Polluted water can at best lead to discomfort and mild illness, at worst to death, so the travelling layman needs to know not only what methods and products are available for water purification, but also how to improvise a treatment system in an emergency.

Three points about advice on water treatment cause misunderstanding. Firstly, there is no need to kill or remove all the micro-organisms in water. Germs do not necessarily cause disease. Only those responsible for diseases transmitted by drinking water need be treated. And even some water-borne diseases are harmless when drunk. Legionnaires' disease, for example, is caught by breathing in droplets of water containing the bacteria, and not by drinking them.

Secondly, in theory, no normal treatment method will produce infinitely safe drinking water. There is always a chance, however small, that a germ might, by virtue of small size or resistance to chemicals or heat, survive and cause disease. But the more exacting your water treatment process, the smaller the risk - until such time as the risk is so tiny as to be discounted. The skill of the experts lies in assessing when water is, in practice, safe to drink. Unfortunately different experts set their standards at different levels.

Thirdly, beware the use of words like 'pure', 'disinfect' and 'protection', common claims in many manufacturers' carefully written prose. Read the descriptions critically and you will find that most are not offering absolutely safe water but only a relative improvement.

Suspended solids

If you put dirty water in a glass the suspended solids are the tiny particles that do not readily sink to the bottom. The resolution of the human eye is about one-hundredth of a millimetre, so a particle half that size (five microns) is totally invisible to the naked eye - and yet there can be over ten million such particles in a litre of water without any visible trace. Suspended solids are usually materials such as decaying vegetable matter or mud and clay. Normally mud and clay contamination is harmless, but extremely fine rock particles including mica or asbestos occasionally remain in glacier water or water running through some types of clay.

Microbiological contamination

Eggs, worms, flukes, etc: These organisms, amongst others, lead to infections of roundworm (Ascaris), canine roundworm (Toxocara canis), guinea worm (Dracunculus) and bilharzia (schistosomiasis). They are relatively large, although still microscopic, and can be removed by even crude forms of filtration. The tiny black things that you sometimes see wriggling in still water are insect larvae, not germs, and are not harmful. Almost any form of pre-treatment will remove them.

Protozoa: In this group of small, single-celled animals, are the organisms that cause giardiasis (Giardia lamblia), an unpleasant form of chronic diarrhoea, and amoebic dysentery (Entamoeba histolytica). Both of these protozoa have a cyst stage in their life cycle, during which they are inert and resistant to some forms of chemical treatment. However, they quickly become active and develop when they encounter suitable conditions such as the human digestive tract. They are sufficiently large to be separable from the water by the careful use of some types of pre-filter. This is not always true for a common water borne protozoan called Cryptosporidium parvum that causes diarrhoea in all parts of the world and which may be lethal in immunocompromised people, e.g. those with advanced AIDS. The cysts are small enough to pass through many filters and are relatively resistant to chlorine, and are best destroyed by boiling water.

Selection of a water supply

Whatever method of water treatment you use, it is essential to start with the best possible supply of water. Learning to assess the potential suitability of a water supply is one of the traveller's most useful skills.

Good sources: Ground water, e.g. wells, boreholes, springs; water away from or upstream of human habitation; fast-running water; water above a sand or rock bed; clear, colourless and odourless water. Fast running water is a hostile environment for the snails that support bilharzia.

Pre-treatment

If you are using water from a river, pool or lake, try to not to draw in extra dirt from the bottom or floating debris from the surface. If the source is surface water, such as a lake or river, and very poor, some benefit may even be gained by digging a hole adjacent to the source. As the water seeps through, a form of pre-filtration will take place, leaving behind at least the coarsest contamination.

Pouring the water through finely woven fabrics will also remove some of the larger contamination. If you have fine, clean sand available, perhaps taken from a stream or lake bed, an improvised sand filter can be made using a tin can or similar container with a hole in the bottom. Even a (clean) sock will do. Pour the water into the top, over the sand. Take care to disturb the surface of the sand as little as possible. Collect the water that has drained through the sand. The longer the filter is used, the better the quality of the water, so re-filter or discard the first water poured through. Discard the contaminated sand after use.

If you are able to store the water without disturbing it, you could also try sedimentation. Much of the dirt in water will settle out if left over a long enough period. Bilharzia flukes die after about 48 hours. The cleaner water can then be drawn off at the top. Very great care will be needed not to disturb the dirt at the bottom. Siphoning is the best method.

If the water you are using has an unpleasant taste or smell, an improvement can be achieved by using coarsely crushed wood charcoal wrapped in cloth. When the 'bag' of charcoal is placed in the water, or the water is run through the charcoal (like a sand filter), the organic chemicals responsible for practically all the unpleasant tastes and smells will be removed. Some colour improvement may also be noticed. The water will still not be safe to drink without further treatment, but you should notice some benefit.

Treatment of a water supply

Boiling

Boiling at 100C kills all the harmful organisms found in water, except a very few such as slow viruses and spores which are not dangerous if drunk. However, as your altitude above sea level increases, the weight of the atmosphere above you decreases, the air pressure drops, as does the temperature at which water boils. A rule of thumb for calculating this is that water boils at 1C less for every 300 metres of altitude. If you are on the summit of Kilimanjaro, at 5895 metres, water will boil at only 80C. At temperatures below 100C, most organisms can still be killed but it takes longer. At temperatures below 70C, some of the harmful organisms can survive indefinitely and as the temperature continues to drop, so they will flourish.

There is one more important consideration. When water is boiling vigorously, there is a lot of turbulence and all the water is at the same temperature. While water is coming to the boil, even if bubbles are rising, there is not only a marked and important difference between the temperature of the water and the temperature at a full boil, but there can also be a substantial difference in temperature between water in different parts of the pan, with the result that harmful organisms may still be surviving. To make water safe for drinking, you should bring water to a full boil for at least two minutes. Boil water for one minute extra for every 300 metres above sea level. Do not cool water down with untreated water.

Filtration

The key to understanding the usefulness of a filter is ensuring that you know the size of the particles that the filter will reliably separate, and the dirt-load the filter can tolerate before it clogs up. If the pores in the filter are too large, harmful particles can pass through. If small enough to stop harmful particles, the pores can block up quickly, preventing any more water from being filtered.

To reduce this problem, manufacturers employ ingenious means to increase the filter area, and filter in at progressively smaller stages. But even in one apparently clean litre of water there can be a hundred thousand million particles the same size or larger than bacteria. And to stop a bacterium, the filter has to take out all the other particles as well. If the filter is small (of the drinking straw type for instance) or if the water is at all visibly dirty, the filter will block in next to no time.

There are three solutions: water can be filtered first through a coarse filter to remove most of the dirt, and then again through a fine filter to remove the harmful bacteria; a re-cleanable filter can be used; or finally, only apparently clean water could be used with the filter. The use of a coarser filter is called pre-filtration. Viruses are so small they cannot be filtered out of drinking water by normal means. However, because they are normally found with their host infected cells and these are large enough to be filtered, the finest filters are also able to reduce the risk of virus infection from drinking water.

A filter collects quite a lot of miscellaneous debris on its surface and, in order to prevent this providing a breeding ground for bacteria, the filter needs to be sterilised from time to time. Some are self-sterilising and need no action, but others should be boiled for 20 to 30 minutes at least once every two weeks.

Where filters are described as combining a chemical treatment, this is for self- sterilisation. The chemical is in such small concentrations and in contact with water passing through the filter for such a short period that its use in improving the quality of the filtered water is negligible.

Chemical treatment

Broadly speaking, there are three germicidal chemicals used for drinking-water treatment. For ease of use, efficiency and storage life, the active chemical is usually made up as a tablet suitable for a fixed volume of water, although the heavier the contamination, the larger the dose required. Germs can also be embedded in other matter and protected from the effects of a chemical, so where water is visibly dirty you must pre-filter first. Chlorine and iodine have no lasting germicidal effect so on no account should untreated water be added to water already treated.

Silver: Completely harmless, taste-free and very long-lasting effect, protecting stored water for up to six months. The sterilisation process is quite slow and it is necessary to leave water for at least two hours before use. Silver compounds are not effective against cysts of Amoeba and Giardia, so use pre-filtration first if the water is of poor quality.

Rules for treatment

Order of treatment: If chemical treatment and filtration are being combined, filter first. Filtration removes organic matter which would absorb the chemical and make it less effective. If of a carbon type, the filter will also absorb the chemical leaving none for residual treatment. In some cases, the filter may also be a source of contamination. If water is being stored prior to treatment then it is worthwhile treating chemically as soon as the water is collected and again after filtration. The first chemical dose prevents algae growing in the stored water.

Storage of water: Use separate containers for treated and untreated water, mark them accordingly and don't mix them up. If you are unable to use separate containers take particular care to sterilise the area round the filler and cap before treated water is stored or at the time treatment takes place. In any case, containers for untreated water should be sterilised every two to three weeks. Treated water should never be contaminated with any untreated water. Treated water should never be stored in an open container. Treated water left uncovered and not used straight away should be regarded as suspect and re-treated.