The CFD Simulation And Experiment Study On The Enhanced Coagulation Treatment For Micro-polluted Water

Water is one of the basic elements for the production and living of the residents and the development of the national economy.However,due to the limitation of hydrogeological condition,the resource of drinking water is scarce in the vast rural area of western China.Along with the rapid economic development of our country,the ecological environment affected significantly in rural area,the water source had polluted in varying degree,and rural residents have problems with drinking water safety.Enhanced coagulation treatment can effectively treat the micro-polluted water,adopt different strengthening measures in the coagulation process to ameliorate the mixing reaction conditions of coagulant and increase the removal of natural organic matter in conventional coagulation process,which is suitable for the treatment of micro-polluted water.This paper takes the enhanced coagulation device as the research object,which is utilizing computational fluid dynamics（CFD）to establish the mathematical model and numerical simulation for the tubular micro-flocculation reactor in the enhanced coagulation device.The results of numerical simulation calculations were verified by enhanced coagulation experiments.Combined with theoretical analysis,experimental study and CFD,propose an optimal solution for the enhanced coagulation device.The main contents and results include:（1）Using Gambit software to establish a mathematical model of the tubular micro-flocculation reactor in the enhanced coagulation device and mesh it.According to the numerical simulation calculation and post-processing results,the velocity distribution and turbulent kinetic energy distribution in the reactor are obtained.Finally,the intra-reactor meshing is in the form of non-uniform structured meshing,and its meshing parameters are:the distance from the first node to the inner wall of the reactor is setting to 5×10^-5m and installing 120 nodes in Radial,the maximum aspect ratio of the axial node grid is stayed within 5.Enhanced wall treatment is used in numerical simulation to take into account the efficiency and accuracy of the numerical simulation.（2）By changing the shape and operating parameters of the tubular micro-flocculation reactor to simulate the different flow conditions.According to the simulation results for the velocity vector and the distribution of velocity and turbulent kinetic energy,the different types of reactor affect the patterns of internal flow field significantly,in which the corrugated round tube micro-flocculation reactor is the best type of reactor.The maximum flow rate in the corrugated round tube micro-flocculation reactor is 1.5 times than the rate of inlet flow.The maximum turbulent kinetic energy is 0.1m²/s² and the kinetic energy action area accounts for4/5 pipe diameters.The streamlines are regularly curved.The hydraulic characteristics of water flow in the reactor remains stable under different operating conditions.Therefore,the corrugated round tube micro-flocculation reactor can enhance flocculation of the colloidal particles in water under different operating conditions and treatment effect in the enhanced coagulation process.（3）Orthogonal experimental design was carried out on the numerical simulation calculation.The influencing factors were the type,the inlet velocity and the length for the reactor.The maximum turbulent kinetic energy of the flow field was taken as the research target inside the reactor.Range analysis results of the orthogonal experiments were 369.34,309.67,110.34,which is indicated the type of tubular micro-flocculation reactor for the patterns of internal flow field has different degree of influence:reactor type>reactor inlet rate>reactor length.（4）Under the condition of turbidity 78.2⁹1.2NTU and COD_Mn 10.5¹3.4mg/L,when the PAC and PAM were combined in the enhanced coagulation experiment,the optimum dosage of PAC and PAM are 60mg/L and 0.8mg/L by beaker stirring coagulation experiment.（5）The optimal reactor type of the tubular micro-flocculation reactor was determined to be the corrugated round tube type by the enhanced coagulation experiment,which is consistent with the Fluent numerical simulation analysis results that the corrugated round tube reactor is most favorable for the coagulation of colloidal particles.The best operating condition for the corrugated round tube micro-flocculation reactor is 0.8m/s at the inlet.The optimal length for the corrugated round tube micro-flocculation reactor is 20meters.（6）Orthogonal experimental design for enhanced coagulation experiments is to determine the sequence of influential factors,which is affecting the removal rate of turbidity and COD_Mn in the effluent.The outcome is:reactor type>reactor inlet rate>reactor length,which is identical with the order of influence for maximum turbulent kinetic energy in Fluent numerical simulation analysis results.Finally,the 20m corrugated tube micro-flocculation reactor in the enhanced coagulation device is the best tubular micro-flocculation reactor.In addition,the optimal operating condition of reactor is 0.8m/s inlet rate.The system can achieve good water yield with micro-polluted water.The effluent turbidity is about 1.8NTU and the COD_Mn is about 1.3mg/L.The enhanced coagulation device can achieve a well effluent effect in the treatment of micro-polluted water.The turbidity is about 1.8NTU and the COD_Mn is about 1.3mg/L in the effluent.