Disinfection of Waters/Wastewaters by Solar Photocatalysis
Danae Venieri
School of Environmental Engineering, Technical University of Crete, GR-73100 Chania,
Greece
E-mail:
The constantly growing demand for clean water of high hygiene standards has led to the
exploration and the development of effective disinfection techniques. Disinfection is referred to a
physical or chemical process that inactivates pathogenic or other virulent microorganisms, without
necessarily reaching the point of killing them. The primary “target” of disinfection is the disease-
causing microorganisms contained in water/wastewater and their concentration control to tolerable
and safe limits for public health protection. Waterborne diseases documented worldwide and their
rapid transmission through the consumption of contaminated water, illustrate the importance of
effective inactivation of pathogens, including bacteria, viruses and protozoa.
Advanced oxidation processes (AOPs) have been recognized as an emerging group of
techniques with high oxidation potential and biocidal effect on various microorganisms in aqueous
samples. The beneficial action of AOPs relies primarily on the
in situ
generation of highly reactive
transitory species, like hydroxyl radicals, which induce oxidative stress to microorganisms and their
ultimate inactivation. Heterogeneous photocatalysis stands out among AOPs as a promising and an
effective biocidal technique, with titanium dioxide (TiO
2
) being the most common catalyst employed
for the purification of aqueous matrices. Upon irradiation, the massive production of reactive oxygen
species (ROS) causes a gradual chain reaction beginning with a first oxidative stress when in contact
with microbial cells, proceeding with deleterious alterations in cellular structure and ending with
microbial inactivation and possible destruction. What makes this method even more attractive is the
prospect of using the solar spectral range after specific modifications of titania, involving doping with
non-metals or/and noble and transition metals and modification of the substrates of the catalyst. In this
way, the absorption spectrum of titania is expanded towards the visible light region, extending the
applications of photocatalysis as a purification process.
Although the environmental applications of photocatalysis, as well as several other AOPs,
have been researched for about three decades now, its potential use for water disinfection is a
relatively new topic for R&D. There is little doubt that economic cost is a key factor that will
eventually dictate process viability and several challenges have to be faced and overcome. In this
respect, one must consider the following points: (i) solar processes have an obvious head start in the
quest of efficient disinfection/decontamination treatment technologies since they exploit a renewable
energy source, avoiding capital and operational costs associated with artificial illumination; (ii) the
synthesis of new solar-active and stable photocatalysts can boost the technology but also increase
treatment cost compared to traditional titania (for semiconductor photocatalysis) or iron-containing
materials (for photo-Fenton and alike processes); (iii) from an engineering point of view, process
scale-up is a challenging task as specific reactor configurations and construction materials may be
needed; (iv) there is no such thing like “zero-cost” technology, therefore, the best one can opt for is
“low-tech, low-cost” technologies.
In this perspective, this talk comprises a detailed presentation of the photocatalytic processes,
in terms of their disinfection potential and mode of action for the inactivation of various
microorganisms. Several issues are discussed, including the difficulty to standardize operational
parameters of such techniques, due to the diverse microbial populations found in the aquatic
environment and their varied behavior when exposed to the stressed conditions of disinfection.
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