Membrane bioreactors biocatalyst

Emerging pollutants removal by biocatalyst membrane bioreactors... 

A membrane bioreactor in which the aqueous biocatalyst solution, with mineral nutrients and an assimilable source of carbon is separated from the feedstock by a membrane which provides the active contact surface for desulfurization. 

Membranes have the property to retain one or more components of a hquid mixture, whereas others may pass. The size of the biocatalyst often differs considerally from the size of the product molecules. The pore size of the membrane must thus be chosen in such a way that the product can pass the membrane, while the biocatalyst is retained. Usually membrane bioreactors consist of ultrafiltration... 

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). 

Membrane bioreactors have been reviewed previously in every detail [3,4,7,8,18], There are two main types of membrane bioreactors (i) the system consists of a traditional stirred-tank reactor combined with a membrane separation unit (Figure 14.1) (ii) the membrane contains the immobilized biocatalysts such as enzymes, micro-organisms and antibodies and thus, acts as a support and a separation unit (Figure 14.2). The biocatalyst can be immobilized in or on the membrane by entrapment, gelification, physical adsorption, ionic binding, covalent binding or crosslinking [3, 7, 18]. Our attention will be primarily focused on the second case where the membrane acts as a support for biocatalyst and as a separation unit, in this study. The momentum and mass-transport process, in principle, are the same in both cases, namely when there is... 

Figure 14.2 Membrane bioreactor with immobilized biocatalysts (enzyme or micro-organism).

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

The OHLM systems, integrating reaction, separation, and concentration functions in one equipment (bioreactor), find a great interest of researchers in the last few years. A bioreactor combines the use of specific biocatalyst for the desired chemical reactions, and repeatedly or continuously application of it under very specific conditions. Such techniques were termed as hybrid membrane reactors. In biotechnology and pharmacology, these applications are termed as hybrid membrane bioreactors or simply bioreactors (see Table 13.11). Experimental setup of the bioreactor system is shown schematically in Figure 13.17. 

The most suitable driving force in BI is the reduction of the diffusion path that already operates in transport processes across biological bilayers. Consequently, biocatalyst membranes and specially designed bioreactors, such as jet loop and membrane reactors, are available to intensify biochemical reactions. " " Supported biocatalysts are often employed to enhance catalytic activity and stability and to protect enzymes/ microorganisms from mechanical degradation and deactivation.f Immobilization of the cells is one of the techniques employed to improve the productivity of bioreactors. 

Some of the efforts, so far, to model such membrane bioreactors seem to not have considered the complications that may result from the presence of the biomass. Tharakan and Chau [5.101], for example, developed a model and carried out numerical simulations to describe a radial flow, hollow fiber membrane bioreactor, in which the biocatalyst consisted of a mammalian cell culture placed in the annular volume between the reactor cell and the hollow fibers. Their model utilizes the appropriate non-linear kinetics to describe the substrate consumption however, the flow patterns assumed for the model were based on those obtained with an empty reactor, and would probably be inappropriate, when the annular volume is substantially filled with microorganisms. A model to describe a hollow-fiber perfusion system utilizing mouse adrenal tumor cells as biocatalysts was developed by Cima et al [5.102]. In contrast, to the model of Tharakan and Chau [5.101], this model took into account the effect of the biomass, and the flow pattern distribution in the annular volume. These effects are of key importance for conditions encountered in long-term cell cultures, when the cell mass is very dense and small voids can completely distort the flow patterns. However, the model calculations of Cima et al. [5.102] did not take into account the dynamic evolution of the cell culture due to growth, and its influence on the permeate flow rate. Their model is appropriate for constant biocatalyst concentration. 

Lipases used in laundry detergents and in other bulk applications do not require enzyme immobilization however, an increasing number of applications in synthesis and biotransformation demand an immobilized biocatalyst for efficient use. It has been claimed that the success of a lipase catalyzed biotransformation for the production of certain pharmaceuticals depends on immobilization. For example, in the industrial preparation of the chiral intermediate used in the synthesis of Diltiazem, the lipase from Serratia marcescens was supported in a spongy matrix, which was used in a two-phase membrane bioreactor (Cowan 1996). 

Abstract This chapter describes membrane bioreactors from an engineering point of view. A membrane bioreactor can be defined as a unit operation or a piece of chemical equipment that combines a biocatalyst-fiUed reaction chamber with a membrane system for the purposes of adding reactants or removing products from a reaction. 

The basic principles of bioconversion, bioreactors and biocatalysis are introduced, together with a description of the most important biocatalyst immobilization techniques. The mass transfer phenomena involved in membrane systems are discussed along with some representative configurations of membrane bioreactors, whose behaviour can be described using a simple mathematical approach. For all the aforementioned systems the most significant parameters have been defined to estimate the system performance. 

A bioreactor is a device within which biocatalysts, usually enzymes or living cells, carry out biochemical transformations. A bioreactor is frequently called a fermenter whether the transformation is carried out by living cells or in vivo cellular components, that is, enzymes. A membrane bioreactor can be defined as a unit operation or a piece of chemical equipment that combines a bioreactor with a membrane system. An enzyme membrane reactor is a membrane bioreactor in which the biocatalyst is an enzyme. In a membrane bioreactor, the membrane can be used for different tasks ... 

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 aim of this chapter is to give a detailed overview of the characterization of biocatalysts and the development of membrane bioreactors, in particular, the main aspects of biocatalyst kinetics and their immobilization/ entrapment, either within the porous membrane structure, or on its surface. Thansport models that can help to predict the behaviour of membrane bioreactors, as well as the most relevant theoretical models and operating parameters, are presented below. This data is then analysed in order to ascertain how to improve effectiveness and productivity of the membrane bioreactors. Some relevant fields of application are also discussed in order to show the potential of such systems. 

Membrane bioreactors with continuous biocatalyst recirculation... 

Schematic of membrane bioreactor with continuous biocatalyst recircuiation in the retentate. 

Steady-state flow reactors, with a constant supply of reactants and continuous removal of products, can be operated as both a continuous stirred-tank bioreactor (CSTB) and as a plug flow bioreactor (PFB). It is possible to have different configurations of the membrane bioreactor where the biocatalyst is immobilized in the fractionated membrane support (Katoh and Yoshida, 2010). In Fig. 1.6 the scheme of a CSMB in which the biocatalyst is immobilized on the surface of the membrane beads is presented. The biocatalyst immobilized in the porous structure of a fractioned membrane can also be operated in CSMB. For example, two configurations are shown in Fig. 1.7 (a) for flat-sheet and (b) for spherical porous structures, respectively. Such structures could also be adopted for PFB, where a bed of membrane support with the immobilized biocatalyst could be utilized, in either a fixed or fluid configuration. 

The basic concepts related to biocatalytic reactions, in terms of the kinetics and mass transport phenomena involved, have been introduced in order to aid formulation of more detailed mass balance in the systems analysed during the study. Some specific aspects, such as biocatalyst denaturation, concentration polarization and activity decay, have been also described. In order to predict the performance of a membrane bioreactor, a detailed analysis of the effectiveness of the biocatalyst processes has been also presented. 

Calabrb V, Curcio S, lorio G (2002), A theoretical analysis of mass transfer phenomena in a hollow fiber membrane bioreactor with immobilized biocatalyst , / Membrane Set, 206(1-2), 217-241. 

Biocatalyst Membrane material Bioreactor configuration Application Reference... 

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