Solar driven advanced oxidation processes for water decontamination and disinfection - Solar Energy Sciences and Engineering Applications - page 407

use global radiation arriving from any direction have a clear advantage, as stated
earlier, compared to expensive imaging/concentrating optic-based systems which
cannot harvest diffuse radiation.
The major advantage with non-imaging collectors is that the collection factor
remains constant for all values of sun zenith angle within the acceptance angle limit.
Therefore, many different non-imaging geometries have been used in the design
of larger-scale solar disinfection systems, including up to small community scale
(Bandala and Estrada, 2007). A solar collecting system for use in, for example, rural,
isolated communities in developing countries should have many specific attributes:
high illuminated volume/total volume ratio; low flow operation to maximize residence
time; serve as a UVA dosimetric indicator (considering that both, photocatalytic detox
and disinfection are dose dependent); high UVA reflectivity; high (90%) UVA trans-
mission in the receiver; robust to harsh environmental conditions; minimal lifecycle
cost; low environmental impact; low maintenance requirements and ease of access to
replacement parts; and minimal external power requirement. In addition, the photore-
actor design must also provide electron acceptors, typically dissolved oxygen, since the
concentration of dissolved oxygen will be rapidly consumed in the initial stages of the
reaction as water temperature increases in static batch systems.
As said earlier, several technological approaches have been reported that strive to
offer the characteristics described previously. However, more research is required to
demonstrate satisfactory full scale performance before widespread deployment of this
technology for point-of-use water or industrial wastewater treatment.
13.5.2 Suspended vs. immobilized photocatalyst
One of the main disadvantages for heterogeneous photocatalytic processes in water
treatment is the generation of catalyst slurries. In agreement with previous studies of
pilot plant scale systems, suspended TiO 2 is more effective than immobilized TiO 2 ,
probably due to limitations imposed by mass transfer processes on the latter's reaction
rate (Bandala and Torres, 2008). Immobilized TiO 2 , however, present some impor-
tant advantages when compared with suspensions, such as reduced material loss, cost
reductions, and the possibility to escape further recovery steps after the water treat-
ment process. Many materials have been tested for the immobilization of TiO 2 as
well as a wide variety of immobilization methodologies (Gelover et al., 2006). How-
ever, the controversy remains over the performance of immobilized TiO 2 photocatalyst
in comparison with suspended TiO 2 , since some authors have found no advantages
at all for the use of immobilized systems (Sordo et al., 2010) whereas others claims
important improvements when using immobilized applications (Gelover et al., 2006;
Acevedo et al., 2012). It is probable that this controversy is related to the lack of a
proper comparison methodology rather than a real difference between the tested mate-
rials. Several chemical techniques for TiO 2 film deposition on solid surfaces have been
described in the past: chemical vapor deposition (CVD), serigraphy, galvanoplasty,
anodization, electrophoresis, electroless deposition, spray pyrolysis, controlled precip-
itation or chemical deposition, sol-gel chemical deposition, and magnetron sputtering.
In the same way, a wide variety of solid matrix materials for TiO 2 immobilization are
reported. It is reasonable to speculate that the variation in results is at least in part due