| The limitation of carbon source is the main reason affecting biological nitrogen and phosphorus removal of urban wastewater.How to achieve good nitrogen and phosphorus removal efficiency under the carbon limited condition is a major difficulty for wastewater treatment.Simultaneous nitrification denitrification phosphorus removal(SNDPR)technology could realize simultaneous removal of carbon,nitrogen and phosphorus from wastewater with low C/N ratio in a single reactor,which has attracted more and more attention.Firstly,this study investigated the fast start-up and long-trem stable operation of SNDPR system with anaerobic/micro-aerobic/anoxic(A/MO/A)operation mode.The performance of SNDPR process under low C/N was enhanced by nitrite stress strategy,and the strengthening mechanism was revealed by pollutant removal and microbial community succession.Secondly,the response of SNDPR system to phosphorus changes was investigated by adopting the strategy of"cutting off phosphorus-supplying phosphorus"for influent water,and the difference of denitrification characteristics and metabolic process between phosphorus-accumulating organisms(PAOs)and glycogen accumulating organisms(GAOs)was further elucidated.Thirdly,the SNDPR system was coupled with granular sludge to study the formation strategy and mechanism of granular sludge,and the stability and N,P removal characteristics of granular sludge under the condition of limited oxygen and low C/N ratio was studied.Finally,based on the reaction process analysis of SNDPR system,the reaction kinetics model of ASM2D was established,and the phosphorus removal mechanism of SNDPR system was explored by using this model.The major conclusions are as follows:(1)In An/MO/A reactor,the SNDPR system was started and PAOs could be enriched by adding sodium acetate during the anaerobic period.It was found that by additional addition of200 mg/L sodium acetate during anaerobic periods,the phosphorus release and phosphorus uptake of PAOs could be effectively increased,and the phosphate concentration of effluent water could be kept below 0.5 mg/L for a long time.Under the condition of C/N ratio of 7,the nitrogen and phosphorus removal efficiencies were 98.76±0.9%and 96.97±1.1%,respectively.and the SND efficiency reached 97.01%.In order to explore the nutrient removal capacity of the system under the condition of limited carbon source,the COD concentration of influent water was reduced,and the COD/N ratio was reduced from 7 to 5.The nitrogen and phosphorus removal efficiencies of the system were significantly affected,reduced to 81.95±2.7%and85.47±6.3%,respectively.Due to the lack of carbon source,the anaerobic phosphorus release amount decreased significantly,and the nitrogen and phosphorus removal efficiency of the system deteriorated gradually under the COD/N ratio of 5.(2)In order to improve the nitrogen and phosphorus removal efficiency of the system at the C/N ratio of 5,the operation mode was changed from An/MO/A mode to An/A mode once a day,with 14 consecutive days.In An/A operation mode,30 mg/L nitrite was added to the reactor at the end of anaerobic period.It was found that high concentration nitrite would increase the level of free nitrite acid(FNA)in the system under acidic environment,and high concentration nitrite denitrification process would inhibit NO reductase and lead to NO accumulation in liquid phase.The accumulation of NO and FNA colud inhibit the activity of unwanted bacteria in the system such as nitrite oxidizing bacteria(NOB)and ordinary heterotrophic organisms(OHOs),thus reduce the consumption of carbon source.NO could stimulate denitrifying phosphorus accumulating bacteria(DPAOs)into continuous phosphorus release,and nitrite addition could promote DPAOs use nitrite as electron acceptor for denitrifying phosphorus uptake.After nitrite strategy,the abundance of DPAOs(Pseudomonas sp.,Acinetobacter sp.and Dechloromonas sp.)showed significant increase,and SNDPR system achieved long-term stable nitrogen and phosphorus removal under C/N ratio of 5,with nitrogen and phosphorus removal efficiencies up to 98.89±0.7%and98.17±0.9%,respectively.(3)In order to investigate the response of the SNDPR system to the phosphorus deficiency environment,a stable run SNDPR reactor was cultured for long-term phosphorus deficiency,and the phosphorus-rich supernatant was discharged at the end of the anaerobic stage to forcibly deprive the intracellular phosphorus in PAOs.The pollutant removal efficiency was observed of the system in the phosphorus deficiency environment.After 90 days of phosphorus deficiency operation,the phosphorus supply to the system was resumed.The response of the microbial community to the change of phosphorus deficient and phosphorus rich environment was explored.The results showed that the nitrogen removal efficiency was reduced by 18%in a short-time phosphorus deficiency,which might due to the denitrification and denitrification of DPAOs was limited by the phosphorus shortage.With the increase of the phosphorus deficiency time,the system nitrogen removal would gradually recover,which might be caused by the glycogen accumulating organisms(GAOs)gradually dominated.The microbial community showed that the long-trem phosphorus deprivation made the domanit gern changed in system from PAOs(Acinetobacter sp.,Zoogloea sp.)to GAOs(Candidatus＿Competibacter sp.,Defluviicoccus sp.).After the resumption of phosphorus supply,PAOs was still in a inferior position in the short term,and the system was dominated by GAOs.(4)In order to explore the metabolic differences between PAOs and GAOs,two reactors with PAOs and GAOs as the dominant bacteria were studied in which acetic acid and glucose were used as carbon sources for acclimation and enrichment.By monitoring NADH/NAD~+of the two systems,it was found that the concentration of NADH increased during the anaerobic phosphorus release process of the PAOs in SNDPR system,while no accumulation of NADH was observed during the anaerobic storage of PHA by the GAOs,which might be caused by the metabolic differences between the two systems in the anaerobic stage.The denitrification rate of DPAOs was higher than that of denitrification glycogen accumulating organisms(DGAOs),which might due to the accumulation of reduced electrons in the cell during the anaerobic period.DPAOs denitrification with a high concentration of nitrite resulted in high concentration of accumulated NO,while DGAOs denitrification only produced a small amount of accumulated NO.NO accumulation was caused by the combined action of nitrite(or FNA)inhibition and NADH accumulation in the anaerobic phase.(5)In order to achieve stable nitrogen and phosphorus removal under lower C/N ratio,the formation of granular sludge was realized by gradually shortening and reducing the settling time,and the SNDPR system was successfully coupled with granular sludge.The changes of microbial community and nutrient removal capacity of the system during the formation of granular sludge were investigated.After the stable formation of granular sludge,the ammonia nitrogen concentration of the system was gradually increased while the influent carbon concentration remained unchanged,and the nitrogen and phosphorus removal efficiency of the granular sludge system under low C/N ratio was investigated.The nitrogen and phosphorus removal efficiencies of 91.59±1.63%and 99.81±0.66%were achieved under the C/N ratio of4.By calculating phosphorus uptake at different electron acceptors,33.03%nitrogen and 31.31%phosphorus were removed by DPAOs,saving 37.66%carbon source consumption.(6)Finally,the process simulation and phosphorus removal mechanism of SNDPR system were studied based on ASM2D model.Based on the detailed analysis of the possible pollutant removal paths in the SNDPR system during the experiment,the reaction dynamics framework of ASM2D model was improved.Global sensitivity analysis method based on Sobol was used to analyze the sensitivity of model parameters and obtain the high sensitivity parameters of each reaction dynamic process.Then,intelligent parameter optimization method based on genetic algorithm was used to take the high sensitivity dynamic parameters and process parameter Kla as the control variables.Taking the mean root mean square error between the model output and the actual measured value as the objective function,the intelligent optimization of parameters was carried out to obtain the best model parameter value.Finally,the established model was used to explore the phosphorus removal mechanism of SNDPR system,and it was found that the denitrifying phosphorus accumulation process played a dominant role in the selected experimental process.The establishment of this model could provide theoretical basis and guidance for real-time operation regulation of SNDPR process. |
