Phosphorus Recovery Through Vivianite Crystallization Enhanced By Magnetic Biochar Combined With Starch

Recovering phosphorus resources from sewage/sludge is an effective way to solve the current phosphorus crisis and phosphorus pollution.Vivianite（Fe₃（PO₄）₂·8H₂O）has become a research hotspot in the field of phosphorus recovery due to its stable properties and high economic value.However,the crystallization efficiency of vivianite is low,the crystal size is small,and it is difficult to separate and recover.Based on this,in this study,starch was used as an external carbon source,and magnetic biochar（MB）was used as a crystal seed to enhance the crystallization efficiency of vivianite during anaerobic digestion of excess sludge,and reduce the difficulty of recovering phosphorus in the form of vivianite.The effects of starch and MB on the formation process of cyanite,phosphorus recovery and methane production under different conditions were explored,and the optimal reaction conditions were determined;On this basis,the characteristics of sludge microbial community before and after adding starch and MB were analyzed,and the promotion mechanism of the combined system on cyanite crystallization was discussed.The main research results are as follows:（1）Using starch as an external carbon source,the effect of dosage on the formation of vivianite in an anaerobic system was studied.The results show that adding starch can effectively increase the iron reduction rate and promote the transformation of phosphorus species,and the XRD results of each group of digested sludge show that vivianite is the main phosphorus crystallization product.Among them,when the dosage was 20%of the sludge VS,compared with the control group,the extractable Fe（II）content increased by 15%on the6th day,and the non-apatite inorganic phosphorus（iron-bound phosphorus in this study）increased by 12%.In addition,the cumulative methane production was 173.32 m L/g VS,an increase of 16%.When the dosage increased to 30%,the methane production decreased by14%,and the optimal starch dosage was 20%.（2）Using MB as the seed crystal,the effect of MB combined with starch on the recovery of phosphorus from vivianite crystallization under different conditions was studied.The results showed that MB material was prepared by dipping-pyrolysis method at 500℃for 2 h using walnut shell as raw material.Compared with the primary biochar prepared under the same conditions,the surface and the inside of the MB were covered with Fe₃O₄ particles.It is covered with magnetic responsiveness and the surface is loose and porous.When the iron loading is m _S:m _Fe=2:1 and the dosage is 1 g/g VS,compared with the control group without MB,the proportion of NAIP increased by 10.21%,and the recovery rate of phosphorus reached 45%;At the same time,the cumulative methane production was 219.71m L/g VS,also increased by 27%.roup,an increase of 27%.The XRD and SEM-EDS analysis results of the phosphorus recovery products proved that limite crystal precipitates were formed on the surface of MB.（3）The results of microbial community structure analysis showed that the addition of starch and MB could increase the abundance and diversity of microorganisms,thereby enhancing the stability of microbial structure in anaerobic digestion.Among them,the iron-reducing bacteria（Clostridium＿sensu＿stricto＿1）were abundantly enriched,which promoted the iron reduction rate;The abundance of acetate-utilizing methanogens（Methanosaeta）and hydrogen-utilizing methanogens（Methanobacterium,Methanolinea,Candidatus＿Methanofastidiosum）also increased to varying degrees.Due to the coexistence of two methanogenesis methods,acetic acid utilization type and hydrogen utilization type,the efficiency of methane production by anaerobic digestion is greatly improved.The research work of the thesis shows that starch and magnetic biochar can effectively promote the recovery of phosphorus from vivianite by affecting the abundance and diversity of sludge microorganisms.