Study On Simultaneous Nitrification,Denitrification And Phosphorus Removal Coupled Side Stream Phosphorus Recovery Of Municipal Wastewater

Phosphorus is an essential nutrient element for all life in nature.In recent years,it has led to a paradox about phosphorus resources:on the one hand,phosphorus is the key factor which could induce water eutrophication.If it is not handled properly,it will lead to the imbalance of water ecological environment.On the other hand,phosphorus resources are non renewable and will face the crisis of scarcity of phosphorus resources.A large amount of phosphorus resources are contained in sewage,and the phosphorus recovered from sewage can meet the phosphorus demand of 12%-15%.Therefore,recovering phosphorus resources from sewage is an important way to solve the double dilemma of"phosphorus pollution"caused by excessive phosphorus in the current water environment and"phosphorus crisis"caused by the depletion of phosphate rock resources.The refore,phosphorus recovery from sewage has attracted more and more attention.However,the phosphorus concentration in urban sewage is generally low.If it is recovered directly,the cost is too high but the efficiency is low.Phosphorus recovery in sewage treatment plants usually adopts enhanced biological phosphorus removal process（EBPR）,which uses the anaerobic phosphorus release of phosphorus accumulating bacteria（PAOs）to obtain phosphorus rich solution much higher than the influent concentration,and then adds chemicals to recover the phosphorus recovery products,which greatly improves the recovery efficiency.In addition,side stream phosphorus recovery reduces the phosphorus load of the mainstream biological system through the efficient removal of point source phosphorus,which can not only save carbon source and energy consumption,but also reduce the output of excess sludge,and achieve sewage treatment and resource recovery.However,sidestream phosphorus recovery is a phosphorus deprivation to the mainstream biological system.But at present,the impact of sidestream phosphorus recovery on the mainstream biological system is not clear,and the operation efficiency and stability of the system under the condition of sidestream phosphorus recovery need to be further studied.Therefore,this study constructed an anaerobic/aerobic/anoxic（A/O/A）simultaneous nitrification and denitrification phosphorus removal coupled side stream phosphorus recovery system（SNDPRr）,and compared the responses of flocculent activated sludge and aerobic granular sludge to pollutant removal performance,sludge characteristics,extracellular polymer（EPS）,phosphorus recovery potential and microbial community dynamics in the side stream phosph orus recovery and recovery stage.In order to explore the impact of the introduction of side stream recovery on the mainstream biological treatment system,different side stream recovery ratios were selected to clarify their impact on the long-term steady-state operation of flocculent sludge biological treatment system,and the optimal side stream ratio was optimized,which provided a basis for the subsequent phosphorus recovery research of granular sludge system.In this study,artificial water distributio n was used to simulate urban sewage,and five groups of SBR reactors operating in A/O/A mode with side flow ratios of 0%,10%,20%,30%and 50%were set respectively.The phosphorus rich solution at the end of the anaerobic section was led away at the reco very frequency of once a day in the first 80 days,and then recovered in the side flow system.The results showed that the removal performance of chemical oxygen demand（COD）,ammonia nitrogen（NH₄⁺-N）and total phosphorus（TP）was not affected by phosphorus recovery,and the average removal rates were more than 88%,95%and 97%respectively.With the increase of side flow ratio,the reduction process of nitrite（NO₂^--N）in the denitration process was inhibited,and the residues in the effluent were mainly nitrate（NO₃^--N）and NO₂^--N.The average concentrations were 3.5-4.5mg/L and 2-3.5 mg/L,respectively.The introduction of side flow reduce d the relative content of inorganic components in the sludge,and improved the sludge sedimentation performance.However,it inhibited the increase of sludge activity.Continuous recovery led to the decrease of phosphorus content,phosphorus release,phosphorus release rate and absorption rate in sludge,and the reduction rate and the length of recovery period were positively correlated with side flow ratio and side flow frequency.The protein（PN）in sludge EPS decreased with the increase of side flow recovery ratio,and the polysaccharide（PS）increased with the increase of side flow ratio.The contents of them were relatively stable in the recovery stage.When each system reaches the stable phosphorus release state,29%-50%phosphorus in the influent can be recovered stably under the side flow ratio of 10%-50%.Considering the balance between energy recovery and slu dge performance,30%side flow ratio was taken as the appropriate side flow ratio under this operation mode.The sludge yield was negatively correlated with the side flow ratio.Compared with the blank group,the recovery group achieved the highest in-situ sludge reduction effect of 81%,indicating that the removal of phosphorus source significantly slowed down the growth and reproduction rate of microorganisms.The relative abundance of GAOs such as Candidatus＿Contentobacter and Candidatus＿Competibacter were positively correlated with anaerobic phosphorus release concentration and side flow ratio,while the relative abundance of PAOs such as Candidatus＿Accumulibacter was negatively correlated with anaerobic phosphorus release concentration and side flow rat io,but its phosphorus removal performance was stable at low level.In order to further understand the impact of sidestream phosphorus recovery on the mainstream biological phosphorus removal system,the operation of granular sludge system with 30%sidestream ratio was explored and compared with flocculent sludge.In this study,the same reactor and operation mode as flocculent sludge and artificial water distribution were used to simulate urban sewage,and the experimental group and blank control group with 30%sidestream ratio were set.The phosphorus rich solution at the end of the anaerobic section shall be led away every day within the first 40 days,and then recovered in the side flow system.The recovery was observed within 41-100 days.The results showed that the removal rates of COD,NH₄⁺-N and TP reached more than 88%,97%and 95%respectively,which was also not affected by the recovery.The simultaneous nitrification and denitrification function in granular sludge made the effluent NO₃^--N and NO₂^--N concentrations lower than that of flocculent sludge,which were 3-3.5 mg/L and 0.5-1 mg/L,respectively.Recycling led to the increase of organic active biomass in granular sludge,and its relative proportion increases with the times of phosphorus recov ery.In the EPS of granular sludge,the recovery reduces the PN concentration,and the PN concentration was lower in the recovery stage,while the PS change d little in the two stages.The phosphorus release of granular sludge decreases with the recovery.1 00%phosphorus in the influent can be recovered within 32 days,and 60%phosphorus in the influent can be recovered stably after 32 days,while maintaining a high level of phosphorus removal performance.Compared with the blank group,the side flow recover y ratio of30%reduced the sludge yield by 14%.In the granular sludge system,the introduction of recycling led to the enrichment of GAOs（such as Candidatus＿Competibacter）and inhibited the growth and reproduction of PAOs（such as Candidatus＿Accumulibacter and unclassified＿o＿Chitinophagales）and DPAOs（such as Dechloromonas）.The rich and diverse microbial community laid a biological foundation for the stability of the system.This study revealed the characteristics and related mechanisms of the long-term steady-state operation of SNDPRr system,and defined the relationship between the process operation parameters,sludge physical and chemical characteristics,pollutant removal efficiency and microbial community of mainstream biological treatment system and side flow recovery system,as well as realized the synchronization of pollutant removal and phosphorus recovery in biological phosphorus removal system,and laid a foundation for the practical application of side flow phosphorus recovery.