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Wednesday, February 4, 2009

Solar Water Disinfection and Pasteurization, by Ariel


This article describes so me simple and practical methods for providing drinkable water in disaster situations. They fit with my motto: "Keep calm, and carry on!"

According to the EPA, if you can’t boil water, you can disinfect it using household bleach. Do not use non-chlorine bleach to disinfect water. Typically, [when freshly-purchased] household chlorine bleaches will be 5.25% available chlorine. Bleach will kill some, but not all, types of disease-causing organisms that may be in the water. If the water is cloudy, filter it through clean cloths or allow it to settle, and draw off the clear water for disinfection. Add 1/8 teaspoon (or 8 drops) of regular, unscented, liquid household bleach for each gallon of water, stir it well and let it stand for 30 minutes before you use it. Store disinfected water in clean containers with covers. There are two big disadvantages of treating water with chemicals. First chlorine can be potentially dangerous if used improperly and it may not be readily available when you need it. But there is a safe, chemical-free, and inexpensive option for disinfecting your water.

Cardboard and aluminum foil are unlikely tools for disinfection water until you factor in sunshine. Solar water pasteurization uses the heat of the sun to raise the temperature of water to a point where microbial pathogens are destroyed. Disease-causing organisms in water are killed by exposure to heat in a process called pasteurization. Water that has been heated to 165 degrees F is free from living microbes including Escherichia coli, Rotaviruses, Giardia and the Hepatitis-A virus. [JWR Adds: The water need not be "held" at a boiling point for ant period of time. Just make the water reach 165 degrees F, and it is done]

Although traditional fuels can be used to pasteurize water, on sunny days solar energy is the better choice. A major problem with boiling water for disinfection is its energy consumption in relation to cost and availability of the fuel supply. If you do not have electricity you might not be able to sanitize your water..

With full sunshine, it can takes up to two hours to reach 165 degrees F pasteurize two liters of water. In order to determine when water has reached pasteurization you will need to invest in a simple device called a Water Pasteurization Indicator (WAPI). The WAPI is a simple reusable device containing a special soy wax. The wax melts at the same temperature as the water is pasteurized. The tube hangs on a string inside the container with the wax end up, and once the water around it becomes hot enough to kill the bacteria the wax melts, running from the top part of the tube to the lower end. Although it is designed for solar pasteurization, the WAPI can be used for pasteurizing over most fuel sources including gas, wood, and charcoal. WAPIs generally cost between $5 and $10.

As described previously in SurvivalBlog, SOlar water DISinfection (SODIS) involves filling clean PETE (Polyethylene Terephtalate) transparent plastic bottles with water and exposing them to full sunlight for six or more hours. [JWR Adds: Do not use polycarbonate water bottles, such as those made up until recently by Nalgene, since that type of plastic blocks ultraviolet (UV) light!] The combination of UV-A radiation and raised water temperature disinfects the water. There are a few drawbacks to this method. SODIS efficiency depends on the physical condition of the plastic bottles, with scratches reducing the efficiency of the SODIS process. There has been some concern over the question whether plastic drinking containers can release chemicals or toxic components into water, a process possibly accelerated by heat. A solar cooker will make the SODIS process more efficient.

While pasteurizing will solve a lot of disease problems, it does not remove other things found in the water such as chemicals, pesticides and heavy metals.

You can purchase elaborate solar cookers or build one using simple materials. You can find a large number of solar cooker building plans at I have included instructions for a simple windshield shade solar cooker as an addendum to this article. I have also included instructions from Needful Provision, Inc. on how to construct a simple solar water-distilling unit.

Building and testing a solar cooker also makes an excellent school science project. The last project I worked on with my child was, “Can you bake cookies in a solar oven in North Carolina in the month of January?” The conclusion was yes. Solar cookers should certainly be one the items at the top of the list for TEOTWAWKI. They are portable, use only the sun as an energy source and they work!

This is one of the easiest cookers to make and it works great. You can make it for less than $10:

Kathy Dahl-Bredine's Auto Windshield Shade Solar Cooker

Materials Needed:
Reflective accordion-folding car sunshade
Wire frame or grill)
4 inches of Velcro
Black pot
Bucket or plastic wastebasket
Plastic baking bag
1. Lay the sunshade out with the notched side toward you.
2. Cut the Velcro into three pieces, each about one inches long.
3. Stick or sew one half of each piece, evenly spaced, onto the edge to the left of the notch. Attach the matching half of each piece onto the underneath size to the right of the notch, so that they fit together when the two sides are brought together to form a funnel. If using stick-on Velcro, you can align the two pieces easily like this: Stick down one side of the Velcro, then press the two pieces of Velcro together, fold the shade into the funnel shape and stick down the second side.
4. Press the Velcro pieces together, and set the funnel on top of a bucket or a round or rectangular plastic wastebasket.
Place a black pot on top of the rack, placed inside a plastic baking bag. A standard size rack in the U.S. is 10 inches. This is placed inside the funnel, so that the rack rests on the top edges of the bucket or wastebasket. Since the sunshade material is soft and flexible, the rack is necessary to support the pot. It also allows the suns rays to shine down under the pot and reflect on all sides. If such a rack is not available, a wire frame could be made to work as well.

1. The funnel should be tilted in the direction of the sun.
2. A stick placed across from one side of the funnel to the other helps to stabilize it in windy weather.
3. After cooking, simply fold up your “oven” and slip the elastic bands in place for easy travel or storage.

Source: (A modification of a design by Kathy Dahl-Bredine, Oaxaca, Mexico)
[JWR Adds: As Reader William B. pointed out, distilled water is NOT good for you, for any length of time, as the minerals your body needs, have been effectively removed! Consider it a very short term contingency method!]

The Needful Provision Solar Water Distiller:

Select a place with good access to sunlight. Dig a hole in the ground about the size of 2-bushel basket, then smooth the bottom of the hole, and add about an inch of sand as necessary to cover any objects that have sharp edges. Line the hole with black plastic (10 mil if possible), and leave about 10 to 12 inches of the plastic around the outside edge of the hole. Use rocks, gravel, or course dirt to hold this edge to the ground. Fill the hole half-full of polluted water (or salt water). Then float a clean, potable water-tub, with open top, on the water. Use ropes and ground-anchors to secure the tub in a center position while floating on the polluted water.

Once the preceding steps are completed, place a siphon-type tube (suitable for potable water) so one end is anchored on the bottom of the tub--and one end extends a few feet beyond the edge of the hole. Now add a clear sheet of plastic over the hole, and allow enough plastic to extend 6 inches beyond the black plastic around the edge of the hole. Place a small, round rock in the center just over the above-described tub, so that the top plastic sags to within 4 to 6 inches of the tub. Now seal the edges of the top and bottom plastic using a layer of dirt at least 6 to 8 inches deep. Cut a circle of white cloth, or similar material, a few inches larger than the collection tub, and then place this over the tub, on top of the plastic, and under the rock (this covering should act to shade water in the tub).

By means of solar water distillation, pure water vapor collects on the underside of the clear plastic where it re-condenses and forms water droplets. The water droplets slide down the plastic, and fall off into the collection tub just below the rock. The siphon tube is then used daily to drain-off the daily ration (1 gallon per person day) of potable water. Hole sizes may be adjusted to meet the water demands of a particular family, as well as adjusting for changes in climatic conditions. If the same water distillation hole is to be used on a regular basis, then a tube and funnel system should be included to add more source water daily, without the need to move dirt or the plastic cover. Pure, potable water was the result of all our prior uses. We do know that there are 2 or 3 chemicals that may evaporate at about the same temperature as water. If such chemicals were present in the source water, then potable water may not always result.
Source: Needful Provision, Inc.


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