SteveS
Member Since: 05 Oct 2010
Location: Devon & Berkshire
Posts: 388
 
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I would not want to rely on sunlight UV as a means of keeping my water fresh as this wikipedia article shows - note the part that sunlight can play in reactivating otherwise 'dead' microbes
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Ultraviolet purification
Main article: Ultraviolet germicidal irradiation
Ultraviolet (UV) light induces the formation of covalent linkages on DNA and thereby prevents microbes from reproducing. Without reproduction, the microbes become far less dangerous. Germicidal UV-C light in the short wavelength range of 100–280 nm acts on thymine, one of the four base nucleotides in DNA. When a germicidal UV photon is absorbed by a thymine molecule that is adjacent to another thymine within the DNA strand, a covalent bond or dimer between the molecules is created. This thymine dimer prevents enzymes from "reading" the DNA and copying it, thus neutering the microbe. Still, there are limits to this technology. Water turbidity (i.e., the amount of suspended & colloidal solids contained in the water to be treated) must be low, such that the water is clear, for UV purification to work well. Also, water treated with UV still has the microbes present in the water, only with their means for reproduction turned "off". In the event that such UV-treated water containing neutered microbes is exposed to visible light (specifically, wavelengths of light over 330-500 nm) for any significant period of time, a process known as photo reactivation can take place, where the possibility for repairing the damage in the bacteria's reproduction DNA arises, potentially rendering them once more capable of reproducing and causing disease.[9] UV-treated water must therefore not be exposed to visible light for any significant period of time after UV treatment, before consumption, to avoid ingesting reactivated and dangerous microbes. For long term usage, the issue of obtaining batteries to power portable water purification UV devices can be a concern.
Another concern with UV portable water purification is that some pathogens are hundreds of times less sensitive to UV light than others. Protozoan cysts were once believed to be among the least sensitive, however recent studies have proved otherwise, demonstrating that both Cryptosporidium and Giardia are deactivated by a UV dose of just 6 mJ/cm sq.[10] However, EPA regulations and other studies show that it is viruses that are the limiting factor of UV treatment, requiring a 10-30 times greater dose of UV light than Giardia or Cryptosporidium[11] .[12] Furthermore, studies have shown that UV doses at the levels provided by common portable UV units are effective at killing Giardia[13] and that there was no evidence of repair and reactivation of the cysts.[14]
Fortunately, Giardia are among the easiest pathogens to filter out of water. A viable two-step portable water purification approach, providing greater protection than UV purification alone, is to first filter suspect water, thereby removing the larger pathogens, prior to using UV purification.
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Tell someone you love them today because life is short.
1st Dec 2012 6:45pm
Tell someone you love them today because life is short.
