In order to eliminate biological activity, pasteurization temperature (150 F or 65 C) must be achieved. This is difficult to do with plastic bottles alone. The simplest solutions to this problem are the solar box and the solar pond. A solar box consists of an insulated box constructed from wood or cardboard with a glass or plastic lid. The inside surfaces should be painted black. A covered vessel with water (ideally, also black) is placed inside. The pot needs to remain in the box until pasteurization temperature is achieved for a few minutes. On average, a solar box can pasteurize about 1 gallon of water in 3 hours on a very sunny day (Rolla, 1998).
From solar cooking.org
If the materials for a box are unavailable, a pond is also a feasible solution. A shallow pit must be dug (about 4 inches deep and 3 feet square) and insulated with grass, leaves, etc. Layers of clear and then black plastic can be used to line the pit. Water is added and then covered with another layer of plastic, rocks or wadded paper for spacing, followed by another layer of plastic. Rocks and dirt can be used to hold down the plastic. At a cost of $4, such a procedure yielded 17 gallons of water on a sunny day in Berkeley, California (Rolla, 1998).
(A basic schematic of a solar pond (Andreatta, 1994)
Links and resources for the solar box and solar pond appear below.
In addition to information on solar water pasteurization, there is information on solar cooking as well:
FAQ -- solar cooking in a nutshell:
Solar cooking documents :
Search the Solar Cooking Archive:
Join the solarcooking-L Mailing List:
Searchable mailing-list archives:
Solar Cooker Review, published two or three times a year:
All of your solar water pasteurization needs:
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. (Andreatta, 1994)
Problems and Limitations of Solar Water Treatment
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. However, the following links provide references and information for those interested in investigating larger scale UV treatment operations.