Does sunlight kill bacteria in water?
 

I've seen products advertised that are supposed to kill bacteria. They are water drinking bottles that have a UV light in it that supposedly kills all the bacteria that may be in the water, making it safe to drink. But what about sunlight? It has UV rays. So wouldn't it have the same affect if you put a bottle of water out in the sun for some time?

Re: Does sunlight kill bacteria in water?
 

The material of the container you put the water in (glass or plastic), if not carefully selected, would likely filter out most of the shortwave UV that is responsible for the germicidal effect.

From http://en.wikipedia.org/wiki/Ultraviolet:
Sterilization
Main article: Ultraviolet germicidal irradiation

"A low pressure mercury vapor discharge tube floods the inside of a hood with shortwave UV light when not in use, sterilizing microbiological contaminants from irradiated surfaces.Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. Commercially-available low pressure mercury-vapor lamps emit about 86% of their light at 254 nanometers (nm) which coincides very well with one of the two peaks of the germicidal effectiveness curve (i.e., effectiveness for UV absorption by DNA). One of these peaks is at about 265 nm and the other is at about 185 nm. Although 185 nm is better absorbed by DNA, the quartz glass used in commercially-available lamps, as well as environmental media such as water, are more opaque to 185 nm than 254 nm (C. von Sonntag et al., 1992). UV light at these germicidal wavelengths causes adjacent thymine molecules on DNA to dimerize, if enough of these defects accumulate on a microorganism's DNA its replication is inhibited, thereby rendering it harmless (even though the organism may not be killed outright). However, since microorganisms can be shielded from ultraviolet light in small cracks and other shaded areas, these lamps are used only as a supplement to other sterilization techniques.

[edit] Disinfecting drinking water
UV radiation can be an effective viricide and bactericide. Disinfection using UV radiation is commonly used in wastewater treatment applications and is finding an increased usage in drinking water treatment. Many bottlers of spring water use UV disinfection equipment to sterilize their water. Solar water disinfection is the process of using PET bottles and sunlight to disinfect water.
New York City has approved the construction of a 2 billion gallon per day ultraviolet drinking water disinfection facility.[27] There are also several facilities under construction and several in operation that treat waste water with several stages of filters, hydrogen peroxide and UV light to bring the water up to drinking standards. One such facility exists in Orange County, California.[28][29] NASA has examined the use of this technology, using titanium dioxide as catalyst, for breaking down harmful products in spacecraft waste water.[30]
It used to be thought that UV disinfection was more effective for bacteria and viruses, which have more exposed genetic material, than for larger pathogens which have outer coatings or that form cyst states (e.g., Giardia) that shield their DNA from the UV light. However, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the microorganism Cryptosporidium. The findings resulted in the use of UV radiation as a viable method to treat drinking water. Giardia in turn has been shown to be very susceptible to UV-C when the tests were based on infectivity rather than excystation.[31] It has been found that protists are able to survive high UV-C doses but are sterilized at low doses.
Solar water disinfection[32] (SODIS) has been extensively researched in Switzerland and has proven ideal to treat small quantities of water cheaply using natural sunlight. Contaminated water is poured into transparent plastic bottles and exposed to full sunlight for six hours. The sunlight treats the contaminated water through two synergetic mechanisms: UV-A irradiation and increased water temperature. If the water temperatures rises above 50 �C, the disinfection process is three times faster."

Hope this helps.:wavey:
R.

Re: Does sunlight kill bacteria in water?
 

I could be wrong, but I think there is a hazard of putting plastic bottles in the sun. Something to the effect of suns rays causing some plastic to get into the water. Plastics, in general, are nasty stuff. But if forced to choose between water that's been disinfected by sun in a plastic bottle and raw giardia, I'll take the carcinogens.

Re: Does sunlight kill bacteria in water?
 

Find a beautiful lake under a bright blue and very sunny sky, and drink some of the water. Get back to us after a week or so.

Seriously...NO, it won't.

Re: Does sunlight kill bacteria in water?
 

the UV purifiers have significantly higher levels of UV than sunlight.....
it's the equivalent of saying 'lasers can cut steel, so i will use my laserpointer to cut my way out of these steel bars of logic'

Re: Does sunlight kill bacteria in water?
 

Of course, one could just boil the stuff...:thumb.aspx:

R.

Re: Does sunlight kill bacteria in water?
 

The bottle I've seen most is the Steri-Pen. It does work as advertised, but then it gives a dose of UV that is far higher than natural sunlight.

Also the water needs to be clear for the Steri-Pen to work; muddy water for example will not sterilize properly.

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There are projects for poor Africans where people made plastic bags that could be filled with water and left on the roof in the sun, where (after so-many-hours) the sunlight would kill most of the more-harmful bacteria. This is not as good as what the Steri-Pen does, but it is cheap, and it would be a lot safer than drinking the water with no treatment at all.

-end-

Re: Does sunlight kill bacteria in water?
 

You need around 1500 times the "sunlight" dose of UV to kill bacteria. I've had a few of these systems installed in buildings to be eco-friendly. They kill off the bacteria in ANY water, but not all parasites if that is an issue.

Re: Does sunlight kill bacteria in water?
 

A survival book I have been reading claims that it works, in a pinch. If you can't boil, don't have chlorine or iodine, he says you can put the stuff in the sun in a clear bottle> I think recommends full 12+ hours in strong sun. Also lots of neat little ideas, like you should have a little vacuum tool if possible, that way you can suck water out of any little puddle if you really need it. Try getting the water from a mud puddle into some kind of container to take back and run through shirts or whatever kind of filters you might have and having to spoon it out with your hands. No way to be prepared, improvisation will be critical if you ever really have to survive.