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Technical and Economic Analysis of Zero Liquid Discharge for Wastewater from Coal Chemical Projects and Treatment of Saline Wastewater Author/Source: Coal Processing & Comprehensive Utilization Date: July 8, 2019 Page views: 7 The treatment and reuse of saline wastewater is the key to achieving true “zero discharge” of wastewater in the coal chemical industry. Through the analysis of the characteristics of saline wastewater, 8 process technology combinations for achieving \"zero discharge\" of wastewater were systematically identified. Under the established baseline scenario, the investment, operating costs, and water-saving effects of each process technology combination were calculated and compared, along with a sensitivity analysis of the operating costs. The study shows that, when the water consumption target is met, the mechanical enhancement of evaporation ponds results in the lowest total cost ； The total cost of the high-efficiency reverse osmosis + MVR crystallization and electrodialysis + multi-effect evaporation crystallization processes is next lowest; furthermore, as technology advances in the future, their energy consumption and overall costs will decrease significantly. Finally, relevant suggestions are put forward, such as coordinating zero wastewater discharge with the overall plant system and providing policy support for zero discharge of highly concentrated brine, in order to achieve a balanced development of economic benefits and environmental protection outcomes for the enterprise. In February 2017, the first **-level special plan for the modern coal chemical industry, namely the \"13th Five-Year Plan for Demonstrating Advanced Coal Processing Industries,\" was officially released. The role of the modern coal chemical industry became increasingly clear: it is necessary for strategic energy technology and capacity reserves, and it represents an important measure to promote the clean and efficient use of coal as well as to ensure energy security. “During the 13th Five-Year Plan period, the development of the modern coal chemical industry was aimed at achieving \"upgrading and demonstration,\" with advanced wastewater treatment and reuse being key aspects of this upgrading process. Most of the modern coal chemical projects that have been built or are planned for construction are located in the central and western regions, where coal resources are abundant but water resources are relatively scarce. Due to water shortages and the lack of suitable bodies of water for wastewater discharge, achieving \"zero wastewater discharge\" from these coal chemical projects has become an inevitable requirement for their development. 1 The significance of zero discharge of coal chemical industry wastewater: During the 11th Five-Year Plan period, China built several of the first modern coal chemical industry demonstration projects, achieving the goals of a seamless process flow and the production of qualified products; however, there were still many shortcomings in areas such as water system optimization and wastewater treatment. At present, if new projects are designed with the water consumption standards of the initial batch of demonstration projects in mind, they often fail to meet the latest industry requirements regarding water consumption; therefore, measures such as upgrading circulating water cooling systems, as well as efficient treatment and reuse of wastewater to achieve a \"zero discharge\" scenario, are necessary to reduce the water consumption of these systems. \"Zero discharge\" of wastewater holds great practical significance for the development of the coal chemical industry. Firstly, through the research and application of advanced water treatment technologies, it is possible to prevent the release of waste liquids, thereby minimizing the impact of such projects on the surrounding environment and meeting environmental protection requirements. Secondly, by reusing water generated during the treatment process, the consumption of fresh water can be reduced, improving water usage efficiency and lowering the water demand of these projects, thus making the most of water resources. Coal chemical industry wastewater includes organic wastewater and saline wastewater. \"Zero discharge\" of wastewater means treating and reusing all the organic wastewater and saline wastewater generated during the production process of a project, so that no waste liquid is discharged outside. Organic wastewater is primarily generated during coal conversion processes, with wastewater from gasification accounting for over 60% of such waste. It is characterized by high levels of COD and ammonia nitrogen. Current treatment technologies are quite advanced; biochemical treatment can effectively reduce the organic content in this wastewater, enabling the reuse of the treated water. Salt-containing wastewater mainly originates from blowdown water in cooling water systems, blowdown water from demineralization plants, effluent from industrial wastewater treatment systems, and boiler blowdown water. It is characterized by high levels of TDS and suspended solids. Through conventional membrane concentration processes, over 70% of this wastewater can be recycled as reclaimed water. However, how to treat and crystallize the concentrated brine—which accounts for 20%–30% of the total volume—at low cost remains a major bottleneck in current water treatment systems. Therefore, the \"zero discharge\" of concentrated saltwater is a key factor in truly achieving \"zero discharge\" of wastewater from coal chemical projects. 2 Analysis of the characteristics of salt-containing wastewater in coal chemical industry 2.1 Sources of salts in the wastewater There are many types of salts present in salt-containing wastewater from the coal chemical industry, such as calcium salts, magnesium salts, chlorides, nitrates, silicates, phosphates, etc. With the increasing water consumption in coal chemical industries and the rising rate of water recycling, both the volume and salinity concentration of saline wastewater gradually increase. If not treated in a timely manner, this can easily lead to serious problems such as scaling and blockages, as well as equipment corrosion. The salts in saline wastewater originate mainly from two sources. The first is the salts brought in by industrial raw water; these salts are concentrated through the circulation system and separated by the demineralization system, accounting for more than half of the total salt content in the system. The second source is the salts generated by chemical agents added during the production process and in the water systems, which make up more than one-third of the total amount. To reduce the scale and investment required for treating saline wastewater from coal chemical projects, it is necessary to control the entry of salts at the source. Selecting high-quality water sources and reducing the use of raw water with high salt content are important preliminary measures for coal chemical projects. Taking the Yellow River as an example, the average TDS across the entire river basin is 453 mg/L, with a gradual increase from the upstream to the downstream areas. The TDS level in the water taken from certain upstream sections is relatively high, so it is necessary to plan industrial water use carefully. At the same time, by optimizing to determine a reasonable cycling ratio and dosing method, the amount of salt added artificially can be effectively controlled, thereby reducing the pressure on subsequent treatment processes. 2.2 Water quality of saline wastewater In the coal chemical production process, saline wastewater mainly originates from the effluent resulting from the biological treatment of organic wastewater, as well as from the circulating water system, the demineralized water system, and the clean wastewater discharged by boilers. According to statistics, the TDS in gasification wastewater that meets the standards is 1000–1500 mg/L, the TDS in recycled wastewater is 1800–2600 mg/L, and the TDS in wastewater from the desalination system is 2500–3500 mg/L. Theoretically, after biochemical treatment, organic wastewater can be 100% fed into subsequent membrane treatment units for saline wastewater to undergo further treatment. When the circulating water system, demineralized water system, boiler drainage, etc., reach a certain circulation rate and wastewater concentration, they need to be discharged directly into the saline wastewater treatment system for unified treatment. Taking a large coal-to-olefins project in Xinjiang as an example, the water quality parameters of the feedwater to the salt-containing wastewater treatment unit are shown in Table 1 below. Generally speaking, the wastewater entering the saline wastewater treatment unit is characterized by a high salt content, high hardness, and low organic matter content; after preliminary pretreatment, it can proceed to the membrane treatment unit for further concentration. 3 Analysis of treatment technology routes for saline wastewater in coal chemical industries 3.1 “Zero-discharge” treatment technology route based on the treatment and reuse of saline wastewater Saline wastewater typically contains various streams such as desalinated water, circulating water, effluents from industrial wastewater treatment systems, and boiler drainage. If saline wastewater is fed directly into the evaporation and crystallization process, it will significantly increase the investment and energy consumption of such systems. Similarly, if the goal is merely to reduce the energy consumption and investment associated with evaporation and crystallization, then the scale and load of membrane concentration processes as well as the equipment used in the preliminary treatment and concentrated brine treatment stages will increase substantially. Therefore, currently, the method of (preliminary treatment + concentration + evaporative crystallization) is generally employed: first, the saline wastewater is concentrated, and then it proceeds to the evaporative crystallization stage. The entire processing procedure needs to be comprehensively optimized to achieve the optimal investment scale and energy consumption. Its main process flow is shown in Figure 1 below. Preliminary treatment often employs reverse osmosis technology. Due to the large volume of wastewater, all new coal chemical projects now include this stage for wastewater reuse. Before treatment, in order to reduce the hardness of the wastewater and prevent microbial growth, the saline wastewater must be softened with chemicals and disinfected; it then proceeds to the (ultrafiltration + reverse osmosis) treatment process, after which the TDS level of the concentrated brine output can generally reach 10,000 mg/L. Through preliminary treatment, 65%–75% of the saline wastewater can be reused; it is sent to the circulating water station or used as makeup water for desalination plants and other industrial processes. If the volume of concentrated brine remains large, further concentration is required; the main process technologies include high-efficiency reverse osmosis (HERO), forward osmosis (FO), electrodialysis (ED), nanofiltration membrane concentration, and vibrating membrane concentration. After membrane treatment, concentrated brine can generally be reused in 70% to 80% of its original amount; the TDS of the highly concentrated brine resulting from this process is usually above 60,000 mg/L. High-concentration brine is no longer suitable for membrane concentration, so evaporation crystallization has become the main method; the primary processes include Mechanical Vapor Recompression cycle evaporation (MVR), Multi-effect Evaporation at low temperature (MED), and Multi-effect Flash evaporation (MSF). For example, the Datang Duolun coal-to-olefins project features a (MVR + mechanical compression falling film crystallization) process system; the Yili Xintian coal-to-natural gas project uses a multi-effect evaporation process system for concentrated brine after reverse osmosis; and the Shenhua direct coal liquefaction project employs a process system in which concentrated brine from reverse osmosis is fed into evaporators and evaporation ponds. Evaporation crystallization of high-concentration brine is a key step in achieving \"zero discharge\" of coal chemical industry wastewater. Currently, the mainstream process technologies are mostly held by foreign patent holders, and the treatment process is complex, requires large investment, as well as high energy consumption and costs. Overall, the process technologies for the concentration, evaporation, and crystallization of saline wastewater in the coal chemical industry are still largely in the experimental and pilot-scale stages, and further improvements are needed in terms of operational stability, large-scale application, and efficiency. 3.2 Technical approach for “zero discharge” treatment of saline wastewater based on sewage treatment. Coal chemical projects are mostly located in the arid northwestern regions with high evaporation rates, which provide natural advantages and geographical conditions suitable for the construction of evaporation ponds. Evaporation ponds require measures for rainwater separation, as well as anti-seepage and anti-corrosion protection. The pre-treated concentrated brine is transported via pipelines to evaporation ponds, where it undergoes natural evaporation crystallization or mechanically enhanced evaporation crystallization; the process flow is shown in Figure 2. The major modern coal chemical projects in China that were the first to use evaporation ponds include the Shenhua direct coal liquefaction project and the Datang Keqi coal-to-natural gas project, among others. The design scale of natural evaporation ponds is directly related to factors such as the regional average evaporation rate, precipitation, and the amount of wastewater discharged. The process requires the installation of adjustment tanks, evaporation tanks, concentration tanks, and crystallization tanks, which means that these ponds generally occupy a large area. In practical engineering applications, due to differences between design and actual operation, an environmental issue arises in which large amounts of concentrated brine accumulate and cannot be discharged. Mechanical forced evaporation technology is an improvement over traditional natural evaporation; by incorporating mechanical atomization evaporators, it increases the air flow rate and the contact area with the wastewater, thereby accelerating the evaporation process. Through mechanically forced evaporation, the land area required can be significantly reduced; however, this increases the system’s energy consumption. For instance, the energy required to evaporate 1 ton of water is equivalent to 0.3–1.5 tons of steam. The evaporation pond process can achieve \"zero discharge\" of wastewater and offers the advantage of low treatment costs. However, since the wastewater is completely released into the atmosphere through air evaporation, it cannot be reused, resulting in incomplete utilization of water resources. 4 Technical and Economic Comparative Analysis of Treatment Technologies for Saline Wastewater in Coal Chemical Industry. At present, domestic scholars have conducted numerous studies on the process technologies for treating saline wastewater generated in coal chemical projects, as well as comprehensive comparative studies on achieving \"zero discharge\" for different types of saline wastewater; however, there are few analyses from a technical and economic perspective. This paper does not analyze the maturity of various technologies or the stability of their operational performance; instead, it conducts a comparative analysis based on available experimental and practical data. 4.1 Comparative analysis benchmarks: The source of saline wastewater in the same project remains relatively stable. Most coal chemical projects carry out preliminary treatment on the saline wastewater with high flow rates, while regarding the concentrated brine generated, due to the high costs of further treatment and the complexity of the related technologies, different treatment approaches are generally chosen based on the specific conditions of each project. Aiming to achieve “zero discharge”, this paper takes the concentrated brine with a TDS concentration of over 6,000 mg/L after preliminary treatment of saline wastewater as the upper limit for influent water quality. Based on actual engineering conditions, 50 m³/h of such concentrated brine was selected as the subject of study. A techno-economic comparative analysis was conducted on eight process routes; the results are presented in Table 2. 4.2 Comparative Analysis of Economic Indicators for Different Process Technologies 4.2.1 Investment and Cost Parameters Investment mainly includes equipment costs, fees for land use related to the process, and other expenses, minus the savings from water rights transfers; among these, equipment costs are based on data from relevant studies, while the water rights transfer fee is calculated at 15 yuan per ton and the land use fee at 400 yuan per square meter, taking into account the local conditions. Operating costs mainly include electricity costs for operation, costs associated with the addition of chemicals, labor costs, and other expenses, minus the savings from using recycled water. The figures related to costs and energy consumption are based on data from relevant studies; the price of recycled water is calculated at 5 yuan per ton, while the electricity cost is 0.5 yuan per ton. 4.2.2 Comparative Analysis of Economic Indicators Different process technology routes can achieve the goal of \"zero emissions,\" but there are significant differences in the key technical and economic indicators; the investment and operating costs as well as related indicators are shown in Table 2. It can be seen that the evaporation pond method requires a high initial investment and has the lowest annual operating costs, but its brine treatment does not produce recycled water. Under the concentrated brine reuse crystallization treatment approach, the initial investment for processes incorporating the MVR technology is higher than that for MED; the operating costs of these various processes are similar to each other, and all of them can achieve a reuse rate of over 90%. 4.3 Selection of process technology routes based on minimizing total costs Different brine treatment process technologies vary in terms of investment and operating costs. From a whole-life-cycle perspective of the entire system, selecting the brine treatment system with the lowest total costs is an important measure for enhancing the economic efficiency and competitiveness of coal chemical projects. Model for minimizing the total cost of brine treatment systems: Based on a 20-year evaluation period typical for coal chemical projects, the total costs associated with different brine treatment systems over their entire lifecycle are calculated and shown in Figure 3. It can be seen that, under the specified boundary and parameter conditions, the total cost of mechanical-enhanced evaporation crystallization and the (electrodialysis + reverse osmosis + multi-effect evaporation crystallization) process is the lowest, while the total cost of natural evaporation crystallization is the highest; the total costs of the other five process technologies are roughly equal. However, considering the constraints of water conservation and the water consumption targets for such projects, the first 6 process technologies enable the saving of 360,000 to 380,000 tons per year in recycled water that needs to be used as makeup water, which is of great significance for reducing the water consumption targets of coal chemical projects. 4.4 Sensitivity analysis: In the treatment of saline wastewater, the concentration and evaporation-crystallization steps have a significant impact on investment costs, while technological advancements play a direct role in reducing the system’s energy consumption and operating costs. Therefore, this paper selects the operating costs, which best reflect the progress in various process technologies, as the factors for sensitivity analysis. Given that the total cost of the natural evaporation crystallization process is too high, only the other 7 process technologies are analyzed, as shown in Figure 4. Through sensitivity analysis, it can be seen that when operating costs are reduced by 10%, the total cost of EDRO-MED-crystallization drops to a level comparable to that of mechanically enhanced evaporation crystallization, at around 81 million yuan. When these costs are reduced by 20%–30%, the economic advantages of the EDRO-MED-crystallization process become most evident; meanwhile, the total cost of the FO-MED-crystallization process falls below that of the EDRO-MVR-crystallization process. As costs decrease, the overall cost of the HERO-MED-Crystallization and HEROMVR-Crystallization processes drops significantly, indicating substantial potential for cost savings. 5 Conclusions and Recommendations (1) The \"zero discharge\" treatment of saline wastewater is the bottleneck in achieving zero discharge for coal chemical projects; at present, most of the treatment technologies are still in the experimental and demonstration stage, with high investment, costs, and energy consumption being major constraints. Increasing technological research and development, as well as reducing investment in process technologies and energy consumption and operating costs, are key to the advanced treatment of concentrated brine in the future. (2) Under the specified boundary conditions, the mechanically enhanced evaporation crystallization process has the lowest total cost, making it highly suitable for widespread adoption. However, it fails to recycle 25% of the saline wastewater generated by the process; as a result, it may not meet the industry requirements for coal chemical projects that face severe water constraints and strict water consumption standards. The total cost of the high-efficiency reverse osmosis + MVR crystallization process, as well as the electrodialysis + multi-effect evaporation crystallization process, is relatively lower. Moreover, with future technological advancements, their energy consumption and overall costs are expected to decrease significantly, giving them great potential for development. (3) The \"zero discharge\" of wastewater from coal chemical projects is not an independent system; therefore, when formulating plans for retrofitting projects or designing new ones, it is necessary to take into account the relationships with the main production processes, investment efficiency, standards for energy and water consumption, wastewater recycling arrangements, and the overall balance of the plant’s water system. This ensures stable production, optimal system performance, and controllable environmental risks. (4) The treatment cost of saline wastewater from coal chemical projects is much higher than the direct benefits of water conservation; for example, the treatment cost of saline wastewater in Shenhua’s direct coal liquefaction project is as high as 54 yuan per ton, which reduces companies’ enthusiasm for advanced wastewater treatment. At present, the technology for \"zero discharge\" treatment of high-concentration brine is still in the stage of engineering trials or demonstrations. While continuing to strengthen research and development as well as process optimization, it is necessary for the relevant regulatory authorities to enhance policy regulation and support, and to establish a comprehensive system of incentives and penalties, in order to achieve a balanced development between corporate economic benefits and environmental protection outcomes.