| Recently,the water quality security of conventional water treatment process has been unable to meet the requirements of humans because of the increasing seriousness of drinking water pollution and the continuous improvement of water quality standards.The unique advantages of ultrafiltration membrane in water treatment have been widely recognized,while membrane fouling is inevitable during the ultrafiltration process.How to guarantee the water quality under the premise of reducing membrane fouling,is a key problem to hinder the promotion using ultrafiltration technology.However,previous studies are always focus on different pretreatments to alleviate the effect of ultrafiltration membrane pollution.The effect of particles morphology on ultrafiltration membrane fouling needs deeply studied.This article firstly study the influence of dispersed colloid particles size distribution on ultrafiltration membrane fouling.The dispersed colloidal particle system was classified on the basis of turbidity,which discussed the evolution of particle size on the effect of ultrafiltration membrane fouling.The results showed that flux decreased by 15% at initial turbidity of 10 NTU,while flux decreased by more than 40% at initial turbidity of 100 NTU.The total number of particles increased in the dispersed system can aggravate the ultrafiltration membrane fouling.The large size particles increased in dispersed colloidal system will obviously alleviate the irreversible membrane fouling.The simulated results showed that particle aggregation process will promote dispersed particles aggregated large size and porosity agglomerates,whicle effectively relief the membrane fouling.The structure of agglomerates changed with the variation of aqueous solution chemistry.When PACl dosing was 20mg/L,the aggregation has large size and loose structure.The large size and loose structure of aggregation are existence in the neural and alkaline aqueous solution.Under low coagulant dosage,the average particle size of aggregation and growth rate increases with the increased concentration of Na+ and Ca+,which is benefit to develop loose aggregation and the proportion of large size particle increased.However,the dosing of Na+ and Ca+ has hardly influenced on the structure of aggregation.There was positive relationship between membrane flux and average size of agglomerates,and negative link between membrane flux and fractal dimension of agglomerates.Breakage-re-growth could effectively improved the morphological characteristics of agglomerates.On the basis of modified three dimensional DLCA model,the breakage-re-groweth model of agglomerates was build.The simulated results showed a agglomerates broken into four parts under high shear force.The number of aggregation increased from 660 to 1630 in the system,which broken agglomerates was two-thirds of the total number agglomerates.Breakage caused by agglomerates size reduced and inceased membrane fouling degree.However,re-growth process could increase the size and porosit of agglomerates and reduce the particle fractal dimension of agglomerates,which effectively alleviate the ultrafiltration membrane fouling.The dynamic suspension layer control micro partice size to change the influence of ultrafiltration membrane fouling was studied.The membrane flux decreased by 17% when the number of micro-particles is about 7000.Micro-particle number has a good correlation between ultrafiltration membrane flux ratio.The hydraulic conditions of forming dynamic suspension layer was simulated.The results showed that 200 rpm of breakage strength was benefit to more uniform size agglomerates getting into sedimentation tank.The new variable velocity-double layers-sedimentation tank increased range of large velocity.Increasing the number of inlets effectively enlarged the region of transition area and increased concentration of particles,which provide favorable conditions to protect more large size agglomerates suspended the transition range.Based on the above results,the reactor of continuous flow flocculation and variable velocity-double layers-sedimentation tank was build.Particle size between 5~8μm was defined as characteristic particles in suspension layer.The factors affected the characteristic particles in suspension layer was analyzed.Setted as 30 mg/L dosage of PACl,250 rpm of breakage strength and shorten sedimentation tank hydraulic retention time by two times could increase in the number of particle number 5-8 μm particles in suspension layer.There was negative correlation between the 5~8μm characteristic particles in suspended layer and ultrafiltration membrane fouling.The existence of dynamic suspended layer can effectively control particle size distribution after sedimentation and reduce the number of micro-particles,which is helpful to alleviate ultrafiltration membrane fouling.The control mechanism of particle characteristic morphology was studied.The results showed that low temperature and low turbidity water are difficult to treat due to the low collision ratio.The adhesion distance and adhesion coefficient of particles can be used to characterize the coagulant.The breakage-regrowth process leads to particle size turns to be uniform,which provide suitable condition for formation of dynamic suspension layer.The permeability relationship between particles and cake layer on membrane surface was studied.The quantitative relationship between particle characteristics parameters and ultrafiltration membrane permeability was build.Smaller fractal dimension and larger porosity of porous particles will have larger penetrative ratio.Higher permeability of particles displayed better permeability of filter cake layer,which indicated the lower fouling degree of membrane.Particle size between 5~8μm in dynamic suspension layer has better relevance with ultrafiltration membrane fouling.The existence of suspension layer can effectively control micro particle size distribution,which reduced the number of particles and alleviate the pollution of ultrafiltration membrane.Addition of 9mg/L PACl after breakage could increase by 30% membrane flux,while addition of 5mg/L non-ionic PAM could increase by 40% membrane flux.The additional coagulant dosing strategy improved characteritic morphology particles agglomorates and reduced the membrane fouling degree. |
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