Tuesday, June 16, 2009

Collect and Purify Water

During short- and long-term emergencies, the most critical resource for you and your family will probably be potable water, especially if you live in an arid region. To prepare for such emergencies, and also to conserve water, you can practice rainwater harvesting.  Rainwater harvesting is one of the easiest ways to move towards self-sufficiency. Because we need water to survive, it is important not to rely completely on the system to provide it; redundancy is desirable in critical life-support systems such as water supply [1].

Because evaporation purifies water, rainwater is usually the purest water in the hydrologic cycle; you can therefore use it to water gardens without treatment. The simplest solution is to place a rain barrel (usually a 55-gallon drum with a valve) underneath the downspout of a rain gutter; the barrel should be placed as high as possible, keeping in mind that any support structure such as a stack of cinder blocks must be able to withstand the 450 pounds of a full barrel. Connect a hose to the valve and place it in your garden. The greater the height difference between the rain barrel valve and the hose outlet in the garden, the greater the pressure that drives the flow, and therefore the faster the water will come out of the hose. Rain barrels should have screens at the top to prevent debris or animals from entering the barrel [2]. Detains on how to construct a rainbarrel are given in [1], or you can purchase one prefabricated.

If during an emergency bottled or municipal tap water are not available and you plan to use rainwater for drinking or food preparation, you will need to go through some extra steps to make sure the water is not contaminated. Waterborne diseases cause nearly 15 million deaths each year. Disinfection kills the pathogens (bacteria, protozoa, parasites, and viruses) that can cause disease. Sterilization kills all living organisms in the water, bad or good. Purification removes potentially harmful chemicals in the water. A simple example makes the distinctions clear. You can disinfect water by pasteurizing it, which requires heating it to 149°F (65°C) for six to twenty minutes ([3], pg. 174). To sterilize the water, simply heat it to a hard boil in a covered pot. Boiling the water requires more energy for heating than pasteurization, but you can reduce the amount of energy required by tightly covering the pot to reduce heat loss. Pasteurization and boiling kill pathogens but do not remove dissolved chemicals such as the salts in seawater. To purify the water, you can remove the dissolved chemicals by boiling the water in an uncovered pot and collecting the condensed steam. Again, this is a very energy-intensive process. Below we will look at a few safe alternatives that require less energy.

The first step in water purification is to filter out suspended sediments that can hold chemical and biological contaminants by forcing the water through clean cheesecloth [2] or by temporarily placing the turbid water in a container to let the sediment settle out. Next, you need to remove biological contaminants that can cause disease. Boiling for ten minutes is the easiest solution. However, if you don't have enough fuel to treat all of your water this way, a more energy-efficient method is to use solar disinfection, termed SODIS ([1], [3]). Ultraviolet light kills the pathogens, and becomes more effective at high temperatures. Simply fix some shelves to a piece of metal painted black, then place bottles filled with water on the shelves and expose them to sunlight for six hours. You can paint the back of the bottles black so they will more effectively absorb sunlight and heat up to higher temperatures, killing the pathogens in as little as one hour [1]. In emergencies or while backpacking you can use iodine tablets, tincture of iodine 2%, betadine, or chlorine bleach to chemically treat water; see Lundin [3] and many other sources for detailed instructions on how to do this safely. There are other ways to kill pathogens, but most of them use high-tech devices such as UV lamps that need to periodically replaced or require electricity and are therefore unsustainable.

After filtering the water and killing the pathogens, most water will be safe to drink. For example, collected rainwater generally has very low concentrations of chemical contaminants, so it usually does not need to be treated to remove them. This is fortunate because it is much more difficult to remove dissolved inorganic chemicals from water. However, if you have reason to believe that your water contains chemical contaminants, you can use sunlight to evaporate the water and then collect the purified condensed water, a form of solar "still" ([4] pg. 471). The Watercone has an ingenious design that allows it to purify 1.6 quarts per day; it can even desalinate seawater ([5], pp. 193-4); see http://www.mage-watermanagement.com/. Or you can set up a still to collect steam produced by boiling water, as described above.

Following the simple procedures described above can help you provide potable water for you and your family during short- and long-term emergencies.

1. Kellogg, S. and S. Pettigrew, Toolbox for Sustainable City Living. 2008, Cambridge, MA: South End Press. 241

2. Bates, A., The Post-Petroleum Survival Guide and Cookbook: Recipes for Changing Times. 2006: New Society Publishers. 236 978-0-86571-568-4.

3. Lundin, C., When All Hell Breaks Loose: Stuff You Need to Survive When Disaster Strikes. 2007, Layton, Utah: Gibbs Smith. 449

4. Tawrell, P., Camping & Wilderness Survival. Second ed. 2006, Lebanon, New Hampshire: Paul Tawrell. 1080 978-0-9740820-2-8.

5. Steffen, A., ed. World Changing: A User's Guide for the 21st Century. 2006, Abrams: New York, NY. 596.

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