Emergency or short-term treatment of drinking water may be necessary due to natural disasters, accidents or other situations caused by humans. In these situations, water can be treated by using heat, chemical treatments, or filtration. Each method has certain advantages and disadvantages that must be considered. In some situations, a combination of these methods may be preferred (e.g., filtration and chemical treatment).
Inspect the water before treatment. Microorganisms may be attached to or embedded in soil or other organic particles suspended in the water. The water to be treated should be allowed to stand so suspended material settles to the bottom of the container. Coarse materials like sand will settle more quickly than finer materials suspended in the water. During and after settling, care should be taken not to agitate the water. Water from the top of the container can be gently poured or drawn off into a second clean container. A second option for removing suspended particles is to strain the water through a clean cloth, layers of paper towels or paper coffee filter; do not use a commercially available portable water filter (see discussion on filters in this guide) for this step as the suspended material may rapidly clog such filters.
Contaminants in water which may cause illness or disease include bacteria, such as E. coli, protozoan cysts such as giardia or cryptosporidium, and viruses such as Hepatitis A. Giardia or cryptosporidium are not likely to be present in some groundwaters but may be encountered in contaminated surface waters. Viruses should be suspected in any water that may be contaminated with human waste.
Heat treatment
Heat kills microorganisms and is the oldest effective means of disinfecting drinking water. The process of bringing water to a boil will kill virtually any disease-causing organism including bacteria, cysts such as giardia and cryptospyridium, and viruses. Heat the water to a vigorous boil and then let it cool. There is some variation in recommendations regarding boiling time required for disinfection. It is important to realize that bringing water to a vigorous boil will adequately disinfect it. If fuel is not limited, however, additional boiling for one minute or keeping the water covered and hot for several minutes can provide an additional margin of safety. Since water boils at a lower temperature as elevation increases, the Centers for Disease Control and Prevention (CDC) recommends boiling for 3 minutes at altitudes above 6562 feet (2000 meters) in order to be certain that viruses are killed.
Though boiling effectively disinfects water for drinking, it does not provide a residual (or long-term disinfection). Therefore, care must be taken not to re-contaminate the water Boiled water may taste flat; the taste can be improved by pouring it back and forth between two clean containers to re-oxygenate it or by adding a pinch of salt to each quart after it has cooled.
Chemical treatments
Chlorine and iodine are the most commonly used chemicals for emergency disinfection of water. The killing effectiveness of the chemical depends on the concentration of the chemical in the water, the amount of time the available chemical is in contact with the water prior to use (contact time), the water temperature and the characteristics of the water supply. A de- creased concentration of the disinfectant or a lower temperature will require a longer contact time for adequate disinfection. If the water temperature is less than 410 F (or 50 C), it should be allowed to warm prior to disinfection or the chemical dose should be doubled. If the water is cloudy, it is recommended to strain it through a coffee filter before treatment.
A common objection to chemical disinfection is the flavor it gives to the treated water. If flavorings of any kind are added to the water to improve taste it should be done after the recommended contact time for disinfection. Flavorings added before adequate contact time has been achieved will “tie up” some of the chemical available for disinfection. Adding about 50 mg of vitamin C (ascorbic acid) per liter or quart of water after the contact time can improve the taste. Vitamin C is often avail able in 250 and 500 mg tablets where vitamin supplements are sold. Tablets should be pulverized and divided before adding to the water. In addition, freshness preservatives containing vitamin C are often available where canning supplies are sold.
Bacteria are very sensitive to chemical disinfectants such as chlorine and iodine. Viruses, cryptosporidium, and giardia require very high dosages of disinfectant or longer contact times with the disinfectant than the standard recommendations. Heat treatment is recommended if these pathogens are suspected in the water.
Chlorine
Regular household chlorine bleach that contains 5% to 6% sodium hypochlorite as the only active ingredient can be used for disinfection. Standard household bleaches are 5.25% sodium hypochlorite; those labeled “Ultra” are generally 6% sodium hypochlorite. Bleaches with labels such as “Fresh Wildflowers,” “Rain Clean,” “Advantage,” or labeled as scented may contain fragrances, soaps, surfactants, or other additives and should be avoided for drinking water disinfection. Using a medicine dropper, add 16 drops per gallon (4 drops per quart). Stir the water and let it stand covered for 30 minutes. For adequate disinfection, the water should have a slight chlorine odor to it after the 30 minute waiting period. If this odor is not present after the 30 minutes, repeat the dose and let it stand covered another 15 minutes. If this odor is not present, the bleach may have lost its effectiveness due to age of the product or exposure to light or heat. Present, the bleach may have lost its effectiveness due to age of the product or exposure to light or heat.
Use the freshest chlorine bleach available. If the chlorine taste is too strong in the treated water, taste can be improved by pouring the water from one clean container to another several times.
Halazone tablets are another form of chlorine for drinking water disinfection. The tablets are convenient and inexpensive but may require high doses and longer contact times. Follow manufacturer directions for use. Chemical treatment with chlorine provides some protection against recontamination since some available chemical remains in the water.
Iodine
Two forms of iodine commonly sold for chemical disinfection of drinking water are tincture of iodine (2%) and tetraglycine hydroperiodide tablets (Globaline, Coghlan’s and Potable-Aqua are examples). Iodine was once widely used, but is no longer recommended because health research has shown that as many as 8% of people have hidden or chronic thyroid, liver, or kidney disease which iodine can make worse. Iodine should not be ingested by children younger than age 14. Do not use iodine-containing products unless you have discussed the risks with your physician.
Filtration
Commercially available portable filters provide widely varying degrees of protection against disease-causing contaminants. The better filters provide adequate protection but less sophisticated filters on the market (often lower cost) may not provide protection. The more sophisticated filters typically operate by a hand pump which draws water into the filter through an intake hose or by slow gravity flow through a filter or series of filters. The filtration process works by physically within the filter medium. The size of contaminants retained depends on the pore size or the space between media fibers or granules. Most filters list an average pore size and are rated by the manufacturer according to the smallest particle they can trap. For example, a one micron (one thousandth of a millimeter) filter traps contaminants one micron in diameter or larger. The removal percentage of contaminants is affected by the amount of time the water is in contact with the filter media; shorter contact time with filter media generally results in less contaminant removal. Some filters have a chemical treatment component such as activated carbon, or iodine-impregnated resins which are effective against bacteria and some viruses. The contact time with the iodine in the filter may be too short to kill protozoan cysts, however.
Portable filters do provide immediate access to drinking water without adding unpleasant tastes or odors. However, as with boiling, the water can become re-contaminated after of 0.1 to 0.3 microns may be acceptable for cysts and bacteria but do not have small enough pore sizes to reliably remove viruses. While the filters may be reliable in remote areas where human waste contamination is unlikely, in populated areas filtration should be followed by either chemical disinfection with chlorine or boiling as described previously.
Proper selection, operation, cares and maintenance of portable water filters is essential for producing safe drinking water in emergency or short-term situations. When considering the purchase of a filter, be aware of the filter’s rating for pore size, output, pump strokes per liter, and pump force (how much effort is required to operate the pump). If size and speed are not critical factors, a gravity-fed drip filter that lets water slowly drip from a reservoir down through a filter may be a good option. Be aware that membranes in some filters can be damaged by chlorine in the water. Also, cloudy or turbid water can quickly clog a filter and shorten the life of the unit. When using a portable water filter, always follow the manufacturer’s instructions for use, care, and replacement.
Summary
Alternative drinking water sources for emergency situations and short-term use may include a stored emergency water supply that has been prepared ahead of time, bottled water, hidden sources of water within the home, and outside water sources. When a stored water supply or bottled water supply are unavailable, alternative water sources may be made acceptable for drinking by use of heat, chemical disinfection, filtration or an appropriate combination of these methods. Each method has advantages and disadvantages which should be considered for individual situations. If local public health department (or water utility) information differs from the recommendations in this guide, the local information should be followed. Local officials will be familiar with site- and event-specific conditions.
No comments:
Post a Comment