Influence of shear breakage of microbial cells on cross-flow microfiltration flux

Abstract

In the cross-flow filtration of fermentation broth, cross-flow pumping applies shear stress onto bacterial cells. The resulting shear field leads to breakage of bacterial cells. In this study, the influence of cell breakage due to shear stress on filtration flux was examined. Shear stress broke baker's yeast cells and induced discharge of granulated matter such as glycogen and cell wall fragments. These smaller particles contributed to the increase of the mean specific filtration resistance. In the cross-flow filtration, cell breakage due to shear stress reduced the filtration flux because of the increase of the hydraulic resistance of the particle-packed layer, R c, which was formed on the membrane during filtration. This flux reduction was explained by considering the lift velocities, which back-transported the particles from the membrane surface. The lift velocity was expressed as follows: V L = K· u 1.0· C p −0.5· d p 0.67· η −1.0, where V L, K, u, C p, d p and η are the lift velocity, filtration constant, feed velocity, particle concentration, particle size and viscosity, respectively. In the mixture of differently sized particles such as a shear-broken cell suspension, the steady-state flux was given as a value equivalent to the smallest V L, which was derived from the calculation of the V L values for each particle representing particles grouped according to size, using the above equation.