The relationship between size-segregated particles migration phenomenon and combined membrane fouling in ultrafiltration processes: The significance of shear stress

Shear stress generated by cross flow velocity (CFV) was employed to investigate the migration of particles with varied sizes ranging from 0.5 µm to 100 µm in a laboratory flat sheet cell, which was implemented to investigate particle size segregation during the formation of combined membrane fouling. Computational fluid dynamic simulations of shear stress on the membrane surface over a range of CFV indicated that the peak value of shear stress (0.95–5.17 Pa) and average shear stress (0.267–1.873 Pa) all increased as the CFV increased (0.05–0.30 m/s). In case of the combine membrane fouling formed under short-term (6 h) filtration of mixed solution, the particle size segregation caused by shear stress was demonstrated using a laser scattering particle analyzer. In details, the volume fraction of small particles within cake layer was larger compared to that in the initial solution, whereas the large particles were susceptible to remain in the end solution. Under lower CFV, increasing shear stress could mitigate combined membrane fouling caused by organic matter and inorganic particles, however, under high CFV, increasing shear stress could enhance combined fouling, and the main reason was that higher shear stress increased the lift force exerted on particles, which could help large particles moving away from the membrane surface, leading to small particles depositing on the membrane surface and causing a severe membrane fouling. The implication is that the option of suitable hydrodynamic condition is preferable to reduce membrane fouling of mixed solution in the field of water treatment.