Yasutoshi Shimizu

Cross-flow microfiltration of bacterial cells

Abstract The influence of operating parameters and fluid characteristics on the steady-state flux during cross-flow microfiltration was studied. An equation to predict flux, which related the influence of various factors on flux, was determined by studying latex. An exponential relationship between the steady-state flux, Jss, and the factors was found and expressed as follows: J ss = V L = K · u 1.0 · C p −0.5 · d p 0.67 · ν −1.0 , where V L , u , C p , d p and η are the lift velocity, feed velocity, particle concentration, particle size and viscosity, respectively. This equation was found to be applicable to the filtration of bakers yeast, Bacillus caldolyticus M1, and methanogenic waste. The filtration constant K , as defined by the equation, was the same (3 × 10 −4 kg 0.5 ·m −2.17 ·Pa·s) for the filtration in each case, while it was different for latex. This difference can be explained by estimation of the water content of the particles, which allows a correction to be made in particle concentration ( C p ).

Cross-flow microfiltration of activated sludge using submerged membrane with air bubbling

Abstract For the purpose of developing a wastewater treatment membrane bioreactor system, cross-flow microfiltration of intermittently aerated activated sludge was carried out, as a part of the solid-liquid separation process in the bioreactor. The tubular alumina microfiltration membrane, having a pore size of 0.5 μm, was submerged in the activated sludge. A cross-flow stream over the membrane surface was produced by air bubbling, generated by a diffuser situated underneath the membrane. An exponential relationship between the steady-state flux, J ss , and the causative factors such as operating parameters and fluid characteristics was found and expressed as J ss =V L =K′u ∗1.0 MLSS −0.5 , where V L , K ′, u ∗ and MLSS are the lift velocity, filtration constant, air-liquid two-phase flow velocity and MLSS, respectively. The relationship, as expressed by the equation, is consistent with that for conventional cross-flow filtration using a circulation pump.

Effect of particle size distributions of activated sludges on cross-flow microfiltration flux for submerged membranes

Abstract The influence of the sludge treatment histories on the cross-flow microfiltration fluxes of submerged membranes was examined using 20 types of sludges such as intermittently aerated, continuously aerated activated sludges and anaerobically digested sludges. The fluxes of the sludges varied from 0.5 to 1.7 m 3 ·m −2 ·d −1 under the same MLSS and filtration conditions. The variation of the flux was not explained by the different physical properties of the sludges such as mean particle sizes, viscosities and specific filtration resistances. Instead, the variation of the flux was quantitatively interpreted by considering the concentration of particles with sizes from 8 to 15 μm, which cause the lowest lift velocity values. The assumption, that the particles of this specific size range controlled the flux, was rationalized by the filtration model proposed for the filtration of the mixture of differently sized particles.

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 bakers 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.