| Identifying sewage treatment unit structure(equipment)and its operation mechanism has great practical significance to enhancing the pollutants removal efficiency.The grit chamber is a kind of pretreatment structure mainly used to remove ISS(Inorganic suspended solids)in urban sewage treatment process.Efficiently removing ISS is a key step for maintaining the sludge activity of the biochemical unit and conducting resourcelization treatment of surplus sludge.The rotational flow grit chamber is widely used in the pretreatment unit.As the foreign patent technology,the key parameters of this introduced technology have not been disclosed.Together with the fact that the inorganic particles in the inflow of China urban sewage treatment plants are so much small which are a far cry from the contents of the design specification and manual,so that the ISS removal ability of the grit chamber is extremely low,the biochemical unit of the urban sewage treatment plant is silted seriously,and the operation efficiency of the equipment is far from satisfactory.Therefore,in view of the grain size of the inorganic sand in the urban sewage,discussing the grit removal process and mechanism of the rotational flow grit chamber presents great significance to unit equipment development,engineering design and the scientific management of the structure.In this paper,based on the characteristics of ISS of different size grading and the optimization of turbulence model,numerical simulation and physical model experiment were combined to study the flow state characteristics of Pista grit chamber and the factors influencing the ISS removal efficiency.Besides,orthogonal regression method was adopted to work out the mathematical relationship between ISS removal efficiency and the factors,and thus to determine the optimal operation parameters and conditions of Pista grit chamber.The main research content and conclusions of this paper are as follows:(1)Based on the type II rotational flow grit chamber(12)in “Water Supply and Drainage Design Manual 5—Urban Drainage”(“Urban Drainage”)and according to flow similarity principle,the numerical simulation of Pista grit chamber was constructed and the physical model for confirmatory experiment was designed as well.(2)Eulerian-Lagrangian method was adopted to simulate solid-liquid two-phase flow of rotational flow grit chamber,UnSteady Discrete Model was adopted to simulate solid phase,liquid phase was simulated by RNG k-?model(RNG model),Realizable k-?model(Real model)and SST k-? model(SST model),respectively,and compared with validation experiments for optimization.The results showed that,1)when ISS particle size was less than 200?m,RNG model could better match the results of validation experiments,and the average error was 7.10%(d?100?m)and 6.87%(100?m?d?200?m),respectively;when ISS particle size was more than 200?m,Real model has the minimum average error(4.16%)with validation experiments.2)The movement characteristics of bed load transport and suspended load transport of ISS with different particle size grading were the important factors resulting in the difference in ISS removal efficiency.In the three kinds of turbulence models,the distribution of pressure gradient and turbulence intensity were significantly different.ISS with different particle sizes had different disturbed effects on pressure difference,which had different degree of influence on the accuracy and reliability of simulation results.(3)According to the results of the optimization of the turbulence model,RNG model and Real model were adopted specific to ISS of different grain sizes to analyze the single factors influencing the grit removal efficiency of Pista grit chamber and discuss the influence of hydraulic retention time,stirring paddle rotate speed,stirring paddle dip angle and the relative height from the stirring paddle to the bottom of the grit chamber on the ISS removal efficiency.The results show that: 1)in all the single factor experiments under the experimental conditions,at the highest removal rate of ISS of different size grading in the Pista grit chamber,the HRT is 30 s,stirring paddle dip angle is 60°,and stirring paddle height is 60mm;2)when the grain size is less than 100?m,within 100?m~200?m or more than 200?m,the ISS removal rate can reach the maximum when the stirring paddle rotate speed is at 25r/min,20r/min and 15r/min,respectively;3)the value ranges of each factor of orthogonal experiment were determined as below: HRT is 20s~50s,stirring paddle rotate speed is 10r/min~30r/min,stirring paddle dip angle is 45°~75°,and the relative height from the stirring paddle to the bottom of the grit chamber is 40mm~80mm.(4)Based on the results of single factor experiment,the orthogonal regression analysis of the four factors was made.Specific to the numerical simulation model,the regression equation of ISS removal efficiency was obtained,and the suitable operating parameters of the optimal ISS removal rate are as below: 1)when the grain size d?100?m,the operation parameters are 35 s,30r/min,60°,and 60 mm,the theoretical optimal ISS removal rate is 16.57%;2)when 100?m< d?200?m,the operating parameters were 27.5s,20r/min,60°and 60 mm,the theoretical optimal ISS removal rate is 84.26%;3)when d>200 m,the operating parameters were 27.5s,15r/min,60°,and 60 mm,the theoretical optimal ISS removal rate is 96.99%.(5)In the results of ISS removal efficiency,single factor experiment is different from orthogonal regression experiment in terms of the value of each factor.This shows that there is correlation and interaction among the influence of hydraulic retention time,stirring paddle rotate speed,stirring paddle dip angle and the relative height from the stirring paddle to the bottom of the grit chamber on the ISS removal efficiency.In the inflow of China municipal sewage treatment plants,the ISS grain size and concentration are different and the inorganic particles have very small diameter.Together with the ambiguous design parameters of the rotational flow grit chamber in urban sewage treatment process,ISS removal rate of the grit chamber is very low.The innovation part of this paper is to figure out the optimal turbulence model for different grain sizes through comparing and verifying the simulation results of different turbulence models and the experimental results,based on which the single factor experiment was conducted to determine the range of each factor of the orthogonal experiment,and thus obtaining the regression equation and optimal conditions specific to different ISS grain sizes.This research method has good theoretical and practical significance for the optimization design,configuration development and operation management of the rotational flow grit chamber. |
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