Numerical Study Of Floc Breakage And Regrowth Behaviors Based On DLA And DLCA Models

In the process of water treatment,coagulation is an essential part,and its central task is to form a floc whose particle size,structure and strength are suitable for settling.For a long time,many researcher have studied the performance of flocs under different control parameters in this process.So far,there are little comprehensive studies on the mechanism of floc morphology and settling rate evolution,because of the complexity of various factors,such as the test means and equipment,the hydrodynamic environment,the type of coagulant and so on.In this work,computer simulation was taken as main research method.The process of floc growth,breakage and regrowth were simulated by MATLAB,based on Diffusion-Limited Aggregation?DLA?model and Diffusion-Limited Cluster Aggregation?DLCA?model.In order to discuss the mechanism of floc formation more comprehensively and systematically,a series of flocculation tests were carried out,attempting to combine the simulation results with the actual flocculation process.And the morphology of the floc was studied by the fractal theory.In the process of computer simulation,the structure of floc was quantitatively characterized by the fractal dimension,and the breakage behavior was introduced into the computer simulation process to study the morphological characteristics and the formation mechanism of floc in the flocculation process more accurately and comprehensively.The simulation of floc growth,breakage and regrowth was realized on the MATLAB platform under different control parameters such as aggregated particles number,particle source,adhesive types,movement-region and adhesive-probability,with the help of the improved DLA model.At the same time,the radius of gyration,the void ratio and the fractal dimension were calculated simultaneously.By analyzing the evolution of these characteristic parameters,it was found that,with the increase of aggregated particles number,the size of floc increased and the floc compaction decreased.So it is helpful to improve the density and structural strength of the floc by reducing the adhesion probability properly.Moreover the structure,density and sedimentation ability of flocs were improved significantly after a suitable degree of breakage and subsequent reformation.When compared with the DLA model,the DLCA model is closer to the process of the actual floc formation.But its simulation mechanism is more complicated,and the computer performance requirements are higher.With the help of DLCA model,the influence of different aggregated particles number,movement-region,adhesion-probability and particle-density on the floc aggregation,breakage and regrowth behaviors and the evolution of the floc morphological characteristics were investigated in the simulation process.By analyzing the variation of the radius of gyration,void ratio and fractal dimension,it can be concluded that,compared with DLA floc,the structure of DLCA floc was more open and porous.With the increase of the iteration step number,the particles in the region collide with each other to form a small floc,and the small floc aggregates together to form a porous structure with a distinctly fractal structure.It was found that a larger aggregated particles number result in a looser and more porous structure of floc.Nevertheless a lower adhesion probability can improve the density and the structure strength of the floc.In the process of simulating floc formation based on DLCA model,the initial particle density has a significant effect on the later floc structure.Except computer simulation method,a series of flocculation tests were carried out in square stirred-tank reactors with different geometrical configurations.Polyaluminum chloride?PACl?was selected as the coagulant,and the testing water sample was made of kaolin clay.In the experiment,the hydrodynamic characteristics of the flow in the reaction system were changed by setting different baffle width and the height-to-width ratios of the square stirred-tank reactors.As a result,the dynamic process of the floc aggre-gation,breakage and regrowth behaviors was changed,at the same time,the change of the floc morphology in the flocculation process was monitored by the online floc monitoring system.In order to investigate the variation of the flocculation performance under different conditions,the turbidity measurements were undertaken after settling for every 4min by the online turbidimeter.It was found that,the shear rate has great influence on the evolution of the floc morphological characteristics in flocculation process,and the most suitable shear rate for flocculation is about G_ave=30s^-1.When the shear rate is low,the collision rate between cluster,small floc and particles was insufficient,as a result the growth process of floc were slowed.On the contrary,the increased shear rate resulted in a higher collision frequency and at the same time and a more pronounced breakage behaviors.The structure,density and sedimentation ability of flocs were improved significantly after a suitable degree of breakage and subsequent reformation,which is same as the conclusion of the numerical study.In addition,the setting of baffle in the reactor can increase the mixing effect of the axial flow in the reactor and improve the mixing efficiency,and simultaneously,the dead zones might be formed.The setting of the height-to-width ratio in the reactor can affect the circulation characteristic of the fluid in the reactor,and improve the mixing effect and the local hydraulic characteristics.The suitable baffle width and height-width ratio can remarkably improve the flocculation effect and the effluent quality.The different height-to-width ratios can significantly affect the circulation characteristics of fluids in the reactors,and then improve the mixing effect and local hydraulic properties.A suitable baffle width and height-to-width ratios can significantly improve the flocculation and effluent quality.