Environmental Engineering Reference
In-Depth Information
by-products, pesticides and herbicides, and other lower molecular weight com-
pounds. RO is another method of desalination widely deployed.
• UV germicidal irradiation:
UV from mercury germicidal lamps is an effective
disinfectant in clear, transparent water; however, monitoring, cleaning, and preil-
tration are required to assure germicidal performance.
• Ion exchange:
This process is effective for targeting speciic minerals for removal,
but does not effectively remove organics, particles, pyrogens, or microorganisms,
and requires frequent resin pack change or regeneration processes.
• Continuous deionization:
This process removes only a limited number of charged
organics, requires very pure feed water for eficient operation, and is commonly
used only in laboratory-grade water applications.
• UV oxidation:
This process uses a deep-UV light (185 nm) to produce ozone,
hydrogen peroxide, and hydroxyl radicals, which are effective at the photodegra-
dation and/or photolysis of organic chemicals. The ozone and hydrogen peroxide
persist beyond the reactor and must be removed, and mercury lamps are very
ineficient at producing this wavelength of light.
• Advanced oxidation processes:
Use of hydroxyl radicals produced by UV activa-
tion of ozone and hydrogen peroxide is effective at oxidizing contaminants, but
requires production and storage of toxic chemicals; therefore, these processes are
generally impractical in smaller-scale, point source drinking water applications.
24.5 Photochemical Water Purification
Puralytics has commercialized a combination of ive photochemical processes that have
been shown to reduce a broad range of contaminants. These photochemical processes are
driven both directly by light and indirectly through light activation of a semiconductor
catalyst. These processes include
• Photocatalytic oxidation
—an advanced oxidation process employing hydroxyl
radicals produced at the surface of a photocatalyst activated by light
• Photolysis
—the direct breaking of molecular bonds by light of appropriate
wavelengths
• Photocatalytic reduction
—reduction of a contaminant to a less toxic state at the
surface of a photocatalyst
• Photoadsorption
—the light-enhanced adsorption of contaminants to a surface
• Photodisinfection
—using one or more bands of light to disinfect water
These new processes provide new tools to address the emerging contaminants entering
our water supplies, as can be seen in Figure 24.5. In fact, these synergistic processes can
improve removal of trace chemical contaminants, reduce maintenance and consumable
replacement frequencies, and reduce water waste, thereby providing environmental and
health beneits and reducing overall cost of ownership.
