Study On Recovery Of Phosphorus Crystallization From Phosphorus Rich Solution In Municipal Wastewater Based On Biofilm Method

Phosphorus resources are a non-renewable resource and a high-demand resource.The current storage capacity of phosphorus has been supported for less than a century.If the phosphorus in urban sewage plants can be recycled,it can meet about 25%of the current phosphorus demand.Because phosphorus resources are polluting,phosphorus recovery is needed.At present,the traditional phosphorus recovery method used in sewage treatment plants is mostly struvite method,but it has the disadvantages of low utilization value and low phosphorus recovery rate.The vivianite method has a good application prospect because of its high economic value and industrial value,and it exists everywhere in the sewage treatment unit.Because the crystalline phosphorus recovery technology is only suitable for high phosphorus concentration liquids(P>50 mg·L^-1),and the phosphorus concentration of phosphorus recovered from sewage treatment plants is relatively low(P<10 mg·L^-1).On the basis of the concept of comprehensive recycling of urban sewage,this research group mainly recycles the organic matter and phosphorus resources in the wastewater in the form of energy or products that can be used effectively,and the biofilm method（aerobic/anaerobic alternate operation mode）is adopted to enrich phosphate directly in the mainstream process,and the phosphorus concentration of the enriched phosphorus solution has reached the requirements of the crystallization method,and the obtained phosphorus-rich solution is subjected to the phosphorus recovery of the vivianite method.This process system not only reduces the phosphorus loss and difficult separation problems of phosphorus recovery from sludge,but also avoids the problem that pH（<5.00）during the anaerobic digestion and fermentation process that is concentrated in the existing research is different from the pH（6.00～9.00）generated by vivianite（Fe（Ⅱ）-P will dissolve）.In order to further promote the formation of this new type of phosphorus recovery process-the formation of film enrichment and recrystallization recovery,this research mainly discusses the feasibility of the vivianite method to recover the phosphorus-rich solution and obtain the reaction conditions of high phosphorus recovery efficiency and the purity of the vivianite.The X-ray diffractometer（XRD）and scanning electron microscope-energy spectrum（SEM-EDS）were used to qualitatively characterize the vivianite under the optimal conditions;The influence of influent phosphorus concentration,residence time and number of crystallization on phosphorus recovery efficiency,the purity of vivianite and crystal size in phosphorus recovery process system were discussed.The result shows that:（1）The phosphorus recovery rate of the high-fold phosphorus-rich solution recovered by the vivianite method has reached 92%,and the qualitative analysis of the recovered products shows that the main crystal phase is vivianite,which shows that the vivianite method can effectively recover the phosphorus-rich solution.（2）When the pH is 7.00～7.50,the ORP drops to about-100 m V,and the molar ratio of iron to phosphorus ions is 3:1,the phosphorus recovery efficiency of the vivianite method（about 95%）and the recovered vivianite（88.35%）are the largest,and the results of qualitative characterization under the optimal conditions also show that vivianite is the only crystal phase in the recovered product and has good crystal form.（3）Both the addition of ascorbic acid and N₂exposure can make the reaction system in a reduced state,but the addition of ascorbic acid can make the ORP lower（-400m V<-100 m V）;the addition of ascorbic acid is slightly larger on the phosphorus recovery efficiency and the purity of vivianite than the exposure to N₂,so from the perspective of experimental optimization,the method of adding ascorbic acid is more suitable;from the perspective of actual process operation,the method of exposure to N₂is more suitable.（4）In the phosphorus recovery process system,the phosphorus recovery rate、the purity of vivianite and the crystal size of vivianite first increase and then become stable with the increase of residence time.The optimal residence time is 5 h;With the increase of influent phosphorus concentration,the phosphorus recovery efficiency first increases and then tends to be stable;the purity of vivianite first increases and then decreases,and the crystal size of the vivianite has little effect.The optimal influent phosphorus concentration is 120 mg·L^-1.5)The particle size of vivianite will take itself as the crystal seed and increase exponentially with the increase of the number of crystallization,and in the process of increasing,the overall particle size will be uneven.When the number of crystallization is 7times,the crystal size of the vivianite in this way is the largest,270.518μm,the phosphorus recovery efficiency is 95.2%,and the purity of the vivianite is 87.91%.