Purify Water With Sunlight

Purify Water With Sunlight (Glass or Plastic?)

Purify Water With Sunlight

Here’s a survival tip… Did you know that you can purify water with sunlight?

The article “How to Purify Water with Sunlight” describes the process in more detail (it’s surprisingly simple). In short, it’s a matter of utilizing UV (ultraviolet) light from the sun to purify the water (with caveats mentioned in the article). This is a good method to know for an emergency.

For the purification process to be effective, you need to use the right kind of container…

The container that holds the water, must be of a material that is transmissive of UVA light.

 

What is Ultraviolet light?

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, in the range 10 nm to 400 nm. It is so-named because the frequencies are higher than those that humans identify as the color violet, and are invisible to humans.

UV radiation is of three types; UVA, UVB, and UVC. Out of these, only UVA and UVB types are harmful to our skin, damaging its surface and inner structure when exposed to the sun too long. The third type, UVC radiation, does not penetrate the atmosphere’s ozone layer and does not reach Earth.

UVA (400 nm – 320 nm)
UVB (320 nm – 290 nm)
UVC (290 nm – 100 nm)

With regards to water purification, UVA is our friend…

UVA interferes directly with the metabolism and destroys cell structures of bacteria.

UVA reacts with oxygen dissolved in the water, believed to also damage pathogens.

As the sun heats the water, and if the water temperatures rises above apprx. 120 °F, the disinfection process is three times faster.

 

Glass or Plastic

Colorless, transparent PET plastic water or soda bottles (2 liter or smaller size) with few surface scratches are best for use. The labels need to be removed and the bottles washed before the first use.

Abbreviated PET, or PETE, Polyethylene terephthalate, most plastic bottles are made from PET and are widely used for bottled water and soft drinks.

Plastic bottles made of ‘clear’ PET will pass most of the UVA radiation into the water inside for eventual purification (~ 6 hours in full sunlight of fairly clear water; bottle no larger than 2 liters).

Glass on the other hand is a pretty good UV blocker; but it depends on the exact type of glass, its thickness, and the UV band.

Normal glass is fairly opaque to the UVB band, blocking nearly 100% of it, which is the UV band usually considered to be the most damaging to your skin.

Normal glass is mostly transparent to UVA (to 330 nm), allowing 80% of the band’s radiation to pass through (UVA penetration is needed for water purification).

Some glass is treated, and will block most of the UVA and UVB. This will not work for water purification.

Knowing this, if you have ‘normal’ clear glass, untreated with anything, then you could expect 80% of the UVA light to provide disinfectant properties to the water contained inside, provided that the size of the bottle or container isn’t more than that of a typical 2-liter plastic soda bottle.

Having said all that, ‘PET’ plastic drinking water bottles or soda bottles (clear, non-colored) are more plentiful, lighter and easier to use for water purification in a situation where safer methods are not readily available (e.g. boiling).

If you are considering a more elaborate method of water purification using ordinary glass in some sort of configuration, be aware that most modern window glass will block much of the UVA due to its manufacture. You will need to check the specifications to know for sure.

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9 Comments

  1. Love the info about purifying water. I save juice jugs, put water in them for emergency. We are on a well but it takes electric to run it. Hey, do you know how to get I water from a well if you lose power? I’m one of those suburban housewives with that, you never know attitude. Was in the army for10 years, Commo was my job. Lots of field time and I worked harder than the guys but smarter. But I work alone, that way I. Am in charge and I don’t have to waste time explaining why we do it like this and like that. Eventually I got soldiers but it was harder than working a lone, the messed up more stuff and I just had to fix more stuff.

  2. What about just putting the water out in an open tray or container? I can see setting your water out in a glass casserole dish or something. With the water exposed directly to the sun, with no glass or plastic blocking those glorious UV rays, wouldn’t that work best? Obviously, you will lose some to evaporation.

    1. I think that the glass or plastic is used as a lens to increase temperature. I doubt that open exposure would work. Try it if you’re interested.

    2. Yes, that would work too… Like you said, open sunshine (UV) and a bit of evaporation. It’s all about the UVA which does the work as described in the article. Ultraviolet water purification, also known as SODIS, has been used and recommended by the World Health Organization for decades in poor countries and regions that have no other methods of purification. Google it for more details… it’s a good thing to know about (just in case ;) )

    3. First, the purification takes a day or two. That’s a day or two of open, standing, stagnant water. That water is exposed to the elements and it will only accumulate even more bacteria. I hate to tell people this, because it usually grosses people out, but their are particles of dust with bacteria and even fecal matter floating around in the air. Yes, that’s the air we breathe. It is a fact of life. That fecal matter settles in open water and begins to feed bacteria immediately, allowing it to propagate greatly. Plus, if any visiting critters like lizards, birds, or bugs, etc. drink from your dish, or just climb into it, that just adds more bacteria, among other things. The plastic bottle both helps seal the water from added bacterial exposure, allows the sun to heat and purify the water, and also keeps it from evaporating away. An almost perfect container for this purpose.

  3. hi there!
    I am actually working on solar disinfection of well water using PET plastics as a project work. I have one sample covered with car windscreen for comparison(with the idea that it would block the UV from entering the sample). Please I will like to know if the sample covered with windscreen will reduce the fluorescence intensity or decrease it via LIF(Laser Induced Fluorescence),if done before!
    Doubts with my supervisor!

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