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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 163

dispersion of the catalyst. The extent of dispersion can also be increased by
ultrasonic irradiation of the slurry at a low frequency (e.g., 20 kHz) using an
ultrasonic bath for approximately 10–15 min (Mazzarino and Piccinini,
1999). However, in the case of slurry reactors, the performance of the reac-
tor might be severely affected by the low irradiation efficiency and low pen-
etration depth of the incident irradiations because of the opacity of the
slurry. Also, after the oxidation treatment, the solid catalyst needs to
be separated from the liquid, which is not easy considering the small size
of the catalyst particles (usually in the range of 100–500 mm). A further prob-
lem is the fouling of the catalyst due to the irreversible adsorption of the
product on the catalyst surface. Thus, the application of slurry reactors for
photocatalytic treatment on a large scale seems to be quite problematic.
An alternative to the use of a catalyst in suspended form is the use of a
supported photocatalyst. The key advantages are the possibility of obtaining
an active crystalline structure and the stability of the catalyst layer in the
reacting media. Some of the laboratory equipment used for carrying out
photocatalytic oxidation is shown in Figure 3.6.

3.4.2 Optimum Operating Conditions
3.4.2.1 Amount of Catalyst
An optimum catalyst concentration should be used because using excess
catalyst reduces the amount of photo-energy being transferred in the medium
due to the opacity offered by the catalyst particles. It should also be noted that
the optimum value will be strongly dependent on the type and concentration
of the pollutant, as well as the rate of generation of free radicals (determined by
the operating conditions of the reactor), and laboratory-scale experiments are
required to compute the optimum value.

3.4.2.2 Reactor Designs
Usually reactor designs should be such that uniform irradiation of the entire
catalyst surface is achieved at the incident light intensity. This is a major
problem associated with the large-scale designs. Moreover, nearly complete
elimination of mass transfer resistances is another point that needs to be con-
sidered while designing large-scale reactors. Efficient reactor design must
allow exposure of the greatest quantity of the activated catalyst to the illu-
minated surface and must permit a high density of active catalyst to come
into contact with the liquid to be treated inside the reactor.