| As non-renewable and limited resources, phosphorus in the world is being exhausted because of overexploitation and overusing. At the same time, the eutrophication caused by great deal of phosphorus wastewater discharged directly is becoming more and more serious.Therefore, developing novel wastewater treatment technology to remove and recover phosphorus from wastewater is crucial and urgent to solve the problems of scarcity of phosphorus and pollution of water environment. Crystallization is an effective method to remove and recover phosphorus from wastewater. However, due to the complexity of crystallization process and difficulty of reaction condition controling, especially for the crystallization in low phosphorus concentration wastewater treatment, there are some problems still existing in the crystallization process which effect the treatment performance and technology development. Ultrasound has recently been widely used in water treatment.The cavitation effect of ultrasound could not only degrade organic matter, but also promote crystallization efficiently. Therefore, based on the previous research work on phosphorus removal from wastewater, a novel technology to remove and recover phosphorus from wastewater by two stage biological filters cooperating with magnesium ammonium phosphate(MAP) crystallization enhanced by ultrasound was developed to provide theoretical and technical reference for removing and recovering phosphorus from wastewater.Firstly, a test device for determination the characteristic of MAP crystallization reaction was designed and assembled. Then the characteristics of MAP and the influence of ultrasonic on the MAP crystallization were investigated and the efficiency of phosphorus removal by MAP crystallization was also studied. Seconedly, two steel slag media biological filters were carried out and operated in alternating anaerobic/aerobic(A/O) mode, to investigate the treatment efficiency and the characteristics of biological-crystal cooperation. Subsequently,the improvement of ammonia nitrogen, phosphorus, and other pollutants removal in the system was investigated while adding magnesium and launching the ultrasound to the inflow.The biological population and community succession under different operation time and reaction condition were analyzed based on the results of PCR-DGGE.The results show that improving pH and reducing the temperature can reduce the solubility and super saturation of MAP in aqueous solution, and the metastable zone became narrow significantly. At the same time, the induction period shorten obviously and enhance the crystallization rate by improving pH.When the power of ultrasound in the aqueous solution increased from 150 w to 350 w, the metastable zone of MAP narrowed down with the increasing of ultrasound power gradually.The induction period of MAP could be shorten under different ultrasound power. And improving ultrasound power could accelerate the crystal rate of MAP significantly.Furthermore, more uniform, more complete of the produced MAP crystal with the increasing of the power ultrasound were observed.With the operation condition of inflow phosphorus concentration 3mmol/L, pH 9.5,reaction time 10 min, P: Mg 1:2.5, P: N 1:3 and stirring intensity 200r/min, the phosphorus can be removed more than 95%, as well as the effluent concentration of phosphorus decreased to 0.5mg/L. However, with the optimal reaction condition of P: Mg: N=1:2:2.5, the removal rate of phosphorus could not be improved by ultrasound.The single filter in the alternating anaerobic/aerobic(A/O) biological filters was first operated without inoculation respectively, And then, transfer to alternating A/O mode. Under the running condition of HRT 2h, the inflow phosphorus concentration 9-11mg/L, alternating time 48 h, the influent ammonia nitrogen concentration 35-40mg/L, COD 250-350mg/L, the removal rate of ammonia nitrogen, COD and phosphorus in the alternating anaerobic/aerobic(A/O) biological filters are 90%, 80% and 40%-60% respectively, as well as the effluent concentration are below to 0.5mg/L, 50mg/L and 3mg/L respectively.The results show that the steel slag composite filter media could releasing alkalinity and ion as well as the PAOs releasing phosphorus under the anaerobic environment would increase the phosphorus concentration in the anaerobic filter, could induce crystallization occoring wich result in white phosphorus crystal formed on the media surface. It is considered that the phosphorus removal in the anaerobic filter was complished by biological-crystal cooperation. The EDS analysis results suggest that the crystal is constitute of several phosphate compound and the hydroxyapatite is included.When the Mg:P increase from 1:1 to 2:1, the removal rate of ammonia nitrogen rise from13-17% to 20-26% in the anaerobic filter. The effluent phosphorus concentration declined from 17-26mg/L to 14-19mg/L. it is found that increasing the magnesium concentration in influent can enhance phosphorus removal by crystallization in the anaerobic filter.Under the influent Mg:P=1:1 and ultrasound power of 150 w, the nitrogen ammonia removal rate is 23-31% and the phosphorus removal rate is 21-30%. Rising the concentration of magnesium cannot enhance the removal of ammonia nitrogen and phosphorus. The COD can be degraded by ultrasound, the removal rate of COD is 35-46% under ultrasound condition, accounting for about half removal rate in the entire system.Inceasing of magnesium concentration in influent and applying ultrasonic, the crystalline product is more and more enrichment in the media surface. The EDS analysis result revealed that magnesium content in the crystal increased with the operation time. Therefore, improving of magnesium concentration played a significant role in promoting the crystallization.The diversity of microorganism species reduced significantly in the alternating operation stage compare to the single filter operation, but recovered in a certain degree with the increasing of running time. It as also found that the diversity of microorganism species decreased with the increasing of influent magnesium concentration and applying ultrasound.In the whole operation time, the dominant population is Proteobacteria, and most of them can perform ammonia oxidation or gathering phosphorus. |
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