Research On Carbon Source And Phosphorus Cooperative Recovery Application Of Low C/N Urban Sewage CASS Treatment Process

With the acceleration of urbanization and economic development,the ratio（C/N）of organic matter concentration（C）to ammonia nitrogen nutrient（N）in the influent of urban sewage plants in the south is getting lower and lower.The CASS process（Cyclic Activated Sludge System）is widely used in sewage treatment plants because of its simple process,good sedimentation effect,and strong adaptability.However,due to the low C/N concentration of influent water in recent years,its total nitrogen（TN）treatment not effectively.This paper aims to solve the problem of poor treatment effect of CASS process TN in low C/N urban sewage plants.First of all,through on-site analysis and research on the CASS treatment effect in the actual operation of the sewage plant,focusing on the three aspects of basic water quality parameters,changes in dissolved organic matter and microbial community structure,to find out the reasons for the insufficient denitrification capacity of the CASS process.Then,based on the analysis results,on-site pilot test research was carried out:（1）Improve the operation parameters of the CASS process,such as drainage ratio,aeration time,and segmented water intake,and compare and analyze the TN removal effect in the CASS process before and after the improvement;（2）Recycle from resources From the perspective of the sludge carbon source and phosphorus reuse,the carbon source recycling of the sludge external cycle and the phosphorus recovery experiment were carried out,so as to improve the treatment effect of the CASS process on TN.At the same time,the phosphorus ammonium magnesium phosphate precipitation method The recovery was analyzed and discussed.This study mainly draws the following conclusions:The effect of water quality treatment in each main operation stage of CASS reactor was analyzed.The results showed that the average concentrations of COD,BOD₅and TN in the influent were about 79.40mg/L,25.49mg/L and 45.70mg/L,respectively.Because microorganisms are affected by the lack of nutrients,COD is rapidly absorbed and degraded in the early stage of aeration（0.5～1.5h）,and low concentrations of organic matter will affect the efficiency of denitrification and nitrogen removal.The average concentration of TN in the effluent water is 12.0mg/L,and the average removal rate of TN is 73.53%.This is the effluent situation when a large amount of external carbon sources are added to the sewage plant.It can be seen that the lack of carbon sources limits the TN removal effect.The qualitative analysis of sewage and sludge was carried out by applying analytical methods such as three-dimensional fluorescence and high-throughput sequencing.The three-dimensional fluorescence results showed that within 0.5 hours of CASS operation,the protein-like substances in the easily degradable carbon source components decreased by89.04%.Within 0.5 to 3.5 hours of CASS operation,due to the low concentration of carbon sources in the water body,microorganisms entered the endogenous respiration stage,and the fluorescence intensity of humic substances increased by 20.86%.The correlation between protein and COD（R²=0.83）and NH₃-N（R²=0.98）is significant,and amino acid plays a decisive role in the denitrification of sewage.The results of high-throughput sequencing showed that the abundance of proteobacteria-dominated microbial communities decreased by50%compared with the sewage treatment system with rich carbon sources.The dominant bacterial phyla in low-carbon source sludge were Chloroflexia,Bacteroidia,Nitrospirota and so on.Due to the low concentration of influent carbon sources,the relative abundance of microbial populations with denitrification functions such asα-Proteobacteria and Nitrospiral were low,accounting for 13.8%and 6.20%,respectively.COD and TN were the two main environmental factors affecting the microbial community,and had little effect on the phosphorus concentration factor（F<0.1）.It can be seen that the low carbon source leads to differences in microbial functional flora,which affects the removal effect of system pollutants.A pilot experiment was carried out,taking the CASS reactor as the research object,and the influence of each main operating parameter on the effluent water quality was studied.The test drainage ratio is set to 30%,40%and 50%respectively.It is found that increasing the drainage ratio can improve the treatment effect of the reactor,and controlling the drainage ratio to 50%is more conducive to the removal of TN.The average removal rate of TN It was85.38%.The aeration time was controlled to 60min,90min and 120min respectively,the removal effect of total nitrogen was improved with the increase of aeration time within 90min,but the removal effect of total nitrogen was poor if the aeration time was too long.Considering the operating cost,the aeration time is controlled at 60 minutes.The removal effect of NH₃-N is the best,and the average removal rate of TN is 64.52%.Adjusting the water inflow in different periods has a weak effect on the removal of organic matter and denitrification of the system,and the average removal rate of nitrogen is lower than 50%.Therefore,the parameters that affect the operation of CASS are in the order of drainage ratio>aeration time>adjustment of water inflow in different periods.The drainage ratio is an important factor for the operation effect of the CASS reactor.The drainage ratio can optimize the allocation of carbon sources.By changing the process parameters,the TN removal capacity of the CASS process in low C/N urban sewage plants can be effectively improved.Investigate the use of carbon sources in excess sludge by different pretreatment technologies for CASS denitrification and denitrification experiments.Select the sludge from the CASS pool as the inoculation sludge,and carry out anaerobic hydrolysis at room temperature in the sludge hydrolysis tank.After 7 days,the concentration of TCOD and SCOD per unit mass in the hydrolysis tank reaches 13.8 mg/g VSS and 12.9 mg/g VSS respectively.The sludge storage should be used in the later stage sludge in the pond.Investigating the anaerobic phosphorus release capacity of phosphorus can maximize the recovery of struvite crystalline phosphorus resources.Controlled the addition of different concentrations of sodium acetate,flour and alkalinity,it was found that under the condition of alkaline p H value=10,the comprehensive release effect of sludge was the best,and the comprehensive release effect of SCOD and TP was the best on the 4th day,respectively4.00mg/g VSS and 1.35mg/g VSS.To explore the optimal parameters of sludge hydrolysis time and struvite recovery in sludge hydrolysis supernatant under alkaline conditions.The research results show that the total phosphorus concentration in the sludge supernatant can reach 154.5 mg/L if the reaction time is controlled at 6 hours,and the SCOD can reach 940 mg/L if the reaction time is controlled at 10 hours.Based on the analysis of the change of molecular weight based on high performance liquid gel chromatography,the increase of small and medium molecular organic matter can reach 53.34%after 6 hours.Therefore,under the condition of p H value 10.0,the sludge hydrolysis time is controlled at about 8 hours,which can meet the requirements of recycling carbon at the same time.The source should produce organic matter with a smaller molecular weight as much as possible and the requirements for struvite recovery of phosphorus in the later stage.The optimal reaction conditions for struvite crystalline phosphorus recovery were p H=10,Mg:P=1.8:1,stirring and settling time were controlled at40min.According to theoretical calculations,although sludge hydrolysis alone cannot completely replace external carbon sources,anaerobic hydrolysis of sludge can replace and save 33.6%of external carbon sources.The use of carbon source reuse in sludge external circulation and coordinated recovery of phosphorus not only recycles sludge resources,but also improves the nitrogen and phosphorus removal capabilities of the original process,reduces the amount of carbon source dosage,and at the same time can recycle sludge sustainable use of phosphorus resources.