Solar Disinfection, or SODIS for short, is a super simple way to clear water of pathogens. For some reason it doesn't some up often when looking at water purifcation methods. You usually find the standard; boiling, chemical tablets, carbon/ceramic filtration advice, but rarely do you see the SODIS method. Actually it is similar to using artificial UV disinfection. In this case however, the UV rays are coming from the sun!
So, basically all you need is a clear bottle, clear (not cloudy) water, and a days worth of sunlight. That's all!
It seems too easy but there is plenty of research to back this up. This method is being used by tens of thousands of people in the third world to purify their water. SODIS is promoted by the World Health Organization (WHO), and many other organizations throughout the third world, as a treatment for disinfecting drinking water and reducing the cases of waterborne diseases.
But apparently, leaving a clear bottle of water in the sun for six hours will completely inactivate dangerous micro-organsims and pathogens. The more exposure to sun, the better. For example, laying the bottle down, rather than standing upright, to get more exposure. As well, the higher the temperature, the faster the method works.
I was skeptical at first as well. How could a way to kill germs and make water safe to drink be so simple yet so overlooked? What I think is great about this is that it costs nothing, it can be done almost anywhere in the world by anyone without any setup costs.
Better yet, it can be done in secret. Laying bottles out in the sun for disinfection means that you don't have to boil the water, which means no fire and smoke to give away your position.
The only two drawbacks is that you need time and sunny days. As well, sunlight will kill the germs but it can't remove chemicals or other contaminants from the water. So the water needs to be taken from an unpolluted source.
If you live in an area where there is cloud cover for most of the year you will have to rely on one of the alternative methods that were mentioned. The counter to that however is that if there is a lot of cloud cover then perhaps there is also plenty of rain, (which is safe to drink) and can be collected directly as it falls.
- A Clear Container - plastic bottle, glass bottle, or plastic bag. It is important that the container NOT be UV. UV resistance will block the necessary rays that need to be there to kill off the pathogens. Most bottles are not UV resistant and would be fine to use.
- Clear Water - This is called the Turbidity of the water. If the water is dirty or cloudy and you can not see through it enough to be able to read newspaper print, then this method will not work. What you will need to do is filter the particulate matter (or let it settle) out of the water first to make the water clear, then use this method to disinfect.
- Sunny Days - You need about six hours of direct sunlight to be absolutely sure it will kill off all germs. If you have a partly cloudy day, the you need to leave the water out for another day before it is safe. UV rays can go through the clouds but just not at full strength.
What Does SODIS Kill?
The SODIS method does not kill all waterborne pathogens, mainly those which cause diarrhea. However, studies have shown many other bacteria, viruses and protozoa are also inactivated by this process. Here is a list of tested organisms that have been reliably neutralized with SODIS:
- E. Coli
- Coxsackie virus
- Polio virus
- Hepatitis A virus
Amoebas are not killed by this method, although they will be killed by temperatures that reach above 50 degrees centigrade.
Benefits Of SODIS
- Simple - All you need is a clear container, clear water, and sun. No other equipment is needed.
- Re-Usability - Bottles can be reused as long as they do not get damaged or scratched up too much. A basic clear water bottle can be used everyday for at least six months before the UV rays start to make the bottle cloudy. Unlike with chemical treatments you can never run out as it is pretty easy to find plastic bottles if one ever needs them.
- Taste - Bottles are closed during this method so the oxygen within the water doesn't escape, unlike boiling which also kills pathogens but gives the water a flat flavor, chemical treatments that also distaste the water.
- Security - You won't have to reveal your location to use this method. No fuel, fire or smoke is necessary. It is easy to pick up a few bottles and relocate or hide them if necessary.
- Storage - Once the bottle of water has been totally disinfected, as long as the cap is not removed, the water is safe to store for an indefinite period of time. It has come to my attention that although it is possible to kill all harmful pathogens with SODIS UV, there is a high probability that not everything will be killed if the procedure is not done optimally. If water is then stored, any living pathogens will then have the ability to regrow. For that reason it is best to drink the water soon after it is irradiated by the sun
- Usability - Just drink from the bottle directly. No need to transfer the water from one container to another. This reduces the chances of contamination.
The hardest part about it is that you don't really know when the water is safe, especially on a partly cloudy day. The SODIS method is not perfect and is not 100% reliable (like boiling is) at killing all bacteria and viruses. But it is preferable than drinking 'raw' water from a stream or lake for example. Depending on your situation, it's just another piece of knowledge that you have in your survival arsenal.