Academic journal article Journal of Environmental Health

Sun and Water: An Overview of Solar Water Treatment Devices

Academic journal article Journal of Environmental Health

Sun and Water: An Overview of Solar Water Treatment Devices

Article excerpt

In the Pacific Northwest, the sun can be unreliable, but water seems to be a constant. In other parts of the world, the sun is a constant, reliable presence, while water, especially safe drinking water, is chancy. Unsafe drinking water causes the majority of deaths and diseases in developing countries (1). People have long used the sun for cooking and preserving food. The sun's energy can also be used to heat water to pasteurization temperature, which kills most pathogens. Many devices have been developed that concentrate the sun's energy. These devices can be as simple as a black box with a pot in it, or as complex as solar-powered battery cells that provide electricity for ultraviolet (UV) or oxidation systems. Most of the simple devices are not intended to "sterilize" water, but to reduce the number of pathogens so that the water is safer to drink.

The simplest solar water purification devices are the solar box and the solar still. Solar boxes are a well-known method for cooking food and can be used for water pasteurization. A solar box consists of a cardboard or wooden box with insulated bottom and sides and a glass or clear-plastic lid. The inside surfaces should be painted black. A covered pot with water (ideally, also black) is placed inside. The pot needs to remain in the box until the water is at 150 F (65 [degrees] C) for a few minutes. Generally, a solar box can pasteurize about 1 gallon of water in 3 hours on a very sunny day in, say, southern California. Pasteurization kills bacteria, viruses, and cysts, but does not remove chemical contaminants (2,3).

A very low-tech method, using direct solar radiation to reduce pathogens, was field-tested by researchers from the Royal College of Surgeons in Dublin, Ireland. The researchers gave 206 Masai children clear, 1.5-liter plastic bottles. The children in the test group were told to fill the bottles (from the contaminated water supply) and place them on the roof, from dawn to midday. The control group kept their bottles inside. Diarrhea incidence in the two groups was tracked over 12 weeks. The researchers found that this solar radiation method may significantly reduce diarrheal disease for communities that have no other way to disinfect water (4).

Boiling water is not necessary to kill pathogens, but reaching water pasteurization temperature is critical. So how do you measure temperature, especially in a remote area, after the glass thermometer breaks? An ingenious solution is the Water Pasteurization Indicator (WAPI). A prototype was developed by Dr. Fred Barrett (U.S. Department of Agriculture, retired) in 1988. The current WAPI was developed by Dale Andreatta and other graduate engineering students at the University of California, Berkeley. The WAPI is a polycarbonate tube, sealed at both ends, and partially filled with a blue soybean fat that melts at 156 [degrees] F (69 [degrees] C). The WAPI is placed inside the water container, with the fat end up. The user can easily tell when the water reaches 156 [degrees] F (69 [degrees] C) because the fat melts and runs to the bottom of the tube. The WAPI is reusable and durable. This device can be placed in a pot in a solar box or over a stove or fire. Since pasteurization occurs at a lower temperature than boiling, less fuel is needed for heating water. This is important where fuel is scarce or expensive (3,5).

If materials for a box are not available, or if cost is a factor, then a solar puddle may be the answer. A shallow pit about 3 feet (1 meter) square and 4 inches deep is dug and is insulated with native materials (paper, straw, grass, leaves). Over this are placed layers of clear and then black plastic, with the edges extending out and over the sides of the pit. The bottom should be flat, except for a trough along one side. A WAPI could go in the trough, the coolest part. A drain siphon is installed in the lowest part of the trough, and weighted down with rocks. Water is added to a depth of 1 to 3 inches. …

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