From Home Power #57, Courtesy of and with permission from Home Power Magazine, http://www.homepower.com

Solar Drinking Water Pasteurization for the Developing World Dale Andreatta )1996 Dale Andreatta Between 1992 and 1994, Derek Yegian, Lloyd Connelly, and I, as graduate students in mechanical engineering at Berkeley, developed several devices for solar pasteurization of drinking water in the developing world. This is an area where simple solar technology can solve a great human need.

Introduction/ According to UNICEF statistics, millions of people die each year in the developing world from water-borne diseases. If a water source is unsafe, the most common recommendation is to boil the water, but this is rarely done due to the time and scarce fuel it would require. Contrary to what many people believe, it is not necessary to boil water to make it safe. It is also not necessary to distill water, unless the water is chemically contaminated or brackish. Heating water to 1490 F (650 C) for 6 minutes, or to a higher temperature for a shorter time, will kill all germs, viruses, and parasites. This process is called pasteurization. The fact that water only needs to be heated to 1490 F reduces the amount of energy needed and puts the required temperature into the range that can be achieved in a simple solar collector. Pasteurization is not the only technique that can purify water, and the selection of the right system should be based on local conditions. Pasteurization is particularly useful if very little money is available and only a small amount of water is needed. For example, drinking water for a family. It is also necessary to educate people about the need for clean water and how to keep their water from being recontaminated.

Basic Methods of Solar Water PasteurizationSolar Cookers/ A simple method of pasteurization is to put blackened containers of water in a solar box cooker, an insulated box made of wood, cardboard, plastic, or woven straw, and having a clear top. One popular type of solar box cooker is made of aluminized cardboard and has a solar collection area of about 23 by 19 inches. With this device, one to three gallons per day is produced in the field. Each person requires about one gallon of water per day, about half of which is for drinking and the other half for dish washing and teeth brushing. The cost for this device is about $20 US, depending on how easily available the basic materials are. Other types of solar cookers can also be used.

Regardless of the type of solar cooker used, a way of knowing that the water reached the pasteurization temperature is needed. An inexpensive device that does this was developed and is shown in Figure 1. It is a plastic tube with both ends heated, pinched, and sealed, and with a particular type of soybean fat in one end that melts at 1540 F. The tube itself is buoyant, but is weighted with a washer so it sinks to the bottom (coolest) part of the water. The fat is in the high end of the tube. If the fat is found in the low end of the tube at any time after immersion, the water reached the proper temperature, even though it may have since cooled down. A nylon string makes it easy to take the tube out without recontaminating the water. The tube is reused by flipping it over and sliding the string through the other way. This device works in any size water container, costs about $3 and is available from Solar Cookers International. This device also works with fuel-heated water. Since heating the water to the pasteurization temperature rather than the boiling point of treated water per day. The cost of the heat exchanger itself is about $15 US, making the cost of the complete system about $65. For a cost increase of about 30% the heat exchanger provides a roughly 400% increase in water output.

The Solar PuddleA Low-Cost Large-Area Device/ While many factors determine the usefulness of a water pasteurizer, an important figure of merit is the amount of water delivered per unit cost. We developed a device made only of low cost materials, which we call a solar puddle. One form of the solar puddle is shown in figure 3, though many variations are possible. The test device was a family-size unit, about 3 1/2 by 3 1/2 feet, but the puddle could be made larger or smaller. If the puddle is made larger there is more water to pasteurize, but also proportionately more sunshine collected. We dug a shallow pit about 4 inches deep, and put in 2 inches of solid insulation. We used wadded paper, but straw, grass, leaves, or twigs could be used. This layer of insulation was made flat, except for a low spot in one corner of the puddle, which is marked trough in Figure 3. A layer of clear plastic and a layer of black plastic went over the insulation with the edges of the plastic extending up and out of the pit. Two layers were used in case one developed a small leak. We used inexpensive polyethylene, though UV stabilized plastic would last longer. We put in some water and flattened the insulation so the water depth was even to within about 1/2 inch throughout the puddle, except in the 1 inch deeper trough. More water was added until the average depth was 1 to 3 inches depending on how much sunshine was expected. A pasteurization indicator went in the trough since this was where the coolest water collects. At this point a drain siphon was installed in the lowest part of the trough so that the most water could be siphoned out before the siphon started to draw air. The end of the siphon was held solidly in place by several small rocks. A layer of clear plastic went over the water to prevent evaporation, again with the edges extending beyond the edges of reduces the energy required by at least 50%, the fuel savings offered by this simple device alone is considerable.

Flow-Through Pasteurization Devices/ In the early 90s PAX World Service produced a flow-through pasteurizer made of 50 feet of black-painted tubing coiled within the same solar box cooker previously mentioned. One end of this tubing is connected to a thermostatic valve and the other to a storage tank for untreated water, which also contains a sand/gravel/charcoal filter. The small amount of water within the tubing allows rapid heating of the water to the valves opening temperature. The thermostatic valve opens, allowing the pasteurized water to drain out of the tubing and into a second storage vessel for treated water. As the treated water drains from the solar box cooker, contaminated water from the storage tank automatically refills the tubing. Once this cool water reaches the valve, the valve shuts and the pasteurization process begins anew. This type of unit can adapt to variable solar conditions which takes the guesswork out of filling jugs in a batch process. This is also an automatic process, freeing time for other chores.

In field trials by PAX World Service and the Pakistan Council of Appropriate Technology, the device produced 4 to 6 gallons per day. The cost of this device is about $50 US.

Although this is a respectable increase, larger improvements can be achieved by recycling the heat in the outgoing pasteurized water. Once the water has been pasteurized and released form the solar box cooker, the heat in this water can be used to preheat the incoming water. Since the temperature of the water entering the solar box cooker is higher, it takes less time and energy to finish the pasteurization process. We built several simple devices which accomplish this preheating. In one version the hot outgoing water flows on one side of a metal plate, while on the other side incoming water flows in the opposite direction (see figure 2). Heat from the hot water is transferred to the cold water, thus preheating the incoming contaminated water. Another version uses a similar idea, but with water flowing in opposite directions on the inside and outside of a metal tube. The flat-plate version is less expensive and produces somewhat more water, but the tubular version is easier to make from purchased parts. Laboratory tests using simulated sunlight showed that the flat plate unit allows around 75 to 80% of the energy to be recycled, and four to five times more water to be pasteurized over a flow-through unit without a heat exchanger. This corresponds to about 20 to 24 gallons.

An insulating air gap was formed by putting a spacer (a large wad of paper) on top of the third layer of plastic and then another clear layer of plastic with at least 2 inches of air between the top 2 plastic layers. Finally, dirt or rocks were piled on the edges of the plastic sheets to hold them down. Once the puddle is built it is used by adding water each day, either by folding back the top 2 layers of plastic in one corner and adding water by bucket, or by using a fill siphon. The fill siphon is NOT the same siphon that is used to drain the puddle, as the fill siphon is recontaminated each day, while the drain siphon MUST REMAIN CLEAN. Once in place the drain siphon is left for the life of the puddle. The only expensive materials used to make the puddle are a pasteurization indicator ($3), a siphon tube (about $1), and 4 sheets of plastic (about $2 for the size tested). Many tests were done in the spring and summer of 1994 in Berkeley, California. On days with steady sunshine the required temperature was achieved even with 17 gallons of water, corresponding to a depth of 2 1/2 inches. With thinner water layers, temperatures as high as 1780 F were reached.

The solar puddle will work under conditions that are not ideal. Small holes in the top layers arent a problem, the device works in wind or if the bottom insulation is damp, and condensation inside the top layer is not a problem. The water temperature is uniform throughout the puddle to within 20 F. The puddle does not work well on cloudy days. After some months, the top plastic layers weaken under the combined effects of sun and heat and have to be replaced.

There are many variations of the solar puddle. The least expensive form is built into the ground, but a puddle could be built with wooden sides on top of the ground, on a tabletop, or on a roof. Weve been able to put the top layer of plastic into a tent-like arrangement that sheds rain. Adding another layer of plastic creates another insulating layer of air and makes the device work even better, though this adds to the cost. As mentioned, the device can cover a larger or smaller area if more or less water is desired. One could help solve the problem of recontamination by putting drinking cups into the solar puddle and pasteurizing them along with the water.

Access/ Author Dale Andreatta, S.E.A., Inc., 7349 Worthington-Galena Rd., Columbus, OH 43085  614-888-4160  Internet E-mail: andreatt@seaohio.com Solar Cookers International, 1724 11th St., Sacramento, CA 95814  916-444-6616

-- Don Kulha (dkulha@vom.com), August 04, 1999