Solar Water Disinfection or SODIS as it is called is a relatively simple and straightforward method for disinfecting drinking water.  In short the SODIS process uses the sun’s energy to purify water that is questionable or known to be unsafe.  It is perhaps one of the most basic forms of water purification and requires nothing other than a little knowledge and a few plastic PET bottles.

The process is actually quite straightforward and with a little creativity on the part of the user can be highly effective.  First you’ll need a few clear plastic bottles made from PET (polyethylene terephthalate).  These bottles do not block the sun’s UV-A rays (which kill bacteria, viruses, and parasites) the way some other glass, pvc, or polycarbonate bottles would.  You can recognize a PET bottle by the recycling symbol found on the bottle.  PET bottles will be labeled with the familiar triangle of arrows around a number “1″.  The bottles should also not be too big.  Bottles larger than 2 liters may prevent the suns rays from penetrating into the bottle and treating all the water.

Once you have the right bottles it’s a simple matter to fill them about three quarters of the way with the water you wish to purify, shake them up to get plenty of oxygen into the water, which will help in the purification process, then fill the bottles the rest of the way and cap them.  If water is too cloudy or hazy it should be pre-filtered with a coffee filter or similar homemade device.  Water that is too turbid may not respond well to the SODIS  method however most clear water is treatable with this method.

Next the filled bottles are simply exposed to direct sunlight for a period of at least six hours.  Anything that can be done to keep the bottles angled towards the sun (such as laying them on a roof) or to help increase the temperature of the water as it sits in the sun will help the process as well.

Obviously the closer you are to the equator the better this method will work as you’re getting stronger rays from the sun.  But as long as you have a good sunny day SODIS has been shown to be very effective.  If the sun is not as strong in your area or it’s somewhat cloudy then it may take as long as two days or so to purify the water.  If it’s raining then the best move is to simply collect rainwater to drink.

The SODIS method is becoming quite common in the developing world and is helping to reduce the amount of pathogens in drinking water thereby reducing the number of cases of diarrhea, which kills over two million people per year.  The SODIS method is recommended by the World Health Organization (WHO), UNICEF, and the Red Cross as an effective and simple method of purifying drinking water.