When it’s not possible to boil your water and there are no other options available, disinfecting water with sunlight is an effective way to make it safe to drink. The World Health Organisation recommends this emergency treatment when water supplies have been contaminated with harmful bacteria.
To disinfect water using sunlight, simply fill a clean, clear container with water and place it in direct sunlight for at least six hours. This will kill any harmful bacteria present in the water, making it safe to drink.
This process is known as solar disinfection and should not be confused with water purification which is the process of removing impurities, parasites, and other causes of turbidity, as well as harmful microbes.
Solar disinfection, or SODIS is a water treatment method that uses solar radiation to disinfect contaminated water. This process is simple, inexpensive, and does not require any electricity or chemicals. SODIS is an effective way to treat water in developing countries where access to safe drinking water is limited.
SODIS was first developed in the 1980s by the Swiss Federal Institute of Aquatic Science and Technology (EAWAG) after a Professor from the American University of Beirut, Aftim Acra, discovered the cost efficiency of water disinfection using the sun.
Since then, SODIS has been adopted by communities around the world as an affordable and effective way to treat water.
How Does Disinfecting Water with Sunlight Actually Work?
It is the UV rays in sunlight that help to disinfect water. UV rays are effective in killing bacteria and viruses. The UV rays work by damaging the DNA of microbes, making it difficult for them to reproduce.
SODIS is most effective against bacteria and does not work as well against viruses. This is because viruses are much smaller than bacteria and can evade the disinfecting effect of the ultraviolet light more easily. Nevertheless, SODIS can still help reduce the spread of viruses by reducing the overall number of harmful microorganisms present in water. For this reason, SODIS is often used in combination with other water treatment methods, such as water filtration, to provide greater protection against both bacteria and viruses.
SODIS is most effective when the water is clear and there is exposure to direct sunlight. This means that cloudy water or shady areas are not ideal for disinfection. However, the process will still work, it just takes a lot longer. UV rays are always present in daylight whether it is cloudy or not.
Steps for Disinfecting Water using SODIS
Transparent PET plastic bottles are the best type of water container to use for solar water disinfection, as they allow the water to be heated by the sun more effectively.
If the water to be disinfected is turbid, or is from an unknown or an untrusted source, it will need to be filtered prior to disinfection. SODIS is only effective against microbes (bacteria, viruses, and chlorella algae) it will not remove heavy metals, nitrates, sulfates, effluents, or any other particles or suspended matter.
- Fill a clear bottle with water, leaving just a few inches of space at the top.
- Oxygenate water by shaking the bottle
- Top up bottle until completely full
- Place the bottle in direct sunlight for 6 hours. If you’ve been unable to remove any turbidity prior to disinfection then the bottle must be left undisturbed in direct sunlight for at least two days.
The disinfection process will be much more effective if you can lay the bottle on a black or reflective surface as this will intensify the effects of the UV rays and produce much higher temperatures.
Survival Benefits of Disinfecting Water with Sunlight
SODIS, although used extensively in third world countries, should be considered a ‘last resort’ water disinfection process. Boiling water is a much more reliable and effective way to disinfect water when in a survival situation, especially if you are short on time.
Also, SODIS, as with other disinfection methods, is only effective if the water is clean in the first place. You will need to pass your water through an effective water filter first.
You also need a clear bottle or plastic bag to use solar to disinfect water, something you may not have to hand in a survival situation.
In short, disinfecting water with sunlight does work, but it has its limitations, especially when it comes to making water safe to drink in a survival type situation.