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Ultraviolet Irradiation

Figure 1: Typical UV disinfection configuration.
Figure 1: Typical UV disinfection configuration.

Ultraviolet (UV) light can be used for the inactivation of drinking water pathogens or the oxidation of micropollutants. In the latter capacity, it is commonly used in combination with hydrogen peroxide as a part of an advanced oxidation process. UV disinfection or oxidation is a physical process that utilizes UV light and does not require addition of any chemicals. This technology is known for its germicidal power in inactivating microorganisms (i.e. bacteria, viruses, algae, etc.) including chlorine-resistant pathogens, such as Cryptosporidium.

UV disinfection uses UV light to inactivate pathogens by disrupting their DNA strands, making them non-viable and non-infectious. UV light is generated by flowing electrons from an electrical source through ionized mercury vapor. Mercury is contained within a UV lamp and, for safety reasons, is isolated from exposure to water by the lamp envelope and the surrounding lamp sleeves. UV lamps commonly used in drinking water treatment are classified as low-pressure (LP) lamps, low-pressure high output (LP-HO) lamps, and medium-pressure (MP) lamps. The LP-HO lamps have special design features that allow for higher UV radiation transmittance and are therefore more efficient than MP lamps. MP lamps produce 10 to 20 times higher UV radiation outputs than LP (40-85 W) and LP-HO lamps (300-400 W); thereby requiring fewer lamps and decreased maintenance. However, power requirements are significantly higher and higher temperatures generated can cause scaling of sleeves in some waters. LP and LP-HO systems deliver the majority of their power at the germicidal wavelength (254 nm) and are typically better suited for small and medium sized systems because of their reliability associated with operating with multiple lamps.

The UV dosage applied for inactivation is a function of UV irradiance and exposure time. The UV irradiance is the rate at which UV energy is delivered to the water and is described in terms of UV power per unit area (mW/cm2). The UV energy is related to UV dose by multiplying the UV irradiance by the exposure time:

UV dosage (mW·.s/cm2 or mJ/cm2) = UV irradiance (mW/cm2) x time (sec) .

The UV dose is analogous to the CT term used in inactivation credits for chemical disinfectants. Since UV dose is primarily based on light intensity, water quality parameters such as turbidity and suspended solids (SS) can lower UV transmittance by screening/shielding UV light from the microorganisms. The presence of some organic and inorganic compounds (such as iron, calcium hardness) can also absorb UV light, lowering UV transmittance. The UV transmittance is the measure of UV energy not absorbed by the water. Water quality parameters such as pH, alkalinity and temperature do not impact the overall effectiveness of UV disinfection. However, these parameters can have an impact on the scaling of UV lamp sleeves.