Travel Journal

Water Purification with Sunlight


Did you know that solar radiation from the sun can purify (disinfect) water and make it safe for drinking from harmful bacteria?Harnessing the solar energy from
the sun to purify (disinfect) water from a lake or stream into safe
drinking water 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
will make the water safe to drink (see the caveat list). In fact,
the effectiveness of terminating harmful bacteria is about 99 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. Use a typical
    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.

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

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.

For better results 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.

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