The Mechanism Of Biologically Induced Chemical Phosphorus Removal In Wastewater Nitrogen Removal Systems

Due to the serious harm of wastewater phosphorus emission to the ecological environment and the scarcity of phosphorus resources,phosphorus removal and phosphorus recovery are crucial in the wastewater treatment process.In order to reveal the unknown phosphorus removal mechanism in high concentrated wastewater biological treatment systems without polyphosphate accumulating bacteria,the migration rule of phosphorus and its microbial mechanism in wastewater biological nitrogen removal systems were explored,and an efficient and low-cost wastewater phosphorus removal technology based on heterotrophic nitrification biologically induced chemical phosphorus removal（BICPR）was developed.On the basis of the identification of BICPR efficiency and pathways in enhanced biological phosphorus removal（EBPR）system,the BICPR pathways in different biological nitrogen removal processes（organic nitrogen hydrolysis,nitrification and denitrification）were systematically explored.The response of BICPR efficiency in heterotrophic nitrification system to environmental factors and the phosphorus removal mechanism were mainly investigated.Through the analysis of the phosphorus and nitrogen removal efficiency,the characteristics of phosphorus precipitation product,the solution physicochemical properties,the microbial community structures in the systems,and the driving mechanism of biological nitrogen removal process on BICPR was clarified.From the perspectives of component contents,chemical characteristics,and binding interactions with ions of microbial extracellular polymeric substances（EPS）,the microscopic roles of EPS in BICPR was revealed.In addition,the practical application efficiency and economic analysis of wastewater BICPR technology based on heterotrophic nitrification were investigated.The main conclusions of the study are as follows:The phosphorus removal in EBPR system were synergistically through biological phosphorus removal pathway of polyphosphate accumulating bacteria and BICPR pathway.Under different influent calcium concentrations,the removal rate of PO₄^3--P in EBPR system was 96.9-99.1%.Calcium cation was involved in the phosphorus removal process of EBPR system,which also affected the metabolic activity of polyphosphate accumulating bacteria and the phosphorus removal pathway of the system.With the increase of influent calcium cation concentration,the chemical phosphorus removal in the system was enhanced,and the contribution of biological phosphorus removal decreased.The content of non-apatite inorganic phosphorus（NAIP）in sludge decreased,while the content of apatite inorganic phosphorus（AP）increased.The proportion of polyphosphate decreased,and the proportion of orthophosphate increased.At high calcium cation concentration（160 mg/L）,hydroxyapatite precipitation with high crystallinity was generated in the system,and its phosphorus content reached 10.04%（wt）.Candidatus＿Accumulibacter and other phosphorus accumulating bacteria genera were detected in the system.The anaerobic phosphorus release can promote the formation of calcium phosphate and hydroxyapatite precipitation.EPS was the main area of BICPR,as the calcium,phosphorus contents and phosphorus form of EPS changed periodically with the metabolism of polyphosphate accumulating bacteria.The organic nitrogen hydrolysis,heterotrophic nitrification and denitrification processes significantly drove the BICPR pathway.Biological nitrogen removal induced the simultaneous removal of calcium and phosphates ions in the system.Organic nitrogen hydrolysis process,heterotrophic nitrification process and denitrification process can significantly promote chemical phosphorus removal,while autotrophic nitrification process has no promoting effect on chemical phosphorus removal.At high calcium cation concentration,the removal rates of PO₄^3--P in organic nitrogen hydrolysis,heterotrophic nitrification and denitrification biological systems were 95.30%,89.75%and 92.93%,respectively.The PO₄^3--P removal rate in the pure chemical phosphorus removal group was only 13.37-20.07%,which was significantly lower than that in the biological nitrogen removal system.Biological nitrogen removal processes produced alkalinity,which increased the effluent p H value and promoted the formation of hydroxyapatite and calcium phosphate precipitation,where phosphorus removed from the liquid phase and migrated to the solid phase.The solid phosphorus component in each system was mainly AP,and the proportions to the corresponding TP were 76.19-82.06%,67.81-80.41%and85.86-86.63%,respectively,indicating that BICPR was the dominant phosphorus removal pathway in nitrogen removal systems.The influent C/N ratio,the type of carbon source and microbial growth mode had significant effects on the BICPR efficiency of heterotrophic nitrification system.With the increase of influent C/N ratio from 10 to 30,the effluent p H value increased from 8.05 to9.08.Under high calcium cation concentration,the removal rate of PO₄^3--P decreased from 89.16%to 71.23%,and the proportion of solid phase AP to corresponding TP decreased from 61.48%to 42.88%.Excessive C/N ratio would adversely affect the BICPR pathway.The phosphorus and nitrogen removal efficiencies of the system with sodium acetate as carbon source were higher than those of the system with starch as carbon source.Compared with activated sludge system,biofilm system has better characteristics of intercepting phosphate and inducing calcium phosphate precipitation.The removal rates of PO₄^3--P in activated sludge and biofilm systems were 49.36%and60.09%at low calcium concentration（55 mg/L）,while the rates were 92.12%and 94.17%at high calcium concentration（175 mg/L）,respectively.The solid AP content of each system was 10.40-20.02 mg P/g SS and 26.16-37.85 mg P/g SS,respectively,and the proportion to corresponding TP was 67.72-75.38%and 64.12-78.81%,respectively.The heterotrophic nitrification process changed the physical and chemical properties of the solution,which thermodynamically promoted the precipitation of calcium phosphate and drove the BICPR.The main precipitation products in each system were hydroxyapatite,calcium phosphate and calcium carbonate.The microbial growth mode had a significant influence on the characteristics of precipitation products.The precipitation product of biofilm system has rock crystal structure,which mainly distributed in the inner layer of biofilm.Its phosphorus content was 10.42-19.34%,which had good phosphorus recovery potential.Heterotrophic nitrification bacteria genera including Paracoccus,Exiguobacterium,Rhodobacter,Microbacterium were detected in the systems.In addition,C/N ratio,carbon source type and microbial growth mode had significant effects on the content and characteristics of EPS.The heterotrophic nitrification biofilm system had strong adaptability to high salinity,while the heterotrophic nitrification activated sludge system was significantly inhibited by high salinity.At high calcium cation concentration,the removal rate of PO₄^3--P in the biofilm system with 3%salinity was 87.56%.The solid AP content of biofilm was 21.59-36.55 mg P/g SS,the proportion to corresponding TP was 71.25-77.27%.The main precipitation products were hydroxyapatite with granular and spherical structure,and the phosphorus content was 7.41-12.45%.Salinity had significant effects on EPS component content and microbial community structure.With the increase of salinity,the protein content in EPS increased,and the relative abundance of heterotrophic nitrification bacteria increased.Microbial EPS plays an important role in BICPR.EPS of heterotrophic nitrification system contains high proportion of monomers such as glutamic acid,aspartate,glucose and glucuronic acid,and abundant functional groups such as hydroxyl,carboxyl and amide groups.EPS molecules could combine with calcium and phosphate ions through functional groups such as hydroxyl and amide groups.Calcium cation was preferentially adsorbed on the molecular surface,and then the phosphate anion was intercepted by the bridging effect of calcium cation,leading to the initial precipitation of calcium phosphate.The contribution of protein components in EPS to calcium adsorption was higher.EPS molecules affected the migration and aggregation of calcium cation and phosphates anion in the solution.The EPS provided nucleation sites for calcium phosphate precipitation and further protected the formed calcium phosphate precipitation,which promoted the BICPR process.BICPR technology based on heterotrophic nitrification can achieve high efficiency and low cost phosphorus removal in high saline mustard wastewater.The TP removal rate of heterotrophic nitrification system for mustard tuber wastewater with salinity of 3%and nitrogen loading rate of 0.21 g N/（L·d）was 75.13%.The AP component content of sludge solid phase was 12.61 mg P/g SS,and the corresponding proportion of TP was 61.29%.The main precipitation products are hydroxyapatite and calcium phosphate,and the phosphorus content is 6.04-9.07%.High abundance of heterotrophic nitrifying bacteria and denitrifying bacteria were detected in the system.Treating high saline mustard wastewater by BICPR technology,the dosing cost was reduced by 19%,compared with pure chemical phosphorus removal technology.The driving mechanism of biological nitrogen removal on BICPR was elucidated in this study.A new phosphorus removal model of nitrogen conversion coupled with phosphorus migration in heterotrophic nitrification system was proposed,which expanded the existing theory of wastewater phosphorus removal.The microscopic mechanism of microbial EPS involved in bioinduced chemical phosphorus removal was revealed.An efficient and low-cost wastewater phosphorus removal technology based on heterotrophic nitrification BICPR was developed,which realized the sustainable treatment and resource utilization of high saline and high concentration wastewater,opened up a new approach for wastewater phosphorus removal and phosphorus recovery,and had important theoretical and practical values.