 mg / L, which greatly reduced the inhibitory effect on the growth and metabolism of microorganisms in the subsequent biochemical treatment. The application of coagulation-air floatation method for coal chemical wastewater can reach about 97%.

02 Characteristics of biodegradable refractory organics

Modern coal chemical wastewater biochemical treatment system has high organic matter concentration and poor biochemical performance in the influent. The water quality of pressurized coal gasification wastewater is more complex than that of coal water slurry gasification wastewater and pulverized coal gasification wastewater. Generally, the COD of pressurized coal gasification wastewater reaches 2 000 to 3 000 mg / L, and B / C is 0.22 to 0.28. The proportion of difficult-to-degrade organic matter is as high as 20% ~ 25%. The feed water contains phenolic compounds, aromatic hydrocarbons, long-chain alkanes, polycyclic compounds and other biologically toxic pollutants. Based on the toxicity mechanism, M. Zheng et al. Established a biotoxicity evaluation of coal chemical-specific hard-to-degrade organic matter, and believed that the cumulative toxicity of nitrogen-containing heterocyclic rings and phenolic compounds severely inhibited the microbial activity of biochemical systems. Xu Peng used anaerobic, anoxic, and aerobic processes to treat difficult-to-degrade organic substances such as quinoline. The removal rate of quinoline, biphenyl, and naphthalene in the anoxic and anaerobic sections was higher than that in the aerobic section (good The removal rates of substances above the oxygen stage were 17.3%, 12.8%, and 19.6%, respectively, indicating that biological toxicity has a synergistic inhibitory effect on the degradation of substances, especially affecting the removal efficiency of the aerobic stage. He continued to study the effects of multi-stage co-metabolism on the removal of heterocyclic rings and polycyclic aromatic hydrocarbons by anaerobic, anoxic and aerobic processes. The results showed that this method can effectively improve the removal rate of heterocyclic rings and polycyclic aromatic hydrocarbons to more than 83.5%.

Unstable operation of the coal chemical wastewater treatment biochemical system can also seriously affect the activity and growth of activated sludge, and it takes a long time to restore the activity of activated sludge. In addition, the stable operation of the biochemical system and the discharge of the effluent quality are the prerequisites to ensure the stable operation of the recycled water treatment system, concentrated brine treatment and the quality and salt system. In short, the stable operation of biochemical treatment and the discharge of effluent water quality standards are the keys to achieving nearly zero discharge of modern coal chemical wastewater.

At present, biochemical treatment of modern coal chemical wastewater is mainly anaerobic + aerobic. This technology is an effective method to achieve stable operation of modern coal chemical wastewater treatment. The goal of the anaerobic process is to improve the biodegradability of wastewater, thereby improving the removal rate of organic matter by the aerobic process. The aerobic process is usually a multi-stage aerobic process. The front stage aerobic process uses high biomass to reduce the concentration of phenolic compounds and biologically inhibited pollutants, and the latter stage aerobic process achieves efficient removal of organic matter. Based on the analysis of the operation status of the coal chemical wastewater treatment project and the application technology of the project under construction (see Figure 2), multi-stage A / O and EBA processes are effective technologies for the treatment of coal chemical wastewater.

03 Characteristics of advanced non-degradable organic matter treatment technology

In order to ensure that the modern coal chemical wastewater meets the discharge standards or the water replenishment water quality standards after treatment, it is necessary to further remove the difficult-to-degrade organic matter, color and suspended matter in the advanced treatment system. H. Zhu et al. Believed that the ozone catalytic oxidation (AOPs) technology can effectively remove nitrogen-containing heterocyclic organics in coal chemical wastewater, and the removal rates of pyridine and indole can reach 90% and 95%, respectively. In actual engineering, advanced treatment will combine and optimize unit treatment technologies based on wastewater water quality, expected effects, etc., as shown in Figure 3. The crushed coal pressurized gasification wastewater has complex water quality and is difficult to treat. The advanced treatment process is longer and the treatment process is more complicated. Generally, ozone + BAF, Fenton + contact oxidation, activated coke adsorption, etc. are used to remove the typical hard-to-degrade organic substances that still exist after the biochemical treatment of coal chemical wastewater.

04 Technical difficulties in using concentrated brine resources

The modern coal chemical concentrated brine comes from the reverse osmosis membrane concentrated water of the coal chemical recycling water treatment system, as shown in Figure 4.

Treatment of high salt water in modern coal chemical industry can effectively improve water resource utilization. Generally, the high salt water content of the high salt water treatment process projects only accounts for less than 10% of the salt water content, and the high salt water content of the high salt water effluent or no evaporation crystallization project accounts for more than 20% to 40% of the total salt water content.

The treatment of concentrated brine in modern coal chemical industry is the last difficult problem to be solved. At present, the concentrated salt water treatment technology is applied to the membrane concentration + evaporation crystallization technology, but the heterologous salt produced by evaporation crystallization is identified as a hazardous waste, which needs to be landfilled by a solid waste treatment plant. This mixed salt treatment method is not only restricted by the site capacity of the solid waste treatment plant, but also has secondary environmental pollution.

The salt concentration of concentrated brine in coal chemical industry is high, mainly sodium chloride, sodium sulfate and sodium nitrate. Coal chemical salt water salting and resource utilization technology can improve the utilization of water resources while recovering the salt in the salt water. In modern coal chemical industry, the salt-separating and resource-recycling technologies are mainly based on membrane separation + evaporation, crystallization, quality and salt. Using nanofiltration technology to separate coal chemical concentrated brine, the removal rate of COD and sulfate can reach 75% and 90%. It is worth noting that the negative rejection rate of chloride ions by the nanofiltration membrane is very helpful for the recovery of sodium chloride in the concentrated brine of coal chemical industry.

3 Outlook

01 Integrated Biochemical Processing Technology

Integrated biochemical treatment technology According to the differences in wastewater water quality, the core biochemical technology, pretreatment technology and advanced treatment technology are reasonably integrated, which can effectively remove the characteristic pollutants of modern coal chemical wastewater. Research by W. Ma et al. Shows that the coupling technology of micro-electrolysis and bioreactor under micro-oxygen conditions can effectively improve the biodegradability of coal chemical wastewater, and the COD removal rate can be increased to more than 86.5%.

At present, the integrated biochemical treatment process has a long process flow and each unit process affects each other. When the unit process operation effect fails to meet the design target, the entire biochemical treatment system cannot run stably. At present, the focus of biochemical treatment technology is still on the application of unit technology, so it is difficult to achieve the standard of biochemical treatment effluent when water quality fluctuations or working conditions change. Therefore, the stable operation of the near-zero discharge system of modern coal chemical wastewater requires overall consideration of the systematic nature of integrated biochemical treatment technology, emphasis on the coordination between unit processes, and the application of the correct operation and operation scheme of the biochemical treatment system.

02 Resource utilization of concentrated brine

A small amount of organics, heavy metals, and other salts will inevitably be mixed in when preparing industrial salts by evaporation crystallization. At this stage, there is no relevant standard for quantifying the trace substances attached to the surface of crystalline salts, which affects the quality and circulation of crystalline salts. The difficulty of industrial salt in brine is to control the quality of crystalline salt. At present, the salt of coal chemical industry is classified according to industrial salt standards of other industries. It is urgent to formulate industrial salt standards for coal chemical wastewater to regulate and guide the resource utilization and circulation of coal chemical industry salt.

03 Application of cleaner production methods to reduce salt content in wastewater

The resource utilization technology and application of concentrated brine in modern coal chemical industry are in the process of development and practice. At present, the process of preparing salt in the coal chemical industry is very complicated and energy-intensive. Coal chemical concentrated brine treatment has huge environmental benefits, but its operating costs have brought tremendous economic pressure to the enterprise. The salt in the concentrated brine of coal chemical industry comes from raw coal, fresh water, production process and chemicals added in wastewater treatment. The added salt in the whole process accounts for more than 50% of the quality of the salt and salt. The amount of medicine added during the process and the application of clean production methods are the prerequisites to reduce the difficulty of salt concentration in coal chemical concentrated brine.

4 Conclusion

The near-zero discharge of modern coal chemical wastewater is the only way to coordinate the contradiction between ecological environment and energy. At present, a reliable technology of pretreatment + biochemical treatment + reused water treatment + concentrated brine treatment and salt and mass separation is formed. Integrated biochemical treatment technology is the core of nearly zero discharge of modern coal chemical wastewater. Characteristic organics in coal chemical wastewater, especially nitrogen-containing heterocyclic rings and polyphenols, have cumulative toxicity suppression effects on microorganisms, and will have a great negative effect on the near-zero emission biochemical treatment process of coal chemical wastewater. Reasonable wastewater treatment technology design and accurate operation can provide guarantee for the stable operation of modern coal chemical wastewater treatment projects. The application of membrane separation + evaporation crystallization technology to treat coal chemical concentrated brine can not only improve the repeated utilization of water resources, but also prepare industrial salts that can be reused in resources, thereby opening up the final barrier of nearly zero discharge of modern coal chemical wastewater. In addition, the coal chemical industry concentrated brine quality and salt need to develop deep metal removal of heavy metals, organics and toxic substances in concentrated brine technology to ensure the quality of industrial salts.

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