Why is COD in coking wastewater difficult to degrade?

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Why is COD in coking wastewater difficult to degrade? Currently, COD has become the key challenge in the treatment of coking wastewater. Industry professionals are aware that coking wastewater contains many substances that are polycyclic and long-chain, with very poor degradability. Many universities and research institutions have conducted experiments showing that no matter how much the biochemical treatment time is extended, the remaining COD level cannot be reduced any further. So why is explaining COD so difficult? First, let’s take a look at the composition of COD in coking wastewater. It is understood that the COD of coking wastewater is mainly composed of organic components, inorganic components, and suspended solids. To investigate the COD composition of coking wastewater, institutions such as Anshan Coking and Refractory Research Institute and Harbin Construction Institute used the biochemical inlet and outlet water from Ansteel Chemical Plant as samples, to determine the contents of major inorganic and organic substances in the coking wastewater as well as its COD value, and also to measure the COD value associated with the suspended solids present in the wastewater. The COD determination method is the potassium dichromate method. Research has shown that the inorganic components in coking wastewater that contribute to COD include SCN-, CN-, S2-, NO2-, etc., among which SCN- is the main component among these inorganic substances contributing to COD. Meanwhile, institutions such as Anshan Coking and Refining Research Institute and Harbin Construction Institute took the biochemical inlet and outlet water from Ansteel Chemical Plant as subjects. By measuring and analyzing various organic components in this wastewater, it was found that the COD value of the main organic components in the biochemical inlet water accounted for about 70% of the total COD in the coking wastewater. These components mainly included phenolic compounds, heterocyclic compounds, and naphthalene derivatives, with phenols accounting for around 30% of the total COD. The COD of the main organic components in the biochemical effluent accounts for about 20% of the total amount; among these, the COD of naphthalene and polycyclic aromatic hydrocarbons is relatively high, each accounting for over 10%. The suspended solids mainly consist of coal dust, coke dust, and water-insoluble oils. Among the three types of substances that make up COD, the organic components and inorganic components are the main constituents. “Breaking down long-chain molecules is a challenging task; there are too many polycyclic aromatic hydrocarbons and heterocyclic aromatic hydrocarbons, and these rings are the main reason for their difficulty in being degraded. Microorganisms can break open a ring at a certain stage through enzymes; what’s needed now is a method that is both effective and low-cost, and this represents a breakthrough point. ”That’s what Professor Wen Donghui from the School of Environmental Science and Engineering at Peking University said. Why is the pretreatment process so important in coking wastewater? The pre-treatment processes for coking wastewater mainly include ammonia evaporation, cyanide removal, oil removal, etc. What is the role of each of these processes? “Biochemical treatment requires certain conditions to be met; for example, coking wastewater contains oils, sulfides, and other substances, so preliminary treatment processes are necessary first, such as oil separation, sulfide removal, and ammonia evaporation. In particular, ammonia evaporation is essential – without it, further treatment becomes practically impossible. ”Li Tianzeng from the Sand Group told a reporter from Shuiwang that \"since the ammonia nitrogen level in typical coking wastewater is 2,000 or 3,000 or higher, methods such as stripping and the addition of alkali can be used to reduce the ammonia level; when these methods are effective, the level can be brought below 100. Some manufacturers are reluctant to use alkali, but it still needs to be added in subsequent treatment steps. In fact, if this principle is understood, adding alkali during pre-treatment will yield better results.\" Preprocessing is done to ensure the proper operation of the biochemical processes. ” Wen Donghui emphasized: “Biochemical treatment technologies can meet the standards, but they must be combined with other physical and chemical treatment methods; pretreatment processes are essential. Certain substances can be recycled, and if pretreatment is not done properly, it has a significant impact on the subsequent secondary treatment.” The main pretreatment processes include ammonia steaming and oil removal, among which ammonia steaming is the most essential. ” Why is research on bioenhancement technology so popular? According to experts, conventional biological treatment of wastewater is generally aimed at substances that are easy to degrade; it is the microorganisms that play a key role in breaking down these substances, and such microorganisms can be found in natural environments, domestic sewage, and soil. However, if the wastewater contains many toxic and harmful substances, not only are these substances indigestible by microorganisms, but they may also be harmful to them; such toxic substances can kill the microorganisms, making it difficult to use biological methods for direct treatment. That’s the contradiction: the most economical and effective method is biochemical treatment, while physicochemical treatment is extremely costly ； However, in the case of coking wastewater, the water quality is very complex, containing many toxic and harmful substances that can have a negative impact on the effectiveness of biochemical treatment. Therefore, when using biochemical methods to treat coking wastewater, there is much research work to be done regarding microorganisms. So what are the research tasks? Firstly, from a process perspective, it is possible to degrade wastewater to some extent by adjusting process parameters and conditions. However, in practice, changes made solely at the process level have limited effects; it is difficult to achieve discharge standards, and toxic and harmful substances still remain in the wastewater and are released into water bodies. Secondly, after more than a decade of development, there has been significant interest in microbial research, with the greatest focus on the development of degradation bacteria. The traditional method for obtaining degradation bacteria involves screening for bacteria or microorganisms with high tolerance from coking wastewater or sludge contaminated by it, and then formulating them into the desired bacterial agents. However, at present, very few bacteria can be isolated from nature, with cultivable microorganisms accounting for only 0.1% of those in nature ； Another issue is that the microbial agents used show excellent results in the laboratory and over short periods of time, but in actual engineering applications various changes occur. In real wastewater, in addition to toxic and harmful substances, there are also other components such as high levels of ammonia nitrogen. Moreover, the volume and quality of the water are unstable, all of which have a significant impact on the practical use of microorganisms. To date, no highly effective microbial agent has been found that can be successfully applied to coking wastewater. Furthermore, experts have also expressed concerns about the treatment plants. “The management of treatment plants needs to be strengthened urgently. When we go to various coking plants to collect wastewater, we find that there are significant problems with their management as well. For example, the production processes often change, and such changing water qualities are very detrimental to biological treatment methods. If the quality of the incoming water keeps changing, the wastewater treatment plant is unable to cope effectively; the sludge ends up in a state between alive and dead, which has a major impact on the efficiency of treatment. “That’s why I suggest that the production process and management should be as stable and precise as possible,” said Wen Donghui.