Effect of crossflow regime on the deposit and cohesive strength of membrane surface fouling layers

Journal article, 2019

Acquiring knowledge of the properties of membrane fouling layers is crucial to mitigating fouling and developing cleaning strategies. The cohesive strength of these fouling layers, which determines the cleaning requirement of the membrane, is nevertheless rarely investigated. Here we introduced fluid dynamic gauging (FDG)to the crossflow microfiltration of a wood material, namely microcrystalline cellulose (MCC, nominal particle size 20 μm, 95% (in volume)of the particles are bigger than 5.4 μm and smaller than 56.4 μm), to study in situ the cohesive strength of the membrane surface fouling formed under different crossflow regimes. Using regenerated cellulose membrane with a nominal pore size of 0.2 μm, filtration experiments with FDG measurement show that the crossflow regime can lead to the formation of surface fouling layers with distinct cohesive strength. Fouling formed in turbulent/transitional crossflow (Reynolds number, Re duct = 4170)was stronger and its removal required more liquid shear stress compared to the layers formed in laminar crossflow (Re duct = 1560). The fouling layers that can withstand the minimum shear of 35 Pa from the FDG sensor with turbulent/transitional crossflow were, on average 294 ± 10 μm thick, in contrast to those formed in laminar crossflow, which were significantly thinner (144 ± 73 μm at 35 Pa shear stress, p < 0.05). On the other hand, turbulent/transitional crossflow reduced material deposition significantly (p < 0.05). After 1000 s filtration, 0.117 ± 0.003 kg m −2 MCC were found on the turbulent/transitional crossflow membranes, compare to 0.134 ± 0.005 kg m −2 in the laminar crossflow situation. Moreover, a similar permeate flux was observed in all experiments. Therefore, this work also highlights the necessity of developing membrane cleaning protocols based on the fouling layer properties, rather than on the permeate flux decline.