This is the most reliable method,
although stoves, kettles and heating coils may be inconvenient to use while
travelling. Boiling for one minute is sufficient to kill all harmful
organisms, even at high altitude when water boils at a lower temperature.
Iodination is a very effective and
convenient method for water purification. Iodine destroys bacteria,
viruses and cysts, and its action is dependent on its concentration, the
water temperature and duration of contact. Thus, a concentration
of 8 mgs per litre at 20 degrees centigrade, will destroy all pathogens
if left for 10 minutes. Lower concentrations and lower water temperatures
require a longer duration of action.
Various preparations of iodine are
available, including tincture of iodine, iodine crystals and tablets.
Manufacturers instructions should be followed in each case. With
tincture of iodine, adequate levels are normally achieved by using 4 drops
to 1 litre of water or one drop to a glass. As with all halogens it is
less effective against cryptosporidia.
Iodine has been used safely for periods
of several months, however it is not known whether there are adverse effects
associated with longer term use. Those with a history of thyroid
disease or iodine allergy, the pregnant and the very young may be advised
against using iodine for water purification.
The taste of iodine can be largely
removed by neutralising the iodine, after it has had time to work,
with dissolvable vitamin C tablets, lime or lemon juice. It should
be noted that this also stops the disinfectant action, so care should be
taken to prevent recontamination of water.
Chlorine and silver
Chlorine and silver based tablets
are available from chemists and specialised travel equipment shops. When
used correctly they destroy most bacteria (e.g. V.cholerae), but
are less effective for viruses and cysts (e.g. hepatitis A virus, giardia
and amoebic cysts, cryptosporidia).
Chlorine alone is readily inactivated
by organic matter and its action varies with pH. However if used in combination
with Phosphoric acid in combination (e.g. Aquamira) it is more effective
and this combination will destroy both giardia and cryptosporidia.
A wide variety of filters are available.
Some versions have cores which can be replaced. Filters remove sand, clay
and other matter as well as organisms by means of small pore size, membranes,
adsorption, exchange resins and osmosis. They effectively remove bacteria
and parasites but not viruses. Good filters are effective against cryptosporidia
and giardia. Due to the inability to remove viruses, filtered water
must also be chemically treated or boiled.
Filtration combined with iodine
Some modern filters incorporate a
method of chemical disinfection to increase their effectiveness.
This is usually achieved by passing water through iodine exchange resins.
When negatively charged contaminants contact the iodine resin, iodine is
instantly released so killing the microorganisms without large quantities
of iodine being in solution. Such filters should be replaced after
a specified volume of water has been treated, although this could sometimes
be difficult to keep track of.
adventure holiday shops and pharmacists
Equipment for sterilising water and
water filters can usually be purchased at 'outdoor' sports, camping and
adventure shops. Chlorine and iodine preparations can often be obtained
from local pharmacies. Filters for sterilising water must not be confused
with cheaper versions (sometimes available in supermarkets) designed only
to remove smells and chlorine from domestic water.