Comparative Study On A / 0 Process And Three - Tank Oxidation Ditch In Urban Wastewater Treatment Plant

With the increase of population, acceleration of economic development and urbanization process, sewage and industrial wastewater emissions is increased year after year, which results in the extremely serious problem of eutrophication of surface water. In order to improve the water environment, and to prevent water eutrophication, the requirements on the sewage plant has become increasingly demanding in terms of effluent quality, operating cost, and energy saving. In recent years, the treatment of the city life sewage has been brought to the attention of government of all levels, and that the choice of wastewater treatment technology to be in line with economic development is extremely critical as well as strengthening the management of industrial pollution sources at the same time.The mostly used urban sewage treatment plants are biological methods. The A/O process and the triple oxidation ditch process used widely. Currently, there are few reports on comparative study of the A/O process and the triple oxidation ditch process. This paper takes the example of Minghang wastewater treatment in Shanghai, and discusses the characteristics of the two processes, and provides a certain basis for selecting the treatment process for similar urban sewage treatment plant. And provide some technical supports of optimal operation, and upgrading reconstruction for the two processes.The paper systematically compares the removal effects of COD、BOD5、 SS、NH3-N、TN、TP in the sewage by the A/O process and the triple oxidation ditch process, and analyzes the influence of DO、inflow COD/TN ration, inflow COD/TP ration. MLSS. Temperature and HRT on denitrification and dephosphorization of the two processes. To comprehensively evaluate the application of the two processes in urban sewage treatment plants, by comparing the site area, construction investment, energy consumption, operating cost and operational management of the two processes. Studies show that:When the influent COD、BOD5、SS、NH3-N and TN are 320 to 400 mg/L, 140 to 180 mg/L,140 to 210 mg/L,26.8 to 34.8 mg/L and 35.1 to 43.4 mg/L respectively in the A/O process, their average effluent concentrations are 9.9 mg/L、26 mg/L、7.6 mg/L and 17.7 mg/L respectively, and the average removal rates reach 92.3%,93.9%,84.7%,74.1% and 52.6% respectively. When the influent COD, BOD5, SS, NH3-N and TN are 270 to 350 mg/L,126 to 153 mg/L,126 to 165 mg/L,21.2 to 30.4 mg/L and 28.4 to 38.9 mg/L respectively in the triple oxidation ditch process, their average effluent concentrations are 33.3 mg/L,12.3 mg/L,27 mg/L,10.4 mg/L and 20.4 mg/L respectively, and the average removal rates reach 89.0%,91.3%,80.1%、 59.4% and 39.5% respectively. The removal effects are significantly better in A/O process than those in the triple oxidation ditch process.In the A/O process, the discharge standard of pollutants for municipal wastewater treatment plant can achieve level B, when the concentration of DO is controlled at 2-3 mg/L in the aeration tank in winter, and MLSS concentration is controlled at 4000 mg/L, the removal rate of TN is at 57%. The total nitrogen removal effect of the A/O processes can reach above 50% if the concentration of DO is higher than 1.1 mg/L in summer, and MLSS concentration is controlled at 4500 mg/L. When the inflow COD/TN ratio is from 8 to 12, with the increase of inflow COD/TN ratio, the removal rate of TN decreases almost linearly, the concentration of organic matter had a serious impact on the nitrification process. The inflow COD/TP ration has no significant influence on denitrification performance of this process.MLSS concentration of middle ditch should be controlled at 1500 mg/L which is appropriate in the triple oxidation ditch process. When the concentration of DO is controlled at 2-3 mg/L in the middle ditch in winter, and the concentration of DO is higher than 1.1 mg/L in summer, the total nitrogen removal rate is expected to improve. Temperature plays an obvious part in the effect of the phosphorous removal. Phosphorus removal efficiency increases with the decrease of temperature. When the temperature is below 15℃, TP of the effluent can achieve the B Level emission standard. When the inflow COD/TN ratio is from 8 to 10, with the increase of inflow COD/TN ratio, the removal rate of TN slightly decreases. The inflow COD/TN ratio has no remarkable influence on the phosphorous removal in the triple oxidation ditch process. The inflow COD/TP ration has small influence on denitrification performance and dephosphorization performance of this process.A comprehensive and economic valuation based on the 5 keys factors such as site area, construction investment, energy consumption, operating cost and operational management, is that A/O process is more suitable for large and medium-size wastewater treatment plants, considering its less costy site area, construction investment and equipment maintenance; and the triple oxidation ditch process is more suitable for medium and small-size wastewater treatment plant, because of its significant energy savings caused by A/O process, and easy operation and management.