Oxidation ditch is an economical and effective sewage treatment technology with stable treatment effect. Since its appearance in 1950s, it has become one of the main biological sewage treatment technologies after many improvements in technology and aeration equipment. Especially for biological denitrification of sewage, oxidation ditch has the advantages of lower cost and higher TN removal efficiency than other biological denitrification processes. However, compared with activated sludge process, oxidation ditch occupies a large area, and its application in cities or regions with expensive land is limited. The new type of three-dimensional circulation integrated oxidation ditch developed in this study not only retains the advantages of simple oxidation ditch equipment and operation, but also reduces the floor space. This technology has been patented. This paper mainly introduces the experimental results of three-dimensional circulating integrated oxidation ditch for treating municipal sewage.
Oxidation ditch is an economical and effective sewage treatment technology with stable treatment effect. Since its appearance in 1950s, it has become one of the main biological sewage treatment technologies after many improvements in technology and aeration equipment. Especially for biological denitrification of sewage, oxidation ditch has the advantages of lower cost and higher TN removal efficiency than other biological denitrification processes. However, compared with activated sludge process, oxidation ditch occupies a large area, and its application in cities or regions with expensive land is limited.
The new type of three-dimensional circulation integrated oxidation ditch developed in this study not only retains the advantages of simple oxidation ditch equipment and operation, but also reduces the floor space. This technology has been patented. This paper mainly introduces the experimental results of three-dimensional circulating integrated oxidation ditch for treating municipal sewage.
Process characteristics of 1
The new integrated oxidation ditch developed in this study adopts three-dimensional circulation and is built together with the precipitation area. Its characteristics are as follows: ① The upper layer of oxidation ditch is an aerobic zone, and the lower layer is an anoxic zone, which is not in contact with the atmosphere, and anoxia forms quickly. The mixed solution completes the biological denitrification process of degrading organic matter and nitrification and denitrification in the process of up-and-down circulation. Compared with the conventional oxidation ditch, the area occupied can be reduced by about 50%. (2) The sedimentation area and the oxidation ditch are built together, and the precipitated sludge can automatically flow back to the oxidation ditch without sludge reflux equipment, which can save investment and energy consumption, and the sedimentation area has no influence on the state of mixed liquid in the oxidation ditch. ③ Compact structure and simple operation.
This study is carried out in the laboratory, and the effective volume of the experimental device is 33L. The test sewage is urban sewage, and its water quality is shown in table 1. During the experiment, the temperature of the mixed liquid in the reactor changed with the seasons, and basically maintained between 1 1 ~ 28℃. At the beginning of the experiment, the treated water volume was 0.33 L/h, and then gradually increased to 6 L/h. The sludge concentration was kept at 2.0 ~ 4.9g/L, and the sludge load was 0.08 ~ 0.1.4 kg BOD5/kg VSS d. ..
In the experimental device, the circulating flow of mixed liquid is driven by a rotating brush. The function of the rotary brush is to aerate and oxygenate, and the other is to make the liquid mix and circulate, and there will be no sludge deposition at the bottom. According to the design requirements, when the submerged depth of the rotary brush is determined, adjusting the rotating speed of the rotary brush can ensure the requirements of dissolved oxygen concentration and water flow speed in the ditch. During the trial operation, the DO in the upper layer of the three-dimensional circulation integrated oxidation ditch remained above 2mg/L, and the lower layer remained anoxic. The average circulating speed of the mixed solution was 0.25m/s, and no sludge was deposited.
2 Results and discussion
Because the oxidation ditch operates in low load mode, the removal efficiency of organic matter in sewage is high. During the stable operation, the COD removal rate reached 95%. When the influent COD concentration is less than 1000 mg/L, the effluent COD is less than 50 mg/L. The removal rate of BOD5 is over 98%, and the average concentration of effluent BOD5 is 5 mg/L, which shows that the experimental system has high removal efficiency of organic matter.
During the experiment, there was no sludge bulking phenomenon. According to the test results, the SVI of oxidation ditch sludge is in the range of 50-260mL/g, the influent SS is about 150mg/L, and the effluent SS is less than15 mg/L. ..
The upper layer of the three-dimensional circulation integrated oxidation ditch is an aerobic zone, and the lower layer is an anoxic zone, and the mixed liquid completes the biological denitrification process in the circulation process. During the test operation, the ammonia nitrogen load in the test system is 0.01~ 0.02kgNH3-n/kgvss d, the BOD5 load is 0.08 ~ 0.10.2kgBOD/kgvss d, the influent NH4-N concentration is 45 ~ 67mg/L, and the influent TN concentration is 70. It can be seen that this experimental system maintains the characteristics of good denitrification effect of conventional oxidation ditch.
Hydraulic retention time is one of the main factors affecting the removal of organic pollutants in oxidation ditch. According to the test results, when the influent COD concentration is about 480mg/L, the water temperature is 23℃ and the hydraulic retention time is less than 5h, the COD removal rate is less than 75%. With the extension of hydraulic retention time, the COD concentration in effluent drops rapidly, and the removal efficiency is obviously improved. When the hydraulic retention time is more than 6 hours, the effluent COD concentration is less than 50mg/L, and the removal rate is more than 90%. However, when HRT is greater than 10h, the removal rate is not obviously improved. Therefore, when the influent COD concentration increases, the HRT of oxidation ditch should be increased accordingly to ensure a higher COD removal rate. In addition, under the experimental conditions, when COD has a high removal efficiency, the removal rate of total nitrogen is always above 90%.
In this experimental system, temperature has a certain influence on the removal effect of COD. The experimental results show that when the water temperature is 12 ~ 15℃, the COD removal rate is about 89%. When the water temperature is higher than 15℃, the COD removal rate is more than 90%. Temperature has obvious influence on the removal rate of total nitrogen. The experimental results show that when the total nitrogen load is less than or equal to ≤≤≤0.08 kgtn/kgMLVSS·d, the BOD5 load is 0.08-0.14 kgbod/kgmlvss d, the temperature is 1 1 ~ 13℃, and the TN removal rate is high. With the increase of temperature, the removal rate of total nitrogen increased obviously. When the temperature is higher than 150℃, the removal rate of total nitrogen reaches over 90%. This is because the temperature has a very obvious effect on denitrification when anoxic state is formed in the oxidation ditch.
3 Conclusion
The experimental results show that the integrated oxidation ditch with three-dimensional circulation can effectively remove organic pollutants from sewage, with the removal rate of COD reaching 95% and the corresponding removal rate of BOD reaching 98%. At the same time, due to the formation of aerobic zone and anoxic zone, it is beneficial to biological denitrification of sewage. In this experiment, when HRT= 10h and SRT=30d, the removal rate of NH3-N reaches 99% and the removal rate of total nitrogen reaches over 90%. Therefore, it is feasible to use the three-dimensional circulating integrated oxidation ditch to treat urban sewage.
The advantages of the three-dimensional circulation integrated oxidation ditch are:
(1) Because the mixed solution of activated sludge is in three-dimensional circulation, it can save about 50% of the floor space compared with the conventional oxidation ditch under the same treatment capacity.
(2) The sludge automatically flows back, and the precipitation separator is placed at one end of the three-dimensional oxidation ditch, which does not change the flow state of the mixed liquid in the main ditch, does not cause energy loss, saves energy and reduces the operating cost.
(3) The whole system has compact structure, less land occupation, less investment and convenient operation, and can realize factory assembly. It is a new technology suitable for sewage treatment in small and medium-sized towns and urban communities in China at this stage.
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