Study Of Coagulation-Membrane Filtration Technology For Drinking Water Treatment Based On Re-flocculation Control

Since the single-stage and instantaneously finished membrane filtration course increases the instability factor of coagulation-membrane filtration process,process optimization has become more and more urgent.The chopped hydraulic retention time?HRT?may lead to the low utilization rate of coagulant,which can not only cause membrane fouling but also make those metal ions of coagulants penetrate membrane without sufficient reaction so as to arouse re-flocculation after membrane filtration.Aiming at above problems,basing on the summary of previous researches,this study has optimized mechanical mixing coagulation-and aerated coagulation-membrane filtrationprocessesandconductedsystemdynamicssimulationfor coagulation-membranefiltrationprocessbytakingtheintegrated coagulation-membrane filtration process as study object,the theories of coagulation kinetics and fluid dynamics as bases,and the reduction of re-flocculation risk after membrane filtration in drinking water treatment as the goal.The main research results are shown as follows:1.Systematic theoretical research was initially conducted about the“re-flocculation after membrane filtration”issue in coagulation-membrane filtration process of which the concept was made clear and the judgement criteria were proposed.It has been pointed out that the main factors influencing the concentrations of residual aluminum and iron in purified water included pH,coagulant species and those physicochemical properties of organic matter in the water.Besides,those methods for lower the concentrations of residual aluminum and iron have been suggested:?1?Utilizations of novel coagulants or flocculants;?2?Pretreatments of coagulation process;?3?Under-dosing“micro-flocculation”and over-dosing“enhanced coagulation”;?4?Controls of aluminum flocs and aluminum speciation.2.Integrated mechanical mixing coagulation-membrane filtration process was conducted for drinking water treatment.In the condition of optimal FeCl₃ dosage of80mg/L and mechanical stirring speed of 60rpm,the impacts of HRT and floc morphology on Fe³⁺concentration of membrane filtration effluent and membrane fouling were investigated.It was found that the generation?broken?re-flocculation process occurred over and over again in the integrated reaction tank.The particle size and fractal dimension of flocs both presented fluctuated variations,and their average values were higher with the longer HRT,which could mitigate membrane fouling effectively.The Fe³⁺concentration variations in the effluent and the reaction tank basically followed the same trend except for the existence of hysteresis effect caused by the formation of membrane cake layer.The risk of re-flocculation after membrane filtration in drinking water treatment could be reduced effectively by adopting the staged decreasing mechanical stirring intensity coagulation strategy of 80rpm?60rpm?40rpm.3.Taking aerated coagulation-membrane filtration integrated process as research object,combining CFD numerical simulation with experimental measurements,the aeration component configuration of the reaction tank was optimized.The aeration hole design index???was proposed.The flow status of gas-liquid two-phase flow in the reaction tank with different?values were assessed.The experimental results indicated that in the condition of aeration intensity at 7L/min and?=0.75,the high liquid velocity region performed a uniform distribution which nearly filled all the sections of membrane module,and surface shear stress reached the peak.With the shortening of HRT,the fouling of membrane module was accelerated,and the Fe³⁺concentration of the effluent rose in a fluctuant way.In the operating mode of fixed flux,keeping membrane flux lower than the critical value was beneficial to the re-flocculation risk control.4.A system dynamics model of coagulation–ultrafiltration process was developed,and the dynamical behaviors of the process were made clear with the combination of experiment and simulation.The research demonstrated that the cake layer resistance?R_c?was the main cause of the total resistance?R_t?.The level and development of membrane fouling could be mitigated through optimizing membrane flux.Moreover,membrane fouling rate was affected by backwashing cycle.The chemical cleaning time point was suggested in the 16^th backwashing cycle.