How to Purify Water with Sunlight
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 2 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.
Camelbak All Clear UV Purifier
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.
How to Purify Water with Sunlight,
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.
the article did say very clearly to know the source of your water, meaning if you’re near a lake, don’t go looking for piss on the freeway. and secondly if you’re going to get all technical about it urine is sterile unless you contaminate it with something else and people have had to drink it in extreme survival situations because they had nothing else available in lived. and thirdly nobody is dumb enough to pick up trucker bombs off the freeway to drink if they have Any Other Option.
If you were truly in survival mode… this was a great tip to know… and that trucker did you a favor by tossing that bottle to the side of the road where it can be retrieved and washed. This trick works better with soda bottles that can take the heat and pressure as the plastic is thicker than with cheap water bottles.
There is a great show on the Blaze called Independence USA. It’s a reality show about this family that does survival planning and are trying to become self sufficient. They set up a rain water collection system to get water for their garden and to drink. Obviously, they were concerned with how to purify it for drinking. They used this very method. They built a lean to with shiny corrugated aluminum. The bottles stacked up quite neatly on the aluminum. The interesting part was that they actually sent samples of the raw rain water and the purified rain water to be tested. They also had their well water tested. The water that was unpurified had multiple types of dangerous bacteria in it, but the water that was purified had 0 bacteria left in it. By the way, the water that was purified by this method was even cleaner than their well water.
@Linda, Thanks for sharing that and reaffirming the method. It is amazingly simple to purify water using this method, and is a very powerful bit of knowledge to know about, should you ever need it. The only caution (the same caution as most other methods) is that this will not remove chemicals that may already be in the water).
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
Ken,
That’s not actually correct. Glass blocks UV-A (the wavelength of UV necessary for water purification) at varying rates depending on its iron content and thickness. Most glass works well for UV sterilization, provided that it’s less than 2mm thick. Some glass (pyrex, corning, etc.).
This statement is actually true… “Most glass blocks some (not all) of the ultraviolet band,”
Its covered in more detail in this follow-on article, “Purify Water with Sunlight (Glass or Plastic)” where I explain that Glass blocks all of UVB and UVC, while it does allow 80% of UVA to pass through (with caveats).
This indicates that the right glass is about 80% efficient compared with a typical ‘clear’ plastic water bottle or 2-liter soda jug.
Given that plastic doesn’t break when dropped, is nearly 100% efficient for this purpose, is lighter, cheaper, and more abundantly available… it would be my personal choice (assuming that I had a choice) for solar water purification.
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
This makes sense since I can purify water in my Global Sun Oven and I never thought to just leave a bottle in the sun. Great idea! I use canning Mason jars and a WAPI in my sun oven. The WAPI came with my sun oven and even using glass it works to purify the water. Wouldn’t the glass jars also work outside of the sun oven?
@Julene, Yes, as indicated in the article, you simply need to expose the water to sunlight for a full 6 hours (no clouds), but the caveat is to use food-grade plastic bottles. I intend to research the UV reduction effects if one chooses to use a glass container instead, and will hopefully come up with a suggestion regarding the increased time required for clear glass. I also have a Global Sun Oven, and we love it.
You can also use a Zipock bag instead of a water bottle. If you get the large sized bags, you can lay them out flat and you get a greater surface area with a shallower depth for the sun to penetrate. Ziplock bags do not contain BPA’s.
That’s a great suggestion Joe. Plus empty Ziplock bags don’t take up much space vs. the same amount of water that would require ‘hard’ containers.
My thoughts about water storage when your on the move…
If you fall on a Nalgene bottle, full of water, you say a few choice words and get up dry. They come in clear, non BPA plastic.
If you fall on a Ziplock bag, full of water, you say a few choice words and get up wet.
A condom does have the advantage of being more elastic. Less likely to break the condom or your ribs. On the down side they probably block a certain amount of UV. On the up side socially, per Cody Lundin, this option allows you to go to the store and request a box of the largest and strongest condoms they carry. ROTFLMAO.
Be well.
Well, the ziplock bag is not for transporting water actually. This article is talking about filtering your water with the solar exposure method and the time involved to do so, which is 6 hours. So you could probably only use the solar method, weather you use a bottle or ziplock bag, when you have the opportunity to stay put for 6 hours and let the sun do its work. So, in that situation a ziplock bag would work very well. Actually, I bet you could affix a bottle/ziploc to the top of your pack if you were on foot all day in the sunlight.
And I buy the Magnums anyway..
The ziplock bag purification can work well during hurricane season at home too. I imagine that I dont want to use up all my coal to boil drinking water during a power outage. I can lay bags of water on foil in the sun all day and let it cool off all night to have cool drinking water the next morning. Rain water can be collected if needed. Survival is not just for campers and wilderness folks. Some of us will need to survive right where you are at home. I would add some gallon or quart size ziplock freezer bags and some foil to my hurricane kit, and the directions to purify water with it.
but plastics that get hot can leach harmful cancer causing toxins into our food/water that have been found in breast cancer. So this is a good idea but should definitely be done in something other than plastic unless it’s an emergency situation.
I agree that long-term usage of plastics may contribute to risks of follow-on health issues, although many plastics are now claimed to be ‘BPA-free’. Having said that, and since this is an emergency survival preparedness site, it is a very valid and potentially life saving method to purify water.
My wife, myself and my daughter are getting ready buy a piece of property and and take a stab at living independently off the grid. We are selling our house and everything we own, except what we know we’re going to need for construction, and starting out, buy a piece of property with a stream on it. We will raise Chickens, (eggs) Goats(milk and meat) and Rabbits(fertilizer and meat) as well as a lifetime hunting, fishing license, and grow and dehydrate our own vegetables.
We plan to excavate a giant living space into a southern facing hillside and shore the heck out of it.
We have been looking at a lot of alternative energy sources but the one thing that concerned me was purifying our drinking water. If we build a large straining system with cloth at he bottom then a layer of charcoal, then sand on top of that, with a layer of small stones on top it will sufficiently strain out and eliminate many of the harmful natural pollutants.
This method would be relatively easy to purify 10, 3 gallon water containers a day and that would be plenty of drinking and cooking water for the three of us.
The water would not have to be so strictly managed for laundry or showering. This article has been very helpful in the final stage of our plan. I saw a segment on TV where they ran the water through an intense UV light to purify it instantly. I can’t remember if it was a laser or just an intense lighting source. I will try to locate the info and get back to you.
Good luck and thanks.
Richard,
While this method won’t get you clean water, a great way to get both vegetables and protein is using aquaponics – vegetables and fish working together.
Your article is a bit off – there’s ample research that shows that glass bottles or jars work just as well as PET bottles, without the potential for harmful chemicals leaching into the water. The only caveat is the the glass must be less than 2mm thick.
Additionally, the best method for SODIS is using SODIS bags – they’re more efficient than PET or glass containers, purify the water faster, and take up far less space.
Can you use milk jug bottles?
I want to try this..here in the southwest desert the sun always shines.
Thank you.
Toni
I would expect that glass milk jugs would work, as I’m fairly confident that they are not treated to prevent ultraviolet light from penetrating. Plastic milk jugs are not clear, and will not work.
More information on glass vs. plastic for water purification by sunlight…
http://modernsurvivalblog.com/health/purify-water-with-sunlight-glass-or-plastic/
I noticed a lot of the plastic vs glass. I know people have been more concerned with BPA from their water bottles and have been making more BPA free water bottles. As long as they were clear, wouldn’t that be a better solution. Doesn’t give you the harmful BPA but still allows all the UV rays through to fully purify the water.