Membrane bioreactors aeration process

The chapter focuses on membrane bioreactors where a UF or MF membrane is employed for biomass retention and filtration. However, membrane bioreactors where the membrane provides a support for biofilms are an alternative form of membrane bioreactor for wastewater treatment application. Two processes, in particular, the membrane-aerated biofilm reactor (MABR) and the extractive membrane bioreactor (EMB), have seen significant interest in recent years. Figure 36.4 shows these two technologies schematically. The application of biofilms reactors for wastewater treatment systems is advantageous in view of... 

Fan, F. S., Zhou, H. D. (2007). Interrelated effects of aeration and mixed liquor fractions on membrane fouling for submerged membrane bioreactor processes in wastewater treatment. Environmental Science Technology, 41(7), 2523-2528. 

Membrane bioreactors combine the activated sludge process for wastewater treatment with biomass separation from the mixed liquor by ultra- or microfiltration membranes. Advantages are the superior effluent quality characterized by complete solids removal and disinfection, the small footprint of the plant resulting from more compact aeration tanks, the absence of a secondary sedimentation tank, and the modular construction. 

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. 

As is the case with pure bubble columns and gas-operated loop reactors, most bioreactors in technical use are aerated with oxygen or air. Reactors with pure surface aeration, such as roller bottles, shake flasks and small stirred reactors or special reactors with membrane aeration, are exceptions. The latter are used for the cultivation of cells and organisms which are particularly sensitive to shearing (see e. g. [28 - 29]). The influence of gas bubbles in increasing stress has been described in many publications (see e.g. [4, 27, 29, 30]). In principle it can be caused by the following processes ... 

Figure 3.53 Membrane bloreactor (MBR) system process flow schematic. MBR combines biological degradation with membrane separation. Raw municipal water flows to an aerated bioreactor where the organic components are oxidised by the activated sludge. The aqueous sludge then passes through a MF or UF membrane filtration unit, separating water from the sludge. The sludge flows back to the bioreactor while the membrane permeate is discharged or reused. Source USFilter.

Hesse F, Ebel M, Konisch N, Sterlinski R, Kessler W, Wagner R. Comparison of a production process in a membrane-aerated stirred tank and up to 1000-L airlift bioreactors using BHK-21 cells and chemically defined protein-free medium. Biotechnology Progress 2003 19 833-843. 

The aeration in the bioreactor differs significantly from that in the shake flask. Aeration in shake flasks was accomplished by gas-exchange at the fermentation-broth-air surface area that was generated by die vigorous shaking motion. The bioreactor was aerated by a pump that delivered the moistened gas air bubble-free via a membrane to the reactor. All transport processes in the bioreactor were based on diffusion. 

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