WATER DISINFECTION

Published on 14/03/2015 by admin

Last modified 14/03/2015

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WATER DISINFECTION

Water purification is the removal of chemical pollutants by filtration through activated charcoal or active resin compounds. This usually improves the taste, but does not decrease the incidence of infectious disease, because microorganisms are not removed. Water disinfection is the treatment of water with chemicals, boiling, or filtration to remove agents of infectious disease, such as bacteria and cysts. Sterilization is the removal of all life forms.

If at all possible, carry disinfected water with you. If you must drink water from a stream or lake that you cannot disinfect, try to use small tributaries that descend at right angles to the main direction of valley drainage. Clean melted snow is of less risk than ice taken from the surface of a lake or stream. Most bacteria that cause diarrhea can survive for months in ice.

The principal offending agents in contaminated water or on unwashed food that cause illness and diarrhea are the bacteria Salmonella, Shigella, E. coli, and Campylobacter, and the flagellate protozoan Giardia lamblia (see page 207). Drinking nondisinfected water in parts of Africa, India, and Pakistan can cause dracunculiasis (guinea worm disease). In countries where water is improperly disinfected, stick to bottled or canned carbonated beverages, beer, and wine. However, be advised that even bottled water in developing countries can be contaminated with bacteria that cause diarrhea, so even bottled water should probably be boiled or disinfected with chemicals or ultraviolet (UV) light before drinking. All containers should be wiped clean to remove external moisture and dirt. All ice should be considered contaminated. For purposes of preserving the environment, it is preferable to carry a reusable water container that is filled with disinfected water than to discard multiple plastic or glass containers.

Do not urinate or defecate (inadvertently) into or near your water supply. Build a latrine 8 to 10 in (20 to 25 cm) deep into the ground at least 100 ft (31 m) and downhill from the water supply. Try to keep the latrine away from a gully or other formation that might become a runoff stream during a thaw or after heavy rainfall.

“Raw” drinking water should be allowed to rest for several hours in order for large particles to settle to the bottom. The top portion can be poured off, if possible, through a filter or fine cloth. Coagulation and flocculation techniques remove smaller suspended particles. Add a pinch of alum (an aluminum salt) to a gallon (3.8 liters) of water and mix well, then stir occasionally for 60 minutes. Allow the water to rest while the aggregated particles settle, and then pour off the upper (hopefully clearer) part through a paper filter (such as a laboratory-grade filter with a pore size of 20 to 30 microns).

Water may be disinfected by any of the following methods:

1. The usual advice—to boil water for 5 to 10 minutes plus 1 minute for each 1,000 ft (305 m) of altitude above sea level—is probably overkill. Giardia cysts are instantly killed in water heated to 158°F (70°C). To play it safe, bacteria and most viruses require a few minutes at this temperature. Hepatitis A virus requires a full minute of boiling to ensure inactivation.

Time and temperature have an inverse relationship with respect to water disinfection: The higher the temperature, the less time is required, and vice versa. For instance, pasteurization of food products can occur at a lower temperature (158°F, or 70°C) if 30 minutes at this temperature are allowed. Sterilization (killing of all microorganisms) occurs after 5 to 10 minutes of boiling at sea level.

The temperature at which water boils varies with altitude because of the surrounding barometric pressure. Barometric pressure is expressed in terms of the height (in inches or millimeters) of a column of mercury (Hg) that exerts a pressure equal to that of a column of air with the same size base. At sea level (barometric pressure 760 mm Hg), water boils at 212°F (100°C); at 5,000 ft, or 1,525 m (632 mm Hg), 203°F (95°C); at 10,000 ft, or 3,050 m (522 mm Hg), 194°F (90°C); at 14,000 ft, or 4,270 m (446 mm Hg), 187°F (86°C).

Thus, boiling water is effective for disinfection at any altitude below 18,000 ft (5,490 m) likely to be attained by a wilderness enthusiast. The time required to heat the water to boiling contributes to the disinfection process. To provide a wide margin of safety, boil the water for 3 minutes.

Halogens, such as iodine and chlorine, are effective chemical disinfectants. The rate at which they kill microorganisms depends on the concentration (measured in mg per liter, or parts per million [ppm], which are equivalent) of halogen and time allowed for disinfection. At a given water temperature and pH, contact time is inversely related to concentration. Thus, you double the contact time if half the concentration of halogen is present. Decreased (cold) water temperature or cloudy (more organic material) water requires a longer contact time or higher halogen concentration. Halogens can create an unpleasant taste if the concentration exceeds 4 mg/liter. They can lose effectiveness after prolonged exposure to moisture, heat, or air, and may be corrosive or stain clothing. In general, to improve taste, use a lower concentration of halogen for a longer contact time. Eight mg/liter (or ppm) is considered the concentration of iodine effective for water disinfection in room-temperature, clear water. A pregnant woman or a person with thyroid disease or iodine allergy should consult a physician before using any iodine compound for water disinfection.

Water disinfection tablets, such as Potable Aqua (see below) and other iodine- and chlorine-based products, may be used inside plastic hydration bladders, such as those found in the CamelBak. While they may discolor the plastic, they do not degrade it.

2. Add one tablet of fresh tetraglycine hydroperiodide (Potable Aqua, Globaline, Coughlan’s, EDWGT) to 1 quart (liter) of water and allow the water to stand for 15 minutes. If the water is cloudy, use two tablets. If the water is cold, allow 1 hour after adding the tablets before drinking. Each tablet releases approximately 8 mg/liter of iodine. Do not leave an open bottle exposed to high heat and/or humidity. Potable Aqua Plus includes oxidizing tablets to remove the iodine taste after disinfection.

ADD TO 1 QUART (LITER) OF WATER