the significant disadvantage of lacking any
indication that the iodine has been exhausted.
SIZE COMPARISON
(MICROMETERS)
Katadyn
®
Filter Pore
0.2
µm
Giardia Cysts
6.0
µm
Cryptosporida Cysts
4.0
µm
Bacteria (Diam) 1.5 to 3.0
µm
Viruses
0.004 to 0.06
µm
Filters are relatively bulky (for backpack-
ing) and expensive, and can be rapidly ob-
structed by sediment in the water. The ob-
struction can be prevented with prefilters, but
can be relieved in some systems only by re-
placing the filter, which costs almost as much
as the entire system. For some filters obstruct-
ing sediment can be removed by scrubbing.
CHEMICAL DISINFECTION SYSTEMS
Chemical disinfection systems provide the
desired features of simplicity, speed, small
size, and lightweight, and are reliable for all
organisms except cryptosporidia (and perhaps
cyclospora.) Although many chemicals are
effective, only preparations that contain iodine
or chlorine (halide) systems are readily availa-
ble and have been proven by extensive use.
Silver compounds are used in other countries
but have not been approved by the FDA for
the United States.
IODINE AS A WATER DISINFECTANT
During World War II, a search for a sim-
ple, reliable water disinfectant was initiated
because chlorine based systems were too un-
dependable. The investigating team found that
diatomic iodine (I
2
) and the various ions resul-
ting from the reaction of molecular iodine
with water consistently and reliably disinfects
water containing as many as ten million bacte-
ria per milliliter, a concentration approxi-
mately ten times greater than grossly polluted
water. (The effectiveness of iodine was dem-
onstrated on raw sewage from the Cambridge,
MA, sewage system.) Unlike chlorine, iodine
is fast, resists inactivation by organic com-
pounds, is active over a wide pH range, and is
available in stable preparations.
At 23ºC (73ºF), even in moderately turbid
water with moderate amounts of organic
color, an iodine concentration of 8 mg/l (8
parts per million) eradicates bacteria, viruses,
parasites, and parasitic cysts other than
cryptosporidia. A contact time of only ten mi-
nutes already includes a considerable margin
of safety. (About ninety seconds is adequate
for eliminating nonparasitic organisms.)
However, an iodine concentration of 8
mg/l (8 parts per million) is only needed to
destroy parasitic cysts. A concentration of 0.5
to 1.0 mg/l eliminates other microorganisms.
If filters are used to eliminate cryptosporidia
and other parasites, such small concentrations
could be used, but no preparations that supply
such a limited quantity of iodine are currently
available. They can only be prepared by dilut-
ing or using smaller quantities of existing
preparations.
IODINE DISINFECTION
PRE-
CAUTIONS
In cold water (0º to 5ºC or 32º to 41ºF) the
chemical activity of iodine is slower, just as
all chemical reactions are slower at lower
temperatures. Contact time should be in-
creased to twenty minutes to insure complete
