has anyone mentioned UV/heat purification with 1or 2 liter plastic soda bottles? i suppose this would work only in sunny areas. apparently you fill the bottles with pond water (or some other relatvely clear but unpure water) and leave them on the roof of your house in direct sunlight for at least 6 hours to kill all the pathogens. i haven't tried this yet but apparently its a simple technique used quite often in many african regions.