Technical Challenges and Prospects for Near-Zero Discharge of Wastewater from Modern Coal Chemical Industry

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Technical Challenges and Prospects for Near-Zero Discharge of Wastewater from Modern Coal Chemical Industry / Author/Source: Huahua Network – Coal Chemical Industry / Date: 2019-09-18 / Clicks: 93. The technology for near-zero discharge of wastewater from modern coal chemical industries is key to reconciling the contradictions between ecological environment needs and energy requirements. Currently, biochemical treatment technologies have evolved from a focus on individual unit technologies to an integrated approach that takes into account process interconnections, treatment system capacity, and source control as key technical aspects. Membrane separation combined with quality and salt separation treatment techniques can improve the utilization rate of water resources while recovering salt resources; therefore, it is the most reliable technology for treating concentrated brine in coal chemical industries at present. The current status of technologies and their applications is analyzed, and by taking into account the technical characteristics, insights are provided to address the challenges in achieving near-zero discharge of wastewater from modern coal chemical industries as well as to outline future development directions. The coal chemical energy industry has developed rapidly due to coal’s primary role in China’s energy reserves. Modern coal chemical engineering is the process of using coal as a raw material and converting it through chemical processing into gaseous, liquid, and solid fuels as well as chemicals. In recent years, the rapid economic development has led to a sharp increase in energy demand. Meanwhile, China’s energy characteristics of “scarce oil and limited natural gas” have further intensified the supply-demand imbalance. Therefore, the development of the modern coal chemical industry is essential for ensuring China’s energy supply and sustainable economic development. The production and construction of modern coal chemical projects are guided by the principle of coordinated development between the ecological environment and energy. Coal chemical industry is a water-intensive sector; the average water consumption per unit of product for coal-to-oil, coal-to-olefins, and coal-to-natural gas is around 10, 27, and 6 tons respectively. However, coal chemical projects are usually located in areas with abundant coal resources but scarce water supplies. In addition, the coal chemical industry generates wastewater during its production process that contains high concentrations of phenols, high levels of ammonia nitrogen, as well as a large amount of toxic and harmful substances. Furthermore, due to the limited environmental capacity for wastewater in the areas where coal chemical projects are located, the requirements for treating coal chemical wastewater from an environmental protection perspective are very strict. To reconcile the ecological and environmental issues arising from coal chemical processing with energy demands and to overcome the bottlenecks in the development of modern coal chemistry, many researchers have applied biochemical treatment techniques, physicochemical treatment techniques, as well as combined biochemical-physicochemical techniques to treat wastewater generated by coal chemical processes. However, there are still some challenges in treating such wastewater, and near-zero emission technologies need to be further developed and optimized. 1 Current Status of Near-Zero Discharge Treatment Technologies for Modern Coal Chemical Industry Wastewater. The water treatment systems in the coal chemical industry include water purification, circulating water treatment, biological treatment, reclaimed water reuse, concentrated brine treatment, and evaporation crystallization treatment (see Figure 1). Therefore, achieving near-zero discharge of wastewater in modern coal chemical industries requires overcoming various technical challenges related to wastewater treatment and utilization, in order to achieve a high rate of water resource utilization without any wastewater being discharged outside. Currently, the treatment of wastewater from modern coal chemical industries follows a technical process that includes pretreatment, biochemical treatment, recycled water treatment, concentrated brine treatment, as well as separation of different water qualities and salts. Among these, biochemical treatment and concentrated brine treatment are key steps for achieving near-zero discharge of wastewater from coal chemical industries. Biochemical treatment technology can remove over 90% of the pollutants in coal chemical wastewater; in particular, characteristic refractory organic compounds such as polyphenols and nitrogen-containing heterocycles require biochemical treatment to be removed. Currently, modern wastewater treatment technologies for coal chemical industries mainly consist of pretreatment, biological treatment, and advanced treatment, including physicochemical + biochemical methods, anaerobic + aerobic processes, as well as optimized treatment techniques. An analysis and statistics of the biochemical treatment processes for wastewater from 18 coal chemical enterprises and 2 park wastewater treatment plants in Inner Mongolia, Shaanxi, Shanxi, and other regions showed that the application of key biochemical processes for treating coal chemical wastewater is related to the quality of the wastewater (see Figure 2). These processes mainly include SBR, CAST, A/O, A2O, MBR, and contact oxidation. The core technologies for water-coal slurry gasification and pulverized coal gasification offer a range of options. There are differences in the biochemical treatment processes used in existing and under-construction projects for treating wastewater from pressurized coal gasification; for example, the wastewater zero-discharge biochemical treatment system at the China National Coal Group Tuket Coal Chemical Project utilizes the EBA technology developed by Harbin Institute of Technology. The COD level in the treated wastewater is less than 60 mg/L, the total phenol content is 10 mg/L, and the ammonia nitrogen level is between 2 and 3 mg/L. The average removal rate for COD is 98%, while the average removal rate for ammonia nitrogen is 99%. As the biochemical process for the stable operation of coal pulverization pressurized gasification projects has been recognized in the industry, the wastewater from ongoing coal pulverization pressurized gasification projects is currently treated using multi-stage A/O processes or EBA processes for biochemical wastewater treatment. Concentrated brine treatment is the final step in achieving near-zero discharge for coal chemical wastewater treatment. The concentrated brine from coal chemical industries refers to the reverse osmosis concentrate discharged from the wastewater recycling systems in these industries; its COD can exceed 1,000 mg/L, while the total dissolved solids level ranges from 30,000 to 100,000 mg/L. This concentrated brine contains a large amount of hard-to-degrade organic substances, various salts, and heavy metals. Currently, the treatment of concentrated brine in coal chemical industries relies mainly on two techniques: \"membrane concentration + evaporation crystallization to produce mixed salts\" and \"membrane separation + evaporation crystallization along with salt separation by quality.\" Mixed salts prepared by evaporation crystallization contain trace amounts of toxic substances and heavy metals, and therefore need to be disposed of as hazardous waste. Moreover, the disposal techniques for such mixed salts pose a risk of secondary environmental pollution, failing to meet ecological and environmental protection standards. Therefore, in modern coal chemical industries, the concentrated brine should first have its salts separated via membrane technology, and then industrial salt should be produced using evaporation crystallization technology, so as to achieve resource utilization. 2 Technical challenges in near-zero discharge treatment of modern coal chemical industry wastewater: The wastewater from modern coal chemical industries has a complex composition, containing large amounts of organic substances; its COD levels range from 10,000 to 20,000 mg/L. It also contains numerous phenols, alkanes, esters, pyridines, quinolines, and heterocyclic compounds that inhibit biological metabolism and are toxic. To date, there are still many technical challenges associated with the treatment of such wastewater. 01 Pretreatment technologies for refractory organic pollutants: Phenol-amine oil is a characteristic pollutant that needs to be removed in the pretreatment of modern coal chemical industry wastewater. The phenolic pollutants in wastewater can reach 2,900–3,900 mg/L, while ammonia nitrogen levels are 3,000–9,000 mg/L. Phenolic pollutants possess a very high degree of biological toxicity toward biochemical systems. Moreover, when the pretreatment system operates unstably, the COD level of the water fed into the biochemical treatment system can increase by 3 times or even more, thereby affecting the stable operation of such systems. Currently, the focus of phenol-amine recovery is on researching different extractants and extraction sequences. Chen Yun et al. studied the treatment efficiency of the extraction solvents methyl isobutyl ketone (MIBK) and diisopropyl ether (DIPE) on wastewater. The results showed that MIBK had a higher distribution coefficient, increasing the total phenol removal rate from 50% to 67%. In the extraction dephenolization technique following single-tower acid and ammonia removal, MIBK is used as the extractant, achieving removal rates of 98%, 99%, and 100% for COD, ammonia nitrogen, and total phenols, respectively. The oil mass concentration in the wastewater after phenol-ammonia recovery treatment is 100–200 mg/L. These oily substances hinder microbial metabolism and generate large amounts of foam in the aerobic stage, leading to sludge loss and thus severely affecting the operation of subsequent biochemical systems. Modern coal chemical projects use air flotation technology to remove oils and various pollutants from wastewater, with processes such as oil separation and sedimentation plus air flotation, multi-stage air flotation, and nitrogen-based oil separation air flotation being commonly employed. After applying the two-stage oil and gas removal flotation pretreatment process, the oil content in coal chemical wastewater was reduced from 90 mg/L to 20 mg/L, significantly reducing the inhibitory effect on the growth and metabolism of microorganisms in subsequent biochemical treatment. When the coagulation-flotation method is applied to coal chemical industry wastewater, the oil removal rate can reach approximately 97%. 02 Biochemical treatment technologies for refractory organic pollutants: The wastewater entering modern coal chemical industry biochemical treatment systems contains a high concentration of organic substances with poor biochemical properties. The wastewater generated from coal pulverization pressurized gasification has a more complex water quality compared to that from water-coal slurry gasification and pulverized coal gasification. Typically, the COD level in this type of wastewater ranges from 2,000 to 3,000 mg/L, while the B/C ratio is between 0.22 and 0.28. The proportion of hard-to-degrade organic substances is as high as 20% to 25%. The wastewater contains various pollutants with strong biological toxicity, such as phenolic compounds, aromatic hydrocarbons, long-chain alkanes, and polycyclic compounds. Based on the toxicological mechanisms, M. Zheng et al. developed a biological toxicity assessment method for recalcitrant organic compounds characteristic of the coal chemical industry; they argued that the cumulative toxicity of nitrogen-containing heterocycles and phenolic compounds severely inhibits the microbial activity in biochemical systems. Xu Peng used anaerobic, anoxic, and aerobic processes to treat recalcitrant organic substances such as quinoline. The removal rates of quinoline, biphenyl, and naphthalene compounds in the anoxic and anaerobic stages were higher than those in the aerobic stage (the removal rates for these compounds in the aerobic stage were 17.3%, 12.8%, and 19.6% respectively), indicating that biological toxicity has a synergistic inhibitory effect on substance degradation, particularly affecting the efficiency of removal in the aerobic stage. It continued to study the impact of multi-stage co-metabolism on the removal of heterocycles and polycyclic aromatic hydrocarbons in anaerobic, anoxic, and aerobic processes, and the results showed that this method could effectively increase the removal rate of heterocycles and polycyclic aromatic hydrocarbons to over 83.5%. The unstable operation of the biochemical system for treating coal chemical wastewater can also severely affect the activity and growth of activated sludge, and it takes a long time to restore its activity. In addition, the stable operation of the biochemical system and the discharge of water meeting quality standards are prerequisite conditions for ensuring the stable operation of the recycled water treatment system, the concentrated brine treatment system, and the system for separating different types of salts. In summary, the stable operation of biochemical treatment and the discharge of wastewater meeting quality standards are key to achieving near-zero discharge of wastewater from modern coal chemical industries. Currently, the biochemical treatment of wastewater from modern coal chemical industries relies primarily on anaerobic and aerobic processes; this technology is an effective way to ensure the stable treatment of such wastewater. The goal of the anaerobic process is to improve the biodegradability of wastewater, thereby enhancing the efficiency of aerobic processes in removing organic matter. In aerobic processes, a multi-stage aerobic process is commonly employed. The initial stage utilizes a high biomass level to reduce the concentrations of phenolic compounds and biologically inhibitory pollutants, while the subsequent stages ensure efficient removal of organic substances. Through the analysis of the operation status of existing biochemical treatment projects for coal chemical industry wastewater and the application technologies in projects under construction (see Figure 2), the multi-stage A/O and EBA processes are effective technologies for treating coal chemical industry wastewater. 03 Advanced treatment technologies for recalcitrant organic pollutants: To ensure that modern coal chemical industry wastewater meets the discharge standards or the water quality requirements for reuse in cooling systems, it is necessary to further remove these recalcitrant organic pollutants, as well as color and suspended solids, within advanced treatment systems. H. Zhu et al. believe that ozone catalytic oxidation (AOPs) technology can effectively remove nitrogen-containing heterocyclic organic compounds from coal chemical wastewater, with removal rates of over 90% and 95% for pyridine and indole, respectively. In practical engineering, advanced treatment involves combining and optimizing various unit treatment technologies based on the quality of the wastewater and the desired outcomes, as shown in Figure 3. Due to its complex water quality and difficulty in treatment, the wastewater generated from pressurized gasification of coal requires longer treatment processes and more complex treatment technologies. Commonly, methods such as ozone + BAF, Fenton + contact oxidation, and activated carbon adsorption are employed to remove the typical refractory organic compounds that remain in coal chemical wastewater after biochemical treatment. 04 Technical challenges in the reuse of concentrated saltwater resources: The concentrated brine used in modern coal chemical industries comes from the concentrated water produced by the reverse osmosis membranes in the wastewater treatment systems for coal chemical processes, as shown in Figure 4. http://img.yf116.cn/image/img/20190918/1430555225586.jpg Treating the high-salinity water generated in modern coal chemical industries can effectively improve the efficiency of water resource utilization. In projects that typically use high-salinity water treatment processes, the amount of high-salinity water accounts for less than 10% of the total saline water volume, whereas in cases where high-salinity water is discharged or no evaporation and crystallization processes are employed, this proportion ranges from 20% to 40% or more of the total saline water volume. The treatment of concentrated brine in modern coal chemical industry is the last key challenge that needs to be addressed in order to achieve near-zero emissions. Currently, the membrane concentration + evaporation crystallization technology is the only technique for concentrated brine treatment that has been put into engineering application. However, the miscellaneous salts generated from evaporation crystallization are classified as hazardous waste and must be landfilled at a solid waste treatment facility. This method of treating mixed salts is not only constrained by the space capacity of waste treatment plants, but also causes secondary environmental pollution. Coal chemical industry brine has a high salt concentration, primarily consisting of sodium chloride, sodium sulfate, and sodium nitrate. The concentration, separation of salts, and resource utilization technologies in coal chemical industries can recover the salts present in concentrated brine, thereby improving the efficiency of water use. For modern coal chemical industries, the technology for salt fractionation and resource utilization of concentrated brine primarily involves a process combining membrane separation with evaporative crystallization to separate components and salts. The use of nanofiltration technology for the separation of concentrated brine from coal chemical processes can achieve removal rates of over 75% for COD and over 90% for sulfate ions. It is worth noting that the negative rejection rate of chloride ions by nanofiltration membranes is highly beneficial for the recovery of sodium chloride from concentrated brines in coal chemical industries. 3 Outlook 01 Integrated Biochemical Treatment Technology: This technology integrates core biochemical processes, pretreatment methods, and advanced treatment techniques in a way that takes into account the differences in wastewater quality, thereby enabling the effective removal of the characteristic pollutants present in modern coal chemical industry wastewater. Studies by W. Ma et al. have shown that the coupling of microelectrolysis with bioreactors under micro-oxygen conditions can effectively improve the biodegradability of coal chemical industry wastewater, with the COD removal rate reaching over 86.5%. Currently, the integrated biochemical treatment process has a long sequence of steps, and the various processes involved interact with one another. When the performance of any individual process fails to meet the design specifications, it prevents the entire biochemical treatment system from operating stably. At present, the focus of biochemical treatment technologies remains on the application of individual treatment processes; as a result, it is difficult for the effluent from biochemical treatment to meet the required standards when there are fluctuations in water quality or changes in operating conditions. Therefore, the stable operation of near-zero discharge systems for modern coal chemical industry wastewater requires comprehensive consideration of the integration of biochemical treatment technologies, an emphasis on the coordination among various unit processes, and the application of proper operating procedures for the biochemical treatment systems. 02 Reuse of concentrated saltwater: When using evaporation crystallization to produce industrial salt, it is inevitable that small amounts of organic substances, heavy metals, and other salts end up mixed in. At present, there are no relevant standards for identifying the trace substances attached to the surface of the crystallized salt, which affects the quality and marketability of such salt. Therefore, the challenge in separating and purifying industrial salt from concentrated saltwater lies in ensuring the quality of the crystallized salt. At present, industrial salts used in the coal chemical industry are classified according to the standards for industrial salts in other industries; there is an urgent need to establish standards for producing industrial salts from coal chemical wastewater, in order to regulate and guide the resource utilization and circulation of such industrial salts. 03 Applying clean production methods to reduce the salt content in wastewater: The technologies and applications for the recycling of concentrated brine from modern coal chemical industries are still in the stage of development and implementation. At present, the processes used for producing industrial salts in the coal chemical industry are very complex and require high levels of energy consumption. The treatment of concentrated brine in coal chemical industries offers significant environmental benefits, but its operating costs impose substantial economic pressures on enterprises. The salts in the concentrated brine from coal chemical processes originate from the raw coal, fresh water, chemicals added during the production process, and wastewater treatment. The salts added throughout this process account for more than 50% of the total salt content in such brine. Therefore, controlling the amount of chemicals used in the coal chemical production process and wastewater treatment, as well as adopting cleaner production methods, are essential prerequisites for reducing the difficulties associated with separating salts from this concentrated brine. 4 Conclusion Near-zero discharge of wastewater from modern coal chemical industries is an essential approach to reconciling the tensions between the ecological environment and energy needs. Currently, reliable technologies have been developed, including pretreatment + biological treatment + reclaimed water treatment + concentrated brine treatment, as well as separation of water and salts based on their different properties. Integrated biochemical treatment technology is the core of near-zero discharge for modern coal chemical industry wastewater. The characteristic organic compounds in coal chemical industry wastewater, especially nitrogen-containing heterocycles and polyphenols, exert a cumulative toxic inhibitory effect on microorganisms, which has a significant negative impact on the biochemical treatment processes aimed at achieving near-zero discharge of such wastewater. Therefore, designing appropriate wastewater treatment strategies based on the differences in wastewater quality, along with accurate operational procedures, can ensure the stable operation of modern coal chemical industry wastewater treatment systems. Using membrane separation plus evaporation crystallization technology to treat concentrated brine from coal chemical industries not only improves the reuse rate of water resources but also enables the production of industrial salt that can be recycled, thereby overcoming the final hurdle toward near-zero discharge of wastewater from modern coal chemical processes. Furthermore, for the separation of different components in coal chemical industry brine, it is necessary to develop advanced technologies for the thorough removal of heavy metals, organic substances, and toxic materials from such brine in order to ensure the quality of industrial salt.