| The accumulation of nitrogen and phosphorus compounds by discharge of wastewater is one of the main causes of eutrophication in lakes and close inshore waters. How to control nutrients pollution high-efficiencily and economically is the most important environmental issue which needs to be solved urgently at present. The removal and recovery of nitrogen and phosphorus from swine wastewater are very important for the pollution control and utilization of the wastewater, and the crystallization of magnesium ammonium phosphate is a main process for this purpose. In this thesis, the process conditions on the removal efficiencies of nitrogen and phosphate with MAP crystallization were investigated in synthetic and actual swine wastewater systems and the three optimum process parameters were obtained. Small-scale and enlargement experiments of three kinds of processes on swine raw sewage and anaerobic digester effluents were carried out respectively, and then running economic costs were analyzed.Firstly, with synthetic wastewater, small-scale batch experiments of MAP crystallization were carried out, adopting MgCl2·6H2O and Na2HPO4·12H2O as precipitants. And by orthogonal experiments the paper drew the sequence of actors and significance level. The effect of P:N molar ratio was the most significant. The optimal reaction condition was obtained by single factor experiments. It showed that P:N molar ratio of 0.9-1.0, pH of 9.5-10.5, Mg:N molar ratio of 1.0-1.1, were found to be beneficial for the higher removal of NH4+-N and the lower residual concentration of phosphate. However, the influences of stirring speed, reaction time and settling time were not significant. From saving energy and reducing the process time perspective,100-200 r·min-1,10-15 min,15-30min could be chosen, respectively.Secondly, in connection with the imbalance of NH4+-N, Mg2+, PO43--P three ions ratio of actual swine wastewater, a systematic study was performed to investigate three types of process conditions to realize the removal of nitrogen and phosphorus from actual swine wastewater. It showed that only by adjusting pH of the wastewater, the optimum pH value was in the range of 9.0-10.0. The pH of the wastewater was raised to 8.91 with continuous aeration standing 60min, and the ammonium-nitrogen, phosphorus and total nitrogen removal efficiencies were 16.8%, 78.4% and 21.4%, respectively. Aeration was especially suitable for swine anaerobic effluents rich in CO2. Adding magnesium source could increase the phosphorus removal efficiency obviously, which was above 95.4%, when the optimum Mg:P molar ratio was 1.6 and the optimum pH value was in the range of 9.0-10.0. The high ammonium-nitrogen removal efficiency could be achieved by adding magnesium and phosphate soures simultaneously, which could reach 87.4% and the residual phosphorous was less than 10mg·L-1 in the solution with the optimum pH 9.5, P:N molar ratio 1.0 and Mg:N molar ratio 1.1.Lastly, with actual swine wastewater, the enlargement experiments of MAP crystallization were carried out. It showed that only adjusting pH of the wastewater and adding magnesium source, capable of phosphorous removal, were suitable for phosphate-rich swine wastewater. Swine raw sewage treated by adding magnesium and phosphate sources simultaneously (Mg:N=1.2, P:N=1.0) had the lowest ammonium-nitrogen removal cost, which was 7.85 yuan·(kgNH4+-N)-1 and the treatment cost of one ton swine raw sewage was 4.52 yuan. Adding magnesium and phosphate sources simultaneously (Mg:N=1.1, P:N=0.9) was suitable for swine anaerobic digester effluents. The treatment cost of one ton swine anaerobic digester effluents was 0.69 yuan and the ammonium-nitrogen removal cost was 4.08 yuan·(kgNH4+-N)-1. Thus it can be seen that the processes should be used after anaerobic treatment not only in removing ability of both nitrogen and phosphorus but also in low cost.
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