288
in the presence of UV wavelength (l < 400 nm) in many studies obeyed the linear
proportionality correlation to the incident radiant flux. This was evidenced by
Glatzmaier et al. [110] and Glatzmaier [109], where they observed that the destruction of dioxin and polychlorinated biphenyls was significantly enhanced in the presence of high-intensity photons. A similar finding was reported in Magrini and Webb
[203] where the organic decomposition rate was reported to increase with the radiation intensity.
Later, it was discovered that the dependency of the reaction rate on radiant intensity behaves indifferently under different lighting conditions [71, 149, 266]. The
linear dependency of the photocatalytic reaction rate on radiant flux (∅) changed to
a square root dependency (∅
0.5
) above certain threshold value. Such a shift in dependency form was postulated owing to the amount of photo-generated holes available
during the electron/hole pair formation. In the TiO 2 catalyst used, the photoinduced
generation of valence-band holes is much less than the conduction-band electrons
available. In this instance, the photo-generated holes are the rate-limiting step and
the detailed derivation of the square root dependency can be obtained from Malato
et al. [205]. At high intensities, the dependency of the photocatalytic reaction rate
on radiant flux reduced to zero (∅
0
). This was explained by the saturated surface
coverage of the catalyst, resulting in a mass transfer limitation in the adsorption and
desorption, thus preventing the effect of light intensity to set in. An increase in the
fluid turbulency in this case might help to alleviate the mass transfer problem on the
surface of the catalyst. The desorbed final products might also affect the dependency of reaction rate on radiant flux, as they might scavenge the electron acceptors
and further promote the electron/hole pair recombination.
Rincón and Pulgarin [275] reported that the residual disinfecting ability of the
photocatalyst largely depends on the duration of light intensity without any temporal interruptions. They investigated the effect of light intensities at 400 and 1000 W/
m
2
on bacterial lethality and regrowth, and found that the higher intensity without
any temporal interruptions can cause irreversible damage to the E. coli. In the intermittent light irradiations with constant interruptions, the bacteria were seen to
regrow during the subsequent 24 or 48 h. Some studies suggested that this regrowth
is due to the dark-repair mechanism where the partially damaged cells recover in the
presence of nutrients [288]. Others have suggested that the damaged but not totally
inactivated cells could recover its viability through photo-repairing under radiation
of 300–500 nm or through resynthesis and post-replication of cells [274, 291]. It
must be noted in this case that for photo-disinfection using different light intensities, a final conclusive point cannot be made directly. The disinfection results of
400 W/m
2
at 2.5-h irradiation might not be the same as the result that arose from
1000 W/m
2
for 1 h. Thus, in order to predict the minimum irradiation required at
constant irradiance, preliminary studies into both the photoreactor performance and
microbial consortia (different resistance) present are important.
13 Wastewater
in the presence of UV wavelength (l < 400 nm) in many studies obeyed the linear
proportionality correlation to the incident radiant flux. This was evidenced by
Glatzmaier et al. [110] and Glatzmaier [109], where they observed that the destruction of dioxin and polychlorinated biphenyls was significantly enhanced in the presence of high-intensity photons. A similar finding was reported in Magrini and Webb
[203] where the organic decomposition rate was reported to increase with the radiation intensity.
Later, it was discovered that the dependency of the reaction rate on radiant intensity behaves indifferently under different lighting conditions [71, 149, 266]. The
linear dependency of the photocatalytic reaction rate on radiant flux (∅) changed to
a square root dependency (∅
0.5
) above certain threshold value. Such a shift in dependency form was postulated owing to the amount of photo-generated holes available
during the electron/hole pair formation. In the TiO 2 catalyst used, the photoinduced
generation of valence-band holes is much less than the conduction-band electrons
available. In this instance, the photo-generated holes are the rate-limiting step and
the detailed derivation of the square root dependency can be obtained from Malato
et al. [205]. At high intensities, the dependency of the photocatalytic reaction rate
on radiant flux reduced to zero (∅
0
). This was explained by the saturated surface
coverage of the catalyst, resulting in a mass transfer limitation in the adsorption and
desorption, thus preventing the effect of light intensity to set in. An increase in the
fluid turbulency in this case might help to alleviate the mass transfer problem on the
surface of the catalyst. The desorbed final products might also affect the dependency of reaction rate on radiant flux, as they might scavenge the electron acceptors
and further promote the electron/hole pair recombination.
Rincón and Pulgarin [275] reported that the residual disinfecting ability of the
photocatalyst largely depends on the duration of light intensity without any temporal interruptions. They investigated the effect of light intensities at 400 and 1000 W/
m
2
on bacterial lethality and regrowth, and found that the higher intensity without
any temporal interruptions can cause irreversible damage to the E. coli. In the intermittent light irradiations with constant interruptions, the bacteria were seen to
regrow during the subsequent 24 or 48 h. Some studies suggested that this regrowth
is due to the dark-repair mechanism where the partially damaged cells recover in the
presence of nutrients [288]. Others have suggested that the damaged but not totally
inactivated cells could recover its viability through photo-repairing under radiation
of 300–500 nm or through resynthesis and post-replication of cells [274, 291]. It
must be noted in this case that for photo-disinfection using different light intensities, a final conclusive point cannot be made directly. The disinfection results of
400 W/m
2
at 2.5-h irradiation might not be the same as the result that arose from
1000 W/m
2
for 1 h. Thus, in order to predict the minimum irradiation required at
constant irradiance, preliminary studies into both the photoreactor performance and
microbial consortia (different resistance) present are important.
13 Wastewater