Effect Of Different Characteristic Foulants And Their Coexistence On NF/RO Membrane Fouling In Raw Water

Nanofiltration（NF）and Reverse osmosis（RO）are widely used in feedwater treatment,reclaimed water reuse and wastewater treatment,but membrane fouling is a bottleneck limiting their popularization and application.Colloidal fouling is considered as one of the major types of fouling for both RO and NF.Humic acid,polysaccharide,protein,colloidal silicon and colloidal iron in natural water are important colloidal foulants.Although water quality characteristics such as colloid size,charge,hydrophilicity and coexistence of different foulants have important effects on membrane fouling,there are still controversies and no in-depth studies at the micro level.The thesis adopted a coupled XDLVO theory and collision-attachment membrane fouling model and deeply explored the effect of colloid characteristic and different foulants coexist on membrane fouling from colloidal-membrane interface interaction,hydrodynamic drag and back diffusion,so from the perspective of the water quality characteristics to provide theoretical basis for the control of membrane fouling.The main research work and conclusions are as follows:（1）In the collision attachment membrane fouling model,membrane fouling is regarded as two processes of colloid transport collision to membrane surface and then attachment to membrane surface.The membrane fouling rate is determined by the collision frequency and attachment coefficient（probability of successful attachment）.The attachment coefficient is determined by boltzmann distribution,and its value strongly depends on the energy barrier,hydrodynamic drag and diffusion.In this thesis,XDLVO theory was used to further relate colloid size,colloid Zeta potential,colloid contact angle with colloid-membrane interface.Experimental validation of protein-fouled NF membrane under different pH values showed that simulated water flux was in good agreement with the experimental results,proving the accuracy and usability of the model.（2）The effects of Zeta potential and contact angle on membrane fouling were studied quantitatively by using the model.The results show that Zeta potential ζ and contact Angle θ have important effects on the membrane fouling ratedm_f/dt.Whenζ~2（≤20~2 mV~2）value is low,serious membrane foulant occurs due to lowΔE_b between foulant and membrane.Increasingζ~2 value can greatly reduce the deposition rate of foulants on membrane surface by increasing the energy barrier.At highζ~2（≥35~2 mV~2）,there is a negative linear relationship betweenζ~2 and the logarithmic form of attachment coefficient and fouling rate.For the influence of contact angle,it is found that there is a critical θ（water θ_w,glycerol θ_g or diiodomethane θ_d）.When the contact angle is higher than（or lower than）the critical θ,the deposition of foulants on the membrane surface will be reduced due to the large increase ofΔE_b,thus the dm_f/dt decreases by several orders of magnitude.Moreover,the critical value of the contact angle θ（θ_w、θ_g、θ_d） occurs at the stage when the AB interaction changes from attractive to repulsive.The results show that it is important to improve critical flux by optimizing contact angle and Zeta potential.（3）The effect of foulant size on membrane fouling was studied based on the model from four mechanisms including the interaction between foulant and membrane,hydrodynamic drag,particle reverse diffusion and cake layer resistance.The results showed that increasing colloid size would increase hydrodynamic drag and decrease Brownian diffusion,thus promoting membrane fouling.At the same time,increasing the colloid size can reduce the cake layer resistance and enhance the foulant-membrane interface exclusion barrier,thus alleviating the membrane fouling.Therefore,the effect of particle size on membrane fouling is the result of competition between these effects.The results further show that when the foulant-membrane interface is dominant,increasing the particle size increases flux stability due to the increased energy barrier against particle deposition.However,when the repulsion of the foulant-membrane interaction is weak,the effect of increasing particle size on flux behavior is reversed.The results of this study can effectively resolve the existing experimental phenomenon about the influence of particle size on membrane fouling.In addition,the regulation direction of raw water pretreatment on colloid size should be determined according to the dominant mechanism.（4）The membrane fouling of colloid blends with different sizes was investigated by using the model.It was found that the flux curves of the blends were between the fluxes of two single size colloid blends.The flux behavior of colloid coexistence with opposite charge was studied,and the collision conglomeration caused by colloid interaction was also considered.The results show that when electrostatic interaction is dominant,the interfacial energy barrier of colloid coexistence with opposite charge is reduced,and the membrane fouling is aggravated.In the case of high ionic strength（i.e.,large inverse Debye constant）,the colloid coexistence with opposite electric properties has little effect on membrane fouling,which is attributed to weak electrostatic action.In addition,under the condition of low initial flux and high ionic strength,colloid blending with opposite charge can alleviate fouling,which is due to the increase of size caused by colloid conglomeration,resulting in the increase of interface repulsion,which makes the colloid particles not easy to deposit.However,in the case of high initial flux,the coexistence of single type or opposite electrical colloid will cause serious membrane pollution,which is due to the dominance of hydrodynamic drag,which will cause serious instability of foulants at the beginning of filtration and lead to rapid flux reduction.Therefore,the effect of colloid coexistence with different charges on membrane fouling needs to be treated dialectically.