Page 190 - Industrial Wastewater Treatment, Recycling and Reuse

164 Industrial Wastewater Treatment, Recycling, and Reuse

UV tube

Effluent in the
annular space

Quartz tube

(a) Hexagonal annular photoreactor

UV source Air

Bubbler

Reaction mixture

Agitator
Cooling coil
(b) Simple suspended type reactors
Outlet
Reflector

Light
source
Lens
Inlet titanium dioxide-coated
hollow glass tubes
(c) Multiple tube reactor
Figure 3.6 Schematic representation of equipment used in photocatalytic oxidation.

3.4.2.3 Wavelength of Irradiation
The threshold wavelength corresponds to the band gap energy for the semi-
conductor catalyst, e.g., for the TiO 2 catalyst having a band gap energy of
3.02 eV, the ideal wavelength is 400 nm (Herrmann et al., 1999). Sunlight
may also be used for excitation of the catalyst in some cases, as discussed
earlier; this results in considerable cost savings (Yawalkar et al., 2001).