Membrane bioreactor advantages

Conversions in two-liquid-phase systems are promising. Although these reactions can be performed in a stirred emulsion system, the use of membrane bioreactors can be advantageous. In addition to retaining the biocatalyst in the reactor, the membrane also serves as a separator between aqueous and organic phase, thus avoiding energydemanding phase separations (Prazeres and Cabral, 1994). 

In recent years, membrane bioreactors, bioreactors combined with membrane separation unit have established themselves as an alternative configuration for traditional bioreactors. The important advantages offered by membrane bioreactors are the several different types of membrane modules, membrane structures, materials commercially available. Membrane bioreactors seem particularly suited to carry out complex enzymatic/microbial reactions and/or to separate, in situ, the product in order to increase the reaction efficiency. The membrane bioreactor is a new generation of the biochemical/chemical reactors that offer a wide variety of applications for producing new chemical compounds, for treatment of wastewater, and so on. 

In particular, membrane bioreactors (MBRs) are today robust, simple to operate, and ever more affordable. They take up little space, need modest technical support, and can remove many contaminants in one step. These advantages make it practical, for the first time, to protect public health and safely reuse water for non-potable uses. Membranes can also be a component of a multi-barrier approach to supplement potable water resources. Finally, decentralization, which overcomes some of the sustainability limits of centralized systems, becomes more feasible with membrane treatment. Because membrane processes make sanitation, reuse, and decentralization possible, water sustainability can become an achievable goal for the developed and developing worlds. 

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... 

Because a blocatalyst-blndlng membrane works as a specific, heterogeneous catalyst that can be used repeatedly. It finds many applications In chemical processes. Several types of reactors have been designed to Incorporate blocatalyst-blndlng membranes. A spiral module reactor has an advantage that pressure-drop Is extremely retarded (22)- The chemical engineering of membrane bioreactors has been reviewed elsewhere (30,31). 

Continuous membrane bioreactors can have advantages. One is the removal of inhibition of cells by the product. This was shown in the oxidation of naphthalene (7.34) by Pseudomonas fluorescens.2ii An ultrafiltration membrane kept the cells and insoluble naphthalene on one side, while the soluble product was removed continuously from the other. The yield was three times that in a batch process. 

Among several techniques used, the advantages offered by membrane bioreactor (MBR) technology have been recognized for some time. An MBR comprises a conventional activated sludge process coupled with membrane separation to retain the biomass. Since the effective pore size is generally below 0.1 pm, the MBR effectively produces a clarified and substantially disinfected effluent. In addition, it concentrates up the biomass and, in doing so, reduces the necessary tank size and also increases the efficiency of the biotreatment process [1]. 

As an immobilization method, both for whole cells or enzymes, membrane bioreactors provide the advantages and drawbacks common to entrapment or adsorption methods. They nevertheless present particular assets. Mass transfer in the porous supports generally used (alginate, k-carrageenan, zeolites, silica) is a diffusion-controlled process, often becoming the overall rate-limiting step. This maybe overcome by the use of membrane modules. This equipment also avoids... 

Consequently, membrane bioreactors are an example of the combination of two unit operations in one step for example, membrane filtration with the chemical reaction. In a typical membrane bioreactor, as weU as acting as a support for the biocatalyst, the membrane can be a very effective separation system for undesirable reactions or products. The removal of a reaction product from the reaction environment can be easily achieved thanks to the membrane selective permeability, and this is of great advantage in thermodynamically unfavourable conditions, such as reversible reactions or product-inhibited enzyme reactions. A very interesting example of a membrane bioreactor is the combination of a membrane process, such as microfiltration or ultrafiltration (UF), with a suspended growth bioreactor. Such a set up is now widely used for municipal and industrial wastewater treatment, with some plants capable of treating waste from populations of up to 80 000 people (Judd, 2006). 

The advantages of using hollow fiber membrane reactors include high membrane surface area that allows high cell density, improvement in mass transfer, reduction in product inhibition and increased productivity. However, there are limitations for the use of membrane bioreactors these include low membrane permeability, selectivity and durabihty and membrane fouling. These factors generally make membrane bioreactors more expensive than conventional bioreactors. 

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. 

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