Simulation Study On Hydraulic Characteristics Of Vertical Flow Secondary Settling Tank Based On Actual Working Conditions

The secondary sedimentation tank is one of the main structures of the activated sludge process.Occuping less space and excluding sluge easily makes the vertical flow secondary settling tank often be used in small sewage treatment plants.However,research on vertical flow secondary settling tank is less.At present,the design of secondary settling tank is mainly based on the ideal sedimentation model,ignoring the influence of reflux ratio on the flow field and sludge concentration and mass distribution of secondary settling tank.The computational fluid dynamics simulation is used to study on the fluid motion characteristics of water treatment structures,so as to optimize and improve the operation design of structures,and it is gradually becoming a new idea in design work.The experimental method requires a lot of time,economy,and labor costs,therefore,this thesis adopts the method of computational fluid dynamics.According to the actual working conditions,the improved RNG k-epsilon two-equation model and the Mixture model are used to simulate the flow field and sludge concentration and mass distribution of secondary settling tank,which provides a theoretical basis for the design and operation management of the actual project.The results show that:(1)The flow in the separation zone of the secondary settling tank under different hydraulic loading is basically vertical upward,which is in line with the ideal distribution of the flow in the vertical flow secondary settling tank.The smaller the hydraulic loading,the flow in the mud is more stable,the bottom sludge is more compressed and the concentration of returned sludge is higher.In comparison,q=0.5 m~3/m~2·h is more suitable for secondary settling tank,which is consistent with the hydraulic loading of the secondary settling tank recommended in many existing design codes.(2)The sludge flocs with good coagulation and sedimentation performance(the critical particle size bigger than 0.10 mm)can indeed improve the efficiency of solid-liquid separation and sludge sedimentation reflux.The larger particle size is more conducive to the removal of suspended matter,the effluent quality is better.Therefore,maintaining good activated sludge flocculation performance is crucial to the stable operation of the system,which is consistent with many existing conclusions.(3)Under the optimal hydraulic loading condition,the lower the concentration of inlet sludge concentration,the stability of the flow field in the separation zone is higher and the sludge concentration in the overflow is lower.2 g/l?5 g/l is the sludge concentration that can be chosen under q=0.5 m~3/m~2·h,but the sludge accumulation of 5 g/l is serious,so 2 g/l?4 g/l is the optimal sludge concentration that can be chosen under q=0.5 m~3/m~2·h.(4)The sludge concentration range that can be withstood by the superior load does not change after changing the reflux ratio.However,the reflux ratio corresponding to the sludge concentration is different.Under the conditions of inlet sludge concentration of 2 g/l,the 100%reflux ratio can meet the effluent standard and the highest return sludge concentration.Under the conditions of inlet sludge concentration of 3 g/l?4 g/l and 5 g/l,the 125% reflux ratio can meet the effluent standard and the highest return sludge concentration.Under the condition of inlet sludge concentration of 6g/l,although the suspended solids of outlet can not reach the standard,the 125% reflux ratio can greatly reduce the concentration of the suspended solids and meet the highest return sludge concentration.(5)The majority of activated sludge can be supplied to the aeration tank by sludge reflux.The larger the reflux ratio,the higher the theoretical ratio of reflux concentration is.To maintain a higher sludge concentration in the aeration tank,the sludge reflux ratio must be increased.However,the increase of the theoretical ratio of reflux concentration is not proportional to the increase of the reflux ratio.In this study,the theoretical ratio of reflux concentration under the reflux ratio of 100% can only reach 150% of the theoretical ratio of reflux concentration under the reflux ratio of 50%.Under the same reflux conditions,the higher the sludge concentration,the higher the sludge concentration of sluge outlet is,but lower the theoretical ratio of reflux concentration is.