摘要
废水中高含量的氮、磷是引起湖泊和封闭近岸海域富营养化现象的重要因素,经济而有效地控制氮磷污染已成为当前亟待解决的环保问题。磷酸铵镁(Magnesium Ammonium Phosphate, MAP)结晶法是脱氮除磷的主要工艺之一,从猪场废水中去除和回收氮、磷营养元素,对该废水的污染控制和资源化利用具有重要意义。本文以模拟和实际猪场废水为处理对象,系统研究了MAP结晶法的工艺条件,以脱氮除磷效果为考察指标确定了三种最佳工艺参数；分别对猪场原污水和厌氧消化后出水进行三种工艺的小试及放大试验,并对其运行的经济成本进行了初步分析。
首先以模拟废水为处理对象,以MgCl2·6H2O和Na2HPO4·12H2O为沉淀剂进行了MAP结晶小试,通过正交设计确定了各影响因素的主次水平及显著性,其中废水中的P：N摩尔比影响最大。处理该模拟废水的最佳工艺条件为：P：N摩尔比=0.9-1.0、pH=9.5-10.5、Mg：N摩尔比=1.0-1.1,此时可获得较高的氨氮去除率同时保证尽可能低的余磷浓度。该工艺中废水搅拌速度、反应时间、沉淀时间等对氮、磷去除影响不太明显,因此从节约能源及缩短处理时间角度出发,可分别选为100-200r·min-1、10-15min、15-30min。
其次以实际猪场废水为研究对象,针对猪场废水中NH4+-N、Mg2+、PO43--P三种离子比例不平衡的问题,设计了三种工艺,并对其工艺条件进行了系统研究。结果表明：pH值调节工艺中,废水的最佳pH值调节范围为9.0-10.0；通过连续曝气60 min可提升废水pH值至8.91,氨氮、磷、总氮去除率分别为16.8%、78.4%、21.4%；曝气尤其适用于富含CO2的猪场厌氧消化出水；补加镁源工艺,可有效提高磷去除率,当废水pH=9.0-10.0,Mg：P摩尔比=1.6时,磷去除率高达95.4%,该工艺较适用于富磷猪场废水；补加镁源和磷源工艺,保证尽可能低的余磷浓度的同时可获得较高的氨氮去除率,当pH=9.5、P：N摩尔比=1.0、Mg：N摩尔比=1.1时,氨氮去除率高达87.4%,余磷浓度小于10mg·L-1.
最后以实际猪场废水为处理对象,进行了MAP结晶法的放大试验。试验结果表明：只调节pH值工艺、补加镁源工艺都具有较高的除磷能力,较适用于富磷的猪场废水。猪场原污水宜选用同时补加镁源和磷源工艺(Mg：N摩尔比=1.2,P：N摩尔比=1.0),单位氨氮处理费用最低,为7.85元·(kgNH4+-N)-1,此时处理1t原污水的费用为4.52元；猪场废水经厌氧消化后的出水也宜选用同时补加镁源和磷源工艺(Mg：N摩尔比=1.1,P：N摩尔比=0.9),单位氨氮处理费用为4.08元·(kgNH4+-N)-1,处理1t厌氧消化出水的费用为0.69元。由此可见,MAP结晶法比较适用于处理猪场废水经厌氧消化后的出水,这样不仅脱氮除磷效果好,而且成本较低。
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.
引文
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