SODIS: Disinfecting water with sunlight

SODIS method on metal roof

There are many ways to make safe drinking water but what if you are caught unprepared and can only scrounge up an old water bottle? There is a way to disinfect water using only sunlight and a plastic bottle. SODIS – “Solar water disinfection”, and since it only requires water and a plastic bottle, it couldn’t be any easier.

According to National Geographic, “An estimated 1.8 million people die every year from diarrheal diseases, mainly by drinking or coming into contact with dirty water.” As a survival minded individual you are aware that just any water shouldn’t be consumed directly, and have probably seen the myriad of products to make it safe. In no way am I saying any of those legitimate products don’t work or shouldn’t be used, but even the most prepared person could end up stranded without their kit. While this system is mainly being pushed for “developing” countries, I see it as a viable backup to filters and purification tablets.

dog drinking from pond

Too bad we all can’t grab a drink like this.

 

The cons pretty much outweigh the pros on the SODIS method, leaving it mainly as a last resort of water disinfecting.

The benefits of SODIS:

  • Reduction of viruses, bacteria, and protozoa in water.
  • Simplicity of use.
  • Free

The drawbacks of SODIS:

  • Specific bottles are required:
    • 0.3 to 2.0 liters in size.
    • Clear PET construction.
    • Scratches or other damage that discolors the plastic will reduce the effectiveness.
  • Requires a minimum of 6 hours direct sunlight. May require two days if the sky is cloudy, or not at all if completely overcast/raining. (pro tip: collect rainwater)
  • Does not remove harmful chemicals or metals.
  • Requires low turbidity water, may need to prefilter water to clear it up.
  • Once removed from sunlight, any remaining bacteria or viruses may begin to reproduce.
  • Does not improve the taste. That nasty pond water will still taste like nasty pond water.
  • Bottle must be placed horizontal to take full effect of the sun.

Despite the drawbacks, SODIS could definitely save your life in certain circumstances.  Humans are messy creatures and these plastic bottles can be found almost anywhere. Most North American ground water isn’t terribly murky either so prefiltering shouldn’t be necessary in most cases unless you want to keep any larger particles out of your bottle. A handkerchief, piece of clothing, or coffee filter would work fine for that.

SODIS coffee filter

Using a coffee filter to strain out the larger particles in the pond water.

These type of filters are all about surface area. By poking the filter down inside the bottle it will filter much faster than just covering the opening.

 This is not a very mobile method and will not create very much safe water without many bottles, however even with just one bottle it should be enough to keep you alive. This would work best at a base camp or dwelling where other means of purification are not available or create an inadequate supply. Where this would not work is if you are on the move, as there is no way to ensure consistent sunlight directly on the bottle.

SODIS bottles in the sun

Place the bottles in direct sunlight for a minimum of 6 hours.

The coffee filter made no difference on the turbidity but did remove any larger particles.

There are studies that say in the real world SODIS is not very effective. This is mainly due to the fact that people are either doing it wrong or not at all. The main problems people have besides just not using the method when other options are not available are:

  • Using the wrong bottles.
    • Too large of bottles – the sunlight does not penetrate far enough.
    • Wrong color or material – absorbs or blocks the sunlight.
  • Positioning the bottles incorrectly.
  • Not allowing enough sun exposure.
  • Recontaminate the water after SODIS is complete.

 

SODIS isn’t really a skill that needs to be practiced or prepared for. It’s more to keep in the back of your mind or on a survival flash card. Despite that it is knowledge that has the potential to save your life.

You can read more below:

http://www.sodis.ch/methode/index_EN

http://news.nationalgeographic.com/news/2009/08/090818-clean-water-sunlight_2.html

https://en.wikipedia.org/wiki/Solar_water_disinfection

https://www.cdc.gov/safewater/solardisinfection.html

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