Analysis of Key Issues in Domestic Coal Gasification Wastewater Treatment

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Analysis of Key Issues in the Treatment of Coal Gasification Wastewater in China Author/Source: Water Treatment Technology Authors: Zheng Pengsheng, Guo Zhongquan Date: 2018-03-21 Clicks: 13 During the 12th Five-Year Plan period, China’s coal chemical industry experienced rapid development, with its technical capabilities and level of industrialization ranking among the best in the world. However, the issue of pollution control in this industry remains a bottleneck to its further growth. “The 13th Five-Year Plan period represents a phase of adjustment for the development of the coal chemical industry, during which water scarcity and environmental emission issues are key problems that need to be addressed urgently. Coal gasification is the cornerstone of the coal chemical industry. In China, coal gasification enterprises are mostly located in the western and northern regions where coal is abundant but water is scarce. Strict water quality emission standards are in place in most of these areas, and in some regions, the lack of suitable water bodies requires zero discharge of coal gasification wastewater. Given the current level of technology for treating gasification wastewater, achieving both zero emissions and high-standard emissions poses significant challenges. Coal gasification enterprises urgently need wastewater treatment technologies that are highly applicable, have low operating costs, and provide stable treatment results, in order to effectively address the water treatment crisis and eliminate concerns associated with coal gasification production. 1 Characteristics of wastewater from coal gasification: The quality of wastewater generated by different coal gasification processes varies significantly; the common features are high concentrations of ammonia nitrogen and the presence of cyanides. Wastewater from fluidized bed gasification is free of tar; it generally contains 400–2,700 mg/L of ammonia nitrogen and 300–1,000 mg/L of COD. The organic compounds present are mainly formic acid derivatives, with phenol concentrations ≤10 mg/L and cyanide concentrations ≤30 mg/L. The water has high hardness and a high concentration of suspended solids. The COD of wastewater from fluidized bed gasification is generally 200–300 mg/L, with a B/C ratio of 0.6–0.65. It contains 10–20 mg/L of tar, a cyanide concentration of ≤5 mg/L, and a phenol concentration of ≤20 mg/L. The wastewater from fixed-bed gasification contains difficult-to-degrade organic substances such as phenols, polycyclic aromatic hydrocarbons, and benzene derivatives. It contains 2,900–8,450 mg/L of monophenols and 1,500–4,250 mg/L of polyphenols; the COD level is as high as 13,500–70,000 mg/L, with a B/C ratio of 0.15–0.25. The ammonia nitrogen concentration is as high as 3,500–10,000 mg/L. The wastewater from fixed-bed gasification is complex in composition, highly toxic, and has poor biodegradability; therefore, effective removal of recalcitrant organic pollutants, phenols, and ammonia nitrogen requires special attention. For the same coal gasification process, the quality of coal also has a significant impact on the quality of wastewater; when bituminous coal or lignite is used as raw material, the quality of the wastewater is relatively poor. For the treatment of gasification wastewater, a combined treatment process consisting of three units—pre-treatment, biological treatment, and advanced treatment—is generally adopted both domestically and internationally. 2 Analysis of pretreatment technical issues: The wastewater pretreatment unit for coal gasification is primarily designed to remove suspended solids, oil, ammonia, and phenols; among these, the removal of phenols from wastewater generated in fixed-bed gasification is the most challenging issue. 2.1 Removal of suspended solids and oil: Some of the suspended solids in coal gasification wastewater can be removed through coagulation sedimentation. This method has advantages such as low investment and simple operation. For different types of coal gasification wastewater, the type of coagulant, the dosage of chemicals, and the reaction and sedimentation processes can be optimized through coagulation mixing tests. Wastewater from fixed-bed gasification contains floating oil, dispersed oil, emulsified oil, and dissolved oil. Free oils such as floating oil and dispersed oil can be removed by taking advantage of the density difference between oil and water; air flotation and oil separation techniques are commonly used for this purpose. However, it is more difficult to remove emulsified oil and dissolved oil. Emulsified oil has good stability in wastewater, and it must be demulsified before it can be removed through methods such as air flotation, adsorption, and flocculation. The main components of the dissolved oils in coal gasification wastewater are phenolic compounds, which can only be partially removed through extraction in the pretreatment unit. 2.2 Recovery and removal of ammonia and phenol: For ammonia removal during pretreatment, single-tank pressurized side-stream stripping and two-tank pressurized stripping processes are generally used; these methods require high energy consumption, and it is necessary to address issues related to ammonium salt crystallization and scaling during operation. To address the existing problems in ammonia removal, some scholars have proposed a bilateral linear stripping tower model and demonstrated its feasibility; others have suggested applying differential pressure thermal coupling along with steam compression technology to the stripping process in order to reduce energy consumption. Dephenolization is a key step in the treatment of wastewater from fixed-bed gasification. Phenol-amine recovery is often optimized as part of an overall process, involving three different process routes: acid removal and extraction for dephenolization followed by deamination; acidification followed by extraction for dephenolization and then acid and ammonia removal; and acid and ammonia removal followed by extraction for dephenolization. Commonly used extractants include benzene, heavy benzene, heavy solvent oil, methyl isobutyl ketone, and isopropyl ether. These extractants have a high distribution coefficient for monophenols, but their extraction efficiency for polyphenols is only 60% to 88%. Excess polyphenols that enter subsequent biochemical treatment units can severely suppress microbial activity, thereby having a significant impact on the stability of the biological treatment process. Common extractants also suffer from issues such as high costs, large extraction losses, and secondary pollution. Some scholars have proposed using processes such as complexation extraction and supported liquid membrane extraction to improve the phenol removal efficiency, but these are currently limited to laboratory-scale tests. There is little research on the industrial application of new extractants, extraction devices, and extraction processes, and their actual performance and operational stability require further analysis. 3 Analysis of biological treatment technology issues. Currently, the biological treatment processes used for coal gasification wastewater include oxidation ditches, A/O, A2/O, SBR, biological contact oxidation, MBR, and aerated biological filters. Research focuses on improving conventional biological treatment processes as well as developing new ones tailored to the characteristics of coal gasification wastewater. 3.1 Conventional biological treatment processes and their modifications For wastewater from fixed-bed gasification, a combined treatment process using various biological treatment techniques is generally employed. However, considering operational costs and labor intensity, the main processes used are still oxidation ditches, A/O systems and their modifications; the most common combination of processes is hydrolysis acidification + A/O + aerated biological filter. The conventional gasification biological treatment process is generally based on the activated sludge method; nitrification and denitrification in the nitrogen removal process are carried out in separate reactors or at different time periods. It features a long hydraulic retention time, high energy consumption, large floor space required for the facilities, frequent sludge bulking problems, and poor ability to handle shock loads. To further improve processing efficiency, some scholars have conducted optimization studies on the operation modes and process combinations of conventional techniques, resulting in an increase in pollutant load and operational stability. 3.2 New biological treatment technologies: To optimize the nitrogen removal methods for coal gasification wastewater, scholars have conducted experimental studies on the application methods and process parameters of efficient nitrogen removal techniques such as simultaneous nitrification-denitrification, short-cut nitrification-denitrification, and anaerobic ammonium oxidation. By employing biological immobilization and biological enhancement techniques, improving reactors, and optimizing control parameters, it is possible to significantly enhance the efficiency of nitrogen removal, thereby achieving effective removal of ammonia nitrogen, total nitrogen, and COD. To mitigate the inhibitory effect of phenols on microorganisms, biological treatment units can also optimize the removal of phenols and other refractory organic compounds through pre-treatment anaerobic processes. Anaerobic processes such as upflow anaerobic sludge bed and anaerobic expanded granular sludge bed are capable of improving the removal efficiency of total phenols. For the treatment of wastewater generated in fixed-bed gasification, common industrial wastewater treatment processes and design parameters should not be applied mechanically; rather, the impact of characteristic pollutants on the efficiency of biological treatment must be taken into full consideration. Researchers have already conducted studies on how typical phenolic pollutants affect the biodegradation capabilities and nitrogen removal efficiency of biological treatment units. The findings from these studies can provide a theoretical basis for process improvements. During operation, the use of bacteria capable of removing phenols can further enhance the efficiency of phenol removal. 4 Analysis of advanced treatment technical issues: For meeting discharge standards, advanced treatment units need to further enhance the removal of ammonia nitrogen, total nitrogen, refractory organic matter, and suspended solids. For projects aimed at water reuse and zero discharge, it is not only necessary to treat coal gasification wastewater to meet the requirements for production water quality, but it is also essential to properly address issues such as concentrated brine treatment, solidification of highly concentrated brine, and membrane fouling. Currently, in China, the discharge of gasification wastewater is generally subject to local emission standards that are stricter than the first-level standards set by the Comprehensive Wastewater Discharge Standards (GB 8978-1996). This increases the difficulty of wastewater treatment to some extent, with the deep removal of ammonia nitrogen, total nitrogen, and organic substances being key challenges in this process. Considering both treatment efficiency and operating costs, biological treatment technology remains the preferred method for nitrogen and carbon removal. In terms of the process flow, advanced oxidation is generally used first after secondary biological treatment to improve the biodegradability of the wastewater, followed by biofilm treatment processes to further remove ammonia nitrogen, total nitrogen, and organic matter. 4.1 Advanced Oxidation For advanced oxidation, common treatment processes include ozone catalytic oxidation, Fenton oxidation, photocatalytic oxidation, and electrochemical oxidation. The common problems of advanced oxidation technologies are high reagent consumption and high energy consumption; high operating costs represent the bottleneck preventing their application in the advanced treatment of coal gasification wastewater. The treatment cost of heterogeneous ozone oxidation technology is relatively low; it also achieves a high decomposition efficiency for ozone. The catalysts can be regenerated, and the development of heterogeneous catalysts is currently a focus of research. Furthermore, some scholars have proposed new technologies such as supercritical water oxidation and plasma deep oxidation through experiments, but their practical effectiveness and operational stability still require further research. 4.2 Adsorption The adsorption technique can enhance the removal of refractory organic pollutants; it is generally used in combination with biological treatment processes in advanced treatment units, and it can also improve the quality of water before it enters membrane separation processes. Common adsorbents such as activated carbon are difficult to regenerate and result in high treatment costs. The development of low-cost new adsorbents is an important research direction in the adsorption technology for coal gasification wastewater. Activated coke, prepared from lignite, possesses well-developed mesopores that enable it to effectively adsorb large molecular organic substances; it has been used for the advanced treatment of wastewater generated by Lurgi coal gasification processes. However, the removal of coke particles requires optimization through coagulation and sedimentation. 4.3 Membrane separation and concentrated brine evaporation Membrane separation technology has been successfully applied to the reuse treatment of certain industrial wastewater, offering advantages such as stable desalination efficiency, low energy consumption, and simple operation. However, its use for the advanced treatment of gasification wastewater still requires solutions to issues such as membrane fouling and the handling of concentrated brine. In membrane separation systems, it is necessary to reduce membrane fouling and the amount of concentrated brine generated. Single-stage reverse osmosis (RO) produces a large amount of concentrated brine, with a recovery rate of only around 70%; two-stage RO can achieve a recovery rate of 90%, but membrane fouling is severe in RO systems, and this fouling is the key issue that affects the long-term stable operation of membrane separation systems. The dual-membrane process consisting of ultrafiltration (UF) and RO can reduce the contamination of the RO membrane by organic substances to a certain extent; however, Ca2+, Mg2+, and certain microorganisms still pass through the UF membrane and deposit on the surface of the RO membrane, causing inorganic salt scaling and microbial contamination. Therefore, the RO membrane needs to be chemically cleaned regularly. To this end, the High-Efficiency Reverse Osmosis (HERO) process removes hardness and other substances that can cause inorganic salt scaling during the pretreatment stage, and prevents microbial contamination of the RO membrane by raising the pH. Common technologies for evaporating concentrated brine from coal gasification wastewater include natural evaporation, multi-effect evaporation, and mechanical vapor recompression evaporation. Natural evaporation has a poor actual evaporation capacity, limited controllability, requires large land area, and has a simple anti-seepage structure; as a result of potential environmental risks, its use in engineering applications is greatly restricted. Multi-effect evaporation technology is mature, features high thermal efficiency and low land use, with a fresh water recovery rate of up to 90%, but it requires high equipment investment. Mechanical vapor recompression evaporation can save energy and cooling water, but it is still in the industrial trial stage in China. 5 Conclusions and Future Prospects 1) Gasification wastewater is characterized by a complex composition, numerous biochemical inhibitors, large fluctuations in water quality, as well as high concentrations of ammonia nitrogen and recalcitrant organic substances. A composite treatment process consisting of pretreatment, biological treatment, and advanced treatment is necessary to address these specific pollutants. The pretreatment stage must effectively remove suspended solids, oils, ammonia, and phenols; the biological treatment stage should focus on removing recalcitrant organic substances, ammonia nitrogen, and total nitrogen; while the advanced treatment stage is required to prevent membrane fouling and improve the efficiency of concentrated brine evaporation. 2) The key issues that need to be addressed urgently in the wastewater treatment process for coal gasification include the efficient removal of ammonia nitrogen and polyphenols, reducing the operating costs of advanced oxidation processes, increasing the water recovery rate in membrane separation processes, suppressing membrane fouling, and the evaporation of concentrated brine. 3) To address the key issues in the treatment of gasification wastewater, it is necessary to focus on both the combination of multiple treatment processes and the optimization of individual treatment units. This approach helps to improve the efficiency of treatment while reducing construction and operational costs. It also enhances the ability of biological treatment units to remove various pollutants such as phenols, organic substances, and total nitrogen, which is crucial for lowering overall treatment costs. The optimized combination of biological treatment processes and advanced oxidation processes will be a key area of research in the future.