| Struvite crystallization is a promising method for removing and recovering phosphorus from wastewater such as biogas slurry which is a wastewater high in phosphorus content.However,there are many problems need to be solved indeed for its practical application:(1) Crystallization condition optimization and continuous flow crystallization reactor should be studyed specifically for cattle farm biogas slurry.(2) The forms of phosphorus in cattle farm biogas slurry and phosphorous transformation in cattle farm biogas slurry during struvite crystallization were not clearly.(3) Nutrient contents analysis and purification of precipitates are fundamental for its slow-release fertilizer use.In this research, static experiment and continuous flow crystallized-settled reactor experiment based on cattle farm biogas slurry have been used to research the struvite crystallization method systematically.The optimum conditions were respectively observed with the removal rate of phosphorus as response indicators. Different forms of phosphorus in cattle farm biogas slurry and phosphorous transformation in cattle farm biogas slurry during struvite crystallization were analyzed,respectively. Precipitates analysis(nutrient content analysis including N and P)and precipitates purification were performed with the precipitates recovered from both static experiment and continuous flow crystallized-settled reactor experiment respectively for its fertilizer use availability.(1)The results from static experiment based on cattle farm biogas slurry indicate that the optimum value of p H and Mg2+∶OP for the reaction were 9.5 and 1.4∶1,and the optimal sedimentation time was 20 min, respectively. The results of static experiment showed that the struvite crystallization has a good treatment effect of phosphorus for cattle farm biogas slurry in which the removal rate of TP and OP is maintained at 65% and 80%,respectively.The removal amount of TP was significantly and positively correlated with the removal amount of OP(p < 0.05).(2)Before struvite crystallization, total phosphorus of 56.21±0.75 mg/L in liquid phase existed in the form of PP, Ortho-P and RDP with contents of 41.01%, 53.69% and 5.30%, respectively. Meanwhile, 5.69%, 25.45%, 12.32% and 32.16% of total phosphorus of 79.97±1.84 mg/L in solid phase was extracted successively by deionized water,0.5 mol/L Na HCO3,0.1mol/L Na OH and 1.0 mol/L HCl, respectively, while the remained 24.38% was residual phosphorus(Re-P). During the struvite crystallization at about p H value 8.5, 9.0, and 9.5,the content of Ortho-P of 30.18±0.37 mg/L in liquid phase decreased to 17.98±0.30, 11.43±0.73 and 6.23±1.32 mg/L,respectively.The content of Na OH-P of 9.85±0.18 mg/L in solid phase decreased to1.47±0.11, 1.29±0.07 and 1.44±0.27 mg/L,respectively.Meanwhile,the content of HCl-P in solid phase increased significantly.In most cases,the fraction of other forms of phosphorus such as PP and RDP in liquid phase and H2O-P, Na HCO3-P and Re-P in solid phase did not vary significantly. According to the experiment, we can speculate that Ortho-P and Na OH-P(Fe-P/Al-P) were transformed into struvite and the other forms. Ortho-P and Na OH-P occupying 29.39% of biogas slurry phosphorus made greatest contribution to struvite formation among all forms, and 80.84% of them were removed from cattle farm biogas slurry, of which 91.28% of them were the struvite formed at about p H 9.5. After the crystallization, 10.96%(mass proportion) of the precipitation could be regarded as struvite at about p H 9.5, and the proportions were 6.97% and 9.25% at about p H 8.5 and p H 9.0, respectively.(3)The results of continuous flow crystallized-settled reactor experiment showed that the optimum value of p H and Mg2+∶OP is 9.5 and 1.5∶1, and the optimal HRT is no less than 40 min, respectively.The removal rate of OP was maintained at 80% under the optimal crystallization conditions while OP and TP concentration in effluent were maintained at 10~15 mg/L and 30~35 mg/L, respectively. The struvite crystallization treatment effect of cattle farm biogas slurry with continuous flow crystallized-settled reactor was lower than the treatment under static experiment. High suspended solid(SS) may has a negative effect on the crystal growth and reduce the mass fraction of struvite as well for its co-precipitation.According to the pilot test with the continuous flow crystallized-settled reactor, we can speculate that struvite crystallization treatment is more suitable for cattle farm biogas slurry with high proportion OP.(4)Struvite can be detected from the precipitates recovered from both static experiment and continuous flow crystallized-settled reactor experiment with X Ray Diffraction(XRD).The results of elemental analysis of precipitates showed that the content of struvite of precipitates recovered from the continuous flow crystallized-settled reactor experiment(17.00%)) was lower than that recovered from the static experiment(22.09%) under the optimal crystallization conditions.And the precipitates purification can increase the static experiment precipitates purity(mass fraction) from 20.09% to 39.59%(almost 40%). |
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