Treatment of industrial wastewater by microbubble pure oxygen aeration technology

When major industrial wastewater from chemical, petrochemical, metallurgical, pharmaceutical, paper, printing, leather and other industries, the wastewater contains many refractory organics, using stripping, extraction wastewater treatment and the like into the biological treatment tank means BOD _5. COD is still relatively high. The biochemical treatment processes mainly used in urban sewage treatment plants and industrial sewage treatment plants in China include: ordinary activated sludge, oxidation ditch, SBR, A/O, A ² /O, A/B, BAF. Etc., and almost all of them use air aeration, and about 80% of the sewage treatment plants use the common activated sludge method. Because China's sewage treatment plants generally have problems such as high investment in construction projects due to backward technology, large floor space, high energy consumption after completion of operation, and large excess sludge production, it is necessary to solve these problems. It is necessary to master advanced, energy-efficient sewage treatment technology. As early as the 1930s, Western countries began to study pure oxygen aeration technology for wastewater treatment in the laboratory, along with the development of oxygen industry technology. Pure oxygen aeration technology has been used in industry. Since the industrialization of pure oxygen aeration technology in the 1960s, it has low investment, low energy consumption, small floor space, high sewage treatment efficiency, impact load resistance, less residual sludge, and simple transformation of old plants. The advantage has been rapidly developed. According to incomplete statistics, by the 1990s, more than 2,000 modified and newly built pure oxygen aerated activated sludge wastewater treatment plants were put into use worldwide. The application of pure oxygen aeration activated sludge process in China is limited to the petrochemical industry. Tianjin, Jinshan, Yangzi, Qilu, Daqing and other petrochemical companies introduced the NUOX pure oxygen aeration activated sludge process developed by American United Carbon Company in Liby, Germany in the 1980s. The sewage discharged by petrochemical enterprises is not only large, but also has complex chemical composition and high concentration of pollutants. The influent BOD _{5 is} as high as 560~5370mg/L, and the COD is as high as 830~7500mg/L. The results of the above-mentioned enterprises show that the treatment of petrochemical wastewater by pure oxygen aerated activated sludge process has a BOD ₅ removal rate of 98% to 99% and a COD removal rate of 90% to 96%, which has achieved good results.

It can be seen from the successful experience of pure oxygen aeration activated sludge process at home and abroad to treat high concentration organic wastewater. The pure oxygen aeration activated sludge process is an efficient and energy-saving sewage treatment technology, which is necessary to make China's sewage treatment industry On the previous step, it is an important way to replace the existing air aeration activated sludge method with the pure oxygen aeration activated sludge method developed by China.

First, the superiority of pure oxygen aeration activated sludge method

The pure oxygen aerated activated sludge process uses a pure oxygen with an oxygen volume fraction of more than 90% instead of air aeration. The principle of pure oxygen aerated activated sludge sewage treatment process is basically the same as that of air aerated activated sludge sewage treatment process. The decomposition of organic matter in sewage is based on the uniform dispersion of activated sludge (aerobic microorganisms) and sewage. Fully exposed, aerobic bacteria require oxygen. The difference between the two processes is that the pure oxygen aeration process uses pure oxygen and the air aeration process uses air, that is, the gas source has different oxygen concentrations. The high oxygen dissolution rate and oxygen concentration of the pure oxygen aeration process accelerate the metabolism of the microorganisms, thereby increasing the number of microorganisms, that is, the concentration of the sludge, accelerating the reaction rate, and greatly improving the sewage treatment efficiency. The pure oxygen aeration activated sludge sewage treatment process has the following advantages:

(1) The oxygen transfer rate is high. At a water temperature of 20 ° C, the oxygen volume fraction of pure oxygen is 4.7 times the volume fraction of air oxygen (21%). The partial pressure of pure oxygen in aeration is 4.7 times higher than the partial pressure of oxygen in air. The saturation value of dissolved oxygen in water. It is also 4.7 times higher, so the dissolution rate of pure oxygen to clear water is also 4.7 times that of air and more than 4.7 times in sewage. The residence time required for pure oxygen aeration is only about 1/3 of that when air is aerated. The test results show that the oxygen utilization rate of the open microbubble pure oxygen aeration and the underwater impeller pure oxygen aeration is 80% to 90%. The general air blast aeration oxygen utilization rate is only 12%. Pure oxygen aeration has strong impact load resistance, and the quality of sewage treatment effluent is stable.

(2) The amount of excess sludge is small. Under high-purity oxygen conditions, organisms are at a high level of endogenous metabolism, ie, self-oxidation, and sludge production is greatly reduced. The excess sludge can be reduced by about 25% compared to the air aerated activated sludge process.

(3) Improving the sedimentation performance of the sludge. The pure oxygen aeration oxygenation capacity is improved, the input power consumption is reduced, and the shear force on the active biological flocs is reduced. High dissolved oxygen can inhibit the growth of filamentous bacteria that cause sludge swelling. The pure oxygen aeration method has a sludge volume index of 30 to 50 mL/g.

(4) Conducive to biological nitrification. The dissolved oxygen with high oxygen aeration can completely nitrate the NH ₃ -N in the sewage.

(5) The sewage treatment efficiency is high. The high oxygen aeration sludge concentration and high sludge load make the volume load 3 to 5 times that of air aeration. The volume of the aeration tank can be greatly reduced, and the energy consumption is reduced by about 25%.

(6) No odor is emitted. Volatile organic compounds decompose quickly and the reaction is complete.

(7) High resistance to organic matter impact. The waste water organic matter emitted by the factory under the opening and stopping accidents will increase. In this case, the treated sewage will be discharged excessively, and the pure oxygen aeration technology can greatly improve the anti-organic impact performance of the sewage treatment device. The pure oxygen aeration system of the Tianjin Petrochemical Company's ethylene plant, after the sewage discharged under the impact of ultra-high concentration organic matter is treated by pure oxygen aeration, the COD can still reach the standard discharge. This is because the pure oxygen aeration system can maintain a high concentration of mixed sludge. MLSS reaches 6000-7000mg/L. The higher sludge concentration can provide more microbial biomass. Pure oxygen aeration can make these more microorganisms get more dissolved oxygen when degrading high concentration organic matter. To maintain a high rate of biological oxidation.

The technical parameters of the pure oxygen aeration activated sludge process and the air aeration activated sludge process are shown in Table 1.

Table 1 Comparison of process parameters of pure oxygen aeration and air aeration activated sludge

Second, the oxygen source of pure oxygen aeration

The process parameters of pure oxygen aerated activated sludge process wastewater treatment are far better than the existing air aeration activated sludge process. Many sewage treatment plants in western countries have adopted pure oxygen aeration activated sludge process and have mature experience. However, this technology has not been widely used in China, and the main reason is the lack of cheap oxygen.

At present, there are two main types of industrial oxygen production technology: one is cryogenic air separation and oxygen generation, and the obtained oxygen has high purity, but the oxygen production cost is high and the investment is large, and it is only suitable for large-scale oxygen use; the other process is Pressure swing adsorption oxygen (referred to as PSA, VPSA oxygen). Oxygen purity is only 90% to 95%, but the investment is small and the energy consumption is low. It is suitable for small and medium-sized applications where high purity oxygen is not required. China's Sinopec 5 companies introduced pure oxygen aeration activated sludge process to treat petrochemical wastewater. The aeration oxygen is obtained by deep cryogenic air separation. Although the cost of cryogenic air separation and oxygen generation is relatively high, the power consumption is about 0.55~0.60kW·h/m ³ , but the petrochemical company's pure oxygen comes from its nitrogen-making air separation unit, which is a by-product, and the petrochemical company treats the sewage. It is economical. For most sewage treatment plants, it is particularly expensive to use oxygen for cryogenic processes. It is much cheaper to make oxygen with PSA and VPSA. Domestic pressure swing adsorption oxygen has many years of experience. However, due to the poor performance of the adsorbent used, the cost of oxygen production is still high. In recent years, Beijing Peking University Pioneer Technology Co., Ltd. has developed a new type of oxygen-producing high-efficiency adsorbent PU-8. Its nitrogen absorption capacity and nitrogen-oxygen separation coefficient are much higher than those of traditional molecular sieves. The pressure-pressure adsorption engineering technology has been developed by using this adsorbent. The oxygen consumption is as low as 0.33 kW·h/m ³ , reaching the international advanced level. The price of the device is about 50% lower than that of the imported equipment, the oxygen production scale can reach 10000m ³ /h, and the adsorbent can be used for more than 10a in one loading. Dozens of installations have been built in the domestic metallurgical and chemical industries, and the operation status is good. The breakthrough of pressure swing adsorption oxygen generation technology of Beijing Beida Pioneer Technology Co., Ltd. has created favorable conditions for the promotion of pure oxygen aeration activated sludge process in China.

Third, the choice of pure oxygen aeration

Pure oxygen aeration is commonly used in the following ways:

(1) The NUOX pure oxygen aeration activated sludge process developed by American United Carbon Company is aerated with a closed aeration tank and an impeller type oxygen meter aerator. The pool body is divided into 3 to 4 sections, and each section is set to 1 unit. Table exposure machine. This technology was introduced by five petrochemical companies of Sinopec. This technology uses surface aeration, which has a higher energy consumption for aeration and a higher cost of aeration tank capping.

(2) The improved activated sludge treatment process (referred to as OASES process) developed by Fiher/Kruger Company of the United States also uses a closed aeration tank. The aeration tank is divided into several compartments, and the compartments are in the upper part of the pool. The opening is such that the gas and liquid are connected in series, and the oxygen is sent from the top to the first liquid level in the tank by a centrifugal compressor. The underwater impeller enters the underwater impeller through the hollow shaft of the underwater impeller, and is dissolved into the treated sewage from the nozzle of the underwater impeller. The sewage and oxygen are all entered by the first grid. The effluent and exhaust are discharged from the last compartment. Oxygen utilization rate can reach 90%.

(3) The Biox-N process developed by MESSER of Germany is called open microbubble pure oxygen aeration activated sludge process. The process uses an open aeration tank. The pure oxygen aeration microbubbles are made of a kind of excellent elasticity. Produced by an oxygen-transparent pad made of a hose made of a special rubber material of durability, the pores are evenly distributed on the wall of the hose, and oxygen can pass through the small hole to generate microbubbles of <2 mm. The contact area of ​​gas (oxygen), liquid (soluble organic matter) and solid (activated sludge microorganism) is enlarged, and the gas, liquid and solid mass transfer efficiency is high, the utilization rate of oxygen is improved, and the reaction time is shortened. The oxygen transmission pad is supported by a stainless steel frame and installed at the bottom of the aeration tank. The test results show that the oxygen utilization rate is 16% in the water depth of 1 m, and the oxygen is used in the water depth of 5 m, and the utilization rate of oxygen is 80%. The small hole on the rubber hose can be closed by itself, which is equivalent to the one-way valve, and will not make the sewage in the pool flow back into the hose and it is convenient to open and stop. The aeration oxygen supply pressure is 0.2-0.4 MPa. The higher oxygen supply pressure can ensure the oxygen distribution of the gas supply hose of the oxygen tank in the whole pool is uniform. The installation level of the oxygen supply mat is also not required, and the oxygen pressure is used. The gas, liquid and solid three-phase mixing in the sewage treatment tank can be evenly distributed to ensure the rapid reaction, and the small hole on the rubber hose can change the opening degree with the change of the pressure, and the oxygen flow rate and the pressure control range are wide. Very flexible. Thereby greatly improving the impact resistance of sewage treatment.

The aerator and air aerator used in pure oxygen aeration can be used universally, but only need to be properly adjusted due to different gas volumes. It has been able to produce a variety of air aerators and good air aerators in China. In principle, it should also be effective for pure oxygen aeration. The performance comparison of several air aerators is shown in Table 2-3.

Table 2 Comparison of performance of several air aerators (1)

Table 3 Comparison of performance of several air aerators (2)

It can be seen from Table 2-3 that the aerator with the best aeration performance is a small bubble type microporous aerator. China has absorbed several microporous aerators from the technology of Finland, Germany and other countries. The oxygenation efficiency of air microporous aerators is 15% to 25%, which is 50% more energy efficient than large and medium bubble aerators. The microbubble aerator has a diaphragm type, a disc type, a tube type, and the like. Among them, the diaphragm type resistance is small, but the long-term use of the membrane hole is easy to break. The disc type and the tube type are rigid and flexible, the rigid holes are easily clogged, and the porous film for the flexible disc surface has problems such that the holes are easily broken. Beijing Beida Pioneer Technology Co., Ltd. has repeatedly used a rubber hose to make a micro-bubble aeration hose, which is durable and has high gas efficiency. The price per meter is only 1/3 of the price of imported rubber hose. It has been tested in the domestic pure oxygen aeration activated sludge process, and the effect is very good.

4. Process plan for construction and transformation of sewage treatment plant with pure oxygen aeration activated sludge process

The aerobic treatment process of most sewage treatment plants in China is almost all using the air aeration activated sludge process, whether it is to use the pure oxygen aeration activated sludge process to build or renovate the existing sewage treatment plant, mainly to change or reform the sewage. The aerobic section of the various processes of the plant. While setting the respective design anoxic, anaerobic denitrification in addition to phosphorus. The primary treatment should be expanded with the expansion of sewage volume, and the post-treatment and sludge treatment can be appropriately changed according to the sludge volume and sludge performance. See Figure 1 for the conceptual design of the pure oxygen aeration section.

Due to the high oxygen content of the aeration section of the pure oxygen aeration, the oxygen content in the aeration tank is gradually reduced in the aeration tank, and the effect of denitrification in the anoxic section of the sewage treatment tank is better. Figure 1 is only a schematic diagram, and the actual project needs to be set according to the specific situation.

After the primary treatment, the sewage enters the anoxic and anaerobic sections of the biochemical treatment tank from the primary sedimentation tank, and merges with the return sludge and the secondary sedimentation tank. The sewage enters after the denitrification and anaerobic section denitrification and dephosphorization. In the pure oxygen aeration section, the oxygen generating unit is ventilated to the tank, and the sewage is removed in the second settling tank to be disinfected, and partially returned to the anoxic and anaerobic section of the biochemical treatment tank. The sludge is partially discharged from the mud collection well. Partial reflux to the anoxic, anaerobic section of the biochemical treatment tank.

V. Initial evaluation of technical and economic indicators for pure oxygen aeration activated sludge process

Because the pure oxygen aeration activated sludge process is different from the pure oxygen aeration method, their technical and economic indicators will not be the same. It is now only possible to make a preliminary assessment in a better way. The high concentration organic wastewater components and COD content of each plant vary greatly. Technical and economic indicators are not easy to compare, while domestic sewage is generally the same and easy to compare. The 150,000 t/d urban sewage treatment plant was designed by Beijing Peking University Pioneer Technology Co., Ltd. VPSA oxygen production and its open microbubble pure oxygen aeration activated sludge process. The initial conservative estimation and the same scale of oxidation ditch air aeration activity Compared with the sludge treatment plant, the construction investment can save about 30%. Per m ³ wastewater treatment can save 25% Click here to estimate, 150,000 t / d urban sewage treatment plant annual energy saving of about 2.8 × 10 ⁶ kW · h, when according to price 0.6 yuan / (kW · h) count The annual operating cost can save 1.7 million yuan.

If the existing sewage treatment plant is retrofitted with a pure oxygen aerated activated sludge process, even if the sewage treatment capacity is doubled, the aeration tank does not need to increase capacity. This alone can save a lot of investment and land. The ratio of pure oxygen aerated activated sludge process to air aerated activated sludge process is an energy-saving process. The aeration portion consumes about 50% to 60% of the energy consumption in the sewage treatment plant. The energy saving effect of the aeration part using pure oxygen aeration process is considerable. Moreover, the sewage treatment plant drainage compliance rate will be greatly improved.

Conclusion

In summary, the pure oxygen aeration activated sludge process has the characteristics of high oxygen transfer rate, less residual sludge, high sewage treatment efficiency, and high resistance to organic substances. The development of oxygen technology and the reduction of oxygen production costs have made it possible to promote the application of this technology. The pressure swing adsorption oxygen generation technology developed by Beijing Beida Pioneer Technology Co., Ltd. and the microbubble aeration technology have created conditions for the sewage treatment of the pure oxygen aeration activated sludge process in China.

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