Study On Phosphorus Recovery By Struvite Crystallization From Wastewater

Phosphorus is a kind of non-renewable resources.Eutrophication and red tide occurred frequently due to the excess of phosphorus discharge.Furthermore,the scaling Phenomenon of struvite is an obstacle to the operation of wastewater treatment plant.However,conventional process of sewage treatment for removing phosphorus barely depends on biological or chemical approachs while regardless of phosphorus recovery. Therefore,phosphorus recovery from wastewater is a vital issue which needs to be resolved urgently.The optimum pH of phosphorus recovery by struvite crystallization from wastewater and the effect of crystallization conditions on struvite purity were theoretically studied in this thesis.The influencing factors of phosphorus recovery rate were analyzed by orthogonal and single factor experiment systematically.The effect of pH and molar ratio on the purity of the precipitate in simulated wastewater with phosphorus concentration of 50mg/L was analyzed by mass comparison and X-ray diffraction.The structures,constituents and physical and chemical properties of struvite were analyzed by X-ray diffraction,microscope and chemical ways.The feasibility of phosphorus recovery by struvite crystallization from wastewater was also analyzed.Here are the general outlines:(1)The optimum pH studied from theory and experiment is equal,the value is 8.0～10.0.The experiment shows that if the initial phosphorus concentration(IPC)is lower than 50mg/L,the optimum pH is 9.5～10.0;if the IPC is higher than 50mg/L,it is 9.0;if the IPC is higher than 150mg/L,the one is 8.0～8.5.(2)Theoretically,the effect of crystallization conditions on struvite purity indicates that the needed concentration of magnesium salt which ensured the struvite precipitated firstly decreased with the increase of IPC and N:P ratio.Within the optimum pH,the IPC should be high enough to ensure the struvite precipitated firstly under the theoretical N:P ratio.When the pH is 8.0,the IPC should be higher than 130mg/L;When the pH is 8.5,the IPC should be higher than 110mg/L;When the pH is 9.0,the IPC should be higher than 90mg/L;When the pH is 9.5,the IPC should be higher than 110mg/L;When the pH is 10.0,the IPC should be higher than 170mg/L.To ensure the struvite precipitated firstly,the lower the IPC, the higher N:P ratio is needed.At the pH is 8.5～9.5,when the IPC is lower than 10mg/L,the N:P should be no less than 32;when the IPC is 10～70mg/L,the N:P should be 2～4;when the IPC is 90～110mg/L, the N:P should be 1～1.5;when the IPC is higher than ll0mg/L,the N:P should be equal to 1.(3)The orthogonal experiment result of simulated wastewater shows that the IPC is the significant factor influencing phosphorus recovery rate for different concentration of wastewater contained with phosphorus; the pH mostly affects phosphorus recovery rate under a given concentration of wastewater,the following influencing factor are Mg:P and N:P ratio.The temperature has little effect on it.(4)The single factor experiment result of simulated wastewater shows that the IPC varies directly with phosphorus recovery rate and has little effect on the concentration of residual phosphorus kept around 2mg/L under the conditions of the excess concentration of magnesium salt and ammonium nitrogen.Phosphorus recovery rate increases with the improvement of Mg:P and N:P ratio,and becomes stable gradually. The experimental optimum ratio is:when the IPC is 30mg/L, Mg:N:P=2:8:1;when the IPC is 50mg/L,Mg:N:P=1.6:4:1;when the IPC is 100mg/L,Mg:N:P=1.4:3:1;when the IPC is 150mg/L, Mg:N:P=I.2:1.5:1.The equilibrium time of struvite crystallization is fairly short,about 10min.(5)The purity of the precipitate produced by simulated wastewater with phosphorus concentration of 50mg/l shows that the precipitate contains none or little struvite when the ratio is 1:1:1;when Mg:N:P=1.6:4:1,pH=10,the purity of the precipitate is quite high.At pH=9.5,when Mg:P=I.6,the ratio of N:P should be no less than 3 for purer precipitate;When N:P=4,the critical molar ratio of Mg:P for purer precipitate was 2;when Mg:P molar ratio was higher than 2,the outgrowth would be produced.(6)The precipitate is a fork rhombus shape crystal,which could be observed clearly through microscope.It rises with the increase of IPC and reaction time.These precipitate have characteristics of great density,good sedimentation capability,and poor water absorption, slow dissolution rate.What's more,they are not inclined to agglomeration during long-term storage,which shows their potential use for slow release fertilizer of crops.(7)The optimum operational condition and the feasibility of phosphorus recovery from filtrate of Songjiang sewage treatment plant were investigated,and the result shows that:the optimum pH is around 9.5, Mg:P is 1.4,the phosphorus recovery rate is 86%.Although the process costs a little more than the economic benefit of the products, nevertheless,it is still feasible based on an overall consideration of various factors such as social,environmental and economic benefits.