Treatment of coal chemical industry wastewater: establishment of technical approach

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Treatment of coal chemical industry wastewater: Determination of technical approaches Author/Source: China Chemical Industry News Date: 10-26-2018 Clicks: 7 China’s coal and water resources are distributed extremely unevenly, in a reverse pattern; water resource and water environment issues have become bottlenecks hindering the development of the coal chemical industry. The implementation of the \"Ten Measures on Water Management\" has further exacerbated this problem. To this end, our country has been committed to developing \"zero discharge\" technologies for coal chemical industry wastewater, and related research has been progressing steadily. The entire set of integrated technologies has gradually matured, establishing the technical approach of \"pre-treatment—biological treatment—advanced treatment—treatment of high-salinity water\" to achieve zero discharge of wastewater. 　　Pre-treatment generally includes oil separation, acidification, sedimentation, air flotation, sand filtration, solvent extraction, etc. Each of these methods has its own advantages and disadvantages; currently, solvent extraction is the most commonly used method. In extraction methods, chelating extractants are being phased out, while solvent extraction has recently gained prominence due to its advantage of not requiring acid-base adjustment, and is gradually taking over the market. 　　Pre-treatment requires targeted technical measures based on different water quality conditions. For example, the phenol-containing acidic wastewater generated by Shenhua Group’s coal direct liquefaction project has high levels of H2S, NH3, and phenol; by using double-tower stripping and isopropyl ether extraction, the concentrations of H2S, NH3, and phenol can be reduced to levels suitable for biochemical treatment. 　　Pre-treatment can also remove or decompose large-molecule, hard-to-degrade organic substances in wastewater in advance; for example, by using n-octanol and cyclohexane as extractants to extract the hard-to-degrade organic substances from coking wastewater, the biodegradability of the wastewater increased from 0.09 liters to 0.29 liters, while the COD removal rate rose from 68.81% to 88.63%. 　　Biological treatment: Internationally and domestically, anoxic and aerobic biological methods (A/O process) are generally used for biological treatment. However, due to the polycyclic and heterocyclic compounds in coal chemical industry wastewater, it is difficult to maintain the COD levels in the treated effluent at acceptable levels using aerobic biological treatment methods. 　　To address the aforementioned problems, some new treatment methods have emerged recently, such as the biochar method (PACT), biological fluidized bed treatment (PAM), immobilized biotechnology, carrier fluidized bed biofilm method (CBR), anaerobic biological methods, and anaerobic-aerobic biological methods. 　　The biochar process (PACT) can treat toxic and harmful organic pollutants that are difficult for microorganisms to degrade, and it is effective in dealing with high-concentration macromolecular organic substances. The biological fluidized bed treatment process (PAM) has a degradation efficiency that is several times higher than that of the suspended sludge activated sludge process, and it also possesses a strong capacity to remove ammonia nitrogen through nitrification. Immobilized biotechnology is a new technology that has emerged in recent years; the optimized microbial strains used in this approach have a degradation capacity for quinoline, isoquinoline, and pyridine that is 2 to 5 times higher than that of ordinary sludge, with degradation rates for substances such as pyridine exceeding 90%. 　　Advanced treatment is used to treat the effluent from secondary biological treatment, in order to overcome the limitations of biological treatment. These mainly include coagulation sedimentation, flocculation sedimentation, multi-media filtration, activated carbon adsorption, and membrane separation; in addition, advanced oxidation and other treatment processes are employed to ensure the quality of the effluent water. 　　Commonly used advanced treatment technologies include electrocatalytic oxidation; other such techniques are coagulation sedimentation, filtration, ozone oxidation, activated carbon filtration, and ultrafiltration. 　　Taking the wastewater from pressurized coal gasification as an example, biological combination technologies are commonly used, with common advanced treatment methods including “ozone + BAF”, “Fenton + contact oxidation”, and LAB. BAF is the core process for advanced wastewater treatment at present. The combination of \"ozone + BAF\" is increasingly favored in new projects due to its reasonable process flow and excellent operational performance. 　　High-salinity water treatment: The wastewater is reused in the system after being treated with dual membranes, while the concentrated brine is evaporated and crystallized to produce salt. Evaporation and crystallization represent the main challenge in the development of treatment processes for coal chemical industry wastewater at present. Evaporators can generally increase the salt content in wastewater to over 20%; the brine discharged as a result is usually sent to evaporation ponds for natural evaporation and crystallization, or to crystallizers where it is dried into solids before being transported to landfills for disposal. 　　Technical challenges: The processes in the earlier stages of coal chemical wastewater treatment are relatively mature. The current technical difficulties mainly lie in the final stages of evaporation and crystallization as well as salt separation. There are three main issues: First, corrosion and blockage problems are severe, affecting the continuous and stable operation of the evaporation equipment. Ions such as calcium, magnesium, sulfate, carbonate, and silicate in wastewater undergo evaporation and crystallization to form substances like calcium sulfate and calcium carbonate, which adhere to form scale layers that can easily cause blockages in equipment and pipes. Second, the operating costs are high; the amount of steam required for multi-effect evaporation and the dosage of chemicals used in mechanical recompression processes represent the main costs associated with these two technologies. Third, the initial investment required for construction is substantial. Due to the strong corrosiveness of concentrated brine at high temperatures, high requirements are placed on the selection of equipment and materials, which leads to increased costs for such equipment and materials. 　　Future development: According to industry insiders who spoke to reporters, it is entirely possible to achieve zero discharge of coal chemical wastewater using existing technologies, regardless of the costs involved. However, considering cost factors and the maturity of related technologies, it is necessary to design the treatment processes reasonably. The appropriate combination of technologies and the most suitable treatment routes must be selected based on the quality of the wastewater and the site conditions; multiple methods should be used in combination for the staged treatment of coal chemical industry wastewater. This represents the basic direction for the development of technologies for treating coal chemical industry wastewater. 　　At the same time, further efforts are needed to develop advanced unit technologies for the treatment of coal chemical industry wastewater. For example, in the initial water treatment processes, appropriate treatment methods must be selected based on the characteristics of each water quality in order to reduce costs effectively. Evaporation crystallization processes account for a large proportion of the total costs associated with such treatment; therefore, there is an urgent need to develop a crystallization method suitable for wastewater with high COD and high salt content from the coal chemical industry. It is also important to separate different types of waste by their composition and salt content, to maximize the value of by-products and thereby reduce the costs associated with handling hazardous crystallized waste.