Thursday, March 25, 2010

SODIS--The Ultimate Water Purification System

by Richard Crews
One billion people around the world do not have safe drinking water. Children typically suffer the most. Third-world children commonly get five to ten bouts of diarrheal disease each year. In fact, diarrhea from contaminated drinking water is the leading cause of death among children under five in many non-industrialized regions, and it is a leading cause of malnutrition and stunted physical development in the children who survive.

SODIS—which stands for Solar Water Disinfection—is a water purification system that is free, requires almost no labor, no expensive equipment, and no electricity or other expensive energy. It works best on small quantities of water--a single drinking bottle at a time.

To apply the SODIS method, begin with any available water that is not too brackish--from surface puddles, roof run-off, ponds, rivers, etc. (Water that is too muddy or brackish may require filtration through a piece of cloth or a grass mat.)

Next, take a recycled plastic water bottle, strip off the label, fill it with water, and leave it in direct sunlight for about six hours. It is best to lay the bottle on a piece of metal, such as a corrugated metal roof. (On a camping trip, you can place a bit of aluminum foil under the bottle to reflect sunlight back up into the water.) If the bottle gets hot in the sun, that's good--it can cut the time down to a couple of hours.

SODIS works because the ultraviolet wavelengths in sunlight kill all viruses, bacteria, and parasites in the water. Then the water is safe to drink. And since the water is sterile, it can be stored safely in that same bottle at room temperature indefinitely.

SODIS works best if you first fill the bottle 3/4 full, cap it, and give it a good shake to allow as much oxygen as possible to become dissolved in the water. Then fill it up the rest of the way and place it in the sun. Don't disturb the bottle for six hours while it is "cooking." If the sun is not shining brightly, a longer period--up to two days--may be required.

How can you be sure when the bottle of water is ready and safe to drink? Simply err on the side of caution. Don't wait until you are thirsty before you begin the process. Have plenty of bottles and keep them in rotation. When a bottle is empty, refill it and put it on the roof. Whenever it rains, collect all the water you can.

The best plastic bottles to use are PET, polyethylene terephthalate; these are bottles with a recycling code "1" in the triangle on the bottom. You cannot use glass bottles because glass will block the crucial ultra-violet wavelengths. Cloudy or whitish plastic does not work well either, and a PET bottle works best when it is not too scratched or beaten up.

SODIS is effective against any and all biological contamination. However it does not remove salt and cannot remove chemical contaminants. But water from rain, roof run-off, or even ground puddles can usually be collected before it has picked up such chemicals. Even for the infamous arsenic poisoning in Bangladesh, arsenic enters the water as it seeps up through underground aquifers. Water that just lands on--or courses along--the surface generally does not contain arsenic. By the way, laboratory test have demonstrated that water heated or stored in PET bottles does not leach any poisonous chemicals from the plastic.

There have been a few problems with acceptance and use of the SODIS method in the third world. In a couple of places there has been a lack of social acceptance--people who have bottles of water out in the sun on their roofs feel they are advertising that they cannot afford to buy "normal" chlorinated water as their neighbors do. Also, there are other possible sources of diarrhea-causing organisms, such as contamination of vegetable gardens by non-potable irrigation water. These problems must be addressed through continuing education.

The SODIS system--effective, free, and remarkably simple--is currently in use by more that four million people around the world. It is recommended and endorsed by the World Health Organization.