The internet is a great resource for survival knowledge, but it also has the potential to spread survival myths and misconceptions at an alarming rate. All it takes is for one ignorant individual to post a blog article or YouTube video to a large online audience, and the idea spreads like wildfire. Pretty soon, you may even see less-than-reputable online publications re-posting these myths without fact-checking them, luring thousands of new readers in with sensational clickbait headlines — Bear Grylls doesn't want you to know about this one cool survival trick! You won't believe it! Click here!!, the headlines insist.

Drinking urine through a LifeStraw is a really bad idea, yet it still spread rapidly online.

Those who know the truth behind these survival myths will roll their eyes and move on, but innocent or uneducated readers may fall for it hook, line, and sinker. The clickbait articles go viral on social media, and the misconceptions continue to spread. If you want to see this phenomenon in action, check out our previous articles on the myths of “safely” drinking urine with a LifeStraw and making a fire-starter from a lemon battery. The source videos for those myths received 6 million and 22 million views, respectively. That's infuriating, but it also motivates us to spread the word about the truth.

When is Water Safe to Drink?

Due to the complex nature of the microbiological science involved, water purification is often the subject of these survival myths, like the urine-through-a-LifeStraw video above. We can't observe the bacteria present in a sample of water with the naked eye, and we can't necessarily smell or taste them either. Obviously foul, murky, and stinking water is a red flag. But crystal-clear stream water or even water direct from the tap can harbor dangerous microbes. This leads to all sorts of folk remedies and tall tales about ways to make water potable. Some have elements of truth; others are obviously false.

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(For an in-depth look at how water filters and purifiers remove or inactivate bacteria, protozoa, and viruses, check out the article H2O Hygiene from Issue 15 of our magazine.)

We recently stumbled upon an interesting water-related misconception in a comment on social media:

“Calm, shallow water exposed to the sun is pretty safe because surface bacteria are killed by the sun and most everything else settles to the bottom. If you really need to drink untreated water, you are generally better off skimming a calm pond than dipping out of a stream or river.”

This statement is almost entirely false, and it contains potentially dangerous misinformation, but there's also a grain of truth as well. To shed some light on this issue — forgive the pun — we'll delve into the concept of solar water purification. First, we'll discuss what's wrong about the statement above, and then we'll establish the truth about the sun's power to kill bacteria in water.

Misunderstandings About Solar Water Purification

So, would direct sunlight really kill pathogens in a calm pond, making its water safe to drink? If you've studied water purification at all, you should know that the answer is emphatically NO. In fact, water in a stagnant pond is likely some of the most dangerous water you could find in a survival situation. The World Health Organization states, “Stagnant pools should not be considered a safe source of potable water.”

The dangers of stagnant water are twofold. First of all, warm and still water provides an ideal breeding ground for pathogens. Giardia thrives in water at a balmy 70°F (21°C), and its harmful cysts can remain viable for almost one month at this temperature, according to the EPA. Even at a scalding 129°F (54°C), hot enough to burn your skin, the EPA states that Giardia cysts can survive for 10 minutes.

Even in the hottest areas, it's extremely unlikely that the sun would heat a pond's surface to a temperature sufficient to kill all pathogens, much less raise the temperature uniformly throughout the stagnant water. A few inches below the surface, the water may still be cool enough to harbor bacteria or other pathogens. Most ponds also have shaded areas and surface vegetation where bacteria can flourish and spread throughout the water. Then consider that animals may be drinking from this water source, defecating in it, or dying and rotting in it.

Secondly, there's the risk of mosquitoes and other disease-spreading organisms. Warm, stagnant water is frequently a breeding ground for mosquitoes and other insects, and a little bit of sunlight won't deter them. In many parts of the world, malaria and Dengue fever are serious risks around stagnant water.

Again, even the warm surface layer of a stagnant pool is likely to be contaminated in some way, and should not be considered safe to drink without boiling or other purification. Then again, the same can be said for running water, like streams and rivers. Regardless of the flow rate of the natural source, you should always purify water before consuming it if at all possible.

The Truth About Solar Water Purification

Now, we mentioned earlier that there's a grain of truth in the original quote — specifically, it's the phrase “bacteria are killed by the sun”. Exposing water to sunlight for extended periods is actually a viable method of water purification, but it's not going to happen naturally in a pond. It takes effort and human intervention to purify water with the sun's rays.

Solar water disinfection, also called SODIS, has been studied extensively by the Swiss Federal Institute of Environmental Science and Technology. This research has also been supported by one of the world's largest humanitarian organizations and providers of aid in developing nations, the United Nations Children's Fund (UNICEF). Here's a quote from a UNICEF document titled “Promotion of Water Treatment and Safe Storage in UNICEF Wash Programmes”:

Solar disinfection, which combines thermal and UV radiation, has been repeatedly shown to be effective for eliminating microbial pathogens and reduce diarrhoeal morbidity (Hobbins 2004) including epidemic cholera (Conroy 2001). Among the most practical and economical is the “SODIS” system, developed and promoted by the Swiss Federal Institute for Environmental Science and Technology.

It consists of placing low turbidity (<30NTU) water in clear plastic bottles (normally 2L PET beverage bottles) after aerating it to increase oxygenation and exposing the bottles to the sun, usually by placing them on roofs. Exposure times vary from 6 to 48 hours depending on the intensity of sunlight. Like filters, thermal and solar disinfection do not provide residual protection against recontamination. Accordingly, householders must have a sufficient number of bottles to allow them to cool and maintain treated water in the bottles until it is actually consumed.

For a complete guide to this solar water treatment process, refer to the SODIS training materials page. These materials are available free of charge in more than a dozen languages. For more information on solar water purification, and the Swiss research which has proven its effectiveness, click here to view a PDF of the 2016 SODIS Manual.

This method of placing water in a sealed clear plastic bottle and exposing it to direct sunlight is highly effective at creating clean potable water. Unlike the exposed warm water on the surface of a pond, this creates a closed system with a small volume of water. Under these conditions, sunlight can effectively purify the water in most cases.

Conclusions

Like all water purification methods, solar water purification is not foolproof, since the research indicates that certain pathogenic viruses and protozoa will still survive the treatment. However, it's far superior to drinking untreated water, and has dramatically reduced the incidence of waterborne illnesses in developing nations. One study showed that children in Bolivia who drank water that had been treated with the SODIS method were 70% less likely to suffer from diarrhea than those who drank untreated water.

If you ever find yourself in a survival situation without access to clean water, remember the simple technique of solar water purification. If you can fill a clean plastic water bottle and expose it to sunlight for a full day, you can create potable water and avoid illness. Just be sure to follow the SODIS method, and don't assume that all water exposed to sunlight is automatically clean.

Written by Patrick McCarthy