Flocculation And Sedimentation Characteristics Of Flocs Under The Influence Of Nonionic Polyacrylamide

Due to the advantages of non-toxicity,environmentally friendly,low cost and high efficiency,Nonionic Polyacrylamide(NPAM)as a common chemical raw material is widely used in sewage treatment and industrial wastewater treatment.The role of NPAM is to accelerate the flocculation and sedimentation of suspended particles or pollutants,then improve the efficiency of water treatment.The flocculation and sedimentation process of NPAM is a dynamic nonlinear process with flocculation-disintegration-reflocculation-redisintegration.It is a cyclical process.When the dosage of flocculant,the settling velocity of floc is unreasonable,it is easy to lead to unreasonable design of sedimentation tank and low efficiency of water treatment.Therfore,how to quantify the flocculation and sedimentation process of flocs reasonably is an important issue need to be studied and solved in the flocculation field.To slove the problem,on the base of some shortcomings in the current researches,with the methords of river dynamics and computational fluid dynamics,three aspects are as follows: quantified the sedimentation process of floc reasonably,simulated the phenomenon of flocculation and disintegration effectively,evaluated the influence of turbulence during flocculation.This research has important practical significance for rational design of sedimentation tank and improvement the efficiency of wastewater traeatent.It also has high theoretical value for deepening research on flocculation mechanism.Then the main work and results are as follows:(1)Based on the sedimentation experiments and microscopic observation tests of NPAM flocs,the law of floc's settling velocity was explored and the flocculation mechansiam of NPAM was demonstrated.During the process,there has an optimal concentration of NPAM flocculant to the highest rate of sedimentation.Flocs formed by sand with 2D fractal dimension between 1.044 to 1.415.Macroflocs is formed through the point and the edge or the edge and the edge of microflocs.The mechanism of NPAM flocculant was demonstrated by the image of microflocs(macroflocs).(2)Settling velocity formula of NPAM flocs was calculated based on the flocculation mechanism of adsorption and bridging of NPAM.It was verified that the calculated results of the improved settling velocity formular agreed well with the measured data.Compared with the other settling velocity formular derived from the mechanical point of view,the influence of flocculant in the wastewater treatment and the morphological characteristics of the flocs are all considered in the formular.It is more in line with the actual wastewater treatment.(3)Settling velocity model of NPAM microflocs/macroflocs based on the turbulent dissipation rate was established,which concerned turbulent shear effect.The turbulent shear eigenvalues,turbulent dissipation rate,microflocs/macroflocs scale and Kolmogorov scale and other correlations were analysised.Then the calculation formular of turbulent dissipation rate and the relationship between flocs scale and Kolmogorov scale were obtained.It was verified that the calculated results were in good agreement with the simulated results,and were also consistant with the experiments results.Compared with the single turbulent shear rate,the model not only considers the influence of turbulent shear effect,but also reflects the relation between the size scale of micro-floc/macro-floc and the scale of turbulent eddy.It has further understanding the relationship between flocculation and turbulent eddy.(4)Model of NPAM flocculation and sedimentation was constructed coupled with the population balance module(PBM).The sedimentation process of flocs is really complicated.It is a dynamic process involving turbulent flow,multiphase flow and flocculationdisintegration.To better understand the process,the Computational Fluid Dynamics Mode(CFD)and the Population Balance Model(PBM)were elaborated.Compared with the DLCA or RLCA model,which requires specified adsorpting spacing and expected results artificially,the calculation criteria of the PBM are from the calculated results of CFD.The external interference factors are more few;the theory is more perfect and they can simulate the flocculation and disintegration more vivid and intuitively during the process.(5)Three stages of flocculation and sedimentation of flocs were recognized by the methord of ESMD(Extreme-point Symmetric Mode Decomposition).The first stage is mainly the sedimentation of particles(0-15s);the second stage is mainly the sedimentation of microflocs(15-24s);the third stage is mainly the sedimentation of macro-flocs(24-30s).Moreover,the frequency oscillation phenomenon is accompanied by the transition at different stages.It is a process of achieving dynamic equilibrium of micro-floc/macro-floc's settling velocity,turbulent kinetic energy and turbulent dissipation rate.