286
photomineralization, a general classification in the bacterial resistance to the disinfectant used has also been proposed. Among all, the most resistance infectious type
of microorganisms is prions, followed by coccidia (Cryptosporidium), bacterial
endospores (Bacillus), mycobacteria (M. tuberculosis), viruses (poliovirus), fungi
(Aspergillus), and Gram-negative (Pseudomonas) and eventually Gram-positive
bacteria (Enterococcus) [205]. Their differences in resistance are explained by the
cell wall permeability, size, and complexity of the specific microorganisms. Each
microorganism might also be of infectious nature, which causes epidemic diseases
when they multiply in water. Most bacteria can be killed easily with TiO 2 photocatalysis, but a complete inactivation might have to be ensured as they are highly
infectious. Similarly, this infectious nature can also be found in viruses (adenoviruses, enteroviruses, hepatitis A and E viruses, noroviruses and sapoviruses, rotaviruses) and the most in protozoa (Acanthamoeba spp., Cryptosporidium parvum,
Cyclospora cayetanensis, Entamoeba histolytica, Giardia intestinalis, Naegleria
fowleri, Toxoplasma gondii) [326]. All these protozoa are highly infectious in low
concentration and the photocatalytic treatment should be targeted on these
microorganisms as the surrogate indicators. This is to ensure adequate photocatalytic treatment to prevent the outbreak of the epidemic diseases in the treated water,
if photocatalytic treatment is chosen.
Light Wavelength
The photochemical effects of light sources with different wavelength-emitting
ranges will have a profound consequence on the photocatalytic reaction rate,
depending on the types of photocatalysts used—crystalline phase, anatase-to-rutile
composition, and any state of photocatalyst modifications. Using commercial
Degussa P-25 TiO 2 , which has a crystalline ratio of anatase 70/80:20/30, a light
wavelength at λ < 380 nm is sufficient for photonic activation [19, 121]. The crystalline phase of rutile TiO 2 has a smaller bandgap energy of E B ~ 3.02 eV, compared to
the anatase TiO 2 of 3.2 eV [103, 127, 144]. This dictates that rutile TiO 2 can be
activated with light wavelength of up to 400 nm, depending on the bandgap threshold for the type of rutile TiO 2 used.
For UV irradiation, its corresponding electromagnetic spectrum can be classified
as UV-A, UV-B, and UV-C, according to its emitting wavelength. The UV-A range
has its light wavelength spanning from 315 to 400 nm (3.10–3.94 eV), while UV-B
has a wavelength range of 280–315 nm (3.94–4.43 eV) and the germicidal UV-C
ranges from 100 to 280 nm (4.43–12.4 eV) [51, 84]. In most of the previous studies,
the UV-A light provides light photons sufficient for photonic activation of the catalyst [27, 48, 247]. As with outdoor solar irradiation, the UV-C is usually absorbed
by the atmosphere and does not reach the earth surface. Only the lamp-driven photoreactor system can utilize UV-C irradiation artificially for photonic activation of
catalyst and reduction of viable microorganisms. The mechanism of UV-C cell
destruction involves the direct induction on pyrimidine and purine and pyrimidine
13 Wastewater
photomineralization, a general classification in the bacterial resistance to the disinfectant used has also been proposed. Among all, the most resistance infectious type
of microorganisms is prions, followed by coccidia (Cryptosporidium), bacterial
endospores (Bacillus), mycobacteria (M. tuberculosis), viruses (poliovirus), fungi
(Aspergillus), and Gram-negative (Pseudomonas) and eventually Gram-positive
bacteria (Enterococcus) [205]. Their differences in resistance are explained by the
cell wall permeability, size, and complexity of the specific microorganisms. Each
microorganism might also be of infectious nature, which causes epidemic diseases
when they multiply in water. Most bacteria can be killed easily with TiO 2 photocatalysis, but a complete inactivation might have to be ensured as they are highly
infectious. Similarly, this infectious nature can also be found in viruses (adenoviruses, enteroviruses, hepatitis A and E viruses, noroviruses and sapoviruses, rotaviruses) and the most in protozoa (Acanthamoeba spp., Cryptosporidium parvum,
Cyclospora cayetanensis, Entamoeba histolytica, Giardia intestinalis, Naegleria
fowleri, Toxoplasma gondii) [326]. All these protozoa are highly infectious in low
concentration and the photocatalytic treatment should be targeted on these
microorganisms as the surrogate indicators. This is to ensure adequate photocatalytic treatment to prevent the outbreak of the epidemic diseases in the treated water,
if photocatalytic treatment is chosen.
Light Wavelength
The photochemical effects of light sources with different wavelength-emitting
ranges will have a profound consequence on the photocatalytic reaction rate,
depending on the types of photocatalysts used—crystalline phase, anatase-to-rutile
composition, and any state of photocatalyst modifications. Using commercial
Degussa P-25 TiO 2 , which has a crystalline ratio of anatase 70/80:20/30, a light
wavelength at λ < 380 nm is sufficient for photonic activation [19, 121]. The crystalline phase of rutile TiO 2 has a smaller bandgap energy of E B ~ 3.02 eV, compared to
the anatase TiO 2 of 3.2 eV [103, 127, 144]. This dictates that rutile TiO 2 can be
activated with light wavelength of up to 400 nm, depending on the bandgap threshold for the type of rutile TiO 2 used.
For UV irradiation, its corresponding electromagnetic spectrum can be classified
as UV-A, UV-B, and UV-C, according to its emitting wavelength. The UV-A range
has its light wavelength spanning from 315 to 400 nm (3.10–3.94 eV), while UV-B
has a wavelength range of 280–315 nm (3.94–4.43 eV) and the germicidal UV-C
ranges from 100 to 280 nm (4.43–12.4 eV) [51, 84]. In most of the previous studies,
the UV-A light provides light photons sufficient for photonic activation of the catalyst [27, 48, 247]. As with outdoor solar irradiation, the UV-C is usually absorbed
by the atmosphere and does not reach the earth surface. Only the lamp-driven photoreactor system can utilize UV-C irradiation artificially for photonic activation of
catalyst and reduction of viable microorganisms. The mechanism of UV-C cell
destruction involves the direct induction on pyrimidine and purine and pyrimidine
13 Wastewater