How to Purify Water with Sunlight
April 19, 2012, Submitted by: Ken Tweet
Did you know that solar radiation from the sun can purify (disinfect) water and make it safe for drinking from harmful bacteria?
While so often we hear or read about the bad things that the sun is capable of doing to us, we should also know that some of the same energy from the sun can be harnessed for doing good. A few good examples include solar electric panels, solar hot water panels, and solar ovens.
Well here is one more good example. Harnessing the solar energy from the sun to purify (disinfect) water from a lake or stream into safe drinking water. It is much simpler than you may think. Here’s how…
UV-A rays from the sun, (Ultraviolet-A, longwave, 315-400 nm), will terminate harmful bacteria, parasites, and viruses in water, given enough sun exposure.
A clear plastic bottle filled with water, exposed to the sun for 6 hours will make the water safe to drink (see the caveat list). In fact, the effectiveness of terminating harmful bacteria is an amazing 5-Nines, that is, 99.999 percent!
List of germs that are terminated from UV-A sunlight exposure at 6 hours
- Bacteria – Escherichia coli
- Bacteria – Vibrio cholera
- Bacteria – Salmonella
- Bacteria – Shigella flexneri
- Bacteria – Campylobacter jejuni
- Bacteria – Yersinia enterocolitica
- Virus – Rotavirus
- Parasites – Giardia
- Parasites – Cryptosporidium (needs 10 hours exposure)
List of caveats to UV-A sunlight water purification
- This method kills germs. If the water is already contaminated with chemicals from pollution, the chemicals will remain.
- If the water is cloudy and dirty, it should be filtered first to allow the UV-A rays to effectively penetrate into the water.
- The plastic water bottle should be no bigger than 3 liters. In moderately cloudy water, UV-A will lose 50 percent effectiveness at a depth of 10 mm (about 0.5 inch), whereas UV-A will only lose 25 percent effectiveness at a depth of 10 mm in clear water. Just use a typical size soda bottle or water bottle.
How to purify water with sunlight
The recommended bottle to use is a “PET” bottle. It is very common and is typically used for soda or other soft drinks.
PET, PolyEthylene Terephthalate, Recycle code #1 on bottom of bottle.
The bottle must not be colored. It must be a clear bottle. Again, it must be a clear plastic bottle. Not glass (glass blocks too much of the UV-A for this purpose).
The bottle must be clean. Common Sense.
Know your water source (as best you can)… if you believe it to be chemically toxic, don’t use it.
Fill the bottle with water. If the water is very cloudy, it must be filtered by first pouring through a cloth or such material to capture sediment.
Lay the bottle down in the sun. Do not stand them up. Ideally the bottles would by placed so that they face the the sun at a similar angle, to maximize the UV-A penetration.
Even better… lie the bottles on a reflective surface to increase the UV-A exposure using direct and reflected sunlight. This is not necessary, however it would shorten the required time and ensure optimum UV-A exposure.
If the sky is partly cloudy with only a few clouds, then 6 hours sunlight exposure will be enough.
If the sky is half filled with clouds, or more, then 2 days will be required.
Note that the outdoor temperature does not matter, so long as the UV-A sunlight exposure has been 6 hours.
A few notes:
An excellent source for more information on this subject is from the Swiss Federal Institute of Aquatic Sciences and Technology.
For those that are concerned about using plastic, the WHO (World Health Organization) states that the limiting values (plasticizers) for drinking water are never exceeded when using PET bottles for this method.
Sediment or other particulate matter should be removed first by a filtering process.
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I’m not sure I would trust this. Consider this: I sometimes collect cans walking alongside the road. I will from time to time see a water bottle with some unknown slightly yellow liquid in it. I understand that sometimes truckers have been known to relieve themselves by peeing in a water bottle and throwing it out the window. This bottle sits in the sun and based on your article it is safe to drink! Is it!! Would you drink it?
Well, my point is if you have to purify or sterilize the water then you are not sure of it’s source (downstream from a sewer or leaky septic system, etc.) or what is in it. Would you drink it after it sat in the sun for 10 hours?
To: GoneWithTheWind
Thanks for your comment. In my opinion, someone would have to be in desperate straits, or off their rocker, to drink a bottle filled as described in your given hypothetical example
I believe that most people have the common sense not to pick up a bottle of yellow colored “something” on the side of the road, and drink it (even if they know that solar UV will kill bacteria).
The use-case for solar water purification spelled out in this post is for the survival-minded to add this tidbit to their knowledge base. A typical example of use may be someone in “survival mode” removing some water from a stream or lake, who had no other means of assuring safe drinking water. It is a proven fact that solar UV water purification works as described in the post.
In fact, UV water purification is beginning to be implemented in several municipalities around the US and is currently being used in many developing or poor areas around the globe to help fight disease.
Very informative article; good to know (esp the caveats) if you get stuck somewhere without a water purification method or means to boil water.
Thanks for the article. I have tried this method while out camping in Idaho. I took the water from a fast flowing stream and placed the 16 ounce bottles on a folded out piece of tinfoil. I did this as to use my time well as I was building an A frame shelter. I figured why not try it and also it was using my time more efficiently. I drank the water later that evening in my shelter after the water was in the sun for 7 hours. I never got sick. I’m sure it worked. For those that scoff at this method, I say something tried is better then doing nothing.
What a fantastic article thank you so much for printing it!! Its amazing to think that it really could be as simple as that.
Well worth knowing.
Thank you so very much for your information. it does ease my mind knowing I can produce clean water. In these day’s we need to know how to survive. Knowledge is power. God Bless You all.
To: Connie (and everyone)
Thanks for your kind words.
I believe that many people would be surprised to discover that UV rays from the sun can kill the bacteria in water, if done in this manner. It’s a nice bit to know.
We also highly recommend that people have at least one quality water “filter”, such as any of these water filters, several of which we own and use. We have one of the Berkey stainless steel water filters, which we have been using for years – it is truly fantastic. We also have several of the smaller portable Katadyn water filters, one for each of our 72 hour emergency kits (one in each vehicle).
“GoneWithTheWind” – I believe the article stated that this process was for collecting water from a lake or stream. Use common sense here, “slightly yellow liquid” in a bottle that has been in a ditch for an undetermined amount of time, could literally be anything…geez
Hi there.
I think the title should be : How to sterilize water with sunlight. As if there is sand or something not alive inside , you don’t get it out with sunlight.
Just imagine if the water has snake poison. it will not get purified with Radiation of the sun.
Another solution is to boil the water , it will kill most of these bacterias.
To: Seuros
True enough, the word “purify” might be a bit strong in this context. Sediment would need to be filtered by a separate process. I’ve added a note at the end of the article to state that the word “disinfect” is probably more appropriate. As mentioned in the article under caveats, if the water source is already contaminated with toxins or chemicals, these toxins will remain.
As you indicate, boiling water is the ultimate, provided you have the fuel and means to do so. In fact, I have a paragraph regarding boiling in a later post titled, Make Drinking Water Safe with Bleach
Thanks for your comment.
The plastic sheet distiller system is safer. Make a tent of thin plastic sheet over a bowl of filtered( a sandy sox will do). The water in the bowl will evaporate in the sun and turn into drops on the plastic sheet. Arrange a cleen spout for the drops to run into a collecting cup and you will have distilled water.
This works, I’ve been drinking water that is placed in the sun for about three years and it has never made me ill. Its only common sense that you have to clear the water before you start killing out the micro- bacteria. To many commercial products on the market don’t want you to know this because they want to sell their goods.
My only concern is the BPA found in plastic. It becomes more of a threat when warmed (out in the sun). So you may have purified it, but you’ve also just leached BPA into it.
I would be cognizant of this, yes. In the context though of ‘surviving’, and using this method as a means to disinfect water for drinking, I would rather take my chances with a ‘potential’ long term affect from BPA leaching from the plastic, than getting sick from ‘bad’ water. I get your point though.
Disinfecting water this way is a good tip to know though, and I’ll bet most people didn’t realize that the sun can disinfect water…
The method Ken describes does work. Do a Wiki on SODIS. Would not be my first choice. While I agree that BPA is a bad thing; bad, or no, drinking water is far worse.
Be well.
Wonderful article! Do you know if a glass blottle will work in this situation?
Glass is not recommended as it blocks some of the UV.
Great article Ken.
Several people have pointed out the flaws they see–obviously this is not a best case scenario course of action.
Sheltering outdoors in cold weather and killing your pets for food aren’t great options either, but given the alternatives they make sense in survival situations.
Thanks for passing along!
You got it exactly right. It’s not the first choice, but it’s a choice. The more of these kinds of things that we know, the more options that we have have…
Interesting to learn this. If you happened to have a glass bottle on hand, is there any reason why that would not work instead of plastic?
@Elaine, Most glass blocks some (not all) of the ultraviolet band, to varying degrees. I would not use glass for water purification. Given that there is an abundance of plastic bottles in our world today, you shouldn’t have a problem finding one
Thanks for the article didn’t know that. I alsways though it was a full day in direct sunlight.
this was a very usefull article. Not only does it help people with their survival skills but it also puts your mind at ease knowing how to purify your water yourself