Phosphorus Recovery From Sewage Or Sludge By Vivianite Crystallization

Phosphorous(P)is an essential element in living organisms and plays a vital role in life activities.The P resource shortage and water eutrophication situation were aggravated due to the increasing of P demand and the neglect of P recovery.Therefore,a feasible P recovery path is needed to realize the sustainable utilization of P resources and to solve the eutrophication situation.Vivianite mediating phosphate immobilization could be a promising measure for phosphorus recovery.In this study,P resources in wastewater and sludge were recovered by vivianite crystallization technology.The P recovery route of release^-recrystallization was established to achieve the maximum P recovery efficiency.Firstly,the effects of different factors on P release were investigated to improve the P release efficiency.And then the effects of metal ions and organic matters on the vivianite crystallization process were investigated.Finally,the parameters of the fluidized bed crystallization(FBC)reactor were optimized to improve the P recovery efficiency.Main contents are summarized as follows:(1)Optimization of P release efficiency from sludge.Considering the low efficiency of P release during AF process,the promotion effects of pH adjustment on P release from sludge were investigated.The results indicated that the pH enhancement for P releasing was related to the sludge P fractions.The leaching of metal ions by H⁺ under acidic conditions promoted the release of AP.Under alkaline conditions,OH^-substituted PO43^-which combined with Fe³⁺/Al³⁺ and promoted the release of NAIP.The effect of acidic conditions on fermentation products and microbial community was insignificant.However,alkaline conditions facilitated sludge disintegration and improved the richness and diversity of bacterial community.The changing of relative abundance of dominant species under alkaline conditions promoted the accumulation of VFAs.The concentration of metal ions and heavy metal ions in the fermentation supernatant increased significantly under acidic conditions,while most of the metal ions and heavy metal ions remained in the solid phase under alkaline conditions.In conclusion,it is feasible to promote the P release from sludge by adjusting pH,but the influence on fermentation process and subsequent crystallization should be considered comprehensively.(2)Effect of iron addition method on vivianite crystallization process.Considering the low Fe/P molar ratio of AF supernatant,iron air fuel cell and iron salts were chosen as the Fe sources,and their effects on P recovery were compared.In addition,the crystallization conditions were optimized to form high purity vivianite from AF supernatant.The results showed that PO₄^3-in liquid phase could be effectively recovered after Fe addition.In situ Fe²⁺ addition by iron air fuel cell has the dual potential of power generation and P recovery.However,limited by the generation rate of Fe²⁺,it is more suitable for small wastewater recycling system.The P recovery process by iron salts addition was susceptible to pH and Fe²⁺ oxidation.Preventing Fe²⁺ oxidation with nitrogen or ascorbic acid could improve the purity of crystallization products.The elemental analysis results showed that the metals and heavy metals in the AF supernatant would be partially transferred to the recovered product during the crystallization process,which had a negative effect on the purity of the crystallization.In addition,the heavy metals in the recovered vivianite were lower than their own risk thresholds.(3)Effect of organic matters(OMs)on vivianite crystallization process.Considering the high organic matter content in sewage and sludge,the effects of four representative OMs,glucose,bovine serum albumin(BSA),humic acid(HA)and sodium alginate(SA),on P recovery by vivianite were investigated in this part.The results showed that P recovery efficiency was inhibited by HA and SA,declining by 3.7% and 12.1% under HA(100 mg/L)and SA(800 mg/L),respectively.BSA,HA and SA affected the aggregated form of vivianite crystals.Vivianite particle size decreased in the presence of HA and SA.Subsequent mechanistic exploration indicated that the complexation between the OM and Fe²⁺ was the main cause of P recovery efficiency reduction.The coprecipitation of HA and SA with vivianite could reduce the zeta potential on the crystal surface,resulting in a smaller particle size.The nucleation sites provided by BSA and SA could transfer vivianite from a single plate-like polymer to a multilayer plate-like polymer.And the effect mechanism of organic matter on crystallization process was further verified by the crystallization process of AF supernatant.(4)Phosphorus recovery by FBC reactor from wastewater.The homogeneous crystallization occurred in vivianite crystallization process could result in the reduction of particle size.In this part,a FBC reactor was constructed to recovery P from P rock drainage.By optimizing the operation parameters,it was found that increasing the flow rate could avoid dead zones.Adjusting the pH and hydraulic retention time of the reaction system could maintain the saturation index in the metastable zone.It was beneficial to the process of heterogeneous crystallization and improved the particle size of the product.The effect of seed addition on P recovery was insignificant.The maximum P recovery rate(91.2%)and crystallization rate(60.3%)can be achieved under optimized conditions and the characteristics of recovered products were similar with vivianite.