Membrane processes can be implemented either in a dead-end mode, cross-flow
mode, and submerged membranes (Chin et al. 2007a, b; Huang et al. 2007; Choo
et al. 2008a, b; Fu et al. 2006a, b). In the dead-end mode, the membrane acts as a
simple filter, and all the feed flowed through the membrane. The main drawback of
this configuration is the accumulation of particles, colloids, and other substance on
the membrane, leading to the formation of a cake, which can clog the pores of the
membrane. Consequently, such a configuration is not suitable for large-scale industrial applications. The submerged mode is close to the dead-end configuration, but
the design of the reactor allows reducing the membrane fouling. In the cross-flow
configuration, the incoming feed moves parallel to the membrane surface that limits
the deposition of foulants on the membrane surface since the feed flowing tangentially to the membrane removes partially the deposit (Mozia 2010).
Generalities About Photocatalytic Membrane Reactors (PMRs)
Photocatalytic membrane reactors (PMRs) are a promising technology to remediate
drawbacks such as separation of the photocatalyst as well as products/by-products
from the reaction medium in the field of wastewater treatment (Ollis 2003;
Augugliaro et al. 2006; Mozia 2010). In such technology, photocatalysis is coupled
with a membrane process. The role of the membrane is to act as a simple barrier for
the photocatalyst and a selective barrier for the molecules to be degraded (Molinari
et al. 2002a, b).
Some advantages of membrane reactors with respect to conventional ones are
(1) the confining of the photocatalyst in the reaction environment by means of the
membrane, which allows the implementation of a continuous process with simultaneous catalyst and product separation from the reaction environment and (2) the
control of the residence time of molecules in the reactor. As a result, postoperational
separation of the catalyst from the reaction medium involving coagulationflocculation-sedimentation operations is avoided. This allows reducing energy cost
and sizing the installation.
As for conventional (photo) catalytic processes, two kinds of PMRs can be
distinguished: (1) reactors where the catalyst is suspended in the reaction mixture
and (2) those where the catalyst is immobilized on/in the membrane (photocatalytic
membrane) (Augugliaro et al. 2006; Ma et al. 2010; Mozia 2010). In the first
configuration (catalyst is suspended in the reaction mixture), the main drawback is
the membrane fouling by the catalyst particles, especially in the case of pressuredriven processes and more particularly in the cases of microfiltration and ultrafiltration. Photocatalytic membranes, where the photocatalytic reaction occurs on the
external surface and within the pores of the membrane, allow overcoming such
drawback. However, the membrane has to be resistant to UV irradiation, and the
coating of the catalyst on and/or in the membrane results in a decrease of surface area
and consequently of the photocatalytic activity.
As expected, when the pore size of the membrane increases (microfiltration
(MF) > ultrafiltration (UF) > nanofiltration (NF) > reverse osmosis (RO)), the
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B. Lebeau et al.
mode, and submerged membranes (Chin et al. 2007a, b; Huang et al. 2007; Choo
et al. 2008a, b; Fu et al. 2006a, b). In the dead-end mode, the membrane acts as a
simple filter, and all the feed flowed through the membrane. The main drawback of
this configuration is the accumulation of particles, colloids, and other substance on
the membrane, leading to the formation of a cake, which can clog the pores of the
membrane. Consequently, such a configuration is not suitable for large-scale industrial applications. The submerged mode is close to the dead-end configuration, but
the design of the reactor allows reducing the membrane fouling. In the cross-flow
configuration, the incoming feed moves parallel to the membrane surface that limits
the deposition of foulants on the membrane surface since the feed flowing tangentially to the membrane removes partially the deposit (Mozia 2010).
Generalities About Photocatalytic Membrane Reactors (PMRs)
Photocatalytic membrane reactors (PMRs) are a promising technology to remediate
drawbacks such as separation of the photocatalyst as well as products/by-products
from the reaction medium in the field of wastewater treatment (Ollis 2003;
Augugliaro et al. 2006; Mozia 2010). In such technology, photocatalysis is coupled
with a membrane process. The role of the membrane is to act as a simple barrier for
the photocatalyst and a selective barrier for the molecules to be degraded (Molinari
et al. 2002a, b).
Some advantages of membrane reactors with respect to conventional ones are
(1) the confining of the photocatalyst in the reaction environment by means of the
membrane, which allows the implementation of a continuous process with simultaneous catalyst and product separation from the reaction environment and (2) the
control of the residence time of molecules in the reactor. As a result, postoperational
separation of the catalyst from the reaction medium involving coagulationflocculation-sedimentation operations is avoided. This allows reducing energy cost
and sizing the installation.
As for conventional (photo) catalytic processes, two kinds of PMRs can be
distinguished: (1) reactors where the catalyst is suspended in the reaction mixture
and (2) those where the catalyst is immobilized on/in the membrane (photocatalytic
membrane) (Augugliaro et al. 2006; Ma et al. 2010; Mozia 2010). In the first
configuration (catalyst is suspended in the reaction mixture), the main drawback is
the membrane fouling by the catalyst particles, especially in the case of pressuredriven processes and more particularly in the cases of microfiltration and ultrafiltration. Photocatalytic membranes, where the photocatalytic reaction occurs on the
external surface and within the pores of the membrane, allow overcoming such
drawback. However, the membrane has to be resistant to UV irradiation, and the
coating of the catalyst on and/or in the membrane results in a decrease of surface area
and consequently of the photocatalytic activity.
As expected, when the pore size of the membrane increases (microfiltration
(MF) > ultrafiltration (UF) > nanofiltration (NF) > reverse osmosis (RO)), the
66
B. Lebeau et al.