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brown or green water that is loaded with organic material, as opposed to inorganic matter
such as fine clay particles. Alum (aluminum sulfate) is added to water (1/8 to 1/4 tsp/gal)
and mixed thoroughly. Then stir or gently agitate occasionally for 5 minutes and allow to
settle. Colloidal particles clump together and then settle by gravity or float. The clear
water can be decanted, filtered or poured through a cloth or coffee filter. The majority of
microorganisms will settle with the floc, but a second disinfection step is recommended.
Alum can be obtained at chemical supply stores or some grocery stores (pickling
powder). If alum is not available, lime or the fine white ash from a campfire can be used.
3. Charcoal filters (purifiers) alone are not adequate for disinfection, although they
improve the taste and appearance of water by absorbing chemicals.
D. Halogens (chlorine and iodine) are effective disinfectants for viruses, bacteria, and
protozoan cysts (excluding Cryptosporidium). Both are available in tablet or liquid form.
Iodine also comes in crystalline or polymolecular resin form. In equivalent
concentrations, iodine has some advantages over chlorine for field use, less reactivity
with organic matter, and less sensitivity to pH.
The effectiveness of a halogen depends on its concentration, the temperature of
the water, and the amount of time it is left in the water (contact time). Weaker
concentrations or colder water necessitate longer contact time. In the wilderness, the
residual concentration cannot be measured, so some uncertainty results. Very high doses
of halogen may be used to overcome the uncertainty, but this results in unacceptable
taste. Smaller doses are effective in clean water if a prolonged contact time is used.
Detection of a faint halogen color, smell, or taste indicates the presence of residual
halogen in the water.
Because iodine and chlorine react with organic impurities to form a relatively
inactive compound, the dose must be increased in grossly contaminated or cloudy water.
Inorganic particulate matter does not react with halogens, but can be removed by
straining, filtering, sedimentation, or coagulation to improve taste. It is best to clarify
water before treatment with halogens. Palatable surface water has a nearly neutral pH,
which is optimal for effective treatment with a halogen.
Taste may be improved by several means:
1) Decrease the amount of halogen while increasing contact time.
2) Add flavored drink mix after adequate contact time.
3) Pour water through a charcoal filter after adequate contact time.
4) Use techniques that do not leave residual halogen, such as heat or filters.
5) Remove the halogen taste by using a zinc brush (see below), sodium
thiosulfate, ascorbic acid (vitamin C), or hydrogen peroxide in combination with
calcium hypochlorite (see below).
An alternative to iodine should be sought for pregnant women (although the
amount ingested from a filter with an iodine resin is safe). Caution should be exercised if
iodine is used for more than 2 weeks with anyone on Lithium (establish stable medication
dose and confirm normal thyroid function) or with an active thyroid disease. Women over
50 are at some risk of hypothyroidism.
Halogen Doses
