| Low-pressure membrane filtration technology,such as microfiltration(MF)and ultrafiltration(UF)has gained increasing popularity in municipal/domestic and industrial wastewater treatment,however,it is not uncommon to encounter surface fouling that limits its productivity and efficiency.Careful manipulation of hydrodynamic conditions is a good approach for mitigating membrane fouling,and the intrinsic memchanism is that shear stress can enhance the particle back transport form the membrane surface,further influencing the transport phenomenon of particles.In the field of water treatment,the filtration of suspension is always consisted of several foulants(defined here as “mixed” suspension),not single compounds,and the fouling induced by inorganic particles and organic collodials,is defined as “combined fouling”.Moreover,the complexity of shear stress on membrane surface contributes to the diversity of combined fouling formation.In this work,computational fluid dynamic(CFD)method was applied to calculate the shear stress on membrane surface,and the hydrodynamic results were related to membrane fouling.The purpose was to investigate the impact of shear stress on combined fouling formation considering particles perperties,particles motion and particle size,and to comprehensively analyze the impact of shear stress distribution on mitigating combined fouling in real application.Firstly,varied mass ratio of inorganic particles/organic collodials was employed to form mixed particles with different properties,and to investigate the impact of shear stress created by stirring on formation of combined fouling induced by particles with varied properties.The cake layer buildup behaviour was monitored using FDG method,which was used to characterize the variation of cake structure.Additionally,shear stress was calculated to analyze the combined fouling controlling conditions,as well as their correlation with various operation flux and inorganic particle concentration.The findings showed that under lower mass ratio of inorganic particles/organic collodials,HA could adhere to inorganic particles,typically induced a change in surface charge and could enter into the voids of cake layer,leading to a relatively dense cake layer and extremely severe membrane fouling.Undesirable membrane fouling could be controlled by shear stress under lower flux because of sufficient back-transport forces,in this case,increasing shear stress could lead to a significant decrease of resistance decline whereas the decline was largely independent of shear stress when the flux was larger.In the case of lower mass ratio of inorganic particles/organic collodials,shear stress could exhibit an excellent performance on combined fouling controlling because of slight interaction forces between particles on the vicinity of the membrane.In the case of high mass ratio of inorganic particles/organic collodials,shear force was unable to overcome the particle-particle interactions.Secondly,shear stress induced by bubbly flow was applied to control combined fouling formed by mixed suspension.In this work,the forces exerted on the fine particles was analyzed in terms of calculated shear stress,which was used to investigate the capacity of shear stress on “deposition particle” and “streaming particle” removal.The results showed that continuous air bubbly flow exhibited a superior impact on alleviating membrane fouling.In detail,aeration intensity increasing(0-3.2 L/min)led to an increasing average shear stress(0.505-2.110 Pa),corresponding to a rapidly decreasing fouling rate.It should be emphasized that small streaming particles were likely to attach to the layer,forming a deposition particle,and a decreasing critical diameter(dc)(2.295-0.785 ?m)was obtained with increasing average shear stress.Additionally,deposited particle could not be scoured away from the layer,which was owing to the fact that Fe force was larger than the corresponding lift force Fl in the lower particle range.Thirdly,relatively homogeneous distribution shear stress on membrane surface was created by cross flow velocity in flat sheet cell,which was implemented to investigate the relationship between particle size segregation caused by shear stress and the formation of combined fouling on the membrane surface.The results showed that the peak value of shear stress(0.95 Pa-5.17 Pa)and average shear stress(0.267 Pa-1.873 Pa)all increased as the average cross flow velocity increased(0.05 m/s-0.30 m/s).When the combine fouling formed under short-term filtration of mixed particles,the particle size segregation caused by shear stress was found.As a result,the volume fraction of small particles was larger than that in the solution before filtration,whereas the volume fraction of large particles in the solution after filtration was larger than that in the solution before filtration.Under lower cross flow velocity,increasing shear stress could mitigate combined fouling caused by mixed particles;under high cross flow velocity,increasing shear stress could enhance combined fouling,and the main reason was that higher shear stress increase the probability of small particles depositing on membrane surface and large particles moving away from membrane surface to bulk solution,leading to a dense cake layer and severe membrane fouling.Based on aforementioned analysis,relatively homogeneous and heterogeneous distribution shear stress was genetated by bubbly flow and slug flow,respectively,which was used to investigate the impact of shear stress distribution on particles combined fouling in terms of particles properties.The findings showed that both the cake layer resistance and mechanical characteristic were significantly influenced by the particle characteristic.The Rc caused by particle in raw water was much larger than that by pre-coagulated aggregates under the optimal coagulant and coagulant-aid dosage,forming a larger particle size than that in the raw water.Both bubbly flow and slug flow were more pronounced for potential deposited aggregates removal than that for cake layer formed in the case of raw water as MF influent,owing to that the shear stress induced by two-phase flow might be below or above the yield shear stress of cake layer in terms of particle characteristic.It should be emphasized that the peak shear stress induced by bubbly flow and slug flow were all increased with the rising viscosity of MF influent,and the peak shear stress value generated by slug flow(15 Pa)was larger than that by bubbly flow(1.4 Pa)at the same air usage.However,bubbly flow creates relatively more homogeneous shear stress,while slug flow creates more regions with lower shear stress,resulting in more aggregate solids coating on the corresponding membrane area.From a practical point of view,bubbly flow generated the more widely spread shear stress and was a feasible approach for aggregates removal during coagulation-microfiltration process in terms of same air usage. |
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