Bubbles submerged membranes

Particular attention is addressed to the permeate flux and to this purpose some preliminary experiments were realized on a different configuration of membrane photoreactor with a submerged membrane module located separately from the photoreactor. Bubbled oxygen on the membrane surface has the roles to reduce the catalyst deposition, to increase the flux through the membrane and to facilitate the photocatalytic reaction. 

Bouhabila, E.H., Ben Aim, R. and Buisson, H. (1998) Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment). Conference on Membranes in Drinking and Industrial Water Production, Amsterdam. 

Over the past decade, there has been an upsurge of interest in the use of gas bubbles to enhance membrane processes. The typical applications include two-phase flow filtration with tubular membranes and submerged membrane systems. A major stimulus for the latter has been the development of MBRs. 

The concept of the helical membrane module has been tested in a submerged membrane filtration mode with bubbling used for the membrane fouling control. Liu et al. [30] showed that the helical membrane with a twisted angle of 180° could achieve a 1.46-1.69 flux enhancement, compared to the membrane modules with 0° twisted angle, in the filtration of 500 mg/L kaolin suspension under a constant TMP of 2.8 and 3.2 kPa. The particle image velocimetry (PIV) analysis [31] showed that the tortured membrane surface of the helical membrane could generate rotational flow near the membrane surface and increase the wall shear rate. 

The idea of intermittent permeation in a submerged membrane module. After the suction pump is turned off, the collection of permeate is stopped and removal of photocatalyst particles from membrane surface by air bubbles starts. 

When necessary, this DIA algorithm can be quickly adapted to account for a specific situation. In Fig. 4.6, a number of postprocessing steps are shown on images created from a fluidized bed with horizontally submerged membranes, where obviously one has to account for the membranes to get the right bubble sizes. 

The early references to submerged membranes came from Japan. Ohkubo et al. (1988) obtained a patent describing hollow fibers in a vertical bundle in a vessel with air scour to vibrate the fibers to remove the cake. The first reported use of submerged hollow fibers in a wastewater membrane bioreactor (MBR) was by Yamamoto et al. (1989), who used fibers in a bundle and air bubbles for aeration, mixing, and induced liquid flow. Permeate was removed by suction. At that time the concept was more of a curiosity, but within a decade the submerged membrane has become the dominant approach for low-pressure membrane processing in the water and wastewater industry. 

The characteristic TMP history for a submerged membrane with cross flow (bubbling) is depicted in Figure 10.2a. Due to deposition TMP rises slowly, and the rate of rise is less with more imposed shear or lower solids. In an idealized situation TMP would remain unchanged at subcritical flux conditions. However, for various reasons (see Section 10.5.1) some degree of TMP rise tends to occur at aU fluxes so the interest is in the acceptable rate of rise and the sustainable flux. Figure 10.2a also includes the potential for a sudden TMP jump that can be observed in prolonged operation at constant flux (discussed further in Section 10.5.1). 

In membrane processing the application of surface shear is required to control concentration polarization and fouling for high solids feeds or to assist cake removal for batch membrane filtration of low solids feeds. For submerged membranes the common practice is to use two-phase bubbly flow to induce surface shear. This section deals with the role of bubbles as well as other hydrodynamic aspects of submerged membranes. 

As pointed out by Cui et al. (2003) there arc two features of submerged membranes in a bubbling system that compheate the critical flux concept for such systems. These features relate to the spatial and temporal variation of filtration conditions, as indicated below. 

In summary, the spatial and temporal features of submerged membranes in bubbly flow complicate the definition of critical flux in these systems. However, in practice it is found that sustainable flux can be achieved and that it is improved by bubbling. 

Wicaksana F, Fane AG, and Chen V, Fibre movement induced by bubbling using submerged hollow fibre membranes, J. Membr. Sci. 2006 271 186-195. 

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