TiO 2 Suspended in the Reaction Mixture
In such configuration, the photocatalytic process is ensured by TiO 2 introduced as a
slurry in the incoming feed and the membrane acts as a filter for TiO 2 particles, thus
ensuring the permeate to be free of TiO 2 particles. Indeed, most of the studies report
the high efficiency of the membrane filtration resulting in a very low concentration of
TiO 2 particles in the permeate (Meng et al. 2005; Huang et al. 2007; Mozia and
Morawski 2009). For reactors where the catalyst is suspended in the reaction
mixture, three main configurations can be distinguished: irradiation of the feed
tank, irradiation in a specific module (photoreactor) located between the feed tank
and the membrane, and irradiation of the membrane.
One advantage of such process is the photocatalytic efficacy given that TiO 2
particles are not embedded in the membrane, allowing a higher surface area available
for the photocatalytic process. It also allows obtaining a higher permeate quality and
prevents the membrane degradation by UV radiation. On the other hand, the main
drawbacks are membrane fouling, especially by TiO 2 particles, leading to the decline
of the permeate flow (Molinari et al. 2006; Molinari et al. 2008). As a general trend,
the higher the TiO 2 particles concentration is, the fastest the fouling of the membrane
is (Sopajaree et al. 1999a, b; Shon et al. 2008). The membrane fouling strongly
depends on the pressure and flow applied in the process (Sopajaree et al. 1999a, b).
A high flow allows preventing the formation of the cake by removing continuously
TiO 2 particles. Under such conditions (high flow) the influence of the pressure on the
fouling of the membrane is negligible. However at low flow, the influence of the
transmembrane pressure becomes predominant, and membrane clogging can occur.
One alternative is the implementation of submerged membrane process, which is less
sensitive to membrane fouling (Chin et al. 2007a, b; Molinari et al. 2008). Low
permeate flux results in higher residence time that favors the process efficiency
(Molinari et al. 2000; Rivero et al. 2006; Chin et al. 2007b). Other parameters such
as pH or ionic strength will also affect the process (Mozia 2010). Especially, the pH
strongly influences TiO 2 particles agglomeration (Xi and Geissen 2001; Paz 2006).
Also, the presence of humic acid was found to render the cake more resistant to the
flow (backpressure increase) (Lee et al. 2001a, b). However, one can expect the
photodegradation of organic products of the cake by the application of UV light
(Choo et al. 2008a, b), especially with the configuration where the membrane is
irradiated. The shaping of the catalyst also impacts the membrane clogging. Indeed,
by using a ball-shaped TiO 2 /SiO 2 catalyst, the fouling phenomenon was found to be
reduced as compared to the standard Degussa P25 (Fu et al. 2006a, b). The batch mode
is generally found to be more effective than the continuous mode, but the latter is better
suited for industrial scale applications (Li and Zhao 1999; Molinari et al. 2002a, b).
Others processes such as dialysis, pervaporation, or direct contact membrane
distillation also exist. For example, Azrague et al. developed a photocatalytic
membrane process where the mass transport through the membrane is not achieved
by transmembrane pressure but thanks to the difference of concentration (dialysis).
The main advantage of such process is that it prevents the fouling of the membrane
observed with pressure-driven processes (Azrague et al. 2007). Authors observed
that under their conditions, the rate-limiting step is the mass transport rather than the
70
B. Lebeau et al.
In such configuration, the photocatalytic process is ensured by TiO 2 introduced as a
slurry in the incoming feed and the membrane acts as a filter for TiO 2 particles, thus
ensuring the permeate to be free of TiO 2 particles. Indeed, most of the studies report
the high efficiency of the membrane filtration resulting in a very low concentration of
TiO 2 particles in the permeate (Meng et al. 2005; Huang et al. 2007; Mozia and
Morawski 2009). For reactors where the catalyst is suspended in the reaction
mixture, three main configurations can be distinguished: irradiation of the feed
tank, irradiation in a specific module (photoreactor) located between the feed tank
and the membrane, and irradiation of the membrane.
One advantage of such process is the photocatalytic efficacy given that TiO 2
particles are not embedded in the membrane, allowing a higher surface area available
for the photocatalytic process. It also allows obtaining a higher permeate quality and
prevents the membrane degradation by UV radiation. On the other hand, the main
drawbacks are membrane fouling, especially by TiO 2 particles, leading to the decline
of the permeate flow (Molinari et al. 2006; Molinari et al. 2008). As a general trend,
the higher the TiO 2 particles concentration is, the fastest the fouling of the membrane
is (Sopajaree et al. 1999a, b; Shon et al. 2008). The membrane fouling strongly
depends on the pressure and flow applied in the process (Sopajaree et al. 1999a, b).
A high flow allows preventing the formation of the cake by removing continuously
TiO 2 particles. Under such conditions (high flow) the influence of the pressure on the
fouling of the membrane is negligible. However at low flow, the influence of the
transmembrane pressure becomes predominant, and membrane clogging can occur.
One alternative is the implementation of submerged membrane process, which is less
sensitive to membrane fouling (Chin et al. 2007a, b; Molinari et al. 2008). Low
permeate flux results in higher residence time that favors the process efficiency
(Molinari et al. 2000; Rivero et al. 2006; Chin et al. 2007b). Other parameters such
as pH or ionic strength will also affect the process (Mozia 2010). Especially, the pH
strongly influences TiO 2 particles agglomeration (Xi and Geissen 2001; Paz 2006).
Also, the presence of humic acid was found to render the cake more resistant to the
flow (backpressure increase) (Lee et al. 2001a, b). However, one can expect the
photodegradation of organic products of the cake by the application of UV light
(Choo et al. 2008a, b), especially with the configuration where the membrane is
irradiated. The shaping of the catalyst also impacts the membrane clogging. Indeed,
by using a ball-shaped TiO 2 /SiO 2 catalyst, the fouling phenomenon was found to be
reduced as compared to the standard Degussa P25 (Fu et al. 2006a, b). The batch mode
is generally found to be more effective than the continuous mode, but the latter is better
suited for industrial scale applications (Li and Zhao 1999; Molinari et al. 2002a, b).
Others processes such as dialysis, pervaporation, or direct contact membrane
distillation also exist. For example, Azrague et al. developed a photocatalytic
membrane process where the mass transport through the membrane is not achieved
by transmembrane pressure but thanks to the difference of concentration (dialysis).
The main advantage of such process is that it prevents the fouling of the membrane
observed with pressure-driven processes (Azrague et al. 2007). Authors observed
that under their conditions, the rate-limiting step is the mass transport rather than the
70
B. Lebeau et al.