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Saturday, April 30, 2022

SODIS Method of Purifying Water


When it comes to prepping supplies and gear, "two is one, and one is none" is good advice to keep in mind. It doesn't matter whether it pertains to having multiples of the exact same item or different skills or methods of achieving a desired goal. It's always having Plans B, C, and D in case Plan A doesn't work out.

For me, the SODIS method of purifying water is somewhere around Plan D. We've got the well, generators for running the pump, large countertop filters, smaller backpacking filters, bleach, activated carbon, iodine, water purification tablets, and so on. However, water being essential to life and all that, having numerous ways to purify it in a variety of circumstances makes sense. SODIS is so easy and cheap that even a caveman can do it.

SODIS (SOlar water DISinfection) utilizes plastic water bottles and the sun to eliminate pathogens from drinking water. It's about as low-tech and inexpensive as you can get. Developed in the 1980s as a means of purifying water in third-world countries, it's a method we can apply to disaster situations in this country, with a few caveats.

Choose the right bottle material. The best containers are clear, uncolored PETE or glass juice or soda bottles. The distinct advantages of PETE are that it is lightweight and highly resistant to breakage. (PETE bottles in the US are designated by the number in the recycling triangle symbol.) These materials allow the ultraviolet radiation to pass through the container to kill bacteria, viruses, protozoa, and other nasties. Contrary to popular belief, it is not the heat, but rather the ultraviolet radiation that does the work here. 

Choose the right size bottle. Ideally, your containers will be one- to two-liter bottles, no more than four inches in diameter. The more water the ultraviolet light has to pass through, the less effective it becomes. 

Clean the bottle. Remove the paper labels and clean the bottles. They must be free from dust and dirt so that the light can penetrate. They must also be relatively free of scratches, as scratches diffuse light and prevent UV radiation from passing into the bottle.

Pre-filter the water if necessary. If the water is cloudy, pass it through a bandana, dishtowel, clean t-shirt, or coffee filter to remove particulates. Adding a pinch of salt is also helpful to bind solids in the water. If you are not sure whether you should have filtered the water first, hold your hand up or put a newspaper on the other side of the bottle. If you can't distinguish your fingers or read a newspaper headline through the bottle, the water needs to be pre-filtered, or disinfection may not occur. 

Fill the bottles. Fill the bottles 75% full and shake well for at least 30 seconds. Fill more bottles than you think you will possibly need. In ideal conditions, SODIS takes only six hours. But it can also take up to two days. 

Lay the bottles out. The filled bottles need to be laid on their sides to maximize exposure. For best results, lay them on a white or reflective metal (mirrors, corrugated roofing) surface to increase the amount of light passing through the bottles. The bottles must also be outdoors. Laying them in front of a sunny window will not work. Residential housing glass doesn't let enough ultraviolet radiation pass through to pasteurize water.

Wait. The requisite exposure times vary in the literature, but it takes careful research to figure out why. SODIS was developed for lower-income people in third-world countries. The vast majority of these people live within 35 degrees north or south of the equator. They've got the most optimal sun exposure for using SODIS to the fullest extent possible. At these latitudes, SODIS will do its job within six hours on a sunny or partly sunny (more than 50%) day. 

However, most of the US is above 35 degrees latitude. You can take a look at a map to see where you fit in, but here's a rough idea:

  • Louisiana, Florida, Mississippi, Alabama, South Carolina, and Georgia are all good.
  • The southern halves of Arizona, New Mexico, and Arkansas are good.
  • Most of Texas is good.
  • The southern third of Oklahoma and the southern quarter of California are good.

In these areas, solar disinfection can be achieved in six hours on sunny or partly sunny (more than 50%) days. But as you move north, the water needs more time to become pasteurized, up to 48 hours. And remember, it is the ultraviolet radiation doing the job, not the temperature. Just like it is the ultraviolet radiation producing a sunburn. And just like it is possible to get sunburned on a cloudy day. It takes a little longer, but it's still entirely possible. My husband does it all the time. 

Keep in mind that the SODIS method is not perfect. It's about 98-99% effective in killing pathogens. Still, it's better than nothing and has been scientifically proven to work. Both the World Health Organization and the Centers for Disease Control recommend it as an option for achieving clean drinking water. 

And finally, clean water does little good if people don't practice proper sanitation and hygiene, especially with food preparation and using the toilet. People can't just pick and choose what practices to observe or ignore and hope to stay healthy. We've got to do it all.

Links to related posts:
Water Treatment Methods Comparison Chart
Katadyn Micropur Water Purification Tablets
Basic Water Purification 

For further reading:

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