Using ultra violet light from the sun is amongst the easiest and most sustainable method for drinking water treatment. The simplest UV treatment scheme requires only clear plastic or glass bottles and bright sun, two commodities that are well supplied in most areas of the developing world. Water is simply placed in the plastic bottle. Because oxygen content of water has been shown to correlate well with inactivation in fecal bacteria (Reed, 1996), the bottle should be shaken vigorously in order to oxygenate the water. The bottles should then be left in full sunlight for at least 3-6 hours with occasional shaking to maintain oxygen content of the water. The bottles can then be stored overnight for use the following day (Reed, 1997). This simple method of disinfection has been shown to be very effective in reducing water borne disease. Reed (1997) presents data indicating a 99.9 % inactivation of coliform bacteria and viral infection using the very simple UV disinfection method. An important consideration in this method is the turbidity of the water, since greater turbidity can results in a longer time to 99.9 % inactivation. However, Rolla (1998) reports that this does not completely
(Photo from Sodis.ch ) eliminate biological activity.
However, there are still problems with UV treatment techniques. Although most biological activity is neutralized by these techniques, giardia cysts will survive UV treatment regardless of the system used for disinfection (Extension Bulletin 795, 2003). In the developing world, it may be difficult to know that pasteurization temperature has been reached. If thermometers are available, they can be used to this end. If not, another method must be found to guarantee the achievement of the appropriate temperature. “An ingenious method developed by Dr. Fred Barrett (U.S. Department of Agriculture, retired) in 1988 is the Water Pasteurization Indicator (WAPI).” This device is a polycarbonate tube, sealed at both ends, and partly filled with a blue soybean fat, which melts at 156 F. It is placed fat end up in the water container, so the user can easily identify when the fat has melted and the water is at the appropriate temperature (Rolla, 1998). A schematic of the “WAPI” is included in the section on “Solar Boxes/Ponds.”
In addition to the inability to inactivate giardia spores, simple UV treatment will not work on cloudy or rainy days. For this reason, it is necessary to have a back up plan for drinking water disinfection. The real advantage to these techniques is that they require little in the way of inputs, either material or monetary. The expertise required is likewise very small. These benefits make UV treatment a viable plan for very small-scale drinking water treatment in the developing world. As mentioned above, a large-scale UV treatment procedure would be more expensive and more difficult to manage.
Links and Resources
Guidelines for Design and Operation of Plastic Bottle Method