Advanced treatment technologies for coal coking wastewater

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1: Coking wastewater. Coking wastewater is an industrial wastewater generated during the processes of coking, high-temperature dry distillation of gas, purification, and recovery of by-products; it contains volatile phenols, polycyclic aromatic hydrocarbons, as well as heterocyclic compounds containing oxygen, sulfur, nitrogen, etc. It is a type of industrial organic wastewater with high CODcr levels, high phenol content, high ammonia nitrogen levels, and is very difficult to treat. There are three main sources of such wastewater. The first is excess ammonia water, which is wastewater generated during coal carbonization and gas cooling; it accounts for more than half of the total amount of coking wastewater and thus constitutes the main source of such wastewater. The second source is wastewater produced during gas purification processes, such as water used for final gas cooling and water obtained from the separation of crude benzene. The third source is wastewater generated during the refining of tar, crude benzene, and in other related processes. Coking wastewater is industrial wastewater containing a large amount of hard-to-degrade organic pollutants; it has a complex composition and includes various toxic and harmful substances such as phenols, cyanides, benzene, and ammonia nitrogen. The discharge of coking wastewater in excess of allowable levels causes severe environmental pollution. Coking wastewater is characterized by large variations in water quality and volume, complex composition, high levels of organic matter especially those that are difficult to degrade, and high ammonia nitrogen concentrations. In coking wastewater plants, nitrogen-containing compounds are organic substances that are present in large quantities and have a very complex composition in the wastewater from coking plants. Quinoline and certain alkyl substituents identified by mass spectrometry are suspected to be carcinogenic substances. There are also many bioactive substances such as aromatic hydrocarbons and aromatic amines. Phthalate esters are another category of carcinogenic substances in wastewater; dimethyl phthalate and diisooctyl phthalate among them are also pollutants that the U.S. Environmental Protection Agency prioritizes for monitoring. In summary, coking wastewater has a complex composition with a wide variety of pollutants, many of which are bioactive substances with carcinogenic and mutagenic effects. The COD level of this wastewater often fails to meet the **discharge standards; therefore, it is important to find effective and cost-effective advanced treatment methods. Coking wastewater treatment can be classified into primary, secondary, and tertiary treatment based on the degree of treatment. Primary treatment, also known as initial treatment or pretreatment, involves removing suspended solids from wastewater through methods such as sedimentation, extraction, and redox reactions, in order to recover valuable substances. Secondary treatment is a further processing of wastewater based on primary treatment. Tertiary treatment, also known as advanced treatment, involves further processing the water that has undergone secondary treatment in order to effectively remove pollutants of various types from it. II: Research and Current Application Status of Advanced Treatment Technologies for Coking Wastewater At present, the advanced treatment technologies for coking wastewater mainly include coagulation sedimentation, adsorption, advanced oxidation processes (such as Fenton oxidation, O3 oxidation, catalytic wet oxidation, etc.), and reverse osmosis technology. 　　Coking wastewater treatment plant: 1. For advanced treatment using the coagulation-sedimentation method, coagulants such as polyaluminum chloride and polyferric sulfate are employed. 　　2. The adsorption method involves using porous adsorbents to absorb one or several solutes in wastewater, thereby purifying the wastewater. Commonly used adsorbents include fly ash, coking dust, activated carbon, resins, etc. 　　3. Advanced Oxidation Technologies 　　(1) The Fenton oxidation method is a homogeneous catalytic oxidation process that uses hydrogen peroxide as an oxidant and ferrous salts as a catalyst. Fenton reagent is a strong oxidizing agent, and the •OH radicals generated in the reaction are highly oxidative free radicals that can oxidize organic substances in wastewater, thereby reducing its color intensity and COD value. 　　(2) Ozone oxidation is a strong oxidizing agent that can rapidly react with most organic substances and microorganisms in wastewater. It can remove pollutants such as phenols and cyanides from wastewater, reduce its COD and BOD values, and also serve to decolorize, deodorize, and sterilize the water. 　　(3) The basic principle of electrochemical oxidation technology is to induce direct electrochemical reactions of pollutants at the electrodes, or to use the highly oxidizing active substances generated on the electrode surface to cause redox transformations of the pollutants. 　　(4) The photocatalytic oxidation method involves light energy triggering reactions between electrons and holes, resulting in electron (hole) pairs with high reactivity. These electron (hole) pairs migrate to the surface of the particles, where they can participate in and accelerate redox reactions. The photocatalytic oxidation method achieves a high removal rate for phenols and other organic substances in water, requires low energy consumption, and holds great potential for further development. 　　4. Reverse osmosis technology is a membrane separation process driven by pressure. A water pump is used to apply pressure to the saline solution or wastewater in order to overcome natural osmotic pressure and the resistance of the membrane, allowing water to pass through the reverse osmosis membrane while preventing dissolved salts and pollutants from passing to the other side of the membrane. 　 III. Conclusion 　　There is extensive research on advanced treatment and reuse technologies for coking wastewater, but few practical applications exist. The main challenges lie in the immaturity of these advanced treatment technologies at an industrial scale, as well as high investment and operating costs. Therefore, on the one hand, efforts should be made to accelerate the industrialization of advanced oxidation technologies; on the other hand, sources for treating these wastewater streams should be found within steel plants, so as to achieve decentralized treatment of coking wastewater and thereby **reduce the scale of advanced treatment processes. This requires systematic analysis and research carried out jointly by water treatment experts and production staff from steel companies, from a bottom-up approach. Currently, some relevant institutions in China are conducting research on areas such as the reuse of miscellaneous wastewater, thermal treatment of steel slag, and integrated treatment of blast furnace flue gases, in an effort to find more ways to dispose of coking wastewater.