Recent progresses in transport phenomena in microfiltration/ultrafiltration, in particular (i) limiting flux in crossflow microfiltration of colloidal suspensions and (ii) mass transfer coefficient in ultrafiltration under high flux conditions, are briefly summarized. This review indicates that we have to consider the situations where the drag force exerted by the flux is balanced by the lift force in the viscous sublayer to discuss (i) limiting flux in cross–flow microfiltration of colloidal suspensions. This strategy is totally different from the gel layer model and the osmotic pressure model, which was developed on the basis of the mass balance concept in boundary layers to discuss transport phenomena in reverse osmosis and ultrafiltration. This review also points out that we have to include the effect of flux on (ii) mass transfer coefficient in ultrafiltration under high flux conditions in the Deissler equation with using Peclet number. This gives us a modified Sherwood relation for estimating the mass transfer coefficient in various cases.

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The research trend of numerical simulation of microfiltration (MF) of particles and its features after 2007 are reviewed. In the around early 2000s, it became possible to perform Direct Numerical Simulation (DNS) that solves the behavior of multi–particles coupled with particles and fluid motion. In the field of MF simulation, a number of simulation studies with a large number of particles have been tried. With the introduction of interaction between particles or between particle and membrane, it became possible to understand qualitative trend and important factors in MF. On the other hand, the fouling time of membrane in the actual scale has not been understood yet. Therefore, the important issue in the future is the scale–up of meso–simulation.

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Microporous membranes for depth filtration capture particles internally, and thus the compaction of the particle layer is partially repressed by the internal structure of the filter to yield a high permeation flux at a low transmembrane pressure. The standard blocking, fiber coating, and deep bed filtration models are typical ones for isotropic depth filters. On the other hand, filtration characteristics of asymmetric filters are much less investigated than that of isotropic ones. We filtered suspensions of polystyrene latex particles (incompressible, spherical, 0.54 µm in diameter) with an asymmetric depth filter, SE20 (nominal pore size ＝ 0.20 µm), at different transmembrane pressures and concentrations. The filtration resistance per unit weight of trapped particles with the depth filter was half of that with a screen filter in the filtration of latex particles. The particles were trapped first near the outlet side of the depth filter and then near the inlet side unlike the filtration models for isotropic depth filters. A filtration model was proposed for the filtration of latex suspension with the depth filter. The model suggested that the asymmetric depth filter showed high performance because the filaments in the filter increased the porosity of the particle layer in contact with them. The permeation flux was 3 ～ 8 times higher with the depth filter than with a screen filter at 10 ～ 150 kPa in the filtration of bacterial cell suspensions. Scanning electron microscopy revealed that the bacterial cells were captured dispersedly from the depth of 2/3 to the inlet side of the depth filter. The dispersed capture was effective to suppress the increase in filtration resistance in the filtration of bacterial cell suspensions with the asymmetric depth filter. The filtration model for the filtration of bacterial cell suspensions has not yet been established. Further studies are necessary to develop the universal filtration model for asymmetric depth filter membranes.

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Microfiltration is widely used to remove microbial cells from the fermentation broth in the downstream processing of biotechnological products and from the mixed liquor in MBR system. Since the filtration behaviors are strongly affected by the properties of the microbial cell cake formed on the surface of the membrane, knowledge of the cake structure provides valuable information for the design and operation of filter equipment. Various approaches have been made on dead–end microfiltration of microbial suspension to examine the filtration characteristics, and it has been clarified that many kinds of microbial cakes exhibit high compressibility. In this article, the analysis of microfiltration behaviors of microbial suspensions based on the compressible cake filtration model is discussed.

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Microalgae can produce various materials for food, pharmaceutical, chemical feedstock and fuel by photosynthesis using carbon dioxide and nutrient–containing water. For the construction of material production system, efficient dewatering of microalgae culture is required because suspended solid (SS) concentration of microalgae culture is low (0.02 ～ 0.05 wt%). Filtration is a promising dewatering method because it consumes relatively low energy and is easy to scale up. In this article, dewatering of microalgae culture by filtration is reviewed. Microalgae cultures are generally dewatered by microfiltration and/or ultrafiltration using polyvinylidene or polytetrafluoroethylene membrane. Although almost complete SS rejection is achieved by the membranes, permeate flux and final SS concentration achieved by filtration are affected by filtration process and operation condition. In the most studies on dewatering of microalgae culture, concentration factor of 10 ～ 100 and permeate flux of 10 ～ 100 L/m²/h by cross–flow filtration are reported. For the further dewatering, filter press, belt press and rotary vacuum filter can be used, and over 15 wt% in SS concentration is achieved. We examined submerged siphon–driven membrane concentration system for a primary concentration and single filter cloth type dehydrator for a secondary concentration as a possible energy–saving process. In this dewatering process, the relatively high permeate flux and high SS concentration were achieved.

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Large volumes of oily wastewater are generated by various industries, such as oil and gas, food, and metal processing. In particular, because of a massive generation of produced water from oil wells, the management of produced water has become an urgent problem. Nowadays, more attention has been focused on the use of ceramic membranes for the removal of stable emulsions from oily wastewater. In this paper, we briefly focus on the produced and refinery wastewater treatment by ceramic microfiltration (MF) and ultrafiltration (UF) membranes, followed by focusing on the fouling during filtration of oil–water emulsions. The fouling mechanism which explains the effect of membrane pore size and surface charges is described. Finally, the development of novel ceramic–based membranes for oily wastewater treatment will be overviewed.

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Application of membrane filtration for large–scale drinking water treatment plant has been significantly expanded in recent years. In addition, development of membrane module for small–scale decentralized water treatment plants is also required. We have developed the submerged membrane module that adopts a PVDF hollow fiber membrane, suitable for the decentralized water treatment plants. By using the lake water, pilot test was conducted with the developed module for membrane performance evaluation. The raw water quality was varied with seasons and rain fall. We demonstrated continuous stable filtration for 6 months under the condition of constant flux (1.2 m³/m²/d) and water recovery rate (98%).

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