The UVA range causes sun tanning in the human skin. The UVB range causes sun burning. The UVC range is absorbed by DNA and thus can cause cancer and mutations. This is also the range that is most effective in inactivating bacteria and viruses. The Vacuum UV range is absorbed strongly by water and air and thus can only be transmitted in a vacuum.
Ultraviolet photons are particularly energetic and when absorbed in molecules can cause bonds to be broken (photochemistry)
Ultraviolet light (along with all others forms of electromagnetic radiation) comes in discrete energy packets called "photons". The energy of a photon is given by: U = hn = hc/l
where h is the Planck constant (6.626755 x 10-34 J s), c is the speed of light (2.997925 x 108 m s-1), n is the frequency (Hz) of the light, is the wave number (cm-1 or m-1) of the light and l is the wavelength (nm or m). Usually photochemical "events" involve absorption of only one photon per molecule.
The portion of the UV spectrum (the germicidal region) that is important for the disinfection of water and air is the range that is absorbed by DNA (RNA in some viruses). This germicidal range is approximately 200-300 nm, with a peak germicidal effectiveness at about 260 nm. The mechanism involves absorption of a UV photon by pyrimidine bases (principally thymine) where two pyrimidine bases are next to each other on the DNA chain. The photochemistry involves formation of a dimer that links the two bases together. This causes a disruption in the DNA chain, such that when the cell undergoes mitosis (cell division), the replication of DNA is inhibited
UV has many commercial applications in society. The major ones are: UV disinfection of water, air and surfaces, UV curing of inks and coatings, UV disinfection of foods, UV-based Advanced Oxidation destruction of pollutants in water and air.
UV-C has been an evolving technology over the past 50 years and has become the leading technology in low cost, efficient and effective disinfection in health care settings and beyond. Microorganisms, viruses, fungus and bacteria can not develop resistance to UV-C due to the photo-chemical sheering of DNA and RNA in the pathogenic genome.
Energy:
UV disinfection requires an excessive amount of energy.
UV lamps have low electrical efficiencies and thus are not "energy-conscious".
The energy costs alone make UV disinfection significantly more expensive (O&M) than chemical disinfection.
Environment / Safety
The mercury contained inside the UV lamp presents a hazard to end-users (or surface waters).
Mercury contained inside the UV lamp presents an environmental hazard after disposal of used lamps.
Lamps break frequently and release mercury to the environment.
UV systems present a safety risk to plant staff because of the potential of exposure to UV-C radiation.
The visible emissions from UV lamps will cause algal growth.
UV creates DBPs that have yet to be discovered.
O&M
Lamps are always burning out and require frequent replacement.
Between lamp replacement and lamp cleaning, operations staff will spend a significant amount of time maintaining a UV disinfection system.
Lamp ballasts are unreliable and require frequent maintenance/replacement.
Effectiveness
UV disinfection cannot be cost effectively used for 4-log virus inactivation.
Since drinking water plants have to add chlorine/chloramines for virus inactivation and residual, adding a UV system does not make financial sense unless required for Cryptosporidium inactivation.
Microorganisms can "re-activate" following UV disinfection.
Regulators won’t approve UV disinfection without significant validation and pilot testing.
