Last updated on June 11th, 2017
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 should be a clear plastic bottle. Some types of glass will block too much of the UV-A spectrum for this purpose.
UPDATE: I discovered this during a re-check of SODIS information in 2017:
Different types of glass have different chemical and physical properties. Certain types (e.g., quartz glass) have a very high transmittance for UV radiation, while other types (e.g., window glass) effectively filter out this part of the solar spectrum.
All commercial glass bottles used for beverages that were tested so far at Eawag had a UV transmittance comparable to PET bottles, and were, thus, suitable for SODIS. No differences were found in studies of glass and PET bottles that compared their SODIS effectiveness.
It is possible, however, that certain glass bottles available in target countries have different UV transmittance properties, and we recommend testing UV transmittance of locally available bottles before promoting them widely for SODIS use. Glass bottles also have certain disadvantages compared to PET bottles which include their greater weight and risk of breaking, limited availability in suitable sizes, and the lack of reusable caps.
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:
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.
Solar water disinfection if performed correctly will eliminate organic contaminants from impure water. Water that contains non-biological contaminants (heavy metals, etc..) may require filtering. One of the best drinking water filters today is the Berkey drinking water filter.