Research On Performance And Mechanism Of Denitrifying Phosphorus And Nitrogen Removal Of The BBSNP Process For Treating Low Carbon Source Wastewater

Increasing with the urbanization process,the problem of water safety and water resources has become more and more prominent.The municipal wastewater treatment plants（WWTPs）play particularly important roles in maintaining the social cycle of water.In recent year,the municipal wastewater had characteristics of low carbon source,high complexity and high fluctuation.Due to the influence of influent carbon source,the traditional wastewater treatment process causes increase of chemicals,energies and indirect CO₂ emissions for meeting nitrogen and phosphorus discharge standards.It is contrary to achieve the target of“carbon-neutral”and“carbon-peak”.The previous research showed that the Bi-Bio-Selector for Nitrogen and Phosphorus removal（BBSNP）process based on denitrifying phosphorus removal（DPR）could achieve efficient nitrogen and phosphorus removal through DPR of“dual use of one carbon source”.Meanwhile,BBSNP could decrease carbon consumption,oxygen consumption and indirect CO₂emissions.Based on actual municipal wastewater,this study will analyze the high-efficiency and mechanism of BBSNP in treating low carbon source wastewater in depth.This study will reveal transformation and removal dynamics of carbon,nitrogen and phosphorus in DPR and microbial mechanism of DPR.It can improve the theory and technology of BBSNP,promote BBSNP engineering application and provide theoretical basis and technical support for sustainable development of WWTPs.Firstly,this study investigated the influence of VFA concentration decrease on removal performance of BBSNP,analyzed the removal performance of BBSNP in low C/N ratio wastewater treatment.Meanwhile,this study analyzed the enhancement effect of adding carbon source to BBSNP performance in treating complex municipal wastewater.The results showed that BBSNP effluent COD,NH₄⁺-N,TN and TP stably reached the Grade A standards of“urban sewage treatment plant emission standards”（GB18918-2002）under VFA of 58 mg/L,SRT of 7 d and MLSS of 2000 mg/L.As the VFA concentration decreased from 58 mg/L to 22 mg/L,the nitrogen and phosphorus removal performance of BBSNP was all inhibited.Among,the TP removal was the most influenced.The enrichment of denitrifying phosphorus accumulation organisms（DPAOs）was closely related to SRT and MLSS.So,when VFA concentration decreased,this study used intensifying measures of prolonging SRT（7 d to 15 d）and improving MLSS（2000mg/L to 4000 mg/L）.These measures enhanced nitrogen and phosphorus removal efficiency.The effluent also reached the Grade A standards.As the C/N ratio decreased from 3.5 to 2,the performance of BBSNP always kept good,but the TN removal performance of parallel AAO process was inhibited.When C/N ratio was 2,due to nitrifying of high NH₄⁺-N concentration,the residual alkalinity was lacking in BBSNP.Adding alkalinity could solve this problem and further enhanced TN removal performance.Facing complex municipal wastewater（contained 40%industrial wastewater）,the glucose as addition carbon source could only improve TN removal efficiency.The sodium acetate,mixed carbon source（sodium acetate:sodium propionate=2.31:1）and Micro C（contained 70%glycerin）as addition carbon sources could all improve TN and TP removal efficiencies.Among,the dosage of mixed carbon source was lowest.So,the mixed carbon source was optimum to improve BBSNP removal performance.Secondly,based on carbon,nitrogen,phosphorus flux and mass balance in BBSNP,this study analyzed transformation and removal law of carbon,nitrogen and phosphorus in BBSNP and established DPR dynamic model based on the VFA removal characteristic.The results showed that when VFA concentration decreased,prolonging SRT and improving MLSS could enhance intracellular carbon source transfer and anaerobic phosphorus release in BBSNP,thus improving DPR capacity.The anoxic selector section of BBSNP created high phosphorus/nitrogen ratio（P/N sludge load ratio=14.87）environment condition.DPAOs achieved 62.89%nitrogen removal amount in anoxic selector section.It indicated that anoxic selector section could strengthen enrichment of DPAOs.Mass balance results showed that under low C/N ratio（C/N=3-2）,BBSNP achieved high-efficient DPR performance through good carbon source utilization.Compared to AAO,the anaerobic phosphorus release amount,DPR efficiency and denitrifying nitrogen removal amount were all better in BBSNP.When C/N ratio was 2,compared to AAO,BBSNP could decrease 9.81%carbon consumption,14.77%oxygen consumption and 17.29%CO₂ indirect emissions.Facing complex municipal wastewater,the sodium acetate,mixed carbon source and Micro C could improve anaerobic COD utilization and phosphorus release performances to enhance DPR efficiency and anoxic nitrogen removal amount in BBSNP.After decreasing carbon source dosage,the capacity of mixed carbon source improving BBSNP performance was the least affected.Relying on Monod equations,from carbon,nitrogen and phosphorus three aspects,the DPR dynamic model of BBSNP was established based on VFA removal characteristic.The operators could predict effluent and optimize operation through regulating model parameters in BBSNP engineering application.Lastly,this study using high throughout sequencing analyzed microbial community structure of BBSNP,clarified the relationship between key functional bacteria and nutrient removal performance,and revealed microbial mechanism of denitrifying phosphorus and nitrogen removal of BBSNP.The results showed that when VFA concentration decreased,BBSNP developed microbial community structure for good phosphorus removal performance through prolonging SRT and improving MLSS.The fermenting bacteria of 1.56%in anaerobic section ensured good intracellular carbon source transfer.The relative abundance of DPAOs was 2.50%higher than phosphorus accumulating organisms（PAOs）.It indicated that when VFA concentration decreased,DPR was key factor of high-efficient phosphorus removal in BBSNP.The relative abundance of DPAOs was above 2 times higher than that of denitrifying glycogen accumulating organisms（DGAOs）.It proved that anoxic selector section had capacities of strengthen enrichment DPAOs and inhibition DGAOs.Under low C/N ratio（C/N=2）,compared to AAO,BBSNP had better bacteria-adjusting ability and developed a unique microbial community structure for higher nitrogen removal efficiency.Compared to AAO,the higher relative abundance of norank＿p＿Saccharibacteria,Candidatus＿Nitrotoga,Nitrospira,Dokdonella,Ferruginibacter and Ottowia in BBSNP was responsible for better carbon source utilization,nitrification,DPR and traditional denitrifying.Facing complex municipal wastewater,addition carbon source types had significant influence on microbial community structure of BBSNP.When the glucose was added,the relative abundance of DGAOs was highest,which inhibited carbon metabolism of DPAOs.Compared to sodium acetate,when the mixed carbon source was added,the relative abundance of DGAOs was lower and the relative abundance of DPAOs was higher,which was more benefit for denitrifying phosphorus and nitrogen removal.Meanwhile,when the mixed carbon source was added,advantage bacteria norank＿f＿Saprospiraceae played an important role in denitrifying phosphorus and nitrogen removal,which ensured that the enhancement capacity of mixed carbon source was better than Micro C.