It does work, but there are some limitations:

1. "The most favourable region for SODIS lies between latitudes 15°N/ S and 35°N/ S... The second most favourable region lies between the equator and latitude 15°N/ S..."

35°N is approximately Oklahoma City. The limitations may not be so limited in more northern climates if the day is warm and clear, and the temperature of the water rises to at least 113F (45 C).

2. The optimum size bottle is 1.5 liters, the max. is 2 liters, and the plastic must be clear, not tinted (the bluish PVC bottles are not as effective as the clear PET).

3. The water has to be fairly clear: fill the bottle, set the bottle on a newspaper headline, and sight down through the uncapped bottle -- if you can read the print through the bottle, it's clean enough.

4. SODIS won't do anything about chemically-contaminated water.

Sue