| In order to take further measures to control the serious water pollution of the environment and speed up the process of converting wastewater into resources, increasingly stringent emission standards for municipal wastewater treatment plants are formulated in our country. Not only the WWTPs which are going to be built have to obey the high emission standards, but the existing WWTPs are also facing a very urgent pressure of upgrading and transformation. At present, A2/O process has been generally accepted and widely applied as one of the simultaneous biological nutrient and phosphorus removal technologies for WWTPs in our country, therefore, mathematical simulation of the A2/O process based on ASM2D, analysis the optimization of control strategies and the best technical parameters of A2/O process further excavating the potential of each existing process units, all enjoy important theoretical and practical significance.First of all, the influential water quality characteristics of the biological treatment system of Longwangzui WWTP in Wuhan were analysised in the paper. The continuous one-year analysis of the conventional water quality had shown that the concentration of pollutant index (except the NH3-N index) in the influent is of great variation along with the seasonal change and could be divided into two obvious stages. The concentration of various pollutant index is high from December 2009 to June 2010, and CODcr, BOD5, SS, TN, TP and PO43- of the influent are 387.8 mg/L, 265.9 mg/L,376.9 mg/L,40.66 mg/L,8.20 mg/L and 2.07 mg/L respectively in average. The CODCr, BOD5, SS, TN, TP and PO43- of the influent from July 2010 to November 2010, are 196.8 mg/L,122.3 mg/L,138.4 mg/L,25.75 mg/L,3.87 mg/L and 1.56 mg/L respectively in average, which shows that the concentration of various pollutant index is low. Environmental factors have little effect on the NH3-N index, with its annual average concentration standing at 18.75 mg/L. Further analysis of the water quality of the influent in Longwangzui WWTP were conducted according to ASM2D standards, and it shows that when the influent total COD was less than 350 mg/L, S1, SF, SA, XS and X1 take up 7.2%,10.9%,7.3%,50.9% and 23.7%of total COD in average respectively. And when it is greater than 350 mg/L, SI, SF, SA, XS and XItake up 2.6%,6.5%,4.3%,29.7%and 56.9%of total COD in average respectively.A 24 m3/d pilot plant with the same process parameters with Longwangzui WWTP was established, the migration and transformation of the pollution in the A2/ O process, and the performance of nitrogen and phosphorus removal were studied and analysised using material balance method under different internal recycle ratio, namely 300%,200%,100% and 0%. The result showed that, the internal recycle ratio had little effect on the effluent COD, BOD5, SS and NH3-N, however, raising internal recycle ratio had contributed to the increase of the nitrification rate, the average nitrification rates were 104gNH3-N/(kgMLVSS·d),102gNH3-N/(kgMLVSS·d), 89gNH3-N/(kgMLVSS·d), 86gNH3-N/(kgMLVSS·d) respectively under the four different operating conditions. In this issue, internal recycle ratios had little effect on the TN removal but with the decrease of the internal recycle ratio, TN removal rates in pre-anoxic and anaerobic zone was reduced gradually; In the absence of the internal recycle ratio, there was an obvious phenomenon of SND lose in the first corridor of the aerobic zone, which accounted for 33.4% of TN removal. In this study, along with the decrease of the reflux ratio, the removal rate of TP and PO43- decreased, and there was anoxic denitrifying phosphorus removal in the anoxic zone, which increases with the decrease of the reflux ratio. However, the percentage of phosphorus removal in the aerobic tank decreased with the decrease of the internal recycle ratio.The initial simulation and calibration of the pilot system was firstly tested with the activated sludge process simulation software based on the ASMs which was developed by the research group of WHUT. The results showed that, the yield coefficients of heterotrophic bacteria (YH) had significant impact on the simulation of the nitrogen and phosphorus removal, while the maximum growth rate of autotrophic nitrifying bacteria (μATU) had great impact on the NH3-N in effluent. The simulated values of the calibrated model conform to the measured values very well; the relative errors of the effluent COD, TN and NO3- were 3.2%,1.2% and 6.5%, respectively. The absolute error of the simulated TP, PO43-and NH3-N value of effluent was less than 0.3mg/L, which itself is relatively small and below the normal range. Therefore, the model could simulate the operation of the process well, which could basically reflect the treatment tendency of the nitrogen and phosphorus removal in pilot system.Secondly, with the help of calibrated model system for process optimization in pilot system, it was found that, in the case of the low concentrations of pollutants in influent, the interval recycle ratio of 50% to 100% was appropriate, the sludge return ratio of 50% to 75% could was appropriate, and the DO of 1.5-2.0mg/L in the aerobic tank was appropriateas well, which would be effective to obtain the better water quality and save energy consumption. Besides, the capacity of the biological treatment unit of the pilot system also had great potential for improvement on treating wastewater. |
