Wastewater treatment process for coal-to-gas production

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1 Water quality analysis: Given the characteristics of the coal-to-gas production process in coking plants, the wastewater mainly originates from the water contained in the coal. This water, along with the volatile components in the coal, enters the gas production process. During the cooling of the gas, water and tar combine to form a mixed condensate, which is then discharged to the ammonia water mechanical clarification tank through the gas-liquid separator and the water seal in the primary cooler. There, tar and ammonia water are separated; the ammonia water goes into the intermediate tank for stored ammonia, while any excess ammonia water is sent for ammonia vaporization, resulting in wastewater from this process； The crude benzene process generates separation water from crude benzene during its production ； All gas water seals in the plant are drained directly ； Gas condensate and other wastewater from storage and distribution stations. 2 Process selection: Based on the quality parameters of the wastewater, this type of wastewater is suitable for treatment using a \"physical-chemical + biological + physical-chemical\" process. The main task of physical-chemical treatment is to remove oils, sulfides, cyanides, high concentrations of ammonia nitrogen, and volatile phenols, so as to ensure the proper progress of biochemical treatment ； The main task of biochemical treatment is to degrade biodegradable substances in wastewater and carry out nitrogen removal ； The biochemical-treated wastewater is then subjected to physical and chemical treatment to further remove pollutants, ensuring that it meets discharge standards. However, there are many types of physical and biochemical treatment processes, each with its own characteristics. The key to this project is to select mature, reliable, and appropriate treatment processes based on local conditions, to arrange them properly, and to reduce investment and treatment costs. Biological nitrogen removal is the application of nitrification and denitrification. Nitrification is a process in wastewater treatment in which ammonia nitrogen is oxidized to nitrite and nitrate under aerobic conditions through the action of aerobic bacteria. Denitrification is a process that occurs under anaerobic conditions, in which denitrifying bacteria use NO2-N and NO3-N produced as a result of metabolic reactions to replace oxygen in the oxidation and decomposition of organic substances. They reduce the nitrogen in NO2-N and NO3-N to nitrogen gas, thereby completing the denitrification process. There are various methods for biological nitrogen removal in China, but among them, the A/O internal circulation process is the most cost-effective when applied to coking wastewater. The treatment process we have chosen is: grid well + oil separation and sedimentation + flotation/aer flotation + reaction and sedimentation + ammonia nitrogen stripping + UASB anaerobic reactor + primary A/O biological treatment + secondary A/O biological treatment + MBR. 3. Description of the wastewater treatment process: The cooling water from coal gas production first enters a regulating tank. This tank serves to regulate the volume of water and ensure uniform water quality. Perforated pipes are installed at the bottom of the tank; through air agitation, wastewater of different concentrations at various times is mixed evenly within the tank, thereby reducing the impact of water volume and quality on subsequent treatment units. The effluent flows by gravity into the oil separation sedimentation tank. Conventional oil separation tanks are primarily used to remove floating oil from water; the particle size of this oil is generally above 150 microns, so they are unable to remove smaller oil droplets. However, this oil separation sedimentation tank has an inclined tube device installed inside it, which increases the area of the tank relative to its volume. It significantly reduces the distance that oil droplets can rise, allowing smaller oil droplets to have a chance of reaching the water surface and thus enabling more oil particles in the water to be separated. At the same time, solid substances and impurities in the water have a better opportunity to come into contact with the surface of the inclined plate, gather together, and settle quickly, further improving wastewater treatment. This is because wastewater impurities contain a large amount of oil. Therefore, the use of inclined tube oil separation devices yields far better results than oil separators of the same scale. The wastewater, after having the pollutants removed, flows by gravity into the regulation tank, while the separated oil is collected in an oil collection tank for regular removal and treatment. The water from the oil separation and sedimentation tank flows into the flotation unit; PFS and PAM are added before this unit. After flocculation, the resulting mixture enters the flotation unit, where the rapidly reduced pressure releases numerous tiny supersaturated gas bubbles. These bubbles combine with the \"floc\" and suspended particles in the water, causing them to rise to the surface and form scum. A skimmer is used to remove this scum regularly, and it is then discharged through pipes into the sludge tank. The wastewater, after having the pollutants removed, flows automatically into the sedimentation reaction tank. After its pH value is adjusted to ≤3 by adding acid, H2O2 and Fe2+ are added; essentially, a chain reaction takes place between the divalent iron ions and hydrogen peroxide, catalyzing the generation of highly reactive ·OH free radicals. These ·OH free radicals react with poorly biodegradable organic substances, causing partial oxidation, cleavage, or oxidation of those substances, thereby forming intermediate products with relatively low molecular weights, which in turn alters their biodegradability, solubility, and coagulability. When the pH is adjusted to an alkaline level and oxygen is present, Fe(OH)3 with coagulation and adsorption properties is also formed, which facilitates further removal of organic substances. It can achieve complete degradation of organic matter in a short time, without being restricted by the type, composition, or concentration of the wastewater, making it particularly suitable for the treatment of biologically difficult-to-degrade organic wastewater. Then, after the pH is readjusted by adding alkali, the effluent flows directly into an inclined tube sedimentation tank for sedimentation. The effluent from the coagulation sedimentation tank is pumped to the ammonia stripping tower for nitrogen removal. After denitrification, the wastewater flows directly into the acid adjustment tank, where H+ is added to adjust the pH; thereafter, it is pumped into the UASB anaerobic reactor. Indegradable aromatic organic compounds in wastewater undergo ring-opening to form linear compounds in the anaerobic stage, and linear compounds break apart to form shorter-chain compounds. Since wastewater contains large amounts of hard-to-degrade compounds such as quinoline, pyridine, and isoquinoline, the purpose of using anaerobic conditions is to leverage the allosteric or depolymerizing effects of anaerobic microorganisms to convert certain substances that are difficult to degrade by aerobic and facultative microorganisms into substances that are easier to degrade. The biochemical system primarily uses the “A/O nitrification-denitrification process” as its main treatment technique. This wastewater contains a high level of organic components, with a BOD5/CODcr ratio of 0.6, indicating good biodegradability; therefore, using biological treatment methods to significantly reduce the organic content in the wastewater is the most economical approach. Due to the high levels of ammonia nitrogen and organic matter in wastewater, especially organic nitrogen, this organic nitrogen is converted into ammonia nitrogen during the biodegradation of organic matter. Ammonia nitrogen is also an important indicator for pollution control; therefore, the A/O biological contact oxidation process is used for wastewater treatment, which involves dividing the biochemical tank into an A-stage tank and an O-stage tank. The wastewater in the regulating tank is lifted to the Class A biological treatment tank using a wastewater lift pump for biological treatment. In the Class A tank, due to the high concentration of organic matter in the wastewater, the microorganisms are in an anaerobic environment. At this time, these microorganisms are facultative organisms; they convert the organic nitrogen in the wastewater into ammonia nitrogen, and at the same time use organic carbon sources as electron donors to transform NO2--N and NO3--N into N2. They also utilize some of the organic carbon sources and ammonia nitrogen to synthesize new cellular materials. Therefore, the Class A tank not only has the function of removing organic matter, thereby reducing the organic load on the subsequent Class O biological treatment tank and facilitating nitrification, but it also relies on the high concentration of organic matter in the wastewater to carry out denitrification, ultimately eliminating eutrophication caused by nitrogen. After the biochemical treatment in Stage A tanks, the wastewater still contains a certain amount of organic matter as well as high levels of nitrogen and ammonia. To further oxidize and decompose these organic substances, and to ensure that nitrification can proceed smoothly once carbonization is nearly complete, an O-stage biochemical tank is provided. The effluent from Pool A flows by gravity into Pool O. The treatment in Pool O’s biological tank is carried out by autotrophic bacteria (nitrifiers), which use the inorganic carbon sources generated from the decomposition of organic matter or carbon dioxide from the air as nutrients to convert ammonia nitrogen in the wastewater into NO2--N and NO3--N. Part of the effluent from the Class O tank flows into the lower-level treatment units, while another part is returned to the Class A tank for internal circulation, in order to achieve denitrification. Fillers are installed in both the Class A and Class O biological treatment tanks, and the entire biological treatment process relies on various microorganisms attached to these fillers. In the Class A tank, the dissolved oxygen level is kept at around 0.5 mg/l ； In the Class O biological tank, the dissolved oxygen level should be maintained above 3 mg/l. Part of the effluent from the Class O biological tank is recycled back to the Class A tank, while another part flows into the MBR tank. A Membrane Bioreactor (MBR) is a new type of efficient wastewater treatment and reuse process that integrates membrane separation technology with traditional biological treatment methods. It uses submerged membrane modules with a unique structure placed in an aeration tank; the water that has undergone aerobic aeration and biological treatment is then pumped out after passing through the filter membranes. This process replaces the secondary sedimentation tank used in traditional biological wastewater treatment processes with membrane modules, enabling complete separation of sludge from water through the efficient filtration action of these membrane modules ； Furthermore, the increased concentration of activated sludge in the aeration tank, along with the presence of specific bacteria within the sludge (especially the dominant microbial communities), enhanced the rate of biochemical reactions ； Due to the filtering action of the membrane, microorganisms are completely retained within the bioreactor, enabling a complete separation between the hydraulic retention time and the sludge age in activated sludge processes, and thus eliminating the problem of sludge bulking associated with traditional activated sludge methods ； At the same time, operating at a low sludge load also reduces the amount of excess sludge produced, thereby addressing the prominent issues associated with traditional biological treatment processes, such as poor water quality in the effluent, large land requirements, complex operation and maintenance, and the tendency for sludge bulking that leads to further deterioration of water quality. Membrane bioreactors offer advantages such as high efficiency in removing pollutants, strong nitrification capacity, the ability to carry out nitrification and denitrification simultaneously, good nitrogen removal performance, stable effluent quality, low production of excess sludge, compact design, and simple operation. It is currently widely used in the treatment and reuse of domestic wastewater and various biodegradable industrial wastewater. Source: Shuibowang