Wilderness Medical Society snowmass 2005 Page 236
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III. TECHNIQUES FOR WATER DISINFECTION
A. Iodine resins. The resin releases iodine on contact that binds to microorganisms. The
exact mechanism of iodine transfer to organisms is not known. Minimal iodine dissolves
in water: effluent contains 0.5 to 2.0 ppm iodine. This dissolved iodine is not responsible
for disinfection, so many filters include a charcoal resin to remove all iodine dissolved in
the water after passing through the iodine resin. Some devices incorporate a 1 micron
filter to remove cysts that are resistant to iodine (Cryptosporidium), or require longer
contact times (Giardia). Potential problems include channeling of water through the
resin, which may allow some organisms through without contacting the iodine resin.
B. Chlorination-dechlorination. This technique uses very high concentrations of chlorine
for disinfection, then "dechlorination" with peroxide, which forms soluble calcium
chloride, a tasteless and odorless compound. Excess peroxide bubbles off as oxygen. The
kit consists of chlorine crystals (calcium hypochloride) and 30% hydrogen peroxide in
separate small Nalgene bottles. This is a very good technique for highly polluted or
cloudy waters, for disinfecting large volumes, and for storing water on boats. Note: 30%
peroxide is extremely corrosive and burns skin.
C. Flocculation-chlorination. Tablets contain both alum as a flocculent and chlorine for a
disinfectant. This has the advantage of cleaning and disinfecting cloudy or foul smelling
water in a one-step process. The tablet is designed to leave 8 ppm free residual chlorine
after flocculation, but 3 to 5 ppm is more common. Extend the recommended 15 minute
contact time for added safety in cold water.
D. Dehalogenation. A small wand with brush-like zinc and copper alloy bristles is used to
stir the water to dechlorinate. It is intended to be used after halogenation. Zinc catalyses
an electrochemical reaction reducing hypochlorite to chloride or iodine to iodide, neither
of which have taste, smell, or color. Zinc is not used up so the life span of the product is
indefinite. The device is practical only for small amounts of water at a time. Larger
volumes or higher concentrations require a considerable amount of time. Very small
amounts of sodium thiosulfate or ascorbic acid will accomplish the same chemical
reduction, removing halogen taste. These techniques should only be used after adequate
contact time.
IV. CONTROVERSIES
A. What is the best technique? The best technique depends on personal preference and
intended use. Use of heat may be limited by fuel supplies. If planned for a large group,
halogenation or high-capacity filters work best. Two-stage techniques are more effective
as water quality deteriorates.
Iodine and chlorine have similar antimicrobial activity, although there may be
some advantages to iodine. Most prefer the taste of iodine over chlorine in equipotent
doses, and iodine is less reactive with nitrogenous wastes in the water. However, iodine is
physiologically active, and may be unsafe for individuals with iodine allergies, for those
with uncontrolled thyroid disease, and for prolonged use in pregnant women. Although
