| Once plenty of nitrogen and phosphorus get into the water,great pressure will be put on the environment.Therefore,huge amount of wastewater containing nitrogen and phosphorus produced by industrial and agricultural industry must be treated to reach a set standard prior to discharge.On the other hand,the general situation of phosphate rock in China is not optimistic,because it has been listed as one of 20 kinds of minerals that could not satisfy the requirement of economic development since 2010.Under such circumstances,if the nitrogen and phosphorus in the wastewater can be recovered during wastewater treatment processes,it will not only conform to the concept of sustainable development and have economic prospect,but also will help to the recycling of phosphorus resources,which will reduce shortage of phosphorus resources in some extent.Struvite(magnesium ammonium phosphate hexahydrate,MgNH4PO4 · 6H2O,hereinafter called MAP),which is slightly soluble under neutral or alkaline conditions,can be used as a slow-release fertilizer.To start with,this article optimized the related parameters affecting the formation of struvite by single-factor experimental analysis with simulation wastewater cantaining nitrogen and phosphorus and got a favorable conditions:stirring speed is 200 rpm,the reaction time is 15 min,the pH of reaction process is about 9.50,and the mole ratio of n(NH4 +):n(PO43-):n(Mg 2+)is 1:1.15:1.15.Precipitation got under the optimized condition were analyzed,and the results showed that the element composition of the precipitation(N,P,Mg = 0.9821:0.9906:1.0288)was very close to the theory composition of struvite(N:P:Mg = 1:1:1).By the calculation,the purity of struvite in the sediment is 98.21%.The above optimized conditions was directly applied to recover phosphorus from actual phosphorus-contained wastewater and recover nitrogen from actual high ammonia-nitrogen wastewater,and excellent results have been achieved.The quality of the recycled product is good.The MAP sedimentation method can recycle more than 98%of total phosphorus from wastewater containing phosphorus(phosphorus concentration was about 1400 mg/L).The element composition of the recovered product was extremely close to that of pure struvite,and the purity of struvite(content)was calculated as 97.92%.When the MAP precipitation method was used for ammonia-nitrogen recycling from high ammonia-nitrogen wastewater(ammonia-nitrogen concentration was about 2200 mg/L),the ammonia-nitrogen recovery rate was up to 94%after primary sedimentation.The residual concentration of nitrogen and phosphorus in the wastewater could be reduced to 25 mg/L and 18 mg/L through secondary sedimentation,respectively.Used phosphorus-contained wastewater as phosphorus source and then mixed with high ammonia-nitrogen wastewater,getting a mixed wastewater.The MAP precipitation method can recover 94.08%of nitrogen and 98.20%of phosphorus from the mixed wastewater.The concentration of residual nitrogen and phosphorus was 31 mg/L and 24 mg/L,respectively.The characterization and elemental analysis of the two sediment products from high ammonia-nitrogen wastewater and mixed wastewater was conducted.XRD results showed that the element composition of the two sediments was also close to the theory composition of struvite,and their main component should be struvite.TOC in the two deposits accounted for less than 0.1%,showing that the two sediments contained few organic matter.The purity of struvite in the sediment from high ammonia-nitrogen wastewater and mixed wastewater was calculated as 98.10%and 97.74%,respectively,and should have high application value. |
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