Travellers may find themselves exposed to the organisms that can cause travellers' diarrhoea. Enterotoxigenic E. coli, Shigella, Entamoeba histolytica, Salmonella, Campylobacter, Giardia, Cryptosporidia, Cyclospora and rarely Vibrio cholerae are spread through the faecal/oral route.
In areas where it is difficult to maintain hygiene and sanitation, travellers are advised to take food and water precautions including water purification. Where water needs to be purified prior to drinking, then it should also be purified for e.g. brushing teeth. Even where bottled water is readily available, it is usually a good idea to have a back up system for purification should bottled water run out. In addition, purification is generally agreed to be more 'environmentally friendly' than bottled water since the same bottle can be refilled with purified water.
There are several different methods of water purification, some are more effective than others. The decision on which method to use should take into consideration the likely level of contamination of water at the destination and also whatever is deemed most practical and acceptable to the traveller.
The main methods of water purification are:
This is the most reliable method, although stoves, kettles and heating coils may be inconvenient to use while travelling. Although varying time periods are quoted, for all practical purposes, boiling for one minute is sufficient to kill all harmful organisms. It is generally accepted that bringing to a rolling boil achieves adequate purification. Water should be boiled for 3 minutes at altitudes above 2000 m (6562 ft) where water boils at a lower temperature.
This may be the most acceptable method for travellers going to visit friends and relatives where access to equipment is possible. Water purified in this way should be cooled and covered to avoid contamination after boiling.
Halogens are cheap and easy to use and may be the method of choice for those on the move such as backpackers and expeditioners.
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. Vibrio cholerae), but are less effective for viruses and cysts (e.g. hepatitis A virus, giardia and amoebic cysts, cryptosporidium).
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 cryptosporidium.
Chlorine dioxide is available in tablet or droplet form and is cheap, safe and effective. Chlorine dioxide will kill bacteria, viruses and cysts in water (including giardia and cryptosporidium). Follow directions in the pack for effective use.
Iodine (no longer recommended)
For many years this was the recommended method of chemical water purification for travellers. However, new EU directives (Oct 2009) have banned iodine for this use and it is no longer available in EU countries for the purpose of water purification.
A hand-held water purifying system has been developed that uses mixed oxidants to make water safe. The pen-sized device works by creating a brine solution using untreated water and salt. A small electrical charge is passed through this solution (using the device) generating the production of oxidants, including hypochlorite. This solution is then added to untreated water and according to the manufacturer, inactivates cryptosporidia, bacteria, giardia and viruses.
Filtration is useful for those who may only have access to brackish water such as rivers and streams, as filters remove particulate matter. A wide variety of filters are available and prices vary accordingly.
Filter pore size determines in the main how effective a device will be, although micro-organisms will also adhere to the filter material. Filter pore sizes between 0.1-0.4 µm are usually effective at removing bacteria and parasites but may not adequately remove viruses. If this type of filter is used, filtered water must also be chemically treated or boiled. Good filters are effective against cryptosporidium and giardia.
There are more technologically advanced filters available that claim to remove viruses as well as other microorganisms due to their smaller pore size. One filter has been developed using Mechanically Advanced Disinfection (MAD) technology based on electroadhesion. The pores in this filter are positively charged which attracts and holds the pathogens which are negatively charged.
Filtration combined with chemicals
Some modern filters incorporate a method of chemical disinfection to increase their effectiveness. Such filters should be replaced after a specified volume of water has been treated, although this could sometimes be difficult to keep track of.
There is evidence that ultraviolet (UV) light can destroy bacteria, viruses and Cryptosporidium in water. The effectiveness of this means of water purification depends on the UV dose and exposure time used. This method requires that the water being treated is clear as the effectiveness of the device may be limited in water that is very cloudy or has a high level of solids in it. Battery-operated pen devices that deliver a metered, timed dose of UV may be an effective means of disinfecting small amounts of clear water, however more testing is required for conclusive evidence.
Outdoor and adventure holiday shops and pharmacists
Equipment for sterilising water and water filters can usually be purchased online or at outdoor sports, camping and adventure shops. Chlorine 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.