| It is difficult for the phosphorus removal effect of traditional activated sludge process to meet the discharge standards which becomes more and more stringent.In the upgrading and reconstruction and new construction projects,various domestic wastewater treatment plants have introduced high density sedimentation tank process and achieved a stable discharge of total phosphorus(TP)concentration up to the standards.A high density sedimentation tank is added in a wastewater treatment plant in Chongqing after the biological treatment process.Even if the TP concentration discharged can reach the standard stably,the polyaluminum chloride(PAC)dosage was higher than that of other sewage treatment plants with the same process and the same treatment scale due to the disorderly dosing of the coagulant PAC.Finally,it has caused the increase of production costs.In order to reduce the dosage of PAC,the influencing factors of phosphorus removal of high density sedimentation tank based on investigation of the operating status of the wastewater treatment plant and high density sedimentation tank were studied in this paper through batch experiments.Moreover,the key factors affecting the phosphorus removal efficiency of the process were also screened out.The phosphorus removal mechanism of return sludge and phosphorus balance in the high density sedimentation tank were analyzed.PAC dosage algorithm was set up on the basis of the experiment and the reliability of the algorithm was also verified by PAC dosage experiment.PAC automatic dosing system was hereby established.At the same time,the economic benefits after optimizing the PAC dosage were also analyzed.The main conclusions are as followings:(1)It was found that the effluent indicators of the plant were able to meet A standards of Class 1 in “Discharge standard of pollutants for municipal wastewater treatment plant”GB 18918-2002 by investigation of the operating status of sewage treatment plant.It was revealed that the grit tank had limited removal effect of TP and OP after observing the concentrations of TP and OP in each process section of the sewage treatment plant.The biological process could merely remove part of the TP,but it had no removal effect on the OP.OP/TP of influent of the high density sedimentation tank was 91%.Meanwhile,it was found that the OP removal rate could be used as an indicator to assess the phosphorus removal effect in the high density sedimentation tank through the analysis of the above data.However,p H and water temperature had the limited effect on the phosphorus removal in the high density sedimentation tank.(2)Through batch experiments of the influencing factors for the phosphorous removal in high density sedimentation tank,it was found that changes in the speed gradient and coagulation sedimentation time could hardly affect phosphorus removal effect.The PAC dosage,reflux ratio and PAM dosage were the key factors affecting the phosphorus removal effect.When PAC dosing coefficient was less than 120,there was a linear relationship between OP removal rate and PAC dosing coefficient and the OP removal rate(%)=0.5856×PAC dosing coefficient+12.5309.When reflux ratio was controlled at 3%-4%,the OP removal rate of the return sludge was positively related to PAC dosage in the high density sedimentation tank.Even if there was no relationship between OP removal rate and PAM dosage,PAM could improve the TP removal rate by removing phosphorus-containing suspended solids,so PAM dosage at 0.2mg/L was economical and reliable.(3)After observing the microstructure,particlesize distribution and metal element content of the sludge,it was found that the PAC dosage of the high density sedimentation tank had little effect on the first two characteristics.However,the Al content of the return sludge increased with the increase of PAC dosage.It was revealed by the phosphorus removal experiment of the return sludge and its filtrate that the phosphorus removal was realized by the adsorption between sludge solids and OP.Meanwhile,the infrared spectroscopy analysis of the return sludge and sedimentation sludge showed that phosphate could replace the hydroxyl on the aluminum oxyhydroxide.Then,the kinetic model of phosphorus adsorption of the return sludge was studied and the result was that the adsorption by phosphorus of the return sludge was in line with the pseudo-second model related of adsorption kinetic.After analyzing of the changes of phosphorus concentration along the high density sedimentation tank,it was found that the removal rate of OP could be calculated in accordance with the OP removal rate by PAC and return sludge.(4)The PAC dosage algorithm based on the iterative method was set up with the influent flow,the OP concentration of influent and effluent and PAC dosage in the high density sedimentation tank as the variables,which was based on the above experiments.It was proved by the experiment that the algorithm was able to accurately predict the OP concentration of effluent from the high density sedimentation tank.Moreover,it was able to ensure that the TP concentration of the effluent could meet the standard on the basis of reducing the PAC dosage.Finally,the PAC automatic dosing system was set up in accordance with the algorithm,and the economic benefits was also analyzed.The results showed that the optimization of the PAC dosage and the establishment of the dosing system would bring 824 000 Yuan as the direct economic benefits to the wastewater treatment plant each year,while the investment cost on the construction of PAC dosing system was only 746 000 Yuan. |
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