| Concentration polarization(CP)in nanofiltration(NF)is one of main factors affecting filtration effectiveness,as CP can increase the concentration of rejected solute on the membrane surface and result in a higher transfer resistance of solute molecules through membrane that eventually contribute to membrane foulng.In the present work,the behavior of CP in NF,especially its stabilization time and property parameters,casued by various organic substances was investigated on the basis of cross-flow batch-test experimental studies and mathematic estimation.Accordingly,an innovative technique involving Laser Induced Fluorescence and Particle Image Velocimetry(LIF-PIV)was introduced to in-situ visualize CP development and to characterize CP properties during the dynamic process accurately.The obtained results from experiments and model s would be very important to understand the formation process of CP and its mechanisms that are eventually significant to fouling control and NF sustainable operation.A series of cross-flow nanofiltration tests were carried out to evaluate the CP formation and stabilization processes,and to estimate CP propery parameters during the process of separating natural organic matter(NOM).Comparative analysis of the determination of mass transfer coefficients,CP stabilization time and thickness,solute concentration near membrane surface un der varied operation conditions according to classic methmatical models and NF flux development curves.The underlying correlations among NF flux decline profiles,CP property parameters and NF operational condition parameters,including membrane pore size,solute concentration,cross-flow velocity and applied transmembrane pressure(TMP)were examined.And the results demonstrated that the primary factors affecting NF flux decline rate and CP characterisitics were solute concentration and cross-flow velocity,respectively.The CP characteristics,including its stabilization time,mass transfer coefficients and intrinsic resistence caused by typical organic substance s were determined.It was observed that the mass transfer coefficient of stable CP layer caused by Sodium Alginate(SA)ranged from 7.69 to 28.81×10-4cm/s,which was much lower compared to that caused by HA that ranged from 2.06 to 4.72×10-3cm/s.These rather low mass transfer coefficients caused by SA induced a great solute transfer resistence through membrane,and thus a short CP stabilization time.The major reason of these discrepancies between SA and HA was the great intermolecular force within polysaccharide matrix,which inevitably resulted in enhancement the formation of gel-like layer with dense structure on the membrane surface,and eventually decrease membrane permeability.Moreover,the contribution of CP made to the total membrane resistance related closely with the solute composition and concentration,crossflow velocity and applied TMP,and their synthetical interactions.Visualization of CP layer on NF membrane was successfully obtained by using developed LIF-PIV technique in the home-made cross-flow NF set-up,in which micro-size glass beads were used as tracing particles to accurately indicate fluid velocity vicinity CP layer,while rhodamine 6G was used as fluorescence reagent to demonstrate solute concentration within and near CP layer.Th e LIFPIV results displayed that the concentration within steady-state CP layer during NF process was more than 2 to 6 times higher compared to inlet solute concentration.Meanwhile,it was observed clearly that the CP thick ness caused by HA ranged from 15 to 30 μm that was determined mainly by the solut e compositions.Increasing cross-flow velocity on the NF membrane surface was evidenced as an effective way to mitigate CP formation,which cause the concentration of HA on the membrane surface decreased from 11.94 mg/L to 10.33 mg/L at the first 5 mins as the cross-flow velocity increased from 0.83 cm/s to 2.50 cm/s,and then the HA concentration within the stabilized CP layer decreased from 29.57 mg/L to 25.40 mg/L. |
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