Particle deposition on flat sheet membranes under bubbly and slug flow aeration in coagulation-microfiltration process: Effects of particle characteristic and shear stress

Empirical and numerical techniques were used to study particle deposition on flat sheet microfiltration (MF) membranes during surface water treatment under single-phase flow (no aeration) and two phase flow (aeration with bubbly or slug flow) in the presence and absence of metal coagulants and polymers. Pre-coagulation of the particles reduced the hydraulic resistance of the cake layer by 87.5% compared with non-coagulated particles (2.0×10¹² m⁻¹ vs 0.25×10¹² m⁻¹) for the same solids load. Moreover, pre-coagulated particles were more susceptible to the effects of aeration and were more readily dispersed from the filter cake on the membrane. Computational fluid dynamic simulations of shear stress on the membrane surface over a viscosity range of 1.10×10⁻³ to 1.38×10⁻³ Pa s, indicated that the peak shear stress induced by slug flow (15 Pa) was far larger than that by bubbly flow (1.4 Pa) at the same volumetric air flow rate, however, the shear stress under bubbly flow was more evenly distributed over the membrane surface. The cake resistance was higher under slug flow (0.05×10¹² m⁻¹) compared to bubbly flow condition (0.025×10¹² m⁻¹). Moreover, cake redispersion was found to be greater under bubbly flow compared to slug flow. The implication is that bubbly flow is preferable to slug flow for removing deposited aggregates that control flux during the coagulation-microfiltration of surface water using flat sheet membranes.