Water Filters and Giardia

Kyoto: Hiking from Kibune to Kurama

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Whenever possible, persons in the Outdoors should carry drinking water of known purity with them. When this is not practical, and water fromstreams, lakes, ponds, and other outdoor sources must be used, time shouldbe taken to disinfect the water before drinking it.

Boiling

Boiling water is one of the simplest and most effective ways to purifywater. Boiling for 1 minute is adequate to kill Giardia as well as mostother bacterial or viral pathogens likely to be acquired from drinkingpolluted water.

Chemical Disinfection

Disinfection of water with chlorine or iodine is considered less reliablethan boiling for killing Giardia. However, it is recognized that boilingdrinking water is not practical under many circumstances. Therefore, whenone cannot boil drinking water, chemical disinfectants such as iodine orchlorine should be used. This will provide some protection against Giardiaand will destroy most bacteria and viruses that cause illness. Iodine orchlorine concentrations of 8 mg/liter (8ppm) with a minimum contact timeof 30 minutes are recommended. If the water is cold (less than 10 deg C or5O deg F) we suggest a minimum contact time of 60 minutes. If you have achoice of disinfectants, use iodine. Iodine's disinfectant activity isless likely to be reduced by un favorable water conditions, such asdissolved organic material in water or by water with a high pH, thanchlorine.

Below are instructions for disinfecting water using household tincture ofiodine or chlorine bleach. If water is visibly dirty, it should first bestrained through a clean cloth into a container to remove any sediment orfloating matter. Then the water should be treated with chemicals asfollows:

Iodine

Tincture of iodine from the medicine chest or first aid kit can be used totreat water. Mix thoroughly by stirring or shaking water in container andlet stand for 30 minutes.

Tincture of Iodine Drops* to be Added per Quart or Liter Clear Water Coldor Cloudy Water**

2% 5 10

* 1 drop = 0.05ml

** Very turbid or very cold water may require prolonged contact time; letstand up to several hours or even overnight.

Chlorine

Liquid chlorine bleach used for washing clothes usually has 4% to 6%available chlorine. The label should be read to find the percentage ofchlorine in the solution and the treatment schedule below should befollowed.

Drops* to be Added per Quart or Liter AvailableChlorine Clear Water Cold or Cloudy Water**

1% 10 20 4% to 6% 2 4 7% to lO% 1 2 Unknown 10 20

* 1 drop = 0.05ml

** Very turbid or very cold water may require prolonged contact time; letstand up to several hours or even overnight.

* 1 drop = 0.05ml** Very turbid or very cold water may require prolonged contact time; letstand up to several hours or even overnight.

Mix thoroughly by stirring or shaking water in container and let standfor 30 minutes. A slight chlorine odor should be detectable in the water;if not, repeat the dosage and let stand for an additional 15 minutesbefore using.

Filters

Newcomers in the battle against waterborne giardiasis include a variety ofportable filters for field or individual use as well as some householdfilters. Manufacturers' data accompanying these filters indicate that somecan remove particles the size of a Giardia cyst or smaller and may be capable of providing a source of safedrinking water for an individual or family during a waterborne outbreak.Such devices, if carefully selected, might also be useful in preventinggiardiasis in international travelers, backpackers, campers, sportsmen,or persons who live or work in areas where water is known to becontaminated.

Unfortunately, there are yet few published reports in the scientificliterature detailing both the methods used and the results of testsemployed to evaluate the efficacy of these filters against Giardia. Untilmore published experimental data become avai lable, there are a few commonsense things that a consumer should look for when selecting a portable orhousehold filter. The first thing to consider is the filter media. Filtersrelying solely on ordinary or silver-impregnated carbon or charcoal shouldbe avoided, because they are not intended to prevent, destroy, or repelmicro-organisms. Their principal use is to remove undesirable chemicals,odors, and very large particles such as rust or dirt.

Some filters rely on chemicals such as iodide-impregnated resins to killGiardia. While properly designed and manufactured iodide-impregnated resinfilters have been shown to kill many species of bacteria and virus presentin human feces, their efficacy against Giardia cysts is lesswell-established. The principle under which these filters operate issimilar to that achieved by adding the chemical disinfectant iodine towater, except that the micro-organisms in the water pass over theiodide-impregnated disinfectant as the water flows through the filter.

While the disinfectant activity of iodide is not as readily affected aschlorine by water pH or organic content, iodide disinfectant activity ismarkedly reduced by cold water temperatures. Experiments on Giardiaindicate that many of the cysts in cold wa ter (4 deg C) remain viableafter passage through filters containing tri-iodide or penta-iodidedisinfectants (28). As indicated earlier, longer contact times (comparedto those required to kill bacteria) are required when using chemicalfilters to proces s cold water for Giardia protection. Presently availablechemical filters also are not recommended for muddy or very turbid water.Additionally, filters relying solely on chemical action usually give noindication to the user when disinfectant activity ha s been depleted.

The so-called microstrainer types of filters are true filters.Manufacturer data accompanying these filters indicate that some have asufficiently small pore size to physically restrict the passage of somemicro-organisms through the filter. The types of filter media employed inmicrostraining filters include orlon, ceramic, and proprietary materials.Theoretically, a filter having an absolute pore size of less than 6micrometers might be able to prevent Giardia cysts of 8 to 10 micrometersin diameter from passing. However, when used as a water sampling deviceduring community outbreaks, portable filters in the 1- to 3- micrometerrange more effectively removed Giardia cysts from raw water than filterswith larger pore sizes. For effective removal of bacterial or viralorganisms which cause disease in humans, microstraining filters with poresizes of less than 1 micrometer are advisable. However, the smaller thepores, the more quickly the filters will tend to clog. To obtain maximumfilter life, and as a matter of reasonable precaution, the cleanestavailable water source should always be used. Keep in mind, however, thateven sparkling, clear mountain streams can be heavily contaminated.

Secondly, because infectious organisms can be concentrated on the filterelement/media, it is important to consider whether the filter element canbe cleaned or replaced without posing a significant health hazard to theuser. Properly engineered portable filters should also minimize thepossibility of contaminating the "clean water side" of the filter withcontaminated water during replacement or cleaning of the filter element.This is especially important for filters used in the field where they areoften rinsed or "cleaned" in a stream or river that may be contaminated.

Ongerth (29) recently evaluated four filters (First Need, H20K, Katadyn,the Pockett Purifier) for their ability to remove Giardia cysts fromwater. Only the First Need and Katadyn filters removed 100% of the cysts.