275
usually replaced by more economical transition or nonmetal doping. The mechanism of such transition and nonmetal doping, however, is different from the noble
metal coupling as the TiO 2 is incorporated into the TiO 2 crystal lattice [10, 136,
137]. Such incorporation introduces impurity in the bandgap of TiO 2 and thus
reduces the photonic energy requirements (Fig. 13.7c). More recently, the use of
nonmetal dopants (e.g., N, C, F, S) can improve the photoactivity and feasibility of
TiO 2 catalysts for industrial application [45, 310]. Further research efforts may be
needed to get a better understanding of the photoactivity kinetics, so as to improve
the photooxidation efficiency for water treatment.
Photocatalytic Reactor Configuration
Photocatalytic reactors for water treatment can generally be classified into two main
configurations, depending on the deployed state of the photocatalysts: (1) reactors
with suspended photocatalyst particles and (2) reactors with photocatalyst immobilized onto continuous inert carrier [264]. Various types of reactors have been used
in the photocatalytic water treatment, including the annular slurry photoreactor, cascade photoreactor, and downflow contactor reactor [43, 62, 247]. The disparity
between these two main configurations is that the first one requires an additional
downstream separation unit for the recovery of photocatalyst particles while the latter permits a continuous operation.
Pareek et al. [255] addressed that the most important factors in configuring a
photocatalytic reactor are the total irradiated surface area of catalyst per unit volume
and light distribution within the reactor. Slurry-type photocatalytic reactor usually
performs a high total surface area of photocatalyst per unit volume, while the fixedbed configuration is often associated with mass transfer limitation over the immobilized layer of photocatalysts. The light photon distribution through either direct or
diffuse paths within the reactors needs to be decided [42]. Direct photon utilization
means that the photocatalysts are directly activated with light photon, rather with
the assistance of various parabolic light deflectors to transfer the photons. To achieve
uniformity in photon flux distribution within the reactor, a correct position of light
source is essential to ensure maximal and symmetrical light transmission and distribution. The use of photoreactors with assisted parabolic light deflectors nowadays
has become unfavorable, owing to the need of special configuration and high operating costs. This type of reactor needs to be specifically designed to ensure the
maximal illuminated reactor volume with minimal pressure requirement for good
catalyst mixing and dispersion. Until recently, the slurry photocatalytic reactor was
still the preferred configuration owing to its high total surface area of photocatalyst
per unit volume and ease of photocatalyst reactivation. The photocatalyst particles
can be separated by settling tanks or external cross-flow filtration system to enable
continuous operation of the slurry reactor. A technically promising solution for
solving the downstream separation of photocatalyst particles after treatment is via
Recent Developments in Photocatalytic Water Treatment Technology
usually replaced by more economical transition or nonmetal doping. The mechanism of such transition and nonmetal doping, however, is different from the noble
metal coupling as the TiO 2 is incorporated into the TiO 2 crystal lattice [10, 136,
137]. Such incorporation introduces impurity in the bandgap of TiO 2 and thus
reduces the photonic energy requirements (Fig. 13.7c). More recently, the use of
nonmetal dopants (e.g., N, C, F, S) can improve the photoactivity and feasibility of
TiO 2 catalysts for industrial application [45, 310]. Further research efforts may be
needed to get a better understanding of the photoactivity kinetics, so as to improve
the photooxidation efficiency for water treatment.
Photocatalytic Reactor Configuration
Photocatalytic reactors for water treatment can generally be classified into two main
configurations, depending on the deployed state of the photocatalysts: (1) reactors
with suspended photocatalyst particles and (2) reactors with photocatalyst immobilized onto continuous inert carrier [264]. Various types of reactors have been used
in the photocatalytic water treatment, including the annular slurry photoreactor, cascade photoreactor, and downflow contactor reactor [43, 62, 247]. The disparity
between these two main configurations is that the first one requires an additional
downstream separation unit for the recovery of photocatalyst particles while the latter permits a continuous operation.
Pareek et al. [255] addressed that the most important factors in configuring a
photocatalytic reactor are the total irradiated surface area of catalyst per unit volume
and light distribution within the reactor. Slurry-type photocatalytic reactor usually
performs a high total surface area of photocatalyst per unit volume, while the fixedbed configuration is often associated with mass transfer limitation over the immobilized layer of photocatalysts. The light photon distribution through either direct or
diffuse paths within the reactors needs to be decided [42]. Direct photon utilization
means that the photocatalysts are directly activated with light photon, rather with
the assistance of various parabolic light deflectors to transfer the photons. To achieve
uniformity in photon flux distribution within the reactor, a correct position of light
source is essential to ensure maximal and symmetrical light transmission and distribution. The use of photoreactors with assisted parabolic light deflectors nowadays
has become unfavorable, owing to the need of special configuration and high operating costs. This type of reactor needs to be specifically designed to ensure the
maximal illuminated reactor volume with minimal pressure requirement for good
catalyst mixing and dispersion. Until recently, the slurry photocatalytic reactor was
still the preferred configuration owing to its high total surface area of photocatalyst
per unit volume and ease of photocatalyst reactivation. The photocatalyst particles
can be separated by settling tanks or external cross-flow filtration system to enable
continuous operation of the slurry reactor. A technically promising solution for
solving the downstream separation of photocatalyst particles after treatment is via
Recent Developments in Photocatalytic Water Treatment Technology